International Concrete Abstracts Portal

Presents an investigation to determine the within-test variability of various nondestructive test methods (NDT) and the correlation between NDT test results and the corresponding compressive strength of cores. The size effects of coarse aggregate on the variability and correlation were also evaluated. The NDT test methods evaluated in the test series include rebound hammer, pulse velocity, probe penetration, pullout, and CAPO (cut and pullout). Companion tests of field-cured standard cylinders and cores were also made at the ages when the NDT tests were made. Results show that the within-test variability of the in situ tests reported (except the pulse velocity test) is two to five times higher than that of the corresponding standard compression test and is affected significantly by the amount of coarse aggregate and its size. There is a good relationship between the results of in situ tests and the compressive strength. In general, the highest degree of correlation is for the pullout test followed by that for the CAPO (cut and pullout) test and rebound test, probe penetration test, and pulse velocity test.

Aim is to analyze the correlations between several nondestructive measured values (ultrasonic velocity and attenuation, rebound number) and the compressive strength of concrete. A computational program performs a step-by-step analysis. First, isolated linear correlations are established for each one of the three nondestructive tests. Then the results are compared with each other in the sense of a general multiple correlation of the values. Since the results obtained from the nondestructive tests are equally scattered, the program determines interactively, in a second step, the multiple coefficients of correlation and restarts the analysis several times by tentatively disregarding the presumably bad experimental results. Since the measured values also comprise a large spectrum of magnitude, limits of validity of the assumed correlations are investigated concomitantly with the process of analysis. A last step is performed to identify a tendency of deviation of the single and multiple correlations from the basic linear ones.

The fracture process of a composite material involves crushing or slippage of adjacent particles, microcracking, etc., resulting in changes in the load-versus-displacement behavior. A study of the fracture process is necessary to develop a rational material model. Laser holographic interferometry was applied to study the whole field deformation pattern. Speckle photography was used to measure quantitatively displacement discontinuities at bond cracks at various stages of loading. Acoustic emission (AE) techniques were applied to monitor microseismic activities resulting from the various fracture phenomena. The rate of microfracture was measured from the AE event rates. A source location algorithm was used to calculate the locations of the AE events. Specimens were made with different aggregate and void sizes to study their effect on crack patterns and load-displacement behaviors.

A new improved prototype ultrasonic pitch-catch (two-transducer) and pulse echo (one-transducer) system has been developed for concrete. Signal generation and detection is done with piezoelectric crystals. A literature search revealed that no piezoelectric pulse-echo system had been developed for the ultrasonic range ( > 20 kHz) and that pitch-catch measurements needed further development. No commercial system could be found on the market for making pitch-catch measurements. Research by the U.S. Army Engineer Waterways Experiment Station has resulted in the development of a 200-kHz pitch-catch system with a signal-to-noise ratio of 18 and a pulse-echo system with a SNR of 8. The mass and dimensions of the improved system have been reduced significantly from the prior state-of-the-art system. The WES system works well for thickness measurements of portland-cement concrete pavement and can indicate the presence of voids.

A nondestructive test method has been developed for locating defects in concrete. The technique is referred to as the impact-echo method and is based on monitoring surface displacements resulting from the interactions of transient stress waves with internal discontinuities. Paper describes the technique and presents results of laboratory studies designed to evaluate the capabilities of the method. These laboratory studies were carried out on 500 mm thick slabs that contained a variety of artificial flaws embedded at known locations. Frequency analysis of recorded time-domain waveforms is explained and shown to be a quick and simple signal processing technique. Finally, results are presented from a field study in which the impact-echo method was used to investigate a 150 mm thick slab believed to contain voids.