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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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Pre-cracking and Loading Sequence

The beams were pre-cracked applying load at the mid-span according to the three-point test setup as depicted in Fig. 4. Notches were not performed to avoid reduction of the section. The distance between supports was 500 mm and the load was transmitted to the beam at a constant rate of 0.3 mm/min. When the beam reached the flexural strength, the loading operation was immediately stopped. This

Fig. 4 Pre-cracking of the beam in a three-point configuration

point was identified by means of both the load-displacement slope variation and the load dropping occurred at that instant. Even though the common procedure in creep tests is arriving to a certain crack width or deflection [6], the procedure here described was systematically followed for all specimens. Moreover, it is worth to mention that no additional damage was undergone by any sample, since in no case the deflection exceeded 1 mm.

Since the span changed from 500 mm at the load press to 1100 mm at the frame, the loads P in each case needed to be recalculated as described in Eq. (1) by keeping the bending moment constant for both lengths. As a result, the load to produce the same moment in the beam of 1100 mm decreased with respect to the load for a beam of 500 mm.

Nevertheless, a different behaviour between wrapped and non-wrapped specimens after the crack strength was detected. In non-wrapped specimens there was a small drop of the strength after the crack load, followed by an increase of the strength even over the peak. In wrapped specimens a sudden drop of the load occurred immediately after the peak load was reached, showing no significant increase of the load from this point onwards. Moreover, higher peak values were obtained in wrapped beams, possibly due to the additional effect of the aluminium since it was not removed.

As a result, two different strategies were defined for loading the beams during the creep test depending on whether the beams were wrapped or non-wrapped. As depicted in the schemes of Table 4, the reference load for non-wrapped specimens was the cracking load (Pcr), whereas for wrapped specimens the reference load was that in which the major cracks stabilize and the residual strength appears (P*r). The load for each beam in the creep test was calculated on the basis of these data.

For non-wrapped specimens the creep load was defined as 50 % of Pcr from the beginning of the test, and no changes on the load were later performed. Wrapped specimens were initially loaded at 25 % of P*r and after a period of 15 days, the

Table 4 Different load levels (kN) in non-wrapped and wrapped HPFRC beams

Scheme

Beam

L = 500 mm

L = 1100 mm

Per

Pc*r

t0 (0 days)

tj (15 days)

to (30 days)

Non-wrapped

|

1 cr 0

О 50% Per Displacement [mm

SF1

1.43

-

0.33 (50 %P„)

-

-

SF2

1.93

-

0.44 (50 %P„)

-

-

SF3

1.61

-

0.37 (50 %P„)

-

-

GF1

1.33

-

0.30 (50 %P„)

-

-

GF2

1.33

-

0.30 (50 %P„)

-

-

GF3

1.52

0.34 (50 %P„)

Wrapped

  • •3
  • 5

OPc*r

. 35% P^.

A 25% P^. Displacement [mm

SF6

2.29

1.25

0.14 (25 %P*r)

0.20 (35 %P*r)

0.52 (50 %P„)

SF7

2.57

1.49

0.17 (25 %P*r)

0.23 (35 %P*r)

0.58 (50 %P„)

SF8

2.49

1.58

0.18 (25 %P*r)

0.24 (35 %P*r)

0.57 (50 %P„)

GF6

1.86

0.80

0.42 (50 %P„)

-

-

GF7

2.19

1.46

0.17 (25 %P*r)

0.23 (35 %P*r)

-

GF8

1.79

1.00

0.12 (25 %P*r)

0.16 (35 %P*r)

Creep Behaviour of Cracked High Performance Fibre ..

load was changed to 35 % of P*r. An exception was made with GF6, which was initially loaded with 50 % of Pcr but immediately collapsed. At 30 days, to compare the creep between non-wrapped and wrapped beams, the creep load of steel fibre reinforced wrapped beams was pushed to the limit and it was increased to 50 % of Pcr.

 
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