Carbon fibre reinforcement, GFRP and basalt rebar: FRP compared

Carbon fibre reinforcement is reinforcement made of carbon fibres in a resin matrix: CFRP, a fibre reinforced polymer. It comes in two forms: as a bar or grid cast into new concrete, and as a plate (laminate) or fabric bonded onto existing concrete to strengthen it, known as plate bonding or externally bonded FRP. Carbon fibre reinforcement does not corrode and is very strong, but it has no yield point: it behaves linear-elastically up to failure and therefore needs its own design rules and product certification.

The same principle exists with glass fibre (GFRP rebar) and basalt (BFRP or basalt rebar). And it is something different from loose fibres in the concrete mix, as in fibre reinforced concrete. This article sets those three worlds side by side: FRP bars in new concrete, bonded strengthening of existing concrete and fibres that come out of the mixer, each compared with traditional steel reinforcement.

What is carbon fibre reinforcement? FRP, fibre reinforced polymer and fibre reinforced concrete

FRP stands for fibre reinforced polymer (fiber reinforced polymer in US spelling), also called fibre reinforced plastic. The material consists of continuous fibres embedded in a resin, usually epoxy or vinyl ester. Bars and plates are often pultruded: fibre bundles are pulled through a resin bath and a heated die, after which the resin cures. The letter in front of FRP gives the fibre: CFRP for carbon, GFRP for glass, BFRP for basalt and AFRP for aramid fibre.

In construction, FRP is used as bars (with a sand-coated, ribbed or helically wrapped surface for bond), as grids and textiles, as flat plates or laminates, and as fabrics wrapped around a member. The word fibre causes confusion here. In fibre reinforced concrete, short loose fibres are dispersed through the whole concrete during mixing; you can compare those in the overview of types of concrete fibres. In FRP, by contrast, the fibres run continuously and aligned in the resin, and together they form a bar or plate placed in a fixed position in or on the concrete.

FRP rebar as reinforcement: carbon, glass fibre and basalt bars

FRP rebar is placed in the formwork to the drawings, just like steel rebar, and tied into cages or mats. It replaces the steel, not the fixing work. The main reason to choose it is corrosion: steel reinforcement starts to rust once chlorides from de-icing salt or seawater reach it (exposure classes XD and XS), which leads to cracking, spalling and eventually serious concrete damage. How that mechanism works is explained in corrosion of reinforcing steel.

FRP does not rust and is non-magnetic; glass and basalt bars are also non-conductive. FRP rebar is therefore used in concrete with heavy chloride exposure, in soft-eye zones that a tunnel boring machine will later cut through, near sensitive electrical or magnetic equipment (for example MRI rooms) and in members where little cover is available.

GFRP rebar: glass fibre reinforcement in concrete

GFRP rebar is the most widely used FRP bar. The bars do not rust, do not conduct and weigh roughly a quarter of an equivalent steel bar, which makes handling lighter. The trade-off is a much lower stiffness: product data sheets give a modulus of elasticity of roughly 40 to 60 GPa for pultruded GFRP bars, against 200 GPa for steel rebar. With the same cross-section, a member reinforced with glass fibre bars therefore deflects more and cracks open wider.

Material specifications for GFRP bars differ per market. In North America, ASTM D7957 specifies solid round GFRP bars; in India, IS 18256:2023 does the same for solid round GFRP bars for concrete reinforcement. In the Netherlands, the Kiwa assessment guideline BRL 0513 for glass fibre bars used as reinforcement in concrete is the basis for a KOMO product certificate, with supplementary provisions to Eurocode 2 because the normal design rules for steel do not simply apply.

Note the difference with glass fibre reinforced concrete (GRC or GFRC): that uses short, alkali-resistant (AR) glass fibres in the mix, for example in thin cladding panels. That is fibre reinforced concrete, not bar reinforcement.

Carbon fibre rebar and grids (CFRP)

Carbon fibre rebar is the stiffest and strongest of the FRP options, and also the most expensive. Unlike glass and basalt, carbon conducts electricity, so CFRP is ruled out where a non-conductive zone is required. In new construction, carbon appears less often as a single bar and more often as a grid or textile in thin, slender members (see carbon reinforced concrete below). Its best-known use, however, is the bonded CFRP plate used to strengthen existing concrete: plate bonding.

Basalt rebar (BFRP bars)

Basalt rebar consists of bars or grids of continuous basalt fibre in a resin. Published test results for BFRP bars are around 1,100 to 1,350 MPa tensile strength and a modulus of elasticity of roughly 45 to 50 GPa, against 200 GPa for steel; values differ per manufacturer. As with glass fibre, deflection and crack width therefore often govern the design. The resistance of basalt to the highly alkaline environment in concrete is a point of attention that has to be demonstrated per product. Basalt rebar is still a young product compared with GFRP and is used mainly in research and pilot projects.

Basalt rebar is something different from basalt fibres in the mix. A basalt bar sits in a fixed position in the cross-section; a basalt fibre works dispersed through the whole concrete. Further down you can read why the macro fibre Basalt Wave is therefore not basalt rebar.

Designing with FRP rebar: no yield point, separate design rules and certification

FRP behaves linear-elastically up to failure. There is no yield point and therefore no ductile, warning deformation as with steel rebar. Designers often size the section so that concrete crushing governs rather than bar rupture. Because glass and basalt bars are less stiff, deflection and crack width usually govern rather than strength.

Cover also changes its role: it is no longer needed as corrosion protection, but it is still needed for bond between bar and concrete and for fire resistance. In a fire, the resin softens at relatively low temperatures; that is the weak point of every type of FRP reinforcement. Bends and links come from the factory, because cured resin cannot be bent.

Separate design rules apply. The new generation of Eurocode 2 (EN 1992-1-1:2023) contains an informative Annex R for non-prestressed FRP reinforcement. In North America, ACI 440.1R is the established design guide for concrete reinforced with FRP bars, and ACI 440.11 is the building code for GFRP-reinforced concrete; in Canada, CSA S806 covers building structures with FRP. Product certification or a technical approval for the chosen bar is always needed as well. An FRP bar is therefore never a one-for-one swap for a steel bar of grade B500: the structural engineer redesigns the reinforcement.

CFRP plate bonding: strengthening existing concrete with carbon fibre

Plate bonding, also called externally bonded reinforcement or externally bonded FRP, is reinforcement bonded to an existing structure after the event. In the past these were mainly steel plates; today they are usually CFRP plates or carbon fibre fabrics bonded with epoxy adhesive to the tension face of a beam, slab soffit or column. Carbon is light, does not corrode and is easy to install even in tight spaces. In Europe, strengthening with fibre composites has been used increasingly since the 1990s.

Typical reasons for strengthening concrete with carbon fibre are a change of use with higher loads, insufficient flexural or shear capacity, missing or deteriorated reinforcement that has to be supplemented, an opening cut after construction, damage from vehicle impact or fire, fatigue and seismic upgrading. Plate bonding comes in four variants:

• CFRP plates (laminates): flat CFRP strips on the tension face, mainly for extra flexural capacity.

• Carbon fibre wrapping: carbon fibre fabric saturated with resin on site and wrapped around beams or columns, for shear and for confining columns.

• Near-surface mounted (NSM) reinforcement: strips or bars bonded into a groove cut into the cover, where they are better protected.

• Prestressed CFRP plates: the plate is bonded and anchored under prestress, so the strengthening works immediately, even without further deformation of the structure.

The main failure mechanism is debonding: the plate comes away before it reaches its tensile strength. The substrate is therefore decisive. After surface preparation (grit blasting or grinding), the concrete is checked with a pull-off test. Data sheets commonly ask for a mean of about 2.0 and a minimum of 1.5 N/mm², but the data sheet of the chosen system governs. The concrete is usually at least 28 days old. The adhesive has to be protected against permanent sunlight and moisture, and because epoxy loses strength at high temperature, the engineer also checks the fire situation.

The international basis for design is fib Bulletin 14, Externally bonded FRP reinforcement for RC structures (2001). In the UK, Concrete Society Technical Report 55 (third edition 2012) gives design guidance for strengthening concrete structures with fibre composite materials, to be read with TR57 on acceptance, inspection and monitoring. In the Netherlands, CUR Recommendation 91 sets the rules for externally bonded carbon fibre reinforcement; it is based on fib Bulletin 14 and limited to the aspects that have been researched sufficiently. Data sheets require the plates to be installed by an applicator trained or approved by the system supplier, with the strengthening designed by a specialist structural engineer.

One thing needs saying plainly: loose fibres cannot strengthen an existing structure. Fibres only work in the concrete they are mixed into. A beam or slab that has to carry more load needs a strengthening design.

Concrete strengthening techniques: how to strengthen existing concrete

Concrete strengthening does not start with a technique but with an assessment. A structural engineer establishes what the structure can carry now and what it will have to carry, based on investigation of the position and size of the reinforcement, the concrete quality and any damage. Where there is concrete spalling from reinforcement corrosion, repairing the cause comes first: adhesive on deteriorated concrete is pointless. Only then is a technique chosen from the table below.

With an overlay or a thickened section, a new layer of concrete is cast onto the existing structure. Fibres in that new layer limit shrinkage and cracking and can replace the mesh in a non-structural overlay; see overlaying with a fibre reinforced topping. Whether a topping contributes structurally is for the engineer to decide: that depends on the bond with the substrate and on any shear connectors or dowels.

Carbon reinforced concrete and textile reinforced concrete: building thin without corrosion

Carbon reinforced concrete is concrete reinforced with carbon fibre instead of steel, usually as a grid or textile of coated carbon fibre yarns (textile reinforced concrete, TRC) and sometimes as bars. Because carbon does not rust, the cover no longer has to protect against corrosion and members can become much more slender: thin cladding panels, shells and strengthening layers of a few centimetres on existing concrete. According to the German InformationsZentrum Beton, the tensile strength of carbon reinforcement is up to six times that of steel rebar, at a quarter of the density.

There are drawbacks too. Producing carbon fibre is energy-intensive, fire resistance becomes the governing design issue, recycling is still under development, and bars or grids have to be shaped before the matrix cures. Development is concentrated in Germany, at TU Dresden and RWTH Aachen among others, and in January 2024 the German Committee for Structural Concrete (DAfStb) published a guideline for concrete members with non-metallic reinforcement. Elsewhere, carbon reinforced concrete is still a niche application.

Steel, FRP rebar and loose fibres compared

The table sets steel rebar, the three types of FRP and loose fibres in the mix side by side. The stiffness values are indicative and come from published values and product data sheets; for a design, the values in the certificate of the chosen product always apply.

The big difference is in the placement row. Steel and FRP bars have to be placed and tied in the position shown on the drawings. Loose fibres come out of the mixer and work throughout the whole volume: they replace the mesh in many floors and pavements, but not designed main reinforcement in a beam or column.

Corrosion-free reinforcement in new concrete: when FRP, when fibres?

If you are considering FRP because steel corrodes, it pays to ask a different question first: which part of the reinforcement can come out of the mixer corrosion-free? For a lot of new construction, that is a bigger share than expected:

• Floors, pavements, yards, greenhouse floors and cycle paths: synthetic macro fibres often replace the mesh here and do not rust, for example TwistR (48 mm polypropylene macro fibre, 2.0 to 6.0 kg/m³) or Wiking 4050 TR (polyolefin, EN 14889-2 class II). A practical example with Wiking 4050 TR is the greenhouse floor without steel reinforcement.

• Precast, edge beams (hybrid), sprayed concrete and structural floors: the basalt macro fibre Basalt Wave.

• Shrinkage control in mortar and thin layers: the chopped basalt fibre Basalt Chopped (4 mm, 0.6 to 1.0 kg/m³).

• Heavily loaded structural members (beams, columns, main reinforcement): steel or FRP bars remain necessary here. Often fewer bars will do in hybrid reinforcement, where fibres take over crack control.

For concrete in a chloride environment (XD or XS: de-icing salt or seawater), read how fibres help prevent corrosion of reinforcing steel. To choose between fibre types, see the comparison of steel fibres or synthetic macro fibres.

Basalt Wave is not basalt rebar

Basalt Wave is made of basalt filaments in an epoxy matrix, the same material principle as a BFRP bar. Yet it is not basalt rebar: it is a macro fibre 50 mm long and 1.2 mm in diameter with a 3D wave profile, dosed into the mix and working dispersed through the concrete, like other macro fibres. Basalt Wave does not replace designed main reinforcement in a beam or column. In edge beams it is used in a hybrid design, together with bar reinforcement, and always on the basis of a fibre reinforced concrete design.

More on its use in members can be found under fibres in precast concrete elements. For project advice on fibres in your element or floor, you can request a quotation or advice.