Ensuring Durable Performance of CFRP-strengthened Concrete

Figure 8: Top photograph shows a failure plane within concrete. Below is a side-view of the test dolly with identification.

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.

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.

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