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Home arrow Engineering arrow Creep Behaviour in Cracked Sections of Fibre Reinforced Concrete: Proceedings of the International RILEM Workshop FRC-CREEP 2016

Drying Shrinkage Test

The drying shrinkage was determined from two load free specimens of the same size and geometry as those used for the creep test. Two LVDTs extended to have a gauge length of 285 mm (more than half the specimen length) were attached to the specimens. The readings were measured electronically using a HBM Spider8 Electronic Measuring System. Since the gauge length was different from that of the creep specimens, shrinkage strain measured was multiplied by the gauge length of the creep specimens to determine the displacements caused by shrinkage in the creep specimens over a gauge length of 70 mm. A deduction of these strains from the measured CMOD gave the actual creep of the specimens under sustained loading. Figure 4 shows the shrinkage specimens during testing.

Fig. 4 Drying shrinkage test specimens [8]

Single Fibre Tests

To understand the mechanisms responsible for the creep of cracked macro-synthetic FRC subjected to sustained tensile loading, time-dependent fibre pull-out and fibre creep tests were performed. The time-dependent fibre pull-out test was pursued by preparing fresh concrete using the mix design in Table 2 without the inclusion of fibres. 100 mm cube specimens were then cast and single macro-synthetic fibres (already marked) were inserted by hand to a depth of 25 mm into the fresh concrete mix at the marked mid-point of the surface area of the cast concrete. The moulds were then gentle vibrated on a vibrating table to ensure closure of any voids created during the insertion of the fibres. Care was taken to ensure that the fibres remain vertical after the gentle vibration of the moulds. After demoulding and curing for 28 days, the specimens were prepared for testing. A hole of 12 mm diameter was drilled on the opposite side to where the fibre was embedded to a depth of about 30 mm. Threaded bars with 10 mm diameter to support the specimens were then fixed in the holes with epoxy glue and left in the controlled climate room to set for 24 h before testing commenced. Specimens were then positioned in a frame for testing as shown in Fig. 5. The free ends of the fibres were gripped with a hand drill chuck and predetermined sustained loads between 50 and 80 % of the average interfacial shear resistance of specimens tested at 25 mm embedment length were applied to the fibres individually to study the time-dependent pullout behaviour.

Measurement of the fibre pullout over time was captured optically with the aid of a microscope using a 3.1 Mega-pixel Leica EC3 camera. Before the application of the weights, a photo of the setup was taken to serve as the reference photo. The height of the mouth grip of the hand drill chuck, hch, 5 mm, was used as a scale reference (Fig. 5). With this scale, the reference fibre length (from reference photo) before pullout, Lr, was determined. Subsequently, photos were taken at regular

Test setup for time-dependent fibre pull-out [8]

Fig. 5 Test setup for time-dependent fibre pull-out [8]

Fig. 6 Fibre creep test setup [8]

intervals of the fibre pullout from the matrix and the new pullout length was then subtracted from Lr to obtain the time-dependent fibre pullout displacement.

The fibre creep test was carried out to understand the creep mechanism of cracked macro-synthetic FRC. Since this type of fibre is well known to creep, its contribution to the overall creep of the specimens is sought. One fibre was used for this investigation. Hand drill chucks of the same type as those used for the time-dependent fibre pull-out test were employed to grip the fibre at its free ends. Damage to the gripped ends of the fibre by the chucks and slippage during test did not occur. Where slippage was noticed, specimens were discarded. Thereafter the specimen was supported by a frame as that in the time-dependent pull-out test and the lower chuck supported the sustained load as shown in Fig. 6.

The same procedure for measuring the time-dependent pullout test was also used to determine the time-dependent elongation of the fibre under sustained load of 143 MPa. This represents 30 % of the average tensile strength of the fibres tested in tension at a rate of 0.5 mm/s.

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