Pamela Turner

Mixing and Compaction Temperatures of Asphalt Binders in Hot-Mix Asphalt

This report identifies improved test methods for determining laboratory mixing and compaction temperatures of modified and unmodified asphalt binders. The report will be of immediate interest to materials engineers in state highway agencies and the hot-mix asphalt (HMA) construction industry.

Testing of Moderate and High Reclaimed Asphalt Pavement Content Mixes: Laboratory and Accelerated Field Performance Testing at the National Center for Asphalt Technology Test Track

This paper describes the performance of test sections containing moderate and high levels of reclaimed asphalt pavement (RAP) at the National Center for Asphalt Technology test track. The test sections included two with 20% RAP, four with 45% RAP, and a control with no RAP. Each mixture contained the same component aggregates and RAP. One of the 20% RAP mixes contained PG 67-22 binder, and the other contained PG 76-22 binder. Different binders in the 45% RAP mixes included PG 52-28, PG 67-22, PG 76-22, and PG 76-22 plus 1.5% Sasobit. All sections performed well for rutting and raveling. Low-severity longitudinal wheelpath cracking was documented in two sections. The 45% RAP section with PG 76-22 plus Sasobit had moderate cracking, which appears to reflect cracking from the underlying pavement. The 20% RAP section with PG 76-22 had less cracking. Other sections had no cracking. Laboratory tests included asphalt pavement analyzer (APA) rutting tests, dynamic modulus, bending beam fatigue, and energy ratio. The APA results corresponded to the effective stiffness of the binder in the mixes. Master curves of dynamic moduli showed the effects of the virgin binder grade on the stiffness of the mixtures. Beam fatigue tests indicated that the 45% RAP mixes have lower fatigue lives compared with those of the other mixes, but this is because of a lower effective volume of asphalt in these mixes.

Effect of rubber characteristics on asphalt binder properties

The use of scrap tyres in ground tyre rubber (GTR)-modified binders has continued to evolve since its introduction in the early 1960s. Currently, many states have developed recipe specifications requiring contractors to blend GTR with asphalt binders using specified rubber sizes, percentages, and grinding methods. While these specifications were developed based on early research, the GTR industry has developed new methods and techniques which might improve the quality of GTR in asphalt binders. The objective of this research was to assess how rubber properties affect the properties of an asphalt binder. This objective was completed by blending 12 unique GTR samples with a singular asphalt binder at a loading of 10% rubber. Two of the selected rubbers were additionally tested at 15% loading. These 14 GTR-modified asphalt binders were then tested using the performance grade (PG), multiple stress creep recovery, cigar tube separation test, and softening point methodologies. Statistical analyses were conducted to determine how particle size, grinding temperature, rubber chemistry, and surface area affected the four test results. GTR particle size was the most influential parameter on the majority of the test results. The smaller particle sizes improved the high- and low-temperature PG and particle separation as tested by cigar separation tubes and the softening point.

Effects of Changing Virgin Binder Grade and Content on High Reclaimed Asphalt Pavement Mixture Properties

Most highway agencies have decades of experience with hot-mix asphalt whose percentage of reclaimed asphalt pavement (RAP) has remained low to moderate because of the general perception that RAP mixtures may be more susceptible to various modes of cracking. As the RAP proportion increases, so does the potential for an increase in mixture stiffness and a decrease in resistance to cracking. Two proposed ways to increase the durability of RAP mixtures are to (a) increase the amount of virgin binder in the asphalt mixture and (b) decrease the performance grade of the virgin binder. To assess these options, 0%, 25%, and 50% RAP mixtures at optimum asphalt content were designed with a standard PG 67-22 virgin asphalt binder. These mixtures were tested to evaluate surface cracking, reflection cracking, and rutting with the use of an energy ratio, overlay tester, and asphalt pavement analyzer, respectively. These tests also were conducted on RAP mixtures with 0.25% and 0.50% higher asphalt contents and at the optimum asphalt content with the use of a softer virgin binder. In addition, the linear amplitude sweep methodology was used to assess the fatigue properties of the blended binders. The results showed that, to improve resistance to cracking, the amount of virgin asphalt should be increased by 0.1% for every 10% of RAP binder in the mixture for up to 30% RAP binder. Once the RAP binder exceeds 30%, a softer grade of asphalt should be used to increase the mixtures resistance to cracking. All mixtures should be assessed for rutting susceptibility.

Effect of rejuvenator on performance characteristics of high RAP mixture

As more reclaimed asphalt pavement (RAP) is utilised in asphalt mixtures, there are increasing concerns about the potential negative effect of the aged RAP binder on the field performance, especially cracking resistance, of the high RAP mixtures. To address the concerns, there have been increasing interests in utilising rejuvenators to improve the cracking performance of high RAP mixtures. The objective of this study was to determine the benefits of using a new rejuvenator made from renewable sources in asphalt mixtures with high RAP contents. The study was conducted by determining and comparing the laboratory performance properties of three mixtures and the binders extracted from the mixes. The three mixtures evaluated in this study included two 50% RAP mixtures (RAP binder ratio = 0.55) with and without the rejuvenator and a comparable virgin mix. Results of this study suggested that the new rejuvenator was effective in improving both the intermediate and low-temperature cracking performance characteristics of the 50% RAP mix close to those of the virgin mix at the short-term laboratory ageing condition without affecting its rutting and stripping resistance. It is recommended that a field study of this rejuvenator be conducted to further evaluate its effect on the long-term field performance of high RAP mixes.