Research Report: Rheology and Workability Testing of Deep Foundation Concrete in Europe and the US-PDF

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The EFFC–DFI research project examined how the rheology and workability of fresh deep-foundation concrete affect its ability to fill bored piles and diaphragm walls while remaining stable and workable during placement. Field trials in Europe and the US, alongside laboratory tests, assessed flowability, viscosity, thixotropic build-up, workability retention, segregation, bleeding and pressure-induced filtration.<br /><br />The findings show that conventional slump or spread measurements alone cannot fully predict concrete performance. Slump-flow diameter correlated well with dynamic yield stress, while the speed of slump flow provided an indication of plastic viscosity. Thixotropy—the concrete’s tendency to stiffen while undisturbed—was better assessed with static yield-stress measurements using a vane rheometer or portable vane tester than by comparing delayed slump-flow or L-box results. Workability retention should be measured after remixing the concrete; undisturbed samples can stiffen through thixotropic build-up and give a misleading indication of lasting workability. Stability also requires separate assessment, since flow tests do not adequately capture segregation, bleeding or filtration.<br /><br />The report identifies the slump-flow test as the most practical onsite method because it provides a measure of yield stress, an indication of viscosity from flow speed, and a visual stability check. It proposes indicative acceptance ranges: slump flow 400–550 mm, flow velocity 10–50 mm/s, Visual Stability Index of 0, flow-table spread 560–640 mm, modified cone outflow 3–6 seconds, workability retention of at least 400 mm, and Bauer filtration of no more than 22 ml.<br /><br />These are recommended ranges, not universal limits. Project-specific target values and tolerances should be selected by a suitably qualified professional, taking account of placement conditions and the required form filling.

Keywords

deep-foundation concrete

rheology

workability

slump-flow test

yield stress

plastic viscosity

thixotropy

segregation

bleeding

filtration