Load-bearing behaviour of functionally graded steel fibre reinforced concrete beams
Author: Matthias Fischer
Language: English
Abstract
The interest in using steel fibre reinforced concrete (SFRC) for structural applications is steadily growing due to benefits such as the reduction of incorporated conventional reinforcement, the reduction of labour for conventional reinforcement placing, the facilitation of the concreting process, and the shape of structural concrete members being basically unlimited. Despite the fact that SFRC has been subject to many research projects since the early 1960s, the application of SFRC in practice has been limited to mostly non-critical structural elements. The lack of understanding of the load-bearing and post-cracking behaviour of SFRC is the main reason for its rare use. The aim of this Master’s Thesis is to present essential findings about the load-bearing and post-cracking behaviour of functionally graded SFRC based on four-point bending tests of SFRC beams. The lower part of the specimens is made of SFRC which provides flexural tensile resistance, whereas the upper compressive zone is composed of unreinforced, conventional concrete. Specimens with different SFRC layer height composed of distinct concrete mixes with variable Dramix® 5D 65/60 fibre contents are tested. In addition to fibre reinforced beams, one hybrid reinforced specimen (minimum conventional steel reinforcement in combination with SFRC) is tested. Similar to conventionally reinforced concrete, strain-hardening behaviour is also requested for load carrying SFRC members. This may be achieved by yielding of the fibres in the cracks during loading while the fibre ends remain anchored in the concrete matrix (no pullout and no anchorage failure). Based on the test results of the first specimens, it can be concluded that the anchorage of steel fibres represents a major issue. Therefore, micro steel fibres (Dramix® OL 13/.16 fibres) are added to the SFRC mixes for the remaining specimens in order to strengthen the matrix and to improve the anchorage capacity. The test results show increased bending resistances and improved ductile and strain-hardening behaviour. Full anchorage of the fibres, however, is not achieved. Hence, anchorage of the steel fibres, related ductility and strain-hardening effects at their fullest potential remain the main issues which need to be addressed in further research and material development projects. The test results of the hybrid reinforced specimen show that the combination of SFRC and conventional reinforcement results in a desirable structural response. Thus, hybrid reinforced concrete represents an adequate reinforcement solution for designing safe and reliable structural members. In addition to the experimental work, considerations regarding the stress transfer from fibres to the surrounding concrete matrix and regarding the development of anchorage capacity of steel fibres are presented. A simple approach to estimate the anchorage capacity of steel fibres is developed. An accurate mechanical model which allows to determine the stress development in the concrete matrix due to increasing fibre stresses of randomly oriented, overlapping fibres which may interact between cracks emerges as the main target for further research regarding the applicability of strain hardening SFRC for load-carrying structural members. Predicting the crack spacing, making efficient alterations to commercially available steel fibres, and modifying the concrete mix based on accurate mechanical considerations to enforce the desired material characteristics would hereby be possible.