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Reflection cracking is a major concern when placing an overlay on a cracked pavement. The opening and closing of joints and/or cracks induced by daily temperature cycles is a major contributor to reflection cracking. This mechanism is currently being simulated in the laboratory at the Texas Transportation Institute (TTI) using a specially modified overlay-tester device. To evaluate the overlay tester concept laboratory results are presented on cores from four Texas projects, three of which performed very poorly and one which performed excellently. The asphalt mixture on US 175 in Dallas was placed on a cracked stabilized base and did not have a single reflection crack after 10 years in service, whereas the mixtures on two projects were badly cracked after only few months. The results clearly show that the upgraded TTI overlay tester can effectively differentiate between the reflection cracking resistance of different asphalt mixtures. It is also found that the reflection cracking resistance of asphalt mixture has a good correlation with the asphalt binder properties. In this report the upgraded TTI overlay tester is also used to quantify the benefits of modified asphalt binders. This benefit is demonstrated with a single mix where specimens were prepared with a variety of asphalt binders. The mix prepared with PG 64-22 plus 3 percent SBR latex demonstrated superior reflection cracking resistance while still maintaining adequate rutting resistance. It is proposed that the overlay tester is a practical device which can be incorporated into mixture design systems, to complement the current systems, which often focus solely on minimizing rutting potential. In many instances it is necessary to optimize both crack resistance and rutting potential to obtain adequate long-term pavement performance.
The focus of many asphalt mixture design procedures over the past 10 years has led to the development of stiffer, drier mixtures. However, these mixes are more difficult to construct and are potentially more prone to reflective cracking. In this research the upgraded overlay tester is introduced and proposed as a simple performance test on reflective cracking. The overlay tester can be run on standard size samples, typically 6 in (150 mm) long by 3 in (75 mm) wide by 1.5 in (38 mm) high. These specimens can be prepared from either field cores or from Superpave Gyratory Compactor (SGC) molded specimens. The test is rapid and repeatable, and poor samples fail in minutes. It characterizes both crack initiation and crack propagation properties of asphalt mixtures. Based on repeatability study results, three replicates are recommended for the overlay tester. Sensitivity studies indicate that the overlay tester provides reasonable test results. Increasing asphalt content will significantly improve the reflective cracking resistance of asphalt mixtures. In a series of tests on Texas mixtures, it was determined that aggregate absorption has a major impact on the performance of specimens in the overlay tester. This topic has not received much attention recently but it obviously needs to be investigated. In the lab these highly absorptive aggregates did not severely impact the rutting performance but they had a major impact on cracking life. The effectiveness of the overlay tester was validated by five case studies in Texas. The overlay tester results all correlated well with the field performance. Furthermore, the overlay tester results have good correlations with beam fatigue test results and low temperature performance of asphalt mixtures in the field. A preliminary framework of asphalt overlay mixture designs and associated criteria have been proposed. Based on the framework, two examples of asphalt overlay mixture designs are presented in this report. This framework and the associated criteria are preliminary and they will need further refinement. Finally, a brand new overlay tester has been manufactured and delivered to TxDOTs central lab at the edar Park office. In addition, training for the operation and analysis has been provided.
Thin hot mix asphalt (HMA) overlays, laid at 1.0 inch or thinner, are cost-effective surface maintenance options. The primary focus of this research was to develop specifications for three such mixes: fine dense-graded mix (fine DGM), fine-graded stone matrix asphalt (fine SMA), and fine-graded permeable friction course (fine PFC). A number of slurry overlay systems were also evaluated, but to a lesser extent. Draft specifications for the three mix types were first developed based on the results of a literature/information search and a field investigation of 11 existing projects. The specifications included minimum material quality levels, laboratory performance criteria, and construction recommendations. To evaluate the design recommendations, extensive laboratory testing was performed on each of the three thin overlay mixes with five different aggregates. Of the 15 mixes attempted, 12 had acceptable designs in terms of the specified performance tests. For the most part, the draft specifications appeared to function well with minor alterations recommended. Testing also included two supplementary studies on the effects of screening type in fine SMA and the effects of recycled materials on both the fine SMA and fine PFC. Compaction of the fine SMAs was highly influenced by packing characteristics of the coarse and fine aggregates. Screening quality did not affect fine SMA rutting resistance, but did affect cracking resistance. Using recycled aggregates reduced rutting problems but increased cracking susceptibility; however, most mixes performed well suggesting that quality, well-engineered mixes can still have good performance when recycled materials are used in limited amounts. Concerning laboratory testing of slurry overlays, the applicability of the overlay tester/procedures and the three-wheel polishing device in testing should be further studied. In particular, a tie-in with actual field performance should be identified, perhaps with the Accelerated Pavement Test program. Six thin overlay projects, comprising 10 unique mix designs, were constructed and evaluated. Most projects were constructed without problems, though some encountered issues with over- and under-compaction. Initial performance has been very good, although, since all the sections are less than two years old, the long-term performance is still undetermined. The researchers recommend adapting the specifications accompanying this report, which require using high quality materials and passing strict laboratory performance tests on both the lab design and trial batch materials. They do not recommend incorporating recycled materials in these mixes, though preliminary results are promising. Guidelines for pavement evaluation and mix selection were also prepared, which recommend the use of certain thin overlay or slurry overlay options given the pavement, traffic, and climate conditions.
The main objective of this project was to develop and verify the overlay tester (OT) based fatigue cracking prediction approach in which the OT is used to determine fracture properties (A and n) of hot-mix asphalt (HMA) mixtures. This approach was developed based on fracture mechanics. However, not only was the fatigue crack propagation characterized by Paris' law fracture concepts, but the crack initiation described by traditional fatigue model was also included in this approach. In this approach, the fundamental HMA fracture properties (A and n) were used to estimate fatigue life of asphalt pavements including crack initiation and crack propagation.
This report provides the user manual for TxDOT's new overlay tester and a draft specification for mix acceptance.