Ultrasonic Monitoring of Setting and Strength Development of Ultra-High-Performance Concrete
In this study, the setting and tensile strength development of ultra-high-performance concrete (UHPC) at a very early age was investigated by performing the penetration resistance test (ASTM C403), as well as the direct tensile test using the newly developed test apparatus, and taking ultrasonic pulse velocity (UPV) measurements. In order to determine the optimum surface treatment method for preventing rapid surface drying of UHPC, four different methods were examined: plastic sheet, curing cover, membrane-forming compound, and paraffin oil. Based on the test results, the use of paraffin oil was found to be the best choice for measuring the penetration resistance and the UPV, and attaching the plastic sheet to the exposed surface was considered to be a simple method for preventing the rapid surface drying of UHPC elements. An S-shaped tensile strength development at a very early age (before 24 h) was experimentally obtained, and it was predicted by a power function of UPV. Lastly, the addition of shrinkage-reducing and expansive admixtures resulted in more rapid development of penetration resistance and UPV of UHPC.
Measurement of the setting time and strength of concrete by the impact-echo method
Tests were performed to evaluate the feasibility of using the impactecho method to determine setting time and monitor strength development of concrete. In the impact-echo method, the test object is subjected to point impact and the surface displacement adjacent to the impact point is monitored. From the measured displacement waveform and the thickness of the object, the P-wave velocity is determined. Changes in the P-wave velocity with time reveal information about the development of mechanical properties . Setting time tests ware made on concrete mixtures of two water -cement ratios and with and without set-controlling admixtures. A strong correlation was found between the time of initial setting of mortars sieved from the concrete, as determined by penetration resistance (ASTM C 403), and the time when the P-wave velocity began to increase. Two approaches for using the impactecho method to define the setting time of concrete are presented. Tests were performed to examine the relationship between P-wave velocity, as determined by the impact-echo method, and the compressive strength of concrete. At early ages (up to about 3 days at standard temperature), the relationship was independent of curing temperature and water cement ratio. It is concluded that the impact-echo method is a promising technique for nondestructively monitoring the development of mechanical properties in concrete from initial setting to ages of several days.
Testing the Core Empirical Implications of Gottfredson and Hirschi's General Theory of Crime
In A General Theory of Crime, Gottfredson and Hirschi propose that low self-control, in interaction with criminal opportunity, is the major cause of crime. The research reported in this article attempts to test this argument while closely following the nominal definitions presented by Gottfredson and Hirschi. A factor analysis of items designed to measure low self-control is consistent with their contention that the trait is unidimensional. Further, the proposed interaction effect is found for self-reported acts of both fraud and force (their definition of crime). Inconsistent with the theory are (a) the finding that criminal opportunity has a significant main effect, beyond its interaction with low self-control, on self-reported crime and (b) the substantial proportion of variance in crime left unexplained by the theoretical variables. Suggestions are offered for modifying and expanding the theory.