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SWUTC Ph.D. Candidate Assistantship Project Description

Fatigue Resistance of Asphalt Mixtures Affected by Water Vapor Movement

University: Texas A&M University

Principal Investigator:
Yunwei Tong
Zachry Department of Civil Engineering
(979) 458-0893

Faculty Supervisor:
Robert Lytton
Zachry Department of Civil Engineering
(979) 845-9964

Funding Source: USDOT Funds

Total Project Cost: $32,079

Project Number: 600451-00019

Date Started: 1/1/13

Estimated Completion Date: 12/31/13

Project Summary

Project Abstract:
Problems in Characterization of Water Transport in Pavement Systems
Hveem (1946) suggested that vapor movement was a significant means of water transport within pavement structures. Since then there is little information available relating to the movement of moisture while in the vapor phase within pavement systems. A literature review suggests that the majority of the research on moisture damage focuses on the hypothesis that surface infiltrated water is the main mechanism of water movement in pavement (Masad, et al., 2007). Hence it is not clear to what extent the permeation of water vapor from the subsurface will affect the performance of asphalt mixtures. However, Carpenter et al (1974) constructed a map of the expected suction levels in the subgrade below a pavement by relating the equilibrium suction to the Thornthwaite moisture index. The obtained subgrade PF values for pavement subgrade reveals that the soil vapor pressure is generally near saturation (100% relative humidity) (Carpenter, et al., 1974). Consequently, the subgrade soil beneath the pavement can be treated as water vapor saturated material. Due to the existence of the Relative Humidity (RH) gradient between the subgrade soil and air above the pavement surface, vapor movement within the pavement layers can be considered as a diffusion process. Therefore, it is necessary to develop a method of predicting and quantifying the amount of water that can enter into a pavement system by vapor diffusion.

Problems in Current Moisture Susceptibility Characterization
Extensive literature studies concerning moisture damage test methods and modeling conducted by Caro et al. (2008) suggests that the majority of current practices for evaluating moisture damage in asphalt mixture focus on comparative measures (wet-versus-dry comparison) such as the simple quantitative Tensile Strength Ratio (TSR) test, Indirect Tensile Strength Ratio (ITS), and three-points beam fatigue test, which yields no fundamental material properties. Although the torsional Dynamic Mechanical Analyzer (DMA) developed at TAMU demonstrates great potential to characterize the moisture susceptibility of Fine Aggregate Mix (FAM), the torsional loading increases the stress state complexity within the specimens by unevenly distributing stress over the cross section area of the cylindrical specimen, which inevitably leads to the cracking opening from the surface of the specimens. Hence, there is an urgent need for a more efficient and reliable performance test method capable of reducing the stress state complexity within the specimens and capturing the fundamental material properties of the asphalt mixture at the meantime.

Problems in Characterization of Moisture Susceptibility of WMA
In recent years, more and more research efforts have been put in investigating the moisture susceptibility of WMA. It is found, however, that the literature review yields inconsistent results with each other. You et al. (2011) evaluated the fatigue crack resistance of foaming WMA with three different levels of water 1.0%, 1.5% and 2.0% added to the asphalt based on asphalt binder weight using the beam fatigue method. It is found, however, that the foaming WMA demonstrated better fatigue resistance than that of HMA, the reason for which is not given. As the production temperature increases from 115 °C to 130 °C, the fatigue life increases considerably. Even through the higher production temperature tends to increase the fatigue potential of mixture, it is believed that improved fatigue resistance is due to the increased binder absorption. Another interesting observation was that as the water content used in the foaming process increases, the fatigue life of the WMA increased, the reason for which is not identified. However, Caro et al. (2012) analyzed the moisture susceptibility of WMA based on the torsional DMA. The results suggest that a water-based (foaming process) WMA technology is the most moisture susceptible fine aggregate mixture (FAM) and the Sasobit (Wax) WMA FAM is more prone to moisture damage than Hot Mix Asphalt (HMA). Kim et al. (2012) characterized the moisture damage of WMA using three methods: Asphalt Pavement Analyzer (APA), a standard test AASHTO T283 and a semi-circular bending (SCB) fracture test. The testing results from T283 and SCB show that the WMA exhibits greater susceptibility for moisture damage. Furthermore, the current fatigue evaluation methods for WMA using T283, SCB, torsional DMA and beam fatigue tests only provide comparative measures to evaluate the filed pavement life. The huge discrepancy between these tests results and the real fatigue life of pavement in the field make it infeasible to predict and match the pavement life in the field. Therefore, it is necessary to develop a mechanistic based analysis method to evaluate the moisture induced fatigue life of pavement and use it to accurately predict the pavement fatigue life.

Project Objectives:
The goal of this research is to address the problems mentioned above pertaining to the modeling of moisture transport in pavement systems and quantifying its destructive effects on asphalt mixtures. The research will focus on achieving the following objectives:

  • Develop a water vapor diffusion model in pavement systems to characterize the vapor transport in pavement;
  • Design an lab experiment to moisture condition FAM based on the proposed water vapor diffusion model;
  •  Develop a controlled-stress Repeated Direct Tension (RDT) test method and perform these tests on the FAM specimens using the DMA;
  • Apply a mechanic based approach to analyze the fatigue crack growth for both dry and moisture conditioned specimens;
  • Apply this new test and data analysis method to the WMA to evaluate its moisture susceptibility.

Task Descriptions:

TASK 1. Conduct Information Search

TASK 2. Establish Moisture Diffusion Model in Pavement

TASK 3. Develop Experimental Design

TASK 4. Testing Protocols Design

TASK 5. Document Findings and Recommendations