International Concrete Abstracts Portal

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.

Describes a laboratory investigation of an ultrasonic method that has the potential to become, through further research, a valuable tool for the nondestructive quality control of concrete during construction. The primary objective of the work is to characterize the development of internal structure of the portland cement paste portion of concrete from very early ages on by making use of the behavior of propagated ultrasonic pulses. To do that, however, a suitable ultrasonic method first had to be developed, since quite a few publications reported difficulties with such measurements in fresh pastes due to high attenuation. Velocity and attenuation of longitudinal ultrasonic pulses were measured at regular intervals in fresh concretes. The first measurements were usually performed 10 min after mixing and continued up to the age of 28 days. Three concretes of different compositions were tested. This paper concentrates on measurements at very early ages. The instrumentation, test setup, and testing procedure are described. The velocity and attenuation results, as well as their interpretation, are then presented. For instance, it is shown that the time of initial set is close to a minimum on the pulse velocity-versus-age relationship, as well as a maximum on the attenuation-versus-age relationship.

The design life of bridge structures is typically 50 years. As highway authorities increasingly consider using fiber reinforced plastics (FRP) to replace steel in reinforced or prestressed concrete structures exposed to aggressive environments, it becomes imperative to develop accelerated test procedures for assessing long-term performance. While acceleration principles for determining long-term material properties, e.g., creep rupture or relaxation, are well known, no similar principles have yet been formulated for determining properties that relate to the interaction between FRP and concrete, such as bond. This is of vital importance since material durability alone cannot guarantee satisfactory performance in concrete. Paper presents a rationale for conducting accelerated tests to evaluate the long-term bond and durability characteristics of pretensioned FRPs used in bridge applications. The principles enunciated are based on recent research findings that have been translated into test setups currently being used to evaluate the long-term performance of pretensioned aramid fiber reinforced plastic (AFRP) and carbon fiber reinforced plastic (CFRP) elements exposed to a marine environment. Preliminary results obtained are quite encouraging and appear to confirm the validity of the approach used. The experimental study is scheduled to end in 1995.

Various researchers have attempted to establish correlation between mechanical properties of brittle materials and ultrasonic measurements. Recently, extensive experiments, including ultrasonic scanning tests, strain gage tests, and combined ultrasonic scanning and strain gage tests, have been processed to study the degradation mechanisms and surface effects in concrete-like brittle materials. In this paper, attention is restricted to the variation of energy dissipation with external load level and the relationship between mechanically dissipated energy and ultrasonically dissipated energy for brittle materials under uniaxial compression. Some typical ultrasonic scanning readings are presented. The load-level-dependent relationship between ultrasonically dissipated energy and mechanically dissipated energy is identified and discussed. It is also pointed out that an energy-based degradation instability theory is verified qualitatively by the energy diagram obtained through the experiments. The findings may be applicable to concrete with minor modifications. However, further work would be necessary to draw a firm conclusion.

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.