Fibre-reinforced Concrete for Industrial Construction - a fracture mechanics approach to material testing and structural analysis

Doktorsavhandling, 2005

ABSTRACT More efficient and industrialised construction methods are both necessary for the competitiveness of in-situ concrete and essential if the construction industry is to move forward. At present, the expenditure on labour (preparation and dismantling of formwork, reinforcing, and casting and finishing of concrete) almost equals the cost of material. Fibre-reinforced concrete (FRC) extends the versatility of concrete as a construction material, offers a potential to simplify the construction process and, when combined with self-compacting concrete, signifies an important step towards industrial construction. However, a barrier to more widespread use of FRC has been the lack of general design guidelines which take into account the material properties characteristic of FRC, i.e. the stress-crack opening ( σ-w) relationship. The presented work has been focused on FRC, showing a strain-softening response, and the interrelationship between material properties and structural behaviour. This has been examined by investigating and developing test methods and structural analysis models. A systematic approach for material testing and structural analysis, based on fracture mechanics, has been presented which covers: (1) material testing; (2) inverse analysis; (3) adjustment of the σ-w relationship for fibre efficiency; and (4) cross-sectional and structural analysis. Furthermore, recommendations for using the wedge-splitting test (WST) method for FRC have been provided. The relative small scale of the WST specimens makes it ideal for use in laboratory studies, e.g. for development and optimisation of new mixes. By conducting experiments, the approach was demonstrated and it was shown that it is possible to adjust the σ-w relationship for any difference in fibre efficiency between the material test specimen and the structural application considered. Full-scale experiments were conducted on beams, made of self-compacting fibre-reinforced concrete, with a small amount of conventional reinforcement. The results indicate that FRC can be used in combination with low reinforcement ratios; the amount of reinforcement could be reduced to half that of conventional reinforced concrete (for the same load-carrying resistance) but still lead to improved structural performance (reduced crack width and increased flexural stiffness). The results also suggest that the approach used for the material testing provides the necessary properties to perform analyses based on nonlinear fracture mechanics. Finally, when comparing the peak loads obtained in the experiments with the results from the analyses, the agreement was good, with a high correlation (>0.9). Hence, this demonstrates the strength of the fracture-mechanical approach for material testing and structural analysis.