Elsevier

Composite Structures

Volume 85, Issue 2, September 2008, Pages 164-174
Composite Structures

Degradation of bond between FRP and RC beams

https://doi.org/10.1016/j.compstruct.2007.10.014Get rights and content

Abstract

Beams and slabs externally reinforced with FRP are often in contact with moisture and temperature cycles that reduce the expected durability of the system. Bond degradation is a frequent cause of premature failure of structural elements and environmental conditions are known to relate to such failures. The study shows the effects of cycles of salt fog, temperature and moisture as well as immersion in salt water on the bending response of beams externally reinforced with GFRP or CFRP, especially on bond between FRP reinforcement and concrete. Temperature cycles (−10 °C; 10 °C) and moisture cycles were associated with failure in the concrete substrate, while salt fog cycles originated failure at the interface concrete–adhesive. Immersion in salt water and salt fog caused considerable degradation of bond between the GFRP strips and concrete. However, immersion did not lower the load carrying capacity of beams, unlike temperature cycles (−10 °C; 10 °C) that caused considerable loss. No significant differences were detected on the behavior of the systems strengthened with GFRP and CFRP, perhaps because the design of the tests impeded failure of the fibres.

Introduction

The external strengthening of slabs and beams with strips of fibre reinforced polymers (FRP) relies on bond developed between concrete and the composite laminate. Bond depends on several parameters and has been the subject of numerous studies, but, despite that wealth of information, the durability of bond under the effect of environmental conditions is not sufficiently understood.

The durability of a structure reinforced with FRP reflects its ability to resist cracking, oxidation, chemical degradation, delamination, wear, and/or the effects of foreign object damage for a specified period of time, under specified load and environmental conditions [1].

Structures externally reinforced with FRP are often in contact with moisture and temperature cycles that reduce their expected durability. Adhesives may deteriorate when exposed to moisture and salt water cycles, ultraviolet radiation, alkalines, and temperature nearing transition vitreous values. Chlorides may be especially severe when the FRP externally strengthens structures in marine environments or cold regions where salt deicing is used. Due to insufficient knowledge of those effects, design rules accounting for the effect of aggressive environments are thought to be overly conservative.

Despite the lack of data on environmental degradation of bond, numerous studies of environmental effects on strength and deformability of FRP laminates and structural elements strengthened with FRP can be found and shed light on the phenomena affecting bond.

It is established that the deterioration of FRP is triggered by the diffusion of light atomic weight free ions, i.e. OH and Cl and water molecules into the polymeric matrix [2]. Absorption accelerated by the cracks and voids in the matrix may cause hydrolysis and polymerization that induce changes of the matrix. When diffused chemicals contact the surface of the fibres, their ensuing degradation decreases the tensile strength and stiffness.

Physical degradation of the resin systems may be due to high temperature or moisture exposure. Temperatures, especially when close to the glass transition temperature of the matrix Tg, may ‘free volumes’, which can be filled by other molecules as the polymer attempts to reach an equilibrium stage and may change mechanical properties of the materials.

Absorbed moisture can cause plasticization of the matrix, i.e. reduction of Tg and mechanical characteristics due to the interruption of Van der Waals bonds between the polymer chains and be responsible for increasing the effective free volumes in the resin system [3].

Alkaline solutions are one of the most severe causes of degradation of glass fibres. Concrete pore water is alkaline and a critical environment for glass fibres due to the hydration of the cement [4]. The most important reaction for the dissolution of the glass fibres in water is “leaching”, the diffusion of the alkali ions out of the glass structure, a temperature dependent process [5], normally more important for reinforcement rods than outer strips.

Kootsookos et al. [6] studied CFRP and GFRP, with polyester and vinylester matrices, immersed in seawater at a temperature of 30 °C and showed that the carbon composites displayed better durability. The moisture absorbed by CFRP composites is lower compared with GFRP, the flexural properties of the glass composites declining slowly for long periods in seawater.

Chajes et al. [7] showed a 36% decrease in ultimate strength for GFRP retrofitted specimens that were subjected to 100 wet/dry cycles, while a 19% reduction was shown for CFRP bonded specimens.

Toutanji and Gomez [8] observed a strength reduction up to 33% on specimens made of different epoxies and subjected to 300 wet/dry cycles in salt water. Failure was reported as a debonding mode that generally took place near the FRP/concrete interface. Karbhari and Zhao [9] reported micro-cracking in the GFRP strip that strengthened cement mortar beams. Specimens were subjected to four-point bending mechanical tests until failure. A 40% reduction in bending capacity was noticed after 120 days of moisture exposure.

Severe losses of pre-stress were found on GFRP subjected to dry/wet cycles with artificial sea water [10]. The authors showed that total immersion of structural composites in water caused degradation of mechanical properties if combined with relatively high temperature.

A study of effects on GFRP laminates due to immersion in water, selected dry–wet (20%RH; 90%RH), thermal (20 °C; 50 °C) and salt fog spray (8 h at 96%RH; 16 h drying) cycles, and cycles of UV radiation with flooding is described in [11]. The results are correlated with those on the strength and deformability of RC cylinders confined with the same GFRP material. Major conclusions reported:

  • Mass gain of GFRP plates immersed in distilled water was higher than for salt water. The results agree with Almeida [12] who showed that the moisture uptake in composites is quicker with deionized water, due to the size of ions in the salt solution where large ions prevent moisture uptake. Although true for short-term degradation the conclusion cannot be extended to long-term exposure when significant strength degradation due to the presence of salt has been found [2].

  • The salt fog spray cycles were the most severe of the actions considered.

  • Noticeable decay of tensile strength was detected on the laminates under salt fog cycles and immersed in water at 22 °C.

  • The results for ultra violet radiation combined with flooding indicate that unprotected material should not be used for applications requiring constancy of strength.

  • The effects of environmental conditioning on confinement of concrete with GFRP jackets are much less severe than those found on the mechanical properties of the GFRP material alone.

Environmental effects, specifically, on bond have been object of fewer publications. A brief survey of such research can be found in [13], comprising tests devised to measure bond on small scale physical models and on environmental degradation.

Temperature effects have been studied more extensively than those caused by moisture, especially due to the importance of temperature on the curing process and, consequently, on the vitreous transition temperature Tg. Glass transition temperature of polymer depends on the chemical bonds in the polymer structure and also on the amount of free volumes and may be used to assess the effects of moisture absorption or chemical diffusion on the physical degradation that can damage the bond in the polymer structure itself.

If Tg of the resin system decreases due to environmental effects, degradation takes place, originating a weaker cross-linked structure with reduced stiffness and strength of the matrix.

Wet lay up techniques used in structural civil engineering are often associated with incomplete matrix cure at ambient temperature. Exposure to elevated temperatures for short time increases the Tg, causing additional cross-linking between the polymer in a process called ‘post-curing’. The fact that additional chemical reactions occur between the unreacted groups may also increase the Tg. Environmental operational conditions, namely with temperature increase, may contribute to the conclusion of the process of curing.

Walker and Karbhari [14] investigated the durability of three FRP composite systems through tests conducted on ring-type specimens. Accelerated aging through the immersion of specimens in water at different temperatures were used to predict long-term durability in terms of tensile strength, modulus and ultimate strain. After correlation with field data, materials properties are predicted and then used for design of seismic retrofit.

Dohnálek [15] presents a brief summary of studies on freeze–thaw effects and, additionally, shows results of lap shear and pull out tests that led to the conclusion that the durability of the adhesive bond depends essentially on the freeze–thaw durability of the concrete. Conclusions reported and drawn from previous freeze–thaw studies include:

  • CFRP has higher environmental durability than either GFRP or AFRP.

  • The ultimate strength of the adhesive bond deteriorates, under freeze–thaw cycles in chloride solution.

  • The combined action of freeze–thaw cycles and chlorides was the most damaging type of freeze–thaw cycles.

Effects of temperature on adhesion strength that develops between an epoxy adhesive and concrete elements, in the presence of water, are also described in [16]. Near the vitreous transition temperature Tg of the adhesive, the bond strength deteriorated, independently of the concrete substrate. The mode of failure was also dependent on temperature, changing from a mixed failure to failure at the interface between concrete and adhesive at higher temperatures. The authors suggest that the Tg of the adhesive should be 5–10 °C higher than the operating temperature.

Studies similar to those reported in this article are described in [17]. The authors tested plain concrete beam specimens 90 cm long, reinforced with externally bonded wet-laid GFRP sheets. The beams were pre-cracked and subjected to different environmental conditions including elevated temperature/dry, and freeze/thaw cycles. Debonding of the GFRP sheet from the concrete was the more common cause of failure. The ultimate strength of the GFRP reinforced beams decreased for specimens subjected to cycles of dry freezing and wet thawing.

Fava et al. carried out some recent experimental tests on environmentally conditioned FRP plates bonded to concrete and deduced these conclusions [18]:

  • Freeze–thaw cycles reduce bond shear strength and peak slip, so that the loading branch (in force-elongation diagram) shows highly non linear bond behaviour at a lower applied force.

  • Beneficial effects of high humidity level characterizing salt spray fog overcame damage due to presence of chloride solution.

  • Higher deformability of interface behaviour (high peak slip) appeared to be the only effect of aggressive environment.

A broad presentation of the unanswered questions of long-term performance and durability issues with respect to the effects of moisture, thermal cycles, freeze–thaw actions, and coupling of environmental and mechanical loading is offered by Buyukozturk [19]. The author believes that the understanding of bond durability requires the acceptance that moisture affected debonding may involve material decohesion and/or interface separation and requires explicit analytical approach, beyond the usual limited strength-based approaches. Physical changes in the bond as well as in the constituent materials are shown in moisture affected debonding. Fracture toughness is shown to decrease by as much 60% and become asymptotic for a certain moisture concentration value for CFRP strengthening. Concrete delamination mode was observed for dry specimens, while the epoxy/concrete interface separation occurred in all wet fracture specimens tested. Interface fracture analysis indicates that this interfacial debonding mode is attributed to an interfacial material toughening and an interface weakening mechanism due to moisture diffusion.

Due to the relative paucity of data on degradation of bond between FRP and concrete and insufficient knowledge of the corresponding mechanisms, design rules accounting for the effect of aggressive environment are currently fairly conservative. These facts warrant studies on environmental aging as part of an evaluation of the long-term durability of adhesively bonded systems.

Section snippets

Experimental study – GFRP reinforcement

Environmental effects on GFRP laminates have been reported [11]. GFRP laminates are vulnerable to moisture because of negative effects of absorption on their physical properties. Sorption is mostly caused by diffusion due to a concentration gradient and also to relaxation swelling of the composite. Tests on the effects of water immersion on plates of GFRP revealed lowering of the mechanical properties in the vicinity of 2000 h, with a recovery around 180 days and a return to initial values after

Experimental study – CFRP reinforcement

The specimens for bending tests and the slabs for pull-off were geometrically equal to those described earlier, except for the replacement of GFRP by CFRP and their study part of another MSc thesis presented elsewhere [25].

The compressive strength of the cubes, extrapolated for the time of each set of bending tests, is shown in Table 4 and it is seen that the resistance is lower than for the GFRP tests.

The average tensile strength, evaluated in cylindrical coupons, was fctm = 2.3 MPa.

The tensile

GFRP

  • In terms of ultimate capacity of beams:

    • The temperature cycles were most detrimental, with a loss of 31% at 10,000 h.

    • The immersion tests showed a gain of 21% at 10,000 h, due to the conjugated effect of increase in tensile strength of concrete and post-curing of the polymers, the decrease in strength of the laminate being negligible.

  • In terms of bond:

    • Aging associated with salt water, i.e. immersion and salt fog, caused considerable degradation of bond between the GFRP strips and concrete.

    • Salt fog

Acknowledgements

The studies were partially financed by Fundação para a Ciência e Tecnologia through Project POCTI/36059/ECM/2000 – Behavior and Design of Concrete Structures Strengthened with FRP considering Ageing Effects. Most of the experimental results on CFRP were obtained by a former student, R. Marreiros, now engaged on different work.

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  • Cited by (0)

    1

    Engenharia Civil, Instituto Politécnico, Portalegre, Portugal.

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