Concrete-to-concrete interfaces : behaviour and modelling

Abstract

Concrete-to-concrete interfaces are widely present in concrete structures. These structures include but are not limited to: i) concrete structures (e.g., concrete beams, slabs and columns) repaired and strengthened through the enlargement of cross-sections, ii) in-situ cast concrete joints between precast concrete components; iii) composite concrete structures comprising multiple components cast at different times. In general, when subjected to tensile or shear stresses, concrete-to-concrete interfaces are the weak links of the structure because the interfacial tensile and shear strengths are normally lower than those of the integrally-cast concrete. However, a thorough understanding of and an accurate model for the behaviour of concrete-to-concrete interfaces are not yet available, making it difficult to fully understand and accurately predict the behaviour of concrete structures in which concrete-to-concrete interfaces play a critical role. Against the above background, the work presented in this PhD thesis was aimed at advancing the understanding of and developing a sophisticated model for the mechanical behaviour of concrete-to-concrete interfaces through investigations of the following three aspects: (1) Analyse the existing test methods for the interfacial behaviour of concrete-to-concrete interfaces and propose an improved test method that is more robust and accurate; (2) Develop a sophisticated interfacial bond-slip model for concrete-to-concrete interfaces based on a comprehensive experimental study using the improved test method; (3) Conduct numerical simulations, in which the proposed sophisticated interfacial bond-slip model is employed, of the structural performance of concrete structures with concrete-to-concrete interfaces. The thesis first presents a literature review on and a finite element analysis of the existing shear test methods for concrete-to-concrete interfaces. These investigations revealed the advantages and disadvantages of the existing test methods, based on which an improved test method suitable for studying the interfacial behaviour of concrete-co-concrete interfaces was proposed. This new test method was validated through a series of trial laboratory tests. A comprehensive experimental programme using the newly developed test method was conducted to investigate the interfacial behaviour of concrete-to-concrete interfaces by considering factors including the concrete strength, interface roughness and normal stress level. Based on the experimental data, a new bond-slip model for the interface was established. The model describes the complete local interfacial behaviour of concrete-to-concrete interfaces, including damage evolution along the interface. The developed bond-slip model was then implemented into a finite element framework, which included the appropriate constitutive modelling of both concrete and steel as well as reliable interfacial models for concrete-to-concrete and concrete-to-steel interfaces. This framework was used to simulate the structural behaviour of composite concrete beams, in which the concrete-to-concrete interfaces play a significant role in the mechanical behaviour. The performance of the framework was validated by comparing the predicted results of the flexural behaviour of the beams with the experimental data. Moreover, the framework was used to conduct a parametric study to investigate the influences of the interfacial parameters of the composite beams on the overall behaviour. The test method, the bond-slip model, and the numerical framework presented in this thesis constitute a major advancement on the understanding and the accurate prediction of the behaviour of concrete-to-concrete interfaces as well as structures containing such interfaces. Future research is needed to improve the current research outcomes to achieve even more accurate predictions of concrete-to-concrete interfaces and to accurately simulate the behaviour of more complicated structures containing such interfaces.