Mechanical performances of composite-strengthened concrete structures with embedded FBG sensors

Abstract

Investigations on the use of fibre reinforced plastic (FRP) composites to strengthen and reinforce civil concrete structures have been found increasingly in recent years. Lightweight and high-strength FRP plate or sheet is laid up on the concrete surface to maintain structural integrity and increase load carrying capacity of deteriorated or aged concrete structures. Meanwhile, the optimisation on the design of using the materials in FRP concrete strengthening with high strength efficiency, low manufacturing and labour costs, long serviceability life and minimum traffic interruption becomes an important issue in the civil and construction engineering applications. In addition, the development on the structural health monitoring system for a structure, which is strengthened by the FRP materials is also significant for inspecting the subsequent damages. Current study aims to investigate the mechanical behaviours of concrete structures, which are strengthened by the FRP materials on the concrete surface subjected to international standard test. The scope of the study also includes the development of smart structure health monitoring system for these strengthened structures by embedding fibre-optic Bragg grating (FBG) sensors inside the concrete and at the interface between the concrete surface and externally bonded composite materials. An extensive review is given to discuss the present practice on the utilisation of the FRP materials in the civil concrete applications. Materials, configurations of the structures, quality control and manufacturing methods for the FRP strengthened concrete structure were also reviewed. Another focus was placed on the adaptability of using the embedded optical fibre sensor as strain- and temperature-measuring devices in engineering applications. The survivability and mechanical performance of the FRP and optical fibre sensor in harsh environments were also addressed. Following the comprehensive review, several results from preliminary experimental studies are given. These studies include fundamental mechanical property tests for the concrete, composite and interfacial bonding behaviour of composite plate with an embedment of optical fibre. The results from the lap joint shear test for the composite-concrete bonded joint shows that an epoxy resin can provide a strong bonding characteristic to the concrete materials. Mechanical behaviour of composite-wrapped concrete structure was investigated. The laboratory-sized concrete cylinders were wrapped around its circumference by glass fibre composite laminates. The composite-wrapped concrete cylinder was then subjected to the uni-axial compression load. Several effects on the composite-wrapped system were also investigated through experimental work and numerical analysis. In the experiment, it was found that the load carrying capacity of the wrapped cylinder increases with increasing the number of laminate layers. Splitting failure occurred at both ends of wrapping laminate and continuously propagated toward the mid-height of the cylinder. Numerical analysis using the finite element method (FEM) interpreted the strengthening efficiency and failure mechanisms of the composite-wrapped concrete structure with different wrapping dimensions, material properties and thicknesses. The maximum load carrying capacity of the composite wrapped cylinder increases with increasing the wrapping size, thickness and modulus of the wrapping material while the maximum hoop stress in the wrapping material increases at both ends of the wrapping laminate. However, the negative hoop stress in the wrapping material occurs when using high modulus and thick wrapping laminate. Optimised design can be achieved by using the FEM analysis. Composite-bonded rectangular concrete beams (with/without crack formation at mid-beam) subjected to the three-point bending were investigated through experimental observation, analytical and numerical approaches. Glass fibre composite plates with different bonding lengths and thicknesses were adhered on tension or shear surfaces of the beam in order to investigate the mechanical behaviour of the whole composite-strengthening system. Flex-ural-cracks in the concrete and debond at the interface between the concrete and composite were observed during the test for long plate-bonded structures. For short plate bonded structures, crack was initiated at plate end region in the concrete. The strengthened beam was then failed catastrophically. No debond at the interface between the concrete and composite was found for the thick and short bonding length system. The overall performance of the composite-strengthened beam was better than that of the plain beam in term of flexural strength properties. Wet lay up technique gave higher ultimate flexural strength than the pre-cured plate-bonded method for the composite strengthened beam. For a notched concrete beam with an injection of epoxy resin, the modulus and ultimate strength were slightly increased compared with the plain beam. Linear elastic analytical model was derived to investigate adhesive shear and peel-off stresses with different strengthening plate geometries and material properties for the composite-strengthened structure. The results show that the adhesive shear and peel-off stresses increases with increasing the plate modulus and thickness. Theoretical prediction in determining the stress intensity factor (KI) for the strengthened structure was also conducted. The KI decreases with increasing the strengthening plate thickness and modulus. The adaptability of using embedded fibre-optic Bragg grating (FBG) sensor for strain measurement was investigated experimentally. The FBG sensors were embedded at the interface between the concrete and composite in both the composite-wrapped and plate-bonded concrete structures. Externally bonded strain gauges were adhered on the surface of the composite to compare the strain measuring results from the sensors. Four-cylinder theoretical model was established to determine the stress transferring properties and minimum embedding length for the embedded sensor. The theoretical prediction reveals that the axial strain measured at the fibre core region of the sensor is lower than the true strain of the host material if using the embedding sensor with thick adhesive layer and short plate bonding length. Experimental results also show that the surface mounted strain gauge could not detect the true strain of the host material when debond at the interface was happened. However, the embedded sensor could give signal to the operator that the structure was operated in strange condition. Environmental effect on composite-strengthened structures is one of the most important aspects in the application of the FRP. The mechanical performance of the plate bonded rein-forced-concrete (RC) structures after immersing into the saline water, fresh water, acidic (PH 4.01) and alkaline (PH 10) solutions for 6 months were studied. Experimental results show that the overall flexural strengths of the composite-strengthened RC beam is increased compared with the plain RC beams at the same chemical environments. The strength of the RC beam is susceptible to acidic solution, a 24.8% drop in flexural strength was measured for the beam submerged into PH 4.0 solution for six months. Finally, conclusions and recommendations for the further study in the design of composite-wrapped and plate-bonded structures, strain measuring system and installation process are presented.