When specialist contractor Mndeni Structural Services was contracted by Stefanutti Stocks to help with the repair of concrete columns in a multi-storey parking garage for the University of Pretoria, a carbon fibre wrap supplied by Mapei South Africa, part of the global Mapei Group, was selected for the project.
The columns were designed to deliver a compressive strength of 30MPa, but after construction they were found to have a compressive strength of 25MPa and therefore needed to be strengthened. Carbon fibre wrapping was the preferred option as it is strong but lightweight and Mapei’s MapeWrap C UNI-AX 600/40 met the specification.
Before the wrapping could be applied, the columns had to be thoroughly prepared. Grinding and sandblasting provided an open textured surface and MapeWrap Primer 1, an epoxy primer specific to the MapeWrap system, was then applied to promote adhesion. All blowholes were filled with Adesilex PG2, a two-component epoxy resin-based product containing special hardeners. It offers extended workability and is ideal for structural strengthening or sealing large cracks.
MapeWrap C UNI-AX 600/40 is a high-strength, uni-directional, continuous carbon fibre fabric with a very high modulus of elasticity and high tensile strength. It was layered tightly around the columns in segments, with overlaps. To impregnate the MapeWrap, MapeWrap 31, a medium-viscosity epoxy resin was applied to the concrete.
In the next step, the MapeWrap fabric was rolled on with a steel roller, to remove any trapped air bubbles, and then blinded with silica sand. After the epoxy hardened, excess sand was brushed off and a skimming mortar was applied to the columns. Mapei’s Planitop 200, a single-component cementitious mortar with high bonding strength was applied in the final step to smooth the surface, protect the wrapping and finish the columns.