Bond failure performances between near-surface mounted FRP bars and concrete for flexural strengthening concrete structures

Near-surface mounted (NSM) fiber reinforced polymer (FRP) has been established as an effective technique for strengthening concrete member. In preview literatures, bond failure was observed usually in the strengthened beam test for increasing flexural capacity. Bond behavior is of primary importance for the transfer of stress between the concrete and the FRP reinforcement to develop composite action. In this paper, a total of 22 tests were conducted to study the bond failure performance between NSM FRP bars and concrete besides only one test as a comparison. Failure modes, load–deflection curves, strain distribution of FRP bars, and local bond stresses at the FRP-epoxy adhesive interface from the tests were analyzed in detail. Some of the factors expected to affect bond performance were presented, namely: diameter of FRP bars, type to FRP material, concrete compressive strength and bonded length. The test results reported in this paper should be useful for further establishing local bond–slip constitute relationship and further verification of numerical simulation models, in addition to gaining a better understanding of bond failures for flexural strengthening concrete structures with NSM FRP bars.     

Ductility analysis of compression-yielding FRP-reinforced composite beams

Ductility is one of the major issues of concern in achieving the widespread acceptance of FRP-reinforced concrete structures in practice. A new technique for improving the ductility of FRP-reinforced concrete beams through compression yielding (CY) instead of tensile yielding in the plastic hinge zone has recently been developed. A CY beam is a relatively complicated composite member that incorporates an additional composite material into the plastic hinge of a FRP-reinforced concrete beam. This paper presents the results of an investigation into the ductility behavior of CY beams. Results from a numerical study are first presented to illustrate the effects of key variables on the ductility performance of CY beams. These effects are then comprehensively examined through an analytical study. Finally, general conclusions on the ductility design of CY beams are provided.     

FRP strengthening of steel and steel-concrete composite structures: an analytical approach

A recent technique for strengthening steel and steel-concrete composite structures by the use of externally bonded Fiber Reinforced Polymer (FRP) sheets, to increase the flexural capacity of the structural element, is described. Several researches developed FRP strengthening of reinforced concrete and masonry structures, but few experimental studies about steel and steel-concrete composite elements are available. Some examples of guidelines for the design and construction of externally bonded FRP systems for strengthening existing metal structures are available, but the method used to predict the flexural behaviour of FRP strengthened elements is usually based on the hypothesis of elastic behaviour of materials and FRP laminate is mainly considered only under the tensile flange. In this paper, an analytical procedure to predict the flexural behaviour of FRP strengthened steel and steel-concrete composite elements, based on cross-sectional behaviour and taking into account the non-linear behaviour of the materials with any configuration of FRP reinforcement, is given. Analytical predictions are compared with some experimental results available in the literature on the flexural behaviour of FRP strengthened steel and steel-concrete composite elements, showing good agreement of the results, even in the non-linear phase, until failure.     

CFRP-PCPs复合筋嵌入加固钢筋混凝土梁裂缝试验及计算方法研究

通过5根嵌入不同张拉控制应力的碳纤维增强塑料预应力混凝土棱柱体(CFRP-PCPs)复合筋加固钢筋混凝土梁受弯试验,对比分析试验梁的裂缝分布与发展,得到最大裂缝宽度与平均裂缝宽度在静力荷载作用下的变化特性。结果表明: 嵌入CFRP-PCPs复合筋能有效的减少被加固钢筋混凝土梁的裂缝宽度和高度。在试验基础上,根据国家现行混凝土规范,对平均裂缝间距和最大裂缝宽度计算公式进行参数修正,建立了CFRP-PCPs复合筋嵌入加固钢筋混凝土梁最大裂缝宽度计算公式,计算值与试验值吻合较好。     

Comparative durability evaluation of ambient temperature cured externally bonded CFRP and GFRP composite systems for repair of bridges

In recent years externally bonded fiber reinforced polymer (FRP) composites have been increasingly accepted as an efficient means of rehabilitating deteriorating and under-strength concrete structures. In the case of the two primary application procedures – wet layup of fabric and adhesive bonding of prefabricated strips, the efficacy and reliability of the approach intrinsically depends on the strength and durability of bond between the FRP and the concrete substrate. This paper presents results from an extensive 24-month study, involving ten different external strengthening FRP systems, to assess the durability of the bond under environmental exposure conditions including immersion in water, immersion in salt water, ponding at different humidity levels, and exposure to freeze conditions of –18 °C. The FRP systems were characterized through determination of moisture uptake characteristics, tensile strength and modulus of the resins and adhesives, and direct tension pull-off testing of the FRP-concrete assembly. As expected from ambient temperature cure systems, both tensile characteristics and pull-off strength show degradation with time under specific environmental exposure conditions. Differences in bond durability are discussed based on system and exposure type and it is seen that exposure to the −18 °C environment results in the highest level of deterioration overall. The results indicate that the use of wet layup, with appropriately selected resins, and well fabricated composites, may at present have advantages over the use of adhesive bonded systems as related to long-term durability of the FRP-concrete bond. They also suggest that the changes in characteristics in resin/adhesive and bond due to environment exposure induced deterioration need to be considered in design to avoid premature failure.