Civil Engineering ETDs

Publication Date

Fall 9-2-2021

Abstract

Warm mix asphalt (WMA) technology reduces the high mixing and compaction temperatures of conventional hot mix asphalt (HMA) by approximately 10 °C to 40 °C. In WMA technology, instead of increasing the binder-aggregate temperature, the volume of asphalt is increased by injecting water into hot asphalt at high air pressure. The amount of water injected in the binder is typically determined by a trial and error process and is termed as foaming water content (FWC). Once the FWC is in contact with the hot asphalt binder, it turns into vapor with a rapid volume expansion due to numerous trapped micro-water bubbles in the binder. As such, foamed asphalt binder has more volume and less viscosity, which help coat aggregate at reduced temperature.

The mechanism that allows mixing and compaction of WMA at reduced temperatures are not well understood yet. It is possible that the frictional behavior or relative motion of binder as a lubricant film between two aggregate particles play a role in the ease of mixing and compaction of WMA. Thus, there is a need for studying the lubricity of a binder-aggregate system to understand the mixing and compactibility, combinedly known as workability of a foamed WMA. This study, for the first time, evaluated the frictional behavior of binder-aggregate system.

Water bubbles, formed during the foaming process, are expected to be expelled out of the foamed WMA during mixing and compaction. However, foaming water may not be completely expelled out, rather some of the microbubbles may be entrapped in the foamed WMA even after compaction. The undissipated microbubbles can diffuse over time and cause damages to the foamed WMA. There is a need for studying whether FWC causes moisture damage in WMA.

The experimental work and analysis of this study are divided into main three tasks: (i) understanding of effects of foaming on binder properties, (ii) understanding of mixing and compaction using tribology concepts, and (iii) evaluation of moisture damage characteristic of an WMA mixture. Foamed asphalt binder and mixtures were produced with varying FWCs. An un-foamed binder (0% FWC) and corresponding HMA mixtures were studied as control binders and mixtures.

This study showed that the foam expansion and stability, rheology, lubrication behavior, and mixture workability depend on foaming water content. During the foaming process, an injection of higher FWC resulted in a higher volume expansion, however lower stability, which suggests that an optimum FWC exists for a specific temperature and pressure. This study found about 2%-3% FWC is an optimum FWC at 140 °C-150 °C and 175 psi air pressure. An addition of higher FWC reduces viscosity, modulus, and more importantly the friction at binder-aggregate system. The friction values were used to evaluate the coating and compactibility of mixture. The reduced friction certainly helps in formation of a better coating during mixing and improves the ability to slide during compaction, thereby, an improved workability at reduced temperature.

Damages in foamed WMA were evaluated in foamed binder, and mixture, and field cores. The study found that foaming of asphalt results in an increase in non-recoverable creep compliance and a reduced recovery in foamed binder, which indicates that foamed WMA might be more prone to moisture damage. Mixture damage analysis showed that foamed WMA exhibits slightly higher rutting and moisture damage than the control HMA. Slightly higher rut susceptibility was also found in field core (five-years old). However, no significant differences were observed in terms of moisture damage in field cores. Overall, damage evaluation shows that foaming softens the binder, which results in slightly higher rutting. In long-term, moisture damage is not found prominent in field cores, however, the rut susceptibility remains slightly higher compared to regular HMA.

Document Type

Dissertation

Language

English

Degree Name

Civil Engineering

Level of Degree

Doctoral

Department Name

Civil Engineering

First Committee Member (Chair)

Rafiqul A. Tarefder

Second Committee Member

Tang-Tat Ng

Third Committee Member

Fernando Moreu

Fourth Committee Member

Yu-Lin Shen

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