Ensuring Durable Performance of CFRP-strengthened Concrete

Bidding
The manufacturer of the CFRP product may offer to provide a sealed set of design drawings for a fee. However, in some jurisdictions, the EOR retains ultimate responsibility for the suitability of the delegated design to the field application. Per Engineers Joint Contract Documents Committee (EJCDC) C-700, Standard General Conditions of the Construction Contract, Section 7.19, the EOR must review submittals, approve, or otherwise take action to ensure conformance with the design intent. Ensure at least the following documents are included in the submittals:

  • sealed design drawings, if outsourced;
  • test data indicating the material strength;
  • installation instructions;
  • working drawings;
  • calculations; and
  • a quality control plan.

The manufacturer may have additional requirements.

It is important to specify the contractor has previous experience with CFRP installation, or completes training provided by the manufacturer. This training might be a requirement for some products regardless of the contractor’s previous experience.

For pricing, it is helpful for the bidders to know the quantity of CFRP material necessary in terms of total surface area. It is useful to assign consistent notations on the bidding documents for each repair location. For example, “1U2(10)” would mean install two CFRP strips with U-wrap configuration at 250-mm spacing along one joist pan bay of the girder.

Construction
Construction monitoring and quality assurance are critical to the performance of the CFRP. Special attention should be given to surface preparation as the primary surface adhesion provides the foundation for the engagement of subsequent CFRP layers. The concrete should meet the minimum strength requirements specified in ACI 440.2R. Poorly consolidated concrete, excessively polished or roughened surfaces, and embedded impurities can result in poor bond development. Moisture and leak issues must be resolved prior to application of the CFRP material.

CFRP fabric is one of many forms available for applying CFRP to the structure. This section discusses the application of unidirectional CFRP sheets with wet layup procedure.

Surface preparation
Loose material and coatings must be removed, and cracks and delaminations repaired in order to provide a uniform surface for installation. The surface should then be abrasive-blasted or ground to obtain a minimum surface roughness equivalent to the International Concrete Repair Institute’s (ICRI’s) Concrete Surface Profile (CSP) Level 3, as defined in Guideline 310.2, Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings, Polymer Overlays, and Concrete Repair.

Consult with the manufacturer about specific surface irregularities that may be too small to require spall repairs. Figure 5 shows a poor surface condition where high-viscosity epoxy paste (“putty”) was employed to smooth the surface as illustrated in Figure 6.

Rounding the concrete girder corners is necessary to avoid kinks in the CFRP. In some cases, sufficient bond can be developed without extending the CFRP sheets beyond the member’s bottom corners, thereby reducing this labor step in construction. In this case, CFRP termination should be specified at a minimum distance from the corner.

Installation
The contractor should be required to have the manufacturer’s instructions and project design specifications available onsite at all times. Many components are involved in the installation, including adhesives and polymer resins. Each needs specific preparation and curing times. It is important to develop action plans where application is disrupted by fast or slow cure of material or other interruptions. The solutions may be as simple as re-priming ahead of the next application or it may be necessary to remove preceding materials or reapply certain layer(s).

Consult the manufacturer for specifying the maximum number of layer applications in a given timeframe. Otherwise, the weight of the fabric and resins can exceed the adhesion of the uncured resin, resulting in slipping and sagging of the fabric. Specify strict daily recordkeeping and document the number of layers applied at each location when total layers exceed the one-day maximum in order to avoid guessing on subsequent applications.

The thickness of the resin and adhesives are critical. The material thickness should be regularly monitored and controlled using a wet-film thickness gauge (Figure 7). The curing process is sensitive to temperature. One must not perform work outside the recommended temperature range, keeping in mind the ambient temperature is often different from the concrete surface temperature. Also, be wary of condensation and high humidity in changing weather conditions.

Specify care in handling and application of the CFRP, including:

  • smoothing the surface to eliminate air gaps and delamination in each layer of CFRP (U-wrap installation is more sensitive than side-only installation because the fabric must turn the corner at the bottom of the girder and has a tendency to sag or bow away from the surface);
  • incorporating specifications for repair of delaminations based on their size and frequency (it is important to consider the delaminated area with respect to the total CFRP area at the location); (For more, refer to the American Concrete Institute (ACI) 440.8, Specification for Carbon and Glass Fiber-Reinforced Polymer Materials Made by Wet Layup for External Strengthen.)
  • ensuring sheets of CFRP are not folded during handling and the unidirectional sheets are applied parallel to the direction of the stirrups;
  • making sure necessary tests, including the adhesion tests described below, are performed prior to installation of the final coating; and
  • consulting the manufacturer in determining the resins and coating appropriate for local weather conditions.
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