Construction Practices and its Effect on Bond Strength of Pavements

Extensive study is carried out to ascertain the effects of various construction practices on the bond strength between different layers of the pavement. In this study various construction practices refers to curing time of the pavement, various equipments used during construction of pavement, surface treatment provided. In this study a review is also carried out of the research work carried out by various researchers for estimating the bond strength between the existing old hot mix asphalt (HMA) layer and the new hot mix asphalt (HMA) layer overlaid. Also a review is taken in study regarding various experiments conducted by the researchers on the bond strength of different layers. Normally milling provides a good strength and good bond at the interface between the old layer and new layer overlaid on it. It is also observed that curing time has least effect on the bond strength. Ankur Arun Kulkarni


Introduction
Any pavement consists of various layers made up of different materials. All layers have different bonds at the interface surface. Till date a very extensive research is carried out regarding performance of the pavement material. But unfortunately very less research is carried out on the performance of adhesion between different layers of the pavement and its effect on the overall performance of the pavement and its life. Due to that very few codes, standards and procedures are existing which specify the quality of interface between different layers of the pavement.
Overall performance of the pavement is extensively depends on the bonding between various layers. Good bonding between various layers of the pavement is highly crucial for achieving the desired results, the higher load carrying capacity and a longer life span of the road pavement. Tack coat is used as adhesion between existing and new asphaltic pavement. Existing literature shows that different types of material is used as a tack coat between two hot mix asphalt (HMA) layers. [4] also found that the fine graded mix of HMA has poor shear strength than the coarse graded mix because the surface roughness increases with coarseness and adds the shear strength. But at the same time West et al. [5] proved that the fine graded mix of HMA has higher bond strength than the coarse graded mix at 25ºC.
Piotr and Wałach [6] conducted the study for evaluation of strength of bond between concrete layers. The experiments conducted by them showed different modes of interface failure depending on the configuration type of concrete in composite specimens.

Experimental work
Extensive experimental work is carried out by Laith Tashman et al. [6].

Result and analysis
The above mentioned three types of tests were conducted to study the influence of various factors on the bond strength of track coat at the interface between the two layers of the pavements.

FDOT shear tester
In this test the road pavement was divided in two type's milled and non-milled sections and surface condition as unbroken, broken and non-tack. Target residual rates of 0.075, 0.215 and 0.33 L/m2 were applied to both broken and unbroken sections. For the milled sections it was observed that there is no considerable difference in the mean shear strength for the three curing time slot categories considered. For both surface condition i.e. milled and nonmilled surfaces there was negligible difference in broken and unbroken sections. Table 1 shows the observed results.   Fig. 1 it may be observed that during FDOT Shear test, the surface condition do not affect the residual rate as well as tracking. From Fig. 2 it is seen that the residual rate varies along with curing time. Also it can be observed that the tracking is having negligible effect over shear strength.

Torque bond test
The torque bond test was performed ac-cording to 'British Board of Agreement standards', according to which the required torque 300 N m capacities. Milled and non-milled sections were tested for torque bond. The cores that could withstand the 300 N m torque are considered as right cores, while the cores that could not withstand the torque of 300 N m were considered as left cores.

Pull off test
In this test the road is divided in to two separate parts as milled and non-milled section. Thus surface condition is considered as main governing factor. Further the surface condition is classified into different residual rates. Testing time is also a factor that affects the results. During the experiments it was observed that the base contact plate has better adhesion with the non-milled section than the milled section. This results in higher tensile strength for non-milled section as compared to milled section. Also it is interesting to note that the pull off strength inversely varies with the time. As time increases the pull of strength reduces. This effect was observed over entire length of the road. Table 3 show the observed results.

Conclusion
The study and review of experimental work presented in this research paper is for the three tests which has different mechanisms. The experiments conducted for FDOT shear tester and tabulated in Table 1, shows that, except tracking factor all other factors are statistically significant. It is also observed that for milled as well as for non-milled sections, increase in residual rate does not improve the shear strength.
The results presented in Table 2, shows that, there is large difference in torque strength between milled and non-milled section. Milled section offers higher torque resistance than the non-milled section. Also it was observed that for non-milled section a significant difference exists for the torque resistance in no tack, broken and unbroken specimens.
The results of Pull-off test presented in Table 3, clearly indicates that the nonmilled sections have higher pull-off strength than that of the milled section. Overall it is observed that the milled section provides considerably higher bond strength at the interface of existing and new pavement surface. The tack coat does not affect the bond strength of milled section. But for non-milled section the tack coat significantly affects the bond strength of the pavement.