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Concrete In Australia : December 2013
Concrete in Australia Vol 39 No 4 43 and yield parameters implemented in the shear flow theory developed for such non-Newtonian, Bingham-plastic materials, as discussed by Alehossein et al.4 SWC incorporates special binders and/or admixtures to increase its workability above the measurable slump (as measured by slump cone apparatus), which made SWC attractive for piling applications5 in the past. The workability of SWC has to be measured by flow tests or other testing methods3 – traditional methods like the slump test as per AS1012.1 - 1993 “Sampling of fresh concrete”6 used for normal concrete are not ideally suited for testing TC and SWC. Consequently, additional test methods for fresh concrete properties need to be introduced.1, 3 Well designed and composed TC mixes combine high flow-ability with no segregation or bleeding. TC has a self- compacting capability or behaviour, and hence no external compaction is required, similar to SWC. The paste is optimised first, then the mortar and then the concrete.7 The accurate dosage of concrete ingredients, admixtures and an optimised aggregate distribution is extremely important for both TC and SWC. The former do not require spread values in the range of 600–700 mm, which are typical for SWC. Spread values in the range of 400–550 mm are sufficient, depending on the aggregate grading and other mix design details. TC for deep foundation is subject to additional hydraulic and hydrostatic pressure, as shown in Figure 3. This additional stress can affect the stability of the concrete mix and free water can be squeezed out of the mix resulting in concrete bleeding. Bleeding usually affects the concrete quality, since the freed water is no longer available for the hydration process. If ground conditions are impermeable, the bleed water will rise to surface creating “bleeding channels” inside the concrete shaft. In permeable soils bleed water can escape into the surrounding soil and in most cases it stays undetected. This case is very critical as concrete workability and stability are both heavily affected by an undetected bleeding cause. There is an increased risk that the concrete will not flow around the reinforcement and will not fill all the voids inside the excavation sufficiently. The installation of structural elements in the pile after concrete placement (e.g. plunged columns for bored piles) might be seriously affected due to reduced workability. Particularly the installation of reinforcement cages for CFA piles – as described later in this paper – will be critically affected by the reduced workability caused by bleeding (lack of stability). 2.2 Performance based fresh concrete tests Due to the advanced quality control measures required for TC practice, particularly on their workability and stability measures, performance based fresh concrete testing is required to determine the suitable characteristics of the TC used in deep foundations. This paper will focus only on the main tests for workability (L-box) and stability (filtration press), which are both described in detail in the Recommended Practice “Tremie Concrete for Deep Foundations”1 , with the background fluid mechanics discussed by Alehossein et al.4 2.2 .1 Workability – L -box test Slump, slump flow and the L-box tests are suitable criteria measures to test and identify concrete workability as a function of concrete stability1. In particular, the newly introduced L-box test provides the opportunity to check workability parameters when concrete had been resting (delayed) for a specified time before being pushed through various obstacles. The behaviour of concrete placed inside a deep excavation can be modeled using this test arrangement as shown in Figure 4. The test method is used to indirectly measure the workability of a concrete with a maximum coarse aggregate size of 20mm or less under a certain, decreasing concrete head pressure. The L-box directly measures: • the concrete flow (flowing ability) • the concrete flow into all spaces within the excavation (filling ability) • the concrete flow through tight spaces and openings (passing ability). The L-box should have smooth, non-absorptive surfaces (preferably galvanised steel), with rigid shape and dimensions as shown in Figure 4. The closed gate prevents the flow of concrete at the time of charging. By opening the gate, concrete flows through the “reinforcement bars” into the horizontal section of the box. Tremie concrete with suitable workability behaviour should reach the end of the L-box after 11s or less. In case the concrete doesn’t reach the end of the L-box, the mix design needs to be reviewed and re-assessed. The L-box test can be carried out even after the concrete has rested for 60min or more inside the upper section (gate closed) if evidence is required about delayed or extended workability. Figure 2: Shear stress vs shear strain rate for normal, tremie and super- workable concrete (1). Figure 3: Additional hydraulic and hydrostatic pressure on a pile in different soil types. 41-48 Larisch.indd 43 41-48 Larisch.indd 43 25/11/13 3:58 PM 25/11/13 3:58 PM