FRP REPAIR OF CORROSION-DAMAGED CONCRETE BEAMS -. WATERLOO EXPERIENCE
Canada Research Chair, Department of Civil Engineering, University of Waterloo,
Waterloo, Ontario, Canada
Abstract: Corrosion of steel reinforcement is one of the main durability problems facing reinforced concrete infrastructures worldwide. This paper gives an overview on a seven year research program conducted at the University of Waterloo, sponsored by ISIS (Intelligent Sensing for Innovative Structures) Canada, to examine the viability of using fibre reinforced polymer (FRP) composites as a repair and strengthening method for corroded reinforced concrete structures. The majority of the research was carried out in the laboratory utilizing large-scale members. The results revealed that FRP repair successfully confined the corrosion cracking and improved the structural performance of corroded beams. Analytical models were developed to validate the experimental data. The FRP repair system was implemented in Fall 2005 to address corrosion damage in a bridge in the Region of Waterloo.
Corrosion of reinforcing steel causes many structures in adverse environments to experience unacceptable loss in serviceability or safety far earlier than anticipated and thus need replacement, rehabilitation, or strengthening. As steel corrodes, there is a corresponding drop in the cross-sectional area. The corrosion products occupy a larger volume than the original steel which exert substantial tensile forces on the surrounding concrete and causes it to crack and spall off. If corrosion cracking can be prevented or delayed, a certain degree of structural strength may be maintained in a corroding RC beam.
Fibre reinforced polymer (FRP) systems are promising alternatives for the rehabilitation of deteriorated and deficient concrete members. In addition to their high strength to weight ratio, durability in adverse environments and high fatigue strength, FRP sheets can be easily externally bonded to reinforced concrete slabs, beams, and columns (ACI Committee 440 1996).
A multi-phase research program was undertaken at the University of Waterloo (see Table 1) to investigate the viability of using externally bonded fiber reinforced polymer (FRP) laminates to rehabilitate corrosion-damaged reinforced concrete beams (Soudki, et al. 2006, Badawi and Soudki 2004a and 2004b; Craig and Soudki 2005, 2002; El Maaddawy et al 2004, 2005a and 2005b; Masoud et al 2005 and 2001, 2000; Soudki and Sherwood 2003 and 2001, 1998). Several reinforced concrete beams (20 small-scale, 24 medium-scale and 50 large-scale beams) with variable chloride levels (0 to 3%) were constructed. The test variables were: level of corrosion damage at the time of FRP repair, location of corrosion damage, effect of short-term static, long-term sustained loading and fatigue loads. The beams were repaired by externally epoxy bonding FRP laminates to the concrete surface bonded to the tension face, with the fibre orientation in the longitudinal direction followed by transverse laminates bonded to the tension face and up each side of the beam, with the fibre orientation in the transverse direction. Two types of FRPs were used: Glass (GFRP) sheets had an ultimate strength of 600 MPa, an elasticity modulus of 26 GPa, and an ultimate elongation of 2.24%. The Carbon (CFRP) sheets had an ultimate strength of 960 MPa, an elasticity modulus of 73 GPa, and an ultimate elongation of 1.33%. The tensile reinforcement of the specimens was subjected to accelerated corrosion by means of impressed current up to 15% mass loss. Strain gauges were used on the FRP laminates to quantify tensile strains induced by the corrosion process. Accelerated corrosion was applied using a constant impressed current of 150 |iA/cm2. The current was impressed through the main longitudinal rebars, which act as the anode while the stainless steel bar in each specimen acts as the cathode. Following the corrosion phase, the specimens were tested in flexure in a four-point bending regime. Details of the test program, test methods, and test results are found elsewhere.
M. Pandey et al. (eds), Advances in Engineering Structures, Mechanics & Construction, 165-173.
© 2006 Springer. Printed in the Netherlands.
Table 1. Overall experimental program
In the following, key findings from FRP repair for reinforced concrete beams subjected to corrosion damage, are given.
Figure 1 shows typical average crack width versus mass loss for corroded specimens. The width of the longitudinal cracks was measured at discrete time periods throughout the accelerated corrosion process for all the corroded specimens. It is evident that the FRP repair process reduced the crack opening by about 88% at the end of corrosion process. This implies a significant enhancement in appearance of FRP repaired corroded specimens by reducing crack opening due to further corrosion.
Figure 1. Crack width vs. time
Steel Mass Loss
Fig. 2 shows the average steel mass loss versus time relationship for various test specimens. A linear regression analysis for the steel mass loss results after repair showed that during the post-repair corrosion phase the steel mass loss rate in the specimens having the continuous-wrapping was on average about 32% lower than the level for the beams having the intermittent-wrapping. The presence of the sustained load during the post-repair corrosion phase increased the steel mass loss rate by about 9% and 12.5% for the specimens repaired with continuous and intermittent wrapping schemes, respectively. The specimens corroded under a sustained load had connected internal microcracks and external flexural cracks which increased the penetration of oxygen and moisture into the concrete and reduced the concrete resistivity and thus increased the steel mass loss rate to a level higher than that for the specimens corroded without load.
Figure 2. Steel mass loss versus time relationship