Although plastic fibres such as nylon and polypropylene have low Young's modulus and contribute little to ultimate tensile strength, the unit reports that their addition can raise the concrete's impact strength by a factor of:

1. The low modulus means plastic fibres do little to stiffen the composite. 2. However, their high ultimate elongation makes them very good energy absorbers. 3. The unit reports an increase in impact strength of 10 to 25 times over plain concrete. 4. This is why polypropylene fibres are used in concrete pile shells and cladding panels. 5. The 2–3 times and 100–200 times brackets misstate the reported range. _Source: IGNOU Concrete Technology, Unit 11 "Special Concrete" (Light Weight, Aerated, No-Fines, High Density, Fibre Reinforced, Polymer Concrete)._