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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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Experimental Program

Many are the parameter that could be investigated for the tensile creep in cracked cross section. Certainly the most important parameter is the load level, together with the pre-damaging: the load or deformation applied to the specimen to generate the crack to be investigate. Regarding the steel fibre, the length/diameter aspect ratio (slenderness), the volume content, the fibre length, the surface property and the fibre orientation along the loading direction seems to influence the tensile creep. Concerning the concrete, the mix design and the presence of coarse aggregate may influence the creep, as well as heat treatment, which modifies the concrete structure and the cement to fibre adhesion. While for ordinary fibre reinforced concrete [5] the compressive strength seems to be decisive, the influence is less important for the UHPC [7].

In Table 1 the test program for the sustained load tests is summarized. There the fibre length, diameter and content, the load level and the adopted test setup are given.

Table 1 Sustained load test program

Series

Test description

Fibre length/0, content

Sustained load [% of tensile strength]

Test setup, number of tests

No.

Axial

Bending

1

Reference

combination

12.5/0.175, 2 %

25/40/80/90

2/4/2/2

2/42/2

2

Pre damaging effect

12.5/0.175, 2 %

40

2

2

3

Fibre aspect ratio

12.5/0.40, 2 %

40

2

2

4

Fibre content

12.5/0.175, 4 %

40/80

2/2

2/2

5

Without heat treatment

12.5/0.175, 2 %

40

2

2

6

Undamaged, creep test

12.5/0.175, 2 %

40/80

2/2

2/2

Total loaded specimen

24

24

Table 2 UHFRC mix design

Material

Cement CEM 52.5 N

728

kg/m3

Water

80

kg/m3

Sand DM 0.125/0.5

816

kg/m3

Quartz flour

510

kg/m3

Microsilica suspension

226

kg/m3

Super plasticizer

29.7

kg/m3

Steel fibre (2 %)

164

kg/m3

A reference combination was tested extensively with a larger amount of specimen at four different load levels as a comparison with the other tests series. The mix design shown in Table 2 was employed, changing in some cases only the fibre diameter and content. Only the straight brass alloy coated fibres were used. This fibre type is mostly preferred and has a good interaction with the concrete paste [10]. For investigating the effect of the fibre content, four tests with 4 % fibre volume were performed in addition to the reference fibre concentration of 2 %. The slenderness of the fibre was investigated with a fibre volume of 2 % also and a length-diameter ratio of 31.3 (fibre length 12.5 mm, diameter 0.40 mm, reference ratio approx. 71 with 0.175 mm diameter).

For each test series six dog-bone specimens and six prisms were tested according to the French guideline [11] for the tensile strength and post cracking behaviour. With these tests, a deformation level was defined for the pre-damaging of the specimens in the sustained load tests (test series 1, 3, 4 and 5). This deformation was vertical displacement of 0.6 mm for the prisms and a axial deformation of 0.01 % for the dog-bone specimen. Since specimen with different fibres have different strength, the sustained load level was defined as a percentage of the respective reference tests. The pre-damaging effect was investigated with test series 2, with a larger predamaging deformation.

In order to exclude the superposition with basic and drying creep of the concrete paste, all the specimen were subjected to a heat treatment of 48 h at 90 °C (except test series 5). Heat treatment gives a very important reduction on the tensile and compressive creep [1, 7, 11, 12]. However since the effects of the uncracked concrete creep cannot be neglected, in test series 6 four uncracked specimen were included in the test program.

Regarding the test setup adopted for this investigation, four point bending and axial tensile tests were chosen. The four point bending test allow to observe the average deformation of a large portion of the material compared to three point bending test with notched specimen. Four point bending tests were performed on prisms 70 x 70 mm according to the French guideline [11] but with a span of 940 mm between the supports, instead of 210 mm. The span was increased to limit the required dead load and to increase the observed surface. The vertical displacement of the midpoint was recorded. A sustained load frame with lever arms for axial tensile tests was built. This allows to test up to 24 specimen simultaneously.

Sustained load test setup and dog-bone specimen

Fig. 1 Sustained load test setup and dog-bone specimen

Dog-bone specimen were designed as shown in Fig. 3. The forms for these specimens were CNC milled in order to guarantee a very precise geometry and avoid unwanted eccentricity (Fig. 1).

One of the decisive issue for the test was the choice of the measuring method. Since there are a lot of specimen that have to be monitored over long time. Beside conventional mechanical displacement indicators, 48 potentiometer displacement transducers were installed on the dog-bone specimen.

Since the expected deformations are quite small, together with the displacement transducers also survey points for extensimeter measurement were fixed at the dog-bone specimen.

 
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