Tarek K. Hassan

Bond Mechanism of Near-Surface-MountedFiber-Reinforced Polymer Bars for FlexuralStrengthening of Concrete Structures

This paper presents both experimental and analytical investigations undertaken to evaluate bond characteristics of near-sutfacemounted (NSM) carbon fiber-reinforced polymer (CFRP) bars. A total of eight concrete beams. strengthened with NSM CFRP bars, were tested under monotonic static loading. Different embedment lengths are studied to determine the development length of the fiber-reinforced polymer (FRP) reinforcement. The performance of two different adhesives used to bond the bars to the surrounding concrete is examined. A general methodology to evaluate the development length of NSM FRP bars of different configurations and types of fibers is presented. A quantitative criterion governing debonding failure is established. The proposed bond model assumes linear elastic behavior jar the concrete, adhesive, and the NSM FRP bars, following the same philosophy oj the ACI provisions for bond analysis and design. The proposed analytical model is validated by comparing the predicted values with test results as well as to nonlinear finite element modelling. The influence of key parameters, including the thickness of the adhesive cover, groove width, groove spacing, and inte171ai steel reinforcement configuration, are discussed.

Analysis and design guidelines of precast prestressed concrete, composite Ioad-bearing sandwich wall panels reinforced with CFRP, grid

This paper presents newly developed design guidelines for precast/prestressed concrete wall panels reinforced with carbon-fiber-reinforced-polymer (CFRP) shear grid to achieve the composite interaction. The analytical approach provides a general methodology to determine the behavior of fully and partially composite wall panels. The effects of an imperfect connection between the two concrete wythes are considered by varying the total shear force transmitted through the shear connectors at the interface. The predicted strains along the thickness of the panel at different load levels compared well with recent test results conducted at North Carolina State University in Raleigh. The shear-flow capacity of the insulating materials and the CFRP shear grid are determined using the proposed approach. The influence of the degree of the composite interaction on the induced curvature and slip-strain behavior is presented. A simple design chart for estimating the flexural capacity of the wall panels with different shear-reinforcement ratios is proposed. The approach is also verified by using finite-element analysis up to the service-load level. The predicted displacement and strains compared well with the measured values reported by the experimental program.

Shear Behavior of Large Concrete Beams Reinforced with High-Strength Steel

This study seeks to quantify the benefits of using high-strength steel for concrete reinforcement and provides experimental evidence of its high strength capabilities. Test results are presented of six large-size concrete beams reinforced with either conventional- or high-strength steel and tested up to failure. The beams were constructed without web reinforcement to evaluate the nominal shear strength provided by the concrete. The shear behavior, ultimate load-carrying capacity, and mode of failure are presented. The applicability of the current ACI design code to large-size concrete beams constructed without web reinforcement is discussed. The influence of the shear span-depth ratio, concrete compressive strength, as well as the type and the amount of longitudinal steel reinforcement is investigated. Findings indicate that using high-strength steel alters the mode of failure from diagonal tension to shear compression failure and results in higher shear strength compared with using conventional steel. Findings also suggest that the current ACI shear design provisions are unconservative for large-size concrete beams without web reinforcement. The expression needs to account for the size effect and reinforcement characteristics.

FRP Shear Transfer Mechanism for Precast, Prestressed Concrete Sandwich Load-Bearing Panels

Synopsis: This paper describes the structural behavior of precast prestressed concrete sandwich wall panels reinforced with carbon fiber reinforced polymer (CFRP) shear grid to achieve composite action. The study included testing of six full-scale sandwich wall panels, each measuring 20 ft (6.1 m) by 12 ft (3.7 m). The panels consisted of two outer prestressed concrete wythes and an inner foam core. The study included two types of foams and several shear transfer mechanisms with different CFRP reinforcement ratios to examine the degree of composite action developed between the two concrete wythes. All wall panels were simultaneously subjected to applied gravity and lateral loads. The paper also presents a general methodology to determine the behavior of fully and partial composite wall panels. The effects of imperfect connection between the two concrete wythes are considered by varying the total shear force transmitted through the shear connectors at the interface. The shear flow capacity of the insulating materials as well as the CFRP shear grid is determined using the proposed approach. The influence of the degree of the composite interaction on the induced curvature and slip-strain behavior is presented. A simple design chart for estimating the flexural capacity of the wall panels with different shear reinforcement ratios is proposed.

Bond Characteristics and Shear Behavior of Concrete Beams Reinforced with High-Strength Steel Reinforcement

This paper evaluates the bond behavior of high strength (HS), steel reinforcing bars and highlights the effect of various key parameters believed to affect the bond characteristics. Nine reinforced concrete spliced beams were constructed and tested. The beams had different splice lengths and levels of confinements. The applicability of different hypotheses for development of conventional steel bars was examined for the HS bars. The study is extended to examine the behavior of the reinforcing bars as shear reinforcement for concrete beams by testing twelve concrete beams reinforced with HS steel stirrups under static loading conditions. The main variables in the study included steel type, concrete compressive strength, web reinforcement ratio and shear span-to-depth ratio. The applicability of various building codes and standards for concrete beams with HS shear reinforcement was also evaluated.

BOND CHARACTERISTICS OF VARIOUS FRP STRENGTHENING TECHNIQUES

Strengthening of reinforced concrete structures using FRP has emerged as a potential solution to the problems associated with civil infrastructure. Many researchers have reported significant increases in strength and stiffness of concrete structures strengthened with FRP. Nevertheless, possible brittle failures of the strengthened system due to delamination of the FRP strips and/or sheets could limit the use of the full efficiency of the FRP system. This paper presents a bond failure hypothesis for near surface mounted FRP bars. Closed-form analytical solutions are proposed to predict the interfacial stresses for near surface mounted FRP strips and externally bonded FRP sheets. The models are calibrated by comparing the predicted behavior to test results. Quantitative criteria governing interfacial debonding failure of near surface mounted FRP bars, strips and externally bonded FRP sheets are established.

Behavior and Flexural Prediction of Special Cementitious Bonding Material for Fiber Reinforced Strengthening Systems

Strengthening of RC structures with externally bonded FRP (fiber reinforced polymers) has become an important challenge in civil engineering. Epoxy is the main bonding agent used so far, but in the case of a fire, it is subjected to complete loss of his bonding capabilities. Mineral based composites strengthening systems consist of FRPs and a cementitious bonding agent which form a repair or strengthening system that is more compatible with the concrete substrata, and roved its efficiency. The current research introduces the use of a special cementitious material “Grancrete” as a bonding agent. Test results of 32 T-section RC beams strengthened with various FRG (fiber reinforced Grancrete) strengthening systems are presented. The results demonstrated that most of the specimens were likely to fail by debonding of the FRP from the concrete either at the ends or at intermediate flexural cracks. This paper presents an in-depth study aimed at the development of a better understanding of debonding failures in RC beams strengthened with externally bonded FRP systems. Different analytical models, published in the literature for plate end debonding, are reviewed and compared to test results. The results also demonstrated that when using U-wraps, the specimens were likely to fail by FRP sheet rupture.

Finite Element Study of Shear Behavior of Spandrel Ledges and Comparison with PCI Shear Design Provisions

Punching shear failure of spandrel ledges has been observed by many researchers and heightened concerns have been raised questioning the safety level of the PCI shear design provisions for beam ledges. This paper presents non-linear finite element analyses conducted to model the behavior of prestressed L-shaped spandrel beams. Special emphasis was given to the behavior of the ledge at the end regions of the spandrel, where punching shear controls the design. The accuracy of the finite element model is demonstrated by comparing the predicted behavior to the results of one major test by another researcher. The influence of different parameters included in the PCI shear design provisions for beam ledges is discussed. The analysis is extended to illustrate the effect of other parameters including the prestressing level, hanger reinforcement and the amount of debonded strands at the end regions of the spandrel beam. Results of the analyses showed that the PCI shear design provisions for spandrel ledges are dangerously unconservative. The provisions do not account for key parameters, which affect the punching shear behavior.