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Concrete In Australia : December 2013
42 Concrete in Australia Vol 39 No 4 FEATURE: RHEOLOGY which is much higher than the pressure at the surface • since piles are commonly installed under drilling fluids or water, concrete must displace the fluid or water completely (excellent concrete flow-ability required) • concrete must be capable of self-leveling to avoid dispersion and migration of drilling fluid, and to ensure sufficient ability of concrete to flow around areas of dense reinforcement • concrete must be capable of self-compacting • in some cases concrete initial setting needs to be retarded for several hours in order to ensure sufficient and constant workability properties throughout the entire pour. Concrete placement in piles is generally a blind process and there are only limited controls. The required workability must be maintained for the entire duration of the pour. 2.0 PREVIOUS WORK In 2012, the Concrete Institute of Australia published the Recommended Practice “Tremie Concrete for Deep Foundations”,1 which is applicable for tremie concrete used for conventional bored piles, diaphragm walls and cut-off walls. Other deep foundation systems, like Continuous Flight Auger (CFA) piles are not included in the scope of the document. However, the authors believe that in some cases the same recommendations1 could also be used for CFA piles and other applications besides deep foundations (e.g . tunneling, pre- cast elements). A successful case study with CFA piles will be presented later in this paper. 2.1 Characteristics of concrete for deep foundations The ease of flow of fresh concrete when unconfined, for example, by formwork or any other obstacles such as reinforcement – is defined as flow-ability. The two main characteristics of tremie concrete for deep foundations are workability and stability as defined below by Beckhaus et al.2 Both characteristics depend on each other, so that a lack of stability significantly influences concrete workability. Workability: Workability describes the ability of concrete to flow through tight openings such as narrow spaces between reinforcement bars without any segregation and blockage (passing-ability or blocking resistance). Stability: Stability of tremie concrete (TC) mainly considers the ability of fresh tremie concrete to retain its water despite being subjected to pressures caused by the supporting fluid or from the fresh concrete above (water retention). It is also the ability of fresh tremie concrete to maintain its flow characteristics, measured by a slump test, over certain time periods, usually controlled by appropriate admixtures (retardation). Conventional concrete requires external energy input (vibration) after placement to remove trapped air. TC should be capable of self-compacting, as any external energy input might be a cause for segregation. It has been argued that tremie concrete is comparable with super-workable concrete,3 because of their similar self-compaction requirements, which allow the mix to de-aerate while filling the formwork and flowing around the reinforcement without any help other than that of the self- weight. Despite some similarities, TC is neither equivalent to conventional concrete nor to super-workable concrete (SWC). It actually fits somewhere in between (with a tendency towards SWC) as shown in Table 1. Figure 2 shows variation of shear stress with shear strain rate for normal concrete, TC and SWC. As shown in the figure, apart from their differences, TC exhibits a very similar line trend to SWC in terms of the slope of the curve (viscosity) and plasticity yield intercept. These are distinct indicators of workability and stability of these two different concretes. It is obvious that the workability criteria for TC needs to be assessed and evaluated using different tests than the standard tests used for normal concrete with much higher plastic yield. When compared to normal concrete, both TC and SWC can be assumed as low or non-plastic – viscous material. However, when comparing TC with SWC, TC should be regarded as a general non-Newtonian plastic-viscous fluid. Indeed the present workability tests proved the importance of the two viscosity Qualitative Parameters Normal Concrete (Wet) Tremie Concrete Super-Workable Concrete Powder Volume Low Medium Medium/High Coarse Aggregates Volume High Medium Medium/High Paste Volume Low Medium Medium/High Paste Viscosity Low Medium Medium/High Concrete Viscosity High (flow by vibration) Low Low/Medium Concrete Yield Value High Medium/Low Low Table 1: Comparing normal, super-workable and tremie concrete.1 41-48 Larisch.indd 42 41-48 Larisch.indd 42 25/11/13 3:58 PM 25/11/13 3:58 PM