International Concrete Abstracts Portal

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.

A new type of short-span bridge system has been developed and implemented over the Albermarle Sound south of Edenton, North Carolina. The new system incorporates precast flat-slab sections that are post-tensioned for continuity. The new system has the potential to replace traditional trestle-type bridges constructed using simple-span prestressed beams with a cast-in-place deck. A continuous two-span, half-scale model of the bridge system was built and tested under various load conditions. The bridge was evaluated analytically and experimentally for the transfer load case (dead load plus prestress), the maximum negative moment service load case, cracking load, and ultimate load. The model bridge performed as expected for all cases. Comparisons between analytical and physical models showed good correlation for all types of tests. At service load levels, the bridge exhibited a linear elastic response with no evidence of cracking. The ultimate load and deflections of the new bridge system were readily predicted by standard behavioral models for prestressed concrete.

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.

Describes details of an extensive monitoring program carried out during the strengthening of the Grand Mere Bridge, a cast-in-place post-tensioned segmental box girder structure built in 1977. The testing program comprised various measurements taken before, during, and after the prestressing application. Electrical strain gages, mechanical strain gages, vibrating wire gages, and thermocouples were among the measuring instruments used. A bridge testing data acquisition system in a vehicle and an autonomous data acquisition system were used, together with manual reading devices. The field measurement program was carried out during strengthening. Some instruments used allow the monitoring of the bridge over a long-term period.

This review paper discusses current and near-future technologies for measurement of full-scale structural performance. Modern instrumentation and measurement methodologies can provide signals and data for use in evaluating the performance of civil structures. Applications exist in model verification, extreme event monitoring, health monitoring, and serviceability monitoring. Examples are provided, and future developments are discussed.