Development of a Design Method for Bonded Concrete Pavement Overlays
Al Sabbagh, Ahmed
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Several asphalt pavements prematurely reached the end of their design life due to wheel loading has dramatically increased on highways. Therefore, transportation and highways agencies often endeavor to provide high-performance pavement to ensure the longest life with low maintenance required. Although asphalt pavement overlay (APO) can provide a quick and low-cost option for pavements rehabilitation, it frequently exhibits excessive rutting especially at highway pavement subjected to stop-and-go traffic conditions. Hence, concrete pavement overlays (CPO) is one of the alternative options for rehabilitating deteriorated pavements. The ultra-thin white topping (UTW) is one type of concrete overlay types and is considered a relatively new technique that extends the service life of distressed asphalt pavements (DAP). The UTW is a fully bonded, short, and thin concretes lab with a thickness ranging from 50 to 100 mm (2.0 to 4.0 in.), which is placed over a DAP. Last three decades, the use of discrete fibers (DF) within UTW has become increasingly popular in pavement restoration to reduce cracks width and required slab thickness. DF are uniformly distributed within a concrete mass and provides bridging forces across initial cracks after the elastic stage. Design methods of UTW depend on the stress ratio (SR) concept to determine the required thickness of UTW, which defines the ratio of total stresses during design life to the UTW capacity. Several cases of UTW exhibited premature failure due to over or under design. Therefore, to achieve a more accurate design and better predict the required thickness, total UTW capacity, and applied stresses should be accurately calculated. There are two main problems related to current UTW design methods; the accuracy of UTW capacity and stresses design during pavement service. The elastic formula of the American Society for Testing and Materials (ASTM)(C78)  has been used to determine the UTW capacity (i.e. Modulus of rupture(MOR)), which is determined from the flexural test of the concrete beam. MOR reflects the maximum concrete capacity within the elastic stage before cracks ap-pear in the concrete matrix. DF in the concrete mix plays a significant role after the elastic stage, therefore using the elastic formula of ASTM C78  to deter-mine the flexural capacity of fiber-reinforced concrete (FRC) structures would not capture the fiber’s benefits and recommend the same thickness for unreinforced concrete. On the other hand, these design methods depend on the thin-plate theory (TNPT) to calculate the design stresses. The main criticism about TNPT is that a transverse deformation cannot be considered, especially for small slabs such as UTW, which leads to overestimated stresses and more required thickness. Hence, thick-plate theory (TKPT) with free four edges should be adopted to include the effects of a transverse deformation in stress calculation. Therefore, there were three main objectives of this dissertation were investigated. Firstly, proposing a test method and procedure to investigate the benefits of DF testing on the flexural capacity of UTW. Secondly, developing the analytical thick-plate model to determine the critical bending stresses in UTW. Finally, determining the required thicknesses of UTW based on the first and second objectives. Recently, an effective MOR (MOReff.) has been proposed, which depend on proportionally increasing the MOR by the post-cracking strength (PCS) to characterize the advantages of DF to flexural capacity. The main shortcoming of these procedures is that the MOReff.is determined depending on testing the concrete layer only, not for FR-C/asphalt composite layers, even though the design assumptions of UTW depends on its behavior as a monolithic section. Hence, pavement capacity should be determined depending on both layers (FRC/asphalt). On the other hand, to include the effects of a transverse deformation in pavement analysis, Shi at al.  modified Reissner’s origin work  and proposed a theoretical solution for a single rectangular thick-plate with four free edges under the vertical load applied. Their solution cannot be used to analyze composite pavements such as UTW since it adopted single thick-plate resting on the foundation. Therefore, composite pavements should be converted to an equivalent, homogeneous, and individual layer to be analyzed by TKPT. To accomplish the first objective of this study, the laboratory experimental investigations included two different phases. The first phase involved four different tests to determine the mechanical properties of concrete layers. While the second phase involved two different tests to determine the flexural properties of the FRC/asphalt composite layers. Three main parameters have been taken into consideration; DF content, UTW thickness, and failure mode. Then, using a PCS concept to determine the actual flexural capacity of the UTW for the critical failure mode. To evaluate the proposed test method, a comparison was made between the current and proposed flexural capacity of the UTW and previous experimental results. Further, the proposed method is verified by comparing the required UTW thickness depending on the current and proposed methods. The results showed that the flexural capacity of the composite beam was higher than the capacity of the concrete layer. Besides, the concept of MOReff. reflects the benefits of DF and takes into account increasing flexural capacity after cracking occurs. Using the elastic formula of ASTM C78  did not reflect the benefits of the inclusion of DF in FRC/asphalt composite layers. The proposed flexural capacity of the composite beam reduced the required UTW thickness compared to the current methods. To accomplish the second objective of this study, the equivalent plate concept (EQPC) was used to convert two bonded layers of the UTW to an equivalent, homogeneous and individual layer to use by the thick-plate model. Two comparisons were made to confirm the accuracy of the proposed analytical solution with data of two previous experimental works. Further, the proposed model has been verified by a comparison with two other analytical thin-plate solutions. Moreover, the proposed model was used to perform a parametric analysis to determine the effects of six different parameters on the critical bending stresses in UTW. The comparison with previous data showed that the proposed analytical model to determine the critical stresses in UTW provided accurate and safe results. Further, the proposed model was generally more reliable than the two other analytical thin-plate solutions. The analysis concluded that the elastic modulus and the thickness of the existing pavement and overlay thickness had a significant effect on critical stresses in a UTW. Furthermore, slab dimensions played an important role in UTW behavior for small slabs-on-ground. Finally, the required thicknesses of UTW for most five design parameters were provided based on the proposed flexural capacity and proposed analytical stresses model.