Dynamic Tensile Strength Of Concrete Research Articles

Mesoscale modelling of the dynamic tensile strength enhancement of concrete in spalling tests using interface elements

The dynamic tensile strength of concrete has been experimentally reported to increase significantly with the increase of imposed strain rates. However, the intrinsic mechanisms accounting for the strength increase are not well understood so far. This paper presented numerical simulations based on the spalling technique to further explore mechanisms of the dynamic responses of concrete under impact loadings. Numerical results have been verified and validated against experimental evidence with various strain rates. The validity of utilizing the Novikov acoustic processing approximation for obtaining the spalling strength of concrete is identified and discussed. Results demonstrate that this indirect processing approach could overestimate the spalling strength because real material behavior tends to deviate from its basic assumption. Mechanisms accounting for the spalling strength increase from key aspects including the meso-structure, the strain rate-dependent material behaviour, the micro-crack inertia, and the structural inertial are also identified accordingly. Results demonstrate that the increment of concrete dynamic tensile strength in spalling tests is mainly caused by the strain rate-dependent material behaviour which should be incorporated in the material constitutive description. Besides that, the material heterogeneity also makes a considerable contribution to the increase of dynamic tensile strength in spalling tests and this contribution becomes increasingly prominent with the increase of the imposed strain rates. On the other hand, the structure inertial and the micro-crack inertial have little effect on the increase of spalling strength of concrete and thus may be ignored within the imposed strain rate range in spalling tests.

Research on energy dissipation and damage evolution of dynamic splitting failure of basalt fiber reinforced concrete

Based on split Hopkinson bar (SHPB) loading system and digital image correlation (DIC) technology, a series of dynamic splitting experiments of basalt fiber reinforced concrete (BFRC) were carried out to reveal the effects of basalt fiber content and loading level on the dynamic tensile properties, energy dissipation and damage evolution. The results show that the dynamic tensile strength and dynamic increase factor (DIF) have obvious strain rate effect, and the appropriate basalt fiber content (0.39%) can remarkably improve the dynamic tensile strength of concrete. According to the changing trend of dissipated energy curve, the dynamic splitting process can be divided into compaction stage, elastic stage, plastic yield stage and softening stage. Appropriate basalt fiber content (0.39%) can effectively improve the dissipated energy and dissipated energy rate of BFRC. The loading level only promotes the dissipated energy of BFRC, but has no obvious effect on the dissipated energy rate. Adding an appropriate basalt fiber content (0.39%) can reduce the shear strain, thereby reducing the damage degree of the shear failure zone, and delaying the cracking time in the center of the specimen to a certain extent.

Mesoscopic investigation on the mechanism of concrete dynamic tensile strength enhancement based on the E(A, B) algorithm

The dynamic tensile strength of concrete shows a certain dynamic enhancement, whereas the potential mechanism of the dynamic enhancement has not been clearly revealed so far. This paper studies the mechanism of concrete dynamic tensile strength enhancement and identifies the contribution of various factors to the dynamic enhancement. In order to study the fracture of concrete under dynamic load, this paper first developed a three-dimensional polyhedral aggregate packing program by adopting the algorithm of Entrance block between A and B (E(A, B)), which greatly simplifies the calculation of contact or overlapping judgment between two blocks in the modeling process. A cohesive model is then proposed to describe the fracture in the microscopic concrete model. Through the verification simulations, appropriate material parameters and mesh size are selected. Finally, the effects of the micro-crack inertia, material heterogeneity, the structural inertia and the material viscosity on the concrete dynamic tensile strength enhancement are discussed. The results show that the dynamic tensile strength of concrete is not significantly affected by the micro-crack inertia but is highly related to the material heterogeneity, the structural inertia and the material viscosity. Specifically, the material heterogeneity has a more pronounced effect on the dynamic tensile strength at higher strain rate. The contribution of the structural inertia effect and the material viscosity to the enhancement of dynamic tensile strength rank the second and first respectively, and their influence degree increase with the increase of strain rate.

Numerical Study of Concrete Dynamic Splitting Based on 3D Realistic Aggregate Mesoscopic Model.

In mesoscopic scale, concrete is regarded as a heterogeneous three-phase material composed of mortar, aggregate and interfacial transition zone (ITZ). The effect of mesoscopic structure on the mechanical behaviors of concrete should be paid more attention. The fractal characteristics of aggregate were calculated, then the geometric models of aggregate were reconstructed by using fractal Brownian motion. Based on the random distribution of aggregates, the concrete mesoscopic structure model was established. And the numerical model was generated by using grid mapping technology. The dynamic compression experiments of concrete under Split Hopkinson Pressure Bar (SHPB) loading verify the reliability and validity of the mesoscopic structural model and the parameters of the constitutive model. Based on these, a numerical study of concrete under dynamic splitting is carried out. By changing the parameters of the constitutive model, the effects of tensile strengths of aggregate, mortar and ITZ on the dynamic tensile strength of concrete are discussed. The results show that the dynamic failure of specimen usually occurs at the interfacial transition zone, then extends to the mortar, and the aggregates rarely fail. However, the increase of strain rate intensifies this process. When the strain rate increases from 72.93 s−1 to 186.51 s−1, a large number of aggregate elements are deleted due to reaching the failure threshold. The variation of tensile strengths of each phase component have the same effect on the dynamic tensile strength and energy of concrete. The dynamic tensile strength and energy of concrete are most affected by the tensile strength of mortar, following by the ITZ, but the tensile strength of aggregate has almost no effect.

Evaluation of dynamic tensile strength of concrete using lattice-based simulations of spalling tests

The mechanical properties and failure modes of concrete are affected by strain rate. Therefore, various experimental methods have been used to quantify the strain-rate dependency of concrete properties, such as the spalling test by using a Hopkinson bar. In the test, pullback velocity at the free-end surface of the specimen is usually measured to evaluate the dynamic tensile strength of concrete, instead of directly measuring the critical stresses at the damaged location(s) due to experimental constraints. Herein, such indirect measurements of tensile strength are compared with direct determinations of tensile strength based on lattice models of the spalling tests. To represent rate-dependent material behavior, rheological units are introduced within the lattice elements. The parameters of the rheological units are calibrated through comparisons with experimental data. The calibrated values remain unchanged in subsequent simulations, which can be regarded as virtual spalling tests at various high strain rates of loading. The separation of viscous and inertial contributions to apparent tensile strength provides insights into the dependence of actual tensile strength on high strain rates. The simulation results indicate indirectly measured dynamic tensile strength, as commonly done in practice, is sufficiently close to directly measured strength.

Study on the Influence of Inertia Effect in Dynamic Splitting Test of Concrete

A group of concrete spilitting tensile test was carried out by using the drop test machine Studying on the inertial effect on the dynamic tensile strength of concrete under different working conditions. The result show:with the increase of the drop height and quailty, peak load, loading rate of the concrete are increasing gradually.peak load of the test group is slightly higher than control group, The inertia effect has a certain influence on the dynamic tensile strength of concrete.

Dynamic size effect of concrete under tension: A numerical study

The macro nonlinearity and size effect of concrete are essentially attributed to the internal heterogeneity of concrete. Considering the heterogeneities and the strain rate effect of meso-components, the concrete was modelled as a three-phase composite composed of aggregate particles, mortar matrix and the interfacial transition zones (ITZs). Taking double-edge notched concrete specimens as examples, a meso-scale numerical model for the simulation of the failure and size effect of concrete under dynamic tensile loading with different strain rates from 10−5/s to 100/s was established. The effect of structural sizes on the dynamic tensile strength of concrete was explored. The numerical results indicate that dynamic size effect on tensile strength of concrete has an obvious discrepancy with the static one. There exists a critical strain rate (ε˙cr = 1/s), and at the strain rate the dynamic tensile strength turns to be independent on the structural size of the specimen. For the case less than the critical strain rate, the tensile strength decreases with increasing the specimen size, and the size effect on the dynamic tensile strength is weakened or suppressed as the strain rate increases. For the strain rate larger than the critical one, there is no obvious difference in the dynamic tensile strength of the concrete specimen having different structural sizes, and the corresponding size effect completely disappears. According to the influencing mechanism of strain rate effect and size effect, a Static and Dynamic unified Size Effect Law (i.e. SD-SEL) for tensile strength of concrete was built. The proposed SD-SEL was also verified by the mesoscopic numerical simulation results.

Mesomechanism of the dynamic tensile fracture and fragmentation behaviour of concrete with heterogeneous mesostructure

As a typical multi-phase material, due to its heterogeneous mesostructure, concrete exhibits complicated mechanical responses when subjected to high-rate loads. Considering concrete is much weaker in tension than in compression, it is of significance to investigate the mesomechanism of the dynamic tensile fracture behaviour of concrete under high-rate loading conditions. In this study, the meso-heterogeneity of concrete is considered by random mesostructure generation with efficient algorithm of Entrance block between A and B (E(A, B)). A 2D meso-scale model is developed by implementing a combined FDEM scheme with zero-thickness cohesive elements in ABAQUS software. The proposed model is validated by quasi-static uniaxial tension and uniaxial compression tests as well as dynamic Brazilian tensile split Hopkinson pressure bar (SHPB) test and then used in a series of Brazilian tensile SHPB tests to investigate the influences of the loading rate, aggregate area fraction and the mechanical properties of the interfacial transition zone (ITZ) on dynamic tensile behaviour of concrete. The numerical results indicate that the dynamic tensile strength increases with increasing loading rate and the fragmentation experiences a transition from parse fracturing to dramatic pulverization. A higher aggregate content has an adverse effect on the dynamic tensile strength of concrete. Moreover, the results also suggest that the mechanical properties of the ITZ play a significant role on the dynamic tensile behaviour of concrete.

Vibration Failure of Young Low‐Temperature Concrete Shaft Linings Caused by Blasting Excavation

The freezing‐blasting method constitutes the only available technique for excavating mining shafts within water‐bearing bedrock. This study explores the effects of vibration damage to young C65 concrete shaft linings caused by close‐range blasting excavation using the finite element method. C65 concrete test specimens were made in the laboratory and then cured at −7°C, and the elastic modulus, compressive strength, and longitudinal wave velocity were tested. The allowable dynamic tensile strength of the concrete for each mold of the shaft lining was obtained according to the observed strain rate of the concrete shaft lining using a regression formula. The finite element simulation results are basically consistent with the in situ measurements, thereby attesting to the validity of the numerical simulation. The blasting‐induced vertical peak vibration velocity of the first mold of the concrete shaft lining reached 20∼25 cm/s, which far surpasses the allowable vibration velocity range (i.e., 2∼3 cm/s) in the Safety Regulations for Blasting for newly cast concrete between the initial setting and an age of 3 d. The tensile stress of the first concrete mold calculated by the finite element method is approximately equal to the theoretical tensile stress, both of which are smaller than the dynamic tensile strength of concrete. The cumulative energy sustained by the shaft lining of each mold and the allowable values of the dynamic tensile strength were obtained. The growth rate of the dynamic tensile strength of the subsequent molds was larger than that of the cumulative energy, and thus the safety of the shaft lining gradually improved. The C65 concrete would therefore not experience tensile failure after the shaft lining has sustained multiple rounds of blasting loads. This finding can provide a basis for safety considerations when employing the freezing‐blasting method to construct mining shafts in water‐bearing bedrock.

Mesoscopic Insight into the Damage Mechanism for the Static Preload Effect on Dynamic Tensile Strength of Concrete

AbstractExperimental studies have showed that the static preload increases the dynamic tensile strength of concrete under certain conditions. But its mechanism still remains open. In this study the...

Enhancement Mechanism of the Dynamic Strength of Concrete Based on the Energy Principle.

This paper analyzes the relationship between the rates of change of elastic strain energy, the strength during the concrete failure process, and proposes that the increased dynamic strength of concrete was caused by the hysteresis effect of energy release—according to the basic principle of energy conversion. Dynamic Brazilian disc tests were carried out on concrete specimens, with diameter of 100 mm, by using the split Hopkinson pressure bar. Test results were obtained through using a gas gun, with an impact pressure of 0.15 MPa, 0.20 MPa and 0.25 MPa, respectively. The dynamic failure process of concrete is then reproduced by numerical calculation methods. Finally, the energy characteristics during the concrete failure process at different strain rates are studied, and the enhancement mechanism of the dynamic strength of concrete is verified. The results showed that the dynamic tensile strength of concrete increased by 9.79% when the strain rate increased by 61% from 60.25 s−1; and when the strain rate increased by 92.8% from 60.25 s−1, the dynamic tensile strength of the concrete rose by 46.28%. The rates of change of both input energy and dissipated energy meet at the peak stress of the material. The increases in rates of change for the two kinds of energy were not synchronized, so excess input energy could be stored as concrete strength increased. As a result, the extra energy stored after peak stress led to a higher degree of concrete fragmentation and greater kinetic energy of the fragment. These results offer research directions for improving the dynamic strength of concrete.

Experimental and numerical investigation on the dynamic increase factor of tensile strength in concrete

A unique design for a vertical tension split Hopkinson bar (TSHB) with steel bars 100 mm in diameter and an aluminum hammer is presented. This facility was used to investigate the behavior of concrete under dynamic tension. Experiments conducted with specially shaped concrete samples demonstrate that the testing system is capable of accurately depicting the dynamic tensile strength of concrete. The experimental data were used to calibrate three concrete material models implemented in the numerical code LS-DYNA: (1) The Karagozian & Case (K&C) concrete damage model (*MAT_072R3), (2) the 072-BGU version of the K&C damage model, and (3) the continuous smooth cap model (*MAT_CSCM, *MAT_159). It was found that the automatic generation option included in these models does not provide satisfactory results. However, by readjusting only a few parameters of each of these material models, very good agreement between experimental and numerical results was achieved. This paper outlines the necessary calibration process and its theoretical background.

On the Processing of Spalling Experiments. Part I: Identification of the Dynamic Tensile Strength of Concrete

The spalling technique based on the use of a single Hopkinson bar put in contact with the tested sample has been widely adopted as a reliable method for obtaining the tensile response of concrete and rock-like materials at strain rates up-to 200 s− 1. However, the traditional processing method, based on the use of Novikov acoustic approach and the rear face velocity measurement, remains quite questionable due to strong approximations of this data processing method. Recently a new technique for deriving cross-sectional stress fields of a spalling sample filmed with an ultra-high speed camera and based on using the full field measurements and the virtual fields method (VFM) was proposed. In the present work, this topic is perused by performing several spalling tests on ordinary concrete at high acquisition speed of 1Mfps to accurately measure the tensile strength, Young’s modulus, strain-rate at failure and stress–strain response of concrete at high strain-rate. The stress–strain curves contain more measurement points for a more reliable identification. The observed tensile stiffness is up-to 50% lower than the initial compressive stiffness and the obtained peak stress was about 20% lower than the one obtained by applying the Novikov method. In order to support this claim, numerical simulations were performed to show that the change of stiffness between compression and tension highly affects the rear-face velocity profile. This further suggests that the processing based only on the velocity “pullback” is quite sensitive and can produce an overestimate of the tensile strength in concrete and rock-like materials.

Effect of Prestatic Loading on Dynamic Tensile Strength of Concrete under High Strain Rates

AbstractThe static bending tests and the split Hopkinson pressure bar (SHPB) dynamic bending tests of concrete are investigated in this paper, and the effects of different initial static loading history on the dynamic tensile strength of concrete are explored. The initial static loadings are divided into five grades, 0, 25, 50, 75, and 90% of static tensile strength, respectively. A large number of mesostructure images of specimens have been obtained by the means of a scanning electron microscope (SEM) and used to analyze the influence of the microstructure and microcrack propagation on the dynamic performance of concrete. In addition, the Weibull distribution model is introduced to analyze the test results, which show that the dynamic tensile strength of concrete increases significantly with an increase in strain rate. When the initial static loading level does not exceed 50%, dynamic tensile strength is basically stable; but when the initial static loading level is more than 75%, dynamic strength begins...

Statistical Analysis of Dynamic Splitting Tensile Strength of Concrete Using Different Types of Jaws

AbstractThe indirect Brazilian test is widely used in engineering practice to indirectly determine the tensile strength of concrete. The Brazilian test (splitting tension test) is performed by applying a concentrated compressive load across the diameter of a disc-specimen. A series of dynamic Brazilian disc tests for concrete specimens were conducted using the split Hopkinson pressure bar (SHPB) technique. The objective of this paper is to investigate the standard Brazilian test by comparing the results obtained using the test with those obtained using a loaded arc, together with a comparison of the experimental results and the statistical analysis. The effect of loading angle on the dynamic splitting tensile strength was studied by one-way analysis of variance. By using linear regression method, the Weibull statistical model was introduced to obtain the relationship between the loading angle and dynamic splitting tensile strength of concrete. It was concluded that dynamic tensile strength of concrete is ...

Advances in experimental assessment of dynamic tensile strength of concrete by the spalling technique

An experimental method to test concrete in dynamic tension by spalling with a Hopkinson bar as loading and measuring tool was developed in 1999. The dynamic strength of concrete specimen and strain rate were indirectly derived from an accurate data processing of the signals measured on the Hopkinson bar surface. This method suggested by late Prof. Klepaczko, allowed for reaching the highest strain rate reported in literature for which an intriguing tensile strength increase was highlighted. This simple and efficient technique has been adopted by many researchers around the world. Some significant improvements in terms of definition and reproducibility of the incident loading pulse travelling along the bar and direct and/or contactless measurements on concrete specimens have been introduced. The very high rate sensitivity of concrete tensile strength was corroborated by the additional experimental data obtained with this experimental technique during the last fifteen years.

Numerical and Theoretical Study of Concrete Spall Damage under Blast Loads

Spall damage is a typical failure mode of concrete structures under blast or high velocity impact loads. At the opposite side from which the structural element was impulsively loaded, spall will occur if the net primary stresses over an area exceed the dynamic tensile strength of concrete. Fragments of structural element could eject with large velocities, and this kind of damage can cause severe threats to equipment and personnel. In the present study, reinforced concrete columns subjected to the blast loading is investigated and the numerical study of concrete spall is conducted. The spall depth is recorded and compared with the theoretical results derived from wave propagation theory. The parameters that affect the concrete spall damage are investigated.

Numerical simulation of dynamic tensile-failure of concrete at meso-scale

Tensile failure behavior of concrete invariably dominates the behavior of concrete specimens as well as structural elements and it is strongly affected by loading rate. The present study focuses on the effects of loading rate and heterogeneity of meso-/micro-structure on the failure pattern and the macroscopic mechanical properties of concrete. For simplicity, concrete is regarded as a two-phase composite composed of aggregate and mortar matrix at meso-scale. The damaged plasticity theory combined with strain-rate effect is employed to describe the dynamic mechanical behavior of mortar matrix, and the aggregate phase is assumed to be elastic. The dynamic tensile failure modes of a single-edge notched concrete specimen and the L specimen under different loading rates are numerically investigated. The simulation results indicate that dynamic failure pattern and the direction of crack propagation of concrete have pronounced loading rate sensitivity. With the increase of loading rate, the failure mode of concrete changes from mode-I to mixed mode. The more complex the meso-structure is, the higher the interaction intensity between the meso components has and the more complicated the crack paths are, resulting in a more obvious crack branching behavior. Furthermore, as loading rate increases much more branching cracks generate within concrete and the width of the damaged region increases, implying that the fracture process at relatively high strain rates requires more energy demand to reach failure. And this should be the main reason for the improvement of the dynamic tensile strength of concrete.

Using Hopkinson pressure bar to perform dynamic tensile tests on SFRC at medium strain rates

The dynamic tensile strength of steel fibre reinforced concrete is experimentally investigated. Cylindrical specimens are first fabricated with three different fibre volume fractions of 0, 0·75% and 1·5%. These specimens are then tested by using the Hopkinson pressure bar technique. Their dynamic tensile strengths are measured under medium strain rate impact. In these tests, the stress histories are recorded by way of the strain gauges symmetrically mounted on the specimen and the post-test fracture modes are photographed. The test results indicate that the superposition of waves induces first compressive stress and then tensile stress. Once the tensile stress reaches its critical value, a spalling phenomenon occurs. Both volume fraction of fibres and strain rate exert a significant influence on the dynamic tensile strength of concrete. Furthermore, the strain-rate-induced enhancement of tensile strength is related to the compressive strength of the concrete matrix.

An Experimental Method to Determine the Tensile Strength of Concrete at High Rates of Strain

In the present work, dynamic tensile strength of concrete is experimentally investigated by means of spalling tests. Based on extensive numerical simulations, the paper presents several advances to improve the processing of spalling tests. The striker is designed to get a more uniform tensile stress field in the specimen. Three methods proposed in the literature to deduce the dynamic strength of the specimen are discussed as well as the use of strain gauges and a laser extensometer. The experimental method is applied to process data of several tests performed on wet micro-concrete at strain rates varying from 30 to 150/s. A significant increase of the dynamic tensile strength with strain-rate is observed and compared with data of the literature. In addition, post-mortem studies of specimens are carried to improve the analysis of damage during spalling tests.