In 2007, the California Department of Transportation (Caltrans) made the decision to answer the question: How does warm-mix asphalt (WMA) affect performance of asphalt concrete mixes?
This decision began the first phase of a comprehensive study into the use of WMA for Caltrans by the University of California Pavement Research Center (UCPRC). The objective of the study was to determine whether the use of warm-mix additives to reduce the production and construction temperatures of asphalt concrete influences the performance of the mix.
The test plan was designed to evaluate short-, medium- and long-term performance of the mixes:
Short-term performance is defined as failure by rutting of the asphalt-bound materials;
Medium-term performance is defined as failure caused by moisture and/or construction-related issues; and
Long-term performance is defined as failure from fatigue cracking, reflective cracking or rutting of the asphalt bound and/or unbound pavement layers.
The phased study began in late 2007 and continues to the present day. Each phase includes accelerated loading testing using a heavy-vehicle simulator (HVS) to assess performance under controlled environmental and loading conditions, and a series of laboratory tests on specimens either sampled from the test tracks or prepared in the laboratory. In 2007 and 2008, testing was performed on dense-graded asphalt concrete mix for Phase 1 (assessing early rutting at elevated temperatures) and Phase 2 (assessing moisture sensitivity) of the experiment. The reports detailing all the work for these phases can be downloaded at www.ucprc.ucdavis.edu. In 2010, testing began on rubberized asphalt concrete for the Phase 3 study. This phase of HVS testing is expected to be completed in January 2011.
Heavy into testing
The Phase 1 study consisted of HVS testing of a control mix, produced and constructed at conventional hot-mix asphalt temperatures, and three test sections with warm-mix additives, produced and compacted at 60ºF lower than the control. The additives tested included Advera WMA, Evotherm and Sasobit. This phase also included a comprehensive laboratory testing study on specimens sampled from the test track. The laboratory test program included rutting-performance testing, wet and dry fatigue testing and moisture-sensitivity testing (Hamburg wheel-track and wet-to-dry tensile strength ratio).
The Phase 2 study entailed HVS testing of the four same mixes under soaked conditions.
In September 2007, the HVS test track was constructed in a cooperative effort between Caltrans, UCPRC, GraniteRock and the three warm-mix technology suppliers. The pavement structure consisted of existing base material, covered with 12 in. of imported aggregate base, and two 2.4-in. lifts of asphalt concrete. Target production temperatures for the control mix were set at 310ºF and at 250ºF for the warm mixes. A PG 64-16 binder was used.
Some notable findings include:
The control, Advera and Evotherm mixes met the project mix design requirements. The binder content of the Sasobit was 0.72% below the target binder content and 0.62% below the lowest permissible binder content. This probably influenced performance and was taken into consideration when interpreting HVS and laboratory test results;
No problems were noted with producing the asphalt mixes at the lower temperatures;
Construction procedures and the final pavement quality did not appear to be influenced by the lower construction temperatures;
Interviews with the paving crew after construction revealed that no problems were experienced at the lower temperatures. Improved working conditions were identified as
an advantage;
The rate of temperature loss in the mixes with additives was lower than on the control mix; and
Skid-resistance measurements indicated that the warm-mix additives tested do not negatively influence the skid resistance of an asphalt mix.
After a six-week curing period, HVS testing started in October 2007 and was completed in April 2008. The testing compared early rutting performance at elevated temperatures (pavement temperature of 122ºF at 2 in.), using a 9,000-lb load on a standard dual-wheel configuration and a unidirectional trafficking mode. Load repetitions continued on the pavement section until a minimum rut depth of 0.5 in. was reached.
The duration of the tests on the four sections varied from 170,000 load repetitions to 285,000 load repetitions (Figure 2). HVS trafficking on each of the four sections revealed that the duration of the embedment phases (high early-rutting phase of the typical two-phase rutting processes) on the Advera and Evotherm sections were similar to the control. However, the rut depths at the end of the embedment phases on these sections was slightly higher than the control, which was attributed to less oxidation of the binder during mix production at lower temperatures. Rutting behavior on the warm-mix sections followed similar trends to the control after the embedment phase. The performance of the Sasobit section could not be directly compared with the other three sections, given that the binder content of the mix was significantly lower.
Based on these results, it was concluded that the use of any of the three warm-mix additives tested in this experiment and subsequent compaction of the mix at lower temperatures will not significantly influence the rutting performance of the mix.
Phase 1 laboratory testing included rutting, fatigue-cracking and moisture-sensitivity tests. Tests on mix properties were carried out on beams and cores cut from the test track after construction. The testing protocol consisted of:
Shear Testing—AASHTO T-320: Permanent Shear Strain and Stiffness Testing;
Fatigue Testing—AASHTO T-321: Flexural Controlled-Deformation Fatigue Test (wet and dry); and
Moisture Sensitivity Testing—-AASHTO T-324 for Hamburg Wheel-Track Testing and Caltrans CT-371 for the Tensile Strength Retained test. This test method is similar to the AASHTO T-283 test; however, it has some modifications for California conditions.
Laboratory test results (see summary bar charts in Figure 3 through Figure 5) indicated that the use of the warm-mix technologies assessed in this study did not significantly influence the performance of the asphalt concrete when compared with control specimens produced and compacted at conventional hot-mix temperatures. However, moisture-sensitivity testing indicated that all the mixes (including the HMA control) were potentially susceptible to moisture damage, but that there was no difference in the level of moisture sensitivity between the control mix and the mixes with warm-mix additives.
One year later
Phase 2 HVS testing commenced approximately 12 months after construction on sections adjacent to each of the Phase 1 test sections. Each section was presoaked with water for a period of 14 days prior to testing using a 6-in.-high soaking dam constructed around each test section. A row of 1-in.-diam. holes was drilled to the bottom of the upper lift of asphalt (i.e., 2.5 in.), 10 in. away from the section and 10 in. apart. During testing, a constant flow of preheated water (122°F) was maintained across the section at a rate of 4 gal/hour to induce moisture damage. The same testing program followed in Phase 1 was used in this phase; however, higher loads were used in later stages of the test to assess load sensitivity and to accelerate the distress. Testing was started in June 2008 and ended in May 2009. The duration of the tests on the four sections varied from 352,000 load repetitions (Evotherm) to 620,000 load repetitions (Advera). A range of daily average temperatures was experienced during the four seasons of testing; however, the pavement temperatures remained constant throughout HVS trafficking.
Rutting behavior (average maximum rut) for the four sections is compared in Figure 6. The duration of the embedment phases on the WMA sections were shorter than the control, opposite to the behavior in the first phase. Binder extractions and testing is currently being undertaken to better understand this observation. Embedment phases were noted at each load change on all sections. There was a distinct difference in rutting performance of the Advera and Sasobit sections compared with the control and Evotherm sections, in that the latter two sections rutted at a notably faster rate than the former two sections. The control and Evotherm sections were predominantly shaded by an adjacent structure for much of the day, while the Advera and Sasobit sections had sun for most of the day. Binder testing is being undertaken to determine if different aging played a role in this behavior. Trafficking was terminated on the Advera and Sasobit sections before the failure criterion was met in the interests of completing the study. None of the sections showed any indication of moisture damage on completion of testing (Figure 7).
Top-down cracking was noted on all four sections, with no significant difference in the crack patterns, crack length or crack density between the sections. Cracks did not appear to penetrate below the top lift of asphalt on any of the sections.
A forensic investigation, consisting of core and test pit assessments, provided no indication of any moisture damage on any of the sections. Rutting on all four sections was confined to the top lift of asphalt only. Debonding of the top and bottom lifts of asphalt was observed on the control section only (Figure 8). Determining the reason for this was beyond the scope of this phase of the study, but may be investigated at a later stage. A tack coat was used between the two lifts of asphalt and it was applied in a single run over the control and Advera sections and later over the Evotherm and Sasobit sections.
This phase of testing further reinforced the findings from the first phase of the study. The results also indicate that the use of the three warm-mix additives did not increase the moisture sensitivity of the mixes compared with the control. Binder aging of the warm and hot mixes and its effect on performance over time deserves further investigation.
Phase 2 laboratory testing is comparing the performance of laboratory-prepared specimens to that of the Phase 1 test-track specimens to assess any influences that laboratory preparation of warm-mix asphalt may have on performance evaluation. This testing is still in progress, but to date, the results have provided no indication that warm mixes prepared in the laboratory perform differently from specimens removed from roads.
Over 10% of the total asphalt concrete produced for Caltrans is rubberized asphalt concrete mix. Rubberized asphalt concrete has been useful as a pavement-preservation tool, in addition to reducing the amount of waste tires in landfills. However, successful placement of rubberized asphalt concrete is highly dependent on production, placement and compaction temperatures.
Paving in the coastal areas and nighttime paving have presented workability challenges in the placement of rubberized asphalt concrete. To overcome these challenges, there have been instances where the production temperatures have been pushed higher than the maximum requirement (325ºF), causing burning of the binder and significant smoke during placement. In 2009, Caltrans began using warm-mix additives in rubberized asphalt in order to lower production temperatures, thus reducing smoke and odors. As an added benefit, these warm-mix additives have improved placement and workability of the rubberized asphalt concrete mix at lower production temperatures.
Warming rubber
In 2010, Caltrans had the opportunity to continue the HVS evaluation of warm-mix additives in rubberized asphalt concrete. Since the number of producers of warm-mix additives had grown since the initial study in 2007, the number of participants in the latest study also increased.
The HVS testing and laboratory testing protocol mirror the testing protocol followed in the Phase 1 and Phase 2 studies.
In April 2010, two HVS test tracks were constructed by Teichert Construction using mix produced by Granite Construction (Gencor plant) and George Reed (Astec plant) at the University of California Pavement Research Center’s Advanced Transportation Infrastructure Research Center (ATIRC) at U-C Davis. Each test track includes a hot-mix control section. Ambient temperatures on the day of construction averaged around 50°F. Haul times from the Granite Construction plant was about 60 minutes, and from the George Reed plant about 120 minutes. Smoke and odors were notably less on the warm-mix sections, and the paving crew commented that the warm mixes were much more workable than the control mixes. Chunking was noted on the two control sections, but not on the warm-mix sections. Each track is being considered as a separate project to account for the different mix designs, aggregate, base binder and plant used:
Project 1 includes: Cecabase RT, Gencor Ultrafoam GX and Evotherm DAT; and
Project 2 includes: Advera WMA, Astec Double Barrel Green, Rediset WMX and Sasobit.
The pavement section consists of 16 in. of aggregate base, 2 in. of conventional dense-graded asphalt concrete and 2 in. of rubberized asphalt concrete. A PG 64-16 binder was used as the base binder.
HVS testing began on Project 1 in June 2010 and in July 2010 on Project 2 after an initial cure period. As of December 2010, HVS testing had been completed on all four of the Project 1 sections and four of the five Project 2 sections.
Initial observations indicate that the Project 2 mix has a higher rut resistance than the Project 1 mix despite having higher binder and rubber contents. Overall, the warm-mix sections are performing comparably to the control sections for both projects (Figures 9 and 10).
Specimens for laboratory testing have been cut from the test track and prepared for testing. Shear, fatigue and moisture-sensitivity testing on these samples are currently under way. Testing is expected to be completed by May 2011.
All findings will be summarized and a Phase 3 report will be completed in mid-2011. Phase 1 and Phase 2 reports are available at the University of California Pavement Research Center website, www.ucdavis.ucprc.edu.
The research findings of this comprehensive study are being used to support implementation of warm-mix asphalt in California. The use of warm-mix asphalt in the state has increased significantly in 2010 and is likely to be fully implemented during the 2011 paving season. AT
