1 Introduction
Externally bonded fiber reinforced polymer (FRP), including wet layup sheets and pultruded plates, has been widely used in the strengthening of the deteriorated structures due to its superior properties such as high strength-to-weight ratio and excellent corrosion resistance \cite{teng2002frp,Hollaway2008}
High temperature close or higher than the glass transition temperature of the epoxy resin (Tg) can cause premature debonding failure at a lower load. Failure mode can be also shifted from a cohesive failure within concrete substrate into an adhesive failure within the interface \citep{ferrier2016mechanical} \citep{firmo2015experimental}. Consequently, some studies recommended a maximum service temperature of 10oC lower than the glass transition temperature of the adessive Tg. The ACI 440.2R \citep{4402008} suggested a more conservative value of 15oC lower than the Tg.
Moisture is another factor influencing the bond behavior of the FRP-to-concrete interface. Epoxy plasticization induced by moisture can greatly decrease the bond strength between FRP and concrete \citep{ghiassi2013water}. Moreover, moisture absorption can reduce both the Tg and the stiffness of the epoxy resin.
The combined effect of moisture and temperature, referred as hygrothermal, can be the most critical environment. Rajan Sen \citep{Sen2015} mentioned that the most critical two environmental conditions that can affect the FRP/concrete composite are: (1) water immersion combined with elevated temperature; (2) moisture exposure or high humid environment combined with elevated temperature. Another study \citep{Ceroni2017} also showed that water combined with elevated temperature can cause the same bond degradation but with a shorter period. This outcome can be utilized in accelerated durability tests. However, despite the extensive literature that was dedicated to investigating the effect of moisture, freeze and thaw, alkane, and temperature, there are a limited number of studies exploring the effect of exposure to humid weather in elevated temperature and water immersion in an elevated temperature.
2 Glass-Transition temperature Tg
Glass-transition temperature of a material is defined as the range temperatures over which material transform from a hard and relatively brittle (glassy) state into a viscous or rubbery state. The effect of the temperature and the hygrothermal conditions on the FRP interface is mostly controlled by that parameter. Since the glass transition temperature of the epoxy resin and epoxy adhesive (Tg) are lower than the glass transition temperature of the FRP strips/laminate (Tg), the behavior of the FRP interface is governed by the first one.
For wet lay-up system, it is normally within the range (45 – 82oC) \cite{4402008} which represents the glass transition temperature for both the matrix and the bonding material because the same epoxy material is usually used. However, this temperature can get up (130oC) \citep{clarke1996eurocomp} \citep{dai2012bond} for prefabricated FRP because the polymer matrix went through higher curing temperature and pressure in the fabrication process.
3 Effect of temperature and Moisture on constitutive materials
3.1 Effect of temperature and moisture on the resin and adhesive
Ferrier et al. \citep{Ferrier2016} investigated the mechanical properties of the epoxy resin and epoxy adhesive used to bond the unidirectional FRP sheets and pultruded FRP strips, respectively, to concrete under both the high and low temperatures. The tensile test was performed based on the ISO527 standard \cite{Organaziation2012}. Test results showed that the tensile strength of the adhesive and resin are significantly decreased at a temperature of 50oC. Moreover, at the temperature of 80 oC and 100 oC (which is slightly higher than Tg) resin and the adhesive totally lost their stiffness, respectively. The results also detected a significant reduction in the tensile strength of the resin and the adhesive at a low temperature of -20oC and -30oC, respectively.
Benzarti et al. \citep{benzarti2011accelerated} studied the effect accelerated aging conditions (40oC temperature and 95% relative humidity) on the behavior of the epoxy resin and epoxy adhesive. Tensile test (performed according to the ISO 527 \cite{Organaziation2012}) revealed that both the resin and the adhesive have initial elastic behavior with an ultimate tensile strength of 49 MPa and 25 MPa, respectively. The aging process shifted the behavior of both epoxies into an elasto-plastic behavior. Then, after 8 months of aging the tensile strength of the resin and epoxy were decreased into 26 MPa and 6.6 MPa, respectively. Modula’s of elasticity of the resin and the adhesive was also decreased from 4.5 GPa and 4.9 GPa to 3.1 GPa and 1.3 GPa under the same aging conditions. It was concluded that the adhesive was more affected by the aging conditions than the resin due to its higher moisture uptake. Therefore, the higher moisture absorbed, the higher deterioration is. Another observation is that epoxy systems based on polyaimde and alkylether-amine hardeners are more sensitive to hydrothermal aging than the epoxy system based on diamine due to the high polarity of the polymer chains.
Maljaee et al. \citep{maljaee2016frp} investigated the effect of the hygrothermal environment on the epoxy resin. Specimens were exposed to 6 hours’ cycles of temperature ranged from 10oC to 50oC and constant relative humidity of 90%. Test result indicated a 26% reduction in the tensile strength and 18% in the modulus of elasticity corresponding to 1.96% water absorption. This reduction in the mechanical properties was attributed to the plasticization of the epoxy resin, microcracks because of the swelling and thermal cycles.
3.2 Effect of the moisture and temperature on concrete
Benzarti et al. \citep{Benzarti2011} investigated the effect of the aging conditions on the compressive strength of the concrete specimens by exposing them into 40oC temperature of and 95% relative humidity for 13 months. Results revealed that a slight increase in the compressive strength due to the continuation of the hydration process in the wet and warm aging environment.
3.3 Effect of the moisture and temperature on FRP
Generally, moisture does not have an obvious effect on the CFRP \citep{cromwell2011environmental}.
Benzarti et al. \citep{Benzarti2011} noticed that after four months of aging in 40 oC and 95 % humidity the mechanical properties of the CFRP plate were not significantly affected.
Maljaee et al. \citep{Maljaee2016} studied the effect of the hygrothermal environment on the GFRP coupons. The tensile strength reduced by 21% corresponding to 0.9% water absorption. Comparison test result of the hygrothermal environment with water emersion revealed an earlier degradation developed in the first case due to the presence of the thermal effect. Hygrothermal conditions had a negligible effect on the modulus of elasticity. Comparison test result of the hygrothermal environment with water immersion revealed an earlier degradation in the first case due to thermal effect.
4 Influence of temperature on the bond strength between FRP and concrete
Studies \citep{Ferrier2016,Firmo2015,Cromwell2011,Blontrock2003} showed that temperatures lower than Tg may have a limited effect on the bond strength in most cases. In other cases, it may result in a useful post-cure effect depending on the heating rate. Nonetheless, temperatures that approach or exceed the glass transition temperature can be harmful on the bond strength.
Kabir et al. \citep{kabir2016effects} studied the impact of the temperature lower than the Tg on the bond strength of CFRP sheet bonded with epoxy resin using wet lay-up technique. In this study, single shear test was implemented to explore the effect of thermal cycles where specimens were exposed into a constant temperature of 40oC for 5 hours followed by 7 hours of gradual cooling down to 30OC. Specimens were tested after 70 cycles (35 days), 180 cycles (90 days) and 730 cycles (365 days). Results indicated a small increment in the bond strength which is attributed for two reasons: (1) the maximum applied temperature was lower than Tg (47oC); (2) the small range of temperature (30-40oC) was not capable to produce thermal strain (thermal expansion and contraction). It was reported that a cohesive failure within the concrete substrate was the dominant failure mode for most specimens due to the increase of the bond strength.
Ferrier et al. \citep{Ferrier2016} utilized double lap shear test to study the effect of high and low temperature on the bond strength between CFRP and concrete. All tested specimens were placed in the thermal chamber for 3 hours before testing. Two strengthening systems were investigated which are unidirectional sheets applied in wet lay-up technique and pultruded FRP strips bonded by the epoxy adhesive. Test results of the first system revealed that decreasing the temperature from 20oC to -40oC can reduce the ultimate load capacity by 5%. However, increasing the temperature from 20oC to 60oC and 80oC can reduce the ultimate capacity by 38% and 70%, respectively. The failure mode was also shifted from a cohesive failure within concrete substrate into an adhesive failure within the bonded interface at a temperature of 80oC, which is higher than glass transition temperature. For the second strengthening system, results revealed that decreasing the temperature from 20oC to -20oC can reduce the ultimate load capacity by 40%. However, increasing the temperature to the range 40-50 oC, which represents the service temperature range in many parts of the worlds, can reduce the ultimate load capacity by 16%. Increasing the temperature up to 100oC can reduce the capacity by 30%. The author concluded that the second system is more sensitive to low temperature than the first one due to the differences in the characteristics of the materials and the higher axial stiffness of the second system.
Firmo et al. \citep{Firmo2015} investigated the influence of elevated temperature on the bond strength of the CFRP strips externally bonded to the concrete substrate with an epoxy adhesive. In this study, a double-lap joint shear test was implanted to study two testing conditions. The first one was a steady condition where specimens were heated first to a specific predefined degree, then they were loaded until failure. In the second testing conditions (transit condition), specimens were placed under a certain level of sustain load, then they were heated by a constant rate until failure. Test results revealed a significant reduction in the bond strength when the temperature raised higher than glass transition temperature. Moreover, it was found that changing testing condition had an only limited effect. It is worth to note that at a temperature of 1.9 and 2.6 times the Tg, the residual tensile strengths were 32% and 23% of the residual strength at the ambient temperature, which represents a relatively high residual strength.
Mikami et al. \citep{mikami2015effect} investigated the effect of high temperature ( 100 and 180oC) on the bond strength between FRP and concrete using a pull-off test \citep{092009}. In this study, all the specimens were strengthened with bi-directional CFRP sheets applied in wet lay-up technique. Then, they were placed into a chamber where specimens exposed to numerous thermal cycles at a temperature of 100 oC or 180oC. The length of each cycle was two hours. Relative humidity was kept constant at 0% or 100% within the cycles. In this research, the accelerated aging behavior of water and temperature was also explored by immersing additional specimens in distilled water and salt water of the same temperature for the same number of cycles. Test results revealed that exposing the specimens to 250 cycles at 100 oC and 0 % relative humidity can reduce the bond strength by to 11 %. However, the reduction in the bond strength increased up to 92 % when the temperature increased to 250oC under the same number of cycles and relative humidity. Most of the tested specimens developed an adhesive failure for both cases.
5 Thermal stresses and effective bond length
Numerous studies \citep{Ferrier2016,Firmo2015} utilized double-lap and single-lap shear tests to explore the effect of elevated temperature on strain distribution along the FRP strips/sheet bonded to the concrete. It is already established that rising the espouse temperature close to the Tg can reduce the bond strength and failure load. Consequently, the strain measured at the failure load will be lower for the case of the elevated temperature. However, for the same load level, it was observed \citep{Ferrier2016,Firmo2015} an increment in the strain distribution. This increment attributed to the thermal stress induced by elevated temperature as shown in Figure 1.
Two more important observations were made by studies \citep{Firmo2015,Benzarti2011,leone2009effect}. First, increasing the exposure temperature shifts the stain distribution into a linear form, particularly for low and moderate load level, due to the dropping in the epoxy stiffness that smooths the stress/strain distribution \citep{Firmo2015}. Second, it is detected that raising the temperature can also increase the effective bond length (least length required to transfer tensile stress from the concrete substrate to FRP strips/sheet by direct shear) more than the minimum length recommended by some design guidelines \citep{Leone2009,Benzarti2011,Kabir2016,Firmo2015}.