Ensuring Durable Performance of CFRP-strengthened Concrete

by Samantha Ashenhurst | June 6, 2018 9:23 am

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by Roya A. Abyaneh, EIT, and Edward S. Breeze, PE
Externally bonded fiber-reinforced polymer (FRP) can be a practical strengthening solution for all concrete structural elements. However, there is little technical literature and design guidance on how one can achieve durable performance from this relatively new repair technique. With the rise of aging buildings and infrastructure, the need for durable reinforcement methods has become more pressing.

Today’s aging infrastructure requires regular maintenance and reinforcement or, in some cases, complete replacement. The 2017 Infrastructure Report Card of America by the American Society of Civil Engineers (ASCE) notes nearly 40 percent of the country’s bridges are more than 50 years old. (For more, consult the 2017 Infrastructure Report Card of America.) Rehabilitation and maintenance are becoming more critical every day, and so is the need for educating the industry in assessment and preservation of existing infrastructure. With the rising popularity of FRP as a means for structural repair, designers require more guidance in design and construction to obtain durable performance.

General-purpose carbon fiber-reinforced polymer (CFRP) has an elastic modulus in the range of 220 to 230 GPa (32,000 to 34,000 ksi), which is close to the stiffness of structural steel at 200 GPa (29,000 ksi). Improved load-carrying capacity through application of CFRP has been reported in various literature. However, its use among practicing engineers is limited when compared with steel and concrete.

One reason for the limited application may be the lack of expertise and familiarity in design with the material. CFRP manufacturers have helped address this issue by offering in-house engineering. In many cases, these services are free in conjunction with purchase of their CFRP products. In such cases, the structural engineer of record (EOR) for the project determines the required additional strength and delegates the task of CFRP design to the manufacturer’s designer (MD). Inherently, the MD will have limited familiarity with the overall structure. This leaves potential risks with the EOR, who must review the design, verify assumptions, and ensure applicability to the project.

Distressed infrastructure
For perspective, the standard procedures in design and construction using CFRP are discussed here in application to a typical reinforced concrete parking garage. Parking structures built in accordance with earlier codes, such as the Uniform Building Code (UBC), were designed for different live load requirements than specified in recent codes. In some cases, the older live load specifications may result in less stringent live load requirements when compared with the more recent International Building Code (IBC). This change in live load is in accordance with the heavier traffic and weight of vehicles. Along with deterioration of aging infrastructure, this code change illustrates the importance of regular assessments for maintaining healthy structures and ensuring public safety.

Once distress is observed, as-built conditions must be determined to perform structural analysis, including:

• determining the concrete density by coring specimens;
• scanning the concrete members to locate reinforcement and compare with design drawings; and
• excavating portions of the members to confirm reinforcing bar size, scan readings, and steel grade.

Assessment
American Concrete Institute (ACI) 562, Code Requirements for Assessment, Repair, and Rehabilitation of Existing Concrete Structures, specifies the required information, such as the tests discussed above, to assess structural distress. In addition to ACI 562, engineering judgement and pertinent research publications are necessary to determine the capacity of the members. Once the cause of distress is identified and the deficit capacity is determined, the repair method should be selected in consideration of various factors, including service interruption, cost, and design life of the repairs.

CFRP fabric is often a suitable choice, when considering:

• speed and ease of application;
• limited debris from demolition and repair activities;
• minimal disruption of traffic and aesthetic influence; and
• structural efficiency (strength-to-weight ratio).

A variety of CFRP products and application methods exist. This article focuses on the wet layup system with CFRP fabric for the purpose of shear strengthening. CFRP strengthening is permitted only when the un-strengthened structural member has the ability to withstand a reasonable level of dead and live loads as described in ACI 440.2R, Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures.

Design
As discussed earlier, many of the CFRP manufacturers offer design services. Consequently, communications between the EOR and the MD become critical. Information about deficit shear capacity, existing concrete strength, reinforcement details, and cross-sectional dimensions of the members are commonly required by the MD. The reported dimensions must consider construction tolerances. The EOR should also clarify whether the reported required strength is the nominal or design (factored) strength.

Shear strengthening using CFRP may be achieved by providing vertical or diagonal strips along the sides of a member similar to the placement of stirrups as shown in Figure 1. Typically, the CFRP sheet wraps the sides and bottom surfaces of the member to form a U-shaped wrap (U-wrap). In repair of concrete girders, maintaining uniform spacing of CFRP strips may not be possible where secondary members frame into the girder. Such information should also be communicated to the MD. Figure 2 shows an elevation of a typical girder and joist spacing.

A frequent challenge is accessibility of the concrete surface for installation of the CFRP wraps. For example, the girders along ramp intersections in a parking garage are typically accessible from one side only; the adjoining up-ramp blocks access to the opposite side of the girder. Therefore, the typical U-wrap configuration would not be feasible.

An alternative solution is applying the CFRP wraps to one side only, but this results in reduced bond surface of the CFRP. In compromise, anchorage is required to help develop higher strains in the CFRP sheets. Providing adequate bond between the CFRP and concrete is more efficient than providing a greater number of CFRP sheets to achieve a certain strength. A common failure pattern in experiments of CFRP-wrapped concrete is debonding of the plies from the concrete. (For more, refer to the 2017 paper, “Use of Carbon Fiber Reinforced Polymer (CFRP) with CFRP Anchors for Shear-Strengthening and Design Recommendations/Quality Control Procedures for CFRP Anchors” by J. O. Jirsa, W. M. Ghannoum, C. Kim, W. Sun, W. A. Shekarchi, N. K. Alotaibi, and H. Wang for the Center for Transportation Research, University of Texas at Austin.)

Anchorage may be achieved through mechanical fasteners, FRP stirrups, and fiber and near-surface mounted (NSM) anchors, as discussed in ACI 440.2R. To provide adequate shear capacity to the intersecting ramp girders, the CFRP sheets can be anchored as shown in Figure 3. This method resembles the NSM anchorage discussed in ACI 440.2R. It is critical the number of anchors and location of the anchors are clarified with the MD, who may specify a typical anchorage method. One-sided wraps would require anchorage at both top and bottom, unless noted. Alternatively, the bottom of the wrap may be extended under the girder or by other methods as specified by the manufacturer.

By the end of the delegated design phase, the MD should provide the required width of CFRP strips, number of plies, and spacing, as well as the configuration of the wrap and specific requirements for anchorage or development length. It is recommended the contract documents identify the required submittals to be obtained by the contractor from the MD. By doing so, the EOR communicates the intent of the delegated design to the contractor. Note ACI 440.8, Specification for Carbon and Glass Fiber-Reinforced Polymer Materials Made by Wet Layup for External Strengthening of Concrete and Masonry Structures, covers the requirements for CFRP wet layup installation.

The EOR should request a copy of the design calculations, and understand and agree with them. One shortcoming of ACI 440.2R is it does not address factors associated with one-sided wraps, which are sometimes the only practical solution due to site obstructions. Therefore, engineering judgement becomes critical in evaluating the assumptions considered by the MD. In addition to the strength reduction factor, ACI 440.2R specifies FRP strength reduction factor based on the variability in experimental results. This factor varies based on the configuration of the wrap (fully-wrapped vs. U-wrap or two-sided wraps).

The EOR must also ensure the recommended design (strip width and spacing) are practical and systematic in consideration of the construction. For example, it would save time for the contractor if all CFRP strips were designed to be 250 mm (10 in.) wide, which can be easily achieved by cutting a 500-mm (20-in.) roll in half. Where possible, specifying uniform spacing in each location can eliminate errors in construction and monitoring. Figure 4 shows the uniform strip width and spacing at a repaired location.

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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Inspection
It is important to perform periodic testing during construction to identify and mitigate improper installation, if any. Some manufacturers also require independent inspection as a warranty criterion. Among many tests listed in ACI 440.2R, acoustic hammer sounding, visual changes in color, and surface irregularity are helpful in identifying delaminations.

Manufacturers may require specific tests. A commonly required field test is ASTM D7522, Standard Test Method for Pull-Off Strength for FRP Laminate Systems Bonded to Concrete Substrate. (This is based on ASTM D7522/D7522M, Standard Test Method for Pull-Off Strength for FRP Laminate Systems Bonded to Concrete Substrate.) ACI 440.3R, Guide Test Methods for Fiber-Reinforced Polymers (FRPs) for Reinforcing or Strengthening Concrete Structures, provides specific requirements not adopted in the ASTM method.

One must employ a pull-off test device suitable for concrete meeting the ASTM requirements. The CFRP manufacturer can provide guidance about the test device suitability, test procedure, and results. In addition to pull-off strength magnitude, it is important to observe and document the surface conditions at failure. The goal is for the failure to occur in the concrete material, as shown in Figure 8, with a strength exceeding 1.4 MPa (200 psi). (This is based on American Concrete Institute (ACI) 440.8, Specification for Carbon and Glass Fiber-Reinforced Polymer Materials Made by Wet Layup for External Strengthen, and ASTM D7522/D7522M, Standard Test Method for Pull-Off Strength for FRP Laminate Systems Bonded to Concrete Substrate.) Depending on the installation quality, the failure plane may include CFRP, resin, or adhesive. Documentation of the failure plane through test reports and photos is essential in evaluation of the test. A map of tested locations should be maintained and manufacturer’s procedures followed for repair of the tested regions.

Conclusion
Regular condition assessments are critical in identifying structural deficiencies and developing repair and maintenance plans.

ACI 562 is a useful resource in the evaluation phase. However, it has a few shortcomings requiring research and engineering judgement. For example, it notes:

The licensed design professional is responsible for determining the appropriate method of analysis…If a linear elastic analysis method is used, the effects of cracking, second-order and other nonlinear effects should be included in the analysis using engineering approximations.

It does not provide guidance to quantify the reduction in load-carrying capacity of concrete members as a function of crack width and spacing. Further, practical and consensus-based methodologies are not readily available to the design professional for quantifying the effects of cracking. This evaluation challenge is an obstacle requiring further research.

FRP repair methods (including CFRP and other materials) are strong and non-intrusive materials for concrete reinforcement. However, many designers are not yet familiar with design and construction for FRP. There are currently no codes that govern design with FRP in the United States. ACI 440.2R provides design guidelines limited to typical FRP wrapping schemes, which are not always practical. Additional study is warranted to determine if one-sided reinforcing results in a lower strength reduction factor as torsion, asymmetry, and girder width vary.

The ASTM D7522 test is specified for bond-critical applications to evaluate the pull-off strength of the CFRP in a direction perpendicular to the concrete surface. Since debonding failures occur as a shear failure at the surface of the concrete, development of a direct shear test may provide additional insight into the CFRP strength and failure mechanisms.

Roya A. Abyaneh, EIT, is a staff engineer at Building Diagnostics Inc. d/b/a Engineering Diagnostics, specializing in the investigation of problems with existing buildings, designing remedies for those problems, and monitoring the construction of the remedies. She participates in the research being performed at The Durability Lab, Building Diagnostics’ testing center at The University of Texas at Austin. She can be reached at RAbyaneh@BuildingDX.com[10].

Edward S. Breeze, PE, is a principal of Building Diagnostics, Inc. d/b/a Engineering Diagnostics. He is a licensed professional engineer specializing in the investigation of problems with existing buildings, designing remedies for those problems, and resolving disputes that arise from them. Breeze also participates in the research being conducted at The Durability Lab. He can be reached via e-mail at EBreeze@BuildingDX.com[11].

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