Effect of concrete heterogeneity on FRP-to-concrete bond behaviour

Abstract

Fibre reinforced polymers (FRP) are widely used for strengthening or retrofitting deteriorated structures, due to their various advantages. In FRP-strengthened concrete structure, one of the main failure modes is the premature bond failure, which prevents the exploitation of the full capacity of FRP strengthening, and leads to brittle performance of the strengthened structure. As a result, the bond behaviour between the FRP reinforcement and concrete plays a critical role in FRP-strengthened RC structures, which has led to extensive experimental and numerical investigations. However, many of the existing studies on debonding failures in FRP strengthened concrete structures ignore the heterogeneity nature of concrete. Some experimental studies reveal that the surface treatment and the coarse aggregates on concrete surface significantly affect the bond strength of FRP-to-concrete interface, and even change the failure mode of the FRP strengthened structures. However, the effect of concrete heterogeneity is still not well understood and has not been investigated numerically. The aim of this study is to investigate the bond behaviour of FRP-to-concrete bonded joint with the focus on mesoscale mechanical behaviour of concrete and the effect of concrete heterogeneity.
A 2D mesoscale modelling framework is proposed, integrating the capabilities of generating mesoscale geometric structures with Matlab programming, incorporating the geometry into ABAQUS with Python scripting, and solving finite element (FE) models with ABAQUS. The mesh sensitivity associated with the strain localisation and softening behaviour of concrete is investigated in both tension and compression conditions under 2D macroscale and mesoscale FE schemes. It has been identified that for compressive damage dominant problems, mesh objectivity cannot be achieved even when the crack band model is adopted. A mesh and fracture pattern dependent model has been proposed, through which mesh objectivity can be successfully achieved under macroscale and mesoscale FE schemes.
Following the development of the above mesoscale modelling framework, an effective and efficient mesoscale modelling method with an explicit representation of concrete heterogeneity for FRP-to-concrete bonded joint was developed. It was found that compressive stress-strain model and damage model play significant roles in mesoscopic simulations of FRP-concrete bonded interface. Various models are assessed and it is proposed that Saenz’s (1964) compressive stress-strain model and Lubliner et al.’s (1989) damage model are the most appropriate to represent the mechanical behaviour of meso-components of concrete in mesoscale simulations. The validity of the proposed mesoscopic modelling scheme is evaluated with a large number of test specimens collected from the literature. Influence of concrete heterogeneity has been studied by comparing the bond behaviour of FRP-to-concrete bonded joints with various coarse aggregate distribution patterns. It is found that the predicted force-displacement curves as well as the micro-crack patterns of different coarse aggregate distribution on concrete surface are distinct, leading to different bond strength and local bond-slip relationship.
Comparative tests aimed to investigate the strain distribution in FRP reinforcement in both FRP-to-concrete and FRP-to-mortar bonded joints are conducted using a beam bending setup. The strain variation across the width of the FRP is about four times higher in an FRP-to-concrete bonded joint than in an FRP-to-mortar bonded joint, which is believed to be caused by the concrete heterogeneity. It is also found that the presence of coarse aggregates is beneficial for enhancing the bond strength.
Another batch of comparative test for the same purpose but with more parameters using a simple single shear test setup is then conducted. In addition to those findings similar to the first batch of test, it is also found that the presence of coarse aggregates would change the failure mode of the bonded joint from FRP debonding to concrete prism cracking in specific cases. When the switch of failure mode happens, the ultimate loading capacity of the FRP-to-concrete bonded joint is improved significantly.
To more realistically model the meso-mechanical processes and the effect of concrete heterogeneity, a 3D mesoscale method and its application in the 3D mesoscopic simulation of concrete uniaxial compressive and tensile behaviour are introduced. Comparison of the 2D and 3D mesoscopic simulations indicates that 3D model gives a similar uniaxial tensile strength with the 2D model, but the uniaxial compressive strength and fracture energy predicted by 3D model is significantly higher than those of 2D model, attributing to the lateral confinement effect existing in 3D models. A multiscale modelling method for FRP-to-concrete bonded joint is then proposed, consisting of both mesoscale and macroscale zone, to balance the computational accuracy with efficiency. Predicted load-displacement behaviour and fracture process are compared with the experimental results, demonstrating performance of the proposed 3D multiscale model.

Date of Award	Jul 2020
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Marios Soutsos (Supervisor), Wei Sha (Supervisor) & Jian-Fei Chen (Supervisor)