International Concrete Abstracts Portal

Fiber-reinforced cement for piezoelectricity and pyroelectricity is introduced, as these phenomena are useful for the sensing of strain and temperature. The use of short steel fibers (8 µm diameter), together with polyvinyl alcohol, as admixtures greatly enhances these effects, thereby attaining longitudinal piezoelectric coupling coefficient 3 x 10-" mN (10kHz), and pyroelectric coefficient 6 x 10$ C/mz.K (10 kHz). The piezoelectric effect is comparable in magnitude to that of PZT. However, due to the high value (2,500) of the relative dielectric constant, the piezoelectric voltage coefficient and pyroelectric voltage are comparable to or even lower than those of plain cement paste or carbon fiber (15 µm diameter) cement paste. Carbon fiber cement paste and plain cement paste are comparable in the piezoelectric coupling coefficient, piezoelectric voltage coefficient and pyroelectric voltage, though the pyroelectric coefficient is higher for carbon fiber cement paste than plain cement paste.

The tensile stress-strain response of cement composites reinforced with newly developed twisted polygonal steel fibers (identified here as Torex fibers) is investigated in this study. The new fibers are produced from a prototype machine developed for this research. Parameters investigated are: 1) volume fraction of the fibers (Vf=1 % to 4%); 2) fiber equivalent aspect ratio (L/de = 60, 80 and 100); and 3) the compressive strength of the mortar matrix (20, 44, 68 and 84 MPa). It is observed that high performance fiber reinforced cement composites that exhibit strain hardening and multiple cracking behaviors can be obtained with a proper combination of these parameters. A comparison with hooked and straight steel fibers is also provided. The new Torex fibers leads to a significantly higher composite performance in terms of strength, ductility and cracking behavior.

Creep of concrete is composed of basic and drying creep components, and the drying creep is primarily caused by two mechanisms: stress induced shrinkage and microcracking. The effects of steel fibers on basic creep, stress induced shrinkage, and microcracking components for two concrete mixtures with w/c of 0.4 and 0.5 are discussed. The steel fibers were found to enhance the basic creep mechanisms and reduce the drying creep mechanisms. The reduction in drying creep offsets the increase in basic creep leading to a net tensile creep similar for both plain concrete and FRC; a conventional conclusion that usually obscures the role of fibers on shrinkage stress relaxation and cracking. A new look that is consistent with material behavior is introduced by dividing the creep mechanisms into beneficial aspects associated with real creep mechanisms and detrimental aspects associated with apparent creep mechanisms (microcracking). Basic creep and stress-induced shrinkage are real creep mechanisms associated with deformation of hydration products, while microcracking is detrimental because of the associated microstructural damage. Steel fibers tend to enhance the beneficial mechanisms and reduce the detrimental ones, thus enhancing the overall performance. The results explain the difference in shrinkage cracking between plain concrete and FRC; an insight that would not be achieved by looking at total tensile creep alone.

The increased use of high strength concrete (HSC) in buildings has raised concerns regarding the behaviour of such concrete in fire. Spalling at elevated temperatures and the resulting reduction in fire resistance is of particular concern. In this paper, the various issues relating to spalling and its impact on fire resistance are discussed. The mechanism of spalling is explained and the main causes for the occurrence of spalling in HSC are described. The various parameters that influence spalling in HSC under fire conditions are discussed. Results from the experimental studies are presented to illustrate the effectiveness of polypropylene fibers in minimising spalling in HSC structural members.

Dynamic fracture studies on fiber reinforced cement-based composites were conducted. Contoured double-cantilevered beam specimens were subjected to one rate of quasi-static loading and three rates of impact loading by using a fully instrumented drop weight impact machine. Steel and polypropylene fibers at two dosage rates were investigated, and an analytical scheme was developed to provide the inertial correction to measured loads and obtain crack growth resistance curves (KR Curves) under static and impact loading. KR-Curves were observed to be highly stress-rate sensitive. Comparison between steel and polypropylene fibers indicated a superior performance of the steel fiber under quasi-static loading, but under impact loading, the polypropylene fiber appears to come up to the level of steel fiber.