• Steel and Composite Structures
• /
• v.24 no.6
• /
• pp.689-695
• /
• 2017

The constitutive relation is an important factor in analysis of confined concrete in composite structures. In order to propose a constitutive model for nonlinear analysis of confined concrete, lateral restraint mechanism of confined concrete is firstly analyze to study the generalities. As the foundation of the constitutive model, peak stress and peak strain is the first step in research. According to the generalities and the Twin Shear Unified Strength Theory, a novel unified equation for peak stress and peak strain are established. It is well coincident with experimental results. Based on the general constitutive relations and the unified equation for peak stress and peak strain, we propose a unified and convenient constitutive model for confined concrete with fewer material parameters. Two examples involved with steel tube confined concrete and hoop-confined concrete are considered. The proposed constitutive model coincides well with the experimental results. This constitutive model can also be extended for nonlinear analysis to other types of confined concrete.

• Structural Engineering and Mechanics
• /
• v.38 no.5
• /
• pp.675-696
• /
• 2011

It is difficult to accurately predict the flexural strength of prestressed members with unbonded tendons, unlike that of prestressed members with bonded tendons, due to the unbonded behavior between concrete and tendon. While there have been many studies on this subject, the flexural strength of prestressed members with unbonded tendons is still not well understood, and different standards in various countries often result in different estimation results for identical members. Therefore, this paper aimed to observe existing approaches and to propose an improved model for the ultimate strength of prestressed members with unbonded tendons. Additionally, a large number of tests results on flexural strength of prestressed members with unbonded tendons were collected from previous studies, which entered into a database to verify the accuracy of the proposed model. The proposed model, compared to existing approaches, well estimated the flexural strength of prestressed members with unbonded tendons, adequately reflecting the effects of influencing factors such as the reinforced steel ratio, the loading patterns, and the concrete strength. The proposed model also provided a reasonably good estimation of the ultimate strength of over-reinforced members and high-strength concrete members.

• Computers and Concrete
• /
• v.11 no.2
• /
• pp.149-167
• /
• 2013

The complete stress-strain behavior of steel fiber reinforced concrete in compression is needed for the analysis and design of structures. An experimental investigation was carried out to generate the complete stress-strain curve of high-performance steel fiber reinforced concrete (HPSFRC) with a strength range of 52-80 MPa. The variation in concrete strength was achieved by varying the water-to-cementitious materials ratio of 0.40-0.25 and steel fiber content (Vf = 0.5, 1.0 and 1.5% with l/d = 80 and 55) in terms of fiber reinforcing parameter, at 10% silica fume replacement. The effects of these parameters on the shape of stress-strain curves are presented. Based on the test data, a simple model is proposed to generate the complete stress-strain relationship for HPSFRC. The proposed model has been found to give good correlation with the stress-strain curves generated experimentally. Inclusion of fibers into HPC improved the ductility considerably. Equations to quantify the effect of fibers on compressive strength, strain at peak stress and toughness of concrete in terms of fiber reinforcing index are also proposed, which predicted the test data quite accurately. Compressive strength prediction model was validated with the strength data of earlier researchers with an absolute variation of 2.1%.

• Computers and Concrete
• /
• v.9 no.2
• /
• pp.153-169
• /
• 2012

The contribution of steel fibers on the 28-day compressive strength of high-performance steel fiber reinforced concrete was investigated, is presented. An extensive experimentation was carried out over water-cementitious materials (w/cm) ratios ranging from 0.25 to 0.40, with silica fume-cementitious materials ratios from 0.05 to 0.15, and fiber volume fractions ($V_f$= 0.0, 0.5, 1.0 and 1.5%) with the aspect ratios of 80 and 53. Based on the test results of 44 concrete mixes, mathematical model was developed using statistical methods to quantify the effect of fiber content on compressive strength of HPSFRC in terms of fiber reinforcing index. The expression, being developed with strength ratios and not with absolute values of strengths, is independent of specimen parameters and is applicable to wide range of w/cm ratios, and used in the mix design of steel fiber reinforced concrete. The estimated strengths are within ${\pm}3.2%$ of the actual values. The model was tested for the strength results of 14 mixes having fiber aspect ratio of 53. On examining the validity of the proposed model, there exists a good correlation between the predicted values and the experimental values of different researchers. Equation is also proposed for the size effect of the concrete specimens.

• Proceedings of the Korean Society of Agricultural Engineers Conference
• /
• 2003.10a
• /
• pp.387-390
• /
• 2003

Direct tensile tests were carried out for the tensile members of steel-reinforced polymer concrete with different steel diameters and steel ratios to figure out the effect of tensile strength of polymer concrete. In the experiments, polymer concrete with $1000kgf/cm^2$ of compressive strength, steel with $5200kgf/cm^2$ of tensile strength, and the tensile members with 100 cm of constant length were used. Experimental results showed that, regardless of steel diameters and steel content, the strain energy exerted by concrete till the initial crack was 14-15% of the total energy till the point of yield: The energy was much larger than the one of high-strength cement concrete. The behaviors of tensile members of steel-reinforced polymer concrete were in relatively good agreement with the model suggested by Gupta-Maestrini (1990), which was idealized by the effective tensile stress-strain relationship of concrete and the load-strain relationship of members, while those showed a big difference from CEB-FIP model and ACI-224 equation suggested for the load-displacement relationship that was defined as the cross sectional stiffness of effective axis. Modified ACI-224 model code about the load-displacement relationship for the tensile members of steel-reinforced polymer concrete and theoretical equation for the polymer concrete tensile stiffness of polymer concrete suggested through the results of this study are expected to be used in an accurate structural analysis and design for the polymer concrete structural members.

• Structural Engineering and Mechanics
• /
• v.58 no.6
• /
• pp.989-999
• /
• 2016

Moist curing of concrete is a time consuming procedure. It takes minimum 28 days of curing to obtain the characteristic strength of concrete. However, under certain situations such as shortage of time, weather conditions, on the spot changes in project and speedy construction, waiting for entire curing period becomes unaffordable. This situation demands early strength of concrete which can be met using accelerated curing methods. It becomes necessary to obtain early strength of concrete rather than waiting for entire period of curing which proves to be uneconomical. In India, accelerated curing methods are used to arrive upon the actual strength by resorting to the equations suggested by Bureau of Indian Standards' (BIS). However, it has been observed that the results obtained using above equations are exaggerated. In the present experimental investigations, the results of the accelerated compressive strength of the concrete are used to develop the regression models for predicting the short term and long term compressive strength of concrete. The proposed regression models show better agreement with the actual compressive strength than the existing model suggested by BIS specification.

• Proceedings of the Korea Concrete Institute Conference
• /
• 1996.10a
• /
• pp.31-36
• /
• 1996

Various strength characteristics of recycled concretes containing different contents of recycled aggregates from waste concretes were compared with one another. Five different contents. 0%, 50%, 60%, 70% and 80%, of recycled concrete were used for this study. Study results showed that the compressive strength, flexural strength, tensile strength, elastic modulus and fracture toughness varied with contents of recycled aggregates. Target strength of the recycled concrete could be difined by nonparametric regression model as a funcion of content of recycled aggregate in the mix.

• Proceedings of the Korea Concrete Institute Conference
• /
• 2006.05a
• /
• pp.490-493
• /
• 2006

The purpose of this study is to investigate the inelastic behavior of reinforced high-strength concrete bridge columns. A computer program, named RCAHEST (Reinforced Concrete Analysis in Higher Evaluation System Technology), for the analysis of reinforced concrete structures was used. Material nonlinearity is taken into account by comprising tensile, compressive and shear models of cracked concrete and a model of reinforcing steel. The smeared crack approach is incorporated. The increase of concrete strength due to the lateral confining reinforcement has been also taken into account to model the confined high-strength concrete. The proposed numerical method for the inelastic behavior of reinforced high-strength concrete bridge columns is verified by comparison with reliable experimental results.

• Proceedings of the Korea Concrete Institute Conference
• /
• 2000.10a
• /
• pp.115-120
• /
• 2000

Currently, in evaluating a flexural strength of a concrete member, the effect of specimen depth has not been systematically studied, even though its effect on ultimate strength of a section is very important. For all types of loading conditions, the trend is that the strength of a member tends to decrease when the member depth increases. In this study, the influence of specimen depth on flexural compressive strength of concrete member was examined experimentally. A series of C-shaped specimens subjected to axial compressive force and bending moment were tested using three geometrically similar specimens with different length-to-depth ratios(h/c=1, 2 and 4) which have compressive strength of 55MPa. The results indicate that the flexural compressive strength decreased as the specimen depth increased. A model equation was derived based on regression analyses of the experimental data. Also the results show that ultimate strain decreases as the specimen depth increases. Finally, a general model equation for the depth effect is proposed.

• Computers and Concrete
• /
• v.33 no.1
• /
• pp.77-90
• /
• 2024

Waste foundry sand (WFS) is the waste product that cause environmental hazards. WFS can be used as a partial replacement of cement or fine aggregates in concrete. A database comprising 234 compressive strength tests of concrete fabricated with WFS is used. To construct the machine learning-based prediction models, the water-to-cement ratio, WFS replacement percentage, WFS-to-cement content ratio, and fineness modulus of WFS were considered as the model's inputs, and the compressive strength of concrete is set as the model's output. A base extreme gradient boosting (XGBoost) model together with two hybrid XGBoost models mixed with the tunicate swarm algorithm (TSA) and the salp swarm algorithm (SSA) were applied. The role of TSA and SSA is to identify the optimum values of XGBoost hyperparameters to obtain the higher performance. The results of these hybrid techniques were compared with the results of the base XGBoost model in order to investigate and justify the implementation of optimisation algorithms. The results showed that the hybrid XGBoost models are faster and more accurate compared to the base XGBoost technique. The XGBoost-SSA model shows superior performance compared to previously published works in the literature, offering a reduced system error rate. Although the WFS-to-cement ratio is significant, the WFS replacement percentage has a smaller influence on the compressive strength of concrete. To improve the compressive strength of concrete fabricated with WFS, the simultaneous consideration of the water-to-cement ratio and fineness modulus of WFS is recommended.