Robert J. Frosch

CONCRETE SHEAR STRENGTH: ANOTHER PERSPECTIVE

The aims of this research were to develop a simple method for the design of fiber-reinforced polymer (FRP)-reinforced beams and to determine if a common approach for steel and FRP-reinforced members is possible. A model is presented for calculating the concrete contribution to shear strength of reinforced concrete beams. The applicability of the model is supported by comparing the computed shear strengths with the experimental strengths of 370 specimens. The shear strength equation developed from the model is simplified to yield a design equation applicable for both steel and FRP-reinforced beams. The design equation is demonstrated to provide conservative results across the range of variables known to affect shear strength.

Another Look at Cracking and Crack Control in Reinforced Concrete

The American Concrete Institute Building Code requires the control of flexural cracking in reinforced concrete structures. Because of durability concerns, the use of thicker concrete covers is rapidly increasing. However, currently used crack control methods that are based strictly on statistical reasoning become unworkable with the use of thick covers. This study investigates the development of the crack control provisions and the crack width equation on which those provisions are based, explores the use of the crack width equation for the calculation of large covers, and presents a new formulation of the equation for calculating crack width that is based on the physical phenomenon. Use of that equation is supported by an evaluation of existing test data; based on that equation, a design recommendation is presented for the control of cracking that addresses the use of both coated and uncoated reinforcement.

SHEAR TESTS OF FRP-REINFORCED CONCRETE BEAMS WITHOUT STIRRUPS

In order to investigate the shear strength and behavior of concrete beams reinforced with fiber-reinforced polymer (FRP) bars, 9 large-scale reinforced concrete beams without transverse reinforcement were tested. Three types of FRP reinforcement and 2 types of steel reinforcement with varying yield strengths were used. The nominal concrete strength was 5,000 psi, and the longitudinal reinforcement ratio was varied from ~0.36-2%. The specimens were simply supported and loaded with 1 concentrated load at midspan. The specimens were analyzed using both the ACI Committee 440 recommended shear design procedures and the ACI 318-99 shear design provisions. These results were compared with test results. For FRP-bar-reinforced beams, the ACI 440 design method resulted in very conservative shear strength estimates, whereas the ACI 318-99 method resulted in unconservative computations of shear strength.

Influence of Beam Size, Longitudinal Reinforcement, and Stirrup Effectiveness on Concrete Shear Strength

Recent research has shown that the current ACI shear design provisions provide unconservative results for large beams and beams with low levels of longitudinal reinforcement; in addition, the research also indicates that some assumptions made in the current design provisions for shear reinforcement may further reduce the level of conservatism. In order to explore these deficiencies, this study investigated the influence of beam size and longitudinal reinforcement ratio on the shear strength attributed to the concrete, as well as the effectiveness of stirrups in transferring shear across a diagonal crack. The experimental portion of the study tested 6 rectangular RC beams. Other research data from the literature was used to supplement the experimental data. Based on test results and a data analysis, conclusions regarding the influence of beam size and longitudinal and transverse reinforcement on shear strength are presented.

BEHAVIOR OF LARGE-SCALE REINFORCED CONCRETE BEAMS WITH MINIMUM SHEAR REINFORCEMENT

Research has indicated that as the depth of a beam increases, a decrease in the shear strength of the section can be expected. This trend has often been termed a size effect. Testing of specimens unreinforced in shear has also demonstrated that it is possible for the shear strength to fall below that commonly assumed in design. Most sections, however, contain at least minimum transverse reinforcement as required by the building code. Therefore, it is important to understand the behavior of these structures as they may also be affected by a size effect. Two duplicate large-scale beams containing minimum shear reinforcement were tested. The tests were conducted to investigate the effect of size on the shear strength provided by the concrete as well as the shear strength provided by the transverse reinforcement. The specimens were subjected to constant shear, and the test results were analyzed. Based on these analyses, conclusions and concerns regarding the shear strength of transversely reinforced sections are presented.

Bond Strength of Lap-Spliced Bars

The performance of reinforced concrete structures depends on adequate bond strength between concrete and reinforcing steel. This article reports on a study of the bond strength of lap-spliced bars. The authors note that calculation methods to evaluate the strength of tension lap splices are based primarily on nonlinear regression analysis of test results. However, the results from these analyses may not be generalizable to situations beyond the domain of the data. In this study, the authors developed an expression for the calculation of bond strength based on a physical model of tension cracking of concrete in the lap-spliced region. Two different types of failure modes are considered: horizontal splitting that develops at the level of the bars (side-splitting failure), and vertical splitting that develops along the bar on the face cover (face-splitting failure). The developed expression was verified using results from 203 unconfined and 278 confined beam tests where the splice region was subjected to constant moment. Results showed that the relation between splice strength and splice length is not linear. The use of the fourth root of the concrete strength provides an improved estimate regarding the behavior of lapped splices as compared with the square root. In addition, the effect of the thickness of the concrete cover surrounding the bar is not linear.

Retrofit of Non‐Ductile Moment‐Resisting Frames Using Precast Infill Wall Panels

Many existing reinforced concrete moment-resisting frames located in seismic zones lack strength and ductility. One approach for correcting these deficiencies is the construction of infill walls to strengthen and stiffen the structure. Cast-in-place construction is often used; however, there are conditions where cost, time constraints, or limiting disruptions to building operations may dictate other solutions. One possible modification is the use of infill walls constructed of precast concrete panels. A precast infill wall system eliminates the need for large formwork during construction. Elimination or reduction of connection hardware between precast panels or between panels and the existing frame element can provide additional efficiency. Problems associated with casting large quantities of concrete in an existing building are eliminated. Construction time and inconvenience to occupants may be reduced along with the costs. The precast system has the potential of reducing the overall costs of rehabilitating existing structures.

Bond Strength of Nonmetallic Reinforcing Bars

Since fiber-reinforced polymer (FRP) reinforcement has different properties than steel, structures reinforced with FRP reinforcement behave differently and thus many of the design equations used for steel-reinforced concrete structures are not applicable for use with FRP reinforcement. This study examines the bond behavior of FRP-reinforced concrete. Three series of beam splice tests were performed on specimens reinforced with steel, glass FRP (GFRP), and aramid FRP (AFRP). The test results are compared to evaluate the influence of the reinforcement type, development length, and reinforcement spacing. The results also are compared with current design expressions provided by ACI Committees 318 and 440 to evaluate their applicability. The findings demonstrate that the bond strength achieved by FRP reinforcement is significantly lower than that achieved by steel reinforcement. The FRP-reinforced specimens exhibited extensive branching of flexural cracks, which was not observed in the steel specimens. The reinforcement modulus of elasticity was shown to be an essential variable affecting bond strength. Increasing bar spacing had a more beneficial effect on bond strength for FRP reinforcement relative to steel. The effect of splice length on bar strength is nonlinear for both steel and FRP bars.

Influence of Flexural Reinforcement on Shear Strength of Prestressed Concrete Beams

The paper presents the experimental results of a series of nine prestressed concrete beams without stirrups failing in flexure and in shear. Some theoretical considerations are also proposed on the basis of a theory previously developed by the authors with respect to shear strength in reinforced concrete members (Tureyen and Frosch 2003). The innovative design of the tests as well as the well-documented data presented by the authors have allowed the discussers to investigate a number of aspects with respect to shear strength in prestressed members. A series of independent conclusions and interpretations derived from this analysis may complete those proposed by the authors. As shown by the tests of the paper, beams are developing shear (diagonal) cracking at a given load level. Such cracking, however, does not lead to failure of the specimens, and load can be significantly increased before failure. For two specimens (V-4-0 and V-4-0.93), the increase meant that yielding of the flexural reinforcement was reached and bending was governing for the strength. For the other seven specimens, failure also developed in shear, but at a load 42% higher on average than the shear cracking load. The increase on the failure load with respect to the shear cracking load can, in the discussers’ opinion, be explained and calculated accounting for the different regions of Kani’s valley (Kani et al. 1979). Figure 12(a) shows a sketch of Kani’s valley and its two governing regimes. The ascending branch (named “crack propagation” in the figure, see Point A) is due to a sudden propagation of a flexural crack as it develops through the theoretical compression strut carrying shear. Such failure (disabling the teeth action as proposed by Kani) is followed by a total loss of load-carrying capacity of the member and is often named diagonal shear failure. The descending branch (named “direct struting” in the figure) has a different nature. Flexural cracks may reach the location of the theoretical compression strut carrying shear and develop through it (Point B in Fig. 12(a)) but they do not progress in an unstable manner. Instead, once such inclined cracks have developed, they can widen progressively as the load increases. A typical crack pattern illustrating this case is plotted in Fig. 12(b)

Shear Strength of Reinforced Concrete T-Beams without Transverse Reinforcement

A design method has been developed that can be used for calculating the shear strength of both steel and fiber-reinforced polymer (FRP) reinforced concrete members. While this procedure has been shown to be an effective method for rectangular sections, this investigation explores its applicability for the design of flanged sections. Several design approaches to provide extension of the method are subsequently developed. The range of applicability of the various approaches is illustrated by comparing the computed results with the test results of 154 T-beams and 370 rectangular beams. In general, the proposed design equations provide reasonable and conservative calculations of shear strength for beams in both the T-beam and rectangular beam databases. The results from the proposed methods are also compared with those using the current ACI design procedure to provide perspective on the benefits of the procedure. Based on this study, recommendations are provided for a simple and consistent method that can be used for the calculation of shear strength for both rectangular and flanged sections.