Desktop version

Home arrow Engineering arrow Creep Behaviour in Cracked Sections of Fibre Reinforced Concrete: Proceedings of the International RILEM Workshop FRC-CREEP 2016

Experimental Program

The experiment was carried out at the University “Ss. Cyril and Methodius”, Faculty of Civil Engineering-Skopje, Republic of Macedonia, in the period October 2011 to February 2013. It involved testing of 24 full scale beams constructed from reinforced concrete and steel fibre reinforced concrete with additional reinforcement. The beams had a cross section proportioned 15/28 cm and a total length of l = 300 cm, Fig. 1. Together with each series of beams, control specimens were cast in order to test the compressive strength, flexural tensile strength, splitting tensile strength, elastic modulus and deformations due to creep and shrinkage. In addition to the tests on mechanical and time-dependent properties of concrete, the used reinforcement was also tested.

All 24 beams were manufactured with concrete class C30/37. According to the used type of material, they were divided into three series:

Geometry, reinforcement and loading scheme of full scale beams

Fig. 1 Geometry, reinforcement and loading scheme of full scale beams

  • - Series A, reinforced concrete (C30/37);
  • - Series B, SFRC with 30 kg/m3 steel fibres and additional reinforcement (C30/37 FL1.5/1.5);
  • - Series C, SFRC with 60 kg/m3 steel fibres and additional reinforcement (C30/37 FL2.5/2.0).

The beams constructed of reinforced concrete were used for comparison with the beams constructed of steel fibre reinforced concrete. In each series, the plain reinforcement was kept the same. The longitudinal reinforcement was ribbed and of RA 400/500-2 quality, while the shear reinforcement was smooth, with GA 240/360 quality. Reinforcement 2010, 208 and 06/10/20 cm was used as tension, compression and shear reinforcement, respectively.

The used steel fibres were hooked-end HE1/50, produced of cold-drawn wire, manufactured by Arcelor Mittal, with a diameter of 1 mm, length of 50 mm and tensile strength of 1100 N/mm* 1 2 3.

The experimental program is presented in detail in Table 1.

The mixture proportioning (Table 2) was done so that it was the same for the three types of concrete. According to many recommendations in the up to date literature, the slump of the concrete without fibres was 120 mm. Since fibres decrease workability, the slump was decreased to 75 and 50 mm with addition of 30 and 60 kg/m3.

Regarding the loading history, the beams were divided into four groups (Fig. 2):

  • 1. The beams from all three series from group “1” (Aj, Bb CO were tested under short term ultimate load at the age of concrete of 40 days. With these testing, relevant dependences had to be found for this age of concrete and the behaviour of the reinforced concrete and two types of steel fiber reinforced concretes had to be compared.
  • 2. The beams from all three series from group “2” (A2, B2, C2) were tested also under short term ultimate load, but at the age of concrete of 400 days. This testing was performed in order to find out the influence of the age of concrete on the behaviour of the beams.
  • 3. The beams from group “3” (A3, B3, C3) were pre-cracked with permanent and variable load “g + q”, and afterwards, a long term permanent load with intensity

Table 1 Experimental program

Series

Group

Number

of

elements

Type of concrete

Steel

fibres

(kg/m3)

Tensile

reinforcement

4 (%)

Type of long term load

Time of ultimate load testing

(days)

i

2

C30/37

0

0.37

/

о

-i-

II

2

2

C30/37

0

0.37

/

t = 400

3

2

C30/37

0

0.37

“g” *

t = 400

4

2

C30/37

0

0.37

“g ± q”

(Atg+q=8 h)

t = 400

1

2

C30/37

FL

1.5/1.5

30

0.37

/

О

ii

2

2

C30/37

FL

1.5/1.5

30

0.37

/

t = 400

3

2

C30/37

FL

1.5/1.5

30

0.37

“g” *

t = 400

4

2

C30/37

FL

1.5/1.5

30

0.37

“g ± q”

(Atg+q=8 h)

t = 400

1

2

C30/37

FL

2.5/2.0

60

0.37

/

О

ii

2

2

C30/37

FL

2.5/2.0

60

0.37

/

t = 400

3

2

C30/37

FL

2.5/2.0

60

0.37

“g” *

t = 400

4

2

C30/37

FL

2.5/2.0

60

0.37

“g ± q”

(Atg+q=8 h)

t = 400

Table 2 Mixture proportions for the three concrete types

Mixture proportions

(kg/m3)

Cement CEM II/A-M 42.5 N

410

Water

215

Water/Cement ratio, w/c

0.524

Aggregate:

0-4 mm (river sand), 50 %

875

4-8 mm (limestone), 20 %

350

8-16 mm (limestone), 30 %

525

Fibres:

C30/37

0

C30/37 FL 1.5/1.5

30

C30/37 FL 2.5/2.0

60

Loading history for the beams of the four groups

Fig. 2 Loading history for the beams of the four groups

“g” was applied at the age of concrete of 40 days and was held up to 400 days, when a short term ultimate load testing was performed. In the meantime, the strains, deformations and crack widths were measured.

4. On the beams from group “4” (A4, B4, C4), a long term permanent load with intensity “g” was applied at the age of concrete of 40 days and was held for a year as a long term load. On the fortieth day, repeated variable load “±q” was also applied in an interval of 8 h +q and 16 h -q, for a year. This means that the beams were loaded additionally with load “q” for 8 h every day, whereas the strains, deformations and crack widths were measured. After 8 h, the beams were unloaded from load “q” and all measurements were performed again.

The beams and control specimens were cured for 8 days and then they were transported to the Laboratory at the Faculty of Civil Engineering—Skopje, where they were kept under almost constant temperature ranging from 15 to 24 °C with an average of 19.5 °C and constant relative ambient humidity ranging from 58 to 62 % with an average of 60.2 %, which was regulated with special humidifiers and dehumidifiers, presented in Fig. 3.

On the 12 full scale beams within all three series from group 1 and 2, short term ultimate load test was performed in 29 load steps. A data acquisition system from

Fig. 3 Ambient conditions in the laboratory of the faculty of civil engineering—Skopje

Hottinger Baldvin-HBM, Germany was used for recording the force, the middle deflection U3, the strains in the compression and tensile reinforcement (R1-R4) and the bottom and top of the concrete (C1-C4), with f = 1 Hz. The load cell and the LVDT as well as the strain gages were a product of Kyowa, Japan. In each step, the strains in the concrete (D1-D15), in the middle section of the beam through the thickness as well as on the top of the beam, were measured by a mechanical deflection meter, type Hugenberger, Switzerland, with a base of 250 mm. The mechanical measurement of the deflections was done at 5 points through the length of the beam and 2 points over the supports by using deflection meters produced by Stopani, Italy. The crack widths were also measured in each load step, in the region with constant moment, by use of a crack microscope—product of Controls, Italy. After 360 days of observing of their behaviour under the effect of long term loads, an ultimate load test was performed in the same manner as described above. The positions of the measurement points are presented in Fig. 4.

The long term load, which consists of permanent sustained load “g” and repeated variable load “q”, was applied by 12 gravitation levers, which enabled an increase of the load for 13 times. The permanent load acts all the time, while the variable load was applied and removed each day by secondary hand gravitation levers.

The bending moments are as follows: from self-weight of the beam, Msw = 1 kN m, from permanent load “g”, Mg = 5.0 kN m, from variable load “q”, Mq = 3.1 kN m, from self-weight, permanent and variable load (service) Msw + g + q = 9.1 kN m. The bending crack moment was Mcr = 6.1 kN m, while the ultimate bending moment Md = 15.6 kN m. The intensity of the load was chosen so that the Mcr is bigger than Msw + g and smaller than Msw + g + q. The permanent load is 0.39 times the flexural strength, while the service load is 0.58 times the flexural strength of the beam without fibres.

Positions of measurement points of full scale beams

Fig. 4 Positions of measurement points of full scale beams

 
Source
< Prev   CONTENTS   Source   Next >

Related topics