International Concrete Abstracts Portal

This paper presents detailed investigations into the effective moment of inertia for concrete beams prestressed with aramid fiber reinforced polymer (AFRP) tendons, including an assessment of the existing predictive methods. A three-dimensional nonlinear finite element analysis (FEA) model is developed, based on three different experimental programs reported in literature, to predict the effective moment of inertia of concrete beams prestressed with AFRP tendons. The investigation includes the effect of different sectional properties and various prestressing levels in the tendons. The solved FEA models are compared with several predictive models. The prestressing level in the AFRP tendons significantly influences the transition of the moment of inertia from uncracked section (Ig) to fully-cracked section (Icr) . The existing design standards may not be applicable for beams having a large Ig/Icr ratio (typically over 50) with a low level of prestress (e.g., below 40% ultimate).

While most studies have focused on short term behavior of fiber reinforced polymer (FRP)-reinforced concrete (RC) members, it is important and necessary to study their time-dependent behavior, if FRP is to be used as a viable alternative reinforcement for the infrastructure. The present study provides an analytical tool to assess serviceability issues of FRP-RC beam-columns under sustained loads. The analytical model accounts for creep of FRP-RC beams and beam-columns. The model is based on age-adjusted effective modulus method for concrete and Findlay’s model for FRP reinforcement. The model has been verified against the FRP-RC beam tests by the authors, as well as those from the literature. A detailed parametric study is then carried out to determine the effects of different types of FRP reinforcing bars on the creep of slender FRP-RC columns, and whether the ACI 318 design provisions for creep of slender RC columns is adequately safe for FRP-RC construction.

To understand and predict the effect of externally bonded reinforcement (EBR) on the serviceability behavior of FRP (fiber-reinforced polymer) strengthened members, four-point bending tests have been executed on reinforced concrete (RC) beams with span length 3.8 m (150 in.). This experimental campaign was further complemented with tests on strengthened tensile members. These so-called ‘tension stiffening’ tests typically consist of a tensile test on a reinforcing bar embedded in a FRP strengthened concrete prism. As the FRP EBR increases the stiffness of the beams and as a denser crack pattern with smaller crack widths is obtained, the serviceability limit state (SLS) of the strengthened members is positively influenced. Hereby, the behavior in terms of deflection and crack widths can be predicted in a fairly accurate way.

The use of externally bonded FRP plates and sheets has been established as an effective mean to strengthen concrete beams in flexure and shear. Several investigators have shown that the direct use of Branson equation does not yield conservative deflection results especially after steel yielding. The authors have developed an analytical solution that generates closed form expressions for deflection in simple beams. These expressions were shown to reproduce results that compared well with experiments. These expressions were dependent on the loading condition while Branson equation is not load specific. Branson equation is modified by replacing the cracked section moment of inertia with an effective moment of inertia at steel yielding to evaluate deflections at the service load levels. This revised formula could be easily implemented in the ACI 440.2R format. The predictions are compared against Branson equation and experimental evidence. Furthermore, another new modification to Branson equation is proposed for the region covering response from steel yielding and up to ultimate capacity.

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.