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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

Creep Tests

Figure 3 shows the deferred crack opening under sustained loads versus time curves, for each specimen. Table 2 summarizes some characteristics of the creep tests including the instantaneous crack mouth opening after loading (CMODi), the applied sustained stress (f) and the relative value of the applied stress referred to fR1, and two Crack Opening Rates (COR6o-3o and COR9o-6o) calculated as chords considering the differed crack openings during the lapse of loading between 30 to 60 and 60 to 90 days respectively. After this period the specimens were unloaded and bending tests were performed to evaluate the residual strength properties after creep; Table 2 also shows the density of fibers on the fracture surfaces.

Left pre-cracking process for creep tests (beams -a to -d); right EN 14651 tests (beams -e)

Fig. 2 Left pre-cracking process for creep tests (beams -a to -d); right EN 14651 tests (beams -e)

Table 1 Results of pre-cracking process and characterization tests (EN 14651)

Specimen

Width

(mm)

h

sp

(mm)

fL

(MPa)

fmax

(MPa)

./R1

(MPa)

./R2

(MPa)

./R3

(MPa)

fR4

(MPa)

50-a

55

120

6.0

6.6

6.4

50-b

59

120

5.6

6.1

5.5

50-c

54

124

5.0

5.4

4.9

50-d

60

121

5.4

6.0

4.3

50-e

51

125

6.1

6.1

3.2

3.0

2.6

2.5

50-f

52

120

5.3

5.9

2.5

50-mean

55

122

5.6

6.0

4.5

50-COV

7

2

7

6

32

100-a

108

120

5.5

5.6

4.5

100-b

107

123

5.5

6.1

5.3

100-c

104

122

5.3

5.3

3.7

100-d

104

122

4.8

5.4

4.3

100-e

98

124

6.1

6.4

4.8

4.1

3.8

3.4

100-f

100

118

6.0

6.0

3.2

100-mean

104

122

5.5

5.8

4.3

100-COV

4

2

9

7

18

150-a

145

124

5.2

5.5

4.9

150-b

148

125

5.2

5.5

4.7

150-c

148

126

5.1

5.7

5.1

150-d

147

126

5.2

5.2

3.4

150-e

149

121

5.0

5.2

3.1

2.5

2.5

2.5

150-f

143

125

5.3

5.7

5.4

150-mean

147

125

5.2

5.5

4.4

150-COV

2

2

2

4

21

Mean

5.4

5.8

4.4

3.2

2.9

2.8

COV

7

7

23

26

25

19

50-mean/mean

1.03

1.04

1.02

100-mean/mean

1.02

1.01

0.98

150-mean/mean

0.95

0.95

1.00

It can be seen that most specimens show very similar creep behavior, with the exception of one atypical response in the case of beam 50-d. In the case of the curves of beams 100-b and 150-c, which have low ffR1 ratios, the deformations are lower, while beam 150-d has the greatest deformations; however no significant differences in the crack opening rate related to the beam width are found. The variability of the response decreases as the beam width increases.

The evolution of crack opening along the time tends to be more stable after the first weeks under loading, showing homogenous increases after the first month; consequently after this age the creep response can be characterized by the concept of crack opening rate.

Table 2 Creep tests

Beam

CMOD;

(mm)

f (MPa)

ffR1

(%)

COR60-30

(mm/year)

COR90-60

(mm/year)

Fiber density (fibers/cm2)

50-a

0.15

3.7

59

0.27

0.15

0.80

50-b

0.18

3.6

66

0.25

0.12

0.68

50-c

0.20

2.8

57

0.27

0.16

0.42

50-d

0.17

2.6

61

1.25

0.42

0.40

100-a

0.24

3.0

66

0.24

0.10

0.55

100-b

0.12

3.0

56

0.27

0.14

0.48

100-c

0.22

2.4

64

0.14

0.08

0.48

100-d

0.12

2.4

54

0.53

0.29

0.49

150-a

0.20

2.9

59

0.27

0.20

0.44

150-b

0.19

2.8

61

0.18

0.14

0.46

150-c

0.15

2.5

50

0.33

0.13

0.47

150-d

0.22

2.6

74

0.24

0.21

0.43

Fig. 3 Evolution of crack opening during creep tests

plots the CMODj after loading as a function of the applied relative stress; it can be seen the consistency of the results between beams of different width

Figure 4 plots the CMODj after loading as a function of the applied relative stress; it can be seen the consistency of the results between beams of different width. Figure 4 also shows the results of crack opening rate between 30 and 60 days, where, with the exception of the beam 50-d previously mentioned, there is a similar response with values near to 0.3 mm/year. In both cases the variability decreases in the standard beams of 150 mm width.

Finally, Fig. 5 plots the results of the crack opening rate during the 3rd month under loading (COR90-60) versus those corresponding to the 2nd month (COR6o-3o), it can be seen a decrease in the crack rate as expected, verifying that a very good correlation exists between those values with no differences regarding the width of the studied beams.

Instantaneous crack opening after loading (left) and crack opening rate between 30 and 60 days (right) as a function of the relative applied stress

Fig. 4 Instantaneous crack opening after loading (left) and crack opening rate between 30 and 60 days (right) as a function of the relative applied stress

Fig. 5 Relationship between crack opening rates calculated for different times under loading

 
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