Discusses important issues relevant to high-rate closed-loop testing of reinforced concrete beams. To obtain a high rate of loading from a closed-loop machine, special considerations are required in the design as well as operation of the machine. These issues are discussed briefly. Useful insight into behavior of a specimen in a high-rate closed-loop test is provided by some analytical expressions supplied here for single-degree-of-freedom (SDOF) and multiple-degree-of-freedom (MDOF) specimen systems. Advantages of displacement control over load control are apparent from the expressions obtained. Preliminary results of displacement-controlled tests conducted on reinforced concrete beams at low and high rates are reported. The specimen deformation-versus-time curve in these tests indicates that, for this setup, the test machine used in this project can apply an essentially constant velocity. Crack pattern obtained for the beams as well as inspection of load and specimen deformation signals indicate that the manner of loading was quasi-static (that is, free of inertial effects) even for the high-rate case. The load-deflection curve for the high-rate case exhibits a down-sloping portion after a small plateau.

Carbon fiber glass fiber reinforced plastic (CFGFRP) is used in concrete structures as a reinforcement material. Appropriate materials design indicates that CFGFRP should be a hybrid of a conductive material with a small ultimate elongation value and an insulating material with a large ultimate elongation value. In the present study, the authors evaluated three types of carbon fiber tows used in CFGFRP composites. They observed a very clear and significant change in electrical resistance at the transition point where carbon fiber tows fractured, and found that this point could be easily controlled though the use of carbon fibers with different ultimate elongation values. The electrical resistance characteristics of CFGFRP-reinforced concrete change along with changing loads. Furthermore, a permanent residual electrical resistance could be observed after the removal of load, and its change was dependent on the maximum load applied. The information on the fracture position was obtained by the arrangement of the CFGFRP composites. Monitoring changes in electrical resistance during and after loading is thus a promising method for anticipating the fracture of CFGFRP-reinforced concrete.

Mortar and concrete samples were subjected to uniaxial compression to determine whether it is possible to distinguish two states of a sample: prior to the subjection to ultimate load, and subsequent to loading but prior to the appearance of visible surface cracks. Stress-strain characteristics, Young's moduli, and frequency characteristics of ultrasonic waves propagating through the samples were studied for each specimen. The qualitative analysis of frequencies and amplitudes of the peaks in resonant P-wave spectra allow the determination of undamaged specimens. The spectral analysis of continuous ultrasonic waves allows the possibility of discovering the specimen damaged by ultimate stress but visually intact.

Describes tests that were performed to evaluate the feasibility of using the impact-echo method to evaluate the in-place strength of concrete in plate-like elements such as slabs and walls. In the impact-echo method, a stress pulse is introduced into an object by mechanical impact on its surface, and this pulse undergoes multiple reflections (echoes) between opposite faces of the object. The surface displacement of the object, caused by the reflected pulse, is monitored at a location adjacent to the point of impact, and the frequency of successive arrivals is determined. Knowing the thickness of the test object, the compression wave (P-wave) velocity is determined. A previously established concrete strength-P-wave velocity relationship can be used to estimate in-place strength. Results indicate that the impact-echo method can be used to determine P-wave velocity through a large volume of early-age concrete such as the slab specimens tested in this study. Use of the impact-echo method to nondestructively estimate the in-place strength of concrete is more appropriately limited to the estimation of early-age strength.

Describes a large-scale pendulum device for testing reinforced concrete structures under impact loading. Details of the facility, including instrumentation of specimens and pendulum mass, are provided. Sample test results relative to full-scale bridge barriers and beams are presented. These tests show differences between responses under static and dynamic loads.