Evaluation of fibres recycled from fishing nets and methods for quantifying plastic shrinkage cracking

Polymeric waste materials were investigated with the aim of being recycled as fibre reinforcement in construction materials. The type of waste fibres studied was made of recycled polyethylene (R-PE) obtained by mechanical cutting of discarded fishing nets. The characterisation of the R-PE fibres showed a relatively low tensile strength and stiffness, which indicated that the fibres for example could be beneficial in controlling plastic shrinkage cracking in fresh cement-based materials.

This type of cracking can be detrimental for the durability and overall lifetime of concrete structures. A well-established technique for preventing these cracks is the addition of randomly distributed fibres of synthetic materials. When studying the formation of plastic shrinkage cracking at laboratory-scale, the use of techniques enabling objective quantification of cracking and its evolution is essential for a more comprehensive understanding of the material behaviour.

However, such a detailed quantification of the degree of cracking has only been reached to a limited extent. To overcome this, an optical digital image correlation (DIC) technique was applied in the present study for monitoring the crack development. The DIC technique was subsequently used to evaluate the influence of the addition of randomly distributed R-PE and commercial polypropylene (PP) fibres.

In addition, an experimental program on the use of R-PE fibres in different types of construction materials with the aim of improving the mechanical performance was carried out for achieving a better understanding of the fibre influence.The application of the DIC technique for quantification of plastic shrinkage cracking and an approach developed for further analysis of the DIC data by numerical post-processing revealed the following advancements: - Data obtained by the DIC technique can display the formation of displacements and strains on a specimen surface and their continuous development over time.

This provides very detailed information on the behaviour of the fresh cement-based material. - A numerical post-processing approach for further analysis of the DIC data was developed. This enabled a quantitative measure of the degree of surface cracking presented as crack width distributions over the entire specimen surface.

The approach facilitated an objective comparison of cement-based mixtures with the addition of different types of fibres. - Generally, it was found that the DIC technique is highly efficient for monitoring shrinkage-induced deformations and cracking. Results obtained by post-processing of the DIC data showed good correlation with manual measurements using optical microscope.

The major conclusions obtained in this project regarding the performance of R-PE fibres are:- The addition of R-PE fibres was found to have a positive influence on the formation of plastic shrinkage cracking: An almost crack free surface was achieved when adding the R-PE fibres in volume fractions of 2.0%.

However, significantly lower fibre additions were required to obtain similar results with commercially available PP fibres. - The fibre performance on the mechanical post-crack performance and ductility of different types of construction materials was found to increase with the addition of R-PE fibres.

However, the effect was observed to be rather poor, especially for high-modulus materials such as cement-based mortars. Hence, the combined material-fibre performance was found to be better for low-modulus construction materials.