Warm mix asphalt (WMA) has been on the horizon of new asphalt technologies and now it is at the forefront of many research and field projects. The process of investigating the implementation of WMA is a task that many state and local agencies are now facing. The typical WMA production temperature ranges from 30 to 100yF lower than typical hot-mix asphalt (HMA). This temperature reduction leads to several benefits for asphalt paving. One of the driving forces of WMA research is the potential for a reduction in energy, fuel consumption and emissions. In accord with emission reduction is the reduced fuel consumption which is an attractive economic benefit. Other benefits include longer haul distances, colder weather paving, reduction of asphalt fumes during paving operations, higher recycled asphalt pavement (RAP) content and a less extreme working environment.

The three main types of WMA are organic wax additives, chemical additives, and plant foaming processes. Presented in this study are performance testing results from field produced WMA (and a control HMA) for each of the three main types of WMA technologies. WMA is showing promising results in laboratory testing throughout the United States and Canada; however, one particular distress that has been documented in laboratory testing is moisture damage. It is hypothesized that the lower aggregate temperatures do not allow for complete drying of the aggregate and can lead to stripping.

There are three main objectives to be addressed though this research. The first is to evaluate field produced WMA mixes with a field produced control HMA mix. The second is to identify potential quality control/quality assurance (QC/QA) concerns and determine if reheating a WMA mixture to prepare a sample will impact the performance testing results. The third objective is to address the WMA moisture susceptibility concerns.

The Iowa Department of Transportation produced four field WMA mixes and four control HMA mixes which were used in this research project. Each mix was produced for a different project at different plant locations. The corresponding control mixes to each WMA mix differed only by the WMA additive. For each project, loose HMA and WMA mix was collected at the time of production and binder from the tank was collected for each mix. Field compacted samples were prepared at the job site and laboratory samples were reheated and compacted at a later date. Indirect tensile strength (ITS) and dynamic modulus samples were procured from each mix produced. Half of the ITS and dynamic modulus samples were moisture conditioned according to AASHTO T283. In total, 284 samples were procured from the field produced mixtures for dynamic modulus, flow number and indirect tensile strength performance testing.

The ITS testing results will include peak loads and tensile strength ratios. Each of these values will be considered when performing the data analysis. The dynamic modulus testing results will help to determine the material stress to strain relationship under continuous sinusoidal loading. The loadings are applied at various frequencies and temperatures to define the material property characteristics over a wide range of conditions. Dynamic modulus testing measures the stiffness of the asphalt under dynamic loading at various temperatures and frequencies, thus it is used to determine which mixes may be more susceptible to performance issues including rutting, fatigue cracking and thermal cracking.

The overall findings of these experiments suggest a difference in the performance of HMA and WMA mixes. The binder results show that the mixing and compaction temperatures are reduced and that the benefits of WMA mentioned in the literature review are realized. While the benefits of the technologies continue to drive the production of more WMA mixes, studying the performance testing results will help to show if there is a net benefit to using WMA. Three of the four field mixes indicate superior performance of the HMA mix to that of the produced WMA in many aspects of the tests performed. There were mixed results for the foaming technology because the WMA mix did perform superior in dynamic modulus and flow number tests but there was a nine day elapse between the production of the foamed WMA mix and the HMA mix due to weather delays. This may have caused a higher degree of variability between the two mixes. The dynamic modulus results show that the interaction of the mix, compaction type and moisture conditioning are statistically significant in all four field mixes. This suggests that the combination of all three factors play a role in determining material response. The master curves do not display a high degree of overall variability but do show differences in mix responses at high temperatures.

Further investigation of WMA technologies will be beneficial to both contractors and owner agencies. The experiments showed statistical differences between the control and WMA for all four field mixes tested. Three field mixes indicate higher laboratory performance results in the HMA mix. The foamed WMA mix showed improved laboratory performance when compared to the control HMA. As WMA is produced in larger quantities and as WMA technologies begin to be used together it is important to continue looking at the pavement performance data and performance testing results in order adapt the QC/QA programs to evolving technologies.