This dissertation provides an in-depth mechanical characterization of slab/base interaction for concrete pavement structures considering the viscoelastic properties of asphalt base under both traffic and environmental loading and different environmental conditions.Concrete pavement structures can be divided into jointed plain concrete pavement (JPCP) for thicker concrete layers or a short jointed plain concrete pavement - concrete overlay on asphalt (SJPCP-COA) for thinner concrete layers. Good performance for concrete pavements relies on the bonding between the concrete and the base layer. The literature shows that JPCP over lean concrete bases (LCB) results in poorer transverse cracking performance than JPCP over asphalt bases. The literature regarding slab/base interactions and the role of the interphase and composite structures with concrete on top of asphalt is scarce. Additionally, current mechanistic-empirical design procedures over-simplify the slab-base interactions. Hence, several gaps and questions about slab/base interactions for concrete pavements were found and are answered in this dissertation.
The goal of this study is to investigate and understand the slab/base interactions, including the bonding of concrete to different bases/interlayers, and how to reduce concrete pavement shear and tensile stresses and strains that cause cracking and, therefore, reduce the cost of the structures. This research developed a laboratory testing framework to address mechanisms of failure related to the base of concrete pavements under testing conditions that replicated temperatures, frequency of loading, and loading modes observed in the field by using available laboratory testing machines and a newly developed device were used to test specimens in shear, tension, and compression. Full-scale test slabs were constructed to analyze different pavement structures under the effect of the environment and falling weight deflectometer load, and the sections were modeled using material properties obtained from the laboratory testing to replicate the behavior observed in the falling weight deflectometer (FWD) testing and study the debonding process of composite structures under the effect of environment loads.
Thirteen laboratory testing procedures in tension and shear were determined adequate for asphalt and composite specimens testing based on the available testing equipment and the development of a new testing device. The shear and tensile tests were frequency sweep, creep, and ramp tests. Additionally, a compressive dynamic modulus was also done. Three testing procedures were already developed and had their corresponding American Society for Testing Materials (ASTM) or American Association of Highway and Transportation Officials (AASHTO) standards, while the other ten were developed under this research project. Nine of those tests were used in the first phase of testing in which a hot mix asphalt (HMA) and a gap-graded rubberized hot mix asphalt (RHMA-G) were extensively characterized under testing conditions that replicated field conditions. A testing protocol consisting of tensile hanging creep, compressive dynamic modulus, and tensile ramp test was narrowed down from the initial 13 tests. It was determined to be enough to characterize the asphaltic base materials and bonding properties precisely while being time-efficient and budget-friendly. Additionally, the final testing phase introduced a test to capture the water-induced damage. Any well-established laboratory or agency can easily replicate all four selected tests.
Material stiffness properties and damage parameters were obtained from the laboratory tests, which determined that temperature and humidity negatively impact the strength of the base materials and, by extension, will also impact the slab/base interaction. Moisture conditioning specimens at 60 °C (140 °F) cause a decrease in the strength of the asphalt material by 11% for HMA specimens and 16% for RHMA-G specimens. Laboratory strength tests conducted at different temperatures determined that temperature increases reduced the material strength and caused the material to behave softer, increasing the deformation to reach a 50 percent integrity between 40 and 100 percent more when testing at 40 °C than when testing at 25 °C.
The literature review showed that JPCP over LCB cracks 2.8 times more than JPCP over HMA bases. A full-scale test track, including four sections, was built to study and better understand the effects of different bases and interlayers for concrete pavements in addition to laboratory experiments. Three of the sections were built over LCB with different interlayer materials. One of the sections was prepared with curing compound, a typical interlayer material widely used in the state of California; another section was built with geotextile as an interlayer, an alternative currently allowed by Caltrans but not commonly used. The third interlayer used was microsurfacing, an interlayer alternative proposed by this research to improve the performance of concrete pavements over LCB. The fourth and last section was built with an RHMA-G base, which is not used as a base but is currently used as a pavement surface, which was also proposed as an alternative to conventional HMA bases. The sections were instrumented with vibrating wire strain gages and thermocouples at different depths. Multiple FWD tests were conducted to analyze the structures under different temperatures and drying shrinkage gradients besides the data recorded from the sensors.
Based on the FWD, it was determined that the deflections in the section of JPCP over LCB are three times the deflections in the sections of JPCP over RHMA-G, which means that less area of the concrete slab is in contact with the base and will cause a higher cracking potential. The two new concrete pavement alternative base/interlayers proposed in this research project outperformed the two alternatives currently allowed by Caltrans. The results obtained support the field observations of poor transverse and longitudinal cracking performance of JPCP over LCB, typically with curing compound, which has already been reported, but there was no clear explanation of the reason behind this until now. The curing compound and geotextile prevent the layers from bonding and do not allow the base to follow the concrete slab deformations due to temperature and moisture gradients.
Based on corner deflections from full-scale test sections and laboratory testing, it is concluded that RHMA-G can be used as a base layer for concrete pavements. An additional benefit of using this type of mix as a base is to fulfill Caltrans desire of using rubber in the paving industry. As of right now, it is only used in the surface layer, but the use of rubber can also be expanded to base layers of concrete pavements. From the laboratory experimental design, it was seen that the interphase in both RHMA-G and HMA composite specimens is not the weakest point in the structure. Placing microsurfacing between the lean concrete base and concrete slabs is considered to be an ideal interlayer. It provides the road paving industry with a new material to be used as an interphase when dealing with concrete pavements, but further investigation in field pilot projects should be conducted. It is an alternative that is cheaper to place than widely used geotextile and produces almost the same behavior as having an RHMA-G base. This outcome is ideal since it still supports the use of lean concrete bases in concrete pavements since they can use the same paving equipment and plants but causes the section to perform similarly to concrete pavements placed on top of asphalt bases. Allowing structures with lean concrete bases to perform similarly to sections with asphalt layers may be a solution to the current issues faced in the state of California, where concrete over LCB layers is cracking at a much faster rate than concrete pavements placed over HMA layers. Based on those results, it is suggested to have RHMA-G base and microsurfacing interlayer in pilot projects.
Lastly, a FEM modeling framework was developed to model the JPCP sections that were built and analyzed. Stiffness laboratory tests provided a detailed characterization of the materials, and the damage initiation and damage evolution laws were obtained from laboratory test models. Two different models for full-scale concrete sections were created with the material parameters from the laboratory testing and material models. First, a complex dynamic model, including asphalt viscoelastic behavior, long-term action of ambient loads, and progressive damage on the interphase, was used to study the performance of the structure and bonding condition under environmental loads. Second, a simplified static model with elastic materials behavior, a preestablished debonding area between the PCC and base obtained from the complex dynamic model, and equivalent static loads to study the performance of the structure under FWD loads.
The damage initiation and damage evolution laws for the materials required for the development of full-scale pavement models were obtained from the tensile hanging damage test and shear ramp test models. Using the complex dynamic model and the simplified static model, it was concluded that the environmental loads have a significant impact on pavement performance, reducing the overall stiffness due to interphase damage and resulting in higher FWD deflections than those observed with an undamaged interphase. Higher deflections result from curled concrete slabs with less base support, which will increase the tensile strains at the bottom and, therefore, increase the cracking potential of the slabs.
