Specifications of fibre-reinforced plastic rebar allow to use it in industrial and civil construction, in surface layers of concrete structures, in road construction, for reinforcement of bridges, in constructions that are prone to be exposed to corrosive environment.
Fibre-reinforced Plastic Rebar for Concrete
Today fibre-reinforced plastic rebar (FRP rebar) is widely used in industrial and civil construction. This is due to a number of factors:
low unit weight of FRP rebar;
high corrosion resistance;
low thermal conductivity of composite materials;
high durability of FRP rebar that exceeds durability of steel rebar by several times;
fibre-reinforced plastic rebar is dielectric.
The main drawback of a traditional low-carbon steel rebar is well-known: it has a low index of corrosion resistance. Because of that traditional rebar is not suitable for marine structures and constructions located in zones with fluctuations of water level. External fiberglass reinforcement can solve the problem of durability for marine structures, while concrete is no longer exposed to corrosive environment.
Advantages of the use of FRP rebar for reinforcement of concrete structures have been analyzed in numerous research works published by Russian research institutes (e.g. Concrete and Reinforced Concrete Research Institute, Kharkov Institute of Civil Engineering, Siberian Energy Research Institute, etc.) and proven by years of experience gained by the foreign companies.
Fiberglass (FRP) Reinforcement
Fiberglass reinforcement of constructions can take three forms: internal, external and combined.
The main field of application is reinforcement of constructions where environment is aggressive to rebar, and not to concrete. In this case steel rebar cannot be used. Internal non-metal reinforcement can be: Discrete – reinforcement with fiberglass bars that are equally durable to the steel ones. Dispersed – chopped fiberglass fibres are added to the concrete mix for reinforcement. In this case the fibres are randomly dispersed in the concrete, however certain measures can ensure that the fibres follow a certain direction.
The main field of application is construction of buildings and structures, where environment is aggressive to concrete. External reinforcement is based on the use of external reinforcement sheets that create an airproof and waterproof envelope for concrete and thus perform their reinforcement functions.
External FRP reinforcement can be:
Uninterrupted – for this purpose solid sheets of material are used;
Discrete – for this purpose separate bands or meshes are used.
Two techniques are used for concrete constructions in fiberglass envelopes. The first one is when the FRP envelope is wrapped around the pre-dehydrated concrete elements. The second technique is when the fiberglass envelope is produced and only then it is filled up with the concrete mix.
For constructions where mechanical stress cannot be taken up by external reinforcement only, internal bar reinforcement can be used for combined reinforcement. These internal reinforcement bars can be made of either plastic or steel.
Pre-stressing of Rebar
The use of FRP rebar is reasonable only in pre-stressed constructions, because the stress-strain modulus of fiberglass bars is several times less (4-5 times less) than that of the metal bars. When fiberglass rebar is used, the following three techniques of pre-stressing of concrete constructions are most frequent:
Tensioning on supports.
When this technique is used, rebar is stretched to the necessary length with the use of special devices and then it is embedded in concrete, which is followed by the thermal and wet treatment of concrete to ensure faster hardening.
Post-stressing in concrete.
When fibre-reinforced plastic rebar is post-stressed in concrete, special channels for FRP rebar are made in concrete. Rebar is stretched with hydraulic jacks. Rebar is fixed in the channels with injection of petrolatum ointment.
When this technique is used, which is by the way not very popular in modern construction industry, flexible fiberglass bars or bands are coiled around the concrete item.
How Temperatures Influence Durability of Rebar
Durability of FRP rebar changes proportionally to fluctuations of temperature: When the temperature goes down to -40 degrees centigrade, durability of fibre-reinforced plastic rebar increases approximately by 40%. When the temperature exceeds +20 degrees centigrade (up to +300) durability of FRP rebar gradually reduces by 60%. Changes in durability of FRP rebar due to fluctuations of temperature between -40…+300 degrees centigrade are reversible.
Fibre-reinforced Plastic Rebar and Concrete Repair Work
Traditionally for repair of reinforced concrete constructions relatively labor-intensive and expensive practices are used, and very often it means that production processes should be stopped. In comparison with these practices external fiberglass reinforcement not only ensures high corrosion resistance and high durability, but also fast repair works. These advantages prove efficiency of FRP rebar for repair and reinforcement of bearing elements in reinforced concrete constructions.
Finally, it should be noted that the use of fibre-reinforced plastic rebar due to its high corrosion resistance not only considerably prolongs service life of concrete constructions, but practically eliminates additional expenses on major overhaul of constructions.