Use of FRP Laminates for Strengthening of RC Beams

Abstract

Existing Reinforced Concrete (RC) structures may be structurally deficient and may require strengthening due to reasons like flaws in design, faulty execution, change in occupancy, up-gradation of design codes, structures damaged due to earthquake, blast, fire, storm, etc. RC beam if under designed or damaged or poorly constructed may lead to failure of structure. Strengthening of RC beam is thus required to be done to regain its strength using Fibre Reinforced Polymers (FRP). FRP materials are available in many forms like Carbon, Glass and Aramid fibres. FRP materials can be effectively used for strengthening of beam in flexure as it is light in weight, easy to install, and has high strength to weight ratio; thus makes perfect material for strengthening of structures. FRP material when used externally for strengthening of beams in flexure has major disadvantage of premature failure due to debonding of FRP cloth with concrete, and FRP cannot be utilized to its ultimate strength. Prestressing of CFRP laminate therefore can be done to solve the problem of debonding. In this technique the CFRP laminates are used as external reinforcement but are prestressed against the beam using different systems developed for prestressing of external CFRP laminate. An attempt has been made hereby to experimentally study the effect of prestressing in strengthening of full scale RC beams as compared to the beams strengthened using CFRP laminate without any prestressing. Experimental behaviour of strengthened beams is compared with behaviour of non-strengthened RC beam. Total 16 RC beams having cross section 300mm x 300mm are casted and tested under two point loading. Two specimens are designated as control specimens. Two beams are strengthened using CFRP laminate having cross section 50mm x 1.2mm without any prestressing. Two specimens are strengthened using CFRP laminate with prestressing level of 20% and 40% of ultimate tensile strength of the CFRP laminate. Eight specimens are cracked partially by applying 60% of its load carrying capacity to develop crack. Two partially cracked specimens are strengthened by grouting. Two specimens partially cracked each are 0%, 20% and 40% FRP prestressed respectively. Prestressing of CFRP laminate is done using hydraulic jack and jacking the laminate against the beam itself after curing of 28 days. Analysis and design of RC beams is done based on IS 456:2000 provisions. Analysis of 0% prestressed beam is made using ACI 440.2R-08 provisions. Measurements taken during testing of beams are ultimate load, displacement, linear strain, failure mode, and crack pattern. Load is applied as two point load for the beams with the help of hydraulic jack mounted on loading frame. Displacement is measured for RC beams with the help of LVDT. Linear strain is measured with the help of mechanical strain gauge for strain in concrete, and with the help of electrical strain gauge using strain indicator as data acquisition system; on CFRP laminate for the RC beams. Percentage increase in load ranging from 5% to 45% has been observed for all strengthened beams as compared to that of control specimen. Rupture of prestressed CFRP laminate is observed in beams strengthened with prestressed CFRP laminate. The beam specimen without any prestressing fails by debonding of the laminate from concrete specimen resulting into premature failure. Improvement in the load carrying capacity and reduction in the deflection of the un-cracked and partially cracked beams strengthened with prestressed CFRP laminate is observed as compared to beams strengthened without prestressing in uncracked beams and partially cracked beams. Anchor plates used for prestressing prevented the debonding of CFRP laminate and thus preventing the premature failure. Partially cracked beams repaired by grouting showed a regain of 82.5 % strength compared to control beam. Prestressing of CFRP laminate for strengthening of RC beams proved to be effective technique when compared to beams strengthened with CFRP laminate without prestressing. Analytical and experimental values for load carrying capacity showed perfect match beams strengthened with 20% and 40% prestressed CFRP laminate. Control beams and beams strengthened with 0% level of prestressing of CFRP laminate showed mismatch in experimental and analytical results, as design is carried out on the basis of codal provisions including various design factors.