International Concrete Abstracts Portal

The purpose of the present investigation was the study of the effect of specimen size on the compressive strength of concrete reinforced with steel spirals and with fibers. Concrete cylinders of different sizes were loaded under uniaxial compression. The cylinders, ranging in size from 60 x 120 mm to 100 x 400 mm, were reinforced with short fibers (steel and polyolefin) at volume percentages of 1% and 2%. Some specimens also contained transverse steel reinforcement (0 5 mm) at pitches of 25 and 50 mm. There was a reduction in compressive strength as the specimen size increased in all cases (plain concrete, fiber reinforced concrete or spirally reinforced concrete). The effect of the fibers and of the steel spirals was a reduction in the brittleness of composite with increasing size of the specimens. The experimental results confirm that the size effect laws proposed in the literature for compressive strength provide a satisfactory prediction of the reduction in compressive strength with increasing specimen size.

A design method for determining the capacity of slab-column connections made with steel fibre concrete at ultimate load is presented. The proposed design equation is based on the authors' theoretical analysis, which considers the physical behaviour of the connections under load and is therefore applicable to both lightweight and normal weight concrete as well as to concrete without fibres. The design equation incorporates the effects of fibre reinforcement on resisting the upward movement of flexural cracking and of increasing the concrete tensile stress. Furthermore, it simplifies the calculation of the neutral axis depth still accounting for the steel strain hardening effect. The approach does not employ fitting factors to match the predictions to experimental data. However, a depth correction factor is used to account for the size effects. The proposed design equation is applied to predict the ultimate punching shear strength of sixty two slab-column connections tested by authors and other investigators, involving a wide range of fibre variables, concrete type and strength, tension steel ratio, size of slab and loaded area. The comparisons between computed values and the experimentally observed values are shown to validate the proposed design equation.

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.

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.

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.