Stronger and lighter. Why didn’t FRP replace steel in concrete reinforcement so far?
Conventional concrete structures are usually reinforced using reinforcing steel bars which are usually protected from the surrounding conditions by the normal alkalinity nature of concrete itself. However, in an aggressive environment, the need for a more resistant material increased. Fiber-reinforced polymer (FRP) materials have emerged as an alternative material for producing reinforcing bars for concrete structures, the research started in as early as the 1960s. Significant progress has been made in the last few years to provide the technical, legal, and contractual framework necessary for the deployment of FRP reinforcement for concrete structures
FRP or Fiber Reinforced Polymer (some refer to as Fiber Reinforced Plastic) is a composite material made of fibers embedded in a polymeric resin matrix. The fibers provide strength and stiffness to the composite and generally carry most of the applied loads. The matrix acts to bond and protect the fibers to provide for the transfer of stress from fiber to fiber through shear stress.
Separately, the two materials (the fiber and the matrix) will not perform as needed; the matrix itself is weak in tension and will not have the required ductility to reinforce the concrete element. On the other hand, the fibers need the matrix to work as a unit with the load equally shared between all the fibers in the same section.
The strength and elasticity are enhanced in a fibre-reinforced polymer depending on the mechanical properties of both the fibre and matrix, their volume relative to one another, and the fibre length and orientation within the matrix. Reinforcement of the matrix occurs by definition when the FRP material exhibits increased strength or elasticity relative to the strength and elasticity of the matrix alone.
Depending on the type of fibers used to reinforce the polymer; various types of FRPs emerged such as:
Aramid Fiber Reinforced Polymer (AFRP).
Carbon Fiber Reinforced Polymer (CFRP).
Glass Fiber Reinforced Polymer (GFRP).
FRP Pros
High Strength
Since there is a wide range of combinations between different types of fibers and matrices, the strength of the FRP widely varies. The range starts at somewhere above 2000 MPa up to around 6000 MPa compared to the much humbler range of carbon steel ranging from 420 MPa to 790 MPa as per ASTM A615-20
Lightweight
The Density, again, varies widely but never as heavy as the steel. The range starts at 1.5 gm/cm3 to 2.1 gm/cm3 where the density of steel is 7.9 gm/cm3
Corrosion Resistant
Given the non metallic nature of the polymers in FRP’s composition; it is insensible to the major concern of the designers of concrete elements in severe conditions; chloride attacks. which could substantially enhance the corrosion resistance for structures.
This becomes of special importance where deicing chemicals are used to help dissolve ice in low-temperature locations.
Dimensionally Stable with Low Thermal Conductivity
The low Coefficient of Linear Expansion easily beats (for most types of FRP) the 11.7 10–6/C of steel which allows for lesser concerns about the thermal expansion while design
Nonconductive and Electromagnetically Transparent
For those who have worked in design and construction of clinics or hospitals with MRI facilities, they must have noticed that special arrangements were to be made to avoid the interference of normal steel bars, given their ferro-magnetic nature, with the magnetic field created by the MRI machine.
Degradation Behaviour
In case of FRP degradation, the degradation mechanism is benign to the surrounding concrete unlike steel that expands and causes failure in members.
FRP cons
High Initial Cost (compared to steel reinforcement)
Although not many of the FRP producers reveal their products' prices, but Historically, fiberglass, whatever the product or the part is—is 1 to 1.5 times more expensive than its counterpart, say in steel. What you get with that is a product that’s going to last a lot longer because your product life cycle is a lot longer.
Susceptibility to Fire (without insulation)
FRP materials are generally more sensitive than steel reinforcing bars to elevated temperatures, the reinforcement type influences the fire resistance of FRP-reinforced concrete elements; these may have lower fire resistance compared to steel-reinforced elements of equivalent ambient temperature capacity (ACI 440.1R-15)
Inability to Bend in the Field
GFRP rebar is orthotropic and non-ductile material, GFRP can be cut using steel saw, a band saw and or grinder with a diamond blade, but not the commonly used shearing machines. For bending, an example of an FRP bars manufactured with Vinyl-Ester Thermoset Matrix, you cannot bend such bars after curing cycle, all the bends have to be fabricated in the FRP’s Manufacturing Plants
Low Strain to Failure (Lack of Ductility)
Except for a very limited combination of Glass FRP and Aramid FRP, the modulus of elasticity of FRPs is far less than that of steel. For the common types of FRP, the modulus of elasticity of FRP ranges between 35 and 86 GPa where that for steel lies at a staggering 200 GPa.
FRP is a relatively new and promising alternative to the normal steel reinforcement bars.
FRP has several types depending on the type of the fiber and the matrix.
FRP has more strength than steel.
FRP is lighter than steel.
FRP has lower modulus of elasticity than steel.
FRP is generally brittle.
Most FRP bars can not be processed (bent) in construction sites.
FRP can have higher initial cost but lower long-term cost compared to steel, especially in an aggressive environment.
ACI 440.1R-15: Guide for the Design and Construction of Structural Concrete Reinforced with Fiber Reinforced Polymer (FRP) Bars.
ASTM D7957/D7957M - 17: Standard Specification for Solid Round Glass Fiber Reinforced Polymer Bars for Concrete Reinforcement
Fiber-Reinforced Polymer: Reinforcement for Concrete Structures, Kiang Hwee Tan Concrete International magazine, volume 43 No.8
Properties evaluation of fiber reinforced polymers and their constituent materials used in structures – A review
ACI STRUCTURAL JOURNAL TECHNICAL PAPER: Tensile Lap Splicing of Fiber-Reinforced Polymer Reinforcing Bars in Concrete