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Concrete In Australia : December 2013
46 Concrete in Australia Vol 39 No 4 FEATURE: RHEOLOGY Foundations” for the design and verification of CFA piling concrete mixes. 5.0 FUTURE OUTLOOK 5.1 Limitation of bleeding by introducing mineral additives and optimised concrete designs For deep foundations bleeding is a recurring problem, which needs to be identified prior to commencement of works on site. With the filtration test, there is a suitable fresh concrete test to detect potential concrete bleeding at a very early stage and to give engineers and concrete suppliers the opportunity to change the mix design accordingly. In December 2012, trials were carried out in Brisbane to demonstrate that concrete bleeding and segregation can be significantly reduced by adding a thixotropic rheology- modifier made from a highly purified form of a hydrous Mg- aluminosilicate nanoclay mineral. This mineral additive has an electro mechanic interaction and due to its uniform particle size and high aspect ratio, this minor additive creates a gel-like structure that substantially improves aggregate suspension, water retention and concrete stability when fully dispersed in concrete at dosages of ~0.03%-0 .075% (total dry weight material basis). During mixing or emplacement, flowability and workability are greatly enhanced because of its frictional shear- thinning behaviour. On removal of shear resistance, the rate of thixotropic rebuilding of the gel structure for this mineral additive is exceptionally fast which provides superior cohesion, early strength, and anti-washout properties that are desired in deep foundation concretes and other applications.10 Bleed water, measured by the filtration test, of a standard 40 MPa piling mix was reduced by more than 50% after adding the mineral additive and slightly optimising the grading of the aggregates (Figure 8). Furthermore, the concrete strength is not negatively affected by the addition of the mineral additive to the concrete mix and all compressive strengths, shrinkage measurements as well as workability requirements were improved. Further research will be carried out in this area, but it is obvious that new products, together with smart concrete mix design, might offer significant help to the industry to reduce concrete bleeding and to achieve better products with less non conformances. 5.2 Modelling concrete as a non-Newtonian frictional viscoplastic fluid This section briefly presents the results of the computational fluid dynamics (CFD) simulations of the slump and L-box tests carried out for TC placed in deep foundations. As mentioned previously, concrete placement in piles is a blind process, especially for deep foundation elements. To control the quality of the foundation, it is desirable to know the concrete flow performance during placement. The advance of CFD modelling provides the possibility to simulate the whole process of pouring concrete in the deep excavation or cavity in the soil. With the aid of graphs or videos of the results, the simulation allows engineers to virtually predict what is happening in the inaccessible space where the concrete is flowing. Before simulating the real concrete placement process, small scale model tests need to be established to simulate the laboratory test and validate the numerical model. Two CFD models are developed using ANSYS Fluent 13.0 software to simulate slump and L-box tests. Each model mimics the same geometry as required by the real slump and L-box tests (section 2.2.1 of this paper). The geometrical models used for CFD discretisation (mesh) are shown in Figure 9. The multiple material phase models (concrete, water, air) are used in the CFD modelling. Flow behaviour in the two tests is treated as transient. The concrete mix is treated as a non-Newtonian frictional viscoplastic fluid.9 For the L-box test, the concrete stays initially in the vertical section of the L-box, i.e. the horizontal section of the L-box is initially empty. The concrete starts to flow due to gravity when the gate opens or is removed. For the slump test, the initial condition is that the test cone is fully filled with concrete. The simulation starts when the cone is lifted or removed. Figure 10 displays the process of the TC flowing along the L-box. The flow length, represented by the distance from the gate to the tip Figure 7: Advanced concrete testing using filtration press (left) and L-box (centre/right). Compressive Strength: 40 MPa Max aggregate size: 10 mm Cementitious material: 450 kg/m3 Retardation: 4 hours TARGET: Target slump: 240 mm Target slump flow: 420 mm Filtration press water loss: <22 ml Filter cake length: <150 mm ACTUAL: Actual slump: 250 mm Actual slump flow: 420 mm Filtration press water loss: 9ml Filter cake length: 75 mm 41-48 Larisch.indd 46 41-48 Larisch.indd 46 25/11/13 3:58 PM 25/11/13 3:58 PM