Sustained Loading Bond Response and Post-Sustained Loading Behaviour of NSM CFRP-Concrete Elements under Different Service Temperatures

Abstract

Nowadays, one of the foremost procedures for strengthening concrete structures is the Near-Surface Mounted (NSM) technique. This paper presents an experimental study on the effect sustained loading and different service temperatures (steady and cyclic) have on NSM Carbon Fibre-Reinforced Polymer (CFRP)-concrete bonded joints and their post-sustained loading load-slip behaviour. Four experimental campaigns using eight NSM CFRP-concrete specimens were performed by employing two different service load levels (15% and 30% of the ultimate load) and combining two groove thicknesses (7.5 and 10 mm) and two bonded lengths (150 and 225 mm). Two steady state temperatures (20 and 40 °C) and two cyclic service temperatures (ranging between 20 and 40 °C) were programmed. The slip obtained was proportional to the sustained load level. Furthermore, higher slips were registered for specimens under higher mean temperatures in the cycle. After 1000 h of sustained load testing, the specimens were tested under monotonic loading until failure (post-sustained loading tests). In general, the ratio between the post-sustained loading ultimate load and the instantaneous ultimate load was close to the unity, although some differences were perceived in series S2 (steady 37.7 °C) with a mean increase of 6.3%, and series S3-B (cyclic temperature ranging between 24.6 and 39.2 °C) with a mean reduction of 9%.

A Novel Approach to Quantifying the Failure Modes of Concrete-Epoxy Interface

“…As shown in Figure 15 , as the IF increases, the fracture energy reduces as the temperature and humidity conditions of the climate-controlled chamber change from 30% relative humidity at 21 °C (RT) to 25% relative humidity at 30 °C (HT) and as the sustained loading period (days) increases. This confirms the inverse relationship between the strength of physical infrastructure (in this case, CEI bond), environmental loads, and sustained loading duration [ 4 , 8 , 10 , 13 , 19 , 32 , 33 ].…”

“…Table 4 shows the ratio of the failure mode on the CEI failure surfaces; the highest (at least 81%) and the least (at most 1%) estimated failure modes are CC and CE, respectively, across the color spaces. This implies that the reinforced polymer bond is more robust and less susceptible to failure than the concrete-concrete cohesive bond and the concrete-epoxy adhesive bond [ 4 , 7 , 8 , 9 , 10 , 12 , 13 , 14 , 15 , 16 , 18 , 19 , 20 ].…”

“…Over the years since the last three decades, FRP (Fiber-Reinforced Polymer) has become the material of choice for strengthening and retrofitting aging and deteriorating infrastructures due to its relative ease of installation, high strength-to-weight ratio, resistance to corrosion, and many other advantages over traditional construction materials used for structural repairs like bolts, steel, and concrete, etc. [ 1 , 2 , 3 , 4 ]. FRP consists of high-strength continuous fibers embedded in a polymer matrix (resin); the embedded fibers are the reinforcement elements, while the resin serves as binders to the fiber and facilitates the transfer of loads to and within the fiber [ 5 , 6 ].…”

“…Over time, under sustained loading and severe environmental conditions, the structural performance of the FRP-repaired concrete weakens, subsequently fails, and debonds [ 4 , 7 , 8 , 9 , 10 , 12 , 13 , 14 , 15 , 16 , 18 , 19 , 20 ]. Failure may occur in either or combination of three ways; CC (Cohesive failure in Concrete), CE (Cohesive failure in Epoxy), and IF (Interfacial Failure), representing crack propagated along the concrete substrate, epoxy layer, and concrete epoxy interface respectively [ 7 , 8 , 10 , 13 , 16 , 20 ].…”

“…The CC is usually more dominant on the CEI failure surfaces than other failure types, and this is because the concrete substrate is more susceptible to failure than the FRP-epoxy bond and the concrete-epoxy interface [ 4 , 7 , 8 , 9 , 10 , 12 , 13 , 14 , 15 , 16 , 18 , 19 , 20 ]. The CE and the IF failure type ratio increases as the adhesive bond between the FRP-epoxy and concrete-epoxy degrades under sustained loading and exposure to high temperature and humidity [ 4 , 7 , 8 , 9 , 10 , 12 , 13 , 14 , 15 , 16 , 18 , 19 , 20 , 21 , 22 , 23 ]. Research efforts have been made toward categorizing the failure modes of CEI, yet more research needs to be made to quantify the failure modes on CEI failure surfaces.…”