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Tilt-up construction was effectively enabled on a wide scale in 1979, when the ACI committee 551 report on Tilt-up construction was published, the Recommended Tilt-Up Wall Design, aka, the Yellow Book and the subsequent ACI-SEASC Task, aka the Green Book, and another Tilt-up design and construction manual developed by the ACI in 1988. The Tilt-up Concrete Association was created in 1986 by a group of industry professionals who had the need of an organization dedicated to the industry. ACI 551 maintains a document outlining the standard practice for contemporary Tilt-up design and construction. The ACI 551 document does not consider walls reinforced with non-ferrous reinforcement. However, recent events have made glass fiber-reinforced polymer rebar a more economical option when compared to traditional steel reinforcement. This white paper is intended to provide the unfamiliar engineer a bridge between the ACI 318, ACI 551 and ACI 440 documents to engineer a tilt-up wall including differences between GFRP reinforcement and steel reinforcement with respect to design. Steel, is an isotropic ductile metal extensively used in construction, mechanical, and electronic devices. Steel is often designed as elastic-perfectly plastic where it has an elastic modulus of 29,000ksi and yield stress of 60ksi. GFRP reinforcement is considered brittle-elastic with an elastic modulus often between 6,000ksi – 8750ksi and guaranteed ultimate tensile strength between 90-150ksi. Figure 1 presents an example of the design-assumed uniaxial stress versus strain comparing traditional steel and GFRP. While GFRP reinforcement has different physical and mechanical properties the differences and behaviors are well understood by the engineering community. There are ASTM material standards for GFRP bars, namely ASTM D7957 and a complete suite of ASTM test method by which the basic properties are verified. The following sections will illustrate the differences between steel and GFRP reinforced concrete members with a focus on slender tilt-up wall panels. |
