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What's the difference between GFRP and steel reinforcement?

What’s the difference between glass fibre reinforced polymer (GFRP) and steel rebar?

 

As concrete is strong in compression and weak in tension, it is usually reinforced to create a homogeneous building material that is strong in both compression and tension. This reinforcement traditionally consists of steel due to its high strength, low cost and high ductility. However, steel’s vulnerability to corrosion costs governments and asset owners billions of dollars in maintenance costs to repair or replace aging infrastructure. In severe cases, corrosion can lead to structural failure and the loss of life. With advances in materials technology, many other materials are being adopted for use in reinforced concrete in lieu of steel rebar. These include steel or carbon fibres, carbon fibre cables, glass fibre reinforced polymer (GFRP) rebar and even bamboo.

 

GFRP rebar, in particular, is increasingly being used as an alternative to steel reinforcement because it doesn’t corrode, is extremely lightweight and is non-conductive. These characteristics make it durable in corrosive environments, easy to transport and install, and beneficial for applications in electrical environments such as rail structures.

 

Composition of GFRP vs Steel Reinforcement

Glass fibre reinforced polymer (GFRP) – sometimes called glass fibre reinforced plastic or fibreglass – is composed of a polymer plastic matrix with embedded glass fibres. The polymer for GFRP rebar usually consists of a vinyl ester, epoxy or polyester thermosetting plastic. Steel is a metal alloy composed of iron with a small percentage of carbon. The material differences between GFRP and steel means their structural performance and durability differ when used as reinforcement in concrete.

 

Structural Performance of GFRP vs Steel Reinforcement

GFRP reinforcement can have more than double the tensile strength of steel; however, it has lower flexural (bending) strength, lower yield strength and lower modulus of elasticity. This means GFRP can tolerate greater levels of force than steel when used in situations that place the element in tension such as at the bottom of a simply supported beam or the top of a cantilevered slab. However, steel rebar will tolerate greater levels of elastic deflection than GFRP before yielding or permanent deformation occurs.

Steel is characterised by high ductility, which means it will deform plastically before failure. Compared to steel, GFRP has an elastic behaviour and is not ductile, which means it has a rupture point rather than a yield point.

These characteristic differences mean GFRP-reinforced concrete is usually designed for concrete crushing failure while steel-reinforced concrete is typically designed for yield failure.

 

 

GFRP Rebar
Steel Rebar

Tensile strength (MPa)

>1000

450

Modulus of elasticity (GPa)

>60

190 to 200

Transverse Shear Strength (MPa)

220

300

Bond strength to concrete (MPa)

>20

>12

Ultimate strain %

1.5 - 2%

15%

Density (Kg/m3)

2100

7800

 

  • Deflection in GFRP-reinforced structures is greater than steel due to GFRP’s lower modulus of elasticity. This may require larger section sizes or a higher reinforcement ratio to be used in structural applications.
  • GFRP has substantially greater bond strength to concrete than steel which makes GFRP-reinforced concrete more resistant to cracking. In addition, crack tolerances in aqueous conditions are higher in GFRP structures (0.7 mm) than steel (0.4 mm) due to GFRP’s corrosion resistance.
  • GFRP has 20 times the fatigue resistance of steel, making GFRP more durable under cyclic loading – 420,684 cycles for GFRP compared to 23,162 cycles for steel.
  • GFRP is a quarter of the weight of steel, making it easy to transport and handle by workers while reducing the dead weight of concrete structures.

 

Durability Performance of GFRP vs Steel Reinforcement

The durability of steel rebar ultimately depends on the amount of concrete cover provided to keep it protected in a stable, alkaline environment within the hardened concrete. Over time, carbon dioxide and chlorides in the air or water penetrate into the pores of the concrete and reduce the alkalinity or pH level of the hardened concrete. Eventually, these carbon and chloride agents will penetrate to the depth of the steel reinforcement and lower the pH level around the steel, breaking down the protective ‘passivation’ layer that protects the steel from corrosion. The design of a steel-reinforced concrete structure for a 100-year lifespan in coastal, saline or otherwise harsh conditions often requires specifying high concrete cover (well in excess of / exceeding the structural requirement), applying a coating on the concrete or using / adopting a cathodic prevention system.

GFRP’s corrosion resistance eliminates carbon- and chloride-induced corrosion, removing the need for excessive concrete cover, protective coatings or an expensive cathodic prevention system. As a result, GFRP bar slightly reduces concrete consumption and substantially reduces whole-of-life asset maintenance costs associated with steel-reinforced concrete structures. These characteristics make GFRP most suitable and economical for harsh conditions and situations where/in which cracking and corrosion are of greater priority than structural capacity. For example, in immersed conditions, splash zones and coastal environments, around corrosive chemicals and de-icing salts, where subjected to high levels of carbon dioxide, and for slabs-on-grade.

 

Conductivity of GFRP vs Steel Reinforcement

In electrical environments such as rail corridors, the conductivity of steel rebar has been known to facilitate the movement of stray currents. Stray currents can lead to safety hazards as well as sacrificial anodic behaviour between dissimilar metals, causing accelerated corrosion of fixtures, fittings and building components. As GFRP is non-conductive it can provide electrical separation and prevent stray currents in electrically sensitive environments. This makes it a suitable alternative for steel in electrical environments such as rail corridors, power plants and substations.