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CROSS Safety Report

GFRP reinforcement in concrete structures

Report ID: 939 Published: 1 August 2020 Region: CROSS-AUS

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A correspondent has become aware of work being undertaken in the field of glass fibre reinforced polymer (GFRP) reinforcement as a substitute for steel rebar in concrete structures and is deeply concerned by what they have observed.

This report highlights the difference in properties of GFRP and steel bar reinforcement in concrete structures. Concerns about behaviour of GFRP-reinforced concrete in fire conditions are also raised.

Key Learning Outcomes

For structural and civil engineers:

  • Design using GFRP reinforcement is significantly different to that using steel reinforcement

  • Deflection (rather than stress) is likely to be the controlling feature when designing with GFRP structural elements

  • Structures constructed with GFRP as a structural element are likely to behave in a significantly different manner to those incorporating steel elements

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A correspondent, who is a Chartered Structural Engineer with many years’ experience in the civil / structural industry, has become aware of work being undertaken in the field of glass fibre reinforced polymer (GFRP) reinforcement as a substitute for steel rebar in concrete structures and is concerned by what they have observed. The correspondent raises the following issues of concern:

Differences between GFRP and steel bar reinforcement

  • There is a lack of recognition that GFRP bars are anisotropic with very low strength and stiffness in the radial direction. This is due to the glass fibres being aligned with the longitudinal direction only and material properties are consistently quoted for the fibre direction alone.

  • When used in columns, the GFRP spirals are being compressed in their radial direction and the stiffness is, at best, the stiffness of the epoxy matrix around the glass fibres (e.g. around 3GPa versus 30GPa for concrete say). This may be considered as being analogous to wrapping rope around the main longitudinal bars.

  • Thus, the GFRP spirals may create an annulus of weakness in the axial direction of the column because of the low radial stiffness. This appears to manifest as complete blow out and loss of the concrete cover zone through concrete buckling.

GFRP reinforcement for concrete columns designed as if made of steel

  • Minimum and maximum longitudinal reinforcement ratios for columns appear to be taken directly from AS3600 for steel reinforcement (1 and 4 % respectively), with no evidence as to the applicability of these design requirements. It is noted that the tensile capacity of GFRP bars is reported to be around 1200 to 1300MPa and is more than double that of conventional steel rebar, hence these steel reinforcement ratios are not applicable for GFRP bars.

  • There will be similar effects when GFRP reinforcement is used in beams. If AS3600 reinforcement ratios are adopted, this could lead to non-ductile failure of the compression zone; and the low radial stiffness of GFRP ligatures may create a weakening effect along the length of the member in the compression zone.

Caution needed

It is the correspondent's view that caution needs to be exercised when GFRP for steel reinforcement substitution is being considered. In particular, the effects of anisotropy and the crucial differences between the materials must be taken into account. In the opinion of the correspondent new combinations of materials must always be critically reviewed to ensure that design rules and test methods remain applicable.

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There is limited published information on the use of GFRP reinforcement bars in Australia and the reporter correctly notes that caution must be exercised when using any new material. The properties of the material must be understood and considered in the design and certainly one cannot treat the design as a straight substitution of GFRP bars for steel.

Guidance from overseas experience

There has been considerable experience in the use of GFRP reinforcement in Canada, Europe, Japan and the USA and to quote from a paper by Feeser and Brown: Guide Examples for Design of Concrete Reinforced with FRP Bars:

“A direct substitution between FRP and steel reinforcement is not possible due to differences in the mechanical properties of the two materials. The modulus of elasticity of FRP is much lower than that of steel; thus, larger strains are needed to develop comparable tensile stresses in the reinforcement. If a direct substitution of FRP for steel reinforcement was used, FRP reinforced sections would have larger deflections and crack widths than comparable steel reinforced sections.”

For use in building structures, reference can be made to American Concrete Institute (ACI) publication 440.1R-15 “Guide for the Design and Construction of Structural Concrete Reinforced with Fiber-Reinforced Polymer Bars”. From this guide the following should be noted:

“The mechanical behaviour of FRP reinforcement differs from the behaviour of conventional steel reinforcement.  Accordingly, a change in the traditional design philosophy of concrete structures is needed for FRP reinforcement. FRP materials are anisotropic and are characterized by high tensile strength only in the direction of the reinforcing fibres. This anisotropic behaviour affects the shear strength and dowel action of FRP bars as well as the bond performance. Furthermore, FRP materials do not yield; rather, they are elastic until failure. Design procedures must account for a lack of ductility in structural concrete members reinforced with FRP bars.”

Deflection likely to control design

Thus, as pointed out by the reporter, there are significant differences in behaviour when GFRP bars are used in reinforced concrete when compared with steel reinforcement as a result of their anisotropic behaviour, low modulus of elasticity and lack of ductility that results in brittle failure of members in bending and in most cases deflection will control the design. It is also noted from the ACI Guide that FRP reinforcement has significantly lower compressive strength than tensile strength and it recommends that the strength of FRP bars in compression should be ignored in design. Caution is therefore required by designers who may not be familiar with the performance of GFRP bars.

Behaviour in fire conditions

Although not raised by the reporter, the behaviour in fire of GFRP reinforced concrete must also be considered. The ACI Guide referenced above contains a section on the effects of high temperature and fire and notes that the type of FRP reinforcement, the aggregate type and concrete cover will all influence the behaviour in fire and the loss of bond due to the softening of the resin is critical. For further work in this area refence can be made to a recent ASCE Technical Paper, GFRP-Reinforced Concrete Slabs: Fire Resistance and Design Efficiency. At present in Australia it is unlikely that the material would achieve an acceptable fire rating for use in building structures, without demonstrating its acceptance in accordance the Australian Building Code Board Developing Performance Solutions.

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