Superpave Asphalt Research Projects

Comparison of Hot Poured Crack Sealant to Emulsified Asphalt Crack Sealant

Sealing and filling cracks has always been an important consideration in pavement maintenance. Hot rubber asphalt has been the most commonly used material for this purpose providing good performance in most cases. However, safety has been an issue with the use of hot rubber asphalt crack sealants. In addition, vehicle tires can easily pick up material if sufficient adherence is not developed between the sealant and the crack sides. Some districts in Texas Department of Transportation (TxDOT) have been using cold pour asphalt emulsion crack sealants to address the safety problem. This material also has the tendency to penetrate into the crack more effectively and easily than the hot pour rubber asphalt. However, cold pour crack sealant requires longer setting and curing time especially in areas of high humidity. In addition, the performance history of these sealants is not known or well-documented so well as the performance of hot pour crack sealants. Furthermore, the cost associated with the use of this material versus hot pour rubber asphalt is not well documented or determined. TxDOT is initiating a research project to address the preceding important issues. The study is intended to compare the cost effectiveness, the ease and safety of installation, the performance, and the life cycle cost for hot rubber asphalt crack sealant, cold pour asphalt emulsion crack sealant, and cold pour asphalt emulsion joint sealant.

Correlation of Field Performance to Hamburg Wheel-Tracking Results

The Hamburg Wheel Device has been gradually gaining acceptance by some state highway agencies within the last five years. The test results from this laboratory equipment have been promising in regard to evaluating the moisture susceptibility of hot mix asphalt (HMA) mixtures. While there is some limited information on the relationship between the laboratory results from this test and the field performance, the information is quite limited. Now that the laboratory test procedure for this equipment is almost well established, it is time to investigate how the laboratory results correlate with the field performance of the HMA mixture. This will be an important step in validating the test and ensuring that the test results could be reliably used to predict performance. For this reason, TxDOT, being among the pioneers in using the Hamburg Wheel Tracking Device, plans to pursue a five year research project for evaluation of the laboratory-field correlation for the equipment.

Develop Guidelines for Designing and Constructing Thin Asphalt Concrete Pavement (ACP) Overlays on Continuous Reinforcement Concrete Pavement (CRCP)

Thin-bonded asphalt overlay on existing Portland Cement Concrete (PCC) pavement is an essential tool for restoring ride quality on a cost-effective basis. This procedure is used very frequently statewide, with mixed results. Some Districts have had good experience with these overlays, while others have reported premature failure of their overlays. Debonding is often cited as the primary cause of early failure, but slippage cracks, stripping, and softening of the asphalt are also occasional problems.

The purpose of this study is to evaluate the causes for these premature failures and ultimately to reduce or eliminate them. Preliminary review of the literature and field experience clearly shows that the causes of early failures can be divided into two categories: (1) uncorrected problems with the existing PCC pavement, and (2) inadequate design or construction of the ACP overlay. PCC problems may include inadequate structural capacity, unrepaired failures and cracks, poor surface preparation, or excessive moisture content, for example. On the asphalt side, proper design of every component of the asphalt must be undertaken with good bonding and water seal in mind. Thus, it is important that the study address both sides of the problem and that a multidisciplinary research team be assembled that has experience with both rigid and flexible pavements.

Case studies and statewide analyses using the Texas Rigid Pavement Database have shown conclusively that timely application of even a thin ACP overlay can extend the useable life of rigid pavements by restoring the ride quality and thereby reducing dynamic loading of the pavement structure. Sealing out water infiltration also has a significant effect in the eastern areas of the state. However, for these overlays to work consistently, a systematic approach must be developed, one that matches the existing pavement to the design of the ACP overlay for optimal effectiveness. The goal of this study is to produce a decision tree taking all these factors into account, so that thin ACP overlays may be used in a variety of situations statewide with minimal early failures. Once this has been proven in pilot projects, it may be incorporated into the specifications to effect significant cost savings and better service to the motoring public in Texas.

Toner Modified Asphalt

This implementation project will transfer the results from project 7-3933, in which the feasibility and potential benefits of utilizing waste toner in hot-mix asphalt concrete were investigated. The research project 7-3933 included procuring a number of waste and spent toner, blending them with asphalt cement at different ratios, and evaluating the binder and mixture properties resulting from the toner addition. At the end of this research study, TxDOT received a patent on blending toner with asphalt to improve hot- mix asphalt concrete performance. To execute this patent TxDOT needs to fully comprehend the performance of different types of toner. In this implementation project, four test sections will be constructed to gather and analyze data to evaluate the benefits of this patent. CTR will provide guidance to district and contractor personnel about toner modified asphalt. For each test section, the binder designs will be developed and consultation on mix designs will be provided. One year after construction, CTR will conduct post-construction pavement evaluation. A technical report will be prepared that summarizes pre-existing pavement conditions, mix designs, binder designs, lab testing results, pavement performance and recommendations.

Effect of Crushed Aggregate Percent on Performance of HMA Mixtures

This project focused on determining the effect of crushed aggregate percent on HMA mixtures. For this study, specimens prepared at different crushed aggregate percentages will be tested by using Asphalt Pavement Analyzer, Hamburg Wheel Tracking Device and MMLS. Other than percentage of crushed gravels, the effect that the nominal maximum aggregate size on mixture performance will be tested. The south Texas districts, in which their primary source of aggregates are gravels, need to know if it is better to use aggregates of larger maximum size or traditional maximum sizes with more crushed faces.

Long Term Research on Bituminous Coarse Aggregate

The Soils and Aggregate Branch of the Construction Division of TxDOT has the primary responsibility for overseeing aggregate quality issues. An overall process review of selected testing and m,onitoring protocols using within the Branch is needed to insure that the needs of TxDOT and the State of Texas are continuously being met. Two key areas of are addressed in this research study: (1) a review and upgrade of the Aggregate Quality Monitoring Program (AQML) and (2) a review and upgrade of the proposed Surface Aggregate Classification System. The assessment of the AQML includes evaluation of other tests that might be required as replacements and/or supplement for the tests currently used to monitor quality. The Surface Aggregate Classification System evaluation will involve defining the best tests and protocols to insureexcellent wet weather skid properties and to insure excellent overall hot mix quality for a safe and durable hot mix surface. The project also includes a long-term field monitoring program to insure that selected test protocols correlate with field experience.

Precision of Test Method Tex-531-C

Test method Tex-531-C is the primary method of determining moisture damage susceptibility of hot mix in Texas. This test has been in use in Texas and several other states for the last 20 years. There are no known precision statements for this test method. Such a precision is required for reliable judgement of moisture susceptibility of the mixture. A study is under progress to determine the precision of test properties determined in Tex-531-C. Such a test precision will be determined through a round robin study involving multiple laboratories and multiple aggregate sources.

Relationship between Aggregate Properties and Hamburg Wheel Tracking Results

Hamburg Wheel tracking device was developed in Germany to predict rutting potential of hot mix. About 10 agency/suppliers in US have used this device. Recent work in Bituminous laboratory of TxDOT construction division has indicated that the device can be used to predict moisture damage susceptibility of hot mix. In addition visual observations of wheel tracked specimens at Bituminous laboratory of TxDOT construction division has indicated that mixtures containing soft limestone undergo severe abrasion and aggregate degradation when tested in Hamburg device.

Research is going on to find a method for quantifying or categorizing the extent of aggregate degradation caused by Hamburg Wheel tracking device. As part of the program, it will be determined if there are any relationships between aggregate properties such as Soundness loss, Micro-Deval loss , LA Abrasion and polish number. In addition British pendulum test will be performed on wheel tracked specimens to determine if wheel tracking could simulate the aggregate polish wear.

The University of Texas at Austin  •  UT's Cockrell School of Engineering