Concrete Stress Research Articles

An experimental study and failure mechanism analysis on dynamic behaviors of plain concrete under biaxial compression-compression

In order to explore the dynamic mechanical properties of concrete under biaxial compression-compression, a true triaxial instrument was used to conduct an experimental study on the dynamic mechanical properties of plain concrete under biaxial compression-compression. Eight different lateral compressive stresses (0 MPa~14 MPa) and four strain (10−5/s, 10−4/s, 10−3/s and 10−2/s) rate were considered. From this, the effects of lateral compressive stress and loading strain rate on the biaxial compression-compression properties of concrete were compared and analyzed by obtaining the failure modes, principal compressive stress-strain curves and related mechanical characteristic parameters of plain concrete under different loading conditions. The research results show that: with the increase of lateral compressive stress, the failure modes of concrete were developed from columnar failure at low lateral compressive stress to sheet-like failure at high lateral compressive stress. With the increase of lateral stress, the development trend of concrete failure mode decreased gradually under the influence of strain rate. With the increase of lateral compressive stress, the variation range of concrete principal compressive stress decreased first and then tended to be relatively stable under the influence of strain rate. With the increase of strain rate, the increase range of concrete principal compressive stress decreased under the influence of lateral compressive stress. The influence mechanism of lateral compressive stress and strain rate on biaxial compression-compression performance of concrete was analyzed. Meanwhile, based on Kupfer biaxial compression failure criterion, a biaxial compression dynamic failure criterion model considering the effect of loading strain rate on plain concrete was proposed. The research results provided an important theoretical basis for the application and development of concrete engineering.

Temperature and Temperature Stress Analysis in Mass Concrete under Cold Environments of Strong Wind and Large Diurnal Temperature Range

Special environmental condition with strong wind and large diurnal temperature range is not conducive for the control of crack development during the curing of mass concrete in the cold environment of Northwest China. In this study, the temperature at four measured points along the vertical profile of a mass concrete was measured over 14 days. Then, a thermal-mechanical model with the optimized initial condition, thermal parameters, boundary conditions, and the effects of rebar on temperature stress of concrete was built. Based on the observations and numerical model, the effects of environmental factor on the peak temperature, temperature fluctuation amplitude, and maximum temperature stress of the mass concrete are analyzed. The result showed that increasing wind speed can reduce the temperature of the concrete, but an opposite result is observed with increase in diurnal temperature range. Moreover, the more obvious diurnal variation of concrete temperature with the increase in wind speed and diurnal temperature range is not conducive to the thermal stability of the structure, especially for horizontal and side surfaces. Accordingly, increasing wind speed and diurnal temperature range will increase the peak temperature stress of the concrete and the overall stress level. Strong wind and large diurnal temperature range significantly increase the temperature stress and its duration for the side surface. The high temperature stress and intensive daily variation of temperature stress at the horizontal surface affect the durability of the concrete material. Therefore, the temperature stresses of the horizontal surface under wind speed levels 3, 4, 5, and 1.25 times large diurnal temperature range easily exceed the tensile strength; then, the temperature crack will appear. Combined with the research results, the temperature control and crack prevention measures are proposed under the condition of ensuring concrete strength. The study will further guide the improvement and optimization of mass concrete construction in an extreme environment.

Bond Strength Behavior in Rubberized Concrete

Through an experimental program of eighteen specimens presented in this paper, the bond strength between reinforcing bar and rubberized concrete that produced from adding waste tire rubber instead of natural aggregate. The fine and coarse aggregate were replaced in 0%, 25%, and 50% with the small piece of waste tire. Natural aggregate replacement ratio, rebar size, embedded length of rebar, yield stress of rebar, cover of concrete and concrete compressive strength were the parameters studied in this investigation. Ultimate bond stress, bond stress-slip response and failure modes were presented. The experimental results reported that a reduction of 19% in bond strength was noticed in rubberized concrete compared with conventional concrete. The bond strength of rubberized concrete increased when the concrete cover, compressive strength of concrete and yield stress of rebar were increased. Meanwhile, increase embedded length of rebar and rebar size decreases the bond strength. The push-out and splitting failure were the failure modes observed in rubberized concrete.

Stress-strain behavior and design-oriented model for FRP spiral strip-confined concrete

Partial fiber-reinforced polymer (FRP) wrapping has become an attractive strengthening technique for reinforced concrete (RC) columns. Apart from FRP full wrapping strengthening technique, FRP partial wrapping with FRP spiral strips leads to some other advantages such as better fire resistance. However, existing study on FRP spiral strip is limited and the confinement mechanism of FRP spiral strip wrapped concrete columns remains unclear. To this end, axial compression tests on 28 concrete columns wrapped with FRP spiral strips are conducted and the test results are presented in this paper. Results show that the ultimate axial stress and the slope of the post-peak linear branch of the axial stress–strain curve are significantly affected by the proposed confinement effectiveness factor. The desired confinement effectiveness factors for specimens can be achieved by varying the key confinement parameters (i.e., spiral angle, width and layers of FRP strips). It is found that the increases in FRP strip width and thickness lead to enhancements in both ultimate axial stress and strain of the FRP spiral strip-confined concrete, whereas a decrease in the spiral angle has a more pronounced effect on the ultimate axial stress enhancement than the ultimate axial strain enhancement. A new design-oriented stress–strain model is proposed for FRP spiral strip-confined concrete based on the new test database assembled in this study. The predictions corroborate reasonably well with the test results, demonstrating the accuracy and robustness of the proposed model.

New Equations to Calculate Failure Stress and Strain of Concrete

The gravel geometry (shape and size of aggregate), and water to cement ratio affect the constitutive law of concrete. In concrete technology, it is intended to increase the failure stress and strain for enhancing the seismic performance during the earthquake. The aim of the present study is to propose mathematical equations that the independent variables are aggregate size and water to cement ratio. These equations evaluate the failure stress and strain of plain concrete in compressive behavior. The samples were prepared to cover a varied range of compressive strength from 17 to 37 MPa. Two sets of samples were selected, the rounded and sharp aggregates with a variety of sizes (9.5, 12.5, 19, and 25 mm) and water to cement ratios (0.35, 0.42, 0.54, and 0.76). Error analysis shows that the proposed relations are intolerable agreement with the test results and the models suggested by the other researchers, especially regarding failure stress.

An analysis model accounting for time-dependent shear lag effects in assembled composite bridge decks

The concrete deck of steel-concrete composite bridges made via prefabricated construction consists of precast slabs and construction joints between the slabs. When the age difference between the longitudinal joint concrete and precast slab concrete (new-old concrete) across a beam section is considered, the time-dependent characteristic of the assembled deck is different from that of the whole prefabricated deck. Based on the virtual work principle, differential equations were established to analyse the influence of the time-dependent characteristics of new-old concrete across a beam section on the shear lag effect of the composite beams. The analysis of examples showed that the proposed model was more accurate than the traditional model, which ignores the concrete age difference across a beam section, especially when analysing the long-term stress of joint concrete. The long-term stress of the joint concrete obtained by the proposed model decreased faster than that obtained by the traditional model with time, which increased the non-uniformity of the distribution of stress along the concrete flange. Overall, the proposed model could reasonably forecast the long-term performance of composite beams with assembled decks, which is beneficial to the life-cycle design and service behaviour analysis of composite bridges.

A data-driven multi-scale constitutive model of concrete material based on polynomial chaos expansion and stochastic damage model

Nonlinearity and randomness are two intrinsic characteristics of the mechanical behavior of concrete material. The structural response under large excitation can barely be predicted without considering these two characteristics. Brilliant works have been done for decades in the material science and computational stochastic mechanics. However, the existed numerical methods are usually parameter dependent and the key mechanical properties of concrete material are determined by empirical recognition. Therefore, in this paper, a data-driven multi-scale constitutive model is proposed for representing the mechanical behavior of concrete material based on the polynomial chaos expansion and stochastic damage model. Several groups of compressive stress–strain data of concrete material are applied to train the proposed model. By cross validation of the prediction and the concrete stress–strain experimental data, the proposed model is firstly verified to have a robust performance to gain accurate prediction results. Afterwards, the proposed method is compared with a neural network method, the results shows that the proposed method is more robust and accurate than the neural network method.

Behaviors of improved multi-cell T-shaped concrete-filled steel tubular columns under eccentric loads

The improved multi-cell T-shaped CFST section is composed of four square or rectangular cavities infilled with concrete, in which the steel plates added in the section are developed as stiffeners to avoid or delay local buckling of the tube wall. In practical engineering, columns are generally subjected to eccentric load. A total of eleven multi-cell T-shaped concrete-filled steel tubular (CFST) columns were tested under eccentric loads in the symmetric axis. Effects of the web height, the slenderness ratio, the load eccentricity, and the eccentric direction of load on the mechanical behaviors of the specimens were investigated. The experimental results showed two failure modes of the specimens: before the peak load, local buckling was observed in the specimen ET-11 owing to the relatively large width-to-thickness ratio of steel plates in the web; before the peak load, slight local buckling occurred at the mid-span of the other specimens but was not observed during the test, and after the peak load, the local buckling gradually increased, which resulted from the large bending deformation. The improved multi-cell T-shaped CFST columns exhibited a better restraint effect of steel tubes on core concrete and the local buckling of steel plates was delayed or restrained. The ultimate bearing capacity of the specimen under eccentric load located on the flange (y-) was relatively larger. Furthermore, finite element (FE) models were developed to simulate the mechanical behaviors of multi-cell T-shaped CFST columns under eccentric loads and verified with the experimental results. The stress and strain analyses of core concrete at the ultimate state were conducted by using the FE models. Based on both experimental and finite element methods, the simplified calculation formulae for predicting the ultimate bearing capacity of multi-cell T-shaped CFST columns subjected to axial and eccentric compression were put forward. The calculated results agree well with the results of the test and the FE models.

An analytical method to determine the post-cracking flexural stress in pretensioned concrete beams

This work presents a novel application of section analysis methods to calculate post-cracking flexural stresses in pretensioned concrete beams, with primary application to railroad crossties (sleepers). The proposed methodology is particularly useful to estimate concrete stresses generated when constitutive relationships between strain and stresses of concrete are complex. To validate the method, bending strength of beams are predicted and the results are compared with laboratory testing data. The proposed procedure is independent of the need for strain computations, and the bending strength calculations are based on the compressive strength of concrete as opposed to a hypothetical crushing strain value (commonly assumed to be 0.003). Experimental results demonstrate very good agreement with the proposed model, with a level of accuracy that exceeds the traditional approach using Whitney’s block. While being structurally conservative by slightly overestimating stresses, the method yielded prediction errors of 9% or less for the investigated beams.

Bond-Slip Relationship between Sand-Coated Polypropylene Coarse Aggregate Concrete and Plain Rebar.

Recycled plastic waste as an aggregate in concrete mixtures is one of the important issues in the construction industry since it allows the reduction of building weight and has beneficial effects on the environment. In addition, the bonding ability of this kind of lightweight concrete to reinforcement is also a prerequisite as a composite material in forming reinforced concrete structures. Therefore, in this study, the bond of plain rebar embedded in artificial lightweight aggregate concrete made from polypropylene plastic waste coated with sand was investigated. A pull-out test of nine group specimens was conducted to study the bond strength of 10 mm, 12 mm, and 16 mm diameter plain rebar embedded in polypropylene plastic waste coarse aggregates lightweight concrete (PWCAC), failure mode, and bond stress–slip relationship. The test results show that the bond–slip relationship and bond strength depend mainly on the bar diameter for PWCAC. Meanwhile, for all PWCAC specimens tested, the pull-out failure modes were observed. A bond equation for PWCAC was formulated by performing a regression analysis on the experimental results and afterward was combined with an existing bond–slip equation for normal concrete to have the bond–slip formulation for the lightweight concrete studied. The comparison between the model and experimental results indicates a close agreement.

Experimental Study on Thermal Stress of Concrete with Different Expansive Minerals Using a Temperature Stress Testing Machine

Experimental Study on Thermal Stress of Concrete with Different Expansive Minerals Using a Temperature Stress Testing Machine

Quantifying Computed Tomography of Basalt Fiber-Reinforced Concrete under Unconfined Compression

The macroscopic aspects of adding basalt fiber (BF) to concrete has been the subject of studies that document great increases in compression strength. Research into the microscopic aspects of the reinforcement mechanism is still in an exploratory stage, and the quantitative analysis and visual observation of fibers in the concrete matrix are difficult. In this paper, the reinforcement effect of fiber on concrete is studied by means of computed tomography (CT) scanning technology and digital image processing (DIP) technology, combined with the macro-mechanical properties obtained from the unconfined compression strength test, and quantitative analysis from the micro point of view. At the same time, the fiber visualization is realized with the help of Avizo. The results show that with the increase in fiber dosage, the peak stress of concrete first increases and then decreases. When the fiber dosage is 3 kg/m3, the peak stress is 44.4 MPa, which is 41.85% higher than that of ordinary concrete. Additionally, the proportion of macropores is the least, which is the best fiber dosage. It is found that when the fiber dosage is 3 kg/m3, the angular distribution of φ is relatively uniform, and the uniform distribution of fibers forms a dense network structure, which significantly increases the peak stress of concrete. However, when the fiber dosage is too high, it will lead to the accumulation of fibers and produce macropores, and these excess fibers mainly appear in the horizontal direction and do not contribute to the compression strength.

The effect of tension stiffening in moment-curvature responses of prestressed concrete members

Although tensile resistance of concrete is often neglected in calculating the nominal flexural capacity of a concrete member, it needs to be taken into account to accurately trace the deformation response of concrete members as it affects pre- and post-cracking behavior. To adequately address tensile resistance contributed by concrete in moment–curvature analysis, it is necessary to exploit a proper concrete tensile stress–strain model. While constitutive stress–strain models of reinforced concrete in tension have been studied by many researchers and well documented, those of prestressed concrete have gained relatively less attention. There are also conflicting results on how prestressing force affects concrete tension stiffening. This study fills the gap in existing studies and suggests an appropriate and simple model capturing the concrete tensile stress–strain relationship of prestressed concrete in a moment–curvature analysis. Based on an inverse-engineering approach using experimental test data and nonlinear analysis results, the effect of concrete tension stiffening varied by the level of prestressing force is investigated. The results show that concrete tensile properties hardly affect the force–deformation response of beams with high-level prestressing. Additionally, several parameters related to concrete tension stiffening are examined in the parameter study. By comparing predicted moment–curvature responses with test data, ideal values for considered parameters are obtained. Based on the parameter study, simple equations are proposed that are expressed in concrete compressive strength to simulate the concrete tensile stress–strain responses of partially prestressed members. The proposed equations are demonstrated using several prestressed beam tests found in previous studies.

Comparison between concrete stress diagrams according to the Brazilian standard ABNT NBR 6118: 2014

This study aims to compare methods for the determination of concrete properties by means of the stress diagrams present in the Brazilian standard ABNT NBR 6118: 2014. The area under the stress diagram, the internal reactions, and the application point of the resulting reactions for the parabola-rectangle and rectangular block diagrams are present in order to compare them. Deductions and numerical examples were used, and different results were obtained for each formulation. This is due to non-consideration of the relationship between stress and strain in the simplified rectangular block. The rectangular block is applicable only for cases in which the concrete reaches the ultimate strain. These cases are those that concrete crushing determines the section failure in compression with steel yielding in tension (domain 3) or without steel yielding (domain 4).

Influence of maintenance methods on temperature and thermal stress of utility tunnel at early-age

In order to reduce and control the generation and development of early cracks in the concrete utility tunnel, ABAQUS finite element software was used to model the temperature field and thermal stress of early-age concrete of a thin-walled box structure. For the accuracy of simulation, the hydration heat of cementitious materials was tested by TAM air method, and the actual working conditions were used. Different curing methods were modeled, such as formwork type, formwork removal time, and curing time. Simulation results showed that steel formwork shall be selected for construction in summer, and the recommended formwork removal time was 3 days, which was better to resist temperature induced cracks. Moreover, the sooner the formwork was removed, the smaller temperature difference between concrete surface and center, but it would bring difficulties to the cooling rate control. Also, higher temperature occurred at bottom plate, especially in the junction area between bottom plate and partition wall. The maximum thermal stress was found at the junction between top plate and partition. Last but not lease, attention should be paid to the occurrence of peak temperature in 26 h, temperature difference control for 26–50 h, and cooling rate control during 2–5 days.

Unified Stress-Strain Model of FRP-Confined Square and Circle Rubber Concrete Columns.

Studying the stress–strain relationship of fiber-reinforced polymer (FRP)-confined rubber concrete (RuC) plays an important role in its application in engineering projects. Most of the existing stress–strain relationship models are established based on the test data of FRP-confined rubber concrete with circular cross-sections, and the effect of the section shape is not considered. Therefore, an analysis-oriented stress–strain model of FRP-confined circular and square rubber concrete columns was studied in this paper for the first time. A database that includes the rubber particle content and section shape on the peak stress-peak strain and axial–lateral strain relationship of FRP-confined rubber concrete was established by collecting 235 test data from the literature. By modifying the key parameters in the existing FRP-confined normal concrete stress–strain relationship model, a unified stress–strain relationship model of FRP-confined RuC with circular and square columns is established. The proposed model is verified, and a good accuracy of the model is proven.

Mechanical properties and constitutive model of steel fiber-reinforced rubberized concrete

This study aims to assess the mechanical behavior of steel fiber-reinforced rubberized concrete (SFR-RuC) with and without mix optimization through experimental investigations, and eighteen concrete mixtures were manufactured for test specimens. Through mix optimization, SFR-RuC with a steel fiber volume fraction no more than 1.5% and a rubber particle volume substitution as high as 20% could successfully achieve a compressive strength of 49 MPa-56 MPa. Cubic and uniaxial compressive tests, direct shear tests and four-point flexure tests were conducted on all specimens. The results showed that optimized SFR-RuC had overall improved mechanical properties and more ductile failure than nature concrete, and the elastic modulus and compressive, flexural and shear strength were enhanced by 13%, 16%, 25% and 50% respectively, when 10% rubber particles and 1.5% steel fibers were added. The peak strain, toughness and post-peak deformation of optimized SFR-RuC under compression increased with the steel fiber volume fraction as well as the rubber particle volume substitution, while the peak deflection, toughness and ductility under flexure and shear increased mainly with the steel fiber volume fraction. An obvious synergistic action was shown between the rubber particles and the steel fibers, through Scanning Electron Microscope (SEM) analyses on the microstructure, as well as test observations on the failure mechanism of specimens. Based on uniaxial compressive test results, formulas were proposed for the compressive strength, elastic modulus and peak strain of optimized SFR-RuC, and a modified stress–strain model was developed based on the classical concrete stress–strain model in the Chinese code (GB50010-2010), for a detailed assessment of the complete stress–strain response.

Full-range Experimental Analysis of Flexural Performance of an Existing PC Hollow Slab

Prestressed concrete (PC) hollow slab bridges are the most common type of short and medium span highway bridges in China. With the increasing load and environmental degradation factor, the flexural capacity of hollow slab bridges in service for many years can no longer meet the new safety regulations. Further research on the flexural capacity of these hollow slabs is necessary to ensure their safety. Experiments on the flexural bearing capacity of a full-scale damaged PC hollow slab are conducted. Elastic, cracking, re-cracking, and destruction tests are conducted to determine the deflection, strain, crack width, bearing capacity, stiffness, failure mechanisms, and dynamic characteristics of the slab. The strain measurements of the steel bar and prestressing tendons in the four-stage bending test indicate new failure characteristics of the PC hollow slab. After the steel bar yields, the concrete stress is redistributed, and the concrete on the top of the hollow slab exhibits tension, compression, and shear, providing science-based information to improve the ultimate bearing capacity of the slab. The cracking load (ultimate load) obtained from the full-scale bending test is 23% (72%) higher than the design value of the main slab’s internal force in normal service limit state (loading capacity limit state). The ultimate displacement in the midspan of the slab is 1/47 of the calculated span length, and the slab exhibits ductile failure characteristics, indicating very good ductility and sufficient strength of the slab after a 23-year service life. This study can provide a reference for the safety assessment and maintenance of numerous hollow slab bridges that have been in service for many years in China.

Stress and strain behavior monitoring of concrete through electromechanical impedance using piezoelectric cement sensor and PZT sensor

In this paper, a nondestructive testing method using piezoelectric sensors to monitor the stress–strain relationship of concrete by using the electromechanical impedance technique is proposed. Two piezoelectric sensors—a lead zirconate titanate (PZT) sensor and a piezoelectric cement (PEC) sensor—were investigated. A direct compression test was performed and a resistive strain gauge was used to measure the stress–strain curve of concrete at an age of 28 days. Furthermore, the impedance–frequency spectrum of a piezoelectric sensor embedded in the concrete was measured at every load in increments of 0.1 fc'. The results indicated that, as the load increased, the concrete conductance as measured by the PEC and PZT sensors in the applicable frequency decreased. Therefore, both sensors could be used to monitor the stress and strain during loading. The applicable frequency range for monitoring the stress and strain of concrete was near the second resonance peak as measured by the PZT sensor and at a high frequency exceeding 800 kHz as measured by the PEC sensor. The applicable frequency range of the PEC sensor was broader than that of the PZT sensor, and the use of the PEC sensor afforded the advantages of a low conductance spectrum fluctuation and short analysis time. A regression analysis of the conductance root-mean-square deviation GR and the stress–strain relationship of concrete indicated that the correlation between stress and GR was logarithmic and that between strain and GR was linear. Overall, the results indicated that the proposed approach can be used to evaluate the stress–strain behavior of concrete.

Numerical and analytical evaluation of stirrup-induced confinement on resistance of RC beams using lower bound limit analysis

In this paper, a modified three-dimensional element is introduced for limit analysis of RC structures, which considers more aspects of structural behaviour. Using the introduced element, the influence of stirrup confinement on the shear and moment resistance of RC beams is evaluated by lower bound limit analysis. To analyse and solve the relevant optimization problem, the semidefinite programming method, SDP, is employed. The modified Mohr–Coulomb yield criterion is utilized to constrain the concrete stress state. Beams with a constant amount of shear reinforcement per unit length, with different values of stirrup spacing, are studied. It means that they only differed in the effect of confinement induced by stirrup reinforcements. The performance of the proposed element is verified by comparing the results of numerical limit analyses with experimental results. This comparison revealed the efficiency and accuracy of the proposed model and the capability of the introduced element to take the effect of stirrup-induced confinements into account. After modelling and investigating the effect of stirrup confinement in load bearing of RC beams, a simplified equation for the design and practical purposes is proposed. Using this equation, the shear resistance of the beams could be modified for different stirrup space choices. The results obtained from this simplified equation are compared with numerical limit analysis and experimental results. The comparisons indicate the reasonable accuracy of the proposed equation.