Size effect research
19 May 2021

Size effect research

In 2018 our company participated in a research analysing size effect of synthetic fibre reinforced concrete.

The head of the research was engineer Desmond Vlietstra, who carried out the research for his dissertation for engineering in advanced concrete technology studies at the University of Leeds. Beams were produced and tested at Czako Adolf Laboratory of Budapest University of Technology and Economics. The Individual Research Project Dissertation was made by the utilization of the research data.

Abstract of the dissertation:

Size effect is a well-documented phenomenon that effects structures in plain and reinforced concrete. Rilem introduced a size effect factor into their design methodology for steel fibres – SFRC, (Rilem TC 162 TDF) after it was found that this design method over- estimated the results based on notched beam tests. No information is given on how this size effect criterion was derived or its background other than stating that it is not well understood and that this is an area that requires more research.

The Model Code 2010 states that any fibre can be used as long as it meets the design requirements. With the lack of codes for macro synthetic fibre reinforced concrete, the obvious starting point is to use an established steel fibre methodology such as Rilem TC 162 TDF. But the question arises as to if there is a possibility that macro synthetic fibre behaves differently to steel fibre with respect to size effect and if so to what extent. The purpose of this thesis is to understand the effect that synthetic fibre has on the well documented fracture mechanics size effect of plain and reinforced concrete and if the existing size effect criterion applied to steel fibre in this design methodology should be applied when considering synthetic fibre.

The question therefore arises. Does structural synthetic fibre reduce or eliminate the well documented size effect phenomena prevalent in plain and reinforced concrete?

This study considered geometrically similar notched beam tests of fibre reinforced prisms with the largest beams being twice the size of a standard EN14651 Beam. The major findings indicated that while the size effect is very obvious at the crack initiation. Post crack the synthetic fibre changes the brittle behaviour of the concrete which is prone to size effect introducing a more plastic behaviour thereby reversing the size effect and introducing what appears to be an increased load bearing capacity relative
to size.

The dissertation can be downloaded from the link below:
Des Vlietstra MSC Eng Advanced Concrete Technology Thesis

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