Rutting Susceptibility of Asphalt Mixes Using Marshall Refusal Density, Superpave Nmax and IDEAL-RT

Abstract: Rutting is one of the major distresses for flexible pavements. Currently, in India, the asphalt mixes are designed using Marshall method. Further, due to lack of advanced test methods, the refusal density (RD) concept is used as surrogate measure to evaluate rutting susceptibility of the asphalt mix. In the refusal density, asphalt sample is compacted using 250 number of blows using Marshall compactor and air voids is checked, and it is speculated that a mix with air void less than 2% will have chance to rut in the field. However, despite following this approach and satisfying refusal density, many of the projects exhibited rutting failure due to high shear stress, heavy slow-moving vehicles. Therefore, in the present research the refusal density obtained from Marshall compactor was compared with Nmax (N=205) density measured using Superpave Gyratory compactor. It is expected that air should not fall below 2% after Nmax. Further, both refusal density and Nmax density were compared with IDEAL Rutting Tolerance test (IDEAL RT) to evaluate any possible correlation.
In the present study, seven plant produced asphalt mixes designed using the Marshall method for National Highways expected to handle over 50 Millions Standard Axles were collected. The Marshall refusal density of all the mixes after 250 blows showed air voids above 2%, indicating the mixes are not susceptible for rutting in the field. However, air voids of samples after Nmax in SGC, indicated that five of the seven mixes were below 2%. Therefore, both Marshall refusal density and SGC Nmax showed opposite trend on rutting performance of these mixes.
The IDEAL RT test revealed that five mixes failed to exceed the minimum Rutting Tolerance Index (RT) of 65, indicating their susceptibility to rutting. These findings aligned with the observations from the Nmax SGC 205 gyratory compaction tests.
The good correlation (R2=0.86) was seen between RD and IDEAL RT, indicating that the RD has a potential for identifying rut susceptible mixes. A strong correlation (R² = 0.94) was observed between the Nmax and IDEAL RT, indicating Nmax can be used as a good indicator for rut susceptible mix. The study also demonstrated a strong correlation (R² = 0.93) between the refusal density obtained using the Marshall 250 blows and the Nmax, suggesting that the 250 blows Marshall criterion may need to be revised from 2% to 3% air voids. The study also suggests RT value of 65 to be used along with 3% refusal density for identifying rut susceptible mixes. The Nmax approach and the revised refusal density of 3% air void could serve as a reliable, efficient method for evaluating asphalt mix performance at the design stage, potentially eliminating the need for extensive rutting test while ensuring the mix meets the performance standard.