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Impact of Anti-Strip Additives on Performance of Asphalt Pavements
AdvisorHajj, Elie Y.
Civil and Environmental Engineering
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Typically, lime and liquid are used as additives to combat moisture damage, and therefore, their impact is only evaluated with respect to their influence on the moisture sensitivity of the HMA mixture. This study extended the evaluation to cover the impact of lime and liquid additives on the structural performance of the HMA mixtures and their impact on the long-term performance of typical HMA pavements.Aggregates and binders were obtained from five different sources: Alabama, California, Illinois, South Carolina, and Texas to produce HMA mixtures that were evaluated in this study. Three mix designs were conducted for each material source: un-treated, liquid-treated, and lime-treated mixtures. All mix designs were conducted following the Superpave Volumetric Mix Design Method. The types of liquid additive were selected by each participating state agency (i.e. mix source) and were added at the rate of 0.5% by weight of binder. A single lime source was used for all five sources at the rate of 1% by dry weight of aggregates. The following performance properties were evaluated for all 15 mixtures:* Resistance to moisture damage: relationship between dynamic modulus and multiple freeze-thaw (F-T) cycles.* Resistance to permanent deformation: relationship between permanent strain in the HMA mix and number of load repetitions under triaxial testing conditions at the un-conditioned and moisture-conditioned stages.The resistance of the mixtures to fatigue cracking was assessed in a separate study. The developed relationship between the bending strain in the HMA mix and number of load repetitions to failure under beam fatigue testing conditions at the un-conditioned and moisture-conditioned stages were used in the mechanistic analysis of various pavement structures.The measured performance properties of the mixtures were used in the AASHTO Mechanistic-Empirical Pavement Design Guide (MEPDG) to conduct 20 years structural designs for actual projects selected from the five sources of mixtures. For each project, a total of three structural designs were established by changing the type of mix used in the HMA layer e.g. un-treated, liquid-treated, and lime-treated.The MEPDG structural designs were used with typical cost figures for the three types of mixtures to estimate the costs of the three types of structural designs for each project. The percent cost savings/additional costs were estimated relative to the cost of the HMA pavement with the un-treated HMA mixtures. Based on the extensive data generated from this research and the analyses of these data, the following findings are warranted.* The use of both liquid and lime additives improved the moisture sensitivity of the HMA mixtures as measured by the tensile strength ratio (TSR) following AASHTO T283 method. However, as the mixtures were subjected to further moisture damage induced through multiple freeze-thaw (F-T) cycling, the un-treated and liquid-treated mixtures had significantly reduced their stiffness properties (i.e. E*). On the other hand, the lime-treated mixtures maintained higher stiffness properties for the entire 15 F-T cycles for all five sources.* Lime either maintained or improved the rutting resistance of the HMA mixtures from all five sources. The impact of liquid on the rutting resistance of the HMA mixtures was source dependent; for the non-moisture sensitive mixtures from AL and IL, the liquid additives reduced their resistance to rutting as compare to the un-treated mixtures. * The cost analysis data revealed the following:o The use of lime additives in HMA mixtures resulted in significant savings, in some cases more than 45%.o The use of liquid anti-strip additives in HMA mixtures may result in additional cost, in some cases as high as 50%.o The data generated on the four mixtures from Alabama, California, Illinois, and S. Carolina show that lime is highly compatible with the use of neat asphalt binders and will always results in savings on the order of 13-34%.o The data generated on the mixtures from Texas show that the lime is highly compatible with the use of polymer-modified binders and will result in savings on the order of 40-45% which is significantly higher than the savings that could be realized with the use of the liquid anti-strip. o This data show that the use of lime additives will always improve the performance of the HMA pavement to a magnitude that always far outweighs its cost. On the other hand, the use of liquid anti-strip additives will not always improve the pavement performance to the magnitude that it will offset its cost.o The cost analysis data showed that the use of lime in HMA mixtures that do not require improvement in their mix design TSR will still result in significant savings such as the case of the mixtures from Alabama and Illinois. On the other hand, the use of liquid in HMA mixtures that do not require improvement in their mix design TSR will result in significant cost increases such as the case of the mixtures from Alabama and Illinois.o The cost analysis data showed that the use of lime in HMA mixtures that require improvement in their mix design TSR will still result in significantly higher savings such as the case of the mixtures from California, S. Carolina, and Texas. On the other hand, the use of liquid in HMA mixtures that require improvement in their TSR will result in mediocre cost savings such as the case of the mixtures from California, S. Carolina, and Texas.Adding the savings realized by the use of lime-treated mixtures from the MEPDG structural designs to the savings realized from the lower thermal cracks per mile resulted in very significant overall cost savings to the highway industry and time savings for the road users.