In the past sub grade deformation was considered to be the primary cause of rutting and many pavements design method applied limiting criteria on vertical strain at the sub grade level however the recent research have shown that most of the rutting occur in the upper part of asphalt surfacing layer which mainly depends on the aggregate gradation and natural properties of aggregates and on the properties of bitumen both of which are the governing constituents’ of mix design and asphalt surfacing layer ,Ahmed et al., 6.
Singh et al.,7 on their study of the role and effect of aggregate gradation on the mechanical response of asphalt mix, say that given a size range of aggregates, numerous aggregate gradations (and thereby numerous asphalt mixes) are possible. Based on these numerous combinations, it is essentially a cumbersome and time consuming task to study the laboratory or field performances for all such possible asphalt mixes. Moreover, type of asphalt binder, shape of aggregates, size of aggregates, ratio of aggregates gradation, They find that a typical asphalt mix, aggregates occupy about 85% and binder about 10% by volume in the mix the rests are voids .Their results shows that the mixes with coarse aggregates in high percentage are more stable and stronger.
Based on the above guidelines, five different gradations of asphaltic concrete are considered and tested in this paper for the purpose of finding an optimum mix to cope with the problem of rutting in ACC.
3.1 Laboratory Characterization of Materials (Based on AASHTO specifications).
3.1.1 Coarse Aggregates: The suitability of aggregates from Chsma baba quarry Karak for use in asphalt construction was determined by evaluating the material in terms of the followings.
• Size and grading
• Particle shape
• impact value
• Abrasion value.
3.1.2 Fine aggregates: Relative Density and Absorption tests are done on this fraction of the HMA
3.1.3 Characteristics of Asphalt Cement: For engineering and construction purposes, three properties or characteristics of asphalt are important i.e. consistency, purity and safety Gudimettla 8.
3.1.4 Marshall test of HMA: To find the behaviour of HMA in Marshall and wheel tracking tests, the considered gradations of the samples are shown in table 3.1.
Sample No. Gradation
1 50% coarse aggregate +40% sand +10% (bitumen +filler + air)
2 60% coarse aggregate +30% sand +10% (bitumen + filler + air)
3 55% coarse aggregate + 35% sand +10%(bitumen + filler +air)
4 45% coarse aggregate +45% sand + 10% (bitumen +filler +air)
5 40%coarse aggregate + 50% sand +10% (bitumen + filler + air)
Table 3.1: Considered gradations and their sample Nos.
The suitability and strength of HMA mixture is measured from a compacted sample prepared from this HMA mixture. The apparatus used for the strength of this sample and HMA is Marshal Stability Meter / machine. The sample is loaded diametrically at the rate of 5cm per minute. The deformation and load are measured in mm and kg respectively. Volumetric relations are also found to establish a sound relation between voids, voids filled bitumen and amount of air which also affects the strength of HMA .So for an HMA design , the followings objectives found this Test:.
• Voids – density relationship
• Marshall Stability and Marshall flow.
The maximum load (kg / kN) carried by the sample at a temperature of 60°C in the machine is called Marshall stability number or value of the HMA mix. The flow value (mm) associated with maximum load is called flow value. It is measured in 0.25 mm units. All related tests are carried out to meet the criteria of NHA specifications (1998) for the asphalt concrete wearing course mixture. Indirect tensile strength tests in conditioned and in unconditioned states are done to know the freeze and thaw response of the mix.
3.1.5 Volumetric relationships: HMA mixture is made of three materials; aggregate, asphalt binders and air. Generally. HMA is described by its constituents’ volume. It is important to know how these three materials relates to one another. Knowledge of these is used to find the amount of optimum binder content which controls the stability of HMA. Theses volumetric includes bulk specific gravity of the compacted asphalts mixture (Gmb), voids filled with asphalt (VFA), voids in mineral aggregates (VMA), air voids (AV) and theoretical maximum specific gravity of bituminous paving mixture (Gmm). 25 samples of considered gradation, five with the same binder content, are tested for optimum binder content to give maximum Marshall Stability and minimum flow. The samples are that mix are used for rutting determination.
3.1.6 Indirect tensile strength: The indirect tensile strength (IDT) of all the gradation is found out both in unconditional and conditional (frozen) state. The IDT plays a vital role in effecting the rutting character of HMA. The values of IDT strength may be used to evaluate the relative quality of asphalt mixture in conjunction with laboratory mix design testing and for estimating the potential for rutting or permanent deformation. The results can also be used to determine the potential for the field pavement moisture damage when results are obtained on both moisture conditioned and unconditioned specim.
3.1.7 Sample testing for Permanent Deformation (Rutting) Wheel tracker test: In this test simulation of the deformation / rutting in the road surface course due to repetitive wheel loads is carried out. The sample in the shape of a slab is prepared in the given formwork and placed on platform of the machine. All data are input for number of passes per minute (frequency), depth recording etc. The computer software records and plots the graph between depth and number of passes in the sample is carried out. This apparatus is available in Sarhad University Peshawar.
Working of Wheel Tracker is as follows: The wheel tracker tray with the sample mounted in it is fixed under the wheel. The machine and the software attached are turned on. Then in the software the speed of the wheel was adjusted to 50ppm (passes per minute). Delay time was fixed as 1 minute. The number of passes was fixed to 12000.The wheel moves to and fro on the mounted sample with contact ;load of 5tonnes simulating road wheel load. LVDT and Load transducers