Development of a Simple Test to Determine the Low Temperature Creep Compliance of Asphalt Mixtures
[ NCHRP 20-30 (NCHRP-IDEA) ]
Project Data
Staff Responsibility:
Dr. Inam Jawed
This project explored the concept of performing bending test on thin asphalt mixture beams to determine low temperature creep compliance of asphalt mixtures. Work in the initial phase focused on developing a methodology for sample preparation and testing. Thin mixture beams were cut using a simple tile saw. The dimensions for 660 thin beams were recorded to obtain the coefficient of variation for thickness. The thickness and width values indicated that beams obtained using the proposed procedure were uniform with very similar sizes. The loading protocol of the existing Bending Beam Rheometer (BBR) device was modified to accommodate higher load levels. Three methods were investigated: one consisting of a two-step loading procedure that did not require any change in the current BBR device, another that used the maximum load of BBR device with software modification that improved resolution of measured deflections and the third involving the use of high and intermediate temperature results and time temperature superposition principle to predict the lowest temperature results. The simplest test method avoided testing at low temperature levels and predicted creep stiffness from data obtained at higher temperatures. Additional testing found no differences in creep compliance results when testing was performed in two cooling media: ethanol and air. Work in the second phase focused on testing and data analysis. The current AASHTO standard for Indirect Tension Test (IDT) and the proposed BBR test method were followed to perform creep tests on laboratory prepared asphalt mixtures and cored field samples. Each asphalt mixture was tested for 1000 seconds in IDT and BBR at three temperatures. The analysis of the experimental results indicated that IDT and BBR creep compliance are slightly different. When similar tests were performed on homogenous polymer specimens, no significant differences were observed. Additional tests were performed on asphalt mixture beams of three different sizes. The creep stiffness results were very similar, which suggests that the differences between IDT and BBR results are due to sample geometry effects and testing artifacts. Based on composite materials models and finite element method simulations, a back calculation procedure was developed to obtain asphalt binder creep compliance from mixture experimental data. A standard test method for determining the flexural creep stiffness of asphalt mixtures using the bending beam rheometer was drafted for consideration by the AASHTO.The final report will be available from the National Technical Information Service (NTIS #PB2010-101388 ).
