Concrete Compressive Strain Research Articles

Study on reinforced lightweight coconut shell concrete beam behavior under flexure

Coconut shell has been used as coarse aggregate in the production of concrete. The flexural behavior of reinforced concrete beam made with coconut shell is analyzed and compared with the normal control concrete. Twelve beams, six with coconut shell concrete and six with normal control concrete, were fabricated and tested. This study includes the moment capacity, deflection, cracking, ductility, corresponding strains in both compression and tension, and end rotation. It was found that the flexural behavior of coconut shell concrete is comparable to that of other lightweight concretes. The results of concrete compression strain and steel tension strain showed that coconut shell concrete is able to achieve its full strain capacity under flexural loadings. Under serviceability condition, deflection and cracking characteristics of coconut shell concrete are comparable with control concrete. However, the failure zones of coconut shell concrete were larger than for control concrete beams. The end rotations of the coconut shell concrete beams just prior to failure values are comparable to other lightweight concretes. Coconut shell concrete was used to produce hollow blocks and precast slab in 2007 and they are being subjected to some practical loading till today without any problems such as deflection, bending, cracks, and damages for the past five years.

Defining Concrete Compressive Strain in Unbonded Post-Tensioned Walls

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Theoretical Prediction on the Shear Capacities of Shear Connectors in Steel Column Bases

Shear connectors are extensively used in steel column bases of steel structures to transfer loading from superstructure to concrete foundations, however the behavior and shear capacity of the steel shear connector subject to complex external loading are not well understood. This paper presents an approach for theoretically predicting the shear capacities as well as behavior of the shear connector in steel column bases subject to external loading such as both shear force and bending moment at the top. The load-displacement behavior at the top of the shear connector and the internal force distribution along the depth of the connector are also investigated at various states with different cases of boundary conditions. Two states of concrete compressive strain, i.e. the critical and ultimate states, are considered for evaluating the critical and ultimate shear capacities of the shear connector. The theoretical predictions for the behavior and load capacities of the shear connector are then compared with finite element results. The results from the verification study show that the proposed approach can provide satisfactory theoretical predictions for the behavior of the shear connector.

Numerical simulation and strength models of FRP-wrapped reinforced concrete columns under eccentric loading

Two finite element models are proposed to simulate the behavior of eccentrically loaded FRP-wrapped RC columns. Comparison between numerical and experimental results indicates that perfect bond can be used to simulate the interfaces between concrete and FRP as well as interface elements. The developing curves of lateral displacement are similar to sine half-wave curves. Twenty specimens were tested based on the finite element model using perfect bond between concrete and FRP composites. The result shows the maximum concrete compressive stress does not change with various parameters and the maximum concrete compressive strain is inversely proportional to eccentricity-to-section height ratio and length-to-section height ratio. An analysis-oriented model and a design-oriented model are proposed to predict the strength.

Experimental Study on Creep and Durability of High-Early-Strength Self-Consolidating Concrete for Precast Elements

Self-consolidating concrete (SCC) typically has high cement paste volumes to achieve the desired fresh characteristics. These high paste volumes may lead to increased creep, which can increase the concrete compressive strain in prestressed concrete members. This increased compressive strain can lead to a reduction in the prestressing force for these elements. Reduced prestressing forces can result in higher deflections and reduced capacities. Accurate estimates of the creep of SCC prestressed members are needed so that these structural elements can be properly designed. The creep of high-early-strength (HES) SCC mixtures was assessed in this study. In addition to the creep characteristics, the durability characteristics of HES SCC need to be assessed. Because sustainability and durability are critical for long-lasting, economically viable systems, implementing the use of a new material that exhibits poor durability can be costly and unsafe. The permeability, diffusivity, and freezing-and-thawing resistance of HES SCC mixtures were assessed in this study. An assessment of the creep compliance of HES SCC values indicates that the 2006 American Association of State Highway Transportation Officials (AASHTO) Load and Resistance Factor Design Specifications predicted the creep compliance with the highest accuracy among all the models considered in this research. The 2004 AASHTO, ACI 209, and CEB-FIP MC90-99 models provide fairly good predictions of the creep compliance for both the conventional concrete (CC) and HES SCC mixture prediction equations. It is noted that for more accurate predictions, however, all models should be calibrated with data from full-scale structural elements. Durability measurements indicate similar performances for the HES SCC mixtures relative to the CC mixtures; however, the 5 ksi (35 MPa) release strength CC and HES SCC mixtures exhibited low resistance to freezing-and-thawing cycles.

Critical Compressive Strain of Concrete under a Long-Term Deformation Effect Part I. Experiments

This paper focuses on the effect of creep on the critical compressive strain (CCS) of concrete. The strain of concrete corresponding to the peak compressive stress is crucial in the selection of the ultimate yield strength of the reinforcing bar used in reinforced concrete columns. Among the various influencing factors, such as the creep, shrinkage, loading rate and confinement, the effect of creep and shrinkage is the most significant. So far, investigations into how these factors can affect the CCS of concrete have been rare. Therefore, to investigate the effect of creep and shrinkage on CCS, an experimental (part I) and a parametric study (part II) were conducted, as presented in these papers (part I considers creep effect, part II considers effect of creep and shrinkage). In part I, experiments pertaining to the loading age, loading rate, loading duration and loading and creep levels were conducted to study the effect of these variables on the CCS of concrete. It was found that the effects of the loading rate, loading age, and level and duration on the CCS of concrete were negligible. However, it is very important to consider the effect of creep.

Square concrete columns strengthened with carbon fiber reinforced plastics sheets at low temperatures

Twenty-one square concrete columns were constructed and tested. The testing results indicate that bonded carbon fiber reinforced plastics (CFRP) sheets can be used to increase the strength and improve the serviceability of damaged concrete columns at low temperatures. The failure of the specimens, in most cases, takes place within the middle half of the columns. And the failure of strengthened columns is sudden and explosive. The CFRP sheets increase both the axial load capacity and the ultimate concrete compressive strain of the columns. The ultimate loads of strengthened columns at −10, 0 and 10 °C increase averagely by 9.09%, 6.63% and 17.83%, respectively, as compared with those of the control specimens. The axial compressive strength of strengthened columns is related to the curing temperatures. The improvement of axial compressive strength decreases with reducing temperature, and when the temperature drops to a certain value, the improvement increases with falling temperature.

강선량 및 긴장력에 따른 외부 강선을 가진 PSC 보의 휨거동 실험

Dept. of Civil Engineering, Kunsan National University, Jeonbuk 573-701, KoreaABSTRACT Recently, the externally prestressed unbonded concrete structures are increasingly being built. The mechanicalbehavior of prestressed concrete beams with external unbonded tendon is different from that of normal bonded PSC beams in thatthe slip of tendons at deviators and the change of tendon eccentricity occurs as external loads are applied in external unbondedPSC beams. The purpose of the present paper is therefore to evaluate the flexural behavior by performing static flexural test accord-ing to tendon area and tendon force. From experimental results, before flexural cracking, there was no difference between externalmembers and bonded members. However, after cracking, yielding load of reinforcement, ultimate load, and the tendon stress ofexternal members was lower than that of bonded members. For the relationship of load-tendon stress, the increasing of tendon st rainwas inversely proportional to the initial tendon force. However, even if the initial tendon force was large, the tendon strain withsmall effective stress was smaller than that with large effective stress. The concrete compressive strain was proportional to t he effec-tive stress of external tendon. From the comparison between test results and codes, the ACI-318 could not consider the effect oftendon force or effective stress, and especially the results of ACI-318 were very small, so it was very conservative. And theAASHTO 1994 could be influenced on the tendon area, initial force and effective stress, but as it was made on the basis of internalunbonded tendon, its results were much larger than the test results. For this reason, the new correct predict equation of externaltendon stress will be needed.Keywords : prestressed concrete, external unbonded tendon, tendon area, tendon force

Nonlinear behavior of concrete beams with hybrid FRP and stainless steel reinforcements

The full-range behavior of partially bonded, together with partially prestressed concrete beams containing fiber reinforced polymer (FRP) tendons and stainless steel reinforcing bars was simulated using a simplified theoretical model. The model assumes that a section in the beam has a trilinear moment—curvature relationship characterized by three particular points, initial cracking of concrete, yielding of non-prestressed steel, and crushing of concrete or rupturing of prestressing tendons. Predictions from the model were compared with the limited available test data, and a reasonable agreement was obtained. A detailed parametric study of the behavior of the prestressed concrete beams with hybrid FRP and stainless steel reinforcements was conducted. It can be concluded that the deformability of the beam can be enhanced by increasing the ultimate compressive strain of concrete, unbonded length of tendon, percentage of compressive reinforcement and partial prestress ratio, and decreasing the effective prestress in tendons, and increasing in ultimate compressive strain of concrete is the most efficient one. The deformability of the beam is almost directly proportional to the concrete ultimate strain provided the failure mode is concrete crushing, even though the concrete ultimate strain has less influence on the load-carrying capacity.

Design Method for Imposed Rotations of Interior Slab-Column Connections

A mechanical model is presented for transfer of unbalanced moment caused by imposed rotation of the slab-column connection of a flat plate without shear reinforcement. The failure criterion is based on the limited concrete compression strain capacity of the slab near the column, which previously is found to accurately capture the ultimate deflection and the concurrent punching capacity of concentrically loaded test specimens of varying size and concrete strength. The extended theory for eccentric loading is shown to give good prediction of the ultimate rotation and the concurrent unbalanced moment of test specimens. The theory predicts that the rotation capacity of flat plates decreases with increasing slab thickness, which indicates that recorded seismic drift capacities of test specimens with affective depth less than 125 mm (5 in. ) might be unsafe for thicker slabs.

Effect of ferrocement confinement on behavior of concrete

In this study, the use of ferrocement as an external confinement to concrete specimens is investigated. The effectiveness of confinement is achieved by comparing the behavior of retrofitted specimens with that of conventional specimens. The primary test variable considered in this study is the concrete compressive strength. All the other parameters, such as size, shape, number of layers of wire mesh, and L/ d ratio of the specimens, were kept constant. The sections chosen are circular cylinders with a size of 150 mm × 300 mm and L/ d ratio of 6:1. The test results showed that the confined concrete specimens can enhance the ultimate concrete compressive strengths and failure strains.

On the prediction of the collapse load of circular concrete columns confined by FRP

The present work is aimed at deriving assessment and design formulae for determining the elastic–plastic response and the ultimate compressive strength of circular concrete columns confined by Fiber Reinforced Polymers (FRP). To achieve this, a constructive method for obtaining closed-form elastic and post-elastic solutions for Functionally Graded Material Cylinders (FGMCs), constituted by an isotropic central core and n arbitrary cylindrically orthotropic hollow phases, is proposed. In the first part of the paper, under the hypotheses of axis-symmetrical boundary conditions, elasticity and perfect bond between the phases, new analytical solutions for self-equilibrated axial forces applied at the extremities of the object are derived. In particular, the rather general mathematical approach has been based on a strategy already proposed by some of the authors in a previous work, and here extended to anisotropic hollow phases. The key of the involved method is to reduce the differential Boundary Value Problem (BVP) to the equivalent linear algebraic one, by means of a special matrix-like arrangement of the governing equations and invoking the Complex Potential Theory for anisotropic materials. The obtained general solution has been then easily particularized to the two phase FGMC representing the circular concrete column confined by FRP sheets. In the second part of the paper, the above mentioned solutions for the concrete column are “moved” within the post-elastic range and we investigate the evolution of the stress field in the solid components when the concrete core is characterized by an Intrinsic Curve or Schleicher-like elastic–plastic behavior endowed with associate flow rule, and the FRP cylindrically orthotropic hollow phases obey to an elastic–brittle Tsai–Hill anisotropic yield criterion. At the end, the elastic and post-elastic response of the overall solid and predictive formulae for estimating the failure mechanism, in terms of concrete ultimate compressive strength, confining pressure and strain at failure, are derived. The obtained results are finally compared with several experimental literature data, highlighting the very good agreement between the analytical predictions and experimental tests.

General stress–strain relationship for concrete at elevated temperatures

A general stress–strain relationship for concrete when subjected to fire is needed, as it allows designing concrete structures to specific fire-performance criteria and improves the understanding of the behaviour of these structures during fire events. Existing relationships are developed based on fire tests of unconfined concrete specimens. They provide significantly different predictions because of uniqueness of each relationship and the existence of numerous formulations for calculating the governing parameters. In this paper, available formulations for estimating the parameters affecting the behaviour of unconfined and confined concrete are presented. These parameters are concrete compressive strength, concrete tensile strength, concrete compressive strain at peak stress, initial modulus of elasticity of concrete, transient creep strain, thermal strain, and yield stress and bond strength of reinforcing bars. Recommendations for choosing specific formulations are made based on accuracy, generality, and simplicity. Suitable compressive and tensile stress–strain relationships at elevated temperatures that utilize the recommended formulations are proposed based on well-established relationships for confined concrete at ambient temperature. The proposed relationships are compared to existing ones and to the available experimental data. They can capture changes in the mechanical properties of concrete resulting from temperature and confinement and are found to be superior to existing relationships. However, additional tests are needed to further validate and improve the proposed relationships.

A computational model for reinforced concrete members confined with fiber reinforced polymer lamina: Implementation and experimental validation

This study presents a computational model to simulate the behavior of confined concrete in column plastic hinge zones. The model describes the distribution of confining stresses within a circular column cross-section and the hysteretic behavior of concrete under passive confinement. Of particular interest is the ability of the model to predict the maximum circumferential strains and thus estimate the limit state of the confining medium rather than relying on empirical limits such as concrete compressive strain or drift ratio. This is performed with fiber-discretized beam column analysis without the computational expenses of a continuum finite element (FE) model. The confined section and material model are implemented in an object-oriented computational platform for structural analysis. New classes are developed and presented for a confined fiber section, a confined concrete material, and associated hysteretic behavior rules. Finally, the results from two experimental programs on columns strengthened using fiber reinforced polymer (FRP) lamina are reproduced using the developed computational model. Comparison of simulation and experiment shows that the computational model can closely match the observed response characteristics and can adequately predict the deformation level leading to FRP rupture.

Experimental Evaluation of the Seismic Performance of Reinforced Concrete Bridge Columns

A current focus in earthquake engineering research and practice is the development of seismic design procedures whose aim is to achieve a specified performance. To implement such procedures, engineers require methods to define damage in terms of engineering criteria. Previous experimental research on bridge columns has focused on component failure, with relatively little attention to other damage states. A research program was undertaken to assess the seismic performance of well-confined, circular-cross-section, reinforced concrete bridge columns at a range of damage states. The test variables included aspect ratio, longitudinal reinforcement ratio, spiral reinforcement ratio, axial load ratio, and the length of the well-confined region adjacent to the zone where plastic hinging is anticipated. The progression of damage was similar for all columns. Analysis of the experimental results suggest that key damage states of residual cracking, cover spalling, and core crushing can best be related to engineering parameters, such as longitudinal reinforcement tensile strain and concrete compressive strain, using cumulative probability curves.

Experimental study on the plastic rotation capacity of reinforced high strength concrete beams

This paper describes an experimental study on the plastic rotation capacity of reinforced high strength concrete beams. Thirty-six beams with various compressive strengths of concrete, tensile reinforcement ratios, compressive reinforcement ratios, and patterns of loading (1 point loading and 2-point loading) were tested to evaluate the plastic rotation capacity, extreme fiber concrete compressive strain and equivalent plastic hinge length, etc. The same quantities were also obtained from numerical analysis and compared with experimental data. According to the results, the yield curvatures obtained from experiments turned out to be quite close to those obtained from theoretical approach. However, the experimental results for ultimate curvatures were significantly larger than those of theoretical prediction based on the assumption of ecu=0.003. Based on these observations, a new formula for ultimate strain is proposed for high strength concrete beams. Also the test results for plastic rotation capacity were found to be closer to those obtained using moment-curvature relationship considering tension stiffening of concrete and shear effect than those obtained using equivalent plastic hinge length. This substantiates that for accurate evaluation of plastic rotation capacity the consideration of tension stiffening of concrete and shear effect is most important.

Ultimate Moment Capacities of Round Prestressed Concrete Poles

ABSTRACTAn alternative technique for the determination of ultimate moment capacities of round prestressed concrete poles is presented in this paper. The traditional concrete compression area of poles is determined using the conventional iterative design process, the compatibility and equilibrium approach. Moreover, the area of annulus defined by the neutral axis is multiplied by an area reduction factor. It is possible to use numerical methods to calculate the centroid of the reduced annulus. However, the concrete compression zone and strain variation in the longitudinal reinforcement makes the calculation awkward. This paper addresses the problem from a different perspective. No reference to area reduction factor is required in the course of locating the neutral axis. The results are compared in a very satisfactory manner with that obtained using PCI approach. The purpose is to provide formulations that can be easily computerized and implemented on relevant software for the design of round prestressed concrete poles.

Holographic interferometry study of deformation of cement and concrete under mechanical and thermal loads

Compressive strain of concrete is accompanied by rotation of the rigid aggregate and by local shifts of the cement matrix, which by analogy with local deformation of metals is the cause of a decrease of the real strength of the material. It is shown that deformation of concrete with haydite and granite aggregates in the presence of a heat supply (within limits of positive operating temperatures) is distinguished by damping of deformations in the first case and by local deformation of the aggregate in the second.

Observed Stress‐Strain Behavior of Confined Concrete

Thirty‐one nearly full‐size reinforced concrete columns, of circular, square, or rectangular wall cross section, and containing various arrangements of reinforcement, were loaded concentrically with axial compressive strain rates of up to 0.0167/s. The circular sections contained longitudinal and spiral reinforcement, the square sections contained longitudinal reinforcement and square and octagonal transverse hoops, and the rectangular wall sections contained longitudinal reinforcement and rectangular hoops with or without supplementary cross ties. The longitudinal stress‐strain behavior of the confined concrete was measured and compared with that predicted by a previously derived stress‐strain model with allows for the effects of various configurations of transverse confining reinforcement, cyclic loading, and strain rate. The measured longitudinal concrete compressive strain when the transverse steel first fractured was also compared with that predicted by equating the strain energy capacity of the tran...

Concrete Slab Stresses in Partial Composite Beams and Girders

Although calculation of compressive flexural stresses in the concrete slab of composite steel beams and girders is a routine procedure, engineers are discovering certain ambiguities in such an analysis when the partial composite concept is used. The problem presents itself when applying the rules in Section of 1.11.2.2 of the AISC Specification. This section states, in part, "The actual section modulus of the transformed composite section shall be used in calculating the concrete flexural stress...." The intent of the Specification is to determine the actual stress distribution corresponding to the actual number of shear connectors in the partial composite design approach. This paper describes a suggested procedure to determine concrete slab stresses based on assumed elastic behavior. The basic justification for the partial composite theory is the fact that shear connectors are inadequate to maintain strain compatibility at the interface of the concrete slab and the top of the steel beam. This results in a limited amount of horizontal slip, which would reduce the concrete compressive strains, and their corresponding stresses