# Design Approach for Composite Pavement Structure Incorporating Reflective Crack Considerations
## 2 Enhancing the Design Methodology of Current Composite  
### Design Methodology for Composite Pavement Structure  
The occurrence of crack damage in composite pavement is depicted in Figure 1. The figure highlights two particularly critical stress locations within the composite pavement structure: the joint of the cement concrete slab corresponding to the bottom of the asphalt layer (referred to as \"asphalt layer bottom position 1\", representing the most unfavorable stress point for the asphalt layer bottom) and the bottom of the cement concrete slab. These stress points give rise to two distinct types of cracks. One type of crack, denoted as a reflection crack, arises due to the presence of joints in the cement board. At these joints, the bottom of the asphalt layer experiences substantial shear forces under both load and temperature conditions, resulting in the formation of reflection cracks. The other type of crack (denoted as a common crack) is attributed to the cracking of the bottom of the cement concrete slab, caused by the bending tensile stress exceeding the material's tensile strength. The design standard outlined in China's \"Design Specification for Highway Cement Concrete Pavement\" aims to prevent common cracks and is expressed through Equations (1) and (2). This design approach employs the bending tensile stress at the bottom of the concrete slab as the key parameter for assessing cracking and effectively addresses the occurrence of common cracks. However, it is noteworthy that the design formula lacks specific control measures for reflection cracks and fails to consider the stress conditions at position 1 of the asphalt layer bottom. Consequently, reflection cracks have emerged as a prevalent issue in composite pavement structures. \[\gamma_{r}\big{(}\sigma_{pr}+\sigma_{tr}\big{)}\ \leq\ f_{r} \tag{1}\]  
\[\gamma_{r}\big{(}\sigma_{pm}+\sigma_{lm}\big{)}\ \leq\ f_{r} \tag{2}\]  
where \(\gamma_{r}\) depicts the reliability coefficient; \(\sigma_{pr}\) is the load fatigue stress of cement concrete slabs at unfavorable locations, MPa; \(\sigma_{tr}\) denotes the temperature fatigue stress of cement concrete slabs at unfavorable positions, MPa; \(\sigma_{pm}\) is the maximum load stress at the most unfavorable position of the cement concrete slab under the heaviest axle load, MPa; \(\sigma_{lm}\) represents the maximum temperature stress at the most unfavorable position of the cement concrete slab at the maximum temperature gradient in the region, MPa; \(f_{r}\) is the standard value of flexural tensile strength of cement concrete, MPa. This article enhances the current design approach for composite pavement structures by revising the design indicators. Specifically, the focus is on two key aspects [18; 19; 20]: (1) The prevention of fatigue cracking at the bottom of cement concrete slabs, which is governed by the bending and tensile strength of the cement concrete as stipulated in the specifications. The corresponding calculation formulas are presented in Equations (1) and (2). (2) The mitigation of cracking at the bottom of the asphalt layer, which is regulated by the bending and tensile strength of the asphalt mixture. The derivation process for this aspect is outlined as follows:  
In the presence of both temperature fluctuations and traffic loads, the bottom of the composite pavement's asphalt layer is susceptible to crack formation when the comprehensive stress exceeds the tensile strength of the asphalt mixture. Hence, the stress condition at the bottom of the asphalt layer, where the comprehensive stress does not surpass the corresponding tensile strength of the asphalt mixture, is defined as the limit state of the asphalt layer.