ACI 440.7R-10 Guide for the Design and Construction of Externally Bonded Fiber-Reinforced Polymer Systems for Strengthening Unreinforced Masonry Structures

ACI 440.7R-10 Guide for the Design and Construction of Externally Bonded Fiber-Reinforced Polymer Systems for Strengthening Unreinforced Masonry Structures. CHAPTER 3—CONSTITUENT MATERIALS AND PROPERTIES The behavior of FRP-reinforced masonry structures depends on the physical and material properties of the existing masonry, as well as the FRP system. The physical and mechanical properties of the masonry should be investigated. The effects of factors such as loading history and duration, temperature, and moisture on the properties of FRP systems, are discussed in this chapter. Fiber-reinforced polymer systems are available in a variety of forms such as wet layup, prepreg, and precured. Factors such as fiber volume, type of fiber, type of resin, fiber orientation, dimensional effects, and quality control during manufacturing, all play a role in establishing characteristics of the FRP system. The material characteristics described in this chapter are generic and do not apply to all commercially available products. Test methods according to ASTM standards should be used for material characterization; for test methods not included in ASTM standards, reference should be made to ACI 440.3R. Preconstruction quality assurance testing of the FRP strengthening system, hereby called FRP system or FRP reinforcement, is recommended. 3.4—Time-dependent behavior 3.4.1 Creep rupture—Fiber-reinforced polymer materials subjected to a constant tensile load over time can suddenly fail after a time period called the endurance time. As the ratio of the sustained tensile stress to the short-term strength of the FRP laminate increases, endurance time decreases. The creep rupture time may also decrease under adverse environ- mental conditions, such as high temperature, ultraviolet- radiation exposure, high alkalinity, wetting-and-drying cycles, or freezing-and-thawing cycles. In general, carbon fibers are the least susceptible to creep rupture, aramid fibers are moderately susceptible, and glass fibers are most susceptible. Creep rupture tests have been conducted on 0.25 in. (6 mm) diameter FRP bars reinforced with glass, aramid, and carbon fibers. The FRP bars were tested at different load levels at room temperature. Results indicated that a linear relationship exists between creep- rupture strength and the logarithm of time for all load levels. The ratios of stress level at creep-rupture after 500,000 hours, which is about 50 years, to the initial ultimate strength of the GFRP, AFRP, and CFRP bars were extrapolated to be 0.3, 0.47, and 0.91, respectively (Yamaguchi et al. 1997). Similar values have been determined by Malvar (1998). Recommendations on sustained stress limits imposed to avoid creep-rupture are given in...