Experimental investigation of the bond behavior of the interface between near-surface-mounted CFRP strips and concrete

Highlights

• 28 specimens were tested to study the behavior of NSM FRP-to-concrete bonded joint.

• The bond performance of the interface increased with increasing concrete strength.

• The bond capacity of the interface increases with bond length until rupture of CFRP.

• The bond at the bonded joint degraded with a decrease in the edge distance.

• A residual friction stress exists at the bonded joint after debonding failure.

Abstract

The technique of strengthening reinforced concrete structures with near-surface mounted (NSM) fiber reinforced polymers (FRPs) has been widely used in structural retrofit, and the load-carrying behavior of the strengthened structures is greatly influenced by the bond between the NSM FRP and concrete. In the present study, a total of 28 specimens, which consisted of concrete prism and NSM CFRP strip, were manufactured and the pull-out test was conducted on them. The failure mode, bond capacity, and CFRP strain distribution of the specimens were investigated. Meanwhile, the influences of concrete strength, bond length, and the distance between the groove and concrete edge on the bond behavior of the NSM FRP–concrete interface were analyzed. The test results revealed that the bond performance of the interface between the NSM CFRP and concrete is improved with the increase in concrete strength. Moreover, increasing the bond length of CFRP strips increases the bond capacity, and as such, the failure mode of specimens transforms from interface debonding into the rupture of CFRP. There exists residual friction stress at the interface between the CFRP and concrete after the interface debonding, which is independent of the bond length. An insufficient distance between the groove and the specimen edge reduces the bond capacity and could result in a splitting failure of the concrete corner. When the edge distance exceeds 60 mm, the bond behavior of the specimens tends to be stable.

Introduction

Due to the deterioration caused by environmental conditions, a large amount of infrastructures, including reinforced concrete bridges, buildings and tunnels, do not satisfy the requirements of structural safety and must be strengthened. Fiber reinforced polymers (FRPs) have been widely used for strengthening reinforced concrete structures due to their high strength, good durability in harsh environments and easy handling. As for an externally bonded FRP, the bond between the FRP and concrete is the crucial issue in strengthening structures with FRPs because the stress shared by the FRP is transferred from the structures to the retrofitted material through the interfacial bond. Poor bonding at the FRP-to-concrete interface often results in debonding and premature failure of the strengthened structures. In the past decade, the technique of strengthening reinforced concrete structures with near surface mounted (NSM) FRPs has attracted much attention [1], [2], [3]. In the NSM FRP method, grooves are first cut into the concrete, and the FRPs (bars or strips) are then embedded and bonded therein with epoxy adhesives. Thus, the bonding capacity of the FRP-to-concrete interface is enhanced by increasing the bond area between the FRP and concrete. The additional benefit of this technique includes better protection for the FRP compared with the externally bonded FRP and good durability [4].

Research studies have been carried out on the bond behavior of the NSM FRP and concrete. De Lorenzis et al. [5] conducted a series of pull-out tests on specimens with a NSM FRP system and investigated the effect of the FRP rod type (material and surface pattern), groove-filling material, bonded length, and groove size and observed the existence of residual friction. Teng et al. [6] studied the debonding failure mechanisms of the reinforced concrete beams strengthened with NSM FRP strips. They found that the failure mode of beams with an FRP strip of intermediate length (1200 mm and 1800 mm) was the concrete cover separation starting at the cutoff section, and when the embedment length was increased to 2900 mm, the failure mode of the strengthened beam shifted to concrete crushing followed by the concrete cover separation. Zhu [7] conducted modified RILEM beam tests on 9 specimens to investigate the influence of bond length and mechanical properties of adhesive on the bond performance between the NSM FRP and concrete and found that the bond capacity increases with the bond length. Oehlers et al. [8] experimentally studied the intermediate crack (IC) debonding resistance at the NSM FRP-to-concrete interface with different cover and thickness of CFRP strips. They reported that embedding NSM FRP strips can substantially increase their IC debonding resistances as well as increase their slip capacities and, through the increased slip capacity, increase the ductility of NSM FRP joints.

As for the technique of strengthening RC structures with externally bonded (EB) FRP, the concrete strength and the bond length have proven to influence the bond performance between the EB FRP and concrete significantly [9], [10], [11], [12], [13]. Studies have also been conducted to investigate the influence of some factors on the technique of NSM FRP. Cruz et al. [14] conducted pullout-bending tests and found that the bond strength, ranging from 13 to 18 MPa, decreased with the increase in bond length and was practically insensitive to the concrete strength. Moreover, the maximum tensile stress recorded on the CFRP increased with the bond length and was independent of concrete strength. Al-Mahmoud et al. [15] found that the concrete strength does not influence the load-carrying capacity of the strengthened beam when the failure occurs due to NSM system failure. However, Kotynia [16] concluded that the increase in the concrete strength delays the CFRP debonding and increases the debonding CFRP strain based on his experimental study. He also reported that the presence of the longitudinal steel bars led to the concrete failure plane moving from trapezoidal to almost horizontal, along or slightly below the longitudinal steel reinforcement. Hassan and Rizkalla [17] experimentally and analytically evaluated the bond characteristics of NSM CFRP strips based on monotonic loading tests of nine concrete beams strengthened with NSM CFRP strips. They reported that the debonding loads were increased by increasing the bond length of the CFRP strips, concrete strength, and groove width. The development length of the NSM CFRP strips was also increased by increasing the internal steel reinforcement ratio but decreased with the increase of both the concrete strength and the groove width.

In addition, Teng et al. [6] and Peng et al. [18] reported that the splitting of concrete corner was observed in their test studies of the NSM FRP strengthened beams, which indicates that the distance from the groove to the concrete edge (edge distance) has a critical influence on the bond behavior between the NSM FRP and concrete. However, there has been little work to date on the influence of the edge distance. Blaschko [19], [20] reported that the edge distance had a significant influence on the bond behavior of the NSM FRP system. According to his test results, the edge distance still influenced the bonded joint until a value of 150 mm, but no cracks were observed in the concrete at bond failure when the edge distance was greater than 30 mm. He suggested that the edge distance should be no less than 30 mm or the maximum aggregate size, whichever is greater, to avoid splitting failure of concrete corners and damage of aggregate during the process of cutting the grooves. Based on finite element modeling for round deformed bars, Hassan and Rizkalla [21] suggested a minimum value for the grooves spacing and the edge distance. However, the splitting failure of concrete was still observed in an experimental study conducted by De Lorenzis [5], which indicates that the value of the edge distance proposed by Hassan and Rizkalla might be insufficient to avoid damage of concrete corners.

It can be concluded from the above review that different opinions still exist among researchers regarding the influence of the concrete strength, the bond length, and the edge distance on the bond behavior of the NSM FRP-to-concrete bonded joint. In the present study, a total of 28 specimens were manufactured, and pull-out tests were conducted to study the bond performance of the NSM FRP–concrete interfaces. The influences of concrete strength, bond length, and edge distance on the interfacial bond were investigated. The failure mode, the bond capacity, and the stress transferring between the FRP and concrete were analyzed.