Shengxing Wu

Effect of Testing Method and Strain Rate on Stress-Strain Behavior of Concrete

The flexural behavior of concrete is often different from its behavior under direct tension. This paper presents an experimental program aimed at the testing method and strain rate effects on the tensile behavior of concrete. Concrete specimens were tested with different testing methods (direct tension and four-point loading) and four strain rates (10(−6), 10(–5), 10(–4), and 10(−3) s(−1)). The results show that the peak stresses increase with an increase in the strain rate; the strain rate dependence of the peak stress is stronger for direct tensile specimens than for four-point loading specimens. The dynamic increase factor is expressed as a function of the strain rate and highly stressed volume method. The continuous damage model is also used to predict the strain rate effects on the ascending stress-strain behavior of concrete. Using the model parameters for the stress-strain curve of concrete at a static strain rate, the stress-strain curves of concrete for dynamic strain rates in four-point loading and direct tension are satisfactorily predicted.

Experimental Study on Dynamic Tensile Strength of Cement Mortar Using Split Hopkinson Pressure Bar Technique

AbstractThe dynamic characterization of cement-based materials under high strain rates is fundamental to understand the material behavior in case of heavy earthquakes and dynamic events. Abram’s law, which was originally formulated for cement-based materials under static loading, is not directly applicable to the dynamic loading condition. In this paper, a modified relationship has been proposed to evaluate the tensile strength of cement mortar. An extensive experiment was carried out to determine the effect of strain rate and water-to-cement (w/c) ratio on tensile strength of cement mortar. The dynamic characterization of tensile strength has been carried out by the split Hopkinson pressure bar at high strain rates. The results of the tests show a significant strain rate–sensitive behavior, exhibiting dynamic tensile strength of cement mortar increases with an increase in strain rate. The influence of w/c ratio on dynamic increase factor is minor.

Strength Values of Cementitious Materials in Bending and Tension Test Methods

A value of tensile strength of cement-based materials is required for the design of pavements, for the analysis of concrete dams to prevent thermal cracking and sometimes for the design of prestressed beams. Cementitious materials exhibit a significant variability in their tensile strength, depending on the test method used. The three common test methods are direct tension, three-point bending, and four-point bending, and it is commonly found that bending tests yield higher strengths than direct tension tests. During this investigation, tests were carried out on cement mortar and concrete for each type of test. A two-parameter Weibull model was found to fit each individual set of data well. For both cement mortar and concrete, the Weibull moduli calculated for the test methods were approximately the same. The Weibull modulus is used to relate the values of strength obtained with different test methods, using the Weibull effective volume concept, the predicted results are in remarkably good agreement with the experimental results.

Compressive Strength of Concrete Cores under High Strain Rates

AbstractA pulse-shaped split Hopkinson pressure (SHPB) was employed to determine the dynamic compressive mechanical responses of concrete cores. The loading pulses in SHPB experiments were precisely controlled to ensure that the core specimen deforms at a nearly constant strain rate under dynamically equilibrated stress during compression. A modified two-parameter Weibull distribution was used to analyze the test data. The Kolmogorov-Smirnov goodness-of-fit test was used to decide whether test data come from a population with this distribution. On the basis of the test data, Kolmogorov-Smirnov goodness-of-fit test, and probability plot, it is found that the modified Weibull model can be applied to compressive strength for concrete cores. In addition, the strain rate effect on the compressive strength of cores can be accurately predicted from the modified Weibull model.

Experimental Study on Split Hopkinson Pressure Bar Pulse-Shaping Techniques for Concrete

AbstractThe dynamic compressive behavior of concrete is investigated using a Φ74-mm variable cross section straight taper split Hopkinson pressure bar (SHPB) apparatus in this study. In order to reduce and eliminate the waveform dispersion effect, two pulse shaping techniques are adopted to improve the incident wave shape. The two techniques include changing the striker (length, shape) and copper pulse shapers (diameter, thickness, shape). Experimental results show that the pulse shaping effect with the small diameter of copper shaper is better than that of the tapered striker and annular pulse shaper, all of which could reach dynamic stress equilibrium, and reduce the high-frequency oscillation, and achieve constant strain-rate deformation approximately.

Variability of Compressive Strength of Concrete Cores

AbstractA comprehensive test program was conducted on the compressive strength of concrete cores. The tests involved eight mixes of concrete. Because over 200 tests were conducted, it was possible to undertake an analysis of the concrete cores using the probabilistic treatment of strength. The present work reports a comparative study of alternative probabilistic models to describe the compressive strength of concrete cores. A large class of probability models including two-parameter Weibull, three-parameter Weibull, normal, lognormal, and gamma distributions were validated using test data. This information is useful in the theoretical description of concrete failure. Furthermore, the results were compared in terms of modified Kolmogorov-Smirnov, log-likelihood, and minimum chi-square criterion. The results suggested that none of the described probability methods are adequate for determining the variability of the compressive strength of concrete cores.

Effect of Fly Ash on Deformation of Roller-Compacted Concrete

In this paper, the effects of fly ash on the strength, shrinkage strain, and deformation of roller-compacted concrete have been described. The results show that fly ash can decrease the shrinkage strain and deformation of roller-compacted concrete. Thus, the cracks in a large concrete dam may be reduced when 50% fly ash is used.

Experimental Study on Acoustic Emission Characteristics of Concrete Failure Process Under Uniaxial Tension

Tension is the main mechanical form for concrete structures. The characteristics of concrete failure process under tension is an important part of concrete failure mechanism study [1], because tension of concrete is weak and concrete tends to crack. The post-peak softening response of concrete in direct tension seems primarily related to widening of a single crack, and the post-cracking resistance of concrete may be due to discontinuities in cracking at the submicroscopic level and to bridging of cracked surfaces by aggregates. Acoustic emission, a kind of real-time technique, detects microcrack activities in high sensitivity, thus it is widely used in concrete study. In this paper, the development and evolution of microcracks are studied by analyzing the AE signals of concrete specimens under uniaxial tension.

Closure to “Effect of Testing Method and Strain Rate on Stress-Strain Behavior of Concrete” by Xudong Chen, Shengxing Wu, Jikai Zhou, Yuzhi Chen, and Aiping Qin

Xudong Chen; Shengxing Wu; Jikai Zhou; Yuzhi Chen; and Aiping Qin Lecturer, College of Civil and Transportation Engineering, Hohai Univ., Nanjing 210098, China. E-mail: cxdong1985@hotmail.com Professor, College of Civil and Transportation Engineering, Hohai Univ., Nanjing 210098, China. E-mail: sxwuhhu@hotmail.com Professor, College of Civil and Transportation Engineering, Hohai Univ., Nanjing 210098, China (corresponding author). E-mail: zhoujikaihhu@ hotmail.com; zhoujikaihhu@163.com Doctoral Student, College of Civil and Transportation Engineering, Hohai Univ., Nanjing 210098, China. E-mail: 596734202@qq.com Former Graduate Student, College of Civil and Transportation Engineering, Hohai Univ., Nanjing 210098, China. E-mail: 191382266@qq.com