Better Roads Digital Magazine
Issue link: https://read.dmtmag.com/i/85927
RoadScience don't mix, moisture damage impairs the bond between ag- gregate and liquid asphalt. "While adhesion failure between the asphalt and aggregate (referred to as stripping) is the most commonly recognized [mechanism], there are others," AI says. "These include moisture-induced cohesion failures within the asphalt mixture, within the aggregate, emulsifi - cation of the asphalt and freezing of entrapped water." "Although moisture damage potential is also possible in some HMA mixtures, due to its method of production, it may be more likely in WMA," say Thomas Bennert, Ph.D., Center for Advanced Infrastructure and Transportation at Rutgers University, and Greg Brouse, Eastern Industries Inc., in their 2011 WMA conference paper, Infl uence of Initial Aggre- gate Moisture Content and Production Temperature on Mixture Performance of Plant Produced Warm Mix Asphalt. "Inadequately dried aggregates at lower production tem- peratures, and even the possible introduction of additional moisture to the WMA from the various WMA foaming technologies, may affect the binder-to-aggregate adhesion, tents, an HMA as control; WMA via foaming system; WMA foaming plus an anti-strip; WMA foaming plus a different anti-strip; and WMA via surfactant additive. Laboratory performance samples were sampled and compacted at the asphalt plant's QC laboratory after similar conditioning periods. Moisture damage tests (Tensile Strength Ratio, Hamburg Wheel Tracking), permanent deformation (AMTP Flow Number, Asphalt Pavement Analyzer), dynamic modulus, and fatigue cracking via overlay tester tests were undertaken on the samples. The study found that at the moisture contents measured, production temperature is more signifi cant than initial ag- gregate moisture content on performance. Moisture dam- age tests did not rank performance the same: the tensile strength ratio results were okay, but the Hamburg were bad based on general criteria. "Not all anti-strips work the same," Bennert and Brouse "Although moisture damage potential is also possible in some HMA mixtures, due to its method of production, it may be more likely in WMA," - Thomas Bennert, Ph.D., and Greg Brouse, moisture susceptibility and general mixture performance," they write. "Preliminary laboratory testing has shown that when producing warm mix asphalt, both the initial aggregate moisture content and mixing temperature can have a dramatic impact on the performance of the mix- tures, especially with respect to the resistance to moisture damage. Unfortunately, it is often diffi cult to control these parameters outside the laboratory setting." That's because, as they say, while some lab tests show WMA specimens often fail the Texas DOT Hamburg Wheel Tracking test criteria, moisture damage has not been wit- nessed in the fi eld, and similar results with other tests have been reported outside Texas. "This may indicate modifi ca- tions to material preparation and/or test procedures are required when evaluating the moisture damage susceptibil- ity of WMA mixtures in the laboratory, so fi eld conditions can be properly simulated," they observe. For this work, two different initial aggregate moisture contents (less than 1.5 percent and greater than 3.0 per- cent) were achieved by the use of covered stockpiles. Five different mixtures were produced at both moisture con- 24 August 2012 Better Roads say. One anti-strip outperformed the other in the TSR test, but with mixed results in the Hamburg. It's important to evaluate each of the mate- rials to ensure they provide the performance required, the authors say. A 10 to 15 degrees F increase in temp clearly increased rutting resistance, but decreased fatigue resistance, they report. Field cores of an actual installation will be under- taken after one year. Another October WMA conference paper, Moisture Damage Potential for Warm Mix Asphalt Containing Reclaimed Asphalt Pavement, by Mariely Mejías-Santiago and Ray Brown, Ph.D., P.E., U.S. Army Engineer Research and Development Center, Vicks- burg; and Jesse Doyle, Ph.D. and Isaac L. Howard, Ph.D., Mississippi State University, stated that increased percentages of RAP in a mix may improve WMA's sensitivity to moisture. Their moisture damage study of a variety of samples indicated: • Gravel WMA mixes showed moisture susceptibility for lower mixing temperatures. Limestone mixes did not show this problem. • WMA additive type did not signifi cantly affect mois- ture susceptibility at HMA mixing temperature • The low mixing temperature results in increased mois- ture susceptibility of WMA for some WMA additives/ processes, and • Increasing RAP tended to increase resistance to mois- ture susceptibility.