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Concrete In Australia : March 2014
24 Concrete in Australia Vol 40 No 1 TECHNICAL PAPER: PEER REVIEWED the fibres, the standard mixing regime suggested in Australian Standard for conventional concrete was modified. Accordingly, the mixing time was increased to three minutes to achieve a completely homogeneous concrete. Modifying the standard mixing sequences in order to achieve a more uniform mix has also previously been investigated by different researchers. For instance, Manolis et al. have suggested a 3-5 minutes of mixing after fibre addition to achieve a proper fibre dispersion. Slump was taken to check the workability and, thereafter, freshly mixed concrete was placed into moulds and compacted. In order to prepare the RC beams, fabricated steel cages were placed into the steel moulds using plastic bar chairs and wheel spacers to provide a uniform concrete cover in all directions of the concrete beam. Thereafter, concrete was cast and compacted by means of an internal (poker) vibrator. Finally, the concrete surface was levelled using a concrete trowel. Moulds were covered with plastic sheets to retain moisture. RC beam samples were wet-cured in moulds for seven days and after that demoulded and air cured until the test dates. Curing of other test specimens (concrete cylinders and prisms) was carried out in accordance with AS 1012.8 . At 24 h for cylindrical specimens and 48 h for prismatic samples, specimens were demoulded and placed in lime-saturated water at a temperature of 20±2 °C until the test date. The RC beams used for this test are designed in accordance with AS 3600 (2009). All specimens were 1.9 m long with a span of 1.8 m and a depth of 200 mm (effective depth (d ) = 159 mm). A concrete cover of 25 mm was also used for all the beams. Figure 1 shows the beam geometry and the reinforcement details. In order to investigate the fibre contribution to flexural behaviour of RC beams, two different types (A and B) of steel reinforcement arrangements as shown in Table 3 was utilised. All beams of series A (made with steel arrangement of type A) and series B (made with steel arrangement of type B) were provided with 10 mm diameter stirrups (Ø10) with a constant spacing of 150 mm (centre-to- centre) over the whole of their length in order to avoid shear failure and have a reinforcement ratio ( ρs) of 1.42%. For concrete beams of series A, both fibre volume fractions of 0.25% and 0.50% were used in order to make FRC beams. However, for making the concrete beams of series B, the volume Figure 1: Schematic of RC beam. Table 3: Steel reinforcement arrangements. Tension steel reinforcement Compression steel reinforcement Diameter [mm] No. fsy (nominal) [MPa] Diameter [mm] No. fsy (nominal) [MPa] Type A 12 3 250 10 2 250 Type B 12 3 500 10 2 250 fraction of 0.50% was only utilised since it is believed (Kim et al, n.d.) to achieve more improved performance by using higher volume fractions of fibre. Accordingly, five RC beams made out of three different concrete mix designs and two different steel reinforcement arrangements as introduced further in details in Table 4, are tested for 4-point static flexure. 2.3 Testing methods For each concrete type, in order to evaluate the material characteristics, the mechanical properties of concrete are assessed using indirect tensile strength, modulus of rupture (MOR) and modulus of elasticity (MOE) tests. Uniaxial compression tests are also conducted for each concrete beam using cylindrical specimens of 100 × 200 mm at the age of 28 days and at the beam test day, in accordance with AS 1012.9 . Splitting tensile tests were performed on cylindrical specimens of 100 × 200 mm at the age of 28 days in accordance with AS 1012.10 specifications and method. Cylindrical specimens were tested under load rate control condition in an 1800 kN universal testing machine with a load rate equivalent to 20±2 MPa per minute for compressive test and 1.5±0.15 MPa per minute for indirect tensile test. Flexural tensile strength or MOR is obtained from four-point bending tests on 100 × 100 × 400 mm prisms at a loading rate of 1±0.1 MPa/min until fracture following AS 1012.11 . MOE test was also carried out on 150 × 300 mm cylinders following AS 1012.17. All tests were conducted at 28 days of ageing and each of the following mentioned results is the average of three test specimens. Literature offers some specific information and research Table 4: Concrete beam designations. Beam label Reinforcement arrangement Fibre length [mm] Vf 1 [%] Control (A) Type A – – Control (B) Type B – – 6 PVA-0.25% (A) Type A 6 0.25 12 PVA-0.50% (A) Type A 12 0.50 12 PVA-0.50% (B) Type B 12 0.50 1 Fibre volume fraction. 22-27 - Peer review.indd 24 22-27 - Peer review.indd 24 3/02/14 12:07 PM 3/02/14 12:07 PM