Properties of Foamed Asphalt for Warm Mix Asphalt Applications (2015)

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S U M M A R Y Economic, environmental, and performance benefits have motivated the reduction of asphalt mixing and compaction temperatures to obtain decreased energy consumption, emissions, odors, and fumes at the plant and at the construction site; extended haul distances and a longer pavement construction season; and improved workability and compactability of the asphalt mixture. The use of warm mix asphalt (WMA) technology results in reduced production and paving temperatures without sacrificing the quality of the final product. There have been a number of products and processes introduced to produce WMA since 2005, including waxes, surfactants, mineral additives, and mechanical foaming processes. The work in this study focused on central plant–produced foamed WMA because foaming asphalt is currently the largest segment of the WMA market. The objectives of this research study were to (1) determine the properties of foamed binders that relate to asphalt mixture performance and (2) develop laboratory foaming and mixing protocols that may be used to design asphalt mixtures. As part of this effort, foamed binder and mixture test methods and analysis procedures were developed. In addition, a mix design procedure was developed and validated with field and laboratory data. The main accomplishments of this research effort are: • An explanation of the fundamentals of foaming as they pertain to WMA. • Development of a laser-based approach to measuring the expansion and collapse of foamed binder to characterize: – The maximum expansion ratio (ERmax) and expansion ratio (ER) with time. – The rate of collapse (k-value). – The foamability index (FI). • Development of a photographic approach to characterize: – Bubble size distribution. – Bubble surface area index (SAI). • Measurements of foamed binder characteristics using both of these methods in labora- tory and field environments. • Validation of compaction shear stress as a workability measure for foamed asphalt mixtures. • Modification of an existing coating evaluation procedure to obtain a coatability index for foamed asphalt mixtures. • Development of a foamed asphalt mixture design procedure to identify the optimum water content for coating and workability. Properties of Foamed Asphalt for Warm Mix Asphalt Applications 1

2The main findings of this research effort are that: • Binder foaming characteristics may vary according to: – Source. – Date of production. – Polymer modification. • Binder foaming characteristics may be improved with the addition of certain additives. • The three commercially available laboratory foamers produce different foaming characteristics. • Increasing water content produces larger expansion ratios but a smaller k-value, FI, and SAI for most of the binders tested (i.e., faster collapse). • The SAI served as an indicator of the quality and longevity of the foam, with larger values for binders that produced smaller-sized, longer-lasting, and better-distributed bubbles. • Increasing water content does not result in improved workability and coatability. • The lowest maximum shear stress (best workability) was identified and generally occurred between 1% and 2% moisture. • Workability improves with higher mixing temperatures for foamed binder. • The mix design procedure for optimum water content was validated through a laboratory study and field trial. • The coatability of foamed mixtures at the optimum water content was better than the coatability of hot mix asphalt (HMA) mixtures.