International Concrete Abstracts Portal

Recommendations for the serviceability design of concrete structures strengthened with externally bonded Fiber-reinforced Polymer reinforcement (EBFRP) are scarce. This paper intends to fill this void by presenting several procedures for the direct and indirect control of deflections in concrete beams and one-way concrete slabs strengthened with EBFRP. Two types of formulations for direct and indirect deflection control are proposed: based on Branson’s effective moment of inertia approach and based on the concept of limiting curvatures. The procedures account for the mechanical properties of both the unstrengthened and strengthened member, including those of the EBFRP, and the load level at which the FRP is installed. The effects of tension stiffening on deflection control are also evaluated.

Serviceability related to deflections and cracking often controls design of fiber reinforced polymer (FRP) reinforced concrete. The existing approach prescribed in ACI 318 for computing deflection of steel reinforced concrete overestimates member stiffness when FRP is used as the reinforcement. Deflection is then underestimated. Numerous proposals have consequently been made for computing deflection of FRP reinforced concrete, and have mostly involved modifications to Branson’s original ACI expression for the effective moment of inertia Ie. This paper reviews the different procedures used in the past by ACI Committee 440 to compute deflection of FRP reinforced concrete members, and discusses deficiencies of past and present ACI 440.1R deflection calculation guidelines. A case is made for the need to adopt a more rational approach to compute deflection and the basis for proposed changes are reviewed and explained in detail. A statistical comparison of past, present, and proposed approaches for computing deflection are compared with an experimental database that justifies the need for a more rational approach to computing deflection. The paper ends with a clear set of deflection procedures for carrying out serviceability design of FRP reinforced concrete.

Due to concerns with corrosion, the use of fiber reinforced polymer (FRP) as a replacement to conventional steel reinforcement has greatly increased over the last decade. However, elastic modulus values of some commercially available FRP reinforcement hardly reach 20% of that for conventional steel. Existing code relationships for conventional steel have been modified to address this proprietary difference but the modifications are restricted by the empirical nature of the expressions. This paper provides an alternate approach to estimate the deflection of concrete beams by considering effects of tension stiffening that incorporate material properties of the reinforcement as well as the effects of concrete non-linearity in compression. A database containing experimental load-deflection records from 139 glass FRP (GFRP) and 48 carbon FRP (CFRP) reinforced concrete beams was used to calibrate a tension stiffening model for the proposed approach and establish its accuracy as well as precision through statistical analysis. Results were compared to those obtained from existing relationships and indicate that the revised approach provides higher accuracy at service conditions ranging from 25% to 80% of ultimate.

Design for deflection control is a critical part of the design of FRP reinforced members due to the relatively low modulus of elasticity and elastic-brittle nature of FRP reinforcement. This paper provides an overview of design for deflection control including a brief review of the basic mechanics of beam flexure and a review of methods to account for cracking, tension stiffening, shrinkage and creep. The basis for selecting appropriate deflection control criteria is also discussed and a framework is presented to incorporate deflection control into the overall design process.

An experimental study was conducted to evaluate the increase in crack width occurring over time in FRP-reinforced concrete as a result of sustained loading. Twelve beams (eight GFRP, two CFRP, and two steel-reinforced) were maintained under a constant sustained service load for nearly three years. Three flexural cracks were monitored on each beam over the duration of the test. The observed increase in flexural crack widths over the study was greater in the FRP-reinforced specimens than in the steel-reinforced specimens. On average, flexural crack widths in FRP-reinforced concrete specimens were observed to double over one year of sustained loading. A simple design approach, based on modification to the existing ACI 440 (2006) crack control procedure, is proposed to account for this observed increase in crack widths with time.