The steel-reinforcement bar, or “rebar”, has been used in concrete construction systems since the mid 1800s. While steel is strong, ductile and has in the past been relatively cheap, it’s susceptibility to corrosion has always been an issue. And with transport costs increasing, its weight is also an issue. That explains why in recent decades we’ve seen a move towards finding better alternatives to steel. Some of these include steel or carbon fibres, carbon fibre cables, bamboo and glass fibre reinforced polymer (GFRP) rebar.
Arguably, the most successful of these substitutes is the GFRP rebar. We’ll discuss the similarities and differences in further detail below, but in short: GFRP doesn’t corrode. It’s got twice the tensile strength of steel and has twenty times more fatigue resistance. It’s safe to use in electrical environments such as rail structures because it’s non-conductive. And it’s easier to transport and install as it’s extremely lightweight. That’s why Reon Products are proud to offer MST bar as an innovative, more sustainable alternative to steel.
How GFRP rebar is made
The raw materials, made up of mostly silica sand, are weighed in exact quantities and mixed, or “batched” and are melted by exposure to a high temperature (1400ºC). After dissolving into molten glass, it flows into the refiner, where the temperature is reduced to 1370ºC. GFRP is then formed by ‘fiberisation’. This occurs when the molten glass is brushed with an erosion-resistant platinum alloy, attenuated to create filaments and cooled with water jets.
Finally, a chemical coating is applied with a polymer plastic matrix, usually consisting of a vinyl ester, epoxy or polyester thermosetting plastic. These filaments are then gathered to form a hardened glass strand composed of multiple filaments.
In comparison, steel is a metal alloy composed of iron with a small percentage of carbon. These material differences means that the structural performance and durability of steel reo and GFRP will be different.
Tensile strength of GFRP rebar VS steel rebar
As noted earlier, GFRP has twice the tensile strength of steel. However, when comparing GFRP rebar VS steel rebar, it must be noted that GFRP has a lower yield strength and a lower bending (flexural) strength than steel. It also has a lower modulus of elasticity, therefore larger section sizes or a higher reinforcement ratio might be required in structural applications.
For example, when used in situations that place the element in tension, e.g. at the top of a cantilevered slab or the bottom of a simply supported beam, MST bar can tolerate greater levels of force than steel. As MST bar has a relatively elastic behaviour compared to steel, and it’s not ductile, it means it’s more likely to rupture than yield.
Modulus of elasticity/ Transverse shear strength/ Ultimate strain of GFRP rebar VS steel rebar
Because of its high ductility, a steel rebar can tolerate greater levels of elastic deflection than an MST bar before yielding or permanent deformation happens. Hence MST bar-reinforced concrete is generally designed for concrete crushing failure while steel-reinforced concrete is classically designed for yield failure.
Because MST bar has 20 times the fatigue resistance of steel, it’s far more durable under cyclic loading: GFRP = 420,684 cycles VS steel = 23,162 cycles.
Bond strength to concrete of GFRP rebar VS steel rebar
MST bar has a significantly greater bond strength to concrete than steel, making it far less likely to crack, especially in wet conditions due to its resistance to corrosion.
Density of GFRP rebar VS steel rebar
MST bar is far lighter (weighing a quarter less) and is therefore more portable than steel. This makes it much safer to work with and allows for faster installation and reduced transportation costs. It also reduces the inert weight of concrete structures.
Conductivity of GFRP rebar VS steel rebar
The ability for steel to conduct electric charges and/or heat has always been a safety concern in electrical environments such as rail corridors. Steel rebar has been known to cause hazardous stray currents as well as sacrificial anodic behaviour between different metals due to its conductivity. This can lead to corrosion of building components and fixtures/fittings over time.
MST bar is not conductive. That means it provides safe electrical separation and will prevent stray currents from happening in electrically sensitive environments such as rail corridors, power plants and substations. Its low thermal conductivity makes it highly useful in the building industry.
Durability of GFRP rebar VS steel rebar
Although steel has been the standard choice in concrete reinforcement for many years, its obvious weakness is the predisposition to corrosion. Over many years concrete will be gradually invaded by the carbon dioxide in the air and chlorides in the water, thus reducing the protective alkalinity of the hardened concrete. This is especially visible in coastal areas where specialist coatings, cathodic prevention systems and/or higher volumes of concrete are required to protect steel rebar from corrosion.
GFRP rebars like MST bar are resistant to corrosion, so excessive concrete cover is not required. This makes MST bar a far more reliable and sustainable option in high-risk areas, e.g. coastal environments, immersed conditions and splash zones. And in areas that are often exposed to corrosive chemicals or de-icing salts.
If you have more questions about GFRP rebar VS steel rebar, contact our team today.