Construction and Building Materials 126 (2016) 573–585

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Construction and Building Materials

journal homepage: www.elsevier .com/locate /conbui ldmat

Review

Long term performance and fire safety aspect of FRP compositesused in building structures

http://dx.doi.org/10.1016/j.conbuildmat.2016.09.0310950-0618/� 2016 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Department of Architecture and Civil Engineering, City University of Hong Kong, Kowloon, Hong Kong.E-mail address: denvid@mit.edu (D. Lau).

Denvid Lau a,b,⇑, Qiwen Qiu a, Ao Zhou a, Cheuk Lun Chow a

aDepartment of Architecture and Civil Engineering, City University of Hong Kong, Kowloon, Hong KongbDepartment of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA

h i g h l i g h t s

� Moisture causes a shift in failure mode in FRP-bonded systems.� Deterioration of FRP-bonded system can be investigated by multiscale modeling.� Non-destructive testing methods are encouraged for assessing interface integrity.� Seawater/sea-sand concrete reinforced by FRP rebar has great corrosion resistance.� Fire behavior of FRP is well recognized but smoke emission issue remains unsolved.

a r t i c l e i n f o

Article history:Received 27 June 2016Received in revised form 9 September 2016Accepted 12 September 2016

Keywords:ConcreteDurabilityFiber reinforced polymerFireInterfaceLong term performanceNondestructive test

a b s t r a c t

Fiber reinforced polymer (FRP) composites are widely used in repairing concrete or wood structures.However, the long term performance of FRP-bonded systems can be seriously affected due to the pro-longed exposure to aggressive environments. Besides, the fire attack towards FRP is another safety con-cern in composite structures. This paper shows the recent research developments on (1) the deteriorationof FRP-bonded system, (2) the evaluation of interfacial integrity, (3) FRP-reinforced system with excellentdurability, and (4) the fire behavior of FRP. Future research perspectives for durability study and recom-mendations towards fire safety issue upon the use of FRP are also provided.

� 2016 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5742. Long term performance of FRP-bonded composites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 574

2.1. FRP-bonded concrete system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5742.2. FRP-bonded wood system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5772.3. Interface integrity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 577

3. Long term performance of FRP-reinforced concrete. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 578

3.1. Hybrid structures with FRP and steel reinforcements used in building structures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5793.2. Innovative use of FRPRC: offshore structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 579

4. Fire behavior of FRP composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5805. Future perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 581

5.1. Future work for better understanding of durability performance towards FRP-bonded systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5815.2. Recommendations for the fire attack issue of FRP-bonded systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 582

574 D. Lau et al. / Construction and Building Materials 126 (2016) 573–585

6. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 582Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 582References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 582

1. Introduction

Concrete is the most commonly used building material aroundthe world due to its relatively high compressive strength and suf-ficient supply of raw materials. However, it has been proven thatconcrete deteriorates over time under the influence of detrimentalenvironmental conditions [1–3]. In order to strengthen the existingdeteriorated concrete structures, FRP composites are increasinglyadopted for the strengthening and retrofitting purposes, due totheir excellent properties including high specific stiffness andstrength, good fatigue performance and damage tolerance,excellent resistance to environmental degradation, non-magneticproperties, and low energy consumption during fabrication ofraw materials [4–6]. Recently, the use of FRP has been extendedbeyond the concrete and its potential use as repair and capacityupgrade of wood structural members has been noticed. Indeed,wood is a renewable construction material with lower productionand processing energy requirements, which can be manufacturedinto strong, versatile, and durable wood elements such as lumber,veneers, flakes, and strands [7–9]. In the future, wood and timberwill be treated as one of the major building materials due to envi-ronmental and energy shortage concerns and it has been men-tioned in the recent ACSE SmartBrief in 2014 [10]. However,wood material possesses a relatively low load-carrying capacitycompared to concrete, which restrains the wide utilization of woodas the construction material [11]. Moreover, wood materials decayover time due to the presence of aggressive environmental condi-tions. Enhancements on the strength and durability of the con-cerned timber structures by the use of FRP composites can makethe most use of wood materials in building construction.

Nevertheless, the FRP-strengthened structures are facing theproblems upon their long term performance, which leads to areduced confidence towards their safety especially when the build-ing age approaches their designed service life. Currently, the longterm durability concerns remain an obstacle to a widespread appli-cation of FRP composites in concrete structures because the safetylevel of building structures using these composites cannot be keptconstant. The durability issue also limits the integration betweenFRP-bonded systems and the concept of green and sustainability.In particular, when it comes to the use of FRP composites forstrengthening the wood structures, in which wood material isregarded as a renewable resource. Concern about the environmen-tal durability of FRP composites in building structures hastherefore been emphasized in many recent research publications[12–14]. The key areas of interest have been mainly focused onunderstanding the degradation mechanism of interfacial bondbetween the FRP layer and the substrate [15–17]. However, theunderlying mechanism of interfacial deterioration due to theaggressive environments has not yet been fully understood, andmore relevant studies should be carried out. In addition, theassessment of the deterioration level for existing composite struc-tures in service is needed so that the maintenance actions can befeasibly implemented and the satisfactory performance of struc-tures is herein safeguarded. To deal with this issue, non-destructive testing (NDT) methods are encouraged for assessingFRP-bonded system. Apart from the environmental durabilityissue, the fire safety of FRP composites is a serious concern becausethe mechanical properties of FRP materials can be seriouslyaffected under fire. Under high temperature, the FRP composites

can release smoke, heat, and toxic fumes, which impose a greatthreat towards both the user’s safety and the structural health. Inview of this fire issue, the FRP composites have not yet reachedtheir full market potential. It is strongly believed that the applica-tion of FRP in construction industry can be further popular giventhat their performance under high temperature is well acknowl-edged and the fire safety guidelines are set up.

In this paper, a state-of-the-art review focusing on the longterm performance and the fire safety aspect of FRP compositesused in building structures is presented. Durability of FRP-concrete and FRP-wood bonded systems under environmentaleffects is discussed. Besides, in order to maintain a reliable FRP-bonded system in a long run, the long term interfacial bond assess-ment with NDT methods is needed and is presented as well. Someinnovative FRP applications, which can establish an improvedstructural system with an emphasis on ductility, durability andsustainability, are presented. In addition, the degradation mecha-nism of FRP composites under fire is highlighted and the solutionsfor fire safety issues are provided. This review work will be bene-ficial to the public safety with the awareness that the long termperformance and fire safety aspect of FRP-bonded system shouldbe carefully considered and designed so that a durable buildingstructure with FRP can be resulted.

2. Long term performance of FRP-bonded composites

Externally bonded FRP composites have been widely used tostrengthen the deteriorated concrete structures. As compared toconcrete, wood material is a naturally renewable resource to beused in building structures but it has a relatively low load-bearing capacity. In order to enhance the mechanical performanceof wood in buildings, the external bonding of FRP composites towood through epoxy has gained an increasing interest during thepast few years. However, both the FRP-concrete bond and theFRP-wood bond can degrade due to the prolonged exposure underthe harsh environmental conditions including moisture, alkalinesolutions, ultraviolet radiation, diurnal temperature change, andfreeze/thaw cycles. Among these environmental effects, the mois-ture entrapment is considered to be the most common and criticalfactor leading to the interfacial bond deterioration.

2.1. FRP-bonded concrete system

The durability of FRP-bonded concrete system under variousenvironmental effects has been studied previously [18,19]. Theexisting experimental work has shown that among the influencesof various environmental conditions (e.g. salt water, moisture, heatand alkalinity), the moisture ingress is identified as the most crit-ical factor that degrades the mechanical performance of FRP-bonded concrete system significantly and causes premature sys-tem failures [16,20–22]. In general, moisture absorption can dete-riorate the mechanical properties of fiber, polymer resin and thefiber/matrix interface. Among commonly used fibers, carbon fiberis relatively immune to moisture attack. However, both aramidand glass fibers are vulnerable to chemical and mechanical degra-dation under moisture condition [23,24]. Moisture uptake canoften lead to fiber pitting and propagation of stress-corrosioncracking, resulting in strength losses [25]. The damaging effect

D. Lau et al. / Construction and Building Materials 126 (2016) 573–585 575

on the fiber can be intensified by salt water [23]. Resin componentin the FRP-to-concrete bond system is also the primary victim ofmoisture absorption since a vast variety of polymer resins (e.g.epoxy and vinyl ester) are hydrophilic. Exposure to moisture canresult in plasticization, hydrolysis, swelling, and fiber debondingfrom the resin component [26,27]. Moisture-induced matrix plasti-cization can lower the glass transition temperature (Tg), soften thepolymer resin, and increase the creep effect. This phenomenon canbe typically reversible when drying the specimens [28,29]. How-ever, hydrolysis of polymer resin is an irreversible process, whichseriously affects both the strength and stiffness of resin material[30]. Besides, moisture absorption is known to degrade thefiber/matrix interface by encouraging fibers to debond from thesurrounding matrix [26]. This interfacial damage is resulted fromthe resin swelling after exposure to moisture which can initiatestresses between the fibers and the matrix and can deterioratethe bond between them. Moreover, the fiber debonding frommatrix allows further paths for moisture penetration, hence esca-lating the growth of deterioration [31]. Reductions in ultimatestiffness and strength of FRP-retrofitted specimens in the presenceof moisture have been generally found [32–34]. Additionally, it hasbeen reported that the structural failure dominantly takes placenear the FRP-concrete bonded region, i.e. at the vicinity of theinterface. A considerable degradation of concrete-epoxy interfaceagainst moisture attack has been demonstrated in both experimen-tal and numerical approaches. The interface fracture toughness cansignificantly drop after moisture conditioning, as stated in the lit-eratures [35–38]. Besides, a shift of failure mode (i.e. before andafter moisture conditioning) has been observed in the FRP-bonded concrete systems [39]. FRP-bonded concrete system indry condition conventionally exhibits a concrete fracture, whilethe specimens in wet state fail mostly at the concrete-epoxy inter-face [19,40–42]. Another study on the concrete-epoxy interface, asshown in Fig. 1(a), with the consideration of moisture effect hasbeen conducted using the four-point bending test setup, whichcan provide a much more accurate measurement on the interfacefracture toughness [43]. The failure modes of the dry and wet sand-wiched bending beam specimens in a four-week moisture durationare shown in Fig. 1(b). It has been found that the dry specimenexhibited failure in concrete itself. The cracks kinked into the con-crete and the specimen failed through concrete delamination. On

Fig. 1. (a) Schematic diagram of the four-point bending test for concrete-epoxy sandwexhibited failure in concrete itself as concrete delamination, while moist specimens exh

Table 1Interface fracture toughness (J/m2) of concrete-epoxy system with different moisture dura

Item Moisture duration (week)

0 2

Interface fracture toughness (J/m2) 13.7 12.1

the other hand, the wet sandwiched bending beam specimenexhibited a distinctive concrete-epoxy interface separation, witha relatively small amount of loose concrete particles adhered tothe epoxy layer. A clear separation between concrete and epoxycan be observed under eight-week moisture conditioning. Thisimplies that the interface becomes the weakest zone in the bondedsystem and the crack does not kink into concrete substrate butpropagates along the moist concrete-epoxy interface. It is alsonoticed that the change of the interface fracture toughness is clo-sely correlated with the shift of failure mode. Table 1 shows thatthe reduction in the interface fracture toughness is more than50% when the moisture duration reaches six weeks. In additionto the studies related to the static moisture ingress, the cyclicmoisture effect has also been explored in recent years [44]. Resultsfrom the moisture reversal and cyclic moisture conditioning testshave shown that the residual bond strength decreases when thenumber of wet/dry cycles increases, as indicated in Table 2.

The traditional experimental based study is able to capture suchdeterioration, but it lacks the ability to explain the associated dete-rioration mechanism. In order to have a comprehensive and funda-mental understanding on the deterioration mechanism, a detailedinsight towards the interaction between water, epoxy and concreteat the material level is needed, and molecular dynamics (MD) sim-ulations have been employed accordingly [45–48]. One of the chal-lenging aspects in MD simulation is to construct an atomisticmodel (length scale: 1–100 nm) which can accurately describethe concrete-epoxy interface system. The molecular structure ofepoxy can be represented and simulated by diglycidyl ether ofbisphenol A (DGEBA) [41] which consists of the key functionalgroup in which most of the construction-related epoxy materialspossess, while the atomistic structure of concrete component ismuch more complicated and is good to be simplified. In general,concrete is a heterogeneous material which mainly consists ofhydrated cement and aggregates. Among all the constituent mate-rials (e.g. cement, aggregates) in concrete, silica accounts for a largeproportion (more than 40% by weight of the solid ingredients) [41].Besides, silica has a well-defined chemical formula which makesthe atomistic model in MD simulation to be easily constructed.Amorphous silica has been used to simulate the cement compo-nent in concrete [49,50]. A recent study has also demonstrated thatcrystalline silica (SiO2) can be adopted to represent both the

iched beam specimens; (b) failure surface after the bending test: dry specimensibited a distinctive concrete-epoxy interface separation.

tions.

4 6 8 10

7.8 5.4 5.6 3.9

Table 2Reduction percentage of interface fracture toughness for FRP-bonded concrete system subjected to wet-dry cycles.

Test scenario Number of wet-dry cycles

0 1 2 4 6 8

Peel test 0% 2.2% 8.9% 22.2% 31.1% 50%Shear test 0% 8.3% 10.6% 31.8% 40.2% 52.3%

576 D. Lau et al. / Construction and Building Materials 126 (2016) 573–585

cement and aggregate components, and the concrete-epoxy inter-face system can be simulated by using crystalline silica and a singlechain of epoxy (DGEBA) [41]. Besides the atomistic model con-struction, the choice of forcefield is crucial to formulate the inter-action between the dissimilar molecular entities (i.e. FRP andconcrete). During the past few decades, some forcefields have beencommonly used such as ClayFF [51], DREIDING [52], PCFF [53],consistent force field (CFF91) [54,55] and consistent valence forcefield (CVFF) [56,57]. These forcefields have been well tested forvarious polymers including epoxy-based materials. Meanwhile,research has been carried out to extend the applicability of CVFFtowards the inorganic materials like silica, as well as the interac-tion between epoxy and silica [41,45]. It has been reported thatthe adhesion energy in an aqueous scenario experienced a reduc-tion by approximately 15% as compared to a dry scenario [58]. Thisreduction at the molecular scale may translate into a greater adhe-sion energy reduction in large-scale structures due to the interac-tion of local defects that significantly reduces the structuralintegrity. The presence of water molecules hinders the attractionbetween epoxy and concrete (represented by silica), as revealedin Fig. 2, and this explains why the adhesion between epoxy andconcrete is weakened.

Considering that the degradation level of composite material isassociated with the amount of moisture absorption, an under-standing on moisture diffusion process in FRP-bonded concretesystem is beneficial to identify the moisture-contaminated loca-tion and predict the service life of FRP composites [59]. Many stud-ies have pointed out that moisture diffusion in FRP compositesfollows Fickian diffusion process [60–62]. This diffusion processrelies on the hypothesis that the FRP plate is infinite in the planedirection, which is mostly applicable in the thin composites. How-ever, deviation from Fickian diffusion may occur for thick FRP com-posites which fail to satisfy the assumption of ‘‘infinite” plate [63].Also, deviation of Fickian diffusion can be initiated when the FRPcomposites are subjected to elevated temperature and immersedin water, which is associated with the polymer relaxation [64].Polymer relaxation is a long term effect to initiate the swelling ofcomposite, leading to the redistribution of voids and free volumes.Moisture absorption due to the polymer relaxation (swelling) canbe increased seriously. On the other hand, the chemical degrada-tion (e.g. hydrolysis of polymer, chain breakage) caused by mois-ture sorption can result in the micro-cracks in the fiber/polymer

Fig. 2. Three snapshots are captured by MD simulation at different time for a better untime = 80 ps; (c) time = 160 ps. The presence of water molecules hinders the attraction b

interface [65]. This chemical damage allows massive moistureuptake, which further makes the Fickian diffusion mechanism lessdominated. In view of the additional effects of polymer relaxationand composite damage, a more versatile model of moisture diffu-sion for polymeric materials has been reported recently [66]. Thediffusion model can be regarded as a linear superposition of phe-nomenologically independent contributions from Fickian diffusion,polymeric relaxation and chemical damage.

It should be noted that the understanding of monotonic deteri-oration mechanism due to the presence of water is only the infantstep for ensuring the good long term performance of FRP-bondedconcrete system. In reality, most FRP-bonded concrete structuresare subjected to more than one deterioration effect, as well as morethan one associated cycle. The coupled deterioration due to mois-ture attack and temperature rise can result in a more adverseimpact on the long term durability of the FRP-bonded concrete sys-tems. Previous study has indicated that the reduction of bondstrength and stiffness as a result of the moisture effect can be esca-lated by the temperature rise [43,67].

Apart from the environmental effects, FRP-concrete interfacesystem is subjected to service loading. The monotonic load-bearing behavior of FRP-bonded concrete members has been inves-tigated by extensive experimental and analytical studies [68–70].Besides, recent studies which investigate the effect of cyclic fatigueon FRP-concrete interface have been conducted [71–73]. From thecyclic bond stress-slip relationship, a quasi-linear hystereticbehavior with very narrow hysteresis loops is shown when cyclicshear test is applied, which indicates the rather brittle behaviorof FRP-to-concrete joint. As compared to monotonic loading, a con-siderable reduction in the interfacial bond stiffness is observedduring fatigue tests. In addition, it is observed that a higher valueof bond slip is obtained when a larger cyclic stress range is applied.Under cyclic loading, a narrower bond width is also found whichcauses a larger slip and a shorter fatigue life compared with thosehaving a larger bond width.

During service life, FRP-bonded concrete system is usually sub-jected to the synergistic effect of mechanical load, moisture, andtemperature variations [74]. The durability of FRP-bonded concretesystem under such synergistic effect is much more complicatedthan that against the single deterioration attack. A recent experi-mental investigation has been carried out to study the perfor-mance of FRP-to-concrete bonded interface subjected to fatigue

derstanding on the interaction among epoxy, silica and water: (a) time = 0 ps; (b)etween epoxy and concrete (represented by silica), which results in a weaker bond.

D. Lau et al. / Construction and Building Materials 126 (2016) 573–585 577

load after a hygrothermal environment exposure (60 �C, relativehumidity = 95%) [75]. The study shows that fatigue life of FRP-bonded concrete system after hygrothermal pretreatment is signif-icantly shorter than that of untreated specimens. Also, the testresults demonstrate that the fatigue life can be reduced by a higherstress level. Another study [76] indicates that the effect of load andmoisture at an elevated temperature causes a significantly largerinterfacial crack length than that at room temperature. Besides, afaster crack growth at the early age and a longer crack lengthbefore failure are resulted at a higher moisture level. However,research work dealing with the synergistic effect on the durabilityof interfacial bond between FRP and concrete is still rare, and alarge test data base is required for developing suitable deteriora-tion model that can describe the behavior of fatigue damage atthe FRP-concrete interface.

In addition to the durability evaluation, it is essential to findeffective ways to enhance the resistance of FRP-bonded concretesystem against the aqueous environments. In general, selectingstrong components (i.e. fiber, matrix type) and a correct manufac-turing process is crucial for the interfacial strength and the resis-tance to environmental degradation. As stated in the literatures[77,78], carbon fiber has the best resistance to acid, alkali, andorganic solvents, while deterioration of glass fiber can occur inalkaline environment. Selection of resin also plays an importantrole in the FRP-to-concrete interfacial durability. Vinylester resinoffers a greater alkali resistance than polyester resin [77]. Besides,the use of a coupling agent on the fiber surface is able to protectthe fibers to a significant degree from moisture or alkalis attack[77]. Meanwhile, it has been reported that organofunctional silanecoupling agents as primer to treat the surface of concrete substratecan greatly improve the durability of FRP-concrete interface bond,due to the mechanism that a hydrophobic organosilicate layer atthe concrete surface is formed to effectively retard the moistureingress [76,79]. Treatment with silane coupling agent can reducethe extent and growth rate of debonding in FRP-bonded concretesystem under aqueous condition. In addition, a recent study hasdiscussed the use of waterproofing spray applied on the surfaceof FRP-bonded concrete system in order to minimize the rate ofmoisture diffusion at the interfacial region [58].

2.2. FRP-bonded wood system

Similar to FRP-bonded concrete system, FRP-bonded wood sys-tem can also deteriorate in ambient moisture that affects its longterm durability [80–82]. Previous experimental work, from themacro-scale perspectives, has demonstrated that the moisturecan diminish the bond strength and weaken the strengtheningeffectiveness of FRP composites for wood members [83,84]. MDsimulation has recently been employed in FRP-bonded wood sys-tem as an attempt to explore the detailed microscopic informationabout how the composite materials interact with the organic woodfibers and the moisture environment [85]. In a recent study, an ato-mistic model representing the epoxy-wood interface has beenbuilt, and the influence of water molecules towards the adhesionstrength has been determined by comparing the adhesion energyunder dry and wet scenarios [85]. It is noted that metadynamicsis a powerful MD technique that has been exploited in complexpolyatomic systems to reconstruct the free energy profile as a func-tion of a suitably chosen set of collective variables (CVs) [86–88].The free energy profiles of the cellulose-epoxy interface describingthe debonding process in both dry and wet cases can be recon-structed by using the metadynamics approach, as demonstratedin a recent study [47,85]. Comparing the epoxy-wood interfacebetween the dry and wet cases, it is found that the adhesion energyin the wet case drops to one third of that in the dry case. Thesefindings indicate that the bond strength between epoxy and wood

decreases with moisture conditioning, and such reduction is evenmore serious when compared with the concrete/epoxy interface.Apart from the experiment at the macro-scale (length scale:>1 m) and the simulation at the atomistic scale (length scale:1–100 nm), the consideration of micro-scale (length scale:1–100 lm) and meso-scale (length scale: 1–100mm) investigationsshould also be conducted in future in view of the hierarchicalstructure of wood materials. Meso-scale study can smooth thescale transition between atomistic and macro-scales so that a com-prehensive durability understanding of FRP-bonded wood systemunder moisture effect can be achieved.

Besides the moisture effect, temperature is another importantfactor that controls the durability of FRP-bonded wood systemsince it can cause the degradation of wood itself and the adhesivebonding [89]. It has been reported that permanent reductions instrength of wood materials can occur at temperatures above100 �C and the wood surface temperature ranging from 80 to100 �C has been regarded as safe for prolonged heat exposure[90]. However, this temperature range can lead to a significantreduction in interfacial bond strength where adhesives with alow glass transition temperature are used [91,92]. More recently,the temperature effect on interfacial bond deterioration for FRP-bonded wood system has been investigated comprehensivelythrough both the experiments and the MD simulations [93]. Theresearch findings imply that the low temperatures (i.e. below0 �C to around 20 �C) have a little influence on the durability ofFRP/wood interfacial bond. However, it is found that the adhesivesunder temperature of 50 �C can experience an increase in vis-coelastic response. Such observation is important for the investiga-tion towards the creep behavior of the FRP-wood bonded system.Exposure to high temperature (i.e. 50 �C or above) can initiate thebond deterioration to a certain degree, especially when the sur-rounding temperature is closed to the glass transition temperatureof the concerned polymers. Practicing engineers should pay extraattention when the FRP strengthened structures are located intropical or hot climate regions where the sunlight exposure is com-mon. Another serious issue related to the temperature effect onFRP composite structures is that the temperature variations andthe freeze/thaw cycling conditions can potentially result indebonding of laminates [94]. Indeed, the durability of FRP-bonded wood system under the coupled effect of moisture and ele-vated temperature, as well as the combined influence coming frommoisture, elevated temperature and sustained loading should becarefully investigated in future.

2.3. Interface integrity

Based on the above discussion, it can be seen that the long termperformance of FRP-bonded system is dependent on the interfaceintegrity. The assessment of the deterioration level for existingcomposite structures in service is therefore essential with the pur-pose that the maintenance actions can be feasibly implementedand the satisfactory performance of structures can be safeguarded.NDT methods are encouraged to be carried out in FRP-bonded sys-tems. Among the current NDT methods, the acoustic-laser tech-nique has been demonstrated to be able to evaluate theinterfacial bond deterioration for FRP-bonded system effectively[95–97]. The schematic diagram of the acoustic-laser techniqueused for evaluating composite materials is presented in Fig. 3(a).In this technique, a loudspeaker is used to provide the acousticexcitation towards the object and a laser beam is employed tocharacterize the vibration of the sample. A photoreceiver is usedto collect the optical signals which contain the vibration informa-tion. During the measurement, the optical signals are convertedinto the electrical signals which can be further digitalized by a dataacquisition system. The amplitude of the electrical signals indi-

Fig. 3. Acoustic-laser technique used in assessing the interface integrity of FRP-bonded wood before and after different temperatures exposure: (a) schematic diagram ofacoustic-laser technique; (b) NDT results from acoustic-laser technique in CFRP-wood sample. The results revealed that the vibration signals over the entire FRP-woodinterface are weak in the case of �5 �C and 20 �C. However, the signals can be amplified at a higher temperature (i.e. 50 �C).

578 D. Lau et al. / Construction and Building Materials 126 (2016) 573–585

cates the vibration level and implies how serious the bond deteri-oration is under various environmental conditions. Fig. 3(b) showsthe measurement results in terms of the electrical responses overthe FRP-wood interface under the temperatures of �5 �C, 20 �C,and 50 �C. It is found that the signals over the entire FRP-woodinterface are weak when temperature is between �5 �C and20 �C. This suggests that the low temperature range (i.e. �5 to20 �C) has little influence on the interface integrity in FRP-bonded wood system. However, the electrical signals can becomesignificant when the material temperature is around 50 �C. Thelarge amplitude of the electrical signals can be conspicuouslyobserved at the edge of bonding area. In order to confirm the reli-ability of acoustic-laser technique, the FRP-bonded wood speci-mens have been tested under shear loading and the failuremodes of the specimens have been recorded and shown in Fig. 4.The separation area between the FRP and wood with respect tothe high temperature (i.e. 50 �C) is found to be the largest amongall the tested specimens. Such observation indicates that thenear-surface defect detection is important for monitoring thestructural health of FRP-bonded systems.

Fig. 4. Failure modes after shear loading test for different temperature conditionings: (a50 �C temperature exposure. Results show that the separation area between the FRP an

Because acoustic-laser technique requires cumbersome equip-ment and devices, it may not be convenient and efficient to beapplied in practice, especially when a large area is required forinspection. More recently, some researchers have developed otherNDT techniques with accurate measurement output and ease ofoperation in detection process, such as laser reflection technique[98,99], modal strain energy based damage detection [100] andhigh speed camera based motion visualization technique [101].These techniques are highly recommended for the future assess-ment of long term interface integrity of FRP-bonded compositessystem, where near-surface defects play a crucial role towardsthe global structural integrity.

3. Long term performance of FRP-reinforced concrete

In conventional concrete structures, the corrosion of steel rein-forcement bars can bring a lot of problematic consequences includ-ing the onset of concrete cracks, the reduction in cross-sectionalarea of the reinforcement bar, and even the spalling of concretecover due to the swelling pressure generated by the corrosion

) after �5 �C temperature exposure; (b) after 20 �C temperature exposure; (c) afterd wood under 50 �C is found to be the largest.

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products [102]. Partially replacing steel reinforcement bars by FRPreinforcement bars appears to be a good solution to the corrosionissue as FRP possesses an excellent corrosion resistance. In thehybrid FRP-steel-concrete system, the steel corrosion can be mini-mized or even mitigated when steel is replaced by FRP as internalreinforcements. Besides the hybrid FRP-steel-concrete system, anewly developed composite system which only contains FRP inter-nal reinforcement and concrete is found to be an efficient struc-tural system that can be adopted in marine environment with asuperior durability performance, given that there is no steel inter-nal reinforcement.

3.1. Hybrid structures with FRP and steel reinforcements used inbuilding structures

Steel reinforced concrete (RC) is widely used in building struc-tures due to its effectiveness in resisting compressive loads andits good deformability. However, this reinforced concrete systemsuffers from steel corrosion due to the penetration of carbon diox-ide or chlorides [103–106]. Additionally, the large weight of steelreinforcing bars (compared with composites) makes transportationand installation difficult. Also, an elaborate and expensive false-work is required to maintain steelwork in position during con-struction [107]. Thus, the embodied energy of steel-reinforcedconcrete structures is high throughout their useful lifetime. In con-trast, FRP materials have an excellent resistance against environ-mental attacks. Besides, FRP composites require less falseworkand less energy per kilogram to produce and transport than steel[107]. This can save a great amount of operational energy in theconstruction industry. In addition, FRP materials are light and havebeen demonstrated to provide much longer lifespans than tradi-tional reinforcement materials, such as structural steel rebarsand stainless steel rebars. This means that FRP composites can savea great amount of operational energy during service life. It hasbeen reported that the cumulative energy (including embodiedenergy and operational energy) of FRP is 68% lower than that ofstainless steel [108]. Owing to these characteristics, FRP is a sus-tainable construction material and has a less energy consumptioncompared to iron and stainless steel. Thus, many researchers havesuggested to replace the internal steel reinforcement bars by FRP[109–112]. The composite system consisting of FRP reinforcementbars and concrete is known as FRP-reinforced concrete (FRPRC), asshown in Fig. 5(a). Even though the FRPRC does not suffer fromsteel corrosion, FRPRCmembers are weak in ductility aspect, whichis crucially important from the structural perspective because duc-tility can offer ample warnings before failure so that the loss of

Fig. 5. Cross section details of reinforced concrete beams: (a) pure FRP-reinforced concretover-reinforced FRPRC beam; (e) over-reinforced FRPRC beam with 135� hook stirrup. PuThe weak ductility of FRP-reinforced concrete can be improved by adding steel reinforFRPRC system rather than under-reinforced and balanced-reinforced FRPRC system. Hybrwith 90� hook stirrup, which is recommended to be used.

human life and property can be significantly reduced. In order tosolve this problem, adding a certain amount of steel reinforce-ments in FRPRC systems appears to be an attractive solution forincreasing the ductility. Reinforced concrete consisting of bothFRP and steel reinforcements is regarded as hybrid FRPRC. Anexperimental study has demonstrated that hybrid FRPRC beamsbehave in a more ductile manner when compared with pure FRPRCbeams [113]. It is found that the ductility increase in view of theadditional steel reinforcement is dependent on the amount ofFRP internal reinforcement, as well as the ratio between FRP andsteel reinforcement [114]. Three FRPRC beams with different FRPcontents (i.e. under-reinforced FRPRC beam, balanced-reinforcedFRPRC beam, over-reinforced FRPRC beam) but the same amountof steel reinforcement are shown in Fig. 5(b)–(d). The ductility per-formance of these specimens has been investigated and it is foundthat the ductility improvement by adding steel reinforcement barsis the highest for over-reinforced FRPRC members when comparedto other counterparts. In over-reinforced FRPRC system, the ratiobetween FRP and steel reinforcement in the range from 1 to 2.5is recommended so that sufficient stiffness and ductility can beensured, as emphasized in the literature [114]. Another beneficialaspect of over-reinforced FRPRC members is that the substantialuse of FRP reinforcements can significantly reduce the amount ofcracks in the concrete surface. This makes the steel reinforcementless susceptible to the chloride attack. Besides, the effect of hookangle in stirrup has been investigated and the results indicate thatthe hybrid FRPRC members with 135� hook stirrup, shown in Fig. 5(e), has better deformability than the one with 90� hook stirrup.Based on the above discussion, the hybrid FRPRC system illustratedin Fig. 5(e) is preferred to be used since it possesses both the sat-isfactory mechanical properties and good durability performance.

3.2. Innovative use of FRPRC: offshore structures

For concrete structures in marine environment, the reinforce-ment corrosion has been a major concern that causes structuraldeterioration and safety problem. The chloride ions from seawaterdamage the passive film around reinforcement and catalyze thecorrosion of steel reinforcement, further leading to the structuraldeterioration and the shortened service life of concrete structures[115,116]. In order to improve the durability and extend the ser-vice life of concrete structures in marine environment, FRP canbe adopted as internal reinforcement for concrete due to its excel-lent corrosion resistant properties compared to steel [117]. Theeffect of chloride on FRP reinforced concrete column has beeninvestigated to verify the effectiveness of FRP reinforced concrete

e beam; (b) under-reinforced FRPRC beam; (c) balanced-reinforced FRPRC beam; (d)re FRP-reinforced concrete has good long term performance but lacks the ductility.cement bars. This ductility improvement is especially effective for over-reinforcedid FRPRC member with 135� hook stirrup behaves more ductile than the counterpart

Fig. 6. Fiber-polymer interface model for FRP composite at different length scales: (a) macroscale model, (b) mesoscale model, and (c) atomistic scale model. The interactioninformation between the material interface and the surrounding can be probed by using MD simulations at the atomistic scale, which is used to develop an appropriatemesoscale model of FRP for investigating the diffusion process of various chemicals. The experimental test of FRP at macroscale level is performed to validate the informationobtained from both the MD simulations and the mesoscale modeling.

580 D. Lau et al. / Construction and Building Materials 126 (2016) 573–585

under chloride attack. The FRP reinforced concrete system castedand cured with two kinds of water possessing two chloride concen-tration levels, i.e. fresh water (0%) and artificial seawater chloridelevel (1.9%), has been studied. The compressive test of FRP rein-forced concrete fabricated with these two kinds of water has beenconducted. The experimental results indicate that the compressionproperties of FRP reinforced concrete fabricated with differentwater are not remarkably different [118–120]. Based on theseresults, it can be concluded that the chloride concentration has lit-tle effect on the mechanical properties of FRP reinforced concrete.In addition, it has been shown that the performance of seawatermixing concrete reinforced by FRP reinforcement bars is reliableand satisfactory [121]. Without the concern of steel corrosion, itis expected that fresh water and river sand used in the concretemixing process can be replaced by seawater and sea-sand respec-tively, and such replacement could be very cost effective for thoseoffshore and marine structures. In particular, a new designapproach using seawater, sea-sand and FRP internal reinforcementbars can be widely used for construction related to artificial islandsince such combination can provide sufficient strength and dura-bility, together with a great convenience.

Excluding chloride, seawater and sea-sand contain other sol-uble salts which may have detrimental effect on FRP, e.g. sulphate.In order to study the effect of various chemicals on FRP properties,a bottom-up multiscale approach can be used. A schematic dia-gram of the bottom-up multiscale approach is shown in Fig. 6. Atthe atomistic scale, the interaction between the material interfaceand the surrounding can be probed through MD simulations[122,123]. An appropriate mesoscale FRP model can be developedbased on the atomistic scale information obtained from the MDsimulations. Through the mesoscale model, the diffusion processof various chemicals can be studied with the correct time andlength scales [124,125]. The findings from the MD simulationsand the mesoscale modeling can be validated through acceleratedFRP test conducted in laboratory [126]. Based on this approach, theeffect of various chemicals in seawater on the mechanical proper-ties of FRP can be fundamentally investigated. Meanwhile, thematerial deterioration mechanism behind the structural perfor-mance at the continuum scale can be well revealed.

4. Fire behavior of FRP composites

Although FRP composites are now widely recognized as effec-tive and efficient materials for civil construction, there is a concernassociated with the behavior of FRP materials in a fire situation.Fire hazard of FRP system should be watched more carefully asmany big fires have been reported for the past few decades

[127]. A room fire begins with ignition of a combustible materialto give a plume, ceiling jet, smoke layer and then flashover. Struc-tural members are exposed to high gas temperature over 600 �Cand radiative heat flux above 20 kWm�2 at floor, 35 kWm�2 atwall and 50 kWm�2 at ceiling in a transition to flashover. Muchhigher gas temperatures and radiation heat fluxes can be recordedin post-flashover fire. Under such fire environment, the FRP systemcannot resist a regular load becuase of the reduced load-bearingcapacity. The polymer resin can become rubbery and viscous whenthe temperature increases to a range between 65 �C and 150 �C[128–130]. At temperatures above 400 �C, FRP systems are suscep-tible to combustion of polymer matrix, and even evaporation.Igniting the polymer matrix at high temperature can lead to therelease of smoke, heat, and toxic fumes [131]. The burning processof FRP systems is very complicated. The fibers can be decomposedand the FRPs can lose their confinement effect. Eventually, concretemembers reinforced by FRPs are not able to perform their load-bearing functions.

One of the first formal studies about the post-fire residualstrength properties of FRPs was reported in 1980 [132]. The ulti-mate tensile strength, tensile modulus, shear strength, and shearmodulus of FRP composites exposed to fire or to high temperaturewere investigated using a simple model. Gas flame burners wereused as the heat source to simulate exposure to fire. Upon exposingFRP to a fire, time to ignition, limiting oxygen index, heat releaserate and flame spread index, and smoke toxicity potency parame-ters should be studied [133]. Data on flammability characteristicssuch as flame spread index, amount of smoke generated, productsof combustion, and the amounts of oxygen required to sustaincombustion on some FRP products [133] were reported. For exam-ple, the mass loss rate, the rate of char formation, and the strengthdegradation of graphite fiber reinforced epoxy matrix compositeswere explored [134].

Thermal response of composite materials was reviewed in theliterature extensively [135]. Basically, the thermal conductionequation was solved to determine the temperature of materials.Physical and mechanical properties at elevated temperatures weremeasured. Decomposition of polymers, such as adhesive was stud-ied. Radiative heat flux from the fire was put in as the boundarycondition to study transient temperature for assessing fire resis-tance rating.

In earlier studies, a semi-empirical approach has beenemployed to describe the failure of composite materials undercombined thermal-mechanical loads [136–138]. Combinedexperimental and theoretical studies were conducted to studythe compressive failures in glass reinforced composites withpolymer matrix, which include vinylester resins and E-glass,quasi-isotropic fabrics with relatively high fiber densities in the

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55 volume percent range, under thermal loading were studied[139]. Effects of fire on the structural integrity of composite mate-rials were reported.

Fire is known as a rapid oxidation of a material which is accom-panied by the process of turbulence, air entrainment and thermalradiation. These phenomena predicted in one instance would betotally unpredictable in a similar situation. There are a numberof scenarios with several possible consequences. In view of this,fire behavior of FRP products should be studied in different scalesin the range from real-scale burning tests to bench-scale tests.

Real-scale tests have the advantage of being representative ofreal-world phenomena. A visual evidence is very important forthe purposes of discovering new phenomena and confirmation ofknown phenomena. In fact, the performance of FRP products canbe evaluated realistically. It is in need of carrying out full-scalephysical experiments for imitating the real-life fire conditions,given that such materials become very popular for strengtheningand retrofitting building structures around the world. The experi-mental data obtained from systematic full-scale burning tests canbe employed to validate the combustion characteristics of FRPproducts in civil construction. The disadvantage of real-scale testsis that expensive human and financial resources are required. Fur-ther, repeatability and reproducibility of the real-scale tests shouldbe watched as such testing results depend on the scenarios andambient conditions, particularly wind effects. Different resultsare yielded for different types of building materials, amounts ofcombustibles, geometry of structures and ventilation provisions.Full-scale burning tests on FRP composites were conducted withina 23-story concrete building in 1964, which are known as the Dal-marnock Fire Tests [140]. The building was erected by reinforcedconcrete slabs with the thickness of 150 mm. The bonded FRP com-posites were tested in two real fire compartment tests to demon-strate how the ventilation of a compartment can be used toprevent the fire from reaching flashover [141]. The fire compart-ment and fire load were identical, but the ventilation parametersdiffered. The tests were performed in living rooms of two identicalflats. The fire load consisted of office furnishings such as a two-seatpolyurethane sofa being the main fuel source, foam padded officechairs, wooden bookcases and desks, and so on. The fuel load inthe test was estimated to be 32 kg/m2 over the floor area. On theother hand, the value of peak heat release rate was 800 kW duringthe fire exposure. The heat flux onwards the ceiling of the compart-ment was measured at 4 min after ignition. This demonstrates thespatial variation in heat fluxes both across the room and across theFRP-strengthened region. The tests focused on the thermalresponse of FRP, but not on the smoke emitted.

Scaled-down tests are easier and less expensive to run. As longas the relations with real full-scale phenomena are known, it ispossible to infer important aspects of the real phenomena in con-junction with theoretical modeling efforts. The objective of thescaled-down tests, together with associated theoretical models isto see how well the aspects of real phenomena are understood. Ifall aspects of the real phenomena are understood, it will be possi-ble just to carry out a small number of real full-scale tests for ver-ification purposes. Real full-scale tests can be carried out to see ifthe results of the small parts, in combination, can be used accu-rately to predict or interpret the real phenomena. The correlationbetween heat release rate and other fire reaction properties ofFRP composite materials was studied by bench-scale experimentswith a cone calorimeter [142]. Thermoset matrix composites rein-forced with combustible fibers (e.g. aramid and extended-chainpolyethylene) and non-combustible fibers (e.g. glass and carbon)were studied under various radiative heat fluxes. The fire reactionproperties measured included time-to-ignition, smoke density,carbon monoxide yield, carbon dioxide yield, mass loss rate andtotal mass loss. Apart from the ignition time, these reaction

properties are found to be linearly related to the heat release ratefor composites containing non-combustible fibers. But for com-bustible reinforcement, the heat release rate is only related tothe carbon monoxide yield, mass loss rate and smoke density insome cases. It is important to correlate the heat release rate withthe smoke density and the carbon monoxide yield, which are thetwo critical reaction properties influencing the human survival infire. Relationship between the peak and average of heat releaserates and the other thermal properties of FRP composites werestudied [142]. The composites were categorized into two maingroups: non-combustible fiber (e.g. woven E-glass, chopped E-glass and unidirectional carbon) and combustible fiber (e.g. wovenaramid and woven extended-chain polyethylene). The polymericresins such as isophthalic polyester, epoxy and three types of vinylester are highly combustible. Hence, these polymers yield a sub-stantial quantity of flammable volatiles when they are thermallydecomposed. Many of the reaction properties, including peak heatrelease rate, increased with the heat flux.

However, smoke emission has not yet been studied carefully. Alimited amount of cone calorimeter results on the combustiblepolymer materials have been presented. As there are numerouscombustibles in the FRP, heat and smoke released upon burningshould be studied. Heat release rate upon burning FRP is an impor-tant fire hazard assessment parameter. A cone calorimeter is favor-ably adopted to study the combustion behavior of FRP compositesunder radiative heat fluxes. Smoke is considered to be the criticalthreat to human life when an accidental fire occurs. Unfortunately,standards pertaining to smoke toxicity have not yet been includedin building fire safety codes and regulations in many countries[143,144]. An expansion to this is that the smoke toxicity is depen-dent on the materials subjected to burning and the fire conditions[141,145]. The toxic potency of fire effluents in different tests hasprovided data on different fire scenarios.

‘‘Non-combustible” materials should be used in buildings asspecified in the building codes in some places including HongKong. Combustible structural materials are allowed in some appli-cations such as protecting prehistoric buildings if they satisfy thefire resistance requirements. However, smoke emitted upon burn-ing FRP should be watched. Therefore, FRP systems do not reachtheir full market potential at this time unless the fire issues areproperly addressed [146,147]. A new approach is to apply flamma-bility data such as time to ignition; limiting oxygen index; heatrelease rate and flame spread index; and smoke toxicity potencyparameters for working out a scaling system on assessing firebehavior of FRP [148,149].

5. Future perspectives

Based on the above discussion, some future research directionsand recommendations related to both the long term performanceand the fire safety issue of FRP composites are outlined in thissection.

5.1. Future work for better understanding of durability performancetowards FRP-bonded systems

The study on the long term durability of FRP composites aremostly relied on the accelerated aging tests. However, the durabil-ity data obtained from accelerated aging tests may differ substan-tially from field data, since the tests conducted over a short periodof time can yield the erroneous results due to the post-curingeffects of polymers and fiber level degradation [94]. Long term datafrom field implementations are essential for the optimal design ofFRP composites in building structures. Besides, evaluation of FRP-strengthened composites under combined conditions (mechanical

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loading, moisture, temperature, and/or other regimes) at both thematerial and structural levels is essential. It is expected that thecoupled effects among these factors will lead to a more complexdeterioration mechanism, which cannot be understood by simplysuperimposing the individual effect.

When using FRP as internal reinforcement bars, the over-reinforced FRPRC design is recommended since it is good in bothstructural performance and durability. In near future, the durabil-ity performance of FRPRC system should be studied in a more com-prehensive manner, which should include the factors related to theFRP and adhesives types, the thickness of concrete cover, and thereinforcement bar diameter as they are all essential componentsto be included in FRP design guidelines.

5.2. Recommendations for the fire attack issue of FRP-bonded systems

The mechanisms contributing to the degradation of advancedcomposites materials (e.g. FRP composites) under fire conditionsstill remain unanswered, posing a major obstacle for implementa-tion of the prescriptive fire specifications. Therefore, a series ofprotections should be provided to reduce the fire harms to FRPcomposites used in the civil construction. Approaches used toaddress the fire issues are grouped into three categories here,namely (1) internal enhancement of fire resistance, (2) externalenhancement of fire resistance, and (3) fire extinguishment. Thefire resistance of FRP composites can be fundamentally enhancedby incorporating the fire retardant additives into the resin andadhesive systems, such as incorporating metallic hydroxides (e.g.alumina tri-hydrate and magnesium di-hydroxide) into resins for-mulations, and developing nano-composites (i.e. polymers blendedwith atomistic-scale fillers like layered silicate or graphite) [150].Regarding the external enhancement of fire resistance, some fire-resistant coating materials (e.g. the intumescent coatings) are cap-able of delaying the temperature rise of FRP composites inside afire. The intumescent coating material can be applied to the surfaceof FRP composites, which can be decomposed into voluminous,multi-cellular char layers when exposed to fire [151]. The porouschar layer acts as a thermal barrier due to its low thermal conduc-tivity, which can prevent the heat penetration forwards the under-lying FRP composites. Additionally, installing active fire protectionsystems such as sprinkler systems or even fire extinguishers on sitecan control the fire. This approach is attempted to control theflame spread and protect the composite structures. Finally, toxicityof smoke emitted from burning FRP composites as pointed out inthe above section should be further explored.

6. Conclusions

In this paper, a state-of-the-art review with respect to the longterm performance and the fire safety aspect of FRP compositesused in building structures is presented. The following major con-clusions are drawn as below.

The review has shown that the mechanical and bond propertiesof FRP-boned system under moisture effect have been widely stud-ied and are now fundamentally well understood. Both the experi-mental work and simulation analysis indicate that moistureingress can severely deteriorate the long-term durability of FRPcomposite structures by depredating the interfacial bond strength,toughness and adhesion energy. Besides, a shift of failure mode hasbeen demonstrated in the FRP-bonded systems when comparingthe dry and wet samples. In addition, the temperature effectshould not be overlooked, especially when the substrate is woodinstead of concrete. It should be emphasized that the multiscalemodeling of FRP-bonded system is necessary for a comprehensiveunderstanding towards the deterioration problem.

The assessment of interface integrity for existing FRP compos-ites under environmental effects is needed in order to sustain thesatisfactory performance of FRP-strengthened structures. Theacoustic-laser technique has been confirmed to be able to evaluatethe interfacial bond deterioration for FRP-bonded system, espe-cially this technique is very effective when the material tempera-ture is high. This technique is economic and easy to be used. It isenvisioned that this technique has a high potential to be adoptedin practice.

Over-reinforced hybrid FRPRC system is preferred to be appliedbecause it is good in both the structural performance and durabil-ity. In the hybrid FRPRC system, the addition of steel reinforcementimproves the ductility of the reinforced system significantly. More-over, the large amount of FRP reinforcement can result in fewerconcrete cracks and can lead to a loss of capillary path for the chlo-ride ion transport. In this circumstance, the steel corrosion in thecomposite system is less likely to occur. FRPRC design is particu-larly useful for the civil infrastructure construction works on arti-ficial islands. Seawater/sea-sand concrete reinforced by FRP rebaris a promising candidate in this situation as the steel corrosionproblem can be fully avoided, while the structural performanceof FRPRC is comparable to that of the conventional reinforcedconcrete.

A lot of studies have pointed out that the FRP composites can bedecomposed under fire and eventually their confinement effect candisappear and the composite structures can collapse. The experi-mental and numerical studies such as semi-empirical approach,real-scale tests, full-scale burning tests, scaled-down tests, andbench-scale experiments have been conducted to understand thethermal behavior of FRP materials. However, smoke emission,which is considered to be the critical threat to human life whenan accidental fire occurs, has not yet been studied comprehen-sively. In order to use the FRP composites in a safe manner,approaches for reducing the fire harms to FRP composites are pro-vided in this paper.

Acknowledgments

The authors are grateful to the financial support from CroucherFoundation through the Start-up Allowance for Croucher Scholars(No. 9500012). The support from the Research Grants Council(RGC) in Hong Kong through the Early Career Scheme (ECS) (No.139113) is also gratefully acknowledged. In addition, the GeneralResearch Fund (GRF) project (No. 11202314) is acknowledged.

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