Richard Willis

Effectiveness of Heavier Tack Coat on Field Performance of Open-Graded Friction Course

Compared with a conventional hot-mix asphalt mixture, an open-graded friction course (OGFC) is more prone to pavement distresses, such as cracking and raveling, which result in a shorter service life. One way to potentially improve the performance of OGFC is to enhance the interface bond between the OGFC and the underlying layer by applying a heavier tack coat. This study evaluated the effectiveness of using a heavier tack coat on the field performance of OGFC by comparing the field performance of the same OGFC mixture with different tack coats placed on Sections N1 and N2 of the National Center for Asphalt Technology pavement test track in Opelika, Alabama. In Section N1, a heavier polymer-modified tack coat was applied with a spray paver immediately before the OGFC layer was placed. In Section N2, a distributor truck applied a trackless tack at a regular application rate. The sections were trafficked to 10 million equivalent single-axle loads by a fleet of heavy trucks for 2 years. The field performance of these two sections was monitored weekly. Field performance characteristics measured included pavement stiffness, pavement structural response, surface functional characteristics, and pavement distresses. The results showed that the OGFC layer in Section N1, in which a heavier tack coat was applied, performed better than that in Section N2, in which a conventional tack coat was used. It is recommended that a heavier tack coat be used to improve the performance of OGFC surfaces.

Evaluation of moderate and high RAP mixtures at laboratory and pavement scales

Abstract The use of reclaimed asphalt pavement (RAP) to replace the virgin materials in asphalt mixtures can result in significant cost savings and provide environmental benefits. As part of the national efforts to support the increasing use of RAP without adversely affecting the long-term performance of asphalt mixtures, a field evaluation of six asphalt mixtures containing moderate and high RAP contents has been conducted on six test sections at the National Center for Asphalt Technology Pavement Test Track since the third research cycle started in 2006. The objective of this study was to present the latest field and laboratory evaluation results of these six RAP mixtures. The field performance evaluated included rutting, roughness, surface texture and cracking. Sections W4 and W3 were constructed with 20% RAP mixes using PG 67-22 and PG 76-22 base binders, respectively. The four other sections, W5, E5, E6 and E7, were paved with 45% RAP mixtures using four different base binder options, consisting of PG 52-28, PG 67-22, PG 76-22 and PG 76-22 + 1.5% Sasobit. The results of this study show that moderate (20%) and high (45%) RAP contents can be used to produce asphalt mixtures that provide good performance, and a softer base binder can be used to improve the performance of a high RAP mixture. In addition, the creep strain rate determined in the indirect tensile test at 10 °C closely matched the field performance ranking – the mix with a higher creep strain rate exhibited more cracking in the field. The difference between the laboratory and field rank order discussed in this study may be affected by the different laboratory and field specimens and testing conditions, illustrating challenges when one uses smaller scale specimens and/or tests to simulate the performance and results of much larger scale pavement sections.

Comparison of Laboratory Cracking Test Results with Field Performance of Moderate and High RAP Content Surface Mixtures on the NCAT Test Track

In 2006, a group of experimental test sections was built on the National Center for Asphalt Technology Test Track to evaluate surface-layer mixtures containing 20 and 45 % Reclaimed Asphalt Pavement (RAP) with variations in the Superpave performance grade of the virgin binder. This paper discusses several laboratory tests that have been proposed as indicators of cracking susceptibility and how the results of these tests conducted on the mixtures used in the test sections compare with the observed cracking performance on the Test Track after five years. The cracking tests performed on the mixtures were the bending beam fatigue test, the Energy Ratio method developed at the University of Florida, the simplified viscoelastic continuum damage method developed at the North Carolina State University, and the Overlay Tester developed at the Texas Transportation Institute. On the track, the test sections performed very well, but exhibited a range of low-severity cracking, mostly near the edges of the wheelpaths. Cores were extracted to confirm that the cracks were limited to the surface layer. The field cracking performance indicates that the performance grade of virgin binder affects cracking resistance. The creep strain rate, measured as part of the Energy Ratio method, and the Overlay Tester results best matched the field performance of the test sections.

Effect of Geosynthetic Material in Reclaimed Asphalt Pavement on Performance Properties of Asphalt Mixtures

Geosynthetic materials have been used effectively at the interface between asphalt pavement layers for waterproofing and mitigating crack propagation. When an asphalt pavement with a geosynthetic interlayer is rehabilitated, the asphalt surface layer as well as the underlying geosynthetic interlayer can be removed for new overlays. The reclaimed asphalt pavement (RAP) as well as the geosynthetic material is reused in new asphalt mixtures because the geosynthetic material cannot be removed from the RAP. This situation leads to the question of whether the presence of the geosynthetic material in RAP has a negative impact on the performance of the new asphalt mixtures. A study was conducted to answer this question. A two-layer test section with a geosynthetic layer installed at the layer interface was constructed. The sections were milled so that RAP with and without the geosynthetic material was collected. Superpave® mix designs were then produced: one with the RAP with geosynthetic material and the other with the control RAP. A variety of hot-mix asphalt (HMA) performance tests were conducted to determine whether the presence of the geosynthetic material in the RAP had an impact on the HMA performance concerning stiffness, moisture susceptibility, rutting resistance, and resistance to low-temperature cracking. The laboratory evaluation concluded that the presence of the geosynthetic material in the RAP did not have a significant impact on the RAP properties or the volumetric and performance properties of the asphalt mixture.