White Paper by by David Trejo, PhD
There has been significant development in the design and use of GFRP reinforcing bars. This
report provides an overview of the literature on the properties, characteristics, and performance
of GFRP bars. This literature review indicates that when compared with steel reinforcement,
GFRP reinforcing bars exhibit lower modulus of elasticity values, lower ductility, and lower barconcrete
bond. The literature also indicates that structures reinforced with GFRP can exhibit
larger deflections and larger crack widths. GFRP bars are also lighter than steel bars and are
non-conductive and non-magnetic, which provide some potential benefits for specific situations.
However, one characteristic that has not achieved consensus in the engineering community is
the durability of GFRP when embedded in concrete. In the large majority of cases concrete has a
high relative humidity within its pore system and this moisture consists of a high pH solution.
When GFRP reinforcing bars are embedded in concrete, the composite surrounding the glass
fibers is supposed to protect the glass fibers from potential deterioration. However, the high pH
pore solution can penetrate this composite material. As the high pH pore solution penetrates
the composite it eventually reaches the glass fiber and these fibers are susceptible to etching
and leaching. This etching and leaching of the glass fibers in the GFRP can result in loss of bond
between the glass fibers and composite material or can result in weakening of the glass fibers.
Both debonding and loss of glass fiber strength result in loss of strength in GFRP reinforcing
bars. Loss of strength in GFRP reinforcing bars has been reported throughout the literature.
Unlike steel reinforcement, which when corroded exhibits signs of degradation (e.g., rust stains
or concrete delamination), when GFRP degrades there are no visual signs of this degradation.
Some recent research indicates that GFRP reinforcing bars embedded in field structures do not
exhibit degradation or loss of strength. However, these research studies contained no direct
measurements of the residual tensile strength of GFRP reinforcing bars. Although indirect
measures of GFRP degradation are improving, direct measurements of GFRP reinforcement
degradation provide a clear and unambiguous measure of performance. Because the literature
indicates that GFRP reinforcing bars do lose strength with time and because degrading GFRP
bars provide no visible warning of this degradation, the engineering community must move
forward in using this reinforcement with caution.