FRP VS Steel Rebar
FRP rebar is quickly gaining ground in many areas of the construction world because of its low weight, high tensile strength, and corrosion resistance. These qualities seem to make FRP a great option for Polymer Concrete manholes, which serve primarily as a lighter, corrosion resistant alternative to traditional concrete manholes. However, the low modulus of elasticity and lack of ductility of FRP bars put them at a severe disadvantage to traditional steel rebar for this use scenario.
Steel rebar has a yield strength of about 515 MPa at the top of its range. This is the point at which the rebar begins to permanently bend. Steel is ductile and can withstand being significantly bent without failing. Glass FRP or GFRP, the most commonly used FRP, is not ductile and does not retain any strength after being pushed past it’s tensile strength of 480-1600 MPa. In addition to this, the low elastic modulus of GFRP (35-51 MPa vs steel’s 200 MPa) means that GFRP bars will begin bending under even small loads. They won’t snap until their tensile strength is exceeded, but they will bend quite easily. Below, you can see FRP cages bending under their own weight.
This major shortcoming limits the proper use cases of FRP bar to prestressed applications where the low elastic modulus can be compensated for. When it is used in precast, the low elastic modulus leads to significant cracking at loads far lower than the point at which steel reinforced concrete cracks. For some design scenarios, this isn’t necessarily an issue, and cracks are even welcome, because they can serve as an early warning signal. Manholes are not one of those cases. Cracks in manholes allow for inflow and infiltration issues, and they can go unnoticed for years while they cause major issues for the surrounding environment and the wastewater treatment facility.
On the point of corrosion resistance, FRP does have good resistance to most chemicals, including those most commonly found in wastewater systems. However, that would only be a true benefit if the manholes were made from traditional, porous concrete. Polymer manholes are non-porous and highly corrosion resistant throughout the entirety of the material. This means that there is no opportunity for the corrosive chemicals to ever contact the reinforcement, be it FRP or steel.
Usually, when FRP is used in polymer concrete applications, it has nothing to do with any perceived mechanical or chemical advantages at all. FRP is often used, because the resins in polymer concrete contain styrene, and they shrink as they cure. When steel rebar is used, it provides nucleation sites during the shrinking for cracks to form. Those cracks start small, but grow during transport and can often lead to critical failure before the structure even makes it into the ground. Some polymer concrete manufacturers have gotten around this issue by using FRP, which doesn't provide the same nucleation sites, because it's made of the same resin as the polymer concrete. Other manufacturers have continued using steel, but they thicken their walls to compensate for the cracking. We developed a resin without styrene that doesn't shrink as it cures.
There is some precedent for using FRP reinforcement for manhole precasting, and P3 Polymers is able to design for FRP using the standards laid out in ACI 440.1R-15. However, the design standards as well as the testing history for FRP are still somewhat limited, and there is nothing specific to manhole reinforcement like ASTM C478 is for steel manhole reinforcement. It is too early to say if FRP is going to be suitable for the wastewater industry or what the best practices will be for its use. That's why P3 Polymers will continue to recommend and design around steel reinforcement.