The knowledge of the concrete materials and properties is essential to better assess their structural applications, not only in terms of instantaneous responses but also in the long-term along the service life.

Creep is a term used to define the tendency of materials to develop increasing strains through time when under a sustained load. Commonly, the increasing strains are accounted for by means of creep coefficients that relate long-term strains with respect to short-term strains. The long-term strains of concrete have been reported to become multiple times larger than the initial instantaneous strain [1]; therefore, creep becomes an important factor to be considered.

Long-term strains can be beneficial to some types of structures because they can lead to stress redistributions that can limit the extent of cracking. At the same time, in the case of decreasing residual strength or significant strains the influence of creep may be unfavourable.

In many applications, fibres have been included in concrete to improve structural serviceability based on benefits in crack control [2]. The mechanical behaviour of fibre reinforced concrete (FRC) has been widely studied over the past decades [3], whereas the analysis of flexural creep behaviour of cracked FRC elements is a relatively new topic which has not been entirely researched yet. Important advances have been achieved regarding the residual strength characterization and applications of FRC [2]. However, there is no standardized method to assess such behaviour at this time, and some researchers are working on proposals [1, 4].

When considering combinations of reinforcing bars and fibres, studies have shown that the addition of steel fibres considerably reduces the time-dependent deflection and crack widening of reinforced concrete [5, 6]. However, in terms of structural design, recommendations and codes do not take into account the long-term behaviour under cracked conditions.

As the contribution of fibres to structural load-bearing capacity is based on the flexural response of FRC, and mainly in the cracked state, the capacity of the material to keep the crack opening values low enough is a topic of interest to be assessed [4]. Although residual strength parameters are applied in structural design, the knowledge of the long-term behaviour of FRC in cracked conditions is limited to a few reports on the subject [1-3, 5, 7-9].

The study of creep behaviour of cracked FRC constitutes a key point of interest mainly for cases in which fibres are the only reinforcement. In these cases, serviceability of the material will depend on its capacity to transfer the sustained stresses through the fibres and the stability of the cracks [5].

A previous study on cracked SFRC [10] has found low coefficients of creep at different long-term load levels for a 90-day period, but the type of fibre, the load level and the concrete strength significantly affected the creep behaviour. Besides, it is not possible to uncouple the behaviour of concrete from that of the fibres because of the post-cracking creep phenomena in SFRC is not caused by deformation of fibres, but rather by pull-out of the fibres from the matrix [5].

This paper studies the influence of fibre reinforcement on long-term behaviour of cracked SFRC by means of a creep test analysing not only creep coefficients but also Crack Opening Rates.

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