An experimental method for dynamic tensile testing of concrete by spalling

: For reinforced concrete structures subjected to blast effects, response at very high strain rates (up to 1000 s(-1)) is often sought. At these high strain rates, the apparent strength of concrete can increase significantly. The dynamic increase factor (DIF), i.e. the ratio of the dynamic to static strength, is normally reported as function of strain rate. For concrete, the DIF can be more than 2 in compression, and more than 6 in tension. Knowledge of the DIF is of significant importance in the design and analysis of structures for explosives safety. DIF curves for concrete have been published in manuals by the Tri-Services, the Defense Special Weapons Agency, the Air Force, and the Department of Energy. However, these curves are typically based on limited data. A literature review was conducted to determine the extant data to characterize the effects of strain rate on the compressive and tensile strengths of concrete. This data support the dynamic increase factor (DIF) being a bilinear function of the strain rate in a log-log plot. The DIF formulation recommended by the European CEB was described, together with its origins. For tension, it was found that the data differed somewhat from the CEB recommendations, mostly for strain rates beyond 1 s(-1), and an alternate formulation was proposed based on the experimental data.

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Dynamic Increase Factors for Concrete

Experimental characterization of concrete in dynamic tension

We investigate the dynamic behavior of concrete in relation to its composition within a computational framework (FEM). Concrete is modeled using a meso-mechanical approach in which aggregates and mortar are represented explicitly. Both continuum phases are considered to behave elastically, while nucleation, coalescence and propagation of cracks are modeled using the cohesive-element approach. In order to understand the loading-rate sensitivity of concrete, we simulate direct tensile-tests for strain rates ranging 1–1000 s−1. We investigate the influence of aggregate properties (internal ordering, size distribution and toughness) on peak strength and dissipated fracture energy. We show that a rate independent constitutive law captures the general increase of peak strength with strain rate. However, a phenomenological rate-dependent cohesive law is needed to obtain a better agreement with experiments. Furthermore, at low rates, peak strength is sensitive to the inclusions' toughness, while the matrix dominates the mechanical behavior at high rates.

Influence of Meso-structure on the Dynamic Fracture Simulation of Concrete Under Tensile Loading

A mesoscale fracture model for concrete

An experimental investigation of the effects of strain rate and size on the tensile strength of concrete was conducted. Concrete specimens of different sizes were tested in the splitting tension configuration at strain rates ranging from 10 to 70 per second. Six different specimen sizes with thicknesses of 0.25 and 0.50 in., and diameters of 0.5, 1.0 and 2.0 in. were tested. A split Hopkinson bar was used to conduct the dynamic tests. Elastic verification tests were performed on specimens instrumented with strain gages at quasi-static and dynamic rates. High speed photography of dynamic tests showed that cracking initiated in the region of maximum tensile stress. The splitting tensile strength of concrete was both size and rate dependent. In this study, the ratio of dynamic to quasi-static strength was about 4.5 at strain rates of 70 per second.

R. John

Papers

Effect of strain rate and size on tensile strength of concrete