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• The variation of strength with loading rate under various loading modes were revealed for cement-stabilized macadam. • The fatigue properties of cement stabilized macadam under different loading modes were uniformly characterized. • The new strength structure coefficient associated with the loading rate was proposed for cement-stabilized macadam. Cement-stabilized macadam is widely employed as a base-layer of pavement in China, because of its high strength and stiffness. However, fatigue cracking of the cement-stabilized macadam based pavement is prone to occur under multiple traffic loads, which affects the service life of the pavement. The main objective of this work is to reveal the fatigue characteristics of cement-stabilized macadam under various loading modes by developing a unified fatigue characteristics model. To this end, an unconfined compressive strength test, indirect tensile strength test, and four-point bending strength test were conducted for the cement-stabilized macadam samples under various loading rates. A power function was employed to the pattern of variation of strength with loading rates. The stress ratio of the fatigue test associated with the loading rate was obtained and defined as the rate-dependent stress ratio. Also, the fatigue tests under various loading modes were implemented based on the rate-dependent stress ratio and nominal stress ratio. The preliminary laboratory experimental results showed that it is challenging to compare fatigue performance under different loading modes based on nominal stress ratio. However, the problems of uncertainty and non-uniqueness of fatigue characteristics under various loading modes were resolved by employing the idea of rate-dependent stress ratio in the correlation analyses. Based on that, the new strength structure coefficient associated with the loading rate was proposed in this study. Therefore, the proposed research results provided the fundamental understanding of the fatigue resistance design of pavement with semi-rigid base-layer. [ABSTRACT FROM AUTHOR]

• Study fatigue based on modulus decay considering variance of compressive-tensile. • A synchronous test method of modulus was used to monitor modulus during fatigue test. • The modulus decay mode of tensile and compressive were established and compared. The cement stabilized macadam base experiences tensile and compressive stresses simultaneously in the actual pavement structures, resulting in the influence of stress states on fatigue performance. In this paper, the fatigue performance of cement stabilized macadam was analyzed based on modulus decay considering the difference between compressive and tensile modulus. A synchronous measurement method of tensile and compressive modulus, which based on convention four-point bending test, was utilized to measure modulus of cement stabilized macadam during a fatigue test. The four-point bending fatigue test was conducted upon four different stress levels of 0.25 MPa, 0.5 MPa, 1.0 MPa, and 1.5 MPa. The average of 10 closest modulus values nearby the first 1% of fatigue life was adopted to determine the initial modulus. The last 5 cycles before the failure of the specimen in the fatigue test was determined as the critical modulus value. It was observed from the tests results that the critical value of compressive fatigue modulus was 8 times higher than that of tensile fatigue modulus. The initial and critical values of both tensile and compression decreased with the increase of stress levels. Under different stress levels, the modulus decay mode of tensile and compressive modulus was similar, but the decay rates of tensile modulus were higher than that of compressive modulus. Based on the modulus decay mode, the concept of critical failure point was put forward to characterize the fatigue behavior of cement stabilized macadam. During the fatigue process, both tensile and compressive modulus decay presented three stages, i.e. preliminary stage, stable stage, and rapid attenuation stage. The stable stage of tensile modulus was shorter than that of compressive modulus, which means that the primary fatigue damage occurred in the tensile zone. [ABSTRACT FROM AUTHOR]

• The Ayyagari model was applied to semi-rigid layers materials. • The unified fatigue model was obtained based on yield criterion. • A new design index of structural resistance was proposed. As the primary high-grade pavement structure, asphalt pavement with semi-rigid layers has been extensively applied in China. Among the semi-rigid layers materials, cement-treated aggregates (CTA) are an essential pavement element, and its fatigue life significantly affects the durability of the asphalt pavement structure. Various fatigue tests have been adopted to evaluate the fatigue properties of CTA. However, the fatigue resistance of the CTA measured under various methods is different, which seriously affects the accuracy of the structural design of the semi-rigid layers. Hence, this work aims to establish a design criterion for CTA to characterize the structural resistance of semi-rigid layers. The uniaxial compression (UC), four-point bending (4 PB), and indirect tensile (IDT) strength and fatigue tests for CTA were carried out. According to the traditional S - N fatigue model, the curves of fatigue performance and stress ratio of CTA under various stress states are quite different. To intuitively compare the structural resistance of CTA under various test types, the mean stress and effective stress of CTA under different loading rates were obtained. The yield surface was then established based on the Ayyagari model, considering the multiaxial yield criterion of CTA. The effective stress ratio was defined, and its relationship with the fatigue life of CTA was established. Based on the Ayyagari model and the normalized equation, the fatigue resistance criterion of semi-rigid layers under the three-dimensional stress state was proposed. [ABSTRACT FROM AUTHOR]

• The strength variation rules of different loading modes under two mixing methods and different loading rates were revealed. • Under two mixing conditions, the strength of different loading modes was uniformed characterized. • The validity and completeness of strength parameters of semi-rigid base asphalt pavement structure design were improved. Cement stabilized macadam base is the primary stress diffusion and bearing layer of semi-rigid asphalt pavement, therefore its mechanical behavior directly affects pavement performance. The indirect tensile strength, unconfined compressive strength, and bending strength are the essential parameters for evaluating the bearing capacity of cement stabilized macadam. In the design of a pavement structure, the conventional mixing strength values of a test sample are analyzed and calculated at a fixed loading rate. By using this procedure, the influence of the mixing method and the different loading rates on the strength values are ignored. Besides, the strength obtained by using the fixed loading rate with different loading modes has significant differences. As a result, the artificial randomness of the strength parameter in the design of a semi-rigid asphalt pavement structure increases. In an attempt, to improve the effectiveness and completeness of the strength parameters of cement stabilized macadam under different loading modes, mechanical tests were conducted on samples prepared with both vibration mixing and conventional mixing methods. This procedure included, indirect tensile strength test, unconfined compressive strength test, and four-point bending strength test under different loading rates. The results of these strength tests each group of samples (vibration and conventional mixing method) were compared. Besides, all samples were subjected to 6 different loading rates when conducting each loading mode. These experimental values were analyzed. The loading rate and the strength value have a power function relationship and the correlation of strength rates was revealed. The results demonstrated that the vibration mixing method increases the indirect tensile strength, unconfined compressive strength, and four-point bending strength by 13%, 14%, and 10%, respectively. A unified strength model (S / S 0 = 0.9841 + 0.16856ln (0.01019 + v / v 0) under two mixing methods and various loading modes were established by using the variation between loading rate ratio and strength ratio. In the unified model, the strength under vibration mixing and conventional mixing methods, and the strength among different loading rates can be converted, which improved the test efficiency. The established unified model solves the problem of uncertainty and uniqueness of strength parameters under different loading modes. The accuracy of the semi-rigid asphalt pavement structure resistance design was effectively improved, thereby providing theoretical guidance for the construction of semi-rigid base asphalt pavement engineering. [ABSTRACT FROM AUTHOR]