Durability of Fabric-Reinforced Cementitious Matrix (FRCM) Composites: a Review

applied sciences

Review Durability of Fabric-Reinforced Cementitious Matrix (FRCM) Composites: A Review

Karrar Al-Lami 1,2 , Tommaso D’Antino 1 and Pierluigi Colombi 1,* 1 ABC Department of Architecture, Built Environment and Construction Engineering, Politecnico di Milano, 20133 Milan, Italy; [email protected] (K.A.-L.); [email protected] (T.D.) 2 Civil Engineering Department, University of Wasit, Al-Rabee st, Wasit 00964, Iraq * Correspondence: [email protected]; Tel.: +39-02-2399-4280

Received: 15 January 2020; Accepted: 27 February 2020; Published: 2 March 2020

Abstract: Strengthening and rehabilitation of masonry and concrete structures by means of externally bonded fabric-reinforced cementitious matrix (FRCM) (also referred to as textile reinforced mortar (TRM)) composites was proposed as an alternative to the use of fiber-reinforced polymer (FRP) composites due to their good mechanical properties and compatibility with the substrate. However, quite limited studies are available in the literature regarding the long-term behavior of FRCM composites with respect to different environmental conditions. This paper presents a thorough review of the available researches on the long-term behavior of FRCM composites. Namely, (i) test set-ups employed to study the FRCM durability, (ii) conditioning environments adopted, and (iii) long-term performance of FRCM and its component materials (mortar and fiber textile) subjected to direct tensile and bond tests, are presented and discussed. Based on the available results, some open issues that need to be covered in future studies are pointed out.

Keywords: fabric-reinforced cementitious matrix (FRCM); textile reinforced concrete (TRC); textile reinforced mortar (TRM); inorganic-matrix; open-mesh textile; long-term behavior

1. Introduction The use of fiber-reinforced polymer (FRP) composites to strengthen and retrofit existing masonry and reinforced concrete (RC) structures was proven to be an effective solution due to the FRP good mechanical properties, resistance to corrosion, and ease of application [1–4]. However, some concerns raised regarding the poor sustainability and compatibility with the substrate of organic resins used in FRPs [5–8]. In addition, organic resins produce toxic fumes when exposed to fire and lose their mechanical properties at temperature close to their glass transition temperature, which for resins commonly adopted in FRPs ranges between 45 ◦C and 82 ◦C[9–11]. Fabric-reinforced cementitious matrix (FRCM) composites, which are also referred to as textile reinforced mortar (TRM) composites, were proposed as an alternative to FRP composites to overcome the drawbacks associated with the organic binders. FRCM are comprised of high strength fiber open-mesh textiles embedded within inorganic matrices. The textile can be made with different types of fiber, such as carbon, alkali-resistant (AR) glass, polyparaphenylene benzobisoxazole (PBO), or basalt. When steel fibers organized in steel cords are adopted, the inorganic-matrix composite is referred to as steel reinforced grout (SRG) [12]. The textile layout (dimension and shape of single yarns, yarn spacing, and direction) can be varied to modify the textile-matrix stress-transfer mechanism and customize the composite behavior [10,13]. In addition, textile yarns can be coated with organic resin to improve their bond with the matrix, FRCM ease of handling and installing, and to protect the fiber filaments. Various types of inorganic matrix can be found in the literature, such as cement-based and lime-based mortars and geopolymers, sometimes modified with fly ash, polymers, silica fume, and short fibers to

Appl. Sci. 2020, 10, 1714; doi:10.3390/app10051714 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 1714 2 of 23 increase the adhesion to the textile and substrate [7,14–18]. FRCM composites are generally employed as externally bonded reinforcement (EBR) for masonry [19] and RC members [20]. Inorganic-matrix composites comprising high-strength fiber textiles embedded within high-performance cement-based matrices were also used to construct high-strength panels often employed for new structural elements. These types of composite, referred to as textile reinforced concrete (TRC), have peculiar properties that make them generally different from FRCM composites [21,22]. Although an increasing number of experimental and analytical studies are available regarding the mechanical behavior of FRCM and TRC composites, limited work has been done to investigate their durability. Since they are externally bonded, FRCM can be exposed to various environmental conditionings, such as wet–dry (WD) and freeze–thaw (FT) cycles, high-alkalinity environments, and salt attack, which may affect their performance. The same holds for TRC composites when high-strength panels are used for outdoor applications. In this study, the available literature is reviewed to gain an insight on the state of research regarding mainly the long-term behavior of FRCM composites. In particular, (i) testing methods adopted to study the FRCM long-term behavior; (ii) accelerated ageing and conditioning environments considered to simulate the outdoor environments; and (iii) possible degradation of FRCM components (matrix and textile), FRCM coupons, and FRCM-substrate interface are analyzed and discussed. Based on this discussion, aspects that need to be further investigated are pointed out. When available, results of TRC composites are also analyzed to discuss their long-term behavior with respect to that of FRCM composites.

2. Test Methods Recently, US acceptance criteria AC 434 (2018) [23] and Italian Initial Type Testing procedures CSLLPP (2019) [24] proposed standard tests to evaluate the durability of FRCM composites. However, the available research does not comply with these recent procedures. In fact, different methods were adopted to investigate the FRCM long-term behavior. The changes in the mechanical properties of FRCM and TRC exposed to a control environment, approximately 23 ◦C and 60% relative humidity (RH), with respect to those of corresponding composites aged in different conditions (discussed in Section3) were studied adopting various test procedures, such as uniaxial tensile, three-point bending, pull-out, and direct-shear tests [21,25–28]. Among them, uniaxial tensile tests were often used in the literature to evaluate the long-term behavior of the matrix, bare (non-impregnated) textile, and composite coupon. Two main tensile test set-ups, which differ for the specimen gripping method, were proposed for the mechanical characterization of FRCM coupons. (i) In the clevis-grip test [23] (Figure1a), metal plates are bonded to the end of the specimens and then connected to the testing machine through a clevis joint. In general, FRCM coupons subjected to clevis-grip tests fail due to cracking of the matrix and subsequent debonding of the fibers at the matrix–fiber interface. The slope of the cracked phase of the axial stress-axial strain curve obtained is referred to as the tensile modulus of elasticity of the cracked composite and is employed in designing the strengthening intervention using the tested FRCM composite [23]. (ii) In the clamping-grip test [24] (Figure1b), the ends of the FRCM coupon are clamped applying sufficient pressure to avoid slippage of the fiber within the matrix [29]. Therefore, after cracking of the matrix, the axial stress is sustained mainly by the textile and the specimen fails due to subsequent rupture of the fiber filaments [28,29]. These two set-ups generally provide completely different results that cannot be easily compared. However, provided that the stress-transfer mechanism between matrix and fibers is properly investigated, the axial stress-axial strain curves obtained by clevis- and clamping-grip tests can be compared, deriving also information on the matrix crack spacing and opening [30]. In addition to the uniaxial tensile test, other test methods were adopted to investigate the durability of the FRCM coupons such as three- and four-point bending tests [21,28,31]. However, these tests do not provide direct information about the effect of conditioning on the matrix–fiber bond behavior since several parameters, such as the matrix tensile and compressive strength, affect the specimen behavior. Therefore, they were mainly used to evaluate the durability of mortar (without textile) [32–35] or they Appl. Sci. 2020, 10, 1714 3 of 23 were coupled with uniaxial tensile and pull-out tests to gain further information combining the results obtainedAppl. Sci. 2020, [21 ,10,28 ].1714 3 of 25

(a) (b)

FigureFigure 1.1. SketchSketch ofof ((aa)) clevis-gripclevis-grip andand ((bb)) clamping-gripclamping-grip uniaxial tensiletensile testtest set-ups.set-ups.

RegardingIn addition pull-out to the uniaxial tests, diff tensileerent methods test, othe andr test procedures methods werewere proposed adopted dependingto investigate on thethe purposedurability of of each the study. FRCM For coupons instance, such Butler as three- et al. and [27] andfour-point Yin et al.bending [21] used tests two [21,28,31]. different However, pull-out set-upsthese tests to investigate do not provide the bond direct between information matrix about and fiber.the effect The of bond conditioning behaviorof on FRCM the matrix–fiber composites bond was alsobehavior studied since using several single- pa andrameters, double-lap such direct-shear as the matrix tests, tensile which and allow compressive for the study strength, of the interaction affect the betweenspecimen the behavior. composite Therefore, and substrate they were [33,36 mainly]. used to evaluate the durability of mortar (without textile) [32–35] or they were coupled with uniaxial tensile and pull-out tests to gain further 3.information Conditioning combining Environments the results obtained [21,28]. TheRegarding best way pull-out to evaluate tests, different the long-term methods behavior and procedures of materials were is to proposed expose them depending to the actualon the environment.purpose of each However, study. thisFor procedureinstance, Butler requires et al. several [27] and years Yin and et would al. [21] hinder used two the use different of the materialpull-out untilset-ups its to conclusion. investigate Inthe addition, bond between the di ffmatrixerences an ind fiber. the weathering The bond behavior conditions of fromFRCM one composites place to anotherwas also and studied the variability using single- of the and exposures double-lap make direct-shear this procedure tests, di ffiwhichcult toallow be standardized. for the study Indeed, of the non-repeatabilityinteraction between of the the compos data andite excessiveand substrate duration [33,36]. of actual environmental exposures led to the development of accelerated laboratory tests. Accelerated ageing was adopted in an attempt to mimic actual3. Conditioning environments Environments and to predict the long-term behavior of the material [37]. Currently,The best way the USto evaluate Acceptance the Criteria long-term AC behavior 434 (2018) of [23 materials] and Italian is to Initial expose Type them Testing to the CSLLPP actual (2019)environment. [24] are However, the only guidelines this procedure available requires regarding several test years methods and forwould FRCM hinder composites. the use Theseof the guidelinesmaterial until recommend its conclusion. test methods In addition, to evaluate the differences the durability in the weathering of the FRCM conditions including from freeze–thaw one place cycles,to another ageing and in high-moisturethe variability condition of the exposures (hygrothermal make environment), this procedure concentrated difficult to solutions be standardized. (chloride andIndeed, sulfate non-repeatability solutions and of alkaline the data environments), and excessive and duration even expositionof actual environmental to fuel (not covered exposures in this led paper).to the development Since these guidelinesof accelerated are laboratory quite recent, tests. diff Acceleratederent test methods ageing was were adopted adopted in in an the attempt past by to somemimic researchers actual environments [14,22,26,36 and,38 ]to and predict the resultsthe long-term obtained behavior could not of the be easilymaterial compared [37]. due to the differencesCurrently, in ageing the US temperature, Acceptance Criteria number AC of 434 cycles, (201 and8) [23] solution and Italian type. Initial In this Type section, Testing the CSLLPP results found(2019) in[24] the are literature the only are guidel discussesines andavailable critically regarding compared test in methods an attempt for toFRCM gain generalcomposites. indications These onguidelines the long-term recommend performance test methods of FRCM to andevaluate TRM th composites.e durability of the FRCM including freeze–thaw cycles, ageing in high-moisture condition (hygrothermal environment), concentrated solutions 3.1. Freeze–Thaw Cycles (chloride and sulfate solutions and alkaline environments), and even exposition to fuel (not covered in thisFour paper). methods Sincewere these employed guidelines in are the quite literature recent, to different evaluate test the methods durability were of adopted FRCM composites in the past subjectedby some researchers to freeze–thaw [14,22,26,36,38] cycles. All theand methods the results recommended obtained could performing not be easily thawing compared in water due or to in the a humiddifferences environment in ageing to temperature, increase the degree number of saturationof cycles, ofand the solution materials, type. which In th canis playsection, a fundamental the results found in the literature are discusses and critically compared in an attempt to gain general indications on the long-term performance of FRCM and TRM composites.

3.1. Freeze–Thaw Cycles Appl. Sci. 2020, 10, 1714 4 of 23 role in the durability of the material. In general, if the material saturation is lower than a certain threshold, the material is considered resistant to frost, i.e., dry materials should not be affected by frost [22]. ASTM C666 [39] recommended a low temperature range of (4 to 18 C) with a short cycle length − ◦ of 2–5 h. EN 12467 [38] adopted freezing at 20 C for 1–2 h and thawing at 20 C for 1–2 h. Pekmezci − ◦ ◦ et al. [14] performed 6 h of freezing at 25 C followed by 2 h of thawing in water at 20 C. Finally, AC − ◦ ◦ 434 (2018) [23,28] and CSLLPP (2019) [24] utilized a wide temperature range and a long cycle period (4 h at 18 C and 12 h at approximately 38 C). − ◦ ◦ The combination of cycle duration and number of cycles affects the material response. Available studies reported that high number of cycles (more than 100 cycles) is required to develop small deteriorations in TRC composites [22,26]. Accordingly, the methods proposed by ASTM C666 [39] and EN 12467 [38], which adopt short cycle lengths allowing for performing a high number of cycles within a short time frame, can be suitable candidates to evaluate the long-term behavior of FRCM. However, it is important to mention that the main purpose of these methods is to determine the resistance of the material to the repeated cycles of freezing and thawing. They can be used to compare between different FRCM composites, but they are not intended to provide a quantitative measure of the material service life [22,39].

3.2. Hygrothermal Environment The simplest and most utilized method to investigate the durability of FRCM composites is accelerated ageing in water, which consists in the immersion of the specimens in hot water for a certain period. However, AC 434 (2018) [23] and Italian Initial Type Testing CSLLPP (2019) [24] recommend ageing in 100% RH at approximately 38 ◦C for 1000 and 3000 h. Donnini et al. [33] considered the recommendations provided by ASTM D7705 [40] to evaluate the resistance of FRP bars exposed to alkaline environments and immersed the FRCM in water heated at 60 ◦C. EN 12467 [38], was also used to investigate the effect of moisture on inorganic-matrix composites. EN 12467 requires water immersion at 60 ◦C for 56 days to evaluate the durability of fiber–cement flat sheets [38]. Moreover, EN 12467 specifies heat rain (HR) tests in which the specimen is heated to 60 ◦C for 3 h using radiant heat and then cooled down to room temperature using water spray for 3 h. De Munck et al. [26] considered this method to study the durability of a TRC composite comprising a glass textile and a cement-based matrix. However, in order to reduce the cycle period, the heating time was reduced to 1 h and the cooling time to 30 min. In addition, the water spray was replaced by immersion in water at 15 ◦C. Finally, Ceroni et al. [35] investigated the effects of water immersion at various temperatures (23–40 ◦C) with different exposure periods (5–74 days) on the FRCM-concrete bond behavior. Although Ceroni et al. [35] found that increasing the ageing temperature from 23 to 30 ◦C did not significantly influence the bond behavior observed, yet this finding should not be extended to higher temperature due to the lack of studies investigating the effect of the ageing temperature on the FRCM long-term behavior. Studies on FRP composites showed that using high temperature can increase water diffusion and increase the degradation rate [41] of GFRP reinforcing bars. These observations allowed for defining accelerated testing procedures that require ageing temperature up to 60 ◦C and a conditioning period of approximately 60 days [6,37,42]. Since in FRCM composites the organic fraction is usually quite limited if not absent, exposure to a relatively high temperature (close to the glass transition temperature of the resin used in FRP) does not represent an issue. Therefore, an ageing temperature of 60 ◦C could be considered adequate for FRCM accelerated testing. It should be noted, however, that the maximum exposure temperature specified by AC 434 (2018) [23] and CSLLPP (2019) [24] is approximately 38 ◦C. Short-term high-temperature tests shall be carried out according to CSLLPP (2019), and the exposure temperature shall be decided by the manufacturer [24]. The limited data available in the literature did not allow to identify a clear effect of accelerating tests in water of FRCM composites. Therefore, further and detailed investigations should be performed. Appl. Sci. 2020, 10, 1714 5 of 23

Another method employed to evaluate the effect of moisture on the durability of FRCM is the exposure to a series of wet–dry cycles. According to EN 12467 [38], 50 cycles of immersion in water at ambient temperature (more than 5 ◦C) for 18 h followed by drying in ventilated oven at 60 ◦C and 20% RH for 6 h are needed to investigate the behavior of fiber-cement flat sheets. Franzoni et al. [36] found that 8 h are enough to saturate masonry specimens with externally bonded SRG strips via capillary absorption and exposure to 60 ◦C in a ventilated oven is enough to dry them. However, since the purpose of the study was investigating salt crystallization within the interfaces, two days of wetting and two days of drying at 60 ◦C were utilized to ensure adequate salt accumulation. In another study, Franzoni et al. [43] utilized 3 days of drying. Donnini et al. [33] utilized two days of wetting and two days of drying in a ventilated oven at 60 ◦C. Yin et al. [21] wetted the FRCM specimens for 12 h at room temperature and dried them again at room temperature for 12 h. Other methods for performing wet–dry cycles can be found in the literature regarding FRP composites (see e.g., [44]). It should be noted that there is no specific standard to perform wet–dry cycles on FRCM composites. Lab tests should be performed to determine the time required to fully saturate and dry the specimens for a given exposure temperature to correctly conduct these types of test.

3.3. Concentrated Solutions Sulfate and chloride attack are one of the common weathering environments that can affect the long-term behavior of inorganic materials. AC 434 (2018) [23] and CSLLPP (2019) [24] require uniaxial tensile tests of FRCM coupons continuously immersed in a saline solution reproducing the seawater conditions. In order to mimic the effect of marine environment (chloride attack) that generally triggers this damage, researchers immersed FRCM specimens in seawater or in aqueous solutions containing 3.5 wt. % (i.e., percentage by weight) of sodium chloride (NaCl) to reproduce a common seawater salinity [23,28,33,34]. Exposure periods of 1000–3000 h were adopted in most cases. Donnini et al. [33] subjected FRCM-brick joints to a series of wet–dry cycles in saline solution containing 3.5 wt. % of NaCl, exposing them to 60 ◦C to accelerate the process rate. Yin et al. [21] utilized saline solution with 5 wt. % of NaCl companied with a series of wet–dry or freeze–thaw cycles. Standards have adopted different procedures to investigate the resistance of porous materials, such as stone and bricks, to the sulfate attack. EN 12370 [45] suggests total immersion in solution containing 14 wt. % of sodium sulfate followed by oven drying at 105 ◦C. Rilem MS-A.1 [46] suggests using 10 wt. % of sodium sulfate and 10 wt. % of sodium chloride These conditions were considered too aggressive by Franzoni et al. [43], which adopted a solution containing 2 wt. % of NaCl and 8 wt. % of sodium sulfate decahydrate (Na SO 10H O) to avoid FRCM failure before the end of the cycles. During the 2 4· 2 ageing procedure, FRCM-masonry joints were laid down and immersed up to 20 mm (measured from the lower face), so that the solution was not in contact with the FRCM strip. Subsequently, they were dried in a ventilated oven at 60 ◦C for 3 days. This procedure was repeated to obtain six cycles [43]. The results indicated that salt accumulation on the FRCM matrix was comparable to that found in old masonry buildings subjected to rising damp and marine aerosol spray for centuries. Indeed, in addition to the chloride and sulfate attack, durability of FRCM used to strengthen masonry structures may be also influenced by salt carried by rising damp. Salt accumulation and crystallization within the matrix-fabric and matrix-substrate interfaces can significantly affect the bond at these interfaces. FRCM durability can be also influenced by alkaline environments, which can have a significant effect on glass fibers embedded within the inorganic matrix [41]. The effect of exposure to alkaline environments was investigated by immersing FRCM coupons in alkaline solutions made of various components for a period ranging between 1000 and 3000 h. Arboleda et al. [28] used a solution with pH > 12 containing calcium hydroxide (Ca(OH2)), sodium hydroxide (NaOH), and potassium hydroxide (KOH). Nobili [34] utilized a solution with pH = 10 made with sodium bicarbonate. Instead of immersing FRCM specimens in alkaline solutions, Butler et al. [47] used three types of mortars with different level of alkalinity to evaluate their influence on the embedded glass fabric textile. The Appl. Sci. 2020, 10, 1714 6 of 23 studies that investigated the durability of FRCM in alkaline environments clearly showed that the results depend on the alkalinity level, exposure temperature, and exposure period. Therefore, a shared standard procedure including also wet–dry cycles is needed to allow the comparison of the results obtained.

4. Durability of Composite The durability of FRCM composites depends on the long-term behavior of their components (matrix and fiber textile) and of the interfaces between them. The durability of each component and of the matrix–fiber and composite–substrate interfaces is discussed in the following sections based on the studies available in the literature. Results are presented in term of retained (or residual) strength ratio, which is defined as the ratio σf/σfu between the strength σf of the conditioned specimens (i.e., exposed to the conditioning environment) and the strength σfu of the unconditioned specimens.

4.1. Durability of Composite Matrix The inorganic matrix employed in FRCM composites is generally cement-based or lime-based, although other types can be found in the literature (see e.g., [14,19–21]). This section focuses on the durability of cement-based and lime-based mortars. In the figures presented in this section, the blue color indicates cement-based mortars, whereas the orange color indicates lime-based mortars. Details of the tests included in the figures below are provided in Table A1. Exposing FRCM matrices to freeze–thaw cycles, alkaline and sulfate attack may influence its durability. For example, the water present within the matrix voids may freeze when exposed to low temperatures, increasing its volume by 9% [22]. Consequently, it may induce damage and cracks if the internal stresses exceed the matrix tensile strength. However, researches available regarding freeze–thaw of FRCM composites did not generally report the specimen moisture content. Figure2 presents the e ffect of the freeze–thaw cycles on the retained cracking (tensile) strength of different FRCM matrices, which was determined by uniaxial tensile test on FRCM [14,28] and TRC [22,26] coupons. The details of freeze–thaw cycles utilized in each study are indicated in the legend of Figure2. All specimens subjected to less than 100 cycles showed an improvement of the matrix cracking strength. However, a large scatter of the results was observed [22]. Moreover, Colombo et al. [19] reported a continuous increase in the matrix tensile strength from 50 to 100 cycles, whereas De Munck et al. [26] observed a reduction for 100 cycles. The difference in the obtained results is mainly attributed to the different performance of the adopted matrices. Colombo et al. [22] considered a high-strength cementitious matrix (average cubic compressive strength and flexural strength equal to 97.5 MPa and 13.6 MPa, respectively [22]), whereas De Munck et al. [26] used a cement-based matrix with average cubic compressive strength and flexural strength of 29.6 MPa and 5.0 MPa, respectively [26]. Studies reported that concrete with a high strength is less susceptible to freeze–thaw cycles due to its low permeability [48,49], which can explain the better performance of the matrix with the highest compressive and tensile strength. In addition, the range of temperature and the period of the freeze–thaw cycles adopted by Colombo et al. [22] were shorter and smaller, respectively, than those adopted by De Munck et al. [26]. However, after 100 cycles, Figure2 shows that the retained matrix cracking strength decreased, which was attributed to the occurrence of microcracks [22,26]. Yin et al. [21] investigated the combined effects of freeze–thaw cycles ( 18 C to 5 C with 3 h − ◦ ◦ cycle length) and saline environment (5 wt. % NaCl) on the cracking strength of TRC composites using four-point bending test. The obtained results are not shown in Figure2, where the e ffect of only freeze–thaw cycles is considered. Yin et al. [21] reported a retained cracking strength after exposing to 50, 70, and 90 cycles equal to 0.98, 0.91, and 0.86, respectively. Similar to the result shown in Figure2, freeze–thaw cycles did not have a significant e ffect for small number of cycles. However, the degradation gradually increased with increasing the number of cycles. The degradation developed with 90 cycles was slightly higher than the one reported by De Munck et al. [26] with a similar number of cycles (100 cycles). This result can be attributed to: (i) presence of the sodium chloride that may Appl. Sci. 2020, 10, 1714 7 of 25 Appl. Sci. 2020, 10, 1714 7 of 23 with 90 cycles was slightly higher than the one reported by De Munck et al. [26] with a similar number of cycles (100 cycles). This result can be attributed to: (i) presence of the sodium chloride that may have increased the degradation process rate and (ii) test method employed (four-point bending test vs. have increased the degradation process rate and (ii) test method employed (four-point bending test uniaxial tensile test). vs. uniaxial tensile test).

Figure 2.2. EffectEﬀect of freeze–thaw cycles on thethe matrixmatrix tensiletensile strengthstrength (blue(blue markermarker == cement-basedcement-based matrix, orange marker = lime-based matrix).

Chlorides andand sulfates sulfates can can interact interact with with some some components components of the matrixof thesuch matrix as calcium such as hydroxide calcium (Ca(OH)hydroxide2) and(Ca(OH) aluminum2) and aluminum oxide (Al2O oxide3) leading (Al2O to3) leading a reduction to a reduction of the matrix of the durability matrix durability [28]. Figure [28].3a presentsFigure 3a the presents effect ofthe continuous effect of continuous immersion immersion in a saline in solution a saline (seawater solution (seawater or 3.5 wt. or % of3.5 NaCl) wt. % onof theNaCl) retained on the strength retained ofstrength the FRCM of the matrices FRCM studied.matrices Nobilistudied. et Nobili al. [34 ]et and al. [34] Donnini and Donnini et al. [33 ]et utilized al. [33] three-pointutilized three-point bending testsbending of lime-based tests of lime-based mortars specimens mortars accordingspecimens to according EN 1015 [ 32to]. EN Arboleda 1015 [32]. [28] studiedArboleda the [28] cracking studied strength the cracking of cement-based strength of FRCM cement-based coupons usingFRCM uniaxial coupons tensile using tests. uniaxial The resultstensile showedtests. The that results although showed Nobili that et although al. [34] utilized Nobili aet lower al. [34] ageing utilized temperature a lower ageing than Donninitemperature et al. than [33], theDonnini degradation et al. [33], of the the mortar degradation was higher of the (up mortar to 60%, was see Figurehigher3 ).(up This to controversial60%, see Figure result 3). might This becontroversial attributed toresult the dimightfference be attributed in the porosity to the and diffe mechanicalrence in the properties porosity and of the mechanical mortar investigated properties byof the the mortar two researchers. investigated A by mortar the two with researchers. high porosity A mortar such with as natural high porosity lime-based such mortar as natural could lime- be morebased influencedmortar could by be saline more environments influenced by than saline hydraulic environments lime or than cementitious hydraulic mortars lime or [cementitious33]. Further studiesmortars are [33]. required Further to studies relate the are physical required nature to relate of the the FRCM physical matrices nature with of theirthe FRCM durability matrices in various with environmentaltheir durability conditions. in various Arboledaenvironmental [28] reported conditions. a remarkable Arboleda increase [28] reported in the cracking a remarkable strength increase of the cement-basedin the cracking matrix. strength This of resultthe cement-based was not significantly matrix. This affected result even was whennot significantly the exposure affected period even was extendedAppl.when Sci. the 2020, toexposure 10, 3000 1714 h. period was extended to 3000 h. 8 of 25 Figure 3b shows the effect of wet–dry cycles in saline solutions on the retained strength of the matrices. Regarding the cementitious matrix, Yin et al. [21] performed 12 h of wetting in a saline solution (5 wt. % of NaCl) followed by 12 h of drying at room temperature on TRC coupons with cement-based matrix, which were subsequently tested using a four-point bending test set-up. The results showed that the matrix flexural strength progressively decreased with increasing the number of cycles. Regarding the lime mortar, Donnini [33] reported similar residual flexural strengths for specimens subjected to continuous immersion and wet–dry cycles, showing a percentage reduction of approximately 10%.

(a) (b)

Figure 3. EEffectﬀect of saline environments on thethe matrixmatrix tensiletensile strength:strength: (a) continuous immersion and ((b)) wet–drywet–dry cyclescycles (blue(blue binbin == cement-based ma matrixtrix and orange bin = lime-basedlime-based matrix). matrix).

FigureAlkali-aggregate3b shows thechemical e ﬀect ofreaction, wet–dry which cycles is inthe saline reaction solutions between on thethe retainedpotassium strength and sodium of the matrices.oxide (K2O Regarding and Na2O) the of the cementitious cement and matrix, the reactive Yin et silica al. [21 (SiO] performed2) present in 12 some h of wettingtypes of inaggregate, a saline may affect glass micro-fibers (if present) dispersed within the mortar [28]. The effect of matrix immersion in alkaline solution on the flexural [34] and uniaxial tensile strength [28] is presented in Figure 4. It can be noticed that the retained flexural strength of the lime-based mortar decreased to approximately 55%, whereas the retained tensile strength of the cement-based matrix was not significantly affected (a slight increase, lower than 10%, was observed).

Figure 4. Effect of the alkaline environment on the matrices (blue bin = cement-based matrix and orange bin = lime-based matrix).

Finally, studies on the effect of immersion of the matrix in deionized water, which is a conditioning typically employed for GFRP reinforcing bars [41], showed that this type of exposure can improve the performance of the cement-based mortar if it is followed by an appropriate drying period due to the resumption of the hydration process [33,35]. It should be noted that the mechanical properties of the matrix affect the FRCM behavior when it is still not cracked. The results gathered show that the exposure of the FRCM to some harsh environmental conditions may lead to the formation of microcracks, which in turn affect the cracking strength and stiffness of the composite. In addition, matrix microcracks can promote the penetration of water and external agents into the composite.

4.2. Durability of the Fibers Textiles used in FRCM can be made of carbon, AR-glass, PBO, aramid fibers, and steel (in the case of SRG). This section is dedicated to the durability of textile. Details of the tests included in the figures below are provided in Table A1. Although carbon fiber showed good durability [50,51], it has a high cost in comparison with other fibers such as AR-glass or basalt. Basalt fiber is often compared Appl. Sci. 2020, 10, 1714 8 of 25

Appl. Sci. 2020, 10, 1714 8 of 23 solution (5 wt. % of NaCl) followed by 12 h of drying at room temperature on TRC coupons with cement-based matrix, which were subsequently tested using a four-point bending test set-up. The results showed that the matrix(a) flexural strength progressively decreased with(b) increasing the number of cycles. Regarding the lime mortar, Donnini [33] reported similar residual flexural strengths for Figure 3. Effect of saline environments on the matrix tensile strength: (a) continuous immersion and specimens subjected to continuous immersion and wet–dry cycles, showing a percentage reduction of (b) wet–dry cycles (blue bin = cement-based matrix and orange bin = lime-based matrix). approximately 10%. Alkali-aggregate chemical reaction, which is the reaction between the potassium and sodium oxide Alkali-aggregate chemical reaction, which is the reaction between the potassium and sodium (K2O and Na2O) of the cement and the reactive silica (SiO2) present in some types of aggregate, may oxide (K2O and Na2O) of the cement and the reactive silica (SiO2) present in some types of aggregate, affect glass micro-fibers (if present) dispersed within the mortar [28]. The effect of matrix immersion in may affect glass micro-fibers (if present) dispersed within the mortar [28]. The effect of matrix alkaline solution on the flexural [34] and uniaxial tensile strength [28] is presented in Figure4. It can immersion in alkaline solution on the flexural [34] and uniaxial tensile strength [28] is presented in be noticed that the retained flexural strength of the lime-based mortar decreased to approximately Figure 4. It can be noticed that the retained flexural strength of the lime-based mortar decreased to 55%, whereas the retained tensile strength of the cement-based matrix was not significantly affected (a approximately 55%, whereas the retained tensile strength of the cement-based matrix was not slight increase, lower than 10%, was observed). significantly affected (a slight increase, lower than 10%, was observed).

FigureFigure 4. EffectEﬀect of of the the alkaline environment on on the ma matricestrices (blue bin = cement-basedcement-based matrix matrix and and orangeorange bin bin == lime-basedlime-based matrix). matrix).

Finally,Finally, studiesstudies on on the the eff ecteffect of immersion of immersion of the of matrix the inmatrix deionized in deionized water, which water, is a conditioningwhich is a conditioningtypically employed typically for employed GFRP reinforcing for GFRP bars reinforcing [41], showed bars that [41], this showed type of that exposure this type can of improve exposure the canperformance improve the of the performance cement-based of the mortar cement-based if it is followed mortar by if anit is appropriate followed by drying an appropriate period due drying to the periodresumption due to of the the resumption hydration process of the hydration [33,35]. process [33,35]. ItIt should should be be noted noted that that the the mechanical mechanical properties properties of of the the matrix matrix affect affect the the FRCM FRCM behavior behavior when when itit is is still not cracked. The The results results gathered gathered show show that that the the exposure exposure of of the the FRCM FRCM to to some some harsh harsh environmentalenvironmental conditions may lead lead to to the the formation formation of of microcracks, microcracks, which which in in turn turn affect affect the the cracking cracking strengthstrength and stistiffnessffness ofof thethe composite.composite. InIn addition,addition, matrix matrix microcracks microcracks can can promote promote the the penetration penetration of ofwater water and and external external agents agents into into the the composite. composite.

4.2.4.2. Durability Durability of of the the Fibers Fibers TextilesTextiles used in FRCM can be made of carbon, AR-glass, PBO, aramid fibers, fibers, and and steel steel (in (in the the casecase of of SRG). SRG). This This section section is is dedicated to to the durabili durabilityty of textile. Details Details of of the the tests tests included included in in the the figuresfigures below are provided inin TableTable A1 A1.. AlthoughAlthough carboncarbon fiber fiber showed showed good good durability durability [50 [50,51],,51], it it has has a ahigh high cost cost in in comparison comparison with with other other fibers fibers such such as as AR-glass AR-glass or or basalt. basalt. Basalt Basalt fiber fiber is is often often compared compared to E-glass and AR-glass due to the similarities in the chemical structure [51,52]. Limited results on the durability of PBO fibers employed in the civil engineering field can be found in the literature. AR-glass fiber is commonly used in civil engineering applications due to its availability and low cost. For these reasons, more results are available regarding the durability of AR-glass fiber than of other types of fiber. In the figures presented in this section, the blue color indicates tests at room temperature, whereas the orange color indicates tests at temperature equal to 60 ◦C. Appl. Sci. 2020, 10, 1714 9 of 25

to E-glass and AR-glass due to the similarities in the chemical structure [51,52]. Limited results on the durability of PBO fibers employed in the civil engineering field can be found in the literature. AR- glass fiber is commonly used in civil engineering applications due to its availability and low cost. For Appl.these Sci. reasons,2020, 10 ,more 1714 results are available regarding the durability of AR-glass fiber than of other 9types of 23 of fiber. In the figures presented in this section, the blue color indicates tests at room temperature, whereas the orange color indicates tests at temperature equal to 60 °C. To thethe authors’authors’ bestbest knowledge,knowledge, specificspecific resultsresults ofof thethe freeze–thawfreeze–thaw cyclescycles long-termlong-term behaviorbehavior of fiberfiber textilestextiles employedemployed in FRCMFRCM compositescomposites are not availableavailable in thethe literature.literature. Similarly, no resultsresults werewere foundfound regardingregarding thethe eeffectffect ofof sulfatesulfate exposures.exposures. The retainedretained tensiletensile strength strength of of AR-glass AR-glass fiber fiber textiles textiles subjected subjected to to various various combined combined immersion immersion in deionizedin deionized water water and and temperature temperature (hygrothermal (hygrothermal environments) environments) observed observed by diff erentby different research research groups isgroups shown is inshown Figure in5 .Figure Portal 5. et Portal al. [ 51 et] reportedal. [51] reported no difference no difference in the tensile in the capacity tensile capacity after 10 daysafter 10 of conditioningdays of conditioning at 20 ◦C. at However, 20 °C. However, Hristozov Hristozov et al. [53] et reported al. [53] areported 15% reduction a 15% reduction after 21 days after of 21 ageing. days Whenof ageing. the ageingWhen the temperature ageing temperature was raised was to 60raised◦C, to both 60 Portal°C, both et Portal al. [51 et] and al. [51] Hristozov and Hristozov et al. [53 et] noticedal. [53] noticed a clear reductiona clear reduction in the strength, in the strength, even larger even than larger 60%. than Nonetheless, 60%. Nonetheless, Donnini [Donnini33] did not [33] see did a significantnot see a significant change of change the textile of the tensile textile capacity tensile at capacity the same attemperature the same temperature (i.e., 60 ◦C). (i.e., This 60 conflict °C). This in theconflict obtained in the results obtained may results be attributed may be attributed to the diff toerence the difference in the coating in the materials coating materials of the fibers of the used fibers by eachused researchby each research group. The group. fiber The used fiber by used Portal by et Portal al. [51 et] and al. [51] Hristozov and Hristozov et al. [53 et] wasal. [53] coated was bycoated 20% ofby styrene–butadiene20% of styrene–butadiene resin (SBR) resin and (SBR) vinylester, and vinylester, respectively, respectively, whereas the whereas fiber used the by fiber Donnini used [ 33by] wasDonnini coated [33] by was polyvinyl coated by alcohol polyvinyl (PVA). alcohol In addition (PVA). to In the addition effect of to coating, the effect the of di coating,fference the in difference the textile layoutin the textile and number layout ofand fiber number filaments of fiber in each filaments yarn mayin each also yarn have may influenced also have the influenced results. the results.

Figure 5. Retained tensile capacity ratio of AR-glass ex exposedposed to hygrothermal environments (blue bin = room temperature and orange bin = temperature equal to 60 °C). = room temperature and orange bin = temperature equal to 60 ◦C).

Figure6 6 presentspresents the retained tensile tensile capacity capacity rati ratioo of of AR-glass AR-glass fibers ﬁbers immersed immersed in seawater in seawater (3.5 (3.5wt. wt.% of % NaCl) of NaCl) for for1000 1000 h (~42 h (~42 days). days). At Atthe the room room temperature, temperature, (ranging (ranging between between 20 20 °C◦C and and 23 °C),◦C), Nobili etet al. al. [34 [34]] and and Hristozov Hristozov et al. et [ 53al.] reported[53] reported a small a degradationsmall degradation of the tensile of the capacity. tensile However,capacity. However, the degradation increased up to 40% when the conditioning temperature was raised to 60 Appl.the degradationSci. 2020, 10, 1714 increased up to 40% when the conditioning temperature was raised to 60 ◦C. 10 of 25 °C.

Figure 6. Retained tensile capacity ratio of AR-glass immersed in seawater (blue bin = room Figure 6. Retained tensile capacity ratio of AR-glass immersed in seawater (blue bin = room temperature temperature and orange bin = temperature equal to 60 °C). and orange bin = temperature equal to 60 ◦C). The effect of the alkaline solution on the tensile capacity of AR-glass textiles is presented in Figure 7. At room temperature, different levels of degradation were reported (minimum textile retained tensile capacity ratio lower than 40%) depending on the alkalinity of the solution, exposure period, and fiber coating materials [34,51,53]. Besides, the degradation level increased with increasing the alkalinity and the exposure period. When the conditioning temperature was raised to 60 °C, the degradation level was increased especially in the case of Portal et al. [51], where the material completely failed (dissolved) before performing the tensile test (these results are not shown in Figure 7 for this reason). Butler et al. [27,47] studied the durability of the AR-glass embedded in three different types of cementitious matrix with different level of alkalinity. Coupons were then not immerged in an alkaline solution as in the previous studies (for this reason, these results are not included in Figure 7). Results indicated continuous reduction in the tensile strength of the coupons with increasing the ageing period. Furthermore, the FRCM coupons made with high alkalinity matrix developed a brittle failure, as explained in Section 4.3. It should be noted that the accelerated ageing environments considered to study the durability of glass fibers represent extremely harsh conditions that are never present in real applications. Therefore, detailed investigations on the effect of these harsh conditionings on the fiber coatings, which protect the fiber from external attacks, should be carried out to correctly understand the results obtained.

Figure 7. Retained tensile capacity ratio of AR-glass exposed to alkaline environments (blue bin = room temperature and orange bin = temperature equal to 60 °C).

Regarding the durability of steel cords employed in SRG composites, unlike the numerous studies on the corrosion of galvanized steel rebar in concrete [54–56], there is limited research addressing the corrosion of steel cords embedded within inorganic-matrix [43]. The performance of steel fibers can be highly dependent on the environment and duration of the exposure. For instance, long exposure periods to an environment containing chloride ions can cause strong steel corrosion. Franzoni et al. [36] investigated the performance of steel SRG-masonry joints under wet–dry cycles Appl. Sci. 2020, 10, 1714 10 of 25

Figure 6. Retained tensile capacity ratio of AR-glass immersed in seawater (blue bin = room temperature and orange bin = temperature equal to 60 °C).

The effect of the alkaline solution on the tensile capacity of AR-glass textiles is presented in Appl.Figure Sci. 7.2020 At, 10 room, 1714 temperature, different levels of degradation were reported (minimum textile10 of 23 retained tensile capacity ratio lower than 40%) depending on the alkalinity of the solution, exposure period, and fiber coating materials [34,51,53]. Besides, the degradation level increased with increasing the alkalinityThe effect and of the exposure alkaline solutionperiod. When on the the tensile conditioning capacity temperature of AR-glass was textiles raised is presentedto 60 °C, the in degradationFigure7. At room level temperature, was increased di ff erentespecially levels ofin degradationthe case of werePortal reported et al. (minimum[51], where textile the material retained completelytensile capacity failed ratio (dissolved) lower than before 40%) performing depending the ontensile the alkalinitytest (these ofresults the solution, are not shown exposure in Figure period, 7 forand this fiber reason). coating Butler materials et al. [2 [347,47],51, 53studied]. Besides, the durability the degradation of the AR-glass level increased embedded with in increasingthree different the typesalkalinity of cementitious and the exposure matrix period.with different When level the conditioning of alkalinity. temperatureCoupons were was then raised not immerged to 60 ◦C, the in andegradation alkaline solution level was as increasedin the previous especially studies in the (for case this of reason, Portal etthese al. [51 results], where are the not material included completely in Figure 7).failed Results (dissolved) indicated before continuous performing reduction the tensile in the test tensile (these strength results of are the not coupons shown with in Figure increasing7 for this the ageingreason). period. Butler Furthermore, et al. [27,47] studied the FRCM the durabilitycoupons made of the with AR-glass high alkalinity embedded matrix in three developed different typesa brittle of failure,cementitious as explained matrix within Section different 4.3.level of alkalinity. Coupons were then not immerged in an alkaline solutionIt should as in thebe noted previous that studies the accelerated (for this ageing reason, en thesevironments results are considered not included to study in Figure the durability7). Results of glassindicated fibers continuous represent reductionextremely inharsh the tensileconditions strength that ofare the never coupons present with in increasingreal applications. the ageing Therefore, period. detailedFurthermore, investigations the FRCM on couponsthe effect made of these with hars highh conditionings alkalinity matrix on the developed fiber coatings, a brittle which failure, protect as theexplained fiber from in Section external 4.3 attacks,. should be carried out to correctly understand the results obtained.

Figure 7. Retained tensile capacity ratio of AR-glass exposedexposed to alkalinealkaline environmentsenvironments (blue binbin == room temperature and orange bin = temperature equal to 60 °C). room temperature and orange bin = temperature equal to 60 ◦C).

RegardingIt should be the noted durability that the acceleratedof steel cords ageing employ environmentsed in SRG considered composites, to studyunlike the the durability numerous of studiesglass fibers on representthe corrosion extremely of galvanized harsh conditions steel reba thatr arein neverconcrete present [54–56], in real there applications. is limited Therefore, research addressingdetailed investigations the corrosion on of the steel effect cords of theseembedded harsh conditioningswithin inorganic-matrix on the fiber [43]. coatings, The performance which protect of steelthe fiber fibers from can external be highly attacks, dependent should on be the carried enviro outnment to correctly and duration understand of the theexposure. results For obtained. instance, long Regardingexposure periods the durability to an environment of steel cords containing employedin chloride SRG composites, ions can cause unlike strong the numerous steel corrosion. studies Franzonion the corrosion et al. [36] of galvanizedinvestigated steel the rebarperformance in concrete of steel [54– 56SRG-masonry], there is limited joints research under wet–dry addressing cycles the corrosion of steel cords embedded within inorganic-matrix [43]. The performance of steel fibers can be highly dependent on the environment and duration of the exposure. For instance, long exposure periods to an environment containing chloride ions can cause strong steel corrosion. Franzoni et al. [36] investigated the performance of steel SRG-masonry joints under wet–dry cycles exposed to a saline solution containing 8% weight ratio of Na SO 10H O. In this study, steel fiber did not show significant 2 4· 2 changes. However, in another study by the same authors [43], steel fibers were corroded when 2 wt. % of NaCl was added to the same solution and the number of cycles was increased from 4 to 6. The limited number of results available in the literature indicate that further studies are needed to gain a deeper understanding of the long-term behavior of steel cords under various environments.

4.3. Durability of the Matrix–Fiber Interface Capacity The effectiveness of externally bonded reinforcements depends on the stress-transfer between its components and between the reinforcements and the substrate. When a single layer of fiber is employed, externally bonded FRCM composites have two main interfaces, i.e., the matrix–fiber interface and the matrix-substrate or composite–substrate interface [57]. Although the long-term bond behavior of the various interfaces of an inorganic-matrix composite should be studied with bond tests, some studies Appl. Sci. 2020, 10, 1714 11 of 23 available in the literature derived information on the effect of various environmental factors on the matrix–fiber interface from uniaxial tensile tests of composite coupons [22,28,34]. In particular, the degradation of the matrix–fiber interface was assessed by direct observation using scanning electron microscopy (SEM) [26] or evaluated by studying the differences in the load response of control and conditioned specimens that can be explained by the matrix–fiber bond behavior. In this section, the effect of various environments on the matrix–fiber bond behavior, either directly studied with bond tests or derived by uniaxial tensile test results, is discussed, whereas the durability of the composite–substrate interface is discussed in Section 4.4. Details of the tests included in the figures below are provided in Table A1. Figure8 presents the e ffect of freeze–thaw cycles on the ultimate strength of FRCM coupons comprising different types of textile. The range of cycles adopted by each research group was discussed in Section 3.1. Colombo et al. [22] investigated the effect of freeze–thaw cycles on uncracked specimens and on specimens cracked by applying a uniaxial tensile load 20% higher than the cracking load. It can be noticed that the retained ultimate strength of the uncracked specimens progressively decreases with increasing the number of cycles until it reaches 0.8 at the end of 500 cycles. However, cracked specimens showed fluctuating results and a clear trend could not be identified. Arboleda [28] reported a slight increment (approximately 10%) in the ultimate strength of PBO-FRCM coupons, whereas the ultimate strength of carbon-FRCM coupons did not change significantly. Pekmezci et al. [14] reported a slight reduction (approximately 16%) in the case of a biaxial fabric. De Munck et al. [26] reported a small reduction (approximately 16%) in the ultimate strength of TRC coupons with AR-glass fibers after 100 cycles. Anyhow, freeze–thaw cycles had a slight effect on the coupon ultimate strength, being the variation always lower than 20%. Appl. Sci. 2020, 10, 1714 12 of 25

Figure 8. EEffectﬀect of freeze–thaw cyclescycles onon thethe retainedretained ultimateultimate strengthstrength ratioratio ofof compositecomposite coupons.coupons.

YinThe eteffect al. [21 of] utilizedsaline environments a pull-out test on to the investigate matrix–fiber the combined bond behavior effectof was freeze–thaw investigated cycles by ( 18 C to 5 C with 3 h cycle duration) and saline environments (5 wt. % of NaCl) on the matrix–fiber immersing− ◦ FRCM◦ coupons in saline solutions (seawater or 3.5 wt. % of NaCl) and observing the bondvariation capacity. of the The ultimate results strength obtained obtained, in [21] are which not listed was attributed in Figure8 tosince the thedegradation authors considered of the matrix– the efiberffect interface of saline solution[28,34]. Figure and used 9 shows a diff erentthe retained test set-up. ultimate Similarly, strength Yin ratio et al. obtained [21] reported by FRCM not significant coupons eimmersedffect of the in ageing saline condition.solutions for different periods. Arboleda [28] reported an increase of the retained ultimateThe estrengthffect of saline for environmentsFRCM including on the PBO matrix–fiber and carbon bond fibers behavior with was cement-based investigated bymatrix. immersing This FRCMincrement coupons (larger in salinethan 40%) solutions was attributed (seawater orto 3.5the wt.continuous % of NaCl) hydration and observing of the mortar, the variation which of may the ultimateimprove strength the matrix–fiber obtained, bond which behavior was attributed leading to to the high degradation ultimate strengths. of the matrix–fiber Nobili et al. interface [34] observed [28,34]. Figurea slight9 showsreduction the (less retained than ultimate 20%) of strengththe retained ratio ultimate obtained strength by FRCM for couponsFRCM including immersed glass in saline fibers solutionsand lime-based for diff matrix.erent periods. It should Arboleda be noted [ 28that] reported a significant an increase scatter ofwas the observed retained among ultimate the strength results, forwhich FRCM indicate including the need PBO to and perform carbon a large fibers nu withmber cement-based of tests to obtain matrix. reliable This results. increment (larger than 40%)Yin was et attributed al. [21] investigated to the continuous the effects hydration of wetting of the in a mortar, saline solution which may (5 wt. improve % of NaCl) the matrix–fiber and drying bondat room behavior temperature leading on to the high bond ultimate behavior strengths. between Nobili a cementitious et al. [34] matrix observed and a a slight hybrid reduction glass-carbon (less thantextile, 20%) using of thea pull-out retained set-up. ultimate Since strength also the for effect FRCM of includingdrying was glass considered, fibers and these lime-based test results matrix. are not reported in Figure 9. However, no significant difference in the maximum pull-out load was observed after 90 and 120 cycles. Nevertheless, a certain degradation was observed after 150 cycles due to the crystallization of the salt that affected the matrix–fiber interface.

Figure 9. Effect of immersion in saline solutions on the retained ultimate strength ratio of fabric- reinforced cementitious matrix (FRCM) coupons.

The effect of moisture and wet–dry cycles on the bond capacity can be controversial. For not entirely hydrated matrix, they can be beneficial due to the continuation of the hydration process [35,36]. However, it should be noted that the continuation of the hydration process can cause a densification of the matrix next to the multi-filament yarn with the penetration of hydration products within the voids among the filaments, which may lead to a reduction of the matrix–fiber bond properties [26]. More studies are needed to identify the dominant effect among these two competing factors. Appl. Sci. 2020, 10, 1714 12 of 25

Figure 8. Effect of freeze–thaw cycles on the retained ultimate strength ratio of composite coupons.

The effect of saline environments on the matrix–fiber bond behavior was investigated by immersing FRCM coupons in saline solutions (seawater or 3.5 wt. % of NaCl) and observing the variation of the ultimate strength obtained, which was attributed to the degradation of the matrix– fiber interface [28,34]. Figure 9 shows the retained ultimate strength ratio obtained by FRCM coupons immersed in saline solutions for different periods. Arboleda [28] reported an increase of the retained ultimate strength for FRCM including PBO and carbon fibers with cement-based matrix. This increment (larger than 40%) was attributed to the continuous hydration of the mortar, which may improve the matrix–fiber bond behavior leading to high ultimate strengths. Nobili et al. [34] observed a slight reduction (less than 20%) of the retained ultimate strength for FRCM including glass fibers and lime-based matrix. It should be noted that a significant scatter was observed among the results, which indicate the need to perform a large number of tests to obtain reliable results. Yin et al. [21] investigated the effects of wetting in a saline solution (5 wt. % of NaCl) and drying Appl.at room Sci. 2020temperature, 10, 1714 on the bond behavior between a cementitious matrix and a hybrid glass-carbon12 of 23 textile, using a pull-out set-up. Since also the effect of drying was considered, these test results are not reported in Figure 9. However, no significant difference in the maximum pull-out load was observedIt should beafter noted 90 and that 120 a signiﬁcant cycles. Nevertheless, scatter was observed a certain among degradation the results, was whichobserved indicate after the150 need cycles to performdue to the a largecrystallization number of of tests the salt to obtain that affected reliable the results. matrix–fiber interface.

FigureFigure 9. 9. EffectEﬀect of of immersion immersion in saline in saline solutions solutions on the on retained the retained ultimate ultimate strength strength ratio of fabric- ratio of fabric-reinforced cementitiousreinforced matrix cementitious (FRCM) coupons. matrix (FRCM) coupons.

TheYin eteffect al. [21 of] moisture investigated and the wet–dry effects ofcycles wetting on th ine a bond saline capacity solution can (5 wt. be %controversial. of NaCl) and For drying not entirelyat room temperaturehydrated matrix, on the they bond can behavior be beneficial between due a cementitiousto the continuation matrix andof the a hybrid hydration glass-carbon process [35,36].textile, usingHowever, a pull-out it should set-up. be Since noted also that the the eff ectcontinuation of drying was of the considered, hydration these process test results can cause are not a densificationreported in Figure of the9 .matrix However, next noto the significant multi-filament di fference yarn in with the maximumthe penetration pull-out of hydration load was observedproducts withinafter 90 the and voids 120 cycles. among Nevertheless, the filament as, certainwhich degradationmay lead to was a reduct observedion afterof the 150 matrix–fiber cycles due tobond the propertiescrystallization [26]. ofMore the saltstudies that are affected needed the to matrix–fiber identify the interface. dominant effect among these two competing factors.The effect of moisture and wet–dry cycles on the bond capacity can be controversial. For not entirely hydrated matrix, they can be beneficial due to the continuation of the hydration process [35,36]. However, it should be noted that the continuation of the hydration process can cause a densification of the matrix next to the multi-filament yarn with the penetration of hydration products within the voids among the filaments, which may lead to a reduction of the matrix–fiber bond properties [26]. More studies are needed to identify the dominant effect among these two competing factors. For completely hydrated matrix, moisture and wet–dry cycles may not directly affect the bond behavior. However, by damaging the fibers (e.g., corrosion of steel cords or swelling of glass fibers [33,43]) and the matrix due to the expansion of water within the pores, they can lead to a decrease of the matrix–fiber bond properties [35,43]. The stress-transfer between the fiber and matrix can be also influenced by the alkalinity of the matrix, as discussed in Section 4.2. Butler et al. [27,47] utilized pull-out tests and uniaxial tensile tests to investigate the effect of the matrix alkalinity on the bond between cement-based matrices and AR-glass fibers. Specimens were aged in a humid chamber at 40 ◦C and 99% RH. One year of this condition was assumed to be equal to 50 years of exposure to the middle European climate. Three cementitious matrices with different level of alkalinity and hydration kinetics were employed. The low alkalinity matrix (M1) had a pH of 12.4 at the mixing time, which gradually reduced to 11.8 after 1 year. The pH of the high alkalinity matrix (M3) was 12.7. For the medium alkalinity matrix (M2), the pH value was between M1 and M3. The results of unconditioned specimens showed that the maximum pull-out force of specimens made with high alkalinity matrix (M3) was higher than the others due to the fast hydration process and homogeneous structure of calcium silicate hydrate (CSH) phase close to the fiber. However, an opposite trend was observed after ageing. Figure 10a shows the behavior of the maximum pull-out force ratio, which progressively decreased with proceeding the ageing period. The reduction was higher for specimens with matrix M2 than with matrix M1. Results of specimens with matrix M3 are not shown in Figure 10a because they failed at the formation of the first crack, without slippage of the fibers within the matrix. Appl. Sci. 2020, 10, 1714 13 of 25

For completely hydrated matrix, moisture and wet–dry cycles may not directly affect the bond behavior. However, by damaging the fibers (e.g., corrosion of steel cords or swelling of glass fibers [33,43]) and the matrix due to the expansion of water within the pores, they can lead to a decrease of the matrix–fiber bond properties [35,43]. The stress-transfer between the fiber and matrix can be also influenced by the alkalinity of the matrix, as discussed in Section 4.2. Butler et al. [27,47] utilized pull-out tests and uniaxial tensile tests to investigate the effect of the matrix alkalinity on the bond between cement-based matrices and AR- glass fibers. Specimens were aged in a humid chamber at 40 °C and 99% RH. One year of this condition was assumed to be equal to 50 years of exposure to the middle European climate. Three cementitious matrices with different level of alkalinity and hydration kinetics were employed. The low alkalinity matrix (M1) had a pH of 12.4 at the mixing time, which gradually reduced to 11.8 after 1 year. The pH of the high alkalinity matrix (M3) was 12.7. For the medium alkalinity matrix (M2), the pH value was between M1 and M3. The results of unconditioned specimens showed that the maximum pull-out force of specimens made with high alkalinity matrix (M3) was higher than the others due to the fast hydration process and homogeneous structure of calcium silicate hydrate (CSH) phase close to the fiber. However, an opposite trend was observed after ageing. Figure 10a shows the behavior of the maximum pull-out force ratio, which progressively decreased with proceeding the ageing period. The reduction was higher for specimens with matrix M2 than with matrix M1. Results Appl. Sci. 2020, 10, 1714 13 of 23 of specimens with matrix M3 are not shown in Figure 10a because they failed at the formation of the first crack, without slippage of the fibers within the matrix. The retainedretained ultimateultimate strength strength ratio ratio of correspondingof corresponding FRCM FRCM coupons coupons subjected subjected to uniaxial to uniaxial tensile tensiletest is shown test is inshown Figure in 10Figureb. The 10b. behavior The behavior observed observed is consistent is consistent with that with of pull-out that of tests,pull-out with tests, the withexception the exception that the ultimatethat the ultimate strength strength of the coupons of the coupons made with made matrix with M1 matrix increased M1 increased after 30 after days 30 of daysageing of dueageing to the due continuation to the continuati of theon matrix of the hydration matrix hydration process. process.

(a) (b)

Figure 10. EffectEﬀect of the matrix alkalinity on the (a) maximummaximum pull-out force and (b) ultimate tensile strength of the FRCM coupons tested by ButlerButler etet al.al. [[27,47].27,47].

4.4. Durability of the Composite–Substrate Bond Capacity Being employed as externally bonded reinfo reinforcements,rcements, FRCM FRCM can can debond debond at at the the composite– substrate interface. However, However, limited number of st studiesudies dedicated to investigating the durability of the composite–substrate bond are available inin the literature.literature. They are summarized in this section and details of the tests included in the figuresfigures belowbelow areare providedprovided inin TableTable A1 A1.. Sulfate attack can reduce the adhesion between the composite and substrate degrading the composite–substrate interfaceinterface [ 33[33,36].,36]. Salt Salt crystallization crystallization within within the the interface interface can alsocan reducealso reduce the bond the bondcapacity capacity and shift and the shift failure the from failure the matrix-fabric from the ma interfacetrix-fabric to the interface composite–substrate to the composite–substrate interface [33,43]. interfaceFigure 11 shows[33,43]. the Figure effect of11sulfate shows and the chloride effect of attack sulfate on theand load-carrying chloride attack capacity on ofthe FRCM-masonry load-carrying capacityjoints subjected of FRCM-masonry to direct-shear joints tests. subjected Donnini etto al.direct-shear [33] exposed tests. FRCM–masonry Donnini et al. joints [33] exposed with an AR-glass FRCM– masonrytextile and joints a lime-based with an matrixAR-glass to 10textile wet–dry and cyclesa lime-based in saline matrix solution to of10 sodium wet–dry chloride cycles (3.5in saline wt. % solutionof NaCl) of at sodium 60 ◦C. After chloride conditioning, (3.5 wt. % theof NaCl) specimens at 60 failed°C. After at the conditioning, composite–substrate the specimens interface failed and at the load-carrying capacity was reduced. Franzoni et al. [36] studied an SRG composite made of a lime-based matrix and steel cords externally bonded to masonry blocks. They found that four wet–dry cycles in a saline solution containing 8 wt. % of sodium sulfate decahydrate (Na SO 10H O) followed 2 4· 2 by drying at 60 ◦C did not have a significant effect on the load-carrying capacity. In addition, all the specimens developed interlaminar failure characterized by fiber slippage from the internal layer of matrix, which remained bonded to the substrate. However, when 2 wt. % of sodium chloride (NaCl) was added to the same solution and the number of cycles increased to 6, the load-carrying capacity decreased. The results analyzed showed that the reduction in the bond capacity and shifting of the failure mode to the composite–substrate interface require a high salt accumulation and crystallization within the interface, which may depend on the porosity of the matrix and on the radius of the pores. For instance, a matrix with pores radius bigger than the radius of substrate pores will allow the solution to pass through it. Consequently, the effect of sulfate and saline attack on the interface may not be as significant as its effect on the fibers and matrix [36]. However, this hypothesis needs to be confirmed by using matrices with different porosity and pore sizes. Appl. Sci. 2020, 10, 1714 14 of 25 the composite–substrate interface and the load-carrying capacity was reduced. Franzoni et al. [36] studied an SRG composite made of a lime-based matrix and steel cords externally bonded to masonry blocks. They found that four wet–dry cycles in a saline solution containing 8 wt. % of sodium sulfate decahydrate (Na2SO4∙10H2O) followed by drying at 60 °C did not have a significant effect on the load- carrying capacity. In addition, all the specimens developed interlaminar failure characterized by fiber slippage from the internal layer of matrix, which remained bonded to the substrate. However, when 2 wt. % of sodium chloride (NaCl) was added to the same solution and the number of cycles increased to 6, the load-carrying capacity decreased. The results analyzed showed that the reduction in the bond capacity and shifting of the failure mode to the composite–substrate interface require a high salt accumulation and crystallization within the interface, which may depend on the porosity of the matrix and on the radius of the pores. For instance, a matrix with pores radius bigger than the radius of substrate pores will allow the solution to pass through it. Consequently, the effect of sulfate and saline attack on the interface may not be as significantAppl. Sci. 2020 as, 10 its, 1714 effect on the fibers and matrix [36]. However, this hypothesis needs to be confirmed14 of 23 by using matrices with different porosity and pore sizes.

FigureFigure 11. 11. EffectEﬀect of of sulfate sulfate and and chloride chloride attack attack on on the the lo load-carryingad-carrying capacity capacity of of steel steel reinforced reinforced grout grout (SRG)–masonry(SRG)–masonry [36] [36] and and FRCM–masonry FRCM–masonry [33] [33] joints.

5.5. Conclusions Conclusions Inorganic-matrixInorganic-matrix compositescomposites have have been been increasingly increasingly used used for strengthening for strengthening and repairing and repairing existing existingmasonry masonry and concrete and concrete structures. structures. However, Howeve limitedr, studies limited are studies available are regardingavailable regarding their long-term their long-termbehavior. Thisbehavior. paper This presented paper presented a review of a review the available of the stateavailable of research state of on research the durability on the durability of FRCM, ofSRG, FRCM, and SRG, TRC composites.and TRC composites. Various testing Various procedures testing procedures and ageing and conditions ageing conditions were used towere investigate used to investigatethe durability the of durability inorganic-matrix of inorganic-matrix composites. Results composites. indicated Results that theirindicated durability that stronglytheir durability depends stronglyon the matrix depends and on fiber the matrix employed. and fiber Furthermore, employed. the Furthermore, literature review the literature showed review that the showed following that theaspects following still need aspects to be still investigated: need to be investigated: 1.1. VariousVarious conditioning conditioning environments environments were were utilizedutilized inin thethe literatureliterature to simulate the effect effect of of outdoor outdoor exposure.exposure. SomeSome ofof themthem werewere proposedproposed byby researchersresearchers and some others ta takenken from standards for for didifferentfferent materials. materials. Therefore,Therefore, thethe resultsresults couldcould notnot bebe easilyeasily compared.compared. In order to standardize specificspecific conditioning conditioning environmentsenvironments forfor durabilitydurability teststests of FRCM,FRCM, a thoroughthorough comparison among thesethese conditioning conditioning environments environments isis needed.needed. 2.2. StudiesStudies reportedreported contradictorycontradictory resultsresults regardingregarding thethe durabilitydurability of FRCMFRCM lime-based matrices exposedexposed to to saline saline environments. environments. The The di differencefference among among the th resultse results was was attributed attributed to to the the variation variation in thein the porosity porosity and and pore pore radius. radius. Further Further studies studies are ar requirede required to correlate to correlate the the physical physical properties properties of the of matrixthe matrix with with its durability its durability against against various various environments. environments. 3.3. LimitedLimited studies studies areare availableavailable regardingregarding thethe corrosioncorrosion ofof steelsteel cordscords embeddedembedded within inorganic matrices.matrices. StudiesStudies ofof thethe performanceperformance ofof steelsteel cordscords in corrosive environments are required to to gain gain informationinformation on on the the durability durability ofof SRGSRG composites.composites. 4. The effect of wet–dry cycles on the durability of inorganic-matrix composites including glass fiber textiles can be influenced by the contrasting effects of hydration and densification. Therefore, more studies are needed to better understand the dominant effect. 5. The durability of the bond between matrix and fiber and composite and substrate was not thoroughly investigated yet. Studies on the effect of freeze–thaw and wet–dry cycles, alkaline environments, sulfate attack, and other exposures are needed to gain a clear and reliable understanding of the long-term behavior of externally bonded inorganic-matrix composites.

Author Contributions: Conceptualization, K.A.-L., T.D., and P.C.;investigation, K.A.-L. and T.D.; writing—original draft, K.A.-L.; writing—draft review, T.D.; writing—editing T.D. and P.C. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: This research was performed with the financial support of the Politecnico di Milano. Conflicts of Interest: The authors declare no conflict of interest. Appl. Sci. 2020, 10, 1714 15 of 23

Appendix A

Table A1. Eﬀects of the ageing environments on the matrix, fabric, coupon, and composite–substrate retained capacity.

Average Retained Average Retained Average Retained Reference Materials Exposure Conditions Tests Matrix Cracking Ultimate Tensile Tensile Strength of Fiber Strength Strength of Coupons FT: retained ultimate • tensile strength of the Hygrothermal: at FRCM made of PBO • 1000 h and 3000 h, ﬁber was 1.10 1. Hygrothermal: 100% retained strength was Saline: retained RH at 37.7 C for 1000 • ◦ 1.56 and ultimate tensile and 3000 h Visual inspection of 1.34, respectively. strength of the PBO 2. FT: 20 cycles with 4 h • PBO–FRCM with the coupon surfaces FT: retained strength FRCM after 1000 h of • at 18 C and 12 h at • cement-based matrix − ◦ with 5 was 1.59 exposure was 1.50. 37.7 ◦C and 100% RH × NA [28] Carbon–FRCM with magniﬁcation lenses Saline: at 1000 h and • However, this value • 3. Saline: (seawater) • cement-based matrix. Uniaxial tensile test on 3000 h, the retained became 1.30 when the • 4. Immersion in alkaline FRCM coupons strength was 1.37 and exposure period solution [Ca(OH)2, 1.4, respectively. increased to 3000 h KOH, NaOH] with Alkaline: retained The retained ultimate pH > 12 • • strength was 1.10 for tensile strength of the both 1000 h and 3000 h carbon-FRCM was increased by 13% after hygrothermal and alkaline conditionings.

FT: The retained Retained ultimate FT (ASTM C666 2015) • • • strength at 25, 50, 75, tensile strength of the TRC with AR-glass (25–500 cycle): 30 min • Uniaxial tensile test on 100, 150, and 500 coupons exposed to 25, ﬁber textile and at +4, 30 min at 18 • NA [22] − the TRC coupons cycles was 1.37, 1.09, • 50, 75, 100, 150, and cement-based matrix with cooling/heating 1.26, 1.59, 0.66, and 500 cycle was 1.03, rate of 11 C/h ◦ 0.72, respectively 0.96, 0.98, 0.89, 0.86, and 0.81, respectively.

TRC coupons made of • Combined eﬀect of FT Retained ﬁrst cracking Retained ultimate cement-based matrix • • • ( 18 C to 5 C with 3 h strength after exposing pull-out strength after and hybrid fabric − ◦ ◦ Four-point bending cycle length) and saline • to 50, 70, and 90 cycles NA exposing to 50, 70, and [21] (warp carbon bundles and pull-out tests • and weft E-glass environment was equal 0.98, 0.91, 90 cycles was 1.00, 0.98 bundles) impregnated (5 wt. % NaCl) and 0.86, respectively and 0.99, respectively with epoxy resin. Appl. Sci. 2020, 10, 1714 16 of 23

Table A1. Cont.

Average Retained Average Retained Average Retained Reference Materials Exposure Conditions Tests Matrix Cracking Ultimate Tensile Tensile Strength of Fiber Strength Strength of Coupons

1. WI at the 23 ◦C for 5–28 days (this condition used to evaluate flexural and Matrix bending and • 2nd condition: 0.57, Fiber reinforced compressive compressive tests • • 0.77, 0.96, and cement-based mortar performance of the Three-point bending 1st condition: 0.67 • • 1.12 respectively. with 4% mortar only) test on small concrete (flexural) and 0.74 NA 3rd condition: 0.6 and [35] polymeric content (compressive) for • 2. WI at 23 ◦C for 5, 9, 40, beams strengthened • 1.01 PBO–FRCM with and 74 days with FRCM. Tests 28 days • 4th condition: 0.73 cement-based matrix 3. WI at 30 C for 9 and were done in an • ◦ 5th condition: 0.72 74 days environment-controlled room • 4. WI at 40 ◦C for 4 days 5. Hygrothermal at 30 ◦C for 9 days FT: 100 cycle with • Average retained Retained ultimate temperature range of • • 20 to 20 C (EN cracking strength for tensile strength of − ◦ 12467 2004) the specimens exposed specimens exposed to 50 cycles heat-rain to FT was 0.81 FT was 0.84 • Uniaxial tensile test on Average retained Retained ultimate (HR): heating to 60 ◦C • • • TRC with AR-glass TRM coupons cracking strength for tensile strength of • within 15 min and fiber and maintained for 45 min, Measure of crack specimens exposed to NA specimens exposed to [26] • • cement-based matrix then cooled with water width and crack HR was 0.7 HR was 0.58 immersion at 15 C spacing via DIC Average retained Retained ultimate ◦ • • Combined effect of the cracking strength for tensile strength of • two previous the specimens exposed specimens exposed to conditions with 100 FT to combined effect of combined effect of FT cycles followed by 50 FT and HR was 0.6 and HR was 0.73 HT cycles Appl. Sci. 2020, 10, 1714 17 of 23

Table A1. Cont.

Average Retained Average Retained Average Retained Reference Materials Exposure Conditions Tests Matrix Cracking Ultimate Tensile Tensile Strength of Fiber Strength Strength of Coupons WI: Small Increment • in the retained flexural and compressive strength (1.03 and Saline: average • 1. WI: tap water at 60 C retained tensile ◦ Flexural tests on 1.06, respectively) WI: average ultimate for 1000 h • strength of the • mortar specimens The retained flexural retained strength was 2. Immersion in saline • specimens exposed to Compressive tests on and compressive 0.89 FRCM with AR glass solution (3.5% NaCl) at • continuous immersion • mortar specimens strength after Saline: average and lime-based mortar, 60 C for 1000 h and WD cycles in • ◦ Uniaxial tensile test on continuous immersion ultimate retained [33] also bonded to 3. WD 10 cycles (960 h): • saline solutions single fiber yarn in saline solution were strength was 0.71 masonry substrate 2 days immersion in 0.91 and decreased to 0.6 and Single-lap shear test WD: average ultimate saline solution (3.5% • 1.08, respectively. 0.36, respectively. • on FRCM bonded to a retained strength was NaCl) at 60 ◦C and WD cycles in saline WI: retained tensile masonry substrate • • 0.8 2 days drying at 60 ◦C solution: slight strength was not decrease in the flexural affected (0.98) and compressive strength of (0.9 and 0.95, respectively) WI: retained tensile • 1. WI in distilled water at strength at 20 ◦C decreased to 0.85 at 20, 50 and 60 ◦C for 500, 1000, 2000, and 500 h and 1000 h. 3000 h. Retained tensile 2. Saline (3.5 wt.% of strength at 60 ◦C Unidirectional glass NaCl) at 20, 50, and decreased to 0.64 at • Uniaxial tensile test 500 h and to 0.56 at fabric impregnated in 60 C for 500, 1000, • NA NA [53] ◦ (ASTM D638 2004) • • vinylester resin 2000, and 3000 h 1000 h. Saline: retained tensile 3. Alkaline (pH = 12.5) at • the same temperature strength was and exposure periods decreased from 0.86 to of the 0.57 when the previous conditions exposure temperature increased from 20 ◦C to 60 ◦C Appl. Sci. 2020, 10, 1714 18 of 23

Table A1. Cont.

Average Retained Average Retained Average Retained Reference Materials Exposure Conditions Tests Matrix Cracking Ultimate Tensile Tensile Strength of Fiber Strength Strength of Coupons Alkaline: retained • tensile strength at 20 ◦C decreased to 0.84 at 500 h and to 0.80 at 1000 h. Retained tensile strength at 60 ◦C decreased to 0.42 at 500 h and to 0.33 at 1000 h.

AR-glass FRCM Alkaline: retained Alkaline: retained • • • coupon with matrix B 1. Immersion in alkaline strength decreased for ultimate strength of Three-point (rich in hydrated lime solution (sodium • mortar B and M to 0.67 the coupons with bending tests Alkaline: retained and pozzolan) bicarbonate, pH = 10) and 0.45, respectively. • matrix B and M is 0.89 Uniaxial tensile test on strength was 0.89 and 0.90, respectively [34] AR-glass FRCM for 1000 h • Saline: retained • the ﬁber textile • Saline: retained Saline: retained coupon with mortar M 2. Immersion in saline strength decreased to • • Uniaxial tensile test on strength was 0.91 ultimate strength of (air-hardening lime, solution (3.5% sodium • 0.68 and 0.43 for FRCM coupons pozzolan and chloride) for 1000 h mortar B and the coupons with marble sand) M, respectively. matrix B and M is 0.85 and 0.89, respectively.

WI: retained strength 1. Immersion in alkaline • solution (0.8% of decreased from 0.98 to NaOH and 2.24% of 0.39 when the KOH) pH = 14 for temperature increased from 10 C to 60 C 5–30 days at 60 ◦C ◦ ◦ AR-glass fiber (ISO 10406-1) Uniaxial tensile test on and the exposure • • Basalt fiber 2. WI in deionized water single yarns NA period increased from NA [51] • • 10 to 60 days • Carbon fiber at 60 ◦C for 30 days Visual inspection • • The retained strength 3. Immersion in the • alkaline solution 1. for of the AR-glass after exposing to alkaline 10 days at 20 ◦C 4. WI in deionized water environment 1. for 10 days at 20 C for10 days in the lab ◦ temperature was 0.66 Appl. Sci. 2020, 10, 1714 19 of 23

Table A1. Cont.

Average Retained Average Retained Average Retained Reference Materials Exposure Conditions Tests Matrix Cracking Ultimate Tensile Tensile Strength of Fiber Strength Strength of Coupons 1. Ageing period: Retained maximum • 28-360 days pull-out forces for 2. M1 mortar: low Retained tensile matrix M1 at period of • alkalinity reduced strength of FRCM 28, 56, 90, 180, and AR-glass ﬁber Uniaxial tensile tests • from pH = 12.4 at • coupons with matrix 360 days are 0.87, 0.72, Three types of mixing to pH = 11.8 on FRCM coupons 0.68, 0.67 and • M1, M2, and M3 after matrices (M1, M2, M3) after 1 year. reinforced with 5 NA 0.62, respectively [47] • 360 days was 0.97, 0.62 with diﬀerent level layers of textile 3. M3 mortar: high and no retained Retained maximum of alkalinity Pull-out tests • alkalinity, pH = 12.7 • load-carrying pull-out forces for 4. M2 mortar: medium capacity, respectively matrix M2 at the same level of alkalinity periods are 0.87, 0.72, ranging between M1 0.68, 0.67, and and M3 0.62, respectively

FRCM coupons with Retained tensile Retained ultimate • • • lime-based mortar and strength of the mortar strength for coupons biaxial glass fabric and 1. FT: 14 cycles of 2 h in Uniaxial tensile tests was 0.63 with biaxial glass • fabric was 0.84 FRCM coupons with water at 20 ◦C and 6 h on mortar and Retained cracking NA [14] • • • lime-based mortar and at 25 C FRCM coupons strength of the FRCM Retained ultimate − ◦ • quadriaxial coupons was 0.99 strength of coupons hybrid fabric after exposure with hybrid fabric was 0.99 Appl. Sci. 2020, 10, 1714 20 of 23

Table A1. Cont.

Average Retained Average Retained Average Retained Reference Materials Exposure Conditions Tests Matrix Cracking Ultimate Tensile Tensile Strength of Fiber Strength Strength of Coupons 1. WD: 4 cycles of 2 days of partially immersion in a saline solution (8 wt. % of Na SO 10H O) and 2 4· 2 2 days drying in ventilated oven at 60 ◦C 2. WD: 4 cycles of 2 days immersion in deionized water and 2 days drying in SGR composite with ventilated oven at 1st condition: 1.03 • • lime-based matrix and 2nd condition: 0.98 60 ◦C Single-lap shear tests NA NA • [36,43] steel cords bonded to • • • 3rd condition: 0.73 3. WD: 6 cycles of 2 days • masonry substrate partially immersed in 4th condition: 0.78 • saline solution (2 wt. % of NaCl and 8 wt. % of Na SO 10H O) at 2 4· 2 20 ◦C and 3 days drying with ventilated oven at 60 ◦C 4. WD: 6 cycles equal to the previous condition 3 except for the deionized water used for drying instead of the saline solution Note: FT: freeze–thaw; HR: heat rain; WD: wet–dry; WI: water immersion. Appl. Sci. 2020, 10, 1714 21 of 23

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