Future studies 132

Recycled steel fibres (RSF) have proven effective for crack width control, as demonstrated in this thesis. Nevertheless, the volume ratios used (0.5% and 1%) are relatively high when compared with commercial steel fibres, and even so the performance obtained in terms of residual stresses is low. This is due to the fact that the fibres used in this experimental programme were generally short.

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Other research groups (Sheffield, UPV, University of Salento…), and even a specifically created company (Twincon), have been working in improving the quality of RSF, and industrialising the production in order to obtain large amounts for use in actual construction sites. An interesting proposal is also to combine RSF with commercial fibres, in order to control with the latter micro- cracking, and with the former larger cracks up to ULS. It would be very interesting to apply these new blends to elements in pure bending or in compression and bending. In fact, although the fibres used in the present study proved effective in crack control, larger improvements are expected when better RSF are used, either alone or as part of a blend with commercial fibres. Moreover, if the volume ratio necessary to achieve adequate residual properties is reduced, other type of concretes (i.e. other than Self-Compacting) could be used without the risk of balling, with economic advantages.

Apart from the improvements of RSF, an important task to perform after their use for crack width control has been validated, both theoretically and experimentally, is the application to real life structures, such as tanks, bridge decks in zones where the use of melting salts is usual, structures in marine environments, and of course columns of integral structures.

It is also advised that RSF could be used in a broader range of applications to partially or totally substitute commercial steel fibres, such for instance in sprayed concrete. The applications of RSFC sprayed concrete could be used in tunnel linings, slope reinforcements, complex geometry concreting, etc.

The design tools for SLS analysis of R/FRC columns of long jointless structures can also be improved by further implementing considerations that are generally not considered in SLS calculations, such as second order effects. Most importantly, to prove that all the assumptions made in order to derive the simplified method are reasonable, it would be necessary to build actual R/FRC columns as part of long jointless structures and measure the behaviour in terms of deformations and crack width. This would lead on one hand to a further validation of the method, and on the other to build stronger confidence among clients and designers about the possibilities of jointless structures.

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