C. Allen Ross

REVIEW OF STRAIN RATE EFFECTS FOR CONCRETE IN TENSION

A literature review was conducted to determine the extant data characterizing the effects of strain rate on the tensile strength of concrete. In particular, additional new data by Ross and colleagues were considered. These data support the dynamic increase factor (DIF) being a bilinear function of the strain rate in a log-log plot, with no increases for strain rates below 0.000001 with a slope change at a strain rate of 1/s. A DIF of approximately 7 was obtained at the highest reported experimental strain rate of 157/s. The DIF formulation recommended by the European CEB was described, together with its origins. It was found that the data differed somewhat from the CEB recommendations, mostly for strain rates beyond 1/s. Therefore, an alternate formulation was proposed based on the experimental data.

Strengthening of Reinforced Concrete Beams with Externally Bonded Composite Laminates

A particularly challenging problem confronting engineers in the revival of U.S. infrastructure is the rehabilitation of concrete structures. This paper summarizes the results of experimental and analytical studies concerning the flexural strengthening of reinforced concrete beams by the external bonding of high-strength, lightweight fiber reinforced plastic (FRP) plates to the tension face of the beam. Twenty-four large-scale beams were tested experimentally to evaluate the strength enhancement provided by the FRP plates. An inelastic section analysis procedure was developed that accurately predicts the load displacement response of the retrofitted beams. A nonlinear finite element method analysis was also conducted that corroborates the results from the experimental study and inelastic section analysis.

Strain rate effects on dynamic fracture and strength

An experimental procedure and accompanying theoretical analysis is presented to produce a well-characterized technique for quantifying dynamic fracture properties of quasi-brittle materials. An analytical and experimental investigation of mode I fracture of concrete was conducted under the dynamic loading of a split Hopkinson pressure bar. Fracture specimens in the form of notched-cavity splitting tension cylinders were subjected to stress wave loading that produced strain rates nearing 10/s. Fracture parameters were extracted by the application of the two-parameter fracture model, a nonlinear fracture model for quasi-brittle materials. Finite element analysis verified the experimental configuration and addressed inertial contributions within the dynamic environment. Ultra-high-speed digital photography was synchronized with the fracture process to provide additional validation and insight to the experimental technique. Results show that the effective fracture toughness and specimen strength both increase significantly with loading rate. The numeric and photographic results validate the experimental technique as a new tool in determining rate dependent material properties.

A strain-rate-dependent concrete material model for ADINA

Abstract The analysis, design and/or evaluation of protective structures and facilities for military use demands the accurate determination of material and structural response to high-intensity, short-duration impulse loadings. There currently exists a preponderance of data supporting increased strength characteristics in concrete, the primary construction material for protective facilities, at high strain rates. This paper summarizes the modification of the nonlinear concrete material model currently employed in the ADINA finite-element computer programs to account for high strain rate effects. The resultant strain-rate-dependent concrete material model encompasses the strain-rate range from 10 −7 s −1 (quasi-static) to 10 3 s −1 , in both compression and tension.

Numerical analysis of dynamic split cylinder tests

Abstract To investigate the effects of strain rate on the tensile strength of concrete, split cylinder tests of plain concrete specimens were conducted on a Split-Hopkinson Pressure Bar (SHPB). To ascertain the stress condition in the material specimens at failure, a comprehensive finite element method (FEM) study was conducted on the SHPB experiments. Both linear and nonlinear analyses were performed. From the results of the numerical analyses, the dynamic states of stress occurring in the split cylinder prior to failure as well as the mode of failure are revealed.

Effect of Compressive Stress on Longitudinal Wave Speed in Cementitious Material

The Air Force has been interested for some time in the development of computer codes that accurately predict the penetrator trajectory created when munitions are fired into concrete and geomaterial targets, as well as the resulting depth of penetration. Recent work has focused on experimental research performed to determine quasistatic, dynamic, unconfined and confined material properties for development of an elastic/viscoplastic constitutive equation. This constitutive equation has shown some promise in predicting stress and strains but lacks a consistent damage parameter to predict damage or fractures exhibited by the target material during experimental impact tests. Current damage level predictors that employ a scalar damage parameter are not sufficient to predict the directional damage or fracture that occurs in simple uniaxial compression tests of concrete and geomaterials. Tensorial or directional damage parameters coupled with constitutive relations are necessary for better understanding and accurate prediction of damage exhibited when munitions impact concrete and geomaterials. The primary objective of the study described herein was to identify, quantify and characterize damage parameters associated with certain constitutive responses of cementitious and geologic materials. To that end, longitudinal wave speed and biaxial strain data were collected simultaneously on a series of grout cubes as they were being loaded to failure in uniaxial compression. The results of these tests, and a comparison to existing related data [1, 2] are presented.Copyright