Conflicting claims have been made in relation to the effects of polypropylene fibres on the compressive strength of concrete. The purpose of this paper is to examine the effects on compressive strength of various dosages of monofilament polypropylene fibres when used in concrete. Compressive strength is widely used as the key indicator of concrete quality and therefore needs accurate determination. Monofilament fibres and air entrainment provide a similar function in that they provide freeze/thaw protection, they are both compared against a plain concrete sample to determine relative strength and density.
Two different concrete design strengths (medium and high) were examined with varying amounts and types of polypropylene fibre fraction/volume to establish a common link between fibre additions and reduced final compressive strength.
The findings from the test programme showed a linear reduction in strength which was observed as being directly related to fibre inclusion in concrete. Density was also found to be reduced with the addition of fibres in a similar degree to that of air entrainment.
The lower density of concrete with polypropylene fibre additions was not scientifically explained and this aspect currently forms part of a long term freeze/thaw research programme, which will examine pore spacing and void formation compared to plain concrete.
This paper is of interest to clients, concrete manufacturers, concrete additive manufacturers, designers, surveyors and specifiers who need to know what effect polypropylene fibre additives have upon the final compressive strength.
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