Abstract
One of COIN’s overall targets is to increase the residual bending strength towards 15MPa. A working hypothesis has been that the ductility of fibrous concrete increases with decreasing concrete compressive strength due to lower crack load and also lower maximum loads. The objective of the following study has therefore been to confirm this hypothesis, and to investigate the influence of paste/matrix composition and properties, workability and compressive strength on the ductility of the test-specimen. Influence of fibre type, fibre combinations and fibre dosages have also been studied.
A number of preliminary mixes were tested before selecting two mixes for incorporation of different fibre types, dosages and combinations. Maximum aggregate size was the main parameter separating these two mixes. Both mixes were workable, but not self-compacting.
The proportionality limit was slightly increased with increasing fibre content (0-1volume %). The increase was larger for steel fibres than for synthetic fibres.
The concrete with smaller maximum aggregate size (dmax= 8mm) had more promising ductility properties than the concrete with larger aggregates (dmax = 16 mm).
The beam specimens cast with dmax= 8mm and 0.5% of both steel and synthetic fibres fulfilled the ductility requirements best. The same concrete with 0.5 or 1.0% steel fibres is also promising considering ductility.
The residual bending strength fR,3 (crack width = 2.5mm) is 4.5MPa for the 11-5 concrete with 0.5% of both fibre types. Although this is far away from the overall COIN-target of 15 MPa, increased fibre content can easily bring us higher up.
Suggestions for further work include
further development of the concretes developed within this study
verification of the ductility-experience of the present investigation in structural elements of larger dimensions
development of new test method for study of fibre distribution under casting