Strengthening structures with CFRP: plates, fabrics and epoxy adhesives

What is externally bonded composite reinforcement?

Strengthening with composites is a way of increasing the load-bearing capacity of reinforced concrete, masonry, timber and steel structures by bonding carbon fibre reinforced polymer (CFRP) elements to them. Two forms are used: pultruded plates — thin flat strips of carbon fibre in an epoxy matrix — and carbon fabrics, which are impregnated with resin on site. Plates can also be bonded into slots cut in the concrete cover — this is known as near-surface mounted reinforcement.

Compared with section enlargement and steel plates, CFRP adds almost no weight or thickness, does not corrode, is supplied in rolls without joints and is installed without heavy equipment, often without temporary props on soffits. But it is a structural solution: the scheme, the cross-section and the amount of composite are calculated by a structural engineer, and the material and the adhesive work only as a system from one manufacturer.

When it is used

  • increased loads: new equipment, a change of use of the building, heavier loads on bridges and floors;
  • changes to the structural scheme: openings in slabs and walls, removal of walls and columns;
  • damage: reinforcement corrosion, ageing of materials, impact, fire, earthquake;
  • design and construction errors: insufficient reinforcement or section depth;
  • seismic retrofitting: fabric jacketing increases the strength and ductility of columns and strengthens masonry walls;
  • improved serviceability: reduced deflections and crack widths, lower stresses in the reinforcement.

Materials

CFRP plates

Used mainly for flexural strengthening of slabs, beams and bridge superstructures. Manufacturers produce plates in several widths and moduli of elasticity — usually 1.2–1.4 mm thick; the data sheets give the mean and characteristic (design) values of strength and modulus. Examples: Sika CarboDur S, Carboplate made by Mapei.

Carbon fabrics

Unidirectional fabrics of various weights and moduli are installed by the “dry” method — onto a layer of resin and rolled in — or by the “wet” method, with prior impregnation. The fabric follows corners and curved surfaces, so it is used for column jackets, shear strengthening and strengthening of walls, pipes and tanks. The corners of the structure are rounded before wrapping, as required by the manufacturer. Examples: SikaWrap-230 C, Fosroc Nitowrap CW, MapeWrap C UNI-AX.

Structural epoxy adhesives and resins

Thixotropic two-component adhesives for bonding plates and steel plates, impregnating resins for fabrics, and bonding agents for bonding fresh concrete to old. Materials for structural bonding are covered by EN 1504-4. Examples: Sikadur-30, Sikadur-330, Sikadur-31 CF Normal, Sikadur-32 Normal, Fosroc Nitobond EP.

Limitations to take into account

  • Substrate strength. Strengthening only works if the concrete reliably transfers the force to the adhesive. Manufacturers specify a minimum pull-off strength of the concrete after preparation: for bonded Sika plates — at least 2.0 MPa on average with a minimum of 1.5 MPa, for Mapei — more than 1.5 MPa; for Sika fabrics — at least 1.0 MPa or as per the design. Deteriorated concrete is first reinstated with repair mortars, and the reinforcement is protected.
  • Adhesive creep. Epoxy adhesives creep under sustained load; the manufacturer states that the long-term design load should, as a rule, be below 20–25 % of the failure load.
  • Temperature. The long-term service temperature of epoxy adhesives is limited — for Sikadur-30, for example, to +45 °C. Hot areas need special solutions.
  • Fire and ultraviolet light. Without protection, the bond line loses strength when heated, and the resin degrades in sunlight. In exposed areas the system is covered with a coating and, where there are fire resistance requirements, with fire protection boards or plaster as calculated; see the “Fire protection of concrete” section.
  • The system as a whole. The fabric, primer, resin and adhesive are matched to one another by the manufacturer; replacing a component with an “equivalent” deprives the system of its tested properties.

Procedure

  1. Survey and design: concrete strength, condition of the reinforcement, loads; the strengthening design is prepared by a structural engineer to the codes of the country of construction and design guides — for example, ACI PRC-440.2-23 of the American Concrete Institute.
  2. Repair of the substrate: reinstating deteriorated concrete, protecting the reinforcement, injecting cracks; the repair mortar is left to gain strength as required by the manufacturer of the strengthening system.
  3. Surface preparation: removal of laitance and contamination to expose the aggregate — by abrasive blasting or grinding; flatness as per the data sheet; rounding of corners for fabric.
  4. Substrate testing: pull-off test on site.
  5. Installation: mixing the adhesive in complete kits at a permissible temperature, applying it to the substrate and to the plate, rolling in with a rubber roller without air voids; for fabrics — primer, impregnating resin, fabric, rolling, final coat of resin.
  6. Quality control: tap testing for voids, pull-off test samples, a temperature and humidity log; protection of the system with a coating.

Common mistakes

  • Bonding plates to weak, unprepared or damp concrete.
  • Components from different manufacturers in one system.
  • Working below the permissible temperature or in the presence of condensation — the adhesive does not gain strength.
  • Wrinkles and voids under the fabric, air not pressed out from under the plate.
  • Fabric over sharp corners without rounding — the fibres break.
  • Strengthening without protection against ultraviolet light and fire where this is required.
  • Design “by analogy” without allowing for creep and service temperature.

What to check with the supplier and the engineer

  • the characteristic values of strength and modulus for design, not just the mean values;
  • requirements for the pull-off strength, flatness and preparation of the substrate;
  • the installation and service temperatures of the adhesive, its pot life and open time;
  • the system’s approvals and tests, method statements for installation;
  • options for protection against ultraviolet light and fire.

Surveys and system selection are part of our engineering services; the strengthening is calculated by a structural engineer. Anchoring of additional reinforcement is covered in the “Anchoring and anchor grouting” section.

Materials in the catalogue

  • Sika CarboDur S — Sika — pultruded CFRP plates for externally bonded strengthening of slabs, beams and bridges.
  • SikaWrap-230 C — Sika — unidirectional carbon fibre fabric for strengthening columns, beams and masonry.
  • Sikadur-30 — Sika — epoxy structural adhesive for bonding CFRP plates and steel plates in strengthening.
  • Sikadur-330 — Sika — epoxy impregnating resin for SikaWrap carbon fabrics and slot-bonded CFRP plates.
  • Sikadur-31 CF Normal — Sika — thixotropic epoxy structural adhesive and repair mortar for bonding.
  • Sikadur-32 Normal — Sika — epoxy bonding agent for fresh-to-hardened concrete and structural bonding.
  • Fosroc Nitowrap CW — Fosroc — unidirectional carbon fibre fabric for external flexural and shear strengthening of concrete.
  • Fosroc Nitobond EP — Fosroc — epoxy bonding agent for placing new concrete and screeds on existing concrete.
  • MapeWrap C UNI-AX — Mapei — unidirectional carbon fibre fabric for strengthening concrete, masonry and timber.
  • Carboplate — Mapei — pultruded carbon fibre plates for flexural strengthening of concrete and timber.
  • Eporip — Mapei — epoxy adhesive for new-to-old concrete joints and bonding precast elements and steel to concrete.

This list is not exhaustive. The final choice is made on the basis of the manufacturer’s documents and the design calculations.

Frequently asked questions

Which should I choose — plates or fabric?

Plates are convenient for flexural strengthening of flat members — slabs and beams. Fabric follows corners and curved surfaces, so it is chosen for column jackets, shear strengthening and members of complex shape. The two are often combined in one project.

Can a structure made of weak concrete be strengthened?

Externally bonded strengthening requires a certain pull-off strength of the concrete. If the substrate is weaker, manufacturers offer other solutions — for example, plates bonded into slots or fabrics with lower substrate requirements — and sometimes section enlargement is the better option.

Is a design needed for CFRP strengthening?

Yes. The number of layers, the width of the plates, the anchorage length and the scheme are calculated by a structural engineer; the technical data sheet describes the properties of the materials and the method of application, but is no substitute for calculation.

How can strengthening be protected against fire?

Fire protection is selected for the required fire resistance rating: board encasement or fire protection plasters, as calculated. The question is settled at the strengthening design stage.

We will select a composite strengthening system and calculate the supply for your project — call us or message us on WhatsApp/Telegram: +998 94 094 52 27.