Issue 41

Y. Yang et alii, Frattura ed Integrità Strutturale, 41 (2017) 339-349; DOI: 10.3221/IGF-ESIS.41.45 340 I NTRODUCTION ue to high intensity and rigidity and good resistance, the cement stabilized semi-rigid base can adapt to heavy traffic and complex climate environment, making it become one of the major types of base course for high- grade highway asphalt pavement in China [1]. Restricted by economic and technological conditions, for a long period of time, semi-rigid base will still be widely applied as a major base course of pavement. Therefore, we should further understand its characteristics and improve and utilize them so as to give full play to the advantages of this semi- rigid base material [2]. The asphalt pavement design code currently in use in China is simple, in which unconfined compressive resilient modulus is used as the parameters in the structural design of asphalt mixture and semi-rigid base material [3,4], and the pavement material is similar to most of the engineering materials, showing different characteristics under different tensile and compression moduli. When the difference between the tensile and compression moduli of the material is large, it is inappropriate to use a single modulus to calculate and analyze the mechanical response [5]. Reference [6,7] studied the constitutive relations of materials with different tensile and compressive elastic moduli and the fracture mechanical responses thereof. Reference [8-11] discussed and analyzed the application of the elastic theory of different moduli like tensile and compression moduli in the theories of beam, shell and plate, established elasticity solutions to beams, shells and plates with different tensile and compression moduli under different loads, and proposed a static equilibrium equation and calculation methods for stress and displacement under external force. For the pavement material, in 1992, Changsha University of Science & Technology (formerly Changsha Communications College) proposed a calculation method when differences between tensile and compression moduli are considered for the rigid pavement [12], then analyzed the drawbacks of flexible pavement design using tensile and compression moduli as the moduli of the structural layer and deduced the basic formula and test method of the double modulus theory [13]. The semi-rigid base is formed through stratified compaction, with significant differences in its tensile and compressive moduli. Under the action of traffic load, the neutral surface of the semi-rigid base course is not at the geometric center of the structural layer; instead, it shifts towards the compressive zone. The existing asphalt pavement design method does not consider the actual stress distribution of the semi-rigid base, and performs the structural load response analysis according to the compressive elastic modulus, which will lead to the unbalance between the working state of the structural design parameter and the actual stress state, and further resulting in large deviation in calculation and analysis results. In order to achieve scientific calculation and analysis, the corresponding design parameters should be determined according to the stress state of the points inside the pavement structure. Therefore, it is necessary to set up the mechanical response analysis method for asphalt pavement structure using the double modulus theory with different tensile and compressive moduli, especially the method to obtain the corresponding material parameters. In this paper, by using the MTS (Material Test System), we propose a new test method for measuring tensile, compression and flexural moduli simultaneously. We test three moduli of the cement stabilized macadam under different loading rates, propose a flexural modulus calculation formula which considers the shearing effect and reveal the change rules of the tensile, compression and flexural moduli with the loading rate and the differences and correlations between the three moduli so as to provide theoretical basis for the selection and optimization of material parameters in the asphalt pavement structure design. S PECIMEN MOLDING AND TEST PREPARATION Specimen Molding he cement used as the raw material of cement stabilized macadam is Xing’an Hailuo ordinary portland cement PC32.5 and its information are shown as Tab.1. The aggregate is the limestone aggregate manufactured by Yangjiaqiao Crushing Plant and its information are shown as Tab.2. The test results show that the technical indexes of these raw materials meet the requirements as specified in the code. In the basis of practical engineering, the gradation of CSM aggregate was designed to realize framework-dense structure according to the Testing Methods of Material Stabilized with Inorganic Binders for Highway Engineering [14].The mineral aggregate gradation adopted is shown in Tab. 3: According to the mineral aggregate gradation listed in Tab. 3, we carry out the heavy compaction test on cement stabilized macadam. First, we determine the optimum water content and maximum dry density at different cement dosages, and then determine the dosage of cement that meets the requirements by performing the unconfined compressive strength D T