Experimental techniques commonly used for high strain-rate testing of concrete in compression, together with the methods used for measurement and recording of stress and strain, are critically assessed in the first part of this paper. The physical capability of each loading method is discussed and some consideration is given to the definitions used for specifying the loading rate. The second part reviews the dynamic compressive strength (mostly uniaxial rather than bi- or triaxial) of plain concrete, while in the third part a review on deformation behaviour indicates that uncertainty and disagreement exist concerning changes in axial strain at high strain rates.

Compressive behaviour of concrete at high strain rates

The purpose of this review is to discuss the development and the state of the art in dynamic testing techniques and dynamic mechanical behaviour of rock materials. The review begins by briefly introducing the history of rock dynamics and explaining the significance of studying these issues. Loading techniques commonly used for both intermediate and high strain rate tests and measurement techniques for dynamic stress and deformation are critically assessed in Sects. 2 and 3. In Sect. 4, methods of dynamic testing and estimation to obtain stress–strain curves at high strain rate are summarized, followed by an in-depth description of various dynamic mechanical properties (e.g. uniaxial and triaxial compressive strength, tensile strength, shear strength and fracture toughness) and corresponding fracture behaviour. Some influencing rock structural features (i.e. microstructure, size and shape) and testing conditions (i.e. confining pressure, temperature and water saturation) are considered, ending with some popular semi-empirical rate-dependent equations for the enhancement of dynamic mechanical properties. Section 5 discusses physical mechanisms of strain rate effects. Section 6 describes phenomenological and mechanically based rate-dependent constitutive models established from the knowledge of the stress–strain behaviour and physical mechanisms. Section 7 presents dynamic fracture criteria for quasi-brittle materials. Finally, a brief summary and some aspects of prospective research are presented.

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A Review of Dynamic Experimental Techniques and Mechanical Behaviour of Rock Materials

Classic Split-Hopkinson Pressure Bar Testing

Hopkinson bar experimental technique: A critical review

A large diameter split Hopkinson pressure bar (SHPB) has been developed. This equipment is briefly described, together with a shaped striker that initiates a half-sine incident waveform to obtain the complete stress-strain relationship of the Bukit Timah granite at medium strain rate. Good constant strain rate was derived, and the dynamic complete stress-strain curves and energy absorption of the granite were measured at a strain rate between 20 and 60 per second. Repeated impact between 60–90% of the static strength of the granite was also conducted. Results from the tests show that the cumulative damage of the granite depends on the peak stress of the dynamic loads with a fixed duration. The dynamic fracture strength of the granite loaded at medium strain rate is directly proportional to the cube root of the strain rate. For the granite loaded at this strain rate, Young’s modulus is unchanged. Energy absorption of the samples loading to fragmentation determined its fragmented size distribution. At high strain rate, the rock possesses large energy absorption and the particle size of the fragments is much smaller.

Dynamic Characteristics of Granite Subjected to Intermediate Loading Rate

Dispersion of an elastic wave propagating in a 76.2-mm-diameter (3 in.) Split Hopkinson Pressure Bar system was investigated with two consecutive pulses recorded in the transmitter bar. Assuming that the dispersive high frequency oscillatory components riding on the top of the main pulse originate from the first mode vibration, the dispersion was corrected by using the Fast Fourier Transform and Fourier series expansion numerical schemes

Dispersion Investigation in the Split Hopkinson Pressure Bar

Finite-element codes for structural response of reinforced concrete use as a parameter the unconfined compressive strength of the concrete, f c ', which is sometimes increased by an arbitrary factor for dynamic loading. The objective of this research is to determine the rate dependence of f c ' and eventually to model the rate-dependent constitutive behavior. Results of tests with a small Kolsky bar system and of a newly built larger system on concrete with a maximum aggregate size 1/2 inch are reported with strain rates at the maximum stress from 50 to 800/sec for mortar and from 5 to 120/sec for concrete. An apparent rate dependence up to almost twice the static strength is observed for both. The mortar shows an apparent linear dependence, while the high-strength concrete shows an approximately logarithmic dependence on the strain rate at the maximum stress, over the dynamic range observed. Some questions about specimen size effects and about how much of the apparent strain-rate effect is really a lateral inertia confinement effect are as yet unresolved. Continuing research is focused on observation of the lateral motion to assess lateral inertia effects in unconfined specimens and on passive confinement by steel jackets. Future efforts will be directed toward constitutive modeling.

Dynamic Compressive Strength of Cementitious Materials

: Large diameter Split-Hopkinson Pressure Bar (SHPB) systems required a correction of the strain signals to account for wave dispersion Using a Fast Fourier transform (FFT) algorithm a wave dispersion correction program was developed for the 30 inch diameter SHPB at the University of Florida and the 2 0 inch diameter SHPB at Tyndall AFB FL Since the data are applicable to only these two systems the report is published as a user's manual for these two systems

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA201314&Location=U2&doc=GetTRDoc.pdf

J. C. Gong

Papers

Dispersion Investigation in the Split Hopkinson Pressure Bar

Dynamic Compressive Strength of Cementitious Materials

Dispersion Correction for Split-Hopkinson Pressure Bar Data