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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

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Test Method to Evaluate the Mid-term Behaviour of Frsc Submitted to Flexural Loading

The temperature and hygrometry conditions of the room are measured throughout the tests and must be consistent if several tests are performed in the same room.

The creep test is performed on three slabs of 60 x 60 cm with a thickness of 10 cm, as in the NF EN 14488-5 test method to measure the energy absorption capacity. Three other slabs made of the same concrete matrix but without fibres are necessary to determine the appropriate loading.[1] Four slabs of each concrete must be sprayed in case a problem arises with one of them.

The test requires a creep test device (Fig. 1) designed and used as follows:

Stage 1: characterisation of the reference concrete (BB)

Seven days after spraying, three unreinforced slabs are loaded with a press according to the NF EN 14488-5 test procedure until the maximum force is obtained. The mean value is called Fel-max BB and is used to determine the loads that shall be applied to the reinforced slabs thereafter.

Stage 2: calibration of the creep test device

During the creep test, one or two loads may be applied on the slab through a square loading block of 10 x 10 cm, depending on the measured deflection: the first one is

Fpre-cracking = [1].2 * Fel-max BB, the second one is Fcreep = °.5 Fpre-cracking. The load

is reduced to Fcreep only if the deflection exceeds 2 mm.

Calibration is carried out using a load cell. A steel slab is placed in the same location as the concrete slabs to be tested; centring is carried out automatically by clamping guides.

Creep test device

Fig. 1 Creep test device

The counterweights are determined firstly to obtain the lowest value: Fcreep. The corresponding weights are then marked. They must remain in place during the whole test. Then the counterweights to be added to obtain the highest load, Fpre-cracking, are determined.

Stage 3: pre-cracking of a reinforced slab and beginning of the deflection measurement

Prior to cracking, both diagonals are drawn on the underside of each slab (in contact with the mould). A square of 1 x 1 cm is marked out in the middle, and then the slab is centred on a metallic frame on which it will stay until the end of the test.

Pre-cracking is carried out seven days after spraying, in two separate phases:

Phase 1: the slab is loaded with a testing machine according to the NF EN 14488-5 test method, until the first drop in the force is detected; then the machine is immediately stopped. As this drop is brutal, it has been found more efficient to rely on the ability of an experienced technician rather than trying to detect the load peak automatically.

Phase 2: the slab, still on its frame, is positioned on the creep test device (the slab and the frame are transported together from the press to the creep device). If necessary the slab is re-centred on its frame, which is also centred within the device, by using clamping guides. A LVDT-type displacement sensor is installed within the central square that was marked out, making sure that it is not positioned over any cracks.

The pre-cracking load Fpre-cracking is applied by unscrewing the screw positioned underneath the loading plate. The recording of deflection values begins from the start of loading. The load is maintained constant for 1 year, except if the deflection exceeds 2 mm.

Stage 4: reduction of the load and continuous measurement of deflections (if necessary)

If a deflection of 2 mm is measured, the load is reduced to the value of Fcreep = 0.5 * Fpre-crackjng. The deflection is continuously monitored.

Stage 5: load removal and measurement of recovery deflection After one year of testing, the load is removed from the slab. The recording of deflection continues for one week after load removal. The slab is then removed from the creep test device.

Stage 6: measurement of the residual energy absorption capacity

The slab is tested according to the NF EN 14488-5 test to evaluate its residual energy absorption capacity.

Stage 7: checking of the calibration of the creep test device

The load value of the creep test device is checked using the same equipment as for the calibration (the metal plate and load cell).

Stages 6 and 7 can be carried out in any order. Photos and additional information on the elaboration of this test method are available in [1].

  • [1] This concrete is considered as the reference concrete (BB).
  • [2] This concrete is considered as the reference concrete (BB).
 
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