Testing fibre-reinforced concrete in some structural applications

In this research the main focus is on the analysis of short dispersed fibres on the cement-based matrix. The aim is to test a few structural ideas and to compare the results with theoretical values. When considering fibre-reinforced concrete as a load-bearing structural material, the main interest lies on the post-crack tensile strength of the fibre composite. Fibres bridge over cracks. This post-crack strength gives the ductility, impact strength and beneficial cracking behaviour. Under bending load the effect of fibres can be observed with a lower fibre amount than under tensile load. Tests were performed in order to verify the feasibility of some structural ideas, which were inverted T-beam, low-pressure concrete pipe, and thin panel. Also, tests elsewhere in VTT are referred to, namely the crack control effect of fibres on beams and fibres as a shear reinforcement. The flange of the inverted T-beam has to be reinforced, because the flange supports the slabs. From the load test results one can draw a conclusion that the fibre amount should be near 1 volume per cent, or higher, to achieve the same level of ultimate load as with conventional reinforcement. For steel this means 30 % more than in the form of reinforcement bars. The idea of using steel fibre in concrete pipes is not new. The difficulties are in watertightness. With all the fibre-types used it is possible to fulfil the requirement of the external load. Tests with fibre-reinforced thin panels were carried out in order to study the differences in deformations due to the non-symmetry. The most important property here is the free shrinkage of concrete. Fibres tend to increase the load level at which visible cracking first appears. The average crack width of a fibre-concrete beam was about 75%, compared to that of beam without fibre. The depth of the crack was smaller with fibre specimens. The ultimate flexural capacity of a concrete beam with conventional reinforcement increases slightly when fibres are added to the concrete. The nature of fracture is brittle when fibres are used. This is because the post-crack strength of fibre-concrete is usually lower than the cracking strength. Also the fracture occurs due to the fibre pull-out. The fibres are dispersed randomly and this also increases the uncertaintity of capacity. The safety level has to be studied carefully case by case. The quality control methods have to be chosen correctly to ensure the desired material properties.