Analysis Of Test Methods Used To Evaluate Hot Mix Asphalt Concrete's Susceptibility To Moisture Damage

Water and/or moisture can cause pavements to suffer premature failure.Moisture damage can be defined as the early loss of strength and durability caused by moisture penetrating through the asphalt-aggregate mixture.The hot mix asphalt(HMA) layer which derives its strength from the cohesive resistance of the binder and grain interlock and frictional resistance of the aggregates may fail prematurely in the presence of water or moisture especially when poorly designed and constructed.The cohesive bond of the mastic(asphalt binder plus mineral filler) and adhesive bond between aggregates and the asphalt binder weakens in the presence of water or moisture.Moisture damage of HMA can occur in all environs as long as moisture is present in the mixture though the external environmental has an effect on what kind of stripping mechanisms will be prominent.There are several test methods that have been developed since the 1930s to help identify mixtures that are likely to be sensitive to moisture damage.However,there is currently no test method that is able to correlate laboratory results to field performance.This thesis describes the evaluation of the interracial bond between aggregate and asphalt cement in HMA using five methods namely Retained Marshall Stability,Modified Lottman,Fatigue test,Lap shear test and the method based on the surface energy of aggregate and asphalt cement.The first three methods have widely been used in the assessment of HMA's sensitivity to moisture damage and are performed on compacted HMA whilst the last two methods are proposed in this research as alternative test methods due to their simplicity.The last two are performed on un-compacted aggregate-asphalt cement mixtures.The capillary rise method has been utilized to determine the contact angle of aggregates and the sessile method for the measurement of contact angle of the asphalt cement.Thereafter,the surface free energy of both the aggregates and asphalt cement was determined using the Young-Dupre equation.Two types of aggregates,a limestone and granite,and two asphalt cements are used to produce four different mixtures of HMA.The four mixtures are subsequently tested using the five methods and then ranked based on their resistance to moisture damage.Based on experimental results,all the five methods indicate that HMA mixture with limestone aggregate had better resistance to water damage than those with granite aggregate. The methods used on un-compacted mixtures offer viable alternatives to the assessment of water susceptibility of HMA especially the method based on surface energy measurements of asphalt cement and aggregates.This method is simple,quicker and the equipment utilized can easily and cheaply be assembled whilst still employing latest advancements in adhesion failure theory.