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Concrete In Australia : December 2013
Concrete in Australia Vol 39 No 4 45 especially for self-compacting, highly flowable mixes). When compared with conventional bored piles, the visual verification of the pile excavation is not possible as concrete is poured while the auger is extracted. CFA piling is a “blind process” and advanced quality assurance is critical to construct a conforming pile. It is obvious that a very workable and stable concrete mix is required for CFA piling. Particularly the installation of the reinforcement cage under self-weight after the placement of concrete under pressure requires exceptional concrete workability and stability. CFA piling mixes also need to withstand segregation due to pumping and hydraulic pressure at the pile toe, which can be up to 40m below working platform level. 4.0 CASE STUDY – High performance concrete for CFA piles in Queensland In 2012, a specialist Brisbane based piling contractor used the recommendations and target values of the “Tremie Concrete for Deep Foundation” booklet to carry out performance based concrete tests for a major CFA piling project in Queensland. The tests were carried out prior to commencement of the project. Besides the characteristic compressive concrete strength, workability and resistance against bleeding were the main requirements for a suitable CFA piling concrete mix. Reinforcement cages up to 24 m length needed to be installed into the fresh concrete under self-weight. The use of vibrators to install the reinforcement had to be avoided as potential segregation of the self-compacting mix might occur. In this project a self-compacting high slump concrete mix (S40-10-250) with 450 kg/m3 cementitious material, 10 mm aggregate and a retarder to ensure workability was maintained for four hours was used for the CFA piles. The mix was designed in accordance with project specifications, which were based on Australian standards. Details of the mix are shown in Figure 7. Prior to commencement of works on site, laboratory tests were carried out to ensure sufficient flow-ability of the mix using the L-box (Figure 7), slump and slump flow tests. The mix reached the end of the L-box at 7.56s after opening the gate which demonstrated sufficient workability. Then the slump (250 mm) and slump flow (420 mm) tests6 were carried out and the results were the benchmark for site testing. When the slump or slump flow values on the site varied more than the allowed tolerances of ±10 mm for the slump and ±20 mm for the slump flow, the concrete was rejected. Six slump cones were filled with concrete after batching and slump tests were repeated every hour. All six tests showed slump values of 250 mm which verifies sufficient retardation. The filtration test had been carried out prior to commencement of concrete placement on site to measure resistance to bleeding. The CFA concrete mix showed excellent results. Measured data were considerably below the recommended limits of the Recommended Practice “Tremie Concrete for Deep Foundations”. 1 Reinforcement cages up to 24 m long for several hundred piles could be installed into the fresh concrete under self-weight, without using the cage vibrator. No indications of concrete bleeding beyond acceptable limits were observed. Thus, the authors strongly recommend the use of the Recommended Practice “Tremie Concrete for Deep Figure 6: Typical CFA piling equipment (left), CFA installation process after Peiffer et al (right).8 Figure 5: Filtration press1 – schematic (left), with supplementary equipment (right). 41-48 Larisch.indd 45 41-48 Larisch.indd 45 25/11/13 3:58 PM 25/11/13 3:58 PM