INFLUENCE OF NATURAL FIBERS AS ADDITIVES ON THE CHARACTERISTICS OF STONE MATRIX ASPHALT
The increase in traffic growth and maintenance expenditures demands the urgent need for building better, long-lasting, and more efficient roads preventing or minimizing bituminous pavement distresses. Many of the principal distresses in pavements initiate or increase in severity due to the presence of water. In Indian highways, where traditional dense graded mixtures are used for the surface courses, major distress is due to moisture induced damages. The Stone Matrix Asphalt (SMA) mixtures provide a durable surface course. Proven field performance of test track at Delhi recommends Stone Matrix Asphalt as a right choice to sustain severe climatic and heavy traffic conditions. But the concept of SMA in India is not so popularized and its application is very limited mainly due to the lack of proper specifications.
Development of stabilized SMA mixtures for improved pavement performance has been the focus of research all over the world for the past few decades. Many successful attempts are made to stabilize Stone Matrix Asphalt mixtures with synthetic fibres and polymers. The concept of using natural fibers to replace these energy intensive synthetic fibres or polymer additives is a recent development in this field. India, being an agricultural economy produces fairly huge quantity of natural fibres such as coconut, sisal, banana, sugar cane, jute etc. In line with these thoughts, this research focuses on the utilization of natural fibres as additives to improve the performance of SMA.
This research is an attempt to study the influence of additives on the characteristics of SMA mixtures and to propose an ideal surface course for the pavements. The additives used for this investigation are coir, sisal and banana fibres (natural fibres). A preliminary investigation is conducted to characterize the materials used in this study. Marshall Test is conducted for optimizing the SMA mixtures (Control mixture-without additives and Stabilized mixtures with additives). Indirect tensile strength tests are conducted to study the engineering properties of stabilized mixtures. The comparison of the performance of all stabilized mixtures with the control mixture and among them is carried out.
Test results have illustrated that, the type of additive and its content play significant roles in the volumetric and mechanical characteristics of SMA mixtures. The volumetric characteristics of SMA mixtures with additives are found to be within the specified limits. Stabilized mixtures with improved stability and Marshall Quotient indicate higher rutting resistance; improved indirect tensile strength value indicates better cracking resistance. All these findings support the influence of additives in Stone Matrix Asphalt mixtures.
Based on the volumetric and mechanical characteristics of the various stabilized mixtures it is inferred that the optimum fibre content is 0.3% fibre by weight of mixture for all fibre mixtures irrespective of the type of fibre.
Regarding the volumetric characteristics, fibre stabilized mixtures show higher air voids and voids in mineral aggregates than the other mixtures. But in all stabilized mixtures, the volumetric results are within the specification range.
The coir fibre additive is the best among the fibres investigated. Sisal and banana fibre mixtures showed almost the same characteristics on stabilization.
The present study brings out the importance of the use of additives in Stone Matrix Asphalt and suggests an eco- friendly alternative to synthetic fibres and polymer additives. Extensive laboratory investigations carried out provide a thorough understanding of the engineering behaviour of the SMA mixture with various additives, which increases the level of confidence in the field application of this material.
Stone Matrix Asphalt, stabilizing additives, volumetric characteristics, stability and strength characteristics.
Road network is vital to the development of economy, social integration and trade of a country. Global competition has made the existence of efficient road transport an absolute imperative. Transport demand in India has been growing rapidly since independence. Easiness in accessibility, flexibility of operations, door-to-door service and reliability has earned road transport an increasingly higher share of both passenger and freight traffic vis-a-vis other transport modes. In recent years this demand has shifted mainly to the advantage of road transport, which carries passenger and freight transport. Road transport has grown, despite significant barriers, to inter- state freight and passenger movement compared to inland waterways, railways and air which do not face rigorous enroute checks/barriers.
According to the Road Network Assessment by National Highway Authority of India, national highways constitute approximately 2% of the total road network of India, but carry nearly 40% of the total traffic. India has 67,000 km of highways connecting all the major cities and state capitals. Most of them are two-lane highways with paved roads. They are widened to four lanes and eight lanes in developed areas and large cities. As per latest reports, 19,064 km of the National Highway system still consists of single-laned roads. The government is currently working to ensure that the entire National Highway network consists of roads with two or more lanes. The total road length in India had increased significantly from 3.99 lakh km as on 1951 to 42.36 lakh km as on 2008. Concomitantly, the surfaced road had increased from 1.57 lakh km to around 20.90 lakh km over the same period (GOI, 2010).
The average increase in maintenance expenditure increases with the increase in vehicle registrations annually. This demands the urgent need for building better, long-lasting, and more efficient roads preventing or minimizing bituminous pavement distresses. This condition of the roads has a direct impact on the cost of travel, operations of vehicle, to traffic delays and to crash-related expenses. Roads in poor condition causes wear, tear to vehicles and even damage. Also, traffic queuing and delays occur when vehicles slow down to avoid important pavement distresses (e.g., potholes) or when the road surface fails to provide safe maneuvering and/or adequate stopping conditions. Many of the principal distresses in flexible pavements initiate or increase in severity due to the stagnated of water. When moisture is available in the pavement, the mechanical properties of the material decreases and the serviceability of the pavement gets reduced.
1.2 SCENARIO IN ANDHRA PRADESH
Andhra Pradesh (AP) can be proud of having developed a good road network compared to other States in India. Transportation infrastructure of Andhra Pradesh consists of 1.23 lakh km of road, 3703.25km of railways, 888 km of inland waterways, 4 airports and 14 ports. Even though it is comparatively better placed than other States as regards to road length, the quality of many of these roads are in poor condition.
Traffic in AP has been growing at a rate of 10–11% every year, resulting in high traffic intensity and pressure on the roads. Inadequate maintenance and the harsh monsoon resulted in damaged roads. The state experiences humid and tropical monsoon climate, with seasonal heavy rainfall, followed by hot summer. The months of March to June are the hottest, with a mean maximum temperature of about 35?C. The summer is followed by the monsoon season, which starts during June and continues till September. This season has heavy tropical rains in Andhra Pradesh. Maximum of rainfall is in June and July, About 1/3of the total rainfall in Andhra Pradesh is brought by the North-East Monsoons around the month of October in the state.
Roads in AP get damaged mainly on account of torrential rains during the Monsoon season. The annual road maintenance and repairs cannot withstand the severity of rains. some other reasons for the faster deterioration of roads are insufficient pavement strength to accommodate the increased traffic (10 –11% every year) and improper drainage system. Pavement shows severe distresses (cracks, large potholes, edge breaks and damaged shoulders with high edge drops).
1.3 BITUMINOUS PAVING MIXES USED IN INDIA
Bituminous mixes are used in a flexible pavement to serve the following three important parameters such as improved structural strength, facilitating subsurface drainage and providing surface friction especially in wet condition. The bituminous paving mixes as specified in MORTH specifications (MORTH, 2001)which are commonly used in India. Mixes like bituminous concrete, semi-dense bituminous concrete, premix carpet, mix-seal surfacing etc., are commonly provided as wearing courses.
Unlike most developed countries, heavy loading is a major concern in India. The axle loads in India are quite heavy and further the speed is low with many stop/start condition which leads to the rutting of currently used bituminous mixes in India. Several studies have shown that permanent deformation (rutting) within flexible pavement is usually up to the top 100 to 150 mm on the pavement. This means that both the binder and wearing course mixes should be designed to be resistant to permanent deformation. That is why in case of heavy traffic loads and high tyre pressures, it is considered to use Stone Matrix Asphalt (SMA) mix which is the apt specification as per international practice (Kandhal, 2002). The loads are carried directly by the coarse aggregate skeleton due to stone-on-stone contact. This results in a long-lasting pavement with minimum maintenance that is going to be the future concern in India. The advantages of such specifications lie not only in long life but also in the reduced cost of travel with better serviceability. In recent days, the Indian Roads Congress (IRC) has adopted a tentative SMA specification, (IRC SP 79:2008) which could be used under such circumstances.
1.4 SCOPE OF STUDY
In highways, major distress on bituminous pavement is due to the rain induced damages. It is a well established fact in developed countries that the water induced damages are expected to be less in gap graded mixes like Stone Matrix Asphalt than other traditional mixes. But the application of SMA in India is limited due to lack of proper specifications. This necessitates the need for thorough experimental and field investigations in various aspects of SMA, in context of India.
In recent days, synthetic fibres or polymers has been using as stabilizing additives in SMA. Replacement of expensive synthetic fibres and polymer additives with renewable material in SMA is an environmental indigences. In this project, a study on the impact of natural fibre material as additives in Stone Matrix asphalt and their role in the volumetric and mechanical characteristics of the mixture is proposed.
1.5 RESARCH OBJECTIVES
The main objectives focused in this research is discussed as below.
The main objective of this study is to propose a durable wearing course with Stone Matrix Asphalt by exploring the application of additives such as natural fibres which are abundantly available and to provide an eco friendly surface for AP highways.
To find out the role of additives on the mechanical and volumetric characteristics
of SMA mixtures.
To study the effect of additives in SMA and to achieve at the optimum additive content of the mixtures.
To intend the best natural fibre additive from the fibre stabilized SMA mixtures.
To suggest the best natural additive from all the SMA mixtures are investigated.
An extensive literature review on bituminous mixtures with various fibres has to be carried out. Based on previous literatures a systematic experimental investigation has to be planned to study the volumetric and mechanical characteristics of the bituminous mix.
Literature review has to be carried out to identify the present conditions of roads in India, issues in maintenance and other problems related with durability. Thorough literature study has to be carried out to examine various researches on bituminous mixtures (Dense graded, Open graded and Gap graded mixtures) with and without addition of additives.
Based on the literature review, experimental research programme has to be formulated. As a preliminary investigation, acquirement of various ingredients of SMA and the evaluation of its properties are carried out. Marshall tests and indirect tensile strength test are proposed for the mix design (with and without additives)and for assessing the strength characteristics of SMA mixtures respectively. A comparative study on different characteristics (volumetric and mechanical) of various stabilized mixtures with the addition of varying additive contents and different types has carried out for optimization. The ideal mix has proposed from the various SMA mixtures with optimum additive content.
1.7 ORGANISATION OF THE THESIS
This thesis consists of six chapters. The contents of various chapters are briefly described below.
Chapter 1 illustrates a clear description of transport infrastructure of India with special accent on the problems of Indian highways. A framework of various bituminous paving mixes used in India and the related issues are mentioned. Scope of study, research objectives and methodology for the present research work is also examined.
Chapter 2 critically reviews the literature on previous studies in the area of bituminous mixtures. The classification on bituminous mixtures regarding to SMA, its history, composition, advantages and disadvantages are discussed. An extensive summary of the literature allied with SMA and other bituminous mixtures with different additives like fibres, polymers and waste materials are also presented.
Chapter 3 on Material characterization gives an outline of the materials utilized for the study and its properties.
Chapter 4 presents the detailed work conducted in the laboratory for the mix, design of different SMA mixtures and their analysis are presented. Marshall method of the mix design and test procedure are discussed. Results of all the stabilized mixtures are analyzed separately and discussed. The effect of additive content on optimum bitumen content of SMA mixture is also evaluated.
Chapter 5 explains the investigations on indirect tensile strength of different SMA mixtures with different types of additive and contents. The tensile strength ratio of various mixtures is determined to attain the water induced damages on surface course. The detailed descriptions of test results of different mixtures within themselves and with control mixture are also conferred in this chapter.
Chapter 6 represents the conclusions derived from the present research work. This chapter highlights the impact of additives on the characteristics of SMA mixtures and supports the exception of ideal stabilized SMA mix from the mixtures that are investigated. Scope for further research work is also mentioned in this chapter.
REVIEW OF LITERATURE
India is the second largest developing economy in the world, in equal with other developmental activities, road infrastructure that are developed at a very fast rate. Large scale road infrastructural development projects like National Highway Development Project (NHDP) and PradhanManthri Gram SadakYojna (PMGSY) are in advancement. A sudden increase in the growth of traffic and overloading of vehicles decrease in the life span of roads laid with conventional bituminous mixes. This imparts the reduction in the riding quality of roads which results in unconscionable vehicle operating costs and frequent maintenance intrusion due to premature failure of pavements. Providing durable roads has always been a issue for a nation like India with varied climatic conditions, landscape condition, rainfall intensities and soil characteristics. A decent measure of research is going everywhere throughout the country in this field to solve the problems regarding with pavements. It is observed that Stone Matrix Asphalt mixture is an ideal mixture for the long lasting Indian Highways. The literature regarding to the bituminous mixtures is reviewed in this chapter with a brief description of SMA mixtures.
2.1.1 Flexible Pavements
Flexible pavements are called “flexible” since the total pavement structure “bends” or “deflects” due to traffic loads. This pavement structure generally composed of several layers of materials which can accommodate this “flexing”. In this type of pavements, material layers are usually arranged in the order of descending load bearing capacity with the highest load bearing capacity material (and most expensive) on the top and the lowest load bearing capacity material (and least expensive) on the bottom. The surface course is the stiffest and contributes the most to the pavement strength. The underlying layers are less stiff but are still important to pavement strength as well as drainage and frost protection. A typical flexible pavement structure (Fig. 2.1) consists of surface course, base course and sub base course (optional).
The surface course is the top layer in contact with traffic loads. This layer provides the characteristics such as friction, smoothness, noise control, rut resistance and drainage. In addition, it serves to prevent the entrance of excess quantities of surface water into the underlying base, sub base and subgrade courses (NAPA, 2001). The topmost layer of the surface course which is in direct contact with traffic loads is the wearing course. This can be removed and replaced as and when it becomes damaged or worn out. The wearing course can be rehabilitated before distress propagates into the underlying intermediate / binder course. This layer which constitutes the major portion of the surface course is meant to distribute the load coming over it. The base course is the layer directly below the surface course which helps in transmitting the load to the subgrade and generally consists of aggregate either stabilized or unstabilized. Bituminous mixes like Hot Mix Asphalt can also serve as a base course. Under the base course layer, a layer of less expensive / inferior quality material can be provided as sub base course material over the subgrade.
The sub base course is optional in many cases.
Fig: flexible pavement structure
2.2 CLASSIFICATION OF BITUMINOUS MIXTURES
A bituminous mixture is a composition of bituminous materials (as binders), properly graded aggregates and additives. Bituminous mixtures used in the pavement applications are classified on the basis of their methods of production, by their composition and characteristics.
On the basis of method of production, bituminous mixtures are classified as Hot- mix asphalt (HMA), Cold-laid plant mix, Mixed-in-place or road mix and Penetration macadam. Hot-mix asphalt is produced in the hot asphalt mixing plant (or hot-mix plant) by mixing a suitably controlled amount of aggregate with a controlled amount of bitumen at a high temperature. The mixing temperature has sufficiently high such that the bitumen is fluid enough for appropriate mixing and coating of the aggregate, but not too high as to avoid extreme stiffening of the asphalt. HMA mixture must be laid and compacted when the mixture is still enough hot so as to have decent workability. These are the most commonly used paving materials in the surface and binder courses in bituminous pavements. Cold-laid plant mix is produced at a bitumen mixing plant by mixing a controlled amount of aggregates with controlled amount of liquid bitumen without the application of heat. It is laid and compacted at atmospheric temperature. Mixed-in-place or road mix is produced by mixing aggregate with the bitumen binders in the form of emulsions (medium setting or slow setting) in suitable proportions on the surface of road by means of special road mixing equipment. Penetration macadam is produced by a construction procedure in which the layers of coarse and uniform size aggregate are distributed on the road and rolled, and sprayed with suitable amounts of bitumen to pierce the aggregate. The bituminous material used in this may be hot bitumen or rapid setting bitumen emulsion.
Bituminous mixtures can be classified on the basis of method of composition and characteristics as Dense-Graded HMA, Open-Graded HMA and Stone Matrix Asphalt (SMA). Dense-graded mixtures (Fig. 2.2) has a dense-graded aggregate gradation (aggregates are evenly distributed from coarse to fine) and had a relatively less air voids after placing and compaction. These are commonly used as surface course and binder courses in bituminous pavements. The term bituminous concrete is mostly used to concern a high-quality, dense-graded HMA mixture. A dense graded HMA mixture with maximum size of aggregate greater than 25 mm is called a large stone dense- grade HMA mix, whereas a HMA mix with 100% of aggregate particles passing through the sieve of 9.5mm is called a sand mix.
Unlike dense-graded mixes, an open-graded HMA mixture (Fig. 2.3) has relatively larger size aggregate that contains very small or no fines, they are designed to be water permeable. Due to less surface area of aggregate, these mixes have comparatively lower bitumen content than that of a dense-graded HMA mix.
Stone Matrix Asphalt (SMA) is a gap graded bituminous mixture having a high proportion of coarse aggregates and filler with comparatively less medium sized aggregates (Fig. 2.4). It contains high binder content, low air voids with high levels of macro texture when laid resulting in waterproofing with good surface drainage.
Fig. 2.2 Dense graded HMA
Fig. 2.3 Open graded HMA
Fig. 2.4 Stone Matrix Asphalt
Fig. 2.5 Comparisons between SMA and conventional HMA
For a comparison, a typical view of SMA mixture and a conventional dense graded mixture (NAPA, 1999) is shown in Fig. 2.5. Cores from SMA mixtures (left) explain the greater percentage of fractured aggregate and higher percentage of asphalt binder, compared to the conventional Hot Mix Asphalt (HMA) mixture (right) which compraise a more uniform aggregate gradation and less bitumen binder.
2.3 ADDITIVES IN BITUMINOUS MIXES
Bitumen modification / reinforcement have obtained a considerable attention as feasible solutions to magnify flexible pavement performance. The introduction of this technology with the transportation industry was predominantly induced by the unsatisfactory performance of traditional road materials exposed to exceptional increase and changes in traffic patterns. Since then, different types of modifiers for bituminous mixtures like fibres and polymers are considered. It was possible to enhance the performance of bituminous mixes used in the surfacing course of road pavements, with the help of different types of stabilizing additives. The additives such as fibres, rubbers, polymers, carbon black, artificial silica, or combination of all materials are used to stiffen the mastic at high temperatures during production and placement, and to get even higher binder contents for increased durability (Pierce, 2000). Since Stone Matrix Asphalt is the focus of the present work, the literature related to that presents a separate session after this. The following is a review of the work done in the bituminous mixes stabilized with different types of additives.
2.3.1 FIBRE AS AN ADDITIVE
The historical backdrop of the use of fibres can be traced back to a 4000 year old arch in China constructed with a clay earth blended with fibres or the Great Wall built 2000 years ago (Hongu and Philips,1990). However, the modern developments of fibre reinforcement started in the early 1960s (Mahrez, 2003). Zube (1956) published the first known study on reinforcement of bituminous mixtures. This study evaluated different types of wire mesh placed under an overlay in an attempt to prevent reflection cracking. The study concluded that all types of wire reinforcement prevents or greatly delayed the formation of longitudinal cracks. Zube proposes that the utilization of wire reinforcement allow the thickness of overlays to be decreased while achieving the same performance. No problem was identified with the steel and bituminous mixture compatibility.
Fibres are added as reinforced materials in bituminous mixtures. Reinforcement comprises of incorporating certain materials with some desired properties within other material which do not have those properties (Maurer and Gerald, 1989). Basically , the principal functions of fibres as reinforced materials are to provide additional tensile strength in the resulting composite and to increase strain energy absorption of bituminous mixtures (Mahrez et al., 2005).
Some fibres have high tensile strength in respect to bituminous mixtures, thus it was found that fibres have the potential to enhance the cohesive and tensile strength of mixes. They are believed to show physical changes to bituminous mixtures (Brown et al., 1990). Research and experience have shown that fibres have a tendency to perform better than polymers in reducing the drain down of bituminous concrete mixtures, that is why fibres are mostly recommended (Hassan et al., 2005). Because of the inherent compatibility of fibres with bitumen and its excellent mechanical properties, the addition of fibres to bitumen improves material strength and fatigue characteristics while at the same time increasing ductility (Fitzgerald, 2000). According to Maurer and Gerald (1989), fibre reinforcement is utilized as a crack barrier instead of reinforcing element whose function is to carry the tensile loads as well as to prevent the formation and distribution of cracks.
The fibres that are finely divided also provides a high surface area per unit weight and behave much like filler materials. Fibres also tend to bulk the bitumen, so it wont run off from the aggregates during construction. In terms of efficiency, mixtures with fibre shows slight increase in the optimum binder content compared to the control mix. In this way, adding fibres to bitumen is very common to the addition of very fine aggregates to it. hence, fibre can stabilize bitumen to prevent leakage (Peltonen, 1991).
It is important to know that the suitable quantity of bitumen required to coat the fibres depends on the absorption rate and the surface area of fibres and also depends on the concentration and type of fibres (Button and Lytton 1987). If the fibres are too long, it may create the so called ”balling” problem, i.e., some of the fibres may lump together, and may not blend well with the bitumen. similarly, too short fibres may not give any reinforcing effect. They serve as an expensive filler in the mix.
Fundamentally, fibre enhance the various properties of the resulting mix. It alters the viscoelasticity of the modified bitumen (Huang and White 1996), magnifies dynamic modulus (Wu, Ye and Li,2007), moisture susceptibility (Putman and Amirkhanian, 2004), creep compliance, rutting resistance (Chen et al., 2004) and freeze– thaw resistance (Echols, 1989), while decreasing the reflective cracking of the bituminous mixtures and pavements (Echols, 1989; Tapk?n et al., 2009, Maurer and Malasheskie,1989). Goel and Das (2004) described that the fibre-reinforced materials develops good resistance to ageing, fatigue cracking, moisture damage, bleeding and reflection cracking.
Serfass and Samanos (1996) analysed the impact of fibre-modified bitumen on bituminous mixtures using asbestos, rock wool, glass wool and cellulose fibres. The test carried out includes resilient modulus, indirect tensile strength, rutting resistance and fatigue resistance. Three studies were carried on a test track in Nantes, France. The first study shows that, fibre modified mixtures maintained the highest percentage of voids with a 13 metric ton axle load for 1.1 million times compared with unmodified and other two elastomer modified mixtures. The authors concluded that decreased susceptibility to moisture related distress in the porous mixtures tested was due to better drainage. In the second study, two million load applications were applied on fibre-modified bituminous mixture was used as an overlay on the pavements with signs of fatigue distress. After the load applications, the pavement surface was noted to a well maintained macrostructure, and in fact no cracking. Fibre modified overlays are also constructed over fatigued pavements in the third study reported conducted by them. After 1.2 million load applications, it was observed that all of fibre modified overlays showed no indication of fatigue related distresses or rutting compared to the unmodified samples which shows the sign of distress. This was in concurrence with the findings of the second study, setting up that the fatigue life of the fibre modified pavement is improved over unmodified bituminous mixtures. Fibre modification allows an increase in film thickness, ensures in less ageing, improved binder characteristics. Addition of the fibres in bituminous mixtures results in the reduction of temperature susceptibility of the bituminous mixtures.
Simpson et al. (1994) conducted an experimental analysis on modified bituminous mixtures using polypropylene, polyester fibres and polymers. Two blends of modified binder were assessed. An unmodified mixture was utilized as a control sample. Mixtures containing polypropylene fibres were found that high tensile strength and resistance to cracking than others.
Studies by Brown et al. (1990, explained that some fibres have high tensile strength relative to the bituminous mixtures, thus it was found that fibres have a potential to improve the cohesive and tensile strength of bituminous mixes. They are supposed to impart physical changes to the bituminous mixtures by the phenomenon of reinforcement and toughening. This high tensile strength may increase the amount of strain energy which can be absorbed during the fatigue and fracture process of the bituminous mix. Finely divided fibres give a high surface area per unit weight and behave like filler materials. Fibres tend to bulk the bitumen so it won’t run off from the aggregates during construction.
Previous researches (Marais, 1979; Chen and Lin, 2005; Amit and Animesh 2004; Tapkin, 2008) showed that the addition of fibre into bitumen mix increases the stiffness of the bitumen binder that results in stiffer mixtures with decreased binder drain down. The fibre modified mixtures shows that improved Marshall properties with increase in stability and bulk specific gravity values compared to the control mix. Fibres seem to be the potential to improve fatigue life and deformation characteristics by increase the rutting resistance. The tensile strength and its relative properties of mixtures containing fibres were found to improve. In terms of workability, mixtures having fibres showed a slight increase in the optimum binder content when compared to the other control mix. It is similar to the addition of very fine aggregates. The proper quantity of bitumen used to coat the fibres is dependent on the absorption and surface area of the fibres and is therefore affected not only by various concentrations of fibres but also by different types of fibres (Button and Lytton, 1987). According to Mills and Keller (1982), the degree of homogeneity of dispersion of fibres within the mix will determine the strength of the resulting mixtures. The results acquire from the various field studies shows that the addition of fibre have a benefit since it helps to produce more flexible mixtures with more resistant to cracking (Jiang et al., 1993).
The design methods of bituminous mixtures mainly include the well-known Marshall design method and Super pave design method. In the design procedure of bitumen content plays a vital role in determining the engineering properties of mixture, which is determined in terms of the volumetric properties of mixture (specific gravity, air void, etc.) in both the Marshall and Superpave mixture design procedures. Anyhow , the volumetric properties of fibre-reinforced bitumen mixture are varries from the ordinary bituminous mixture (Serfass and Samanos, 1996). Therefore, it is essential to examine the volumetric properties of the bituminous mixtures to design more reliable ones. Fibre content plays a key role in determination of the volumetric and engineering properties of bituminous mixtures. It reports that there exists some optimum fibre content to attain maximum tensile strength and toughness (Chen et al., 2004). In many cases, fibre content is determined by engineering practices or manufacturer’s recommendation.
Bushing and Antrim (1968)are used cotton fibres as additives in the bituminous mixtures. These were degradable and are not suitable as long term reinforcement in the bituminous mixes.. Metal wires has been proposed by Tons and Krokosky (1960), but they were liable to rusting with the penetration of water. Asbestos fibres were also used in pavement mixes till it has been determined as a health hazard (Kietzman, 1960; Marais, 1979). With the new advancement in the innovation of production, natural fibre reinforced bituminous mixtures can be cost competitive when compared with other modified bituminous binders.
The natural coir fibre which is a less expensive and an ecofriendly alternative to synthetic fibre, can be efficiently used as a stabilizing additive in bituminous concrete (Bindu and Beena, 2009). The rate of increase in retained stability of the mixture as compared to the conventional mix was around 14% at the optimum fibre content of 0.3% and the decrease in bitumen content is 5% giving an appreciable saving in binder.
2.4 HISTORY OF SMA
Stone Matrix Asphalt (SMA) is a hot-mix asphalt, developed in Germany during the mid- 1960’s. SMA was referred to over the years as Stone Mastic, Split Mastic, Grit Mastic, or Stone Filled Asphalt. It is a gap-graded hot-mix asphalt that is designed to maximize deformation (rutting) resistance and durability by utilizing a structural basis of stone-on-stone contact.
SMA mixture is an impervious wearing surface that provides rutting resistant and durable pavement surface layer (Ibrahim, 2005). It was first introduced in Europe for more than 20 years for resisting damage from the spotted tires better than other type of HMA (Roberts et al., 1996). In recognition of its better performance, a national standard was set up in Germany in the year 1984. Since, the concept of SMA has been spread throughout Europe, North America and Asia Pacific. A few individual countries in Europe now have a national standard for SMA, the European Committee for Standardisation is building up an European Product Standard. As a result of its acheivement in Europe, some States, through the cooperation of the Federal Highway Administration, constructed SMA pavements in the United States in 1991 ( Brown et al., 1997). Wisconsin was the first SMA project followed by Michigan, Georgia, and Missouri (NAPA, 1999). From that time the use of SMA in the US has been increased significantly.
The word SMA is derived from the German term Splitt Mastix (Keunnen, 2003) revealing its German origin (Splitt-crushed stone chips and mastix- the thick asphalt cement and filler). The some of the definitions of SMA are stated here.
Austoroads (1993) defines SMA as ‘a gap graded wearing course mix which has a high proportion of coarse aggregate content that interlocks to form a stone-on-stone skeleton to resist permanent deformation. The mix is filled with a bitumen mastic and filler to which fibres are added to provide adequate stability of the bitumen and to stop drainage of the binder during transport and placement’.
The European definition of SMA (Michaut, 1995) is ‘a gap-graded asphalt concrete made up of skeleton of crushed aggregates bounds with a mastic mortar’.
The BCA (1998) defines SMA as ‘a gap graded bituminous mixture contains a high proportion of coarse aggregate and filler, with relatively small sand sized particles. It has low air voids with high levels of macro texture when laid resulting in waterproofing with good surface drainage’.
Technically, SMA consists of discrete single sized aggregates sticked together to support themselves by a binder rich mastic. The mastic is composed of bitumen, fines, mineral filler and stabilising agent. The stabilising agent is required to provide enough stability to the bitumen and to resist the drainage of bitumen during transport and placement.
Japan and Saudi Arabia have started to utilize SMA paving bituminous mixtures with good success (Brown et al., 1997; Ibrahim, 2005).
Findings from the European Asphalt Study Tour ahead with subsequent seminars and demonstrations have illustrated the aid of this product on roadways throughout the U.S. (NAPA, 1999). In many nations like United States, Australia, New Zealand, China, South Korea, Taiwan and other major countries in Asia, the utilization of SMA is increasing in popularity among the road authorities and asphalt industry. The Wisconsin Department of Transportation and the asphalt paving industry constructed a trial installation of SMA. The success of that trial was on the basis of decision to conduct a thorough investigation of SMA. Subsequently, more projects were constructed at different locations around the state. Each project contained six test locations utilizing different fibres and polymer modified SMA mixes. At the completion of the five year evaluation period, SMA performed better than that of the standard Bituminous Concrete Pavements. (Schmiedlin and Bischoff, 2002)
The Indian Roads Congress (IRC) was adopted a tentative SMA specification, (IRC SP 79:2008). One test road have been constructed in Delhi in October 2006, using SMA as a wearing course.
SMA Test track in India
In India a field trial on the design and construction of the Stone Matrix Asphalt (SMA) Surfacing had conducted between KhajuriChowk and BrijPuriChowk on Road No.59 in Delhi in October 2006 (Highway Research record No: 34). A test section was laid in by the Central Institute, New Delhi on the intersection (Loni Flyover to KhajuriChowk) of Road No. 59 which has busiest corridor having mixed traffic conditions including heavy vehicles. These test sections were monitored for their performance, at six months interval (pre monsoon and post monsoon) to determine the performance of SMA surfacing on intersections. By considering the advantage of the field performance of this test track and in other regions of developed countries like USA with climatic conditions which are close to that of India, SMA can be considered as the right choice for our long lasting pavements.
2.4.1 Stone Matrix Asphalt Composition
SMA is a mixture of crushed coarse aggregate, crushed fine aggregate (or sand), mineral filler, bitumen binder and a stabilizing agent. The components of SMA mixtures are shown in Fig. 2.6. A stone skeleton mixture of SMA provides the gap-graded stone-on-stone structure to carry heavy traffic loads, prevent rutting, and procured long term durability. The mineral filler, fine aggregate and bitumen provides the binder adhere to bond the stone skeleton together and provides a cohesive mixture. Finally, additives such as fibres or polymers are utilized as a stabilizer to ensure the mastic within the overall structure. They stiffen the resulting mastic and anticipate the draining off during storage, transportation and placing of SMA. The mastic fill up the voids, hold the chips in position and has an additional stabilizing effect as well as providing low air voids which results in highly durable bitumen (AAPA, 1993). Mineral fillers and additives play the key role of minimizing the binder drain down and increasing the amount of binder utilize in the mix thus improving the mix durability.
The SMA mixture consists of high coarse aggregate content (typically 70-80%), a high bitumen content (typically over 6%) and high percentage of mineral filler content (approximately 10% by weight) (Roberts et al., 1996). High coarse aggregate content in the mix results in stone-on-stone contact which produces a mixture of highly resistant to rutting. These gap graded (minimizing medium sized aggregates and fines) results a structurally tough skeleton in the bituminous mix(Fig. 2.7). In summary, the high stone content forms a skeleton type mineral structure which offers high resistance to rutting because of stone to stone contact, which is independent of temperature.
Fig. 2.6 Major components of SMA mixture
Fig. 2.7 SMA aggregate skeleton (NAPA, 1999)
Selection of materials is an important factor in SMA mix design. The coarse aggregate should be a durable, fully crushed1rock with a cubicle shape (maximum of 20% elongated or flat aggregate). Fine aggregate must be at least 50% crushed. Filler can be of grounded limestone rock, hydrated1lime or flyash. In general, materials of same quality that used in dense graded bituminous wearing course are required.
The strength, toughness and rutting resistance of SMA depends majorly on aggregates present in the mix with 100% crushed aggregate having good shape (cubicle) and binding limits for abrasion resistance,1flakiness index, crushing strength and impact resistance. Fine aggregate used must be crushed as the internal friction of the fine fraction largely1contributes to the overall stability of SMA.
184.108.40.206 Mineral Filler
Mineral filler is a portion of passing the 0.075 mm IS sieve. It consists of finely divided mineral material such as stone dust, Portland cement, hydrated lime, fly ash. A high Percentage of filler may stiffens the mixture extreamly, and difficult to compact that may be resulting in a crack susceptible mixture, (Brown and Haddock, 1997). In general, the amount of material passing through 0.075 mm sieve is relatively 8-12 percent of the total amount of aggregate in the bituminous mix. Hence the amount of material passing through the 0.075-mm IS sieve is relatively large, the SMA mixtures effects very differently from other HMA mixtures. These fines present in the mix, along with the bitumen and fibre, make the mastic that holds the mix together. SMA mixtures are very perceptive to the amount of 0.075 mm (No. 200) in the mix. Hence handling, storing and the introduction of the mineral filler is an important concern. Brown and Mallick (1994) found that fines has much less drain down than the mixtures that contains marble dust. This is1because the smaller particles provide more surface area for the given weight and tends to stiffen the binder more than coarse fines. This clearly shows the importance of the particle size passing the 0.075 mm sieve in SMA.
Stone matrix asphalt has more binder than conventional dense graded mixes, with percentages ranging from about 6.0% to 7.5%. The performance of the mix is usually increased with th addition of polymers and fibres. These help to provide a thick coating to the aggregate and prevents the drain down during transportation and placement. Polymer modified binders can be used to give a greater deformation resistance. Brown et al. (1997a) reports that SMA combined with styrene butadiene styrene produces more rutting resistant on mix than SMA with unmodified binder.
Stabilizing additives was introduced into SMA mixes to reduce the drain down effect and bleeding problems (fat spots)on wearing course. Because1of the compaction issues, storage and placement temperatures cannot be lowered. Additives was added to the mix to stiffen the mastic at high temperatures. Fibres do the good job of preventing drain down, where as polymer improves the bitumen properties at low and high temperatures. Brown and Cooley (1999) also concluded in their report that fibres do a better job than polymers to reduce drain down.
The addition of fibres or polymer during the mixing process as a stabilizing agent had several advantages including increased binder content, increase film thickness on the aggregate, increase mix stability, interlocking between the additives and the aggregates that improve strength and reduction in the possibility of drain down during transport and paving. There are so many additives available in the market including cellulose, wool fibres, glass fibres, siliceous acid (artificial silica), rubber powder1and rubber granules and polymers.
2.4.2 Stone Matrix Asphalt Properties
SMA is a hot mixture with relatively large proportion of stones and a considerable quantity of bitumen and filler. The main concept of gap gradation of 100% crushed aggregates is to increase the stability of pavement through interlock and stone-to-stone contact. This mixture is designed having 3–4% air voids and relatively high bitumen content due to the high amount of voids present in the mineral aggregate. The mixture containing high filler content (10% passing the 0.075-mm sieve), and a polymer or fibre (cellulose or mineral) in the bitumen has to prevent the drainage of bitumen. This mixture had a surface appearance similar to that of open graded friction course, however it shows low air voids similar to a dense graded HMA.
SMA provides a mixture that gives maximum studding reistance of tyre wear. It has high plastic deformation resistance under heavy traffic loads with high tyre1pressures and has a good low temperature properties (Brown et al., 1997b; Cooley and Brown, 2003).
At the bottom, and in the layers, the voids in the aggregate structure are mostly filled by the mastic, but at the surface the voids are only partially fill up. This gives good skidding resistance at all speeds and facilitates the drainage for surface water (Nunn, 1994).
Stone Matrix Asphalt was an excellent deformation and durability characteristics, along with the fatigue resistance. Stone matrix asphalt has a rough surface texture that offers lower noise characteristics than dense graded bitumen mixtures.
The increased deformation resistance, or resistance to rutting, compared to dense graded bituminous mixture is achieved through mechanical interlocking of high coarse aggregate content that forms a strong stone skeleton. In dense graded asphalt mix, the lean mastic provides the stability.
The improved durability of SMA comes from its slow rate of deformation obtained from the low permeability of the binder rich mastic which cementing the aggregate together.
The increased fatigue resistance is due to high bitumen content, a thicker bitumen film over the aggregates and low air voids in the mix. The higher binder content should give good flexibility and resistant to reflection cracking of underlying cracked pavements. Fat spots that appear is the biggest problem in surface course. These are caused by segregation, drain down, and high bitumen content or improper adding of stabiliser (Brown, et al., 1997). The rich mastic gives good workability and aggregate retention. The high binder and filler content provides durable, fatigue resistant, long life bituminous surfacing for heavily loaded traffic areas.
The difficult task in designing the SMA mix is to evalute a strong stone skeleton with the correct amount of binder. Too much addition of binder results in pushing the coarse aggregate particles apart, while little amount of binder results in a mix that is difficult to compact.
In Germany, surface courses of SMA has proved that it to be exceptionally resistant to permanent deformation and durable to surfaces subject to heavy traffic loads and severe climatic conditions (DAV, 1992). Stone Matrix Asphalt wearing courses are reported to show excellent results in particularly stable and durable in traffic areas with maximum loads, a variety of weather conditions (Wonson, 1996).
The gap-graded aggregate mixture gives a stable stone-to-stone skeleton that is held together by a rich bitumen mastic, a mixture of bitumen, filler, sand and stabilizing additives. Stabilizing additives are organic or mineral fibres, or less often, they considered as polymers. They stabilize the asphalt mortar and lead to thicken or bulk the bitumenhence it prevent binder run-off from the aggregate. Thus, they assure the homogeneity of the mixture. Aggregate interlock and particle friction are maximized thus gives the structures stability and strength (Susanne, 2000).
Because of the aggregates are in contact, rut resistance relies on aggregate properties than that of binder properties. Hence aggregates are not deformed as much as bitumen binder under loads, this stone-on-stone contact greatly reduces the rutting. The improved rutting resistance of the SMA mixture is attained the fact that it carries the load through the coarse aggregate matrix (or the stone matrix). The usage of polymer or fibre in the mix, that increase the viscosity of the mixture. The use of high filler content, which increases the stiffness of the binder, allow the SMA mixtures to a higher binder film thickness and higher binder content without the problem of drain down of bitumen under construction. The increased durability of the SMA mixtures attributes to thick film thickness and the high binder content (Chen and Huang, 2008).
Hence, the properties of a well designed and constructed SMA can beevaluated as
The stone skeleton of high internal friction, will give excellent shear resistance.
The bitumen mastic with rich binder and low voids will provide good durability and good resistance to cracking.
The high concentration of large stones, three to four times higher than a conventional dense graded mixture gives a superior resistance to wear.
Rough surface texture than that of dense graded mixture will assured a good skid resistance and proper light reflection and
The surface texture also provides an anti-splash features during wet and rainy conditions and reduces hydroplaning which results from water draining through the voids present in the matrix.
Advantages1and Disadvantages of Stone Matrix Asphalt
The advantages of stone matrix asphalt over dense graded asphalt are illustrated as follows:
Resistance to permanent1deformation or rutting (30-40% less permanent deformation than that of dense graded bituminous mixtures).
The mechanical properties of SMA depends on the stone to stone contact so they have less sensitive to binder variations than that of conventional mixes (Brown, et al, 1997a).
They have good durability due to high binder content (slow ageing), which results in longer service life (up to 20%) over conventional mixes.
They possess good wear resistance.
The coarser surface texture characteristics may decrease sound from tyre and pavement contact as well as glare and water spray.
SMA produced and compacted with the same plant and the equipment available for dense grade asphalt.
SMA that can be used at intersections and other high traffic stress situations where as open graded aggregate is unsuitable.
SMA surfacing may provide decreased reflection cracking from underlying cracked pavements due to flexible mastic.
Apart from good stability and durability that provides a long service life, other advantages claimed for SMA are:
It can be laid over on a rutted or uneven surface because it compresses very low during compaction. This also helps to provide a good longitudinal and transverse evenness (Nunn, 1994). There is no harmness to the final evenness of the surface even when it is applied in different mat thicknesses.
If the pavement losts its stiffness, such that dense graded mixtures with conventional binder which suffer premature1fatigue induced cracking, then it may be1beneficial to place SMA because of its improved fatigue resistance properties (Austroads, 1997).
An anticipated secondary benefit of SMA is the1retardation of reflection cracks (Austroads, 1997).
The disadvantages of SMA may include:
Increased cost allied with higher binder and filler contents, and additive.
High filler content in the SMA results in reduced productivity. it can be overcome by suitable plant modifications.
Possible delays in opening to traffic, as SMA mix should be cooled to prevent flushing of the binder1surface.
Initial skid resistance is low until the thick binder film is worn off from the surface top sby traffic.
2.4.4 Life Cycle Costing
The initial costs of SMA are 20-40% higher than the conventional dense graded mixtures. To analyze the SMA is more cost effective than conventional dense graded surfacing, the higher initial cost and the longer life expectancy of SMA is taken into account.
The increased initial costs of SMA compared to that of conventional dense graded mixtures results the usage of higher bitumen content and the use of fibres, increased quality control requirements and lower production rates due to increase in mixing time. However, costs may vary considerably with the project size, and also on haul distances.
Collins (1996) reported that the State of Georgia produces a set of life cycle costs on the basis of State’s experience and reasonable mix designs. The analysis shows that there savings in the order of 5% using SMA over dense graded mixtures for overlay project. The analysis used for the assumptions of rehabilitation intervals of 7-10 years for dense graded mixes and 10-15 years for SMA mix.
However, considering the potential for increased costs, the Georgia Department of Transport (DOT) has found that the use of SMA to be quite cost effective on the basis of improved performance and the potential for increased service life.
The Alaska DOT (NAPA, 1998), founds approximately 15% increase in SMA cost compared to conventional mixtures is more than offset by 40% additional life from a reduction in rutting. It appears that SMA is cost effective with high performance, durability and frictiona1 requirements.
2.5 PREVIOUS STUDIES ON SMA
The study conducts in Ontario, Canada, by the Ministry of Transportation on SMA pavement slabs trafficked with a wheel-tracking machine gives less rut depths when compare to that occurring in a dense friction course (Brown et al., 1997). In the United States, the Georgia Department of Transportation has performed a considerable amount of wheel tracking tests on SMA mixtures, gives positive results. The SMA has a rough surface texture, which provides good properties of friction after the surface film is removed by traffic. Other essential factors that improves the feasibility of SMA in contrast to conventional hot mixture are increase in durability, improves ageing properties and reduces the traffic noise (NAPA, 1994).
Brown et al. (1997) carries a study to analyse the performance of SMA in the United States by evaluating 86 SMA projects. Data was collected on material and mix properties, and the performance was evaluated based on rutting, raveling, cracking, and fat spots. The major conclusions from their study were: (i) 60% of the projects were more than 6.0% bitumen content (ii) over 90% of the SMA projects has a rutting measurements less than 4 mm; (iii) SMA mixtures appears to be more resistant to cracking than dense graded mixtures; (iv) there was no evidence for raveling on the SMA projects and (v) fat spots appeared to be the1biggest performance problem in the SMA mixtures.
HRB Report, No. 34 described the laboratory study undertaken to evaluate the performance of Stone Matrix Asphalt mix with draft specification of Indian Roads Congress. Marshall Mix design method was carried out to determine the optimum binder content of SMA mixtures. The mix was designed by using 50 blows to sustain heavy traffic, using three different binder contents of 6.5%, 7.0% and 7.5% by the weight of mix. The target mixing and compaction temperatures are 175?C and 143?C respectively. SMA mixtures are prepared by two different stabilizing additives, by adding 0.3% by weight of total mixture with VG-30 viscosity grade paving bitumen. The OBC was estimated at which the air voids and the minimum voids in mineral aggregates are 4.5% and 17% respectively.
Production of SMA is similar to that of standard hot mix asphalt (HMA). All the feed system for HMA facility must be carefully calibrated prior tothat of the production of SMA. Manufacturers of stabilizing additives are generally assist in setting up, calibrating and monitoring the additive delivery system to hot mix producer. Production temperatures of SMA mixtures varies accordingly to aggregate’s moisture content, weather conditions, bitumen grade and type of stabilizing additives. Temperature of 145?C-160?C can be utilized for the production of SMA. By the addition of stabilizing additives (fibres) to the aggregate mixture, the mixing time should be increased slightly. This additional time allows the effective distribution of fibres in the aggregate mixture. After that required amount of bituminous binder should be injected and mixed thoroughly in a mix plant. The additional time, in both the dry and wet mix cycles, increased from 5 to 15 seconds each.
SMA is a unique flexible pavement which has demonstrated its ability to bear the heavy truck loadings and resist the wear1of the super wide, single truck tires and the studded tires used throughout Europe (Schimiedlin and Bischoff, 2002). SMA mixes has provided excellent service on bridge decks as a wearing surface to protect deck membranes. The two main benefits of SMA mixtures are its rut resistance and long term durability provides an extended performance life from 30 to 40% longer than that of a conventional dense-graded HMA pavement (Watson and Jared, 1995).
In addition to that, the SMA mix has safety features like improved skid resistance due to high percentage of crushed aggregate particularly on wet pavements (NAPA, 2001). Although water does not drain through SMA, its surface texture is similar to the open-graded mixture, so that the noise generated by traffic is low than the dense graded mixture but equal to or slightly higher than the open graded mixture. Hence the coarser surface texture characteristics may reduce tyre sound and the pavement contact and water spray, glare.
SMA that can be produced and compacted with the same mix plant and equipment available for the normal hot mix,using Marshall Procedure. SMA may be utilized at intersections and other high traffic stress situations where open graded mixture is unsuitable. For pavements with cracking or raveling it suggests that SMA can be considered to be used as an overlay because it can be reduced severe reflection cracking from underlying cracked pavements due to the flexible mastic. The durability of SMA should be equal to be or greater than of dense graded mixtures and significantly higher than open graded mixtures.
2.61STABILIZING ADDITIVES IN SMA
Since SMA mixes has a high bitumen binder content, the binder tends to drain off the aggregate and down to the bottom – a phenomenon known as “mix drain down”. This happens while the mix is present in the HMA storage silos, trucks for transporting and during placement. Mix drain down is usually combated by the addition of stabilizing additives and may be organic or mineral fibres or polymers. They stabilize the mixture and lead to thicken or bulk the bitumen to prevent binder run-off from the aggregates. Thus, they can be ensured the homogeneity of the mixture. Aggregate interlock and particle friction can be maximized and gives the structure stability and strength (Susanne, 2000).
Most of the SMA projects, fibres or polymers can be used as additives. Fibres are added to SMA mixtures to overcome the draindown problem usually encountered during mixing, transporting and compaction. Loose organic fibres, like cellulose, and mineral fibres are generally used at the rate of 0.3% and 0.4% by the weight of mixture, respectively (Brown and Manglorkar, 1993). A high percentage of mineral fibre is used; it is typically heavier than the cellulose (Roberts et.al., 1996). Other types of fibres, including glass fibre, rock wool, polyester, and even natural wool, has been found to be suitable. But the cellulose fibre is mostly a cost-effective one (Mangan and Butcher, 2004).
When polymer is used, it blends together with the binder prior to the delivery to the plant but in some conditions it was added at the plant itself (Roberts et al., 1996). The polymers can increase the stiffness of bitumen due to loading at high and low temperature, and drain down resistance. In addition, it give better binder adhesion to the aggregates particularly in wet condition (Robinson and Thagesen, 2004). Polymers that can be added to the mixture, at a rate of 3.0–8.0% by weight of the binder (Ibrahim, 2005).2.6.1 Fibre as an additive
The application of fibres in dense graded bituminous mixtures and their reinforcing effects, improved performance of pavements was already discussed in detail in section 2.3.1. This section deals with the review of previous works done in SMA with different synthetic fibres, waste fibres and some natural fibres like jute fibre and oil palm fibre. Fibre stabilizes the bitumen to prevent binder leakage especially for the open-graded-friction courses (OGFC) and stone matrix asphalt (SMA) mixtures d,uring the material transportation and paving (Hassan et al., 2005; Serfass and Samanos, 1996; Peltonen, 1991). Fibre can be changed the viscoelasticity of mixture (Huang and White, 2001) and improves dynamic modulus (Wu et al., 2008), moisture susceptibility (Putman and Amirkhanian, 2004), creep compliance1and rutting resistance (McDaniel, 2001; Chen et al., 2004). It reduces the reflective cracking of bituminous mixtures and pavements (Tapkin, 2008; Maurer and Malasheskie, 1989).
Polyester, polyacrylonitrile, lignin and asbestos fibre stabilized SMA mixes was suggested by Chen and Huang (2008) and by studying1the volumetric and mechanical properties, they arrived at the design method of fibre reinforced bituminous mixtures. Polyester and polyacrylonitrile fibres has high stability due to their high networking effect, while the lignin and asbestos fibres results in higher optimum bitumen content and VFA due to their high absorption of bitumen. The design procedure for the fibre reinforced bituminous mixture elects the type of fibre based on the characteristics of both fibre and bituminous mixture, designs the optimum bitumen content following the Marshall method, and then determines the1optimum fibre content in terms of performance test results.
Behbahani et al. (2009) founds the variation of fibre type and content leads to considerable changes in the rutting performance of SMA mixtures. The principal functions of fibre reinforcement in bituminous mixes are to provide the additional tensile strength and increasing strain energy absorption of the bituminous mix. This attains the formation and propagation of cracks that can reduce the structural integrity of the pavement. The idea was based on the general concept of HMA is strong in compression and weak in tension, then reinforcement could be used to provide the required resistance to tensile stresses (Al-Qadi et al., 2003; Bushing et al., 1968; Wu et al., 2007).
Pawan Kumar et al. (2004) determined the possibility of using coated jute fibres in SMA mixes replacing of synthetic fibres. The test results indicate that the natural jute fibre can replace synthetic fibres. A slightly high accumulated strain and subsequently low creep modulus were observed to SMA with synthetic fibre. Permanent deformations has observed the same for both mixes and tensile strength ratio (TSR) is more than the prescribed limit.
Muniandy and Huat (2006) stated that the cellulose oil palm fibre enhance the diametric fatigue performance of the SMA design mix. The fatigue life increased to a maximum extent at a fibre content about 0.6% and the tensile stress and stiffness also showed a similar trend in performance. The initial strains of the mix are lower at a fibre content of 0.6%.
2.6.2 Polymer as an additive
Additives such as styrene1based polymers, polyethylene based polymers, gilsonite, various oils, polychloroprene and many other modifiers were added to bitumen to enhance various engineering properties of bitumen mix (Denning and Carswell, 1993). Goodrich (1998) reported that modifier enhance the properties of bituminous mixtures. For ordinary SMA, the use of unmodified bitumen together with fibrous material as a drainage1inhibitor is sufficient. Under high temperatures and heavy loading, a hard bitumen grade is needed.Polymer such as polypropylene, polyethylene or styrene– butadiene–styrene modified binder may also be used to substitute the fibrous material. It is possible to enhance the capability of resistance to permanent deformation and to reduce the pavement failure, thereby ensuring a good bituminous mixture. SMA can control and limits the distress failure such as shoving, rutting, stripping, etc., through the polymer modification.
Al- Hadidy and Tan Yi- qiu (2008) investigates about the improvement in service life of the pavement or the reduction in thickness of SMA and base layer due to the addition of SMA mixtures with polyethylene. The thickness of SMA is reduced by 34% than the unmodified mix. They exhibits better service life and reduced temperature susceptibility.
2.6.3 Waste materials as an additive
Putman and Amirkhanian (2004) used waste tire and carpet fibres in SMA and compared the performance of these stabilized mixtures with cellulose and polyester fibre stabilized mixtures. The outcomes showed that adding wastes increases the toughness of SMA mixtures, without making any significant difference in permanent deformation.
Richard et al. (2009) reported that the volume of waste polymer produced is increasing rapidly and the disposal is very difficult resulting in exceeding the waste beyond the acceptable levels. The possibility of incorporating waste polymer into bitumen as a modifier was examined. A wide range of recycled polymers were tested, including polyethylene, polypropylenes, polyether polyurethane, ground rubber, and truck tire rubber. Tests included viscosity, penetration, softening point, ageing, and rheology. Stiffness tests on samples of bituminous mixes were made using different grades of binders. The blend with 3% low density polyethylene substituted for 1% styrene butadiene had similar properties to that of Polyflex 75, although it had lower stiffness. The most impressive was a combination of low density1polyethylene, bitumen and ethyl vinyl acetate. Recycled plastics comprising predominantly of polypropylene and low density polyethylene was incorporated into conventional bituminous road surfacing mixtures. Greater durability and fatigue life was reported for these modified mixes when compared to conventional mixes (Zorrob, 2000).
It is hoped that in future we have a strong, durable and eco-friendly roads which will relieve the earth from all type of plastics. The use of the innovative technology not only strengthened the construction of road but also increased the life span of the road.
A critical literature review showed that Stone Matrix Asphalt is an ideal paving mixture for Indian conditions especially to our AP Highways. Literature shows that it was possible to improve the performance of bituminous mixtures used in the surface course with the help of different types of additives like fibres, polymers and waste1materials. Synthetic fibres are commonly used in the construction of Stone Matrix Asphalt. They are not manufactured at India and are imported at high cost. The excessive usage of the synthetic fibres leads to environmental pollution. This ecological crisis necessitates the use of bio-renewable resources and plant fibres. Resources in terms of materials utilized for construction and maintenance of pavements are very unique and limited. Therefore, there is a great need to identify new technologies that can work with alternative resources like agro based materials and renewal of existing resources without affecting the performance. Some limited studies were reported on the use of natural fibres in SMA. India produces huge quantity of naturally occurring agro based fibres such as Coconut, Sisal, Banana and Jute fibres etc., which need to be explored for their potential applications in the field of bituminous road construction. This results in improving the characteristics and service life of bituminous surfacing, eventually leading to conservation of1construction materials.
The performance of bituminous surfacing depends on the correct choice of quality and quantity of materials that are used. Materials need for the production of Stone Matrix Asphalt mixtures includes high quality aggregates, bituminous binder, mineral filler and a stabilizer. The materials used in this research work are locally available construction materials in Nellore. The properties of the different materials used for the research are described in this chapter.
Aggregates form the major constituent of road construction materials. Since they have to bear the brunt of traffic, they should be strong enough to resist the degradation and should have enough structural stability which is offered by the mechanical interlock of aggregate in the layer. IS 2386-1963 gives the methods of tests for aggregates in road construction. Aggregate of sizes 20mm, 10mm and stone dust acquire from a local quarry is used in the present investigation. The values obtained fordifferent properties of aggregate are given in Table 3.1.
3.3 MINERAL FILLER
The role of mineral filler is essentially to stiffen the rich binder SMA. It is designed to fulfill the voids and forms a stiff mastic with bitumen binder and stabilizing additive. It increases the cohesion of the mix which results a significant increase in the shear resistance. The high percentage of filler may stiffens the mixture excessively and making it difficult to compact and may be resulting in a crack susceptible mixture, (Brown et al., 1997). In general, the amount of material passing through 0.075 mm sieve is 8-12 % of the total amount of aggregate in the mix. The commonly used mineral fillers are fly ash, hydrated lime, finely ground limestone dust and ordinary Portland cement (OPC). OPC available from a local market which makes a better bond with aggregate, bitumen and additive was used in the present work. The physica1 properties of filler material used are shown in Table 3.2.
Table 3.1 Physical properties of the aggregate
Property Values obtained Method of Test
Aggregate impact value (%) 14 IS:2386 (IV)
Los Angeles Abrasion Value 24 IS:2386 (IV)
Combined Flakiness and Elongation Index (%) 18 IS:2386 (I)
Stripping Value Traces IS 6241:1971 (R2003)
Water Absorption (%) 1.1 IS:2386 (III)
Specific gravity 2.65 IS:2386 (III)
Table 3.2 Physical properties of the cement
Physical property Values obtained
Specific gravity 3.11
% passing 0.075 mm sieve (ASTM C117) 95
3.4 STABILIZING ADDITIVES
Stabilizing additive which is rich in binder content must be used to hold the binder in SMA mix during mixing, transporting and placement operations. To prevent the unacceptable drain down, fibres or polymers as stabilizing additives can be added to the mixture. The three natural fibres namely coir, sisal and banana fibre are used as stabilizing additives for the present study. The description of these three materials is given below.
3.4.1 Fibre stabilizer
India has a vast resource for different natural fibres viz., jute, sisal, banana, coir fibre etc. and can be advantageously used for many construction activities. A,t present the production of natural fibres in India is more than 400 million tonnes.
The addition of fibres during the process of mixing as a stabilizing agent has several advantages including increased binder content, film thickness and the mix stability. This results in better interlock between the aggregates and thereby improving the strength and reduces the possibility of drain down during transport and paving.
There are various types of fibres used in SMA mixtures like polymer fibre, mineral fibre, natural fibres etc.. In this study, three natural fibres are used namely, coir, sisal and banana fibre at different percentages by weight of mixture. The photographs of the three fibres are shown in Fig. 3.1.
220.127.116.11 Coir fibre Kerala is the home land of Indian coir industry, accounting for 61 per cent of coconut production and over 85 per cent of coir products. Coconut fibre/ coir fibre derived from the mesocarp tissue or husk of the coconut fruit. The individual cells of coconut fibre are narrow and hollow, with1thick walls made up of cellulose. These fibres are pale when immature but later they becomes hardened and yellowed as a layer of lignin gets deposited on their walls. Brown coir fibres are stronger because of more lignin than cellulose, but they are less flexible. Coconut fibres are relatively water proof and decomposition of the fibre is generally much less than other natural fibres due to high lignin content. The peelings of ripe coconut are collected locally; dried and neat fibres are taken out manually. The lengths of such fibres are normally in the range of 15mm to 280 mm and diameter varies from 0.1mm to 1.5 mm. The average tensile strength of fibre was found to be 70.58N/mm2. Compared with other vegetable fibres, the coconut fibre has a cellulose content of 36% to 43%, and lignin content of 41% to 45%. The coir fibres for the present work had obtained from Alappuzha and its properties are given in Table 3.3.
18.104.22.168 Sisal fibre Sisal plantations in India yield about 2.5 tonnes dry fibre per hectare in a year. The fibre is usually obtained from sisal leaves by decortications in a machine called Raspador. Sisal is a leaf fibre derived from the plant Agave Sisalana. The lustrous strands are usually creamy white, 80cm to 120 cm in length and 0.2 to 0.4 mm in diameter. Sisal fibre consists of 66-72% cellulose, 12% hemi cellulose and 10–14% lignin. The superior engineering properties (diameter 50–200 micro meter, Ultimate Tensile strength 468–640 Mpa and elongation 3-7%) makes it an excellent material for manufacturing of high strength textile and reinforcement in composites for different applications (Saxena and Ashokan, 2011). Sisal fibre is fairly coarse and inflexible. Generally they are used for rope making, paper industry etc,. Very few works are done by using sisal fibres in the bituminous mixtures. In this study the sisal fibres is used as a stabilizing additive in Stone Matrix Asphalt mixtures and is procured from Alappuzha. Its properties are given in Table 3.3.
22.214.171.124 Banana fibre The entire sheath of banana fibre yields good quality fibre which is highly valued in the market for its durability and strength. A large quantity of bio waste is generated every year owing to banana cultivation which needs to be disposed off. By extracting banana fibre, the waste can be effectively utilized and provide additional income to the banana farmers.
Banana fibre is a multicellular fibre. The suitability of fibre for utilization in products can be identified by the degree of polymerization of cellulose. It is a cellulose rich fibre (70%) with low lignin content (12%). Tensile strength, elongation and density are the most important mechanical properties of the fibre. The high tensile strength exhibited by the fibre indicates its resistance to wear and tear, thus facilitating its use in the pavements. It is observed that single fibre tenacity, fibre bundle tenacity, fibre porosity and moisture regain is the highest for this fibre when physical and chemical properties of different natural fibres are compared (Sinha, 1974). They can be extracted from the pseudostem by removing non fibrous tissues and other plant parts from the fibre bundles. Fibre extraction is usually practiced either by hand extraction or by mechanical extraction(Suma, 2009). The banana fibre required for carrying out the work had procured from Kerala Agricultural University, Banana Research station, Kannara, Thrissur, Kerala and the properties of the fibre are given in Table 3.3.
Coir fibre Sisal fibreBanana fibreFig. 3.1 Fibres used for the present study
Table 3.3 Properties of fibres used
Property Coir fibreSisal fibreBanana fibreDiameter (µm) 100 – 450 50 – 200 80 – 250
Density (g/Cm2) 1.45 1.40 1.35
Cellulose content (%) 43 67 65
Lignin content (%) 45 12 5
Elastic modulus(GN/m2) 4-6 9 -16 8-20
Tenacity (MN/m2) 131 – 175 568 – 640 529 – 754
Elongation at break (%) 15 – 40 3 – 7 1.0 – 1.2
Bitumen acts as a binder in the SMA mix. Different grades of bitumen are used in different mixes like hot-mix or gap-graded mix or dense-graded mix. Bitumen of VG-30 grade obtained from Vishakhapatnam Refineries was used in the preparation of mix samples. The Physical properties of bitumen are found and the results are given in Table 3.6.
Table 3.6 Physical properties of bitumen
Property Result obtained Test procedure as per specification
Specific Gravity @ 27?C 1.03 IS:1202 – 1978
Softening Point (?C ) 52 IS:1205 – 1978
Penetration @ 25?C,0.1 mm 100g, 5Sec 64 IS:1203 – 1978
Ductility @ 27?C (cm ) 72 IS:1208 – 1978
Flash Point (?C )
Fire Point (?C ) 240
270 IS:1209 – 1978
Viscosity at 60 ?C(Poise ) 1200 IS:1206 – 1978
Elastic recovery @ 15?C 11 IRC: SP:53 – 2002
The procurement of the materials and their properties utilized for making the samples of stone matrix asphalt mixtures are discussed in this chapter. Using these materials, Stone Matrix Asphalt mixtures are prepared and mix design, analysis are carried out by Marshall method of design which is explained in the next chapter.
MARSHALL MIX DESIGN AND ANALYSIS
Suitable mix design will withstand heavy traffic loads under adverse climatic conditions and also fulfill the requirement of structural and pavement surface characteristics. The objective of the design of bituminous mix is to determine an economical blend through several trial mixes. The gradation of aggregate and the corresponding binder content should be such that the resultant mix has satisfy the following conditions.
Sufficient binder to ensure a durable pavement by providing a water proofing coating on the aggregate particles and binding them together under suitable compaction.
Sufficient stability for providing resistance to deformation under sustained or repeated loads. This resistance in the mixture is obtained from aggregate interlocking and cohesion which generally develops due to binder in the mix.
Sufficient flexibility to withstand deflection and bending without cracking. To obtain desired flexibility, it is necessary to have proper amount and grade of bitumen.
Sufficient voids in the total compacted mix to provide space for additional compaction under traffic loading.
Sufficient workability for an efficient construction operation in laying the paving mixture.
There are three principal bituminous mix design methods are generally used. They are Marshall Method, Hveem Method and Superpave Method. Marshall mix design is the widely used method throughout India. In this method, the load is applied to a cylindrical specimen of bituminous mix and the sample is monitored till its failure as specified in the ASTM standard (ASTM D1559). For the present work, the bituminous mix is designed by using the Marshall Method and arrived at the volumetric properties.
4.2 MARSHALL MIX DESIGN
This test procedure is used in designing and evaluating bituminous paving mixes and is extensively used in routine test programmes for the paving jobs. There are two major features of Marshall Method of designing mixes namely, density – voids analysis and stability – flow test.
Strength is measured in terms of ‘Marshall’s Stability’ of the mix following the specification ASTM D 1559 (2004), which is defined as the maximum load carried by a compacted specimen at a standard test temperature of 60?C. In this test compressive loading was applied on the specimen at a rate of 50.8 mm/min till it brokens. The temperature 60?C represents the weakest condition for a bituminous pavement.
The flexibility is measured in terms of the ‘flow value’ which is measured by the change in diameter of the sample in the direction of application of load between the start of loading and at the time of maximum load. During the loading an attached dial gauge measures the specimen’s plastic flow (deformation) due to the loading. The associated plastic flow of specimen at material failure is called flow value.
The density- voids analysis is done by using the volumetric properties of the mix, which will be described in the following sub sections.
4.2.1 Gradation of aggregates
Gradation of aggregates is one of the most important factors for the mix design of SMA mixture. The sieve analysis, blending and the specified limits of SMA mix are given in Table 4.1 as per NCHRP – 425, TRB.
4.2.2 Volumetric properties
Fundamentally, mix design is meant to determine the volume of bitumen binder and aggregates necessary to produce a mixture with desired properties (Roberts et al., 1996). Since weight measurements are typically much easier, weights are taken and then converted to volume by using specific gravities. The following is a discussion of the important volumetric properties of bituminous mixtures.
The properties that to be considered, include the theoretical maximum specific gravity Gmm, the bulk specific gravity of the mix G mb, percentage air voids VA, percentage volume of bitumen V b, percentage void in mineral aggregate VMA and percentage voids filled with bitumen VFB.
Table 4.1 Gradation of aggregates and their blends for SMA mixture
Sieve size (mm) Percentage passing
20 mm (A)
10 mm (B)
Stone dust (C)
A: B: C: D NCHRP, TRB
50:30:11:9 25.0 100 100 100 100 100 100
19.0 98 100 100 100 99 90 -100
12.5 20 100 100 100 60 50 – 74
9.50 4 58 100 100 39 25 – 60
4.75 0 6 100 100 22 20 – 28
2.36 0 0 92 100 19 16 – 24
1.18 0 0 77 100 17 13 – 21
0.6 0 0 64 100 16 12 – 18
0.3 0 0 45 100 14 12 – 15
0.075 0 0 6 96 9 8 – 10
Theoretical Maximum Specific Gravity of the mix (Gmm)
Gmm=Wmixvol of the mix-air voids
Where, Wmix is the weight of the bituminous mix,
Gmm is calculated as per ASTM D 2041 – 95.
Bulk specific gravity of mix (Gmb)
The bulk specific gravity or the actual specific gravity of the mix Gmb is the specific gravity considering air voids and is found out by
Gmb=WmixBulk vol of mix It is obtained by measuring the total weight of the mix and its volume. Volume is determined by measuring the dimensions of the sample or for better accuracy it can be measured by the volume of water it displaces. However, while the sample is immersed in water, some water may be absorbed by the pores of the mix. Therefore, the mix is covered with a thin film of paraffin and the volume of the sample is measured by knowing the volume of paraffin used to coat its surface. The bulk specific gravity of paraffin-coated specimen is determined in accordance with ASTM standard test procedure D1188-96.
The phase diagram of the bituminous mix is given in Fig. 4.1. When aggregate particles are coated with bitumen binder, a portion of the binder is absorbed into the aggregate, whereas the remainder forms a film on the outside of the individual aggregate particles. Since the aggregate particles do not consolidate to form a solid mass, air pockets also appear within the bitumen-aggregate mixture. Fig.4.1 illustrates, the four general components of HMA are: aggregate, absorbed bitumen, bitumen not absorbed into the aggregate (effective bitumen) and air.
Fig. 4.1 Phase diagram of the bituminous mix
Effective Bitumen Content (Pbe)
It is the total bitumen binder content of the mixture less the portion of bitumen binder that is lost by absorption into the aggregate.
Volume of Absorbed Bitumen (Vab)
It is the volume of bitumen binder in the mix that has been absorbed into the pore structure of the aggregate. This volume is not accounted for the effective bitumen content.
Air voids percent (VA)
It is the total volume of the small pockets of air between the coated aggregate particles throughout a compacted paving mixture, expressed as a percent of the bulk volume of the compacted paving mixture. The amount of air voids in a mixture is extremely important and closely related to stability, durability and permeability.
The voids in a compacted mixture are obtained in accordance with ASTM standard test method D3203-94. The following equation represents the percentage of air voids in the specimen.
VA=Gmm-Gmb*100GmmWhere Gmm is the theoretical specific gravity of the mix and Gmb is the bulk specific gravity of the mix.
Voids in mineral aggregate (VMA)
The total volume of voids in the aggregate mix (when there is no bitumen) is called Voids in Mineral Aggregates (VMA). In other words, VMA is the volume of intergranular void space between the aggregate particles of a compacted paving mixture. It includes the air voids and the volume of bitumen not absorbed into the aggregate. VMA is expressed as a percentage of the total volume of the mix.
When VMA is too low, there is not enough room in the mixture to add sufficient bitumen binder to coat adequately over the individual aggregate particles. Also, mixes with a low VMA are more sensitive to small changes in bitumen binder content. Excessive VMA will cause unacceptably low mixture stability (Roberts et al., 1996). Generally, a minimum VMA of 17% is specified. VMA can be calculated as,
VMA=1-Gmb*PsGsb*100Where Ps is the fraction of aggregates present, by total weight of the mix and Gsb is the bulk specific gravity of the mixed aggregates.
Voids Filled with Bitumen (VFB)
VFB is the voids in the mineral aggregate frame work filled with bitumen binder. This represents the volume of the effective bitumen content. It can also be described as the percent of the volume of the VMA that is filled with bitumen. VFB is inversely related to air voids and hence as air voids decreases, the VFB increases.
VFB=VMA-VAVMA*100Where, VA is air voids in the mix and VMA is the voids in the mineral aggregate.
The decrease of VFB indicates a decrease of effective bitumen film thickness between aggregates, which will result in higher low-temperature cracking and lower durability of bitumen mixture since bitumen perform the filling and healing effects to improve the flexibility of mixture.
4.2.3 Role of volumetric parameters of mix
Bitumen holds the aggregates in position, and the load is taken by the aggregate mass through the contact points. If all the voids are filled with bitumen, the one to one contact of the aggregate particles may lose, and then the load is transmitted by hydrostatic pressure through bitumen, and hence the strength of the mix reduces. That is why stability of the mix starts reducing when bitumen content is increased further beyond a certain value.
During summer season, bitumen softens and occupies the void space between the aggregates and if void is unavailable, bleeding is caused. Thus, some amount of void is necessary in a bituminous mix, even after the final stage of compaction. However excess void will make the mix weak from its elastic modulus and fatigue life considerations. Evaluation and selection of aggregate gradation to achieve the specified minimum VMA is the most difficult and time-consuming step in the mix design process.
In the Volumetric method of mix design approach proportional volume of air voids, binder and aggregates are analyzed in a compacted mixture, applying a compaction close to that of field compaction. SMA mixture design requirements is given in Table 4.2
Table 4.2 SMA mixture design criteria
Design Parameter Design Criteria
Percent Air Voids 3- 5 %
Percent voids in mineral aggregate (VMA) 17 (minimum)
Stability value 6200 N(minimum)
Flow value 2 – 4 mm
Retained Stability (LS-283). 70% (minimum)
4.3 MIX DESIGN
Laboratory mix designs of SMA mixtures are done by Marshall test procedure.
4.3.1 Specimen preparation
Approximately 1200g of aggregates and filler put together is heated upto a temperature of 160-170?C. Bitumen is heated to a temperature of 160?C with the first trial percentage of bitumen (say 5.5% by weight of the mineral aggregates). Then the heated aggregates and bitumen are thoroughly mixed at a temperature of 160 – 170?C. The mix is placed in a preheated mould and compacted by a hammer having a weight of 4.5 kg and a free fall of 45.7 cm giving 50 blows on either side at a temperature of 160?C to prepare the laboratory specimens of compacted thickness 63.5+/-3 mm. Seventy five compaction blows were not given as in the case of dense graded bituminous mixes for heavy traffic condition, since in the gap graded mixes, this would tend to break down the aggregate more and would not result in a significant increase in density over that provided by 50 blows. SMA mixtures have been more easily compacted on the roadway to the desired density than the effort required for conventional HMA mixtures. In this research, the compaction of all the SMA samples are performed using fifty blows of the Marshall hammer on either side of the sample. The heights of the samples are measured and specimens are immersed in a water bath at 60?C for 35±5 minutes. Samples (Fig. 4.2) are removed from the water bath and placed immediately in the Marshall loading head as shown in Fig. 4.3. The load is applied to the specimen at a deformation rate of 50.8 mm/minute. Stability is measured as the maximum load sustained by the sample before failure. Flow is the deformation at the maximum load. The stability values are then adjusted with respect to the sample height (stability corrections).
For the proposed design mix gradation, four specimens are prepared for each bitumen content within the range of 5.5 – 7.5% at increments of 0.5 percent, in accordance with ASTM D 1559 using 50 blows/face compaction standards. All bitumen content shall be in percentage by weight of the total mix. As soon as the freshly compacted specimens have cooled to room temperature, the bulk specific gravity of each test specimen shall be determined in accordance with ASTM D 2726. The stability and flow value of each test specimen shall then be determined in accordance with ASTM D 1559. After the completion of the stability and flow test, specific gravity and voids analysis shall be carried out for each test specimen to determine the percentage air voids in mineral aggregate and the percentage air voids in the compacted mix and voids filled with bitumen. Values which are obviously erratic shall be discarded before averaging. Where two or more specimens in any group of four are so rejected, four more specimens are prepared and tested.
The average values of bulk specific gravity, stability, flow, VA, VMA and VFB obtained above are plotted separately against the bitumen content and a smooth curve drawn through the plotted values. Average of the binder content corresponding to VMA of 17 % and an air void of 4% are considered as the optimum binder content (Brown, 1992). Stability and Flow values at the optimum bitumen content are then found from the plotted smooth curves and shall comply with the design parameters given in Table 4.2.
The optimum bitumen content (OBC) for the SMA mixture is determined and is found to be 6.42 % (by wt. of total mix). This SMA mixture without additives is considered as the control mixture for the subsequent studies.
Fig. 4.2 Marshall test apparatus
Fig. 4.3 Sample in the mould179070013716000
Fig. 4.3.a Marshall Samples
4.3.2 Stabilized SMA
SMA mixtures with additives are taken as the stabilized SMA. An optimum bitumen content of 6.42 % (by wt. of total mix) as found from Marshall Control mix design is used in preparing all the stabilized mixes to maintain consistency throughout the study.
126.96.36.199 Preparation of Marshall Specimens
Marshall Stability test is conducted on stabilized SMA in more than 100 samples of 100 mm dia and 63.5 mm height by applying 50 blows on each face as per ASTM procedure (ASTM D1559, 2004). Bituminous mixes are prepared by mixing the graded aggregates with 60/70 penetration grade bitumen and additives. Three different natural fibres are used as additives in SMA mixture viz., coir, and sisal and banana fibres. The fibre content in this research is varied between 0.1%, 0.2%, 0.3% and 0.4% by weight of mix. The procedure adopted for the preparation of Marshall Specimen is the same as used in control mixtures (sec.4.3.1), except, the additives are added in heated aggregate prior to mixing them with heated bitumen (dry blending method). The fibre length in the mixture is preserved as a constant parameter with a value equal to 6 mm. The mixing and compaction temperatures are kept as 165?C and 150?C respectively (Brown and Manglorkar, 1993). A total of 120 Marshall Samples for all percentages of different additives are prepared.
4.4 MOISTURE SUSCEPTIBILITY TEST
It is well known that presence of moisture in a bituminous mix is a critical factor, which leads to premature failure of the flexible pavement. The loss of adhesion of aggregates with bitumen is studied by utilising Retained Stability test to examine the effect of additive on resistance to moisture induced damage. This test measures the stripping resistance of a bituminous mixture. The test is specified in IRC: SP 53-2002 and is conducted as per ASTM D 1075-1979 specifications. The standard Marshall specimens of 100 mm diameter and 63.5 mm height are prepared. Marshall Stability of compacted specimens is determined after conditioning them by keeping in water bath maintained at 60?C for 24 hours prior to testing. This stability, expressed as a percentage of the stability of Marshall Specimens determined under standard conditions, is the retained stability of the mix. A higher value indicates lower moisture susceptibility (higher moisture damage resistance).
4.5 MARSHALL TEST RESULTS AND DISCUSSION
Results of mix design and their discussion for the fibre stabilized mixtures are given in this section.
4.5.1 Fibre stabilized mixtures
Test results of volumetric and mechanical properties of SMA mixtures using different fibres are tabulated in Table 4.3 and discussed in this section.
188.8.131.52 Marshall stability and flow value
From Table 4.3, it is evident that the presence of fibre in the SMA mixtures effectively improves the stability values, which will result in an improvement of mixture toughness. This result indicates that the mixture using fibre would result in higher performance than using the control mixture. Variation of Marshall Stability and flow value with different fibre contents are given in Fig. 4.4.a and Fig. 4.4.b
Fig. 4.4.a indicates that the stability of fibre stabilized mixtures increases initially, reaches a maximum value and then decreases with increasing fibre content. Bituminous mixture is an inconsistent, non-uniform, multi-phased composite material consisting of aggregates and sticky bitumen. Therefore, excessive fibres may not disperse uniformly, while coagulate together to form weak points inside the mixture. As a result, stability decreases at high fibre contents.
Table 4.3 Variation of Marshall Properties of SMA with different % of fibres as additive
(kN) Flow (mm) Marshall Quotient
(kN/mm) Air void (%) Bulk specific gravity VMA
(%) VFB (%)
Nil 0 7.416 3.18 2.332 4 2.32 18.865 78.796
Coir fibre0.1 8.19 3.14 2.609 4.14 2.318 18.935 78.135
0.2 10.073 3.05 3.303 4.31 2.315 19.039 77.363
0.3 12.58 2.83 4.445 4.46 2.308 19.284 76.872
0.4 7.936 2.72 2.918 4.64 2.298 19.634 76.368
Sisal fibre0.1 7.743 3.17 2.443 4.09 2.31 19.214 78.714
0.2 8.701 3.07 2.834 4.24 2.3 19.564 78.328
0.3 11.862 2.86 4.148 4.37 2.291 19.879 78.017
0.4 8.742 2.77 3.156 4.54 2.278 20.333 77.672
Banana fibre0.1 7.732 3.16 2.447 4.09 2.308 19.284 78.791
0.2 8.703 3.09 2.817 4.22 2.296 19.704 78.583
0.3 11.854 2.86 4.145 4.34 2.286 20.030 78.333
0.4 8.643 2.76 3.132 4.50 2.275 20.438 77.982
Fig. 4.4.a Variation of stability with different fibre %
Fig. 4.4.b Variation of flow value with different fibre %
It may be noted that all fibre stabilized mixtures gave the maximum stability at 0.3% fibre content. Comparing different fibre stabilized mixtures, it is evident that the mixtures with coir fibre have the highest stability (12.58 kN), indicating their higher rutting resistance and better performance than mixtures with other fibres. The percentage increase in stability with respect to the control mixture is about 70% for SMA with coir fibre and about 60% for SMA with other fibres. This result could be attributed to fibre’s adhesion and networking effects in the stabilized mixtures. The spatial networking effect was regarded as the primary factors contributing to fibre’s reinforcement (Chen and Lin, 2005). This trend could be explained as follows: fibre performs as ”bridge” when cracking of bitumen mixture appears and thus resists the propagation of cracking development, which is called bridging cracking effect (Li., 1992). In addition, due to the absorption of light component of bitumen (Serfass and Samanos, 1996), fibre improves the viscosity and stiffness of bitumen (Huang and White, 2001).
Flow value of SMA mixtures decreases after adding fibres, as shown in Fig.
4.4.b. Owing to the stiffness of fibres in the mixture, the mixes become less flexible and the resistance to deformation increases resulting in a low flow value. However, flow values are located within the required specification range of 2 to 4 mm (AASHTO T 245).
Marshall Quotient (MQ) also known as rigidity ratio is the ratio of stability to flow value of the mixture and the Marshall Quotient values of SMA with different fibre contents are shown in Fig. 4.4.c. It is found that MQ of the coir fibre stabilized SMA at 0.3 % fibre content is almost doubled with respect to the control mixture. It can be inferred that these stabilized SMA provide better resistance against permanent deformations due to their high stability and high MQ and also indicate that these mixtures can be used in pavements where stiff bituminous mixture is required.
Fig. 4.4.c Variation of Marshall Quotient with different fibre %
184.108.40.206 Bulk specific gravity
The bulk specific gravity of bituminous mixture decreases with increasing fibre content in SMA as depicted in Fig. 4.4.d.
Fig. 4.4.d Variation of bulk specific gravity with different fibre %
This trend is in agreement with other research (Tapk?n, 2008; Saeed and Ali, 2008). This result would be attributed due to the different specific gravities of different fibres and the much lower specific gravity of fibre than that of aggregates. Meanwhile, the elastic behavior of mixture increases with increase in fibre content, due to the elastic nature of fibres. As a result, at the same compaction effort (50 blows on both sides of Marshall sample), adding fibre reduces the specific gravity of the control mixture. However, it is noted that the coir fibre stabilized SMA has the highest specific gravity which is due to the fact that coir fibre has the maximum density (Table 3.3) as compared to other fibres. Considering the fact that higher specific gravity results in better design mixes, it can be inferred that coir fibre stabilized mixtures perform better than the other stabilized mixtures.
220.127.116.11 Air void, VMA and VFB
Excessive air voids in the mixture would result in cracking due to insufficient bitumen binders to coat on the aggregates, while too low air void may induce more plastic flow (rutting) and bitumen bleeding. Here the test results (Fig. 4.4.e) show that air void increases after adding fibres into bituminous mixtures. This may be due to the net working effect of the fibre within the mix (lower Gmb correlates to higher air voids). The mixtures with coir fibre have the highest air voids than the other mixtures. However, the air voids of mixtures are located within the specification range of 3% to 5% (AASHTO T 312) which support the use of these additives.
Fig. 4.4.e Variation of air void with different fibre %
Increasing the fibre content increases the VMA of SMA mixtures as shown in Fig. 4.4.f, while reduces VFB as shown in Fig. 4.4.g. With respect to the control mixture, when fibre content increases from 0% to 0.3%, air void increases by about 11.5%, VMA increases by 2.2%, while VFB decreases by 2.4% for coir fibre stabilized mixtures and the corresponding percentage changes are respectively about 9.25% increase, 5.4% increase and 1% decrease for sisal fibre stabilized mixtures and 8.5% increase, 6.2% increase and 1% decrease for banana fibre stabilized mixtures with respect to the control mixture. But all the results are within the required specification range which also supports the use of these additives.
Fig. 4.4.f Variation of VMA with different fibre %
Fig. 4.4.g Variation of VFB with different fibre %
4.5.2 Moisture susceptibility
From Table 4.5, it can be observed that the retained stability is significantly higher in the stabilized SMA mixtures as compared to the control mixture. Retained stability value of more than 70% (Table 4.2) is suggested as a criterion for a mixture to be resistant to moisture induced damages. It is seen that for the control mixture, it is only 69 %, supporting the need for an additive in SMA mixture. It also shows that the retained stability of the mixture increases with increasing additive content initially up to 0.3% for fibre beyond these contents, the value is found to be decreasing. Among the fibre stabilized mixtures, coir fibre stabilized mixture exhibits the maximum value (95%). These results show that the presence of additives in the Stone Matrix Asphalt mixture leads to a higher protection against water damage.
Both the cohesive properties of the bitumen and the adhesion of the bitumen to the aggregate surfaces may affect as a result of exposing the bituminous mixtures to moisture. Additive incorporation into bituminous mixtures helps to reduce the high level of moisture damage that was noted in the control mix. Among the fibre stabilized mixtures, the coir fibre stabilized mixes showed lower moisture susceptibility than those of the other fibre mixes at the same fibre concentration. 0.3% fibre concentrated mixes showed better resistance to water damage than that at other concentration. Higher fibre concentration may have far too high void contents (balling effect) which allow more water penetration into SMA mixtures.
Table 4.5 Retained stability of SMA mixtures with fibresAdditive (%) Retained stability (%)
Coir fibreSisal fibreBanana fibre0 69 69 69
0.1 84 82 81
0.2 90 89 88
0.3 95 93 93
0.4 92 90 90
4.6 COMPARISON OF VARIOUS STABILIZED MIXTURES
Test results have illustrated that type of additive and its content play significant role in the volumetric and mechanical properties of bituminous mixtures. Meanwhile, results have clearly shown that different additives have different reinforcing effects. Therefore, choice of appropriate additive type, design of optimum bitumen content, and design of optimum additive content would be among the primary objectives for the design of additive -reinforced bituminous mixtures.
Based on the Marshall test results discussed previously, an optimum fibre content of 0.3% is recommended for fibre stabilized SMA mixtures, with which fibre mixture exhibits the highest stability, Marshall Quotient and the residual stability and also the specified volumetric characteristics. The choice of additive type would consider both additive characteristics and its reinforcement effects.
The variations of volumetric and mechanical properties of SMA at the optimum additive content with different additives are shown in the table4.3. It is observed that the additives have great impact on the properties of the gap graded SMA mixture with rich binder content. There is significant improvement in the characteristics of control mixture after adding additives, showing the influence of additives on Stone Matrix Asphalt.
The percentage increase in stability value is significant at the optimum additive content. The flow value of SMA mixtures decreases with an increase in additive content. Stability and the Marshall quotient are almost doubled. Retained stability result indicates that the extent of moisture induced damage is more for the control mixture and it doesn’t fulfill the minimum criteria of 70%. But for all stabilized mixtures, the value is more than 90% which supports the role of additives in SMA mixture to reduce the moisture induced damages. Regarding the voids, fibre stabilized mixtures show higher air voids and voids in mineral, but the voids filled with bitumen is more in banana fibre stabilized mixtures. But in all stabilized mixtures, all the volumetric characteristics are within the specification range which also supports the use of these additives.
Among the fibre stabilized mixtures, coir stabilized SMA mix gives the best results as compared to the other two stabilized mixtures.
The mix design and analysis of SMA mixtures stabilized with three natural fibres (coir, sisal and banana).
While increasing the percentage of additives in the mixture, Marshall stability and retained stability of the mixture increases with respect to the control mixture and obtained the maximum value at 0.3% fibre content, beyond which these values show a decreasing trend. The flow value of the mixtures decreases with respect to the control mixture. At any stage in all cases, the values are within the required specified limits. As percentage fibre additive increases in the SMA mixture, bulk specific gravity and VFB decreases while VMA and air void increases irrespective of the type of fibre. In the case of other additives, the increase in additive content resulted in an opposite trend for the above volumetric properties. But all the results are within the specified limits.
Adding additives to Stone Matrix Asphalt mixture has shown improvement in the volumetric and mechanical properties of the mixture. It can be inferred that these stabilized SMA provide better resistance against permanent deformations (rutting) and also indicate that these mixtures could be used in pavements where stiff bituminous mixture is required.
Among the natural fibres, based on Marshall Mix design, coir fibre gives the best result at 0.3 % fibre content with a percentage increase in stability value of about 70% and Marshall Quotient of 90% with respect to the control SMA. The retained stability value is 95%. It can be observed that the highest Marshall stability is achieved by specimens with 7% waste plastics and the percentage increase is about 82% with respect to the control SMA. This mixture also exhibits the highest retained stability of 95%. The Marshall quotient is also doubled with respect to the control mixture. It can be concluded that Natural fiber stabilized Stone Matrix Asphalt mixture provide better resistance against permanent deformations due to their high stability and high MQ and it contributes to the protection of the environment. This will also lead to an ecofriendly sustainable construction method.
INDIRECT TENSILE STRENGTH CHARACTERISTICS
The tensile properties of bituminous mixtures are of interest to pavement engineers because of the problems associated with cracking. Although SMA is not nearly as strong in tension as it is in compression, SMA tensile strength is important in pavement applications. The indirect tensile strength test (IDT) is used to determine the tensile properties of the bituminous mixture which can further be related to cracking properties of the pavement. Low temperature cracking, fatigue and rutting are the three major distress mechanisms. A higher tensile strength corresponds to a stronger cracking resistance. At the same time, the mixtures that are able to tolerate higher strain prior to failure are more likely to resist cracking than those unable to tolerate high strains (Tayfur et al., 2007).
A lot of research work was reported on the performance of the bituminous pavements related to the tensile strength of bituminous mixtures (Zhang et al., 2001; Behbani et al, 2009; Anderson et al., 2001). A higher tensile strength corresponds to a stronger low temperature cracking resistance (Huang et al., 2004). The test provides information on tensile strength, fatigue characteristics and permanent deformation characteristics of the pavement materials.
The resistance of bituminous mixtures to fatigue cracking is dependent on its tensile properties, notably its tensile strength and extensibility characteristics. Fatigue was defined in the literature, as the phenomenon of fracture under repeated or fluctuating stresses. The layers in a flexible pavement structure are subjected to continuous flexing as a result of traffic loads that they carry that results in tensile stresses and strains at the bottom of bituminous layers of the pavement. The magnitude of the strain depends on the overall stiffness of the pavement. Indirect tensile strength test is an indicator of strength and adherence against fatigue, temperature cracking and rutting.
Tensile strength is typically used as SMA performance measure for pavements because it better simulates the tensile stresses at the bottom of the SMA surface course when it is subjected to loading. Tensile strength is difficult to measure directly because of secondary stresses induced by gripping a specimen so that it may be pulled apart. Therefore, tensile stresses are typically measured indirectly by conducting splitting tensile test.
5.2 LABORATORY TESTING FOR INDIRECT TENSILE STRENGTH
The tensile characteristics of the bituminous mixtures are evaluated by loading Marshall specimen along a diametric plane with a compressive load at a constant rate acting parallel to and along the vertical diametrical plane of the specimen through two opposite loading strips. This loading configuration develops a relatively uniform tensile stress perpendicular to the direction of the applied load and along the vertical diametrical plane, ultimately causing the specimen tested to fail by splitting along the vertical diameter. A 13 mm (1/2?) wide strip loading is used for 101 mm diameter specimen to provide a uniform loading with which produces a nearly uniform stress distribution. The static indirect tensile strength of a specimen is determined using the procedure outlined in ASTM D 6931. A loading rate of 51mm/minute is adopted. Tensile failure occurs in the sample rather than the compressive failure. Plywood strips are used so that the load is applied uniformly along the length of the cylinder. The compressive load indirectly creates a tensile load in the horizontal direction of the sample. The peak load is recorded and it is divided by appropriate geometrical factors to obtain the split tensile strength using the following equation:
St=2000P?tDSt = IDT strength, kPaP = maximum load, N
t = height of the specimen immediately before test, mm
D = diameter of the specimen, mm
The values of the indirect tensile strength may be used to evaluate the relative quality of the bituminous mixtures in conjunction with the laboratory mix design, testing and for estimating the resistance to cracking. The results can also be used to determine the resistance to field pavement moisture when results are obtained on both water- conditioned and unconditioned specimens. Many researchers used this test (Wallace and Monismith, 1980; Kennedy and Hudson,1968; Kandhal ,1979; Ibrahim, 2000). The method was standardized by both of the British Standard Institutions and the ASTM. The indirect tensile mode of testing like the one presented in Fig. 5.1 can be used to establish the tensile properties of bituminous mixtures to evaluate the performance of the pavement. The forces acting during the test is shown in Fig. 5.2.
Fig. 5.1 Schematic of Indirect tensile test setup
Fig. 5.2 Forces acting during split tensile test
5.2.1 Tensile strength ratio (TSR)
Moisture damage in the bituminous mixes refers to the loss of serviceability due to the presence of moisture. The extent of moisture damage is called the moisture susceptibility.
The ITS test is a performance test which is often used to evaluate the moisture susceptibility of a bituminous mixture. Tensile strength ratio (TSR) is a measure of water sensitivity. It is the ratio of the tensile strength of water conditioned specimen, (ITS wet, 60?C, and 24 h) to the tensile strength of unconditioned specimen (ITS dry) which is expressed as a percentage. A higher TSR value typically indicates that the mixture will perform well with a good resistance to moisture damage. The higher the TSR value, the less will be the strength reduction by the water soaking condition, or the more water-resistant it will be.
A total of 120 Marshall Specimens of Stone Matrix Asphalt stabilized with different additives are prepared. 40 specimens are prepared for each additive and divided into two groups (20 specimens each). The first group of specimen was immersed in a water bath at 60?C, for a period of 24 hours (conditioned sample). The samples are then removed from the water bath and kept at a temperature of 25?C for a period of 2 hours. Other set of samples (unconditioned sample) are kept at a temperature of 25?C for a period of 2 hours without soaking. These specimens are then mounted on the conventional Marshall testing apparatus and loaded at a deformation rate of 51mm/min and the load at failure is recorded at each case. Then the tensile strength of water conditioned as well as unconditioned specimen for each additive stabilized mixture is determined. Samples before and after failure are shown in Fig. 5.3
Fig. 5.3 Sample before (left) and after (right) failure
5.3 RESULTS AND DISCUSSIONS
The indirect tensile strength results of SMA mixtures with different additives at various percentages both for conditioned and unconditioned samples are given in Table 5.1. It is evident that all the stabilized SMA mixtures showed higher tensile strength than the control mixtures irrespective of type of the additive. This is because of improved stiffness of stabilized mixture than that of control mixture. The presence of additives in bituminous mixture strengthens the bonding between the aggregate provided by the binder and as a result, the mixtures had the highest stiffness. The results also indicate that the tensile strength increases as the additive content increases, reaches a maximum value and then decreases.
It is also observed that for all SMA mixtures, the tensile strength decreases with conditioning regardless of the type of additive. But the percentage decrease in strength due to the conditioning of the sample decreases with the increase in additive content. For the control mixture, the percentage decrease in strength due to conditioning is about 48 %, while at higher additive contents, for all the additives, the percentage decrease is very less (< 3%).
5.3.1 Fibre stabilized SMA
The variations of indirect tensile strength of Stone Matrix Asphalt mixtures with different percentages of fibre contents are given in Fig. 5.4 to 5.6. The tensile strength of SMA mixes with fibre additive shows increasing trend up to 0.3% and it is found to be decreasing at 0.4% fibre content. This behavior is because, the tensile strength is related primarily to a function of the binder properties, and its stiffness influences the tensile strength. Presence of fibre in the mixture makes it stiffer. The addition of fibre beyond a certain level can increase the viscosity of binder, which results from the effects of increase in volume of fibre particles due to the absorption of binder. Therefore, this increase in viscosity inhibits the ability of the binder to coat adequately on the surface of aggregates, thereby lead to the potential loss of bonds between the fibre, binder and the aggregate.
Table 5.1 Indirect tensile strength results for stabilized SMA mixtures
Additive % ITS, Unconditioned (MPa) ITS, Conditioned (MPa) % TSR
Nil 0 0.8143 0.4253 52.23
Coir fibre0.1 0.851 0.709 83.31
0.2 1.0983 1.059 96.42
0.3 1.1242 1.1048 98.27
0.4 1.0831 1.0521 97.14
Sisal fibre0.1 0.8313 0.6915 83.18
0.2 1.0619 1.0114 95.24
0.3 1.1057 1.0766 97.37
0.4 1.0538 1.0153 96.35
Banana fibre0.1 0.8272 0.6941 83.91
0.2 1.065 1.0107 94.90
0.3 1.1018 1.0762 97.68
0.4 1.054 1.015 96.30
A comparison of tensile strength characteristics for the three fibre stabilized SMA mixtures both for unconditioned and conditioned are given in Fig. 5.7.a and Fig. 5.7.b. All the fibre stabilized SMA mixtures have the maximum tensile strength at 0.3% fibre content by weight of mix for both conditioned and unconditioned SMA mixtures. The percentage increase in strength for the coir fibre stabilized mixture (0.3% fibre content) with respect to the control mixture is 38% and 160% respectively for unconditioned and conditioned samples. This increase is about 36% and 153% respectively for both sisal and banana fibre stabilized mixtures. The improvement in indirect tensile strength would be due to fibre’s absorption and adhesion of bitumen which improves the interface adhesion strength and fibre’s networking and bridging cracking effects. Fibres possess greater modulus and elongation than bitumen, performing as bridges to resist cracking propagation and material failure. It sustains greater stress and strain before material failure, resulting in improved toughness. Fibre reinforcing effect increases initially with increasing fibre content; but at high fibre content (more than 0.3%) may induce coagulation and thus reducing its reinforcing effect. The higher amount of fibre in the mixture may not have any beneficial effect and may deteriorate its deformation properties. In fibre stabilized mixtures, large amount of fibre leads to higher surface area that must be coated by bitumen, and consequently, the aggregate particles and fibre would not be fully coated with bitumen. This results in less stiff mixture which leads to the failure of the mixture. Test results show that coir fibre stabilized mixtures has the highest tensile strength as compared to the other two mixtures.
Fig. 5.4 Variation of indirect tensile strength of SMA with coir fibre contents
Fig. 5.5 Variation of indirect tensile strength of SMA with sisal fibre contents
Fig. 5.6 Variation of indirect tensile strength of SMA with banana fibre contents
Fig. 5.7.a Variation of indirect tensile strength of SMA (unconditioned) with different fibre contents
Fig. 5.7.b Variation of indirect tensile strength of SMA (conditioned) with different fibre contents
5.3.2 Comparison of different SMA mixtures
It is evident that the percentage increase in strength is very high in case of all conditioned samples of stabilized SMA with respect to the control mixture. The maximum percentage increase is for SMA with coir fiber and the minimum for that with banana fibre (160% and 153% respectively). The tensile strength value for specimens with coir fibre is higher as compared to that with sisal and banana fibres (shows approximately similar strength). The mixtures had the highest stiffness and the percentage increase in strength is about 11% at both unconditioned and conditioned SMA mixture as compared to coir fibre stabilized mixture.
5.3.3 Moisture susceptibility of SMA mixtures
As given in Table 5.1, the tensile strength ratio (TSR) values of the control mixture is nearly 52% which is less than 70%, a minimum TSR value set forth by AASHTO T283. This illustrate that the control mixture has more significant moisture susceptibility. The tensile strength ratios for the mixes containing the additives are greater than the specification limits. From these results, it can be concluded that the presence of additives significantly reduces the moisture induced damage of the SMA mixture. This also indicates that the additives do not cause the mixture to weaken when exposed to moisture. The results also indicate that the tensile strength ratio which represents the moisture susceptibility increased up to a certain percentage of additives and after that, it is found to be decreasing. In fibre stabilized mixtures, the decrease in TSR at higher fibre content may be due to the balling effect of the fibres in the mix.
Fig. 5.8 Variation of tensile strength ratios of SMA with different fibre contents
From Fig. 5.8, it is evident that among the fibre stabilized mixtures SMA with coir fibre gives a slightly higher tensile strength ratio than the SMA with other fibres. The specimens containing sisal and banana fibre produced almost similar results.
It is increasing with respect to the control mixture when additives are added and the increase is somewhat similar. It can be concluded that all stabilized mixes satisfy the minimum required tensile strength ratios of 70% indicating their better moisture resistance than the control mixture.
The indirect tensile strength test is used to determine the tensile properties of the Stone Matrix Asphalt mixture which can be further related to the cracking properties of the pavement. The tensile strength ratio of bituminous mixtures is an indicator of their resistance to moisture susceptibility and a measure of water sensitivity.
Based on the test results, it can be concluded that the SMA stabilisation improves the cracking resistance of the mixture as compared to the control mix. All the additives improve the adhesion property of the bitumen to aggregate. The indirect tensile strength values are found to be much higher when additives are incorporated in SMA mixtures and the effect is more influential in the conditioned state. This substantiates the need of additives in SMA mixtures.
Coir, sisal and banana fibres have improved indirect tensile strength with respect to the control mixture. Even though all stabilized SMA mixtures show higher indirect tensile strength and tensile strength ratio, addition of 0.3% coir fibre in the SMA mixture resulted in the highest tensile strength and exhibit superior water resistance property.
SUMMARY AND CONCLUSIONS
Annual increase in maintenance expenditures demands the urgent need for building durable and efficient roads. Severe climatic conditions, growing traffic volume and insufficient drainage conditions results in faster deterioration of the pavements in AP. Stone matrix Asphalt (SMA) has proved to be the right choice to handle such situations. This research is an attempt to study the influence of additives in improving the characteristics of SMA mixtures and to propose a durable surface course.
This chapter presents summary and conclusion of this study based on the various objectives addressed.
Additives are added to improve the characteristics of SMA, which is a gap graded mixture with rich binder content. The influence of additives as natural fibres on the characteristics of SMA is studied. The natural fibres like coconut fibre, sisal fibre, and banana fibre are used as additives. The SMA mixture without any additive is taken as the control mixture. The volumetric and stability characteristics are studied by Marshall tests and the strength characteristics by indirect tensile strength tests. Major conclusions of this research are discussed in the succeeding sections.
The major conclusions deduced from this study are presented in these sections.
6.3.1 Influence of additives in SMA
Additives influences the characteristics of SMA mixtures by showing an enhancement in strength and stability, reduction in water induced damages, while maintaining the specified volumetric characteristics.
Volumetric and stability characteristics
Marshall Stability test results show that:
With the increase in percentage of additives in the SMA mixture, Marshall stability and Marshall quotient values increase with respect to the control mixture showing its better resistance against permanent deformations. The maximum values for these properties are obtained at 0.3% fibre content irrespective of the type of fibre. Beyond this percentage of additives, the results show a decreasing trend.
Flow value of SMA mixtures decreases due to the addition of fibres. The mixes become less flexible resulting in a low flow value. However, flow values are located within the required specification range of 2 to 4mm (AASHTO T 245) supporting the use of additives in SMA mixtures.
The air voids of the stone matrix asphalt mixtures increase after adding fibres into the mixture due to networking effect of the fibres within the mix. However, the air voids of all mixtures are located within the required specification range of 3 to 5% (AASHTO T 312) which support the use of these additives.
Increasing the fibre additive content in the SMA mixture resulted in an increase in the voids in the mineral aggregate (VMA) and a reduction in voids filled with bitumen (VFB) values. But VMA values are within the required specification range of 17% minimum, which also supports the use of these additives.
The bulk specific gravity of SMA mixtures slightly decreases with the addition of fibre additives. This is due to the variations in air voids in the mixture as compared to the control mixture.
Based on the indirect tensile strength test, the following conclusions can be drawn.
The mixtures containing additives have higher values of indirect tensile strength at failure under static loading as compared to the control mix, indicating the improved cracking potential of SMA misx.
The effect of additive in increasing the indirect tensile strength value of SMA mix is more influential in the conditioned state due to the improved adhesion property.
18.104.22.168 Moisture susceptibility
The retained stability value, tensile strength ratio and index of retained strength which are the indicators of the ‘extent of moisture induced damage’ shows that all stabilized SMA mixtures exhibits higher values against the lower values of control mixture. This support the influence of additives in significantly reducing the water induced damages of the SMA mixture. In addition, it also indicates that additives do not cause the mixture to weaken when exposed to moisture.
The values of retained stability, tensile strength ratio and index of retained strength for the control mixture is less than the required minimum values of 70% (LS 283), 70% (AASHTO T 283) and 75% ( ASTM D 1075) respectively. When additives are added, these values are enhanced to above 90%.
6.3.2 Fibre stabilized SMA
Optimum fibre content of fibre stabilized mixtures and the best fibre additive is arrived by analysing the volumetric, mechanical and moisture susceptibility characteristics of the SMA mixture with various fibre additives such as coir, sisal and banana fibre. In this research, fibre length is kept as constant (6 mm) and the content is varied from 0.1% to 0.4% at an increment of 0.1% by weight of mixture.
Stability and volumetric characteristics
Irrespective of the type of fibre, the maximum values of stability, Marshall Quotient and bulk specific gravity of SMA mixtures are obtained at 0.3% fibre content. Comparing different types of fibre stabilized SMA mixtures, mixtures with coir fibre have the highest stability (12.58 kN), indicating their higher rutting resistance and better performance than mixtures with other fibres. The percentage increase in stability with respect to the control mixture is about 70% for SMA with coir fibre and about 60% for SMA with other fibres.
Flow value of SMA mixtures decreases after adding fibres. Owing to the stiffness of fibres in the mixture, the mixes become less flexible resulting in a low flow value. However, flow values of all SMA mixtures are located within the required specification range of 2 to 4 mm.The Marshall quotient of coir fibre stabilized SMA mixture at 0.3 % fibre content is almost doubled with respect to the control mixture, indicating its better resistance against permanent deformations and also indicates that these mixtures can be used in pavements where stiff bituminous mixture is required. Coir fibre stabilized SMA has the highest bulk specific gravity when compared to mixes with other fibres. Since higher specific gravity results in better design mixes, the coir fibre stabilized mixtures perform better than other stabilized mixtures considered. Considering the volumetric characteristics, at 0.3% fibre content, air void increases by 11.5%, VMA increases by 2.2%, while VFB decreases by 2.4% for coir fibre stabilized mixtures. The percentage changes are respectively 9.25% increase for air void, 5.4% increase for VMA and 1% decrease for VFB in sisal fibre stabilized mixtures. Whereas 8.5% increase in air voids, 5.9% increase in VMA and 1% decrease in VFB are observed in banana fibre stabilized mixtures. But all the volumetraic characteristics are within the required specification range which also supports the use of these fibre additives.
All the fibre stabilized SMA mixtures has the maximum tensile strength at 0.3% fibre content. The coir fibre stabilized SMA exhibits the highest tensile strength showing its higher cracking resistance as compared to the other fibre stabilized mixtures. The percentage increase in strength with respect to the control mixture is 38% for unconditioned and 160% for conditioned samples for the coir fibre stabilized mixture, whereas around 35% and 153% for both sisal and banana fibre stabilized mixtures respectively.
Fibre reinforcing effect increases initially with increasing fibre content in SMA, but at high fibre content (more than 0.3%) induce coagulation and thus reduce its reinforcing effect, resulting in less stiff mixture with lower strength values.
The test results converge to the conclusion that the best performance of the Stone Matrix Asphalt mixture is at 0.3% fibre content and with coir fibre.
22.214.171.124 Moisture susceptibility
The presence of fibres in SMA mixtures gives the higher retained stability, tensile strength ratio and index of retained strength at 0.3 % fibre content by weight of mix and the best performance is exhibited by SMA with coir fibre indicating its higher resistance to moisture induced damages.
Based on the volumetric and mechanical characteristics of the various fibre stabilized mixtures it can be concluded that the optimum fibre content of the fibre stabilized Stone Matrix Asphalt mixture is 0.3% by weight of mixture and the coir fibre additive is the best among the fibres investigated.
SCOPE FOR FURTHER RESEARCH
Further research is recommended on the following aspects:-
Many properties of SMA mixes such as Marshall and strength Properties was studied in this investigation.
Only VG-30 grade bitumen, coconut, sisal and banana fibres have been tried in this investigation. Some other synthetic and natural fibres and other type of binder can also be tried in mixes and compared.
Only one gradation was adopted here, so an attempt can be made to compare different gradations suggested by various agencies.
The fibres used in this study is a low cost material, therefore a cost-benefit analysis can be made to know its effect on cost of construction.
Moreover, to ensure the success of this new material, experimental stretches may be constructed and periodic performance of these pavements with modified mixes can be evaluated.
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