Carbon Fibre and Reinforced-Concrete Strengthening — Columns, Slabs and Walls

The question asked most often on strengthening projects is this: "should we go for carbon fibre or jacketing?" This project shows that the right answer is usually "both".

The building’s performance assessment showed a serious axial capacity shortfall in some of the columns and inadequate flexural capacity in the slab. Columns with a large shortfall were enlarged by reinforced-concrete jacketing; those with a limited shortfall were wrapped in carbon fibre, and the slab was reinforced with embedded carbon laminate.

The four methods applied

For each member, the method best suited to that member’s need was chosen:

• Column jacketing — a reinforcement cage was built around the columns with the largest shortfall, formwork erected and new concrete cast.
• Carbon fibre column wrapping — columns needing ductility and shear capacity were wrapped in CFRP fabric.
• Slab carbon laminate — parallel channels were cut into the slab, carbon laminate strips were embedded in them with epoxy, and the channels were closed with repair mortar.
• Wall and shear wall fabric overlay — wall surfaces were overlaid end to end with carbon fabric for in-plane strength.
• Epoxy injection — existing cracks were filled with low-viscosity epoxy before the fabric was applied.

Why embedded laminate was chosen for the slab

Embedding the carbon laminate in channels cut into the slab, rather than bonding it to the surface, brings two advantages. First, because the laminate sits inside the concrete it takes bond on three faces, so the risk of debonding falls markedly. Second, once the floor finish goes back there is no build-up at the surface — the floor level does not change. The channel depth and spacing were set by the calculation in the strengthening design. In the literature this method is called NSM (near-surface mounted).

Surface preparation: the invisible half of the job

In carbon fibre work, almost every failure traces back to surface preparation. On this site the following steps were carried out in order:

• Existing render and loose concrete were stripped back to sound concrete.
• Corroded reinforcement was cleaned and coated with an anti-corrosion primer.
• Voids and segregation were filled with structural repair mortar.
• Column corners were rounded so that the fabric would not be cut on a sharp arris.
• Epoxy primer was applied and surface irregularities levelled with epoxy putty.
• The fabric was laid in saturating resin and air bubbles rolled out.