Carbon Fibre Reinforcement (CFRP) for Concrete Strengthening: When Plate Bonding Is the Right Solution

What Is Carbon Fibre Reinforced Polymer (CFRP) Strengthening?

Carbon fibre reinforced polymer (CFRP) structural strengthening — also called carbon fibre plate bonding or carbon fibre reinforcement — is a technique for increasing the structural capacity of existing concrete, masonry, timber or steel elements by bonding high-strength carbon fibre composite plates, strips or fabrics to the external surface of the element. The carbon fibre material carries tensile stress in parallel with the existing reinforcement, effectively augmenting the steel reinforcement area and increasing the element's resistance to bending, shear or confinement loading without requiring the element to be demolished and rebuilt. In the UK commercial sector, CFRP strengthening is increasingly used as an alternative to structural demolition and replacement when existing buildings are modified to carry increased loads, converted to new uses, or when a structural deficiency is identified that would otherwise require major reconstruction.

CFRP materials have a tensile strength approximately 10 times that of structural steel and a stiffness (elastic modulus) of 150–500 GPa depending on the fibre type — compared to 200 GPa for steel — at a density approximately one quarter that of steel. This exceptional specific strength allows significant structural capacity enhancement with minimal additional weight or profile added to the existing structure, making CFRP particularly attractive in applications where headroom, clearance or aesthetic impact is constrained. CFRP strengthening systems are specified under the requirements of the Concrete Society Technical Report 55 (Design Guidance for Strengthening Concrete Structures Using Fibre Composite Materials) and are supplied by several UK-active manufacturers, including Sika, whose SikaWrap and Sika CarboDur product families are widely used on UK commercial strengthening projects and available through our Sika product range.

MPS Concrete Solutions works alongside structural engineers to install CFRP strengthening systems on commercial and industrial buildings in London and the South East. Our Industrial Concrete Repairs service covers the full range of structural remediation techniques we apply to concrete structures, and this guide is intended to help building owners and project managers understand when CFRP strengthening is the right solution and what the installation process involves.

When Is CFRP the Right Solution? Common Applications

CFRP structural strengthening is appropriate in a specific subset of situations where existing structural elements are deficient in strength or stiffness and where the objective is to increase capacity without structural replacement. The most common applications on UK commercial buildings are: increasing the bending capacity of beams or slabs to accommodate higher imposed loads following a change of use (for example, converting an office floor plate to a data centre or archive store); restoring the flexural capacity of a beam or slab that has been partially compromised by reinforcement corrosion and section loss; providing additional shear capacity to beams that are deficient in stirrup steel relative to current code requirements; and confining columns to increase their compressive capacity and ductility on structures being upgraded for seismic loading or where the column section has been partially reduced by deterioration.

A change of use from office to a higher-imposed-load occupancy — retail storage, library stacks, plant room, server room — is a very common trigger for CFRP specification on existing commercial buildings. Office floor slabs are typically designed for imposed loads of 2.5–4.0 kN/m², while storage uses may require 7.5–15 kN/m² and server rooms up to 20 kN/m² for raised floor systems with high UPS battery loads. If a structural assessment confirms that the existing slab is inadequate for the proposed new loading, CFRP plate bonding to the slab soffit can provide the required additional bending capacity at a fraction of the cost and disruption of slab replacement — particularly in occupied buildings where demolition and reconstruction would require the floor above and below to be cleared for extended periods.

CFRP is also increasingly specified as a complementary measure to concrete repair on structures where reinforcement has been significantly reduced by chloride-induced corrosion: the physical repair — breaking out, steel treatment, mortar reinstatement — restores the concrete section, while CFRP plate bonding compensates for the structural capacity lost as a result of the reduced steel cross-section that could not be restored by mortar reinstatement alone. This combined approach — repair plus CFRP — is particularly efficient on car park decks, bridge beams and retaining walls where localised corrosion has reduced the effective reinforcement area at a critical section.

CFRP Plate Bonding vs Fabric Wrapping: Which System to Use

Two primary forms of CFRP material are used in structural strengthening: pultruded CFRP plates (or strips), which are pre-manufactured composite laminates of defined width, thickness and fibre volume fraction, bonded to the substrate using a structural epoxy adhesive; and wet lay-up CFRP fabric, which is a woven or unidirectional carbon fibre fabric applied in a saturant epoxy resin to form a laminate directly on the structure surface. Each has specific advantages and is suited to different applications.

Pultruded CFRP plates — such as Sika CarboDur S, M or H grades — offer consistent mechanical properties (guaranteed by the manufacturing process), higher fibre volume fraction and a pre-cured, stable geometry that makes installation straightforward and quality-controllable. They are the preferred choice for bending strengthening of horizontal soffits (beams and slabs), where the plate is bonded parallel to the span direction to carry tensile stress at the soffit face. Installation involves abrading the concrete substrate, applying a consistent bed of structural adhesive, positioning the plate against the adhesive, and providing temporary clamping or propping until the adhesive has cured sufficiently to carry the plate's own weight. Pull-off adhesion testing of the bonded plate — testing to confirm bond strength exceeds the minimum specified — is the key quality assurance check.

Wet lay-up CFRP fabric — such as SikaWrap systems — is applied as a flexible sheet that conforms to complex or irregular surfaces including curved soffits, column sections, pile caps and connection zones that a rigid pultruded plate cannot accommodate. Fabric systems are preferred for column confinement (wrapping around the full column perimeter to provide hoop restraint), for shear strengthening (diagonal wrapping around beam webs), and for strengthening elements with non-planar surfaces. The disadvantage of wet lay-up relative to pultruded plate is that the mechanical properties of the cured laminate depend on the quality of the saturant application — the fibre volume fraction, the absence of voids and the uniformity of the laminate thickness — all of which are process-dependent and require careful quality control. Fabric systems should only be installed by contractors specifically trained and accredited in the manufacturer's application process.

Structural Design and Engineering Responsibility

CFRP strengthening must be designed by a qualified structural engineer in accordance with Concrete Society Technical Report 55 and, where applicable, the relevant Eurocodes. The design process includes: a structural assessment of the existing element to establish the current capacity and the magnitude of the capacity shortfall; the selection of the CFRP system type and grade; the calculation of the required plate or fabric width, thickness, length and number of layers to achieve the target capacity increment; the design of the end anchorage to prevent peel failure at the end of the plate; and the specification of the adhesive system, the substrate preparation standard and the quality assurance regime.

The structural engineer's responsibility does not end at design — they should also review and approve the contractor's method statement, witness key installation stages (substrate preparation standard, adhesive application, plate positioning and end anchorage), and issue a formal structural sign-off on the completed strengthening after the post-installation testing programme has been completed. A CFRP installation carried out without engineer involvement — or where the engineer's role has been reduced to issuing a drawing without site supervision — carries a significant risk of installation errors that are not detected until a load test or an inspection reveals inadequate performance.

Building owners and project managers should ensure that the CFRP strengthening scope in the construction contract clearly assigns design responsibility to the engineer, installation responsibility to the accredited contractor, and post-installation inspection responsibility to the engineer. The three-party framework — owner, engineer, contractor — each with distinct and documented responsibilities, is the basis for a defensible, auditable structural strengthening project.

Installation Process, Quality Assurance and Long-Term Performance

A typical CFRP plate bonding installation on a commercial floor slab follows a well-defined sequence. The concrete soffit in the strengthening zone is blast-cleaned or ground to remove laitance, coatings and surface contamination to expose the aggregate, achieving a surface profile equivalent to ICRI CSP 3–4. Concrete compressive strength is verified at the installation location by in-situ testing (rebound hammer calibrated against cores, or pull-off testing) to confirm it meets the minimum value assumed in the structural design — typically 25–30 N/mm². The substrate is primed with a compatible epoxy primer to reduce porosity and improve adhesive bond. The structural adhesive (a two-component epoxy such as Sikadur 30) is applied by notched trowel to one or both bonding faces to achieve the specified bed thickness (typically 1–3 mm), and the CFRP plate is pressed into the adhesive, aligned and temporarily secured. Adhesive bleed-out at the plate edges confirms adequate coverage. The installation is left to cure undisturbed for a minimum of 7 days at temperatures above 10°C before load is applied.

Post-installation quality assurance includes: adhesion testing of the bonded plate by pull-off test at the required frequency (typically one test per 5–10 metres of plate, or as specified by the engineer); visual inspection of the plate edges for voids or debonding indicated by hollow-sounding areas when tapped; and review of the installation records against the specification requirements. A formal inspection report from the engineer confirming acceptance of the installed strengthening is the basis for the structural sign-off and for the building owner's maintenance records.

Long-term performance of bonded CFRP is excellent in dry interior environments, with service lives of 50 years or more documented in research and field studies. In exposed exterior environments — marine, chemically aggressive, UV-exposed — a protective coating over the CFRP plate surface is required to maintain the epoxy adhesive and the plate surface in good condition. Regular inspection (recommended every 5–10 years as part of the building's principal structural inspection programme) should check for debonding, impact damage, moisture ingress at plate ends and any signs of epoxy degradation. If you are a building owner or structural engineer considering CFRP strengthening for a commercial structure in London or the South East, MPS Concrete Solutions can advise on the installation process and can work alongside your appointed structural engineer to deliver the project. Contact our team to discuss your requirements, and review our related guides to Structural Concrete Repair for Multi-Storey Car Parks and BS EN 1504 Explained for further technical context.