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U of MD Lottman test results
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Tensile Strength Ratio To evaluate the moisture susceptibility of the mixtures investigated in this study, the modified Lottman procedure was used (AASHTO T 283-89). Each mixture was evaluated at the selected design binder content and an air void level of 7 plus or minus 1% by using the SHRP gyratory compactor (4 inch diameter) in preparation of samples. Replicate tests were conducted under both the unconditioned and conditioned states and the ensuing indirect tensile strength ratio (TSR), for each mixture, was calculated.Test Procedure For each of the six asphalt mixtures studied in this report, the test procedure used was as follows. For each mix (at the respective design asphalt content value noted [above]), six specimens 2.5" height and 4.0" diameter were prepared to a target air void level of 7 plus or minus 1% using the SHRP gyratory compactor. The six specimens were divided into two sets of three so that the average air voids in each set were nearly equal.T'he initial set of three specimens were then tested in a dry (unconditioned state), while the second set was tested in a saturated (conditioned) state. For the dry set, the samples were placed in a water bath maintained at 77 degrees F for 20 minutes and then tested to obtain their indirect tensile strength. This was achieved using the Lottman breaking head under an applied load with a rate of 2 inches/minute. The load at failure was recorded and the indirect tensile strength computed using the following relationship: St = 2P/(pi)td where:
For the conditioned set of specimens, the samples are placed in a jar filled with water at room temperature and a vacuum of 20 inches Hg is applied for 10 minutes. The specimens are then placed in a water bath maintained at 140 degrees F for 24 hours. Before evaluating the indirect tensile strength, the specimens are placed in another water bath at 77 degrees F for one hour. The average value of the indirect tensile strength for the conditioned set (SII) is then computed. The indirect tensile strength ratio (TSR) is computed as: TSR = (SII) / (SI) Test Results The modified Lottman procedure was used to evaluate the moisture susceptibility for each of the six mixtures investigated in this study. The detailed test results are shown in Appendix D [omitted*] for all specimen tests. [The following table] is a summary of the TSR values for the AC 120/150 and AC-10 binder studies; while a bar chart graphically illustrates the results of this study.Summary of Moisture Susceptibility Tests
It is obvious that for both virgin (conventional) mixtures, the addition of the Elvaloy® percentage greatly enhances the moisture susceptibility of the mix. At a 0% modifier level (i.e., virgin binder); both mixtures (AC 120/150 and AC-10) are marginally unsatisfactory at the "design" AC % (note: most specifications commonly require a minimum TSR value of 75%). In contrast, it can be observed that there is no difference, practically speaking, in the dry and conditioned (saturated) indirect tensile strengths at the "design AC %" for both mixtures using 2.0% Elvaloy® polymer. It can therefore be concluded, that moisture susceptibility, of properly design Elvaloy® polymer modified mixtures, is greatly enhanced compared to comparable mixture designs using a non-modified binder.
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