Validating Rapid Rutting Tests

As more state agencies move toward Balanced Mix Design (BMD), there is an increasing need for rapid tests that can be employed during QA to evaluate mixture performance. Two rapid rutting tests – the high-temperature indirect tension test (HT-IDT, ALDOT Method 458) and the IDEAL-RT test (ASTM D8360) – have gained significant interest with practitioners in recent years for their faster turnaround times compared to traditional wheel-tracking tests (Figure 1).

   

Figure 1: HT-IDT Test (left) and IDEAL-RT Test (right)

A study was conducted at NCAT in conjunction with the 2021 Test Track research cycle to compare laboratory test results of the HT-IDT and IDEAL-RT tests to those of the Hamburg Wheel-Tracking Test (HWTT, AASHTO T324) and the Asphalt Pavement Analyzer test (APA, AASHTO T340). The preliminary results of that study were published in the Spring 2023 NCAT newsletter. Since the publication of that article, the trafficking of the 2021 NCAT Test Track and the construction of the 2024 NCAT Test Track have been completed. We can revisit the previously published lab results and compare them to how these mixtures performed in the field in terms of rutting now that trafficking is complete. This comparison also provides insight into the lab test results we are collecting in conjunction with the 2024 NCAT Test Track.

To review the results of the laboratory evaluation, 14 unique plant-produced mixtures were sampled and tested during the construction of the 2021 NCAT Test Track. The HT-IDT, IDEAL-RT, and HWTT tests were conducted on plant-produced mixtures without reheating (Production PMLC). The HT-IDT, IDEAL-RT, HWTT, and APA tests were also conducted on the same mixtures after re-heating (RH PMLC). This dataset showed the two rapid rutting tests (HT-IDT and IDEAL-RT) correlated very well with one another, with a Pearson correlation coefficient (rp) greater than 0.97 for both Production PMLC and RH PMLC. Practically, this means the two rapid rutting tests provide very similar indications of the rutting resistance of the mixtures. The HWTT showed a very strong correlation with both rapid rutting tests for RH PMLC (rp > 0.85) and a strong correlation for the Production PMLC (rp > 0.60). Based on this strong correlation with HWTT, laboratory threshold values for both the HT-IDT and IDEAL-RT were established to correspond to the common HWTT failure criteria of 12.5 mm (0.5 inches) of rutting at 20,000-wheel passes. These threshold values agreed well with those found in literature – 20 psi for HT-IDT and an RT Index of 60 to 75, depending on binder type.

With the completion of traffic for the 2021 Test Track, the laboratory rutting test results from this study were compared with the final field rutting for those mixtures on the Track. The correlations between the HWTT and HT-IDT and field rutting on the Track are shown in Figure 2. Similar trends were observed with the IDEAL-RT, given its strong correlation with HT-IDT. For the relationships in Figure 1, only the sections on the Track tangents (straightaways) were included.

   

Figure 2. Field Rut Depths (NCAT Test Track) vs. Laboratory Rut Test Values – Tangent Sections – a) HWTT vs. Field Rutting b) HT-IDT vs. Field Rutting

This was done because 13 of the 14 mixtures in this study were paved in the tangents and only one mix was paved in the curves. Because these mixtures represent a variety of pavement structures, some scatter in the lab-to-field correlation is to be expected. Figure 2 shows the correlation between the field rut depths at the Track and the laboratory HWTT and HT-IDT results was generally low, with R2 values less than 0.4 and Pearson rp values in the 0.5 to 0.6 range.

A few things were notable from the lab-to-field relationships for the HWTT and HT-IDT tests. First of all, the lab-to-field relationships trend in the proper direction. In other words, as field rut depths increase, HWTT rut depths generally increase, and as HT-IDT tensile strengths increase, field rut depths generally decrease. At the Test Track, 0.5 inches of rutting is considered the threshold value at which section rehabilitation is considered. The maximum rut depth for any of the test sections in this study was less than 0.25 inches, with most of the field rut depths occurring in a very narrow range between 0.1 and 0.2 inches. This narrow range limits the power of the lab-to-field correlation.

Based on the low field rut depth magnitudes observed, it appears the laboratory-calibrated thresholds discussed previously for rutting test results are likely on the conservative side. It is evident that mixtures exhibiting less than 12 mm of rutting in the HWTT and greater than 20 psi in the HT-IDT consistently demonstrate excellent rutting resistance. However, it remains uncertain how much further these thresholds could be pushed while still maintaining strong field performance. The HT-IDT test was recently performed on 14 dense-graded mixtures on production mix samples taken during the construction of the 2024 Test Track. The production PMLC HT-IDT ITS results ranged from 15.9 psi to 43.8 psi. Two of the mixtures had results below the 20 psi HT-IDT threshold value.

Moving forward, it will be important to see how these mixes perform in the field regarding rutting to see if the limits can be adjusted and still have good field performance.

   

Contact Adam Taylor or Chen Chen for more information about this research.