A CFRP strengthening design is only as reliable as the information used to prepare it. Before carbon fibre plates or wrap are specified, the designer needs enough evidence to understand the existing concrete structure, the proposed change, the load path, the reinforcement assumptions, the substrate condition and the site constraints that will affect installation.
This is where many refurbishment projects become risky. CFRP strengthening is often discussed after an opening, load change or structural concern has already appeared on site, but the design cannot be treated as a simple product selection exercise. A plate, wrap or bonded strengthening system must be matched to the actual structure, not just to a drawing mark-up or a late project instruction.
This article sits within London Construction Magazine’s practical CFRP series and follows the main hub on carbon fibre strengthening in existing concrete structures, where the central point is that design intent, concrete substrate and installation evidence must align.
What the Designer Must Understand First
The first requirement is the structural problem. The designer needs to know why strengthening is being considered. The reason may be a new slab opening, increased loading, change of use, beam strengthening, column confinement, service penetration, local reinforcement deficiency, construction defect or concrete repair requirement.
The design information must then explain how the existing element currently works. A one-way spanning slab is not the same as a two-way slab. A beam soffit strengthening detail is not the same as a column wrap. A local service opening through a slab does not create the same design question as a full change in building use.
This is why the designer normally needs general arrangement drawings, structural sections, reinforcement drawings, loading plans, alteration drawings, opening locations and any previous investigation records. Where drawings are incomplete, intrusive checks, GPR scanning, cover meter surveys and concrete testing may be needed before the CFRP layout can be trusted.
Why Existing Concrete Assumptions Matter
CFRP design depends heavily on the assumed existing concrete. A design based on C30/37 concrete is not the same as one based on C16/20, C32/40 or C40/50. The concrete grade affects the design model, the bond expectation, the repair strategy and the confidence the engineer can place in the existing element.
The designer also needs reinforcement information. Existing bar diameter, spacing, cover, direction, laps, top steel, bottom steel and local reinforcement around columns or openings can all affect whether carbon fibre plates or wrap are suitable. If reinforcement assumptions are wrong, the CFRP may be strengthening a structural model that does not match the real slab, beam or column.
Condition matters as much as geometry. Carbonation, corrosion, spalling, delamination, honeycombing, previous repair mortar, damp concrete, coatings, plaster or weak laitance can all change whether a bonded system is practical. CFRP cannot simply be bonded over uncertain material and expected to solve the structural problem.
| Design Information Needed | Why It Matters on Site |
|---|---|
| Existing drawings | They show the intended slab, beam, column and reinforcement arrangement, but must be checked against site reality. |
| Loading information | The designer needs to know current and proposed loads before deciding whether CFRP is suitable. |
| Concrete strength | Concrete class assumptions such as C30/37 or C40/50 affect design capacity and substrate confidence. |
| Rebar scanning or opening-up | The real reinforcement layout may differ from archive drawings and can change the strengthening decision. |
| Substrate and pull-off testing | The concrete surface must be capable of achieving the required bond before plates or wrap are installed. |
Where Surveys and Testing Fill the Gaps
Where drawings are missing, old or uncertain, site investigation becomes part of the design process. GPR scanning and cover meter surveys may be used to locate reinforcement, identify zones of congestion, check bar direction and reduce the risk of cutting or fixing through critical reinforcement.
Concrete strength may need to be verified by core testing or other accepted investigation methods where the design relies on a specific strength class. The designer may also need opening-up works to confirm whether a slab is solid, ribbed, post-tensioned, clay pot, composite or affected by previous alterations.
This is particularly important before penetrations are formed. Cutting into a slab before understanding reinforcement, services, tendons or the intended CFRP layout can create avoidable risk. London Construction Magazine has already covered this wider issue in its article on concrete core drilling and slab damage, where incomplete information can turn a small opening into a structural concern.
Pull-off testing then addresses a different question: not what the slab is, but whether the prepared surface can accept the bonded system. A typical minimum value used on many bonded CFRP installations is 1.5 N/mm² concrete tensile bond strength, but the required value must always follow the project specification, engineer’s design and selected system. The failure mode should be recorded because substrate failure, adhesive failure and interface failure are not equivalent.
What the Site Team Must Give Back to the Designer
CFRP design is not finished just because a drawing is issued. The site team must feed back differences between the design assumption and the actual condition. If the substrate is not level, if coatings are thicker than expected, if reinforcement is found in a different position, or if the opening location changes, the designer needs to know before installation proceeds.
Surface preparation also affects design confidence. Laitance removal, mechanical grinding, grit-blasting and dust extraction are not just practical tasks. They determine whether the adhesive bonds to sound concrete or to a weak surface layer. A suitable ICRI CSP 3 to CSP 5 type surface profile may be required depending on the system, but the actual requirement must follow the selected product, specification and engineer’s instructions.
Environmental information should also be recorded. Epoxy resin systems depend on substrate temperature, ambient temperature, humidity and dew point. The substrate temperature should normally be at least 3°C above dew point to reduce condensation risk, while curing conditions must remain within the limits set by the selected resin system.
This is why the choice between carbon fibre plates and carbon fibre wrap is not only a design-office decision. Plates and wrap each create different site checks, different QA risks and different feedback requirements before the final system can be signed off.
Where Early Information Protects the Project
The best CFRP projects normally have early information flow. The designer receives the drawings, proposed openings, loading changes, concrete assumptions and reinforcement information before works are rushed. The site team understands which areas must be opened, scanned, prepared, tested and protected. The contractor understands where drilling, fixing or cutting is restricted after installation.
Poor information has the opposite effect. It can lead to late redesign, failed pull-off tests, unsuitable substrate, missing fire protection, clashes with services, incorrect plate positioning, incomplete wrap coverage or future damage by follow-on trades. In those cases, CFRP may still be physically installed, but the project may not have the evidence needed to prove that it performed the intended strengthening role.
In practical terms, design information is a risk-control tool. It protects the engineer, the installer, the contractor and the client because it reduces guessing. CFRP is strongest when the project treats investigation, design, installation and QA as one continuous evidence chain, not as separate disconnected tasks.
Design Information Takeaway
Before a CFRP strengthening design can be trusted, the project needs clear information about the existing structure, proposed alteration, load path, reinforcement, concrete strength, substrate condition and installation constraints. The design must be based on evidence, not assumption alone. Drawings, scans, opening-up, testing, pull-off results, environmental checks and ITP records all help convert a strengthening idea into a controlled structural intervention. Without that information, carbon fibre plates or wrap may look correct on site while the underlying design risk remains unresolved.
CFRP Design Information Questions
What information is needed before CFRP strengthening is designed?
The designer normally needs existing drawings, proposed alterations, loading information, reinforcement details, concrete strength, substrate condition, access constraints, fire requirements and any investigation or testing records.
Why are rebar scans important before CFRP design?
Rebar scans help confirm reinforcement position, direction and congestion where drawings are uncertain or where new openings, fixings or strengthening zones may affect the load path.
Does the designer need concrete strength results?
Concrete strength may be needed where the design relies on a specific concrete class or where existing drawings are uncertain. The requirement depends on the engineer’s assumptions and the project specification.
Can CFRP be designed without pull-off testing?
A preliminary design may be prepared before pull-off testing, but bonded CFRP installation commonly requires pull-off testing before works proceed so the concrete tensile bond strength can be verified.
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Expert Verification & Authorship: Mihai Chelmus
Founder, London Construction Magazine | Construction Testing & Investigation Specialist |
