Unconfined Concrete Strength Research Articles

Sustainable FRP-Confined Symmetric Concrete Structures: An Application Experimental and Numerical Validation Process for Reference Data

The use of fiber-reinforced polymer (FRP) jackets as external confinement is becoming popular, especially in seismic areas, because of its ability to enhance the strength and ductility of reinforced concrete to perform as a sustainable symmetric structural member. Therefore, various researchers have worked out for the prediction of strength and strain models of FRP-confined concrete. This study presents the improved strain models for the FRP confined cylindrical concrete members. Different previously proposed models of axial strain of FRP-confined concrete were evaluated based on a large database of 678 specimens from previous experiments and an improved model was proposed using the general regression analysis technique. Furthermore, the proposed model was validated using the previous experimental work of FRP-wrapped concrete cylinders and their finite elements analysis (FEA) using the ABAQUS software. The accuracy of the proposed strain model was quite satisfactory in comparison with the previous experimental and FEA results of the present study. Moreover, the proposed empirical strain model was used for the parametric study to investigate the effect of different geometric and material parameters such as the compressive strength of unconfined concrete, diameter of the cylinder, elastic modulus and thickness of the FRP layers, on the axial strain of FRP-wrapped cylinders. A close agreement among the proposed strain models and experimental outputs was observed. This study will help in understanding the behavior of sustainable FRP-confined symmetric concrete members.

Design methods for spiral stirrups confined concrete columns by evaluating the lateral performance of transverse reinforcements

Transverse reinforcements may not yield at the peak stress of confined concrete when the tensile strength and volumetric ratio of lateral steel do not be coordinate with concrete strength. The compressive strength and ductility of confined concrete are closely related to the lateral strain of transverse reinforcement at peak stress. However, determining the lateral strain of transverse reinforcements at peak stress is difficult due to many parameters should be considered. In this paper, a regression analysis model and an artificial neural network (ANN) model were developed to predict the lateral strain in transverse reinforcements and an ANN model was established to determine the volumetric ratio ranges of transverse reinforcements which transverse reinforcements yield at the peak stress of confined concrete. To reach the purpose, 111-group data sets assembled from previous studies were applied to test the effects of influential parameters on the lateral strain and judge whether transverse reinforcements yield at the peak stress of confined concrete. The aspect ratio of specimens, the tensile yield strength and volumetric ratio of transverse reinforcements, the compressive strength of unconfined concrete, as well as longitudinal reinforcements ratio were considered. For lateral strain prediction models, the regression analysis equation was proposed based on theoretical analysis, while the ANN model was determined by collected data; for appropriate volumetric ratio ranges determination model, the outputs of ANN model were based on the classification method to judge the lateral steel yields. The results showed that the prediction models of lateral strain had high efficiency for predicting lateral strain. Besides, the accuracy and the prediction ability of the ANN model was higher than regression analysis model. Furthermore, the appropriate volumetric ratio ranges of transverse reinforcements in different confined concrete were calculated by proposed ANN model. Finally, the recommended lateral strain prediction models and volumetric ratio of transverse reinforcements determination model can be utilized to instruct the design of confined concrete.

A new numerical model for polymer-confined rectangular concrete columns

A new numerical model to estimate the strength of unreinforced and reinforced concrete columns of rectangular cross-section confined with fibre-reinforced polymer wraps is introduced. The model is based on data from 456 tests compiled from 30 studies available in the technical literature. The test database covers unconfined concrete strengths varying from approximately 10 to 80 MPa and the width and depth of the cross-sections ranged between 90 and 500 mm. The effects of aspect ratio and size of the cross-section, effective rupture strain of the polymer, corner radius and internal hoop steel reinforcement are all accounted for in the model. Based on the test database, thresholds of lightly and heavily confined concrete are proposed to ensure that columns have a sufficient level of polymer confinement. Model predictions were found to be in good agreement with the test results.

A Stress-Strain Model Considering Bilinear and PostPeak Softening Behaviors for FRP-Confined RC Columns

Objectives: A model with more generalized scope than those in the literature has been proposed to predict the stressstrain response of Fiber-Reinforced Polymer (FRP) confined rectangular RC columns under axial compression. Methods/ Statistical Analysis: 255 columns with their experimental data were used as a basis for the model expressions. Their database included columns with aspect ratios ranging from 1.0 to 2.0. Furthermore, the cross sectional depth varied from 100 to 500 mm and the unconfined concrete strength varied from 18.3 to 51.5 MPa. Findings: Based on interpretation of the test results, bilinear and post-peak softening stress-strain responses are proposed as referencing models, respectively, for columns having a sectional depth of smaller or larger than 300 mm. The softening model is composed of a parabolic expression for describing the first stage of the response and two linear expressions for the second and third stages. The influences of aspect ratio and size of cross sections, number of FRP layers and internal hoop steel bars on the key features of the envelope stress-strain curve model (i.e. ultimate axial stress and strain) are all considered in the model. Application/ Improvements: Reasonable agreement was revealed between the analytical responses of the proposed model with the selected tested responses, confirming its ability to simulate the stress-strain behavior of FRP-confined rectangular RC columns of small or large sizes. Keywords: FRP-confined Concrete, Rectangular RC Columns, Stress-strain Model

Experimental Investigation and Statistical Modeling of FRP Confined RuC Using Response Surface Methodology

Scrap tires that are dumped to landfill is a serious problem in China and rest of the world. The use of rubber in concrete is an effective environmental approach to reduce the amount of scrap tires around the world. However, the loss in compressive strength of concrete is a major drawback of rubberized concrete. In this paper, the fiber reinforced polymer (FRP) confinement technique is used to overcome the drawbacks of rubberized concrete (RuC). A total of sixty six RuC cylinders were tested in axial compression. The cylinders were cast using recycled rubber to replace, a) 0-50 percent fine aggregate volume, b) 0-50 percent coarse aggregate volume, and c) 40-50 percent fine and coarse aggregate volume. Twenty seven cylinders of the latter mix were then confined with one, two and three layers of CFRP jackets. Concrete suffered a substantial reduction in compressive strength up to 80 percent by fine and coarse aggregate replacement with rubber content. However, CFRP jackets recovered and further enhanced the axial compressive strength of RuC up to 600% over unconfined RuC. SEM was performed to investigate the microstructural properties of RuC. Statistical models were developed on the basis of experimental tests for FRP confined RuC cylinders using response surface method. The effect of variable factors; unconfined concrete strength, rubber replacement type and number of FRP layers on confined compressive concrete strength was investigated. The regression analysis was performed to develop the response equations based on quadratic models. The predicted and experimental test results were found in good agreement as the variation between experimental and predicted values were less than 5%. Furthermore, the difference between predicted and adjusted R2 was found to be less than 0.2 which shows the significance of the statistical models. These proposed statistical models can provide a better understanding to design the experiments and the parameters affecting FRP-confined RuC cylinders.

Innovative models for prediction of compressive strength of FRP-confined circular reinforced concrete columns using soft computing methods

There are several methods for predicting experimental results such as empirical methods, elasticity and plasticity theory. Among these methods, the use of soft computing has been expanded due to good capabilities and high accuracy in predicting the target. Soft computing contains computational techniques and algorithms to provide useful solutions to deal with complex computational problems. In this study, three methods including Artificial neural networks, Group method of data handling and Gene expression programming are utilized to predict the compressive strength of columns confined with FRP. Total of 95 experimental data were selected to form the model. The height of the column, the compressive strength of unconfined concrete, the elastic modulus of FRP, the area of longitudinal steel, the yield strength of longitudinal steel and confinement pressure provided by FRP and transverse steel were considered as input parameters, while the compressive strength of FRP-confined columns was considered as the target. The proposed methods are compared with the existing models and provide great accuracy in predicting the results. Among the utilized methods, the ANN model showed the highest accuracy.

Experimental evaluation of the residual compression strength and ultimate strain of chloride corrosion‐induced damaged concrete

In this paper, the effects of chloride corrosion of steel reinforcement on the axial compressive strength and the ultimate strain of concrete materials in reinforced concrete columns have been investigated experimentally. In addition, the effects of reduction in the effective mechanical properties of concrete materials on moment‐curvature response of a corroded bridge pier were numerically studied. This investigation has resulted in clear quantification of the relationship between the degree of reinforcement corrosion and the effective compression strength and ultimate strain of unconfined concrete. The results clearly show that corrosion of steel reinforcement resulted in a deterioration of the effective axial compression strength and ultimate strain of column's concrete cover.

Evaluation of ultimate conditions of concrete confined by GFRP wire jackets

This study investigated the ultimate conditions of concrete confined by glass-fiber-reinforced polymer (GFRP) wire. For this purpose, the experimental data obtained from a previous study was examined. The ultimate strain in wires was investigated through a strain reduction factor, which was the percentage of the rupture strain of the GFRP jackets to the ultimate material strain of GFRP. It is believed that the factor declined with an increase in the unconfined concrete strength. The concrete confined by using a single layer showed a higher maximum strain of the wire than concrete confined by multiple layers. An increase in the number of layers in the multilayer cases did not significantly affect the maximum strain of the wire. Equations to predict an ultimate stress and ultimate strain of the concrete with respect to the ultimate strain of the wire were introduced. The proposed equations achieved acceptable results in comparison to experimental results and results obtained from literature equations.

Concentrically Loaded Circular RC Columns Partially Confined with FRP

Wrapping reinforced concrete (RC) columns with fiber reinforced polymer (FRP) composites is effective in increasing their capacity. The current state of the art concentrates primarily on fully wrapped columns and few studies dealt with partially wrapped ones. The objective herein is to evaluate the effectiveness of partial wraps (or strips) and to develop a confined concrete compressive stress–strain (fc − εc) model that accounts for partial wrapping. Three-dimensional finite element (FE) models are generated to evaluate the influence of different parameters on the behavior of concentrically loaded RC circular columns that are partially and fully wrapped with FRP. The results indicated an increase in ductility as the number of FRP strips is increased, and revealed that longitudinal steel had little influence on the confined fc − εc relationship. The proposed fc − εc model, derived from the parametric study, accounts for the effect of partial and full confinement, the unconfined concrete strength f^{prime}_{c} , and yielding of transverse steel. Comparison of the results generated using the proposed model with FE and experimental results are in good agreement.

A Parametric Study of Confinement Effects to the Interaction Diagram of P-M for High-Strength Concrete Columns

In reinforced concrete columns design, P-M interaction diagram is used as axial load control and column bending without taking into account the effect of lateral reinforcement bars. Design principles that ignore the effect of reinforcement bars will result in low value of actual axial capacity of column. This paper presents the effect of lateral reinforcement with a case study of square-sectional columns on high-strength concrete. The equation of unconfined concrete strength enhancement is based on Mander, Legeron, Imran, Antonius and Muguruma models. The reviewed parameters include compressive strength of concrete, reinforcement configuration, and spacings of confinement. The result of the analysis is a comparison of P-M diagram based on confinement models. It shows that confinement significantly influences axial capacity of column, yet it has a certain collapse point at point P = 0 in all models. This is caused by longitudinal reinforcement system. Configuration of reinforcement, spacing and confinement models greatly affects the collapse behavior of column, whether the collapse is classified as compressive collapse or tensile collapse based on the P-M diagram.

Influence of the measurement method on axial strains of FRP-confined concrete under compression

Accurate determination of axial strains of fiber reinforced polymer (FRP)-confined concrete is of significant importance in determining the axial compressive behavior of the material. This is challenging, however, because of the fact that axial strains are often influenced by the methods used in their measurement. Two measurement methods, involving the use of linear variable differential transformers (LVDTs) either along the full- or mid-height region of the specimens, have been extensively used to measure the axial strains of FRP-confined concretes. This paper presents a targeted study investigating how the axial strains obtained from these two measurement methods compared. A complete database of FRP-confined normal- and high-strength concrete (NSC and HSC) cylinders containing both full- and mid-height LVDT (i.e. FLVDT and MLVDT) axial strain data was assembled. The analysis results of database show that axial strains obtained from FLVDT and MLVDT are close to each other in the case of NSC specimens. However, significant differences are seen in the axial strains of the HSC specimens obtained from these two measurement methods, indicating that the axial strains of HSC are highly sensitive to the instrumentation arrangement used in their measurement. It is shown that, in addition to the significant influence of the unconfined concrete strength, the lateral stiffness of FRP confinement influences the relationship between the axial strains obtained by FLVDT and MLVDT methods. Finally, with the aim of establishing a unified framework for future design and modeling efforts, an expression is developed to describe the relationship between the axial strains of FRP-confined concrete obtained by the two measurement methods.

Axial-strength model for FRP-confined concrete based on the improved twin shear strength theory

External confinement by wrapping with FRP sheets is a very effective method for the retrofit of a concrete structure subject to either static or seismic loads. For the reliable and underlying analysis of the confinement, an accurate and theoretical strength model is required for FRP-confined concrete. In this paper, the math-deductive method is introduced based on a review of the existing models. The theoretical method uses the failure criterion as the foundation to establish the axial-strength model. To consider the stress condition of FRP-confined concrete, an improved failure criterion is developed based on twin shear strength theory. Based on a proposed failure criterion, an axial-strength model is proposed for FRP-confined concrete columns. The coefficients in the proposed model are computed with respect to the range of the unconfined concrete strength, and the ranges of the coefficients show obvious trends. The predictions of the proposed model agree well with the comprehensive database.

Loading paths of confined concrete in circular concrete loaded CFT stub columns subjected to axial compression

So far, the loading path effect on the compressive strength of confined concrete has been poorly investigated. To clarify the compressive strength of confined concrete in concrete-filled steel tube (CFT) columns, experimental tests with a total of 18 specimens are conducted to investigate the loading paths of confined concrete in concrete loaded CFT stub columns. The parameters that are varied in the experiment include the steel strength, the unconfined concrete strength and the D/t ratio. Two evaluation indices, namely the laterally dominant index and the effect index, are proposed to characterize the loading path and its effect on the compressive strength of confined concrete, respectively. A detailed parametric study is conducted on the loading paths and the corresponding effects on compressive strength. The results suggest that the loading paths of confined concrete in CFT column are affected by the column parameters. Depending on the loading paths by which the concretes are confined, the loading path effects on the compressive strength of the confined concrete in CFT columns could be totally different. Based on the test results, a compressive strength model incorporating the loading path effect is developed and good performance was found in comparison with the experimental results.

Modeling the axial compressive stress-strain behavior of CFRP-confined rectangular RC columns under monotonic and cyclic loading

This paper presents a stress-strain model for rectangular reinforced concrete (RC) columns confined with carbon fiber-reinforced polymer (CFRP) composites and subjected to cyclic axial compression. The model is mainly based on experimental observations and results from a series of tests reported in the companion study, in which 24 large-size CFRP-confined rectangular unreinforced and reinforced concrete columns under monotonic and cyclic axial compression were tested. Relative experimental results compiled from the available literature are also used. The test database covers unconfined concrete compressive strengths ranging from 17.6 MPa to 51.5 MPa and cross-sectional side lengths ranging from 90 mm to 400 mm. The proposed stress-strain model is composed of three main components: (1) a monotonic stress-strain model for the envelope curve of cyclic response; (2) a parabolic expression for the unloading path; and (3) a straight line for the reloading path. The model also considers the effects of key parameters including cross-sectional size, aspect ratio, effective rupture strain of the CFRP, and internal longitudinal and hoop steel reinforcement. The performance of the model is finally verified through comparison of predictions made by this model with selected test results, as well as the predictions of more than 20 models of other researchers.

An Experimental Study on Shrinkage Strains of Normal-and High-Strength Concrete-Filled Frp Tubes

It is now well established that concrete-filled fiber reinforced polymer (FRP) tubes (CFFTs) are an attractive construction technique for new columns, however studies examining concrete shrinkage in CFFTs remain limited. Concrete shrinkage may pose a concern for CFFTs, as in these members the curing of concrete takes place inside the FRP tube. This paper reports the findings from an experimental study on concrete shrinkage strain measurements for CFFTs manufactured with normal- and high-strength concrete (NSC and HSC). A total of 6 aramid FRP (AFRP)-confined concrete specimens with circular cross-sections were manufactured, with 3 specimens each manufactured using NSC and HSC. The specimens were instrumented with surface and embedded strain gauges to monitor shrinkage development of exposed concrete and concrete sealed inside the CFFTs, respectively. All specimens were cylinders with a 152 mm diameter and 305 mm height, and their unconfined concrete strengths were 44.8 or 83.2 MPa. Analysis of the shrinkage measurements from concrete sealed inside the CFFTs revealed that embedment depth and concrete compressive strength only had minor influences on recorded shrinkage strains. However, an analysis of shrinkage measurements from the exposed concrete surface revealed that higher amounts of shrinkage can occur in HSC. Finally, it was observed that shrinkage strains are significantly higher for concrete exposed at the surface compared to concrete sealed inside the CFFTs.

Predicting Strain Reduction Factor for Concrete-Filled FRP Tube Columns Incorporating Interface Gap and Prestress

This paper reports the findings from an analytical study into the influence of fiber reinforced polymer (FRP)-to-concrete interface gap and prestressed FRP tubes on strain reduction factor (kε) for concrete-filled FRP tube (CFFT) columns. A database that consists of a total of 45 aramid FRP- (AFRP) confined normal-and high-strength concrete (NSC and HSC) specimens with circular cross-sections is presented. All specimens were cylinders with a 152 mm diameter and 305 mm height, and their unconfined concrete strengths ranged from approximately 45 to 110 MPa. Analyses of the experimental databases that consisted of 22 specimens manufactured with FRP-to-concrete interface gap and a further 23 specimens prepared with lateral prestress is presented and discussed. Based on close examination of the hoop strain development on the FRP confining shell, expressions to predict strain reduction factors (kε) are proposed. The comparison of the proposed model predictions with the experimental test results of specimens prepared with an interface gap or prestressed FRP tubes shows good agreement.

Behavior of partially and fully FRP-confined circularized square columns under axial compression

Fiber-reinforced polymer (FRP) jacketing has become an attractive strengthening technique for concrete columns. Existing research has demonstrated that FRP confinement is effective for circular columns, whereas it is less effective for square columns. The lower FRP confinement effectiveness in a square column is predominantly attributed to the non-uniform FRP confinement in the column, while the concrete in an FRP-confined circular column is uniformly confined. Although rounding the corners of a square column can enhance the effectiveness of FRP confinement, its benefit is still not satisfactory as the radius of rounded corners is usually limited owing to the presence of internal steel reinforcement. An appropriate approach to enhancing the confinement effectiveness of FRP strengthening technique for square columns is to circularize a square column before FRP jacketing. This paper aims to study the compressive behavior of circularized square columns (CSCs). A total of 33 column specimens were prepared and tested under axial compression in this paper. The column parameters include FRP wrapping schemes (including fully wrapped and partially wrapped), FRP volumetric ratio, sectional shapes and unconfined concrete strength. The test results have indicated that the section circularization of square columns can significantly improve the effectiveness of FRP confinement, and strengthening square columns using section circularization in combination with partial FRP confinement is a promising and economical alternative to the fully FRP strengthening technique. Comparisons between the theoretical predictions and the test results were conducted, and the accuracy of existing partially FRP-confined concrete models were also examined.

Performance of bridge piers under vehicle collision

Both the peak dynamic force (PDF) and the equivalent static force (ESF) of a vehicle collision with reinforced concrete bridge columns were examined as part of an extensive finite element (FE) analyses study. An extensive parametric study of 13 parameters, including the concrete material model, the unconfined concrete compressive strength fc′, the material strain rate, the percentage of longitudinal reinforcement, the hoop reinforcement, the column span-to-depth ratio, the column diameter, the top boundary conditions, the axial load level, the vehicle’s velocity, the vehicle’s mass, the roadside distance between errant vehicle and unshielded bridge column, and the soil depth above the top of the column footing was conducted. Three approaches were used to investigate the ESF. The ESF in the first (stiffness-based) approach was defined as the static force producing the same maximum displacement that is produced by a vehicle collision at the point of impact. The ESF examined in the second approach was calculated according to the Eurocode. The ESF studied in the third approach was defined as the Peak of the Twenty-five Milli Second moving Average (PTMSA). The different ESFs were compared to the ESF in the American Association of State Highway and Transportation Officials-Load and Resistance Factor Design (AASHTO-LRFD; 2670kN [600kips]). In general, the ESF calculated according to the Eurocode presented the lower bound while those from the stiffness-based approach presented the upper bound. Furthermore, the recommended ESF of the AASHTO-LRFD was found to be non-conservative for heavy and/or high speed vehicle impacts; it was found to be too conservative for light and/or slow vehicle impacts. Hence, rather than a constant design impact force, a variable design impact force should be used. An equation was developed to calculate a design impact force, which is the function in the vehicle’s mass and velocity. A simplified equation based on the Eurocode equation of the ESF was proposed. These equations, however, do not require cumbersome FE analyses.

Dynamic and Static Behavior of Hollow-Core FRP-Concrete-Steel and Reinforced Concrete Bridge Columns under Vehicle Collision.

This paper presents the difference in behavior between hollow-core fiber reinforced polymer-concrete-steel (HC-FCS) columns and conventional reinforced concrete (RC) columns under vehicle collision in terms of dynamic and static forces. The HC-FCS column consisted of an outer FRP tube, an inner steel tube, and a concrete shell sandwiched between the two tubes. The steel tube was hollow inside and embedded into the concrete footing with a length of 1.5 times the tube diameter while the FRP tube stopped at the top of footing. The RC column had a solid cross-section. The study was conducted through extensive finite element impact analyses using LS-DYNA software. Nine parameters were studied including the concrete material model, unconfined concrete compressive strength, material strain rate, column height-to-diameter ratio, column diameter, column top boundary condition, axial load level, vehicle velocity, and vehicle mass. Generally, the HC-FCS columns had lower dynamic forces and higher static forces than the RC columns when changing the values of the different parameters. During vehicle collision with either the RC or the HC-FCS columns, the imposed dynamic forces and their equivalent static forces were affected mainly by the vehicle velocity and vehicle mass.

FRP-Confined Recycled Coarse Aggregate Concrete: Experimental Investigation and Model Comparison.

The in situ application of recycled aggregate concrete (RAC) is of great significance in environmental protection and construction resources sustainability. However, it has been limited to nonstructural purposes due to its poor mechanical performance. External confinement using steel tubes and fiber-reinforced polymer (FRP) can significantly improve the mechanical performance of RAC and thus the first-ever study on the axial compressive behavior of glass FRP (GFRP)-confined RAC was recently reported. To have a full understanding of FRP-confined RAC, this paper has extended the type of FRP and presents a systematic experimental study on the axial compressive performance of carbon FRP (CFRP)-confined RAC. The mechanical properties of CFRP-confined RAC from the perspective of the failure mode, ultimate strength and strain, and stress–strain relationship responses were analyzed. Integrated with existing experimental data of FRP-confined RAC, the paper compiles a database for the mechanical properties of FRP-confined RAC. Based on the database, the effects of FRP type (i.e., GFRP and CFRP) and the replacement ratio of recycled coarse aggregate were investigated. The results indicated that the stress–stain behavior of FRP-confined RAC depended heavily on the unconfined concrete strength and the FRP confining pressure instead of the replacement ratio. Therefore, this study adopted eleven high-performance ultimate strength and strain models developed for FRP-confined normal aggregate concrete (NAC) to predict the mechanical properties of FRP-confined RAC. All the predictions had good agreement with the test results, which further confirmed similar roles played by FRP confinement in improving the mechanical properties of RAC and improving those of NAC. On this basis, this paper finally recommended a stress–strain relationship model for FRP-confined RAC.