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Concrete In Australia : December 2013
Concrete in Australia Vol 39 No 4 41 Performance control tests and numerical simulations for concrete in deep foundations M. Larisch, National Technical and Quality Manager, Piling Contractors Dr Z. Qin, Researcher, CSIRO Dr H. Alehossein, Principal Researcher, CSIRO High quality concrete for deep foundations needs to meet particular performance criteria to achieve the required workability (plastic flow-ability) and stability (resistance to bleeding). Workability is normally measured by the concrete slump test and most concrete specifications, codes and standards largely rely on this test alone. However, the slump test alone is not a reliable method to determine workability, although it can reasonably indicate the consistency of a fresh concrete mix. Stability requirements to control concrete bleeding and aggregate segregations are usually not part of the current specifications and no standard test methods are recommended. For instance, it is important to consider the pressure effects from the hydraulic head and self-weight of the concrete mass on the fresh concrete at the base of deep foundations causing potential risks of bleeding and other associated stability and quality issues. Concrete for deep foundations must have excellent workability criteria and sufficient resistance against bleeding to guarantee a non-defective end-product. Over the past decade, concrete technology has changed considerably, and the test methods recommended by the current codes and standards need to be adjusted to reflect these changes. This paper highlights the new requirements, particularly the introduction of performance based testing criteria as recommended in “ Tremie Concrete for Deep Foundations”, published by the Concrete Institute of Australia in 2012. Concrete viscosity and plastic yield are the two important characterisation parameters to be considered. A case study will demonstrate how these characteristics can be achieved by applying the newly introduced performance tests. New testing criteria recommended here will meet the progressive demands of modern concrete technology for deep foundations as a next step for the entire concrete industry. To prove the capability of numerical methods to mimic the complex process of concrete behaviour, CFD modelling has been used to simulate the laboratory tests. These simulations demonstrate that the behaviour of a highly workable concrete is similar to a non-Newtonian frictional plastic-viscous fluid, and CFD modelling is a suitable tool to model the behaviour of fresh concrete inside deep foundation elements. 1.0 INTRODUCTION Concrete for deep foundations, like piles and retaining structures, is subject to special conditions within a distinctive environment (soil, rock, water). Special placement techniques as well as particular fresh concrete properties are required to ensure an excellent, fully compliant end-product free of damage. Depending on the ground conditions and load requirements, deep foundations can reach depths up to 125 m below the ground surface with foundation excavations usually full of drilling fluids such as bentonite or polymers used to support the excavation walls. Concreting in deep excavations normally has different requirements to those for superstructures since the former is subjected to additional pressures and requires replacing the drilling fluid with concrete during placement (Figure 1). The following aspects are essential for concrete mix designs for deep foundations: • concrete must have sufficient stability against segregation in order to withstand the hydrostatic pressure at the pile base, Figure 1: Concrete for deep foundations needs more quality control than structural and shallow foundation concrete, due to much higher hydrostatic pressures. 41-48 Larisch.indd 41 41-48 Larisch.indd 41 25/11/13 3:58 PM 25/11/13 3:58 PM