Strengthening of RC Beams Using NSM FRP Reinforcements

Abstract

Fiber-reinforced polymer (FRP) materials are used in different configuration and techniques for strengthening of Reinforced concrete (RC) elements to ensure their longer service life. Near surface mounted (NSM) method is one of the innovative strengthening techniques that consists of placing FRP reinforcing bars for flexural and shear strengthening of deficient RC members. The reinforcement is embedded in grooves cut onto the surface of the member to be strengthened and filled with appropriate binding agent such as epoxy paste. Limited studies are reported to date on the use of NSM FRP reinforcement for strengthening of RC elements in literature. An attempt has been made to investigate the structural performance of simply supported RC beams strengthened with NSM FRP bars. Flexural and shear strength- ening of RC beams is conducted using CFRP and GFRP bars of 9.53 mm and 12.7 mm, respectively. Total 27 beam specimens are cast and subsequently are strength- ened using NSM FRP reinforcements. 7 damaged beams and 8 undamaged beams are strengthened in flexure on the other hand, 6 damaged beams and 6 undamaged beams are strengthened in shear. Damaged beams are subjected to additional 80% load cor- responding to failure load of beam specimens before strengthening. Size of beam is kept 150mm_ 200mm _ 2100mm. Beams are designed using IS provisions. The beams are made deficient for flexure and shear by reducing percentage of tensile re- inforcement and required shear reinforcement, respectively. Analysis of strengthened beams is conducted using ACI provisions. Variables considered in flexure strengthen- ing of beams are change in diameters and numbers of FRP bars by keeping total area of bars nearly same. Variables for shear strengthening of beams include change in spacing of FRP bars. The RC beams are subjected to two point loading during test- ing.FRP strengthened beams deficient in flexure exhibited increment in flexure load ranging from 49.78% to 120.26% as compared to that of control beam. Results have shown that by changing number of FRP bars, increment in failure load and decrement in displacement is observed for FRP strengthened beams. For beams strengthened vi using CFRP bars, 10.13% to 14.71% higher load is observed compared to that for as beams strengthened with GFRP bars. Shear strengthened beams have exhibited 18.06% to 68.75% increase in load as compared to that for control beams. Increment in failure load and decrement of displacement is observed by reducing spacing of FRP bars. Higher failure load and change in failure mode from shear failure to combined failure is observed for beams strengthened with reduced spacing of FRP bars, respec- tively. Marginal reduction in strength of damaged beam compared to undamaged beam is observed. Tensile test and bond test are performed for 9.53 mm diameter GFRP bar and the results are compared with that of 10 mm diameter HYSD bars. GFRP bar gives 50 % increase in tensile strength as compared to that of HYSD bar. GFRP bar is installed at center of the cube of size 150_ 150_ 150mm and its bond strength is measured and results are compared with HYSD bar. GFRP bar gives higher bond strength at corresponding slip compared to HYSD bar. Bond test on NSM FRP bar is conducted to compare change in bond strength at different bond lengths for the bar. GFRP bar is installed in pre-cut groove along surface of the concrete block of 150mm_150mm_300mm. Failure in epoxy and concrete is seen for specimens having bond length equal to five times bar diameter. The failure between epoxy and FRP bar is observed for specimens having higher bond length. It is observed that RC beams strengthened in flexure and shear with NSM GFRP and CFRP bars have given beneficial results in term of load, moment, displacement, strain, crack patterns etc. for damaged as well as for undamaged beams. Thus use of various type of FRP reinforcement is strongly recommended for strengthening and Retrofitting of elements especially beams by NSM techniques.