Strength Of Concrete Material Research Articles

Suggesting a new testing device for determination of tensile strength of concrete

A compression to tensile load transforming (CTT) device was developed to determine indirect tensile strength of concrete material. Before CTT test, Particle flow code was used for the determination of the standard dimension of physical samples. Four numerical models with different dimensions were made and were subjected to tensile loading. The geometry of the model with ideal failure pattern was selected for physical sample preparation. A concrete slab with dimensions of $15{\times}19{\times}6cm$ and a hole at its center was prepared and subjected to tensile loading using this special loading device. The ratio of hole diameter to sample width was 0.5. The samples were made from a mixture of water, fine sand and cement with a ratio of 1-0.5-1, respectively. A 30-ton hydraulic jack with a load cell applied compressive loading to CTT with the compressive pressure rate of 0.02 MPa per second. The compressive loading was converted to tensile stress on the sample because of the overall test design. A numerical modeling was also done to analyze the effect of the hole diameter on stress concentrations of the hole side along its horizontal axis to provide a suitable criterion for determining the real tensile strength of concrete. Concurrent with indirect tensile test, the Brazilian test was performed to compare the results from two methods and also to perform numerical calibration. The numerical modeling shows that the models have tensile failure in the sides of the hole along the horizontal axis before any failure under shear loading. Also the stress concentration at the edge of the hole was 1.4 times more than the applied stress registered by the machine. Experimental Results showed that, the indirect tensile strength was clearly lower than the Brazilian test strength.

Seismic Performance of a Concrete Highway Tunnel under Earthquake Sequence Using a Concrete Damage Plasticity Model

The aim of this paper was to investigate the dynamic response of a concrete tunnel subjected to mainshock–aftershock seismic sequence. Three components of the registered seismic saquence were applied as seismic excitation acting in three directions. A three-dimensional FE model of a tunnel section (600 m long) was prepared with the ABAQUS software. The soil layers interacting with the tunnel lining were also taken into consideration. To represent the inelastic behavior of the concrete material under the earthquakes, a concrete damage plasticity model (CDP) was assumed a constitutive model for the concrete. The analysis proved that strongly nonlinear behaviour of the concrete lining of the tunnel was observed under the sequence of seismic events. The plastic strains as well the tensile damage (cracking) were noticed in some zones of the concrete lining after the first and the second event. The crack patterns were in good agreement with damages observed on concrete tunnels during real earthquakes. The results indicate that aftershocks can enlarge zones affected by irreversible strains or cause damage evolution. The analysis also revealed that aftershocks, even being much weaker than main events, may result in additional loss of concrete material strength while performing in mainshock-aftershock seismic sequences and striking a structure which is already degraded by a mainshock.

Dynamic behaviour of a concrete building under a mainshock–aftershock seismic sequence with a concrete damage plasticity material model

ABSTRACTThe aim of this paper was to investigate the dynamic response of a concrete structure subjected to a mainshock–aftershock seismic sequence. In the dynamic analysis, three components of the registered mainshock and aftershock were taken into account. The peak ground accelerations of about 0.5 g were assumed for both shocks. A one-storey shed was modelled with the ABAQUS software to represent a large concrete structure under the repeated earthquakes. For proper characterization of concrete structure behaviour under the sequence of shocks, a concrete damage plasticity model was assumed as a constitutive model of concrete. The obtained results indicate that aftershocks can have considerable effect on dynamic behaviour of concrete structures in terms of enlarging zones affected by irreversible strains or additional damage evolution. The analysis revealed that aftershocks, which are usually not as strong as mainshocks, may result even in total loss of concrete material strength while performing in mainshock–aftershock seismic sequences.

A predicting model for thermal conductivity of high permeability-high strength concrete materials

The high permeability-high strength concrete belongs to the typical of porous materials. It is mainly used in underground engineering for cold area, it can act the role of heat preservation, also to be the bailing and buffer layer. In order to establish a suitable model to predict the thermal conductivity and directly applied for engineering, according to the structure characteristics, the thermal conductivity predicting model was built by resistance network model of parallel three-phase medium. For the selected geometric and physical cell model, the thermal conductivity forecast model can be set up with aggregate particle size and mixture ratio directly. Comparing with the experimental data and classic model, the prediction model could reflect the mixture ratio intuitively. When the experimental and calculating data are contrasted, the value of experiment is slightly higher than predicting, and the average relative error is about 6.6%. If the material can be used in underground engineering instead by the commonly insulation material, it can achieve the basic requirements to be the heat insulation material as well.

Correlating strength of concrete to its early-age temperature rise

Compressive strength is the most commonly used property for characterising the performance of concrete materials. In this study, semi-adiabatic temperature evolutions and compressive strength developments of a series of mortar and concrete mixtures were experimentally investigated. In order to predict compressive strength, it was correlated to the initial temperature rise induced by early-age hydration. The results indicate that both properties are strongly dependent on the raw materials and the water to cementitious materials ratio. Estimated results of temperature rise and strength from the modified equations match well with the measured data. A linear relationship between compressive strength and the peak value of temperature rise is developed and presented. The linear correlation exists for all mixture components and proportions of concrete materials in this study. In conclusion, predicting the strength of concrete materials from their early-age hydration-induced temperature evolutions is enabled.

Determination of concrete quality with destructive and non-destructive methods

In this study, the availability of Schmidt hammer has been investigated as a reliable method to determine the quality of concrete in irrigation networks. For this purpose, the 28-day compressive strength of concrete material used in the construction irrigation channel of Bafra lowland, which is one of the most fertile plains in Turkey was examined by means of concrete compression and as well as concrete Schmidt hammer in laboratory conditions. This study was carried out on cylindrical samples to represent the everyday concrete party (<TEX>$150m^3$</TEX>) produced by contractor firm as 3 replications. The statistical analysis of experimental data showed that the correlations between the values of 28-day compressive strength of Schmidt hammer and the rebound number was found to be 0.98. Differences of the compressive strength between compression testing and Schmidt hammer were statistically significant at P<0.01. In this context, it was found that the reliability of compressive strength of the concrete compression test are excellent, also the reliability of compressive strength of Schmidt hammer are fair in assessing the quality of concrete irrigation channels.

Predicting Flexural Strength of Concretes Incorporating River Gravel Using Multi-Layer Perceptron Networks: A Case Study of Eastern Nigeria

Cases of collapsed buildings and structures are prevalent in Nigeria. In most of these cases the cause of the collapse could be traced to the strength of the construction materials which is usually concrete. Secondly, experimental determination of the strength of concrete materials used in buildings and structures is quite expensive and time consuming . This work shows the development of a computational model, based on artificial neural networks for the determination of flexural strength of concrete materials made from prevalent coarse aggregate components from Nigeria. The work involves building a multi-layer perception neural network model which uses experimental data obtained from flexural strength test of concrete made from washed gravel. The flexural strength predictions were compared with predictions from an alternative model based on regression analysis. The results of the study show that for the washed gravel based concrete the regression model prediction has a correlation coefficient of 0.92687 and a sum of squares error of 0.51954100, while the neural network model prediction has a correlation coefficient of 0.98364 and a sum of squares error of 0.00629630. Generally, the models predicted well, but the neural network model predicted better than the regression model. The result of the study has adequately demonstrated a cheap, simple, very quick and accurate alternative to experimental method of concrete strength determination. The method is also simpler and quicker than analytical methods based on regression analysis. Results obtained suggests an option of immense benefit to civil engineers and construction professionals, which would help in economically determining the strength, as well as economical selection of appropriate mix of construction materials, that is a prelude to building strong and cheap buildings and structures. http://dx.doi.org/10.4314/njt.v34i1.2

A three-dimensional elastoplastic constitutive model for concrete

Strength of concrete materials can be described by the theory of nonlinear unified strength criterion with four independent parameters. The failure surface of the strength criterion in the principal-stress space is smooth, and therefore it has continuous partial derivatives. The theory of nonlinear unified strength criterion is applied as a yield function, plastic shear strain is used as the hardening and softening parameters, and the hardening and softening laws are given by the stress-strain relationship under uniaxial compression. Then a three-dimensional elastic-plastic constitutive model for concrete is established. A user-defined material mechanical behavior (UMAT) is developed based on the finite element analysis software ABAQUS. The load resistance and the deformation capacity of a reinforced concrete column under eccentric compression are simulated using the present model. The present theoretical results are comparable to the available material-test data, and the numerical results have a good agreement with the experimental observations of the concrete column. This indicates that the developed constitutive model is capable of describing the strength and deformation behaviors, and representing the strain softening behavior of concrete under three-dimensional stress state. Furthermore, the model has high efficiency and accuracy in the numerical analysis.

Influence of the concrete DIF model on the numerical predictions of RC wall responses to blast loadings

Reliable prediction of structural responses to blast loadings requires an accurate dynamic material model. The dynamic strength of concrete material is normally higher than the static strength. Based on extensive experimental data, a number of empirical DIF (dynamic increase factor) relations have been proposed to model concrete material strength increment at high strain rates. Most of these empirical relations are obtained by fitting the scattered dynamic testing data. It is commonly acknowledged that the induced structural effects such as lateral inertia confinement effect are inevitable in high-speed impact tests. Therefore directly fitting the laboratory testing data may not necessarily obtain the true dynamic concrete material properties. Some recent studies investigated the contributions of lateral inertia and end friction confinement effects on DIF of concrete materials in laboratory tests, and proposed relations to remove these influences to obtain the true DIF for concrete materials. The present study carries out numerical simulations of a reinforced concrete wall under different blast loadings. Different DIF relations including CEB defined DIF, DIF proposed in previous studies after removing structural effect in laboratory tests, and NO DIF, i.e., neglecting dynamic strength increment, are considered in numerical simulations. Verification of the numerical model is made through the comparisons of the numerical simulation results with field test data. The results demonstrate that using the new DIF model yields the best prediction of the structural responses. The responses of a typical RC wall under blast loads with a 5m stand-off distance but different TNT explosive charge weights are also simulated using different DIF relations. The responses of RC wall obtained from numerical models with different DIF relations of concrete material are compared. From the numerical simulation results, the range of scaled distance that DIF relation has significant influences on the numerical simulation results is identified.

3D meso-scale modelling of concrete material in spall tests

Tensile strength is one of the key factors of concrete material that need be accurately defined in analysis of concrete structures subjected to high-speed impact loads. Dynamic tensile strength of concrete material is usually obtained by conducting laboratory tests such as direct tensile test, Brazilian splitting test and spall test. Concrete is a heterogeneous material with different components, but is conventionally assumed to be homogeneous, i.e., cement mortar only, in most previous experimental or numerical studies. The aggregates in concrete material are usually neglected owing to testing limitation and numerical simplification. It has been well acknowledged that neglecting coarse aggregates might not necessarily give accurate concrete dynamic material properties. In the present study, a 3D meso-scale model of concrete specimen with consideration of cement mortar and aggregates is developed to simulate spall tests and investigate the behaviour of concrete material under high strain rate. The commercial software LS-DYNA is used to perform the numerical simulations of spall tests. The mesh size sensitivity is examined by conducting mesh convergence tests. The reliability of the numerical model in simulating the spall tests is verified by comparing the numerical results with the experimental data from the literature. The influence of coarse aggregates on the experimental test results is studied. The wave attenuation in concrete specimen is analysed, and empirical equations are proposed for quick assessment of the test data to determine the true dynamic tensile strength of concrete material. The contributions of aggregates to dynamic strength in spall tests are quantified for modifying the test results based on mortar material in the literature.

Predicting the Compressive Strength of Concretes Made with Unwashed Gravel from Eastern Nigeria Using Artificial Neural Networks

Cases of collapsed buildings and structures are prevalent in Nigeria. In most of these cases the cause of the collapse could be traced to the strength of the construction materials, mainly concrete. Secondly, experimental determination of the strength of concrete materials used in buildings and structures is quite expensive and time consuming. This research seeks to develop a computational model based on artificial neural networks for the determination of the compressive strength of concrete materials made from a prevalent coarse aggregate component from Nigeria. The study involved building a multilayer perceptron neural network model which was trained using experimental data obtained from compressive strength test of concrete made from unwashed gravel. Compressive strength predictions were compared with alternative model based on regression analysis. Results show that for the unwashed gravel based concrete the regression model prediction has a sum of squares error of 9.808 and a mean absolute percentage (relative) error of 1.167, while the neural network model prediction has a sum of squares error of 0.025 and a mean absolute percentage (relative) error of 0.015. Generally, the models predicted well, but the neural network model predicted better than the regression model. This study has ably demonstrated a cheap, simple, very quick and accurate alternative to experimental method of concrete strength determination. It is simpler and quicker than analytical methods based on regression analysis. Keywords: Artificial Neural Network, Concrete, Unwashed gravel, Regression, ModellingNigerian Journal of Technological Research, 18(2), 2013

Effect of Radial Inertia Confinement on Dynamic Compressive Strength of Concrete in SHPB Tests

This paper extends the study of Dharan on the stress state in an elastic solid specimen subjected to axial strain acceleration to compressible materials. And an improved Dharan model is presented. Based on the Drucker-Prager strength criterion, analytical equations of the radial inertia confinement effect on dynamic compressive strength of concrete in split Hopkinson pressure bar tests is derived. Comparison and discussion on the analytical, experimental and numerical results are performed. It is proved that the strain rate effect of compressive strength of concrete materials is a pseudo material property partly caused by the radial inertia confinement. Special attentions should be paid to dynamic tests of low-strength concrete to get the real mechanical properties.

Experimental confirmation of some factors influencing dynamic concrete compressive strengths in high-speed impact tests

This paper reports probably the first systematic experimental investigation of the influences of lateral inertia confinement, end friction confinement and aggregates in high-speed impact tests on dynamic compressive properties of concrete. The influences of the concerned factors on the dynamic increase factor (DIF) are discussed. The proposed DIF relations in Hao and Hao [36] are verified by the experimental data in this study. The results confirm the observations made before based on numerical simulations that lateral inertia confinement effect is dependent on the specimen diameter and strain rate. The influences of aggregate size and volume to DIF are considerable, and aggregates cannot be neglected in experimental or numerical studies. It also confirms that the end friction confinement effect is sensitive to the specimen geometry and strain rate. The equation proposed in Hao et al. [40] based on numerical simulations satisfactorily removes the friction confinement effect on the DIF of concrete material strength obtained in impact test.

Determination Method for Damaged Concrete Strength

AbstractThis study aims to reflect the original strength of concrete material objectively and accurately by determining the concrete damage degree quantitatively and identifying the relation between damage variable and strength. According to uniaxial compressive stress-strain curve and the strain equivalent principle, the stress-strain curve of damaged concrete is obtained while the concrete subjected to circulate loading has different degrees of damage and strain is beyond that corresponding to peak stress. The relation between the damage variable and concrete strain at different strength grades is derived. The corresponding strain is obtained by testing the damage variable of damaged concrete. The original strength of the concrete material is calculated according to the testing strength of core samples of damaged concrete with a height and diameter of 100 and 150 mm, respectively. Finally, the conclusion is verified by a test.

Influence of end friction confinement on impact tests of concrete material at high strain rate

The compressive strength of concrete material increases with the strain rate. The dynamic compressive strength of concrete material is usually obtained by conducting laboratory tests such as split Hopkinson pressure bar (SHPB) test or drop-weight test. It is commonly agreed now that the dynamic increase factor (DIF) obtained from impact test is affected by lateral inertia confinement. In addition, friction at specimen-bar interfaces also constrains the lateral deformation of the specimen under high-speed impact and thus might influence the testing results as well. However, no systematic study that devotes to investigating the influence of end friction on the dynamic compression test of concrete specimen can be found in the literature. Moreover, owing to the complication of including aggregates in concrete specimens in high-speed impact tests, and complexity of including aggregates in numerical simulation of high-speed impact tests of concrete materials, coarse aggregates are usually neglected in both the laboratory tests and numerical simulations, which may not give accurate concrete material dynamic properties. In the present study, a mesoscale concrete model with distinctive consideration of different components in a concrete specimen is developed to simulate SHPB tests and to study the influence of the confinement due to end friction between specimen and pressure bars on impact tests of dynamic concrete material properties. The commercial software AUTODYN is used to carry out the numerical simulations of SHPB tests. The friction coefficient between steel bar and concrete specimen is varied from 0.0 to 0.5 in the simulation. The results confirm that the end friction confinement does affect the testing results, and its influence depends on the L/D ratio (specimen length to diameter). This observation is also verified by experimental tests. The influences with L/D ratio and friction coefficient on stress and strain distributions and failure process of mesoscale concrete specimen under different strain rates are discussed. Based on the results from numerical simulations, an empirical formula is proposed to remove the influence of end friction confinement on dynamic strength increment of concrete material obtained in SHPB tests.

Numerical analysis of concrete material properties at high strain rate under direct tension

The tensile strength of concrete material increases with the strain rate. The dynamic tensile strength of concrete material is usually obtained by conducting laboratory tests such as direct tensile test, spall test or splitting test (Brazilian test). It is commonly agreed now that the DIF obtained from dynamic impact test is affected by lateral inertia confinement. Therefore, those derived directly from testing data do not necessarily reflect the true dynamic material properties. The influence of the lateral inertia confinement, however, is not straightforward to be quantified in laboratory tests. Moreover, concrete is a heterogeneous material with different components, but is conventionally assumed to be homogeneous, i.e. cement mortar only, in most previous experimental or numerical studies. The aggregates in concrete material are usually neglected owing to testing limitation and numerical simplification. In the present study, a mesoscale concrete material model consisting of cement mortar, aggregates and interfacial transition zone (ITZ) is developed to simulate direct tensile tests and to study the influences of the lateral inertia confinement and heterogeneity on tensile strength of concrete material with respect to strain rates between 1/s and 150/s. The commercial software AUTODYN with user provided subroutines is used to perform the numerical simulations of SHPB tests. The model is verified by testing data obtained by others. Numerical simulation results indicate that the lateral inertia confinement contributes to the dynamic increase factor (DIF) of concrete material tensile strength. The lateral inertia confinement effect is specimen size and strain rate dependent. Based on the numerical results, discussions on the relative contributions from the lateral inertia confinement and the material strain rate effect on DIF of concrete material tensile strength are made. Empirical relations are proposed to remove the influence of the lateral inertia confinement in dynamic impact tests on dynamic concrete material strength. The effect of aggregates inside the concrete specimen on its dynamic strength is also investigated. The results demonstrate that it is very important to include aggregates in experimental and numerical studies of concrete material dynamic strength, otherwise significant inaccuracy might be induced.

Numerical Evaluation of the Influence of Aggregates on Concrete Compressive Strength at High Strain Rate

The dynamic strength of concrete materials is usually obtained by conducting laboratory tests such as drop-weight test or split Hopkinson pressure bar (SHPB) test. It is widely accepted that the uniaxial compressive strength of concrete and concrete-like material increases with strain rate. Many empirical relations of concrete material dynamic increase factor (DIF), which are proposed for use in the design and analysis, are given in the literature. However, most of these empirical relations were obtained from testing data of concrete-like materials, i.e. the testing specimens were made of mortar matrix only without coarse aggregates owing to constraints in preparing the concrete specimens for high-speed impact tests. Because concrete is a composite material with mortar matrix, interfacial transition zone (ITZ) and aggregates, and these components have different material properties, using specimens made of mortar material alone in tests may not give accurate concrete dynamic material properties. It is also known that the lateral inertia confinement affects the dynamic strength of concrete specimens obtained in impact tests. A number of studies to investigate and quantify the lateral inertia confinement effect on dynamic strength of concrete materials obtained in impact tests have been published. Previous studies also indicate that including aggregates in concrete specimens affects the dynamic strength. However, no systematic study that devotes to investigating the influence of aggregates in concrete specimen on its dynamic strength has been reported yet. In the present study, a mesoscale concrete material model is used to simulate impact tests and to study the influences of aggregates on concrete material compressive strength increment at high strain rates. The commercial software AUTODYN is used to perform the numerical simulations. A method to remove the influence from lateral inertia confinement is proposed and verified. The influence of ITZ on compressive behavior of concrete specimen is discussed. Numerical simulations of concrete specimens with different volumetric percentages, e.g. 20% 30% and 40%, of aggregates under impact loads of different loading rates are carried out. The influence of the aggregates on DIF of concrete material is examined and quantified.

비파괴시험에 의한 콘크리트 항만시설물의 주요 물성치 평가

Recently harbor remodeling projects are seriously considered to enhance the loading and unloading capability of old container terminals and to make decrepit ports as eco-friendly harbor and waterfront spaces in many countries. In such a case, quantitative and qualitative evaluations on concrete harbor facilities are mandatory to determine the current structural integrity condition of aged materials. Once the remodeling project is determined to be carried out, the reusability of individual structural members and facilities including caissons, cell-blocks, and tetra-pods need to be decided based on the simple and economic visual inspection and/or nondestructive testing. In this study, the systematic quantitative evaluation procedure for determining the structural integrity condition and the reusability is studied based the nondestructive testing and evaluation methods. Conventional methods including Schmidt hammer test and ultrasonicpulse velocity methods and elastic wave based methods including impact echo test and surface wave test are applied to the old harbor facilities in five different sites. The compressive tests are also carried out to determine the elastic modulus and compressive strength of concrete materials.

Numerical Analysis of Concrete Material Properties at High Strain Rate Under Direct Tension

The tensile strength of concrete material increases with the strain rate. Dynamic tensile strength of concrete material is usually obtained by conducting laboratory tests such as direct tensile test, flexural test, spall test or splitting test (Brazilian test). Some codes of practice such as Comite Euro-International du Beton (CEB) give empirical relations of concrete material dynamic increase factor (DIF) based on testing data. However, the reliability of the dynamic testing and the derived DIF are under debating. It is commonly agreed now that the DIF obtained from dynamic impact test is affected by lateral inertia confinement effect. Therefore, those derived from testing data do not truly reflect the dynamic material properties. The influence of the lateral inertia confinement, however, is not quantified. Moreover, concrete is a heterogeneous material with different components, but is conventionally assumed to be homogeneous, i.e. cement mortar only, in most previous experimental or numerical studies. In the present study, a mesoscale concrete material model consisting of cement mortar, aggregates and interfacial transition zone (ITZ) is developed to simulate direct tensile tests and to study the influences of the lateral inertia confinement and heterogeneity on tensile strength increment of concrete materials with respect to strain rates. The commercial software AUTODYN is used to perform the numerical simulations. The influence of lateral inertia confinement on tensile DIF of concrete material is examined.

Compressive Strength of Hardening Fluid Concrete by Explosion

The explosion of fluid concrete under different explosive dosage is achieved by experiment as the main study means. After hardening, we carry out its compressive strength test by Shimadzu AG-X250kN electronic universal pressure machine. The experiment shows that compressive strength of hardening fluid concrete by explosion is higher than that in the same state. Explosive can increase compressive strength of concrete material.