Bond degradation between FRP bars and concrete under sustained loads

Abstract

In recent times, fibre reinforced polymer (FRP) bars have been used as a replacement for conventional steel reinforcing bars in reinforced concrete members where the corrosion of steel is a major problem as a result of severely aggressive environments. The application of FRP bars as reinforcement in concrete structures has thus been proposed as a potential solution to the problem of steel corrosion in aggressive environments; however, the use of FRP could introduce other durability problems such as alkali resistant resins/fibres, de-icing chemical resistance, creep rupture of fibres, ultra-violet resistance, and freeze-thaw performance. One of the problems is long-term bond behaviour between FRP bars and concrete. FRP bars are coated with a polymer which is known to experience time-dependant deformation (creep) under sustained loads. It is anticipated that creep of the bars' coating and/or the bars' surface features (lugs) could be a factor contributing to the total creep in the reinforced concrete system. Also, glass FRP (GFRP) bars may deteriorate in water and alkaline environments if the coating is damaged and the glass fibres are exposed to water and alkaline solutions which are harmful for the glass fibres. An experimental program was conducted in two phases to study the long-term bond behaviour between FRP bars and concrete under sustained loads. Also, the effects of water and alkaline solutions on the bond between FRP bars and concrete under sustained loads were investigated. In Phase I, two types of carbon FRP bars (CFRP) and one type of glass FRP bar (GFRP), in addition to conventional steel rebar, were used to make pullout specimens which were tested under various levels of sustained loads at room temperature and exposed to air. In Phase II, three different types of GFRP bars were employed in pullout specimens that were tested under two different sustained load levels in three different environments at room temperature. In this phase, the specimens were either submerged in water or an alkaline solution, or were exposed to air. To help explain the test results, image analysis, scanning electron microscopy (SEM), and electron microprobe analysis (EMPA) techniques were used to examine the materials and the tested specimens. It was concluded that one of the two CFRP bars and the GFRP bar used in Phase I exhibited poor bond performance under sustained loads when compared to companion steel specimens. Deterioration of the bars' surface features was observed in these FRP bars. The other type of CFRP bar tested performed well in terms of long-term bond behaviour under sustained loads. For the FRP bars used in Phase II, bond degradation between the bars and concrete was not a problem. All three bars showed good bond behaviour under sustained loads. Instead, shear failure at the fibre-matrix interface was found to be a problem, causing a decrease in tensile strength of the bars under sustained loads. SEM investigation revealed circumferential cracks at the fibre-matrix interface in the bars' cross-sections, especially in the specimens tested at higher load levels. X-ray maps, obtained using EMPA, indicated no sign of calcium ion ingress in any of the specimens tested in Phase II. However, some specimens tested in water and alkaline solutions failed sooner than the companion specimens tested in air. This may have been because of degradation of the matrix due to absorption of water, although no evidence of deterioration of the matrix could be found.