Mechanical Properties Of Concrete Materials Research Articles

Experimental study on compressive properties of mixed fiber concrete after fire

In recent years, the application of high performance self-compacting concrete with high strength, high durability and high workability has become more and more widespread, and has gradually become the main building material. However, high-performance self-compacting concrete increases brittleness while increasing strength, and at the same time, explosion resistance and fire resistance are significantly reduced. With the development of cities, natural gas and intelligent electrical equipment are becoming more and more popular, but people's awareness of fire safety has not been improved, making the incidence of fire in buildings on the rise. In recent years, the development of hybrid fiber self-compacting concrete (HFRSCC), which combines the high workability of self-compacting concrete (SCC) and the high toughness and high crack-resistant properties of fiber concrete (FRC), giving full play to the fiber's anti-cracking, reinforcing and toughening effect, and after the fire and high temperature effect, the components of the use of hybrid fiber self-compacting concrete to maintain a certain degree of residual load-bearing capacity. The introduction of fibers for reinforcement and toughening, to improve the mechanical properties of concrete materials, blast resistance and fire resistance performance points to a direction.

Experimental Research on the Impact Resistance Mechanical Properties and Damage Mechanism of Rubberized Concrete under Freeze–Thaw Cycling

To improve the long-term performance of concrete engineering in high-altitude areas, waste tire rubber was added to a concrete mix, and freeze–thaw and impact tests were conducted. The effects of waste tire rubber with different particle sizes (10, 20, 30 mesh) and freeze–thaw cycles (0, 25, 50, 75, 100, 125) on the dynamic mechanical properties of concrete materials were studied. The stress–strain curves, peak stress, and fracture morphology of the specimens were analyzed. The microstructure changes of the specimens were also analyzed using scanning electron microscopy (SEM). The results showed the following: (1) Both macroscopic and microscopic analysis results showed that the internal damage of rubber concrete specimens was smaller after freeze–thawing, and the integrity was better after impact, maintaining a loose but not scattered state. The addition of waste tire rubber significantly improved the material’s impact resistance to a certain extent. (2) As the impact pressure increased, the strain rate of the specimens increased linearly, and the dynamic peak stress was linearly positively correlated with the strain rate. (3) After 125 freeze–thaw cycles, the peak stress of the specimens with 30-mesh added rubber decreased significantly less than that of ordinary concrete under 0.3, 0.45, and 0.6 MPa impact pressure. The dynamic peak stress was higher than that of specimens with 10-mesh and 20-mesh added rubber, and the addition of 30-mesh rubber significantly improved the frost resistance and impact resistance of concrete materials. This study can provide new ideas for the engineering application of rubber concrete.

Influence of cooling methods on high‐temperature residual mechanical characterization of strain‐hardening cementitious composites

AbstractResidual strength tests are commonly used to characterize the high‐temperature mechanical properties of concrete materials. In these tests, the specimens are heated to a target temperature in a furnace and then cooled down to room temperature, followed by mechanical testing at room temperature. This research investigates the influence of the cooling method on the residual strength of Strain Hardening Cementitious Composites (SHCC) after exposure to 400°C and 600°C. Two types of cooling methods — furnace‐cooling (within a closed furnace) and water‐cooling (immersed in a water tank) — were adopted. Four different SHCC previously investigated by the authors for high‐temperature residual mechanical and bond behavior with steel were studied. Two different specimen sizes were tested under uniaxial compression and flexure to characterize the residual compressive strength and modulus of rupture. The effect of the cooling method was prominent for the normalized residual modulus of rupture at 400°C, but not at 600°C. The cooling method had no effect on the normalized residual compressive strength of any material at either of the two temperatures, except one of the SHCC (PVA‐SC) at 400°C. Specimen size also had no effect on the normalized residual compressive strength and modulus of rupture irrespective of the cooling method.

The analysis of the destabilizing motion of a hyperbolic cooling tower during demolition blasting

The destabilizing motion characteristics of the hyperbolic cooling tower in demolition blasting are thoroughly investigated through the establishment of a numerical simulation calculation model, leading to the following conclusions regarding its destabilizing motion. The tensile-compression elastic-plastic model, which possesses the characteristics of parameter modification function and independence from unit size, can more effectively capture the mechanical properties of concrete materials and find better application in the simulation and calculation research of reinforced concrete structures. The self-oscillation frequency check and collapse morphological analysis are employed to validate the accuracy of the simulation calculation model for hyperbolic cooling towers, as well as to assess the rationality of parameters in the tensile-compression elastic-plastic model. The collapse of a cooling tower induces flexural deformation in the lateral wall, tensile disturbance in the upper and middle sections of the cylinder, and compressive disturbance in the vertical cross-section. The cylinder body has incurred damage as a result of the tower wall’s front end striking the ground at the directional window position on the front side of the throat, leading to a significant extrusion deformation issue. The buckling deformation in the central and lower sections of the rear wall propagated towards the back side of the tower wall upon reaching the ground, ultimately resulting in an “inverted V-shaped” damage along the buckling deformation line. The research findings hold significant relevance for future endeavors.

Experimental investigation on electrical response and mechanical performance of cementitious materials at low temperatures

In cold regions, concrete structures suffer from low temperatures, where damage will initiate due to the expansion of ice formation and accumulate with water supply to keep penetrating into the pores and cracks. Meanwhile, it is found that the mechanical properties of cementitious materials can be enhanced owing to the pores filled by ice. Therefore, it is important to evaluate the freezing process, in which the freezing action can have both strengthening and damaging effects on the mechanical properties of concrete material, depending on several environmental and material factors. Besides, if temperature remains negative, the capacity of cementitious materials in service with ice presented inside pores and cracks are also important. However, there is still lack of effective evaluation methods for the mechanical properties of frozen concrete. In this study, cementitious materials with/without air entraining agent (AEA) of different water-to-cement ratios (W/C) are prepared and tested at low temperatures (−10, −20, −30 and −40 °C). The results show that the mechanical properties of the materials (compressive strength, splitting tensile strength, dynamic elastic modulus) are closely related to electrical resistivity, which is further explained by the impact of air voids and pore structures from a multiscale scheme. It proves that the change of resistivity could be adopted as the index to evaluate the performance of frozen concrete, and an empirical model is derived based on the experiment results.

Seismic damage characteristics of high arch dams under oblique incidence of SV waves

This study investigates the seismic damage characteristics of high arch dams considering the oblique incidence of seismic waves. Based on the dynamic implicit finite element method, a viscous-spring artificial boundary is applied to consider radiation damping of the infinite foundation, the seismic wave input is converted into an equivalent nodal force of the artificial boundary, and the equivalent nodal loads of the wave input under oblique incidence of the SV wave is deduced. Taking JP Ⅰ arch dam as an example, combined with the concrete damaged plasticity (CDP) model, the nonlinear dynamic response of the dam under 25° oblique incidence of the SV wave is conducted considering the geometrical nonlinearity of contraction joints and the dynamic mechanical properties of concrete materials. Based on the incremental dynamic analysis (IDA) method, the relative displacement and transverse joint opening are selected as the damage measures for fragility analysis of the arch dam. According to the damage distribution from the analysis of a large number of seismic time-history responses, the damage levels of the arch dam are classified. Three limit states of slight damage, moderate damage and severe damage are proposed for the arch dam. The average responses of the 10 earthquakes are considered as the limit values to quantitatively describe the damage of the arch dam. By fitting the results of incremental dynamic analysis, the seismic probabilistic demand model and seismic fragility curves for the two damage measures to predict the probability of the arch dam reaching damage levels under different seismic actions have been established. The fragility curve shows that the probability of severe damage is almost zero under the maximum design earthquake (PGA = 0.1971 g) and less than 5% under the maximum credible earthquake (PGA = 0.2299 g). It is concluded that JP Ⅰ arch dam has a high degree of safety redundancy and fully satisfies the “two levels” design requirements.

Fracture characteristics of sustainable crumb rubber concrete under a wide range of loading rates

The dynamic mechanical properties of concrete materials significantly influences the safety of construction under impact loads. In this study, a comprehensive experiment on the crack straight-through flattened Brazilian disc specimens of crumb rubber concrete (CRC) is undertaken. Three specific experiments were performed to obtain the wide loading-rate ranges of fracture characteristics, i.e., the quasi-static test, drop-hammer impact test, and split Hopkinson pressure bar test. The measured and calculated results are also discussed. The results reveal that the fracture toughness and energy are both positively correlated with loading rate. Moreover, a transition loading rate (around 1 GPa × m1/2/s) exists between the high and low loading rates, and the enhancing effect of high loading rate on fracture toughness cannot be neglected. The analysis of the dynamic increased factor (DIF) on the fracture toughness (DIFk) and fracture energy (DIFg) of CRC shows that DIFk and DIFg are related to the rubber content. Furthermore, the analysis demonstrates that crumb rubber particles can enhance the crack resistance of concrete under dynamic loads. Accordingly, empirical models for the DIFk and DIFg of CRC are proposed based on the experimental data. In addition, the mechanisms by which the loading-rate affects the fracture characteristics were preliminarily identified by observing the failure modes under various loading-rate loads. This study is intended to help spread the application of crumb rubber in concrete materials.

Research on Dynamic Strength and Inertia Effect of Concrete Materials Based on Large-Diameter Split Hopkinson Pressure Bar Test.

The Split Hopkinson Pressure Bar (SHPB) test device is an important tool to study the dynamic characteristics of concrete materials. Inertial effect is one of the main factors that cause inaccurate results in SHPB tests of concrete materials. To solve this problem, Large-diameter SHPB tests on concrete and mortar were performed. A dynamic increase factor (DIF) model considering strain rate effect and inertia effect was established. This model provides a scientific reference for studying the dynamic mechanical properties of concrete materials. The experimental results indicate that the strain rate effect of concrete is more sensitive than that of mortar, but the inertia effect of mortar is more sensitive than that of concrete. Under the same strain rate, the energy utilization rate, average fragment size, and impact potentiality of mortar are higher than concrete.

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.

Influence of Fe2O3 Content on Acoustic Properties of Backing Materials and Development and Utilization of Acoustic Emission Sensors

Fe2O3 and epoxy resin sensor substrate materials (Fe2O3/epoxy resin composite material) are prepared in this article. A series of acoustic emission sensors are prepared by using composite material as sensor substrate, and the effect of Fe2O3 content in composite material on sensor performance is studied. The effect of Fe2O3 content in the composite material on the performance of the sensor is studied through the simulated lead‐breaking experiment, and the self‐made sensor is calibrated by using the calibration system. Nondestructive testing of concrete materials with different strengths is carried out by selecting the sensor with the best performance. The results show that the content of Fe2O3 in the composite increases to 50%, the epoxy resin matrix reaches saturation point of accommodate Fe2O3 powder, and the propagation velocity of ultrasonic waves reaches the lowest point when it passes through the composite. The sensor with 50% Fe2O3 content as a backing layer has the highest sensitivity and signal‐to‐noise ratio. The relationship between the strength of concrete material and acoustic attenuation and frequency is established by using this sensor, and the law can be used to evaluate the strength and mechanical properties of concrete material without destroying them.

Fire damaged ultra-high performance concrete (UHPC) under coupled axial static and impact loading

The load bearing structural components such as columns could experience axial static loads during the service life. High temperature induced by fire would have a significant detrimental impact on the mechanical properties of concrete materials. The structure could be severely damaged as the column was simultaneously loaded by other impact loads. In this study, the behavior of fire damaged ultra-high performance concrete (UHPC) with a compressive strength of 128.3 MPa under coupled axial static and impact loading was studied. UHPC specimens were heated to the target temperatures (250, 500 and 750 °C) in an electric furnace and then naturally cooled down to room temperature. The results demonstrated that the P-wave velocity and compressive strength of the heated-cooled treatment UHPC degraded significantly as the target temperature exceeded 250 °C. The impact tests were then conducted on the heated-cooled treatment UHPC specimens with axial static compression. The experimental results indicated that the axial static compression could enhance the dynamic mechanical properties such as compressive strength and elastic modulus in the elastic phase and weaken the dynamic mechanical properties in the plastic phase. In addition, the dynamic increase factor (DIF) of UHPC exhibited an increase with the temperature. The UHPC specimen could withstand a temperature of 250 °C, but lost most of its strength at temperatures of 500 and 750 °C. Thus, the axially loaded static force accelerated the failure of the specimen after being heated to above 250 °C.

Influence of steel fiber on dynamic failure property of twin fibers pull-out specimens

By using the realistic failure process analysis RFPA^2D-dynamics software, we establish a dynamic numerical model for twin fibers pull-out test. The microscopic non-uniformity of mechanical properties of concrete materials in different phases is accepted to study the influence of steel fiber volume content under dynamic load on mechanical properties. According to the elastic damage constitutive relationship of the meso-element, the dynamic tensile failure process of specimens with various steel fiber volume content, from micro-cracks initiation, propagation to crack penetration, is studied in details. Furthermore, curves of load-displacement and acoustic emission cumulative energy could also be obtained. The results show that variation of steel fiber volume content has significant influence on the failure pattern of the specimen under dynamic load. With the increase of steel fiber volume fraction, the energy consumption level of specimens first increases and then decreases. When the steel fiber volume fraction is 1.6%, the energy consumption reaches the highest, which is about 1.36 times of that of the plain concrete specimens.

Effect of fly ash and sand stone slurry on mechanical properties of concrete materials

Abstract Due to the simultaneous growth of urbanization and industrialization globally, the demand for the infrastructures like building structure, roads, bridges, canals, dams. is inevitable. These activities are increasing day by day to fulfil the requirement of people of the country for their comfortable living and enjoyment. In the past when our country was young, No crisis with the natural resources of raw materials like cement, sand, aggregate. These three materials are the key elements of any type of construction work. In recent days, the natural resources get depleted due to the increasing construction activities. In the current study, the attempt has been taken with the valuable alternate materials like Fly ash (FA) and sandstone slurry (SSS) by replacement of amount of cement and sand with different proportions of SSS. Six mixes were prepared, in which, one is the standard and the other five are the trial mixes. Three samples from each mix were prepared and tested after 14 days of curing to determine the mechanical properties of the prepared mix and the compressive strength, flexural strength, splitting tensile strength were determined.

Utilization of coal gangue as coarse aggregates in structural concrete

In this paper, coal gangue is utilized as a replacement of coarse aggregate in structural concrete. The effects of coal gangue on the mechanical properties of concrete material, circular reinforced concrete column and concrete-filled steel tubular column are experimentally studied and discussed in detail. The mechanical behavior of coal gangue concrete is reduced by increasing the coal gangue replacement ratio. A model is proposed for predicting the compressive strength of coal gangue concrete. Some constants are proposed based on test results to predict the splitting tensile strength and elastic modulus of coal gangue concrete. The increase of coal gangue replacement ratio decreases the axial compressive strength and elastic modulus of reinforced coal gangue concrete stubs and coal gangue concrete-filled steel tubular stubs linearly. The enhancement factor and the confinement factor both approximately increase with the increase of coal gangue replacement ratio which indicates that concrete-filled steel tube is a more preferable product than reinforced concrete for the usage of coal gangue as coarse aggregates. A modified design method for reinforced concrete and concrete-filled steel tube towards cleaner products is proposed by considering the coal gangue replacement ratio. The full-range prediction analysis for the strength degradation of coal gangue concrete-filled steel tube implies that relative higher steel strength and lower concrete strength should be used in the preparation of concrete-filled steel tube using coal gangue as coarse aggregates in structural stubs.

Effect of Initial Static Load and Dynamic Load on Concrete Dynamic Compressive Failure

Concrete structures often suffer from multiple dynamic loads, or prior to being subjected to dynamic loads have already withstood initial loads. The mechanical properties of concrete materials under dynamic loads may be closely linked to initial loading history that concrete suffers. This paper deals with the investigation of the effect of initial static load and initial dynamic load on dynamic compressive failure of concrete materials based on numerical simulations. A meso-scale modeling method was established and a total of 147 three-dimensional concrete cubic models were simulated. It is demonstrated in research results that the strain rate effect of concrete is relatively weak under low strain rates, while the strain rate effect is significantly enhanced under high strain rates. Dynamic compressive strength under different dynamic loads gradually decreases with the increasing initial static load and the increasing strain rate can weaken the influence of initial static load on dynamic compressive strength of concrete. When dynamic load changes in hardening stage, tangent modulus has a sudden increase and compressive stress reaches a new peak strength. A sudden increase of dynamic load in softening stage reverses the postpeak softening to postpeak hardening followed by a second peak strength. Initial dynamic load plays an important role in dynamic compressive failure of concrete, especially under low strain rates. The numerical results compare well with the existing test results.

Experimental determination of the residual compressive strength of concrete columns subjected to different fire durations and load ratios

Purpose At elevated temperatures, concrete undergoes changes in its mechanical and thermal properties, which mainly cause degradation of strength and eventually may lead to the failure of the structure. Retrofitting is a desirable option to rehabilitate fire damaged concrete structures. However, to ensure safe reuse of fire-exposed buildings and to adopt proper retrofitting methods, it is essential to evaluate the residual load-bearing capacity of such fire-damaged reinforced concrete structures. The focus of the experimental study presented in this paper aims to investigate the fire performance of concrete columns exposed to a standard fire, and then evaluate its residual compressive strengths after fire exposure of different durations. Design/methodology/approach To effectively study the fire performance of such columns, eight identical 200 × 200 × 1,500-mm high reinforced concrete columns test specimens were subjected to two different fire exposure (1- and 2-h) while being loaded with two different load ratios (20% and 40% of the column ultimate design axial compressive load). In a subsequent stage and after complete cooling down, residual compressive strength capacity tests were performed on each fire exposed column. Findings Experimental results revealed that the columns never regain its original capacity after being subjected to a standard fire and that the residual compressive strength capacity dropped to almost 50% and 30% of its ambient temperature capacity for the columns exposed to 1- and 2-h fire durations, respectively. It was also noticed that, for the tested columns, the applied load ratio has much less effect on the column’s residual compressive strength compared to that of the fire duration. Originality/value According to the unique outcomes of this experimental study and, as the fire-damaged concrete columns possessed considerable residual compressive strength, in particular those exposed to shorter fire duration, it is anticipated that with proper retrofitting techniques such as fiber-reinforced polymers (FRP) wrapping, the fire-damaged columns can be rehabilitated to regain at least portion of its lost load-bearing capacities. Accordingly, the residual compressive resistance data obtained from this study can be effectively used but not directly to adopt optimal retrofitting strategies for such fire-damaged concrete columns, as well as to be used in validating numerical models that can be usefully used to account for the thermally-induced degradation of the mechanical properties of concrete material and ultimately predict the residual compressive strengths and deformations of concrete columns subjected to different load intensity ratios for various fire durations.

Impact of freeze-thaw environment on concrete materials in two-lift concrete pavement

The freeze-thaw (FT) damage is a potential severe distress on concrete pavements in seasonal frozen regions. This study focuses on the FT resistance of a two-lift concrete (2LC) pavement, including a polypropylene fiber reinforced concrete (PFRC) top lift and a conventional portland cement concrete (PCC) bottom lift. The rapid FT test was conducted on PCC, PFRC, and 2LC specimens and the relative dynamic modulus and mass loss were measured. The mechanical properties of concrete materials were investigated in laboratory, including compressive, tensile, flexural strengths, and flexural modulus. In the end, the influence of FT damages of concrete materials on pavement responses was evaluated. It is found that with the addition of the PF, the PFRC and the 2LC exhibited better mechanical properties and less mass loss than the PCC after FT cycles. The PFRC and the 2LC behaved better FT resistance than the PCC. The Voigt model can predict mechanical properties of 2LC in freeze-thaw process. Through pavement response analysis, the FT damage was found shortening the service life of 2LCP. In summary, the proposed 2LC pavement was justified a better alternate concrete pavement type on resisting the FT environment in seasonal frozen areas.

Influence of embedment depth on dynamic failure property of twin fibers pull-out test

Weibull statistical distribution function is introduced to establish dynamic numerical simulation model of twin fibers pull-out test with the software RFPA^2D (realistic failure process analysis)-dynamics. The model is used to describe the non-uniformity of mechanical properties of concrete materials in different phases and study the influence of embedment depth on mechanical properties under dynamic load. According to the elastic damage constitutive relation of the meso-element, the dynamic tensile failure processes of specimens with various embedment depth are studied from micro-cracks initiation, propagation to crack penetration. The curves of load-displacement and acoustic emission cumulative number curves are also obtained. Influence of the embedment depth on properties of the twin fibers pull-out test specimens is scrutinized. The results show that variation of embedment depth of fibers has significant influence on the failure pattern of the specimen. Under dynamic loading, a phenomenon called 'vibration springback’ appears obviously in specimens with larger embedment depth. In addition, with increasing embedment depth, the twin fibers coupling effect is enhanced and the pull-out process needs more energy, which leads remarkable higher strengthen and toughness.

Destructive and non-destructive testing of bronze-waste tire-concrete composites

In this study, the effects of finely-milled bronze and waste tire on the mechanical properties of concrete have been investigated. Approximately 2.5% and 5% by weight for each additive (bronze sawdust and waste tire) were added to dry concrete. The open porosity, density, compressive strength values of cured concrete have been determined. In addition, the Schmidt rebound hammer (SRH) and the ultrasonic pulse velocity (UPV) tests, which are non-destructive test methods, were applied. The microstructure and fracture surfaces of these materials were characterized by scanning electron microscopy (SEM). It was observed that the density of pure concrete was 2.35 g/cm3 while the density was 2.19 g/cm3 for a C+5%B+5%T material. Similarly, pure concrete had an almost three times better compressive strength and a two times better SRH value than those of the C+5%B+5%T material. The density and mechanical properties of concrete materials containing bronze and waste tire decreased due to micro crack formations, weak bonding and deep cracks forming especially between the concrete and additives.

Performance of concrete targets mixed with coarse aggregates against rigid projectile impact

For effective structure and personnel protection against projectile impact, accurate prediction of the depth of penetration (DOP) of projectile into concrete is essential. Many empirical and semi-empirical formulae have been developed to yield accurate predictions as compared with testing data. In most of these formulae, the concrete target was assumed as homogeneous and the mechanical properties of concrete material were characterized by the compressive strength of concrete. The deviation of DOP predications is identified for the concrete targets mixed with coarse aggregates because of the homogeneous assumption which leads to errors in DOP prediction. In this study, the effects of mortar strength, aggregate strength, maximum size and volume fraction of coarse aggregate on penetration resistance are quantitatively studied by using 3D meso-scale modeling. The applicable condition of the homogeneous assumption and the upper bounds of aggregate size and volume fraction that affect the penetration resistance are determined with respect to the aggregate configurations. The factors dominating the penetration resistance of concrete target are investigated and it is found that the sensitivity of penetration resistance to aggregate size is lower than other mesoscopic factors, i.e., mortar strength, aggregate strength and volume fraction. In addition, a formula incorporating the effect of multi-factors is proposed to predict the DOP of rigid projectile penetration into concrete target mixed with coarse aggregates. The proposed formula, which considers the effects of the mesoscopic factors on the penetration resistance, can give better predictions of DOP into concrete targets.