Fibres in reinforced concrete structures - analysis, experiments and design

Licentiate thesis, 2008

Potential benefits from fibres in concrete are improved crack control and the possibility of more slender structures. The extent of the crack control depends, among other factors, on the amount of fibres added, and plays a great role for durability. As of today there exist no generally accepted design and analysis procedures, and if the technique with fibres is to move forward, there is a need for development of such methods. As a part of the present work, an investigation of currently available design methods (proposed) was made. In addition a selection of analysis methods for fibre-reinforced concrete specimens in bending was studied. The main characteristics and comparisons of the investigated design and analysis methods are presented in a report Jansson (2007), and also in an article based on the report, Jansson (2008). Although several technical committees have proposed design methods, these methods are mainly intended for design in the ultimate limit state (ULS). Therefore, in order to control and understand crack growth in fibre-reinforced concrete, methods aimed at serviceability limit state design are needed. The present work has been carried out with this in mind and the aim, in the long run, is to develop a method which can be used to predict crack widths, i.e. small crack widths relevant for the serviceability limit state (SLS). The work includes experimental evaluation in the form of four-point beam-bending tests to investigate flexural behaviour and wedge-splitting tests to obtain material properties in the form of a stress-crack opening (σ-w) relationship. Finite element analyses (FEA) of the tested beams were performed. This is the tool which, in combination with fracture mechanics, is believed to have the potential to provide the desired results regarding crack-width prediction. From the work presented here, the FEA results indicate that a rather simplified (σ-w) relationship is sufficient for calculations in the ULS, while for the SLS a more refined σ-w) relationship may be required. The multi-linear (σ-w) relationship, which was investigated and used in the present work, appears to yield more accurate results during the early stage of the analysis, i.e. the cracking stage.