Quantitative Characterization and Elastic Properties of Interfacial Transition Zone around Coarse Aggregate in Concrete

Backscattered electron images（BSE） obtained by scanning electron microscope was used to quantitatively characterize the microstructure of interfacial transition zone（ITZ） in concrete. Influences of aggregate size（5, 10, 20, and 30 mm）, water to cement ratio（0.23, 0.35 and 0.53） and curing time（from 3d to 90d） on the microstructure of interfacial transition zone between coarse aggregate and bulk cement matrix were investigated. The volume percentage of detectable porosity and unhydrated cement in ITZ was quantitatively analyzed and compared with that in the matrix of various concretes. Nanoindentation technology was applied to obtain the elastic properties of ITZ and matrix, and the elastic modulus of concrete was then calculated based on the Lu ＆ Torquato model and self-consistence scheme by using the ITZ thickness and elastic modulus obtained from this investigation. The experimental results demonstrated that the microstructure and thickness of ITZ in concrete vary with a variety of factors, like aggregate size, water to cement ratio and curing time. The relative low elastic properties of ITZ should be paid attention to, especially for early age concrete.

Simulation of Fatigue Stiffness Degradation in Prestressed Concrete Beams under Cyclic Loading

In order to investigate and research the fatigue cracking of prestressed concrete fatigue properties and loading and stiffness degeneration process,cyclic loading tests were carried out on six prestressed concrete beams and the stiffness degradation under fatigue was investigated. A simulation model of stiffness degradation is proposed based on the stiffness analysis of the fatigue-damaged section. The elastic modulus of damaged concrete and the effective residual area of steel were introduced as well as an adjusted three-stage concrete fatigue damage evolution model. The strip method was used to analyze concrete damage due to changing stress along the depth of the beam section. The simulation and test results were compared and a method of predicting fatigue deflection was presented based on the simulation model. The predicted results were compared with that of the neural network method. It is in good agreement for the simulation results with the test results. It is only less than5% error for the simulation model which can reveal the two-stage degradation of prestressed concrete beams under cyclic loading. It is more precise for the simulation prediction method under proper conditions.

Development of combined transitional pavement structure for urban tram track-road grade crossings

The grade crossings and adjacent pavements of urban trams are generally subjected to complex load conditions and are susceptible to damage.Therefore,in this study,a novel pavement structure between tram tracks and roads constructed using polyurethane(PU)elastic concrete and ultra-high-performance concrete(UHPC),referred to as a track-road transitional pavement(TRTP),is proposed.Subsequently,its performance and feasibility are evaluated using experimental and numerical methods.First,the mechanical properties of the PU elastic concrete are evaluated.The performance of the proposed structure is investigated using a three-dimensional finite element model,where vehicleinduced dynamic and static loads are considered.The results show that PU elastic concrete and the proposed combined TRTP are applicable and functioned as intended.Additionally,the PU elastic concrete achieved sufficient performance.The recommended width of the TRTP is approximately 50 mm.Meanwhile,the application of UHPC under a PU elastic concrete layer significantly reduces vertical deformation.Results of numerical calculations confirmed the high structural performance and feasibility of the proposed TRTP.Finally,material performance standards are recommended to provide guidance for pavement design and the construction of tram-grade crossings in the future.