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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

Results

Creep was evaluated measuring the vertical displacement at the mid-span of the beams. In this section, the evolution of the deflection and the evolution of the creep coefficient are analysed. The differences on these two parameters are discussed according to the influence of the type of fibre and curing method.

Evolution of the Deflection

The total deflection at a time t, dtot(t), is the direct sum of the initial deflection, d(to), due to the instant effect of the loading of the beams and the deflection due to creep 8u(t). In Fig. 5 it is shown the deflection due to creep of each beam with steel fibres (SF) or glass fibres (GF), as well as their average value. The results also reveal the differences between non-wrapped and wrapped specimens. The data of non-wrapped beams are shown for a time of 150 days as no changes on the load level were introduced within that period. In wrapped specimens the level loads were changed two times. Only data until the day 30 appear, since during that period the two level loads were the same for both types of fibre.

Figure 5a indicates that both SF and GF present a similar 8u(t) until the day 30. At this point, when the deflection in GF is only 2.9 % higher than in SF, the trend starts to change and the deflection detected in the GF beams gradually increases

Table 5 Deflection due to Deflection (mm) creep at 15 and 30 days

Non-wrapped (50 % PCr)

Wrapped (35 % P*„)

SF

GF

SF

GF

dp (15)

0.254

0.241

0.338

0.306

dp (30)

0.363

0.374

0.447

0.452

with respect to SF. At 150 days, the deflection due to creep in GF is 36 % higher than in SF. These results highlight that the biggest differences between the two types of fibres in terms of dp(t) are evident from 30 days onwards.

One of the beams reinforced with glass fibres (GF2) revealed an unexpected behaviour. Since the beginning of the test the deflection was significantly higher than the deflection of the other GF beams. This inconsistency with respect to the other results may be due to a defective manufacture of this individual beam, since its pre-cracking and loading procedure was identical to the rest of the beams.

The performance of wrapped specimens is gathered in Fig. 5b. As it happened in non-wrapped specimens, the average deflection of the beams resulted to be very similar regardless of the type of fibre used as reinforcement. Moreover, when the load was increased at the day 15 from 25 to 35 % of P*r the deflection in SF increased from 0.136 to 0.338 mm, whereas in GF this grew from 0.169 to 0.306 mm. At 30 days, the average difference of dp(t) between beams with SF and GF was only 1 %, thus excluding the type of fibre as a factor influencing the deflection in such low load levels. Further study should be undertaken to identify at which load level the fibres become the distinguishing feature affecting dp(t).

Comparing the results gathered in Table 5 regarding the curing method between beams, it was noticed that dp(t) at 15 and 30 days was higher in wrapped specimens when the load level was 35 % of P*r. At 15 days, the deflection due to creep in SF and GF was 25 and 21 % higher than in non-wrapped specimens, respectively. At 30 days, these percentages decreased respectively to 19 and 17 %, still remaining the deflection in wrapped specimens greater than in non-wrapped.

The latter results seem to be contradictory in relation to the load level since the load in wrapped specimens was in every case lower than in non-wrapped. This could be attributed to the different curing process produced by the lack of external water contribution during the set of concrete. This might have caused a lower amount of hydrated cement paste and, consequently, a lower elastic modulus. In this case, deformation would be greater in wrapped specimens in comparison to others with a higher degree of hydration and higher elastic modulus.

 
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