Showing papers in "Aci Structural Journal in 1998"

Shear Strengthening of Reinforced Concrete Beams Using Epoxy-Bonded FRP Composites

Abstract: The paper deals with the application of fiber reinforced polymer (FRP) laminates or fabrics as shear strengthening materials for reinforced concrete beams. The study aims at increasing the experimental database on shear strengthening of concrete using composites and, most importantly, developing an analytical model for the design of such members within the framework of modern code formats, based on ultimate limit states. The experimental part of the study involved testing of eleven concrete beams, strengthened in shear with carbon FRP at various area fractions and fiber configurations, while the analytical part resulted in a model for the contribution of FRP to shear capacity in analogy with steel stirrups, with an effective FRP strain that decreases with increasing FRP axial rigidity. It is shown that the effectiveness of the technique increases almost linearly with the FRP axial rigidity and reaches a maximum, beyond which it varies very little.

Prediction of Failure Load of R/C Beams Strengthened with FRP Plate Due to Stress Concentration at the Plate End

Abstract: Epoxy-bonding a composite plate to the tension face is an effective technique for repair and retrofit of reinforced concrete beams. Experiments have indicated local failure of the concrete layer between the plate and longitudinal reinforcement in retrofitted beams. This mode of failure is caused by local stress concentration at the plate end as well as at the flexural cracks. This paper presents a method for calculating shear and normal stress concentration at the cutoff point of the plate. This method has been developed based on linear elastic behavior of the materials. The effect of the large flexural cracks along the beam has also been investigated. The model has been used to find the shear stress concentration at these cracks. The predicted results have been compared with both the finite element method and experimental results. The analytical models provide closed form solutions for calculating stresses at the plate ends that can easily be incorporated into design equations.

Numerical Modeling of the Behavior of Concrete Structures in Fire

Abstract: Finite element thermal and structural computer programs were developed to predict the behavior of three-dimensional reinforced concrete structures in fire. The thermal program uses an implicit recurrence scheme to improve convergence. The structural program was based on recent experimental data obtained at Imperial College on the transient thermal strain behavior of concrete heated for the first time under load. The thermal program was validated against data from an experimental test on a steel ceramic element. The structural model was also validated against data from experimental tests on concrete slabs and a concrete column. Good correlation between predicted and experimental results was obtained for both temperature distribution in the thermal module and deformation in the structural model. The results highlight the important role that transient thermal creep plays, particularly in the column example.

Behavior and design of single adhesive anchors under tensile load in uncracked concrete

Abstract: A user-friendly model for the design of single adhesive anchors subjected to tension loading in uncracked concrete is presented. Descriptions of the various types of adhesive anchor systems is included as well as a summary of previously published design models. The development of the user-friendly design model includes a comparison of the model and previously published models to a database including 888 European and American tests. The comparison shows that the simple user-friendly model provides a better fit to the database than more complicated design models previously presented. Although the model is limited to anchors located away from free edges, it provides the basis for development of models that account for the effect of edges, anchor groups, and other design conditions.

Flexural Behavior of Concrete Beams Reinforced with Deformed Fiber Reinforced Plastic Reinforcing Rods

Abstract: Fiber reinforced plastic (FRP) reinforcing bars are being used as an alterntative to steel reinforcement to overcome the corrosion problem in bridge decks, parking garages, water and wastewater treatment facilities, marine structures, and chemical plants. This paper presents test results of concrete beams reinforced with FRP and conventional steel reinforcement. The beams were tested under static loading to investigate the effects of reinforcement ratio on cracking, deflection, ultimate capacities, and modes of failure. Based on this investigation, theoretical correlations for the prediction of crack width, maximum deflection, and ultimate load-carrying capacity are proposed.

Moment tensor analysis of acoustic emission for cracking mechanisms in concrete

Abstract: A quantitative analysis of acoustic emission (AE) waveforms, which was developed as a Simplified Green's Function for Moment Tensor Analysis (SiGMA) code, is revised. By the analysis, cracks of AE sources are located, classified into a tensile crack and a shear crack, and their orientations are determined. For practical application to concrete, both experimental and analytical procedures are reexamined on the basis of the background theory of moment tensor. To apply the SiGMA analysis to AE waveforms recorded, a basic experimental procedure is discussed and established. In the tensile test of an L-shaped reinforced concrete model, nondestructive evaluation (NDE) of cracking mechanisms is investigated. To improve the accuracy of SiGMA solutions, a postanalysis is developed. Unreliable solutions are screened out, based on the discrepancy between SiGMA solutions of the experiment and those of synthesized waveforms. The mechanisms of the fracture process zone are studied in the bending test of a notched mortar beam. These results demonstrate the applicability of the moment tensor analysis to both NDE of concrete structures and to experimental fracture mechanics in concrete.

Pivot Hysteresis Model for Reinforced Concrete Members

Abstract: For nonlinear dynamic seismic analysis of bridge structures to be practical, only the dominant nonlinear characteristics of the structure should be included. Based on capacity design principles, the current seismic bridge design philosophy is generally to force all member nonlinearities into the ends of ductile columns. Therefore, nonlinear characteristics of the columns must be adequately defined. In this paper, a hysteresis model is presented that accurately captures the nonlinear behavior of reinforced concrete members in terms of a force-displacement, or moment-rotation, response. What makes this model attractive, when compared with other hysteresis models, is that unsymmetric sections (nonsymmetric cross-section geometry and/or tension reinforcement amounts in the two loading directions), a cyclic axial load, and strength degradation may be included. The model is based on a few simple rules. Results based on the proposed hysteresis model show close agreement with various experiments on reinforced concrete members.

Predicting the service life of concrete marine structures: an environmental methodology

Abstract: A methodology is presented that is particularly useful for evaluating mixture designs and multiple design criteria when predicting the service life of concrete structures. The methodology incorporates surface environment, chloride transport, temperature of surrounding medium, seasonal effects, and construction variability into a model that can be used to predict the service life of a reinforced concrete structure in different environments. Three components (submerged, splash zone, and superstructure) on the same concrete structure in two temperature environments are used in examples to illustrate the technique. Several concrete mixtures of different quality are evaluated in these applications. Silica fume is shown to increase significantly the estimated service life of the structures by decreasing chloride transport in the concrete and by decreasing the buildup of chloride in the near surface region of the structure.

Bond between Normal Strength and High-Strength Concrete (HSC) and Reinforcing Bars in Splices in Beams

Abstract: This paper presents the second part of a study on bond. It reports on the influence of several parameters on bond in splices. The parameters covered are concrete strength, splice length, concrete cover, ratios between side cover, bottom cover, and the spacing between the spliced bars, rib face angle of the reinforcing bar, and admixtures in the concrete mix. The study was conducted in two stages. In the first stage, an analytical equation was developed to calculate the bond strength of splices in normal strength concrete. The mean value of test/calculated bond strength for the proposed equation is 1.007 with a standard deviation of 0.084. These values indicate a significant improvement in the calculation of the bond strength when compared with other proposed equations. The second stage of the research involved tests on 22 beams made of high-strength concrete (HSC). In each beam, the tensile steel was spliced in the constant moment zone. The test strengths are compared with the calculated values. Good correlation between calculated and test strengths of splices in the case of HSC is found.

Flexural Behavior of One-Way Concrete Slabs Reinforced by Fiber Reinforced Plastic Reinforcements

Abstract: Fiber Reinforced Plastic (FRP) reinforcements are currently used for special concrete structures in areas sensitive to magnetic fields and severe environmental conditions that accelerate corrosion of the steel reinforcements, and consequently leads to deterioration of the structure. This paper presents test results of eight one-way concrete slabs reinforced with glass-fiber, carbon-fiber, and conventional steel reinforcements. The slabs were tested under static loading conditions to determine their flexural and shear limit states, including the behavior prior to cracking, cracking, ultimate capacities, and modes of failure. Based on this investigation, design recommendations and guidelines are proposed.

Response of Reinforced Concrete Beams at High Strain Rates

Abstract: A closed-loop servohydraulic testing machine was used to conduct high rate tests on reinforced concrete beams. Seven pairs of singly reinforced beams (without shear reinforcement) were tested under displacement control. From each pair, one beam was tested at a "static" rate (piston velocity = 0.00071 cm/sec) and the other at a "high" rate (piston velocity = 38 cm/sec). The total number of cracks reduced significantly at the high rate. Peak load and energy absorption capacity were found to increase with the rate of straining. For three of the pairs tested, final failure mode shifted from shear failure at the static rate to flexural failure at the high rate. This phenomenon is the opposite of the brittle transition in the mode of failure reported by some other researchers.

Local bond strength of reinforcing bars in normal strength and high-strength concrete (hsc)

Abstract: The paper presents the first part of a study on bond. It reports the research conducted on bond strength of short-length specimens (local bond strength) in normal strength and high-strength concrete (HSC). For normal strength concrete, using the test results available in the literature, Tepfers' partly cracked thick cylinder theory is modified to account for the variation of the bursting angle and the tensile plastic deformation of the concrete. The study of local bond in HSC included tests of 45 short-length specimens using two types of bars with different rib face angles. For different ranges of concrete strength, a relationship for bond strength normalized with respect to the tensile strength of concrete is obtained. The normalized bond strength increased with the compressive strength of concrete. The bond strength of the bars with rib face angles between 23 deg (0.4 rad) and 27 deg (0.5 rad) is smaller than that of bars with rib face angles between 40 deg (0.7 rad) and 47 deg (0.8 rad).

Ultimate Shear Capacity of Reinforced Concrete Beams Strengthened with Web-Bonded Fiber-Reinforced Plastic Plates

Abstract: The ultimate shear capacity of reinforced concrete beams can be increased by epoxy-bonding fiber-reinforced plastic (FRP) plates to the web of the beam. The shear crack inclination angle is changed as a result of bonding of the plate. In this paper, truss analogy and compression field theory are used to determine the effect of the FRP plate on the shear capacity and crack inclination angle of reinforced concrete beams at ultimate state. Following the calculation of the crack inclination angle, the equilibrium and compatibility equations are used to obtain the shear force resisted by the plate. A parametric study was carried out to reveal the effect of important parameters such as plate thickness and fiber orientation on the crack inclination angle and shear capacity. The upper bound value of crack inclination angle found in this study is suggested as a conservative value for determining the shear capacity of the retrofitted beam. Knowing the inclination angle of cracks, the shear force in the composite plate and concrete beam can be calculated and used for the design of this type of beam. The results of this method have shown close agreement to experimental results.

Experimental studies on high-strength concrete deep beams

Abstract: Sixteen high-strength concrete deep beams were tested to destruction. Variables considered in the investigation were shear-span to depth ratio, concrete strength of 50-120 MPa (7,250-17,400 psi), and the provision of secondary reinforcement. The investigation examined deep beam behavior and compared the experimental results with the CIRIA Guide 2 design method, the American Concrete Institute (ACI) 318 method, and the plastic truss model of Rogowsky and MacGregor, using the efficiency model proposed by Warwick and Foster. The results showed that good load predictions were obtained using the plastic truss model when combined with the Warwick and Foster efficiency factor. It is also concluded that the design methods given by CIRIA Guide 2 and ACI 318 are generally conservative for deep beams fabricated with high-strength concrete.

A two parameter stress block for high strength concrete

Abstract: The rectangular stress block parameters in the current ACI Code are limited to concrete strengths in the range 20-50 MPa (2,900-7,250 psi). This paper looks at the applicability of the ACI rectangular stress block parameters to high-strength concretes. New rectangular stress block parameters are proposed that are based on a probabilistic analysis using a stress-strain relationship for high-strength concrete and that include estimates of variability and distribution of the input properties. A sensitivity analysis is also carried out to ascertain the effect of parameter uncertainty. The probabilistic models proposed can be used in a code calibration of design formula for high-strength concrete. It is shown that for a ductile singly reinforced rectangular section, the ultimate moment capacity is relatively insensitive to the stress block model. Estimates of the ductility level at both ultimate and column capacity in primary compression failure, however, are significantly affected by the choice of the stress block model.

Shear strength of high-performance concrete beams

Abstract: Tests on 48 reinforced high-performance concrete (HPC) beams with vertical shear reinforcement under combined bending moment and shear are reported. The test parameters included the concrete cover-to-shear reinforcement cage, shear reinforcement ratio, longitudinal tensile reinforcement ratio, overall beam depth, shear span-to-depth ratio, and concrete compressive strength. The loading configuration was also varied. The shear strength calculated using a stress analysis of the web region of the beam shows good correlation with the test values from the present and previous investigations. The predictions by the shear design provisions contained in the Australian Standard, ACI 318-95, Canadian Standard, and Eurocode are also compared with the test shear strengths of the beams.

Analytical study of reinforced concrete beams strengthened with web-bonded fiber reinforced plastic plates or fabricsanalytical study of reinforced concrete beams strengthened with web-bonded fiber reinforced plastic plates or fabrics

Abstract: Bonding fiber reinforced plastic (FRP) plates or fabrics to the web of reinforced concrete beams can increase the shear and flexural capacity of the beam. This paper presents analytical models to calculate the stresses in the strengthened beam, and the shear force resisted by the composite plate before cracking and after formation of flexural cracks. The anisotropic (orthotropic) behavior of the composite plate or fabric has been considered in the analytical models. The companion paper extends this discussion into post cracking behavior at the ultimate load, where the diagonal shear cracks are formed. The method has been developed assuming perfect bond between FRP and concrete (no slip) and using compatibility of the strains in the FRP and the concrete beam. The validity of the assumptions used in this method has been verified by comparing the results to the finite element method. A parametric study has been performed to reveal the effect of important variable parameters, such as fiber orientation angle on the shear force resisted by the FRP plate. The method has been developed for both uncracked and cracked beams, and it can be used for stress analysis of these types of beams.

Analytical Study of Reinforced Concrete Beams Strengthened with Web-Bonded Fiber Reinforced Plastic Plates or Fabrics

Abstract: Bonding fiber reinforced plastic (FRP) plates or fabrics to the web of reinforced concrete beams can increase the shear and flexural capacity of the beam. This paper presents analytical models to calculate the stresses in the strengthened beam, and the shear force resisted by the composite plate before cracking and after formation of flexural cracks. The anisotropic (orthotropic) behavior of the composite plate or fabric has been considered in the analytical models. The companion paper extends this discussion into post cracking behavior at the ultimate load, where the diagonal shear cracks are formed. The method has been developed assuming perfect bond between FRP and concrete (no slip) and using compatibility of the strains in the FRP and the concrete beam. The validity of the assumptions used in this method has been verified by comparing the results to the finite element method. A parametric study has been performed to reveal the effect of important variable parameters, such as fiber orientation angle on the shear force resisted by the FRP plate. The method has been developed for both uncracked and cracked beams, and it can be used for stress analysis of these types of beams.

Vertical Deflection of a Pre-Stressed Concrete Bridge Obtained Using Deformation Sensors and Inclinometer Measurements

Abstract: The serviceability of a bridge is generally analyzed by a comparison between the vertical deflections expected by the engineer and those measured during a load test or in the long term. The existing methods do not allow the determination of the vertical displacements from the measurements carried out by a network of deformation sensors placed inside the bridge. The mathematical model presented allows the determination of the displacement field from internal horizontal deformation measurements and helps in the design of the required sensor network. This model was tested on an experimental model and on the Lutrive Highway Bridge in Switzerland by comparing the changes in vertical displacements under daily temperature variations obtained with the proposed method, with those measured directly using an absolute hydrostatic leveling system. Fiber optic deformation sensors and electrical inclinometers were used to carry out the measurements. With this deformation monitoring system, featuring a precision of 10 micrometers on 1 m long deformation sensors, it is possible to retrieve the vertical displacement field of a beam with a global error less than 8%.

Effect of alkali silica reaction expansion and cracking on structural behavior of reinforced concrete beams

Abstract: A laboratory study was carried out to investigate the effect of deleterious alkali-silica reaction (ASR) expansion on the structural behavior of reinforced concrete beams and on mechanical properties of cylinders made with the same concrete. The specimens were conditioned by immersion in a cyclically heated alkali solution for one year to accelerate ASR. To simulate in-service conditions, two beams were held under load that caused flexural cracking while being conditioned. Cracks were first observed in the cylinders at an age of 125 days. Before ASR cracking occurred, changes in the mechanical properties of the reactive cylinders were minor. After cracking, the compressive strength, splitting tensile strength, and dynamic modulus of the cylinders were significantly reduced. Cracks were first observed on top of the beams after 6 months of conditioning and were oriented in the direction parallel to the reinforcement. However, after conditioning for one year, the flexural strength of the reactive beams that experienced ASR cracking was nearly the same as that of the nonreactive concrete beams. Effect of ASR on flexural strength of the preloaded and cracked beams was also insignificant.

Punching Shear Strength of Restrained Concrete Bridge Deck Slabs

Abstract: This paper focuses on the development of a theoretical model for predicting the behavior of laterally restrained, fiber-reinforced concrete bridge decks. Based on observations of the failure modes in experimental testing and a review of models used for the punching failure of reinforced concrete slabs, a rational model is developed. The assumptions and equations used in the model development are presented along with the solution algorithm. The theoretical model is checked through comparison with several half-scale and full-scale tests on laboratory bridge decks. The results of the experimental programs are recorded elsewhere and listed in the references.

Modeling Punching Shear of Reinforced Concrete Slabs Using Layered Finite Elements

Abstract: The paper examines the applicability of the finite element, layered, shell formulation in the global analysis of reinforced concrete slabs when subjected to high, concentrated transverse loads. The formulation incorporates quadratic, degenerate, isoparametric shell elements. A significant feature of these elements is their ability to take into account out-of-plane shear response. This allows the implementation of a three-dimensional constitutive model for the evaluation of the stiffness matrix. The proposed formulation allows the use of out-of-plane reinforcement in the elements. Through the consideration of the three-dimensional states of strain and stress in each layer, the model can predict the failure of structures caused by either flexure or punching shear. The paper presents and discusses the essential features of the proposed formulation that are related to the transverse shear model. The formulation has been checked against experimental results of tests on slabs subjected to punching shear. A parametric study has also been undertaken to determine the influence of several parameters on the behavior of slabs with and without stirrups.

Nonlinear Analysis of Cyclically Loaded Reinforced Concrete Structures

Abstract: This paper presents a simple and reliable constitutive model for predicting the response of reinforced concrete under cyclic loading. The proposed model is based on the findings of previous experimental and analytical studies. The model adopts the concept of a smeared crack approach with orthogonal cracks and assumes a plane stress condition. It is comprised of two independent functions: the normal stress function and the shear stress function. The proposed model is simple enough to be incorporated into any nonlinear finite element analysis program used in analyzing full structures. The accuracy of the proposed model is in agreement with available analytical and experimental data.

Reliability-based strength reduction factor for bond

Abstract: The formulation of a reliability-based strength-reduction (phi) factor for developed and spliced bars is described. Conventional and high relative rib area bars, both with and without confining reinforcement, are considered. The phi-factor is determined using statistically based expressions for development/splice strength and Monte Carlo simulations of a range of beams. The overall approach is applicable to the calculation of phi-factors for all types of loading on reinforced concrete. A strength-reduction factor of 0.9 is obtained for the design expressions for development/splice length, based on a probability of failure in bond equal to approximately one-fifth of the probability of failure in bending or combined bending and compression. The value phi=0.9 is incorporated into two expressions for development/splice length in a manner that is transparent to the user. A major advantage of each of the final expressions is that they provide identical values for development and splice length, removing the need to multiply development length by 1.3 or 1.7 to obtain the length of most splices.

Effect of High-Strength Concrete Slab on the Behavior of Slab-Column Connections

Abstract: Slab-column connections were tested under combinations of gravity and lateral loads to investigate the effect of using high-strength concrete slab on the structural behavior of the slab-column connections. The variables selected for this study are the strength of concrete slab, the flexural steel reinforcement ratio, and the moment to shear ratio. As the concrete slab strength increases from 35 MPa to 75 MPa, the shear strength increases by 7% and 15% for loading cases of zero and high moment-shear ratio, respectively. However, the current design codes provisions for interior connections have greatly reduced the factor of safety regarding the punching shear strength of high-strength concrete slabs. The use of high-strength slab has a significant effect on the load-deflection characteristics for specimens subjected to high-moment. The ultimate deflection increases and the failure mode becomes less sudden and more gradual, if high-strength concrete slab is used. The first yielding for specimens constructed with high-strength concrete slab occurs at loads considerably lower than those for specimens constructed with normal-strength concrete. The radius of yielding significantly increases for specimens constructed with high-strength concrete slab; therefore, the steel reinforcement is utilized better and a much more desirable steel stress distribution is produced in the area around the column by the use of high-strength slab. In the meantime, connection displacement and rotation ductility increases by 75% for specimens with high-strength concrete slabs under high moment. Therefore, the use of high-strength concrete slabs can be justified economically when good ductility and higher levels of absorbed energy of slab-column connections are required under high moment-shear ratios. On the other hand, the use of high-strength concrete slabs for slab-column connection subjected to gravity loads only is not recommended because of the small increase of the punching shear capacity of the connection compared with the high cost of using high-strength concrete.