A B S T R A C T R i~ S U M I ~Effects of fiber length on the tensile and flexural performance of cast and extruded PVA fiber reinforced cement composites were investigated. Microstructural characterization, image analysis, and statistical tools were used to study the influence of processing and fiber length on fiber-matrix bond, fiber dispersion and fiber orientation in the composites. In the extruded composites, shorter fibers improved the performance. In the cast composites, longer fibers gave the best performance. This contradictory trend was found to be a result of differences in fiber failure mechanism, fiber distribution and fiber orientation. Microstructural observations indicated a strong matrix-fiber bond for the extruded composites. Statistical quantification of image analysis indicated a better distribution and alignment of shorter fibers in extruded composites. Les effets de la longueur des fibres sur les performances de tension et deflexion des composites moulds et des composites extru

The in-situ testing of early age concrete strength is crucial for determining the time of form removal from concrete elements, opening highways to traffic or applying of prestress to steel reinforcement. A nondestructive ultrasonic technique, which measures the reflection loss of ultrasonic transverse waves at a concrete-steel interface, is presented in this paper. The focus is to compare wave reflection measurements on mortar and concrete to strength. It is shown that the reflection loss is linearly related to the strength gain of mortar and concrete at early ages. The experiments have revealed a relationship between the homogeneity of the tested materials and the consistency of the reflection measurements. The repetition of simultaneous measurements of wave reflections and compressive strength on mortar results in similar strength-reflection loss relationships. Multiple measurements on the same concrete gave multiple strength-reflection loss relationships. The accuracy of the strength predictions made with the proposed method is discussed and compared to other nondestructive test methods.

A one-sided, nondestructive, ultrasonic technique for monitoring the setting and hardening process of concrete has recently been developed. The technique is based on monitoring the reflection coefficient of ultrasonic transverse waves at the surlhce of a laboratory scale hardening concrete in steel molds. The technique has been shown to reliably estimate the rate of strength gain of concrete under isothermal and outdoor conditions. Results of an experimental study to investigate the influence of curing temperature and mix design on the rate of strength gain and the ultrasonic transverse wave reflection loss are presented in this paper. Simultaneous measurements of wave reflection loss and compressive strength have been performed on various concrete compositions under different curing conditions. The ultrasonic technique is shown to produce reliable estimates of the rate of strength gain at early ages. I~SUMEUne technique de contrtle de la cristallisation du btton a rdcemment 6t6 ddveloppte. Cette mtthode non destructive et ne requtrant qu'une seule surface d'essai est fondte sur l'ttude de la diminution de la rdflexion des ultrasons sur la surface du btton en cours de cristallisation. L'influence de la temptrature de cristallisation et de la formulation du btton sur la vitesse de durcissement et l'atttnuation de la rtflexion des ondes ultrasoniques transversales a 6t6 raise en 6vidence par le biais d'une 6tude expdrimentale dont les rtsultats sont prtsentts. La mtthodologie de cette 6rude consiste en la mesure simultante de la rtflexion des ultrasons et de la rtsistance en compression du btton sous difftrentes conditions de cristallisation. II a ainsi 6t6 dtmontr6 que cette technique de contr61e produisait des estimations fiables de la vitesse de durcissement du btton aux jeunes gges, dans un environnement isotherme comme sous des conditions exttrieures.