Development of the saturation ageing tensile stiffness (SATS) test

Abstract

This paper describes the development of a combined ageing/moisture sensitivity laboratory test to evaluate the performance of coated macadam binder course and base mixtures with relatively low binder contents and relatively high air void contents. The test is known as the saturation ageing tensile stiffness (SATS) test, and consists of initial saturation prior to placing compacted asphalt core samples in a high-temperature and high-pressure environment in the presence of moisture for an extended period of time. The stiffness modulus measured after the test divided by the stiffness modulus measured before the test (retained stiffness modulus), and the specimen saturation after the test (retained saturation), are used as an indication of the sensitivity of the compacted mixture to combined ageing/moisture effects. The test was used to reproduce in the laboratory the loss of stiffness modulus observed on a trial site where high modulus base (HMB) had been used. Two different aggregate types (basic and acidic) and four different binders were used in the HMB mixture that was investigated. For the mixtures containing the acidic aggregate, the general trend is for the retained stiffness modulus to reduce from approximately 0·6 at a retained saturation level of 10% to 0·2 at a retained saturation level of between 80% and 100%. At a retained saturation level of 40% the retained stiffness modulus is approximately 0·4, which compares favourably with the 60% reduction in stiffness modulus observed from the HMB trial site (for nominally identical mixtures). For the mixtures containing the basic aggregate, the retained stiffness modulus remains at an approximately constant value of 0·7 over a wide range of retained saturation levels (between 15% and 80%), indicating a lower sensitivity to moisture. For the low binder content, high air void content mixtures investigated, the effect of changing the binder (source and grade) was small compared with the effect of the aggregate type.