Materials and Test Programme

Materials and Mix Design

A concrete with a 60 MPa cube strength was used for the tests. The maximum coarse aggregate size was 6 mm with a water/cement ratio of 0.48. The mix design is shown in Table 1. The steel fibres used were supplied by Bekaert and with the brand name Dramix 3D-65/60-BG. The fibres were 60 mm long with hooked-ends. For the tensile tests, the fibres were added at a volume of 0.5 %. For the single fibre tests the same concrete was used except that the fibres were not added to the matrix. This did result in an increased workability for the concrete used for the single-fibre tests.

Table 1 Concrete mix design

Material type


Cement (CEM I 52.5)


Stone (6 mm nominal size)






Steel fibre (0.5 % by volume)


Superplasticiser (0.5 % by weight of binder)


Single Fibre Tests

The single fibre pull-out tests were done over 5 orders of magnitude of the pull-out rate in order to determine whether there is a rate-effect in the range of rates tested. The slowest test was done at 0.25 ^m/s and rate was increased in multiples of 10 up to 2.5 mm/s. Five tests were done per pull-out rate. The average maximum load over all the rates was used as the reference load (i.e. 100 %) for the single fibre pull-out creep tests.

For the single fibre pull-out creep tests, the fibres were loaded at 30, 50, 70 and 85 % of the average maximum load resisted in the pull-out rate tests. Three specimens were tested for each load level.

Additional tests were also done to verify whether pre-damaging the single fibres would have an effect on the pull-out creep results. In the case for the uni-axial tensile creep tests, where the specimens were pre-cracked, the fibres would have experienced some form of pull-out before the creep loading started. This test was done in order to investigate the effect of this pre-damage on the creep behaviour. Three samples were pre-damaged by pulling out the fibre by 0.53 mm, then unloaded after which the creep load was applied.

Lastly, the samples from the single fibre creep tests were analysed using a micro X-ray computed tomography (ц-CT) scan in order to investigate the damage around the hooked-end of the fibre in the matrix.

Uni-Axial Tensile Creep Tests

All the specimens used in the uni-axial creep tests were pre-cracked using the setup presented in Sect. 2.3. Each specimen was loaded until a crack width of 0.53 mm and the average stress for the specimens at this crack width was 2.19 MPa. Three specimens were loaded at each load level which are the same as for the single fibre pull-out creep tests, namely 30, 50, 70 and 85 %. Note the actual loading level was calculated according to the specific specimen’s residual strength at 0.53 mm crack opening.

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