Workability Assessment of Pumpable Structural-Grade Heavy-Weight Concrete Using Novel Coaxial Cylinder Method

The mixture proportioning of heavy-weight concrete (HWC), as seen from the literature, is primarily based on a trial-and-error approach; even so, data from the literature indicate that obtaining high-slump mixtures for high density levels (more than $3.1\mathrm{g}/{\mathrm{cm}}^3$) is difficult. Because heavy-density aggregates are prone to segregation at high slump, a low water to cement ratio is generally used for HWCs, and full-scale pumping trials are needed for checking pumping placement. For structural-grade HWCs used in nuclear facilities and specific infrastructure such as tunnel cross-rail floating slabs, rheological properties are needed to quantify the workability for pumping placement. Because use of rheological assessment methods using sophisticated rheometers cannot be realized easily onsite, a novel coaxial cylinder method is proposed in this study for determining the yield stress of HWCs delivering 3,800 to $\mathrm{3,900}\mathrm{kg}/{\mathrm{m}}^3$ density. The device proposed in this study is portable and can be used also onsite. Four structural-grade hematite aggregate HWCs, with design strengths varying from 40 to 70 MPa, were designed with ordinary portland cement. To understand the effect of fly ash addition on rheological behavior, one of the structural-grade HWC with three different levels (15%, 25%, and 35%) of fly ash was investigated. For referring the initial workability and workability retention properties of HWCs used in jobsite pumping, the conventional slump test values of the mixture were compared with yield stress measurements. Experimental investigations suggested that the method proposed in this study are able to adequately capture the workability characteristics required for pumpable mixtures. Further, it was seen that 35% partial replacement of fly ash resulted in 100% reduction in yield stress of HWC mixtures. Fly ash replacement (15% to 35%) did not significantly alter the hardened concrete density ($\mathrm{3,800}\mathrm{kg}/{\mathrm{m}}^3$); however, attainment of the specified compressive strength depended on the replacement level of fly ash and extended curing period.