Writer: Dr. Shobana Sivanendran
Derived from excerpts of the PhD thesis of Dr. Shobana Sivanendran, University of Cambridge, 2017
Traditionally, concrete structures are internally reinforced with steel bars. However, there are many problems that result from the corrosion of this steel reinforcement. The corrosion products of the steel reinforcement occupy a greater volume than the original steel causing bursting forces to build up within the concrete cover. This leads to cracking and spalling of the concrete, which could significantly weaken the structure. In prestressed concrete, corrosion could lead to sudden snapping of the pre-tensioned steel reinforcement, which has in some cases led to catastrophic damage of large structures. In 1985, a post-tensioned single span concrete bridge in Ynys-y-Gwas, Wales, collapsed suddenly with no signs of weakening prior to failure. The failure was attributed to severe localised corrosion of the prestressed tendons.
To this day, there has been extensive research conducted to find ways of mitigating this problem of corroding steel reinforcement. The addition of silica fume to concrete mixes significantly decreases the permeability of the concrete to water. This makes it more difficult for external moisture to penetrate into the concrete and instigate the corrosion process in the steel reinforcement. However, it does not prevent moisture from penetrating through cracks in the concrete. To solve this, research has also been conducted to find ways to initiate a self-healing process for cracked concrete that could prevent environmental moisture from reaching the reinforcement. However this idea is still in its infancy and has yet to produce crack healing at the required rate and robustness for field applications. The other main alternative to preventing corrosion in reinforced or prestressed concrete is to replace the steel entirely with a non-corrosive material. Following from the successful applications of fibre reinforced polymer (FRP) materials in the aeronautical and maritime industries, the idea for the use of such materials as construction materials has been explored. The three fibres commonly considered were glass (GFRP), carbon (CFRP) and aramid (AFRP).
The main challenge in using FRP rods as a replacement for steel lies in their inherently different material properties. FRPs fail in a brittle manner. This is unlike steel, which yields significantly prior to ultimate failure i.e. steel fails in a ductile manner. Therefore if a concrete structure were to be under-reinforced with FRP rods, there could be limited warning signs if the rods were close to their ultimate load capacity. Particularly for prestressed FRP rods, any failures could occur suddenly. Additionally, FRPs have a much higher strain capacity than steel, approximately 10 times higher, and are also significantly more expensive. Therefore the most economical method of utilising them would be as prestressed reinforcement. These considerations mean that a different design approach to that of steel reinforced concrete is required when using FRPs.
While CFRP rods with a high strength and stiffness offer an attractive alternative to steel as prestressed reinforcement because of their non-corrosive nature, other modes of deterioration of these rods when exposed to high moisture environments cannot be ruled out. Any deterioration over the long-term should be accounted for during the design stage to ensure safe structures for the duration of their design life.
The CFRP tendons used in pre-stressed applications are cylindrical in shape with diameters ranging from 3mm to 10mm. They are made of thousands of continuous unidirectional carbon fibres of diameter approximately 5-6µm set in an epoxy matrix.
The tendons are most commonly manufactured using the pultrusion method, where the carbon fibre strands are pulled through a resin bath and then through a heated die thus forming a continuous carbon-epoxy tendon. They are then surface treated to encourage better bond with the concrete. Spiral wound fibres, indentations and sand coatings may be added during the manufacturing process to improve the CFRP-concrete bond. Bundles of rods may be combined to form strands, also used in prestressed concrete members.
The tendons derive most of their strength from the fibres while the matrix functions primarily as a binder of the fibres and a distributor of the imposed loads. Because of the unidirectional orientation of the fibres, the resulting polymer is anisotropic and possesses different material properties in the longitudinal and transverse directions. Material properties in the direction parallel to the fibres are fibre dominated and material properties in the transverse orientation to the fibres are matrix dominated. Thus when considering the durability of CFRP tendons, it is important to take note of these directional effects in relation to the directions of the applied loads.
Currently in Malaysia, there are some applications of CFRP sheets for the strengthening of concrete structures, however the use of CFRP as internal prestressed reinforcement in concrete is still yet to be applied. Potentially with further research, the benefits of such applications for structures that are more prone to corrosion, such as sea bridges, could be better understood to pave the way to the use of CFRP as internal prestressed reinforcement here.
Image 1: Photograph of sand-coated CFRP tendons (Source: Dr. Shobana Sivanendran)
Image 2: 500x optical microscope photograph of transverse cross section of CFRP rod showing the carbon fibres and epoxy matrix (Source: Dr. Shobana Sivanendran)