Experimental and FE analysis of NSM FRP bond in low- and normal-strength natural and recycled aggregate concretes

Abstract

Poor mechanical properties of construction materials constitute one of the major reasons for structural deterioration. Since the extent of the weakening in the material quality could reach an alarming threshold, their suitability for strengthening needs assessment prior to adopting any strengthening strategy. This paper experimentally investigates the bond behavior between near-surface mounted (NSM) fiber-reinforced polymer (FRP) strengthening in low- and normal-strength concrete with compressive strength in the range of 11.6 – 14.8 MPa for the former and 39.4 MPa for the latter. The use of two aggregate types – natural and recycled aggregates – was also investigated in the low-strength concretes. Steel fiber reinforcement was used in some of the mixes to check whether some enhancement is possible in low-quality concrete that can result from inferior recycled material, compromised workmanship, or material deterioration due to durability challenges. Ultimately, the variables of the study include recycled coarse aggregate incorporation ratio of 0%, 30%, 60%, and 100% replacement of the natural aggregate for each of the steel fiber’s percentages of 0%, 0.5%, and 1% of the specimen’s volume. The control NSM FRP-concrete specimen made from normal-strength concrete (39.4 MPa) was prepared and tested as a reference. The low-strength concrete mixtures demonstrated a decrease in the NSM FRP-concrete bond strength with an increase in the RCA incorporation. However, the addition of steel fibers marginally increased the bond failure capacity. Compared to the reference specimen of normal-strength concrete, at least, a 44% reduction in the NSM-concrete bond failure load was observed. Because some design guides (including the ACI 440) suggest a minimum concrete compressive strength for NSM FRP strengthening, a finite element model (FEM) was used to check the full spectrum of the mixture’s quality (both low and high-strength materials) and other key parameters affecting the NSM FRP-concrete bond behavior. The so-developed model is first validated against the experimental observations, followed by using it to run a series of parametric studies using the bar diameter, concrete compressive strength, FRP modulus, and steel fiber content as variables.