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One challenge of using self consolidating concrete (SCC) for cast-in-place structural applications is that the rheological properties of the concrete (low yield stress and low plastic viscosity) can lead to increased lateral pressure exerted on the formwork. High structural build up and thixotropy can reduce the lateral pressure, thereby allowing for simpler formwork and faster construction. The addition of CO 2 to SCC, in proportions from 0.063 % to 0.250 %, by weight of cement, was investigated as a rheology modifier. The CO 2 served to increase the plastic viscosity with little impact on the dynamic yield stress up to a dose of 0.125 %; higher dosages increased the dynamic yield stress with a lesser effect on plastic viscosity. The initial formwork pressure was reduced, pressure decay increased, and the pressure cancellation time was decreased. The effects are attributable to an increased product volume. [ABSTRACT FROM AUTHOR]

The rheological properties of mortar samples with different additions of CO 2 during mixing were determined. Flowable cement-based mortars at three different w/c (0.55, 0.45 and 0.35) were produced and the effects of CO 2 gas were examined on the fresh properties of slump flow, yield stress and plastic viscosity. According to the experimental results, the mortar yield stress and plastic viscosity increased faster over time than in a reference system with the increasing addition of CO 2 and greater effects were seen at the lowest w/c of 0.35 than at the others. The impact of the CO 2 on the yield stress was more pronounced than on the viscosity. In the w/c 0.35 system the properties showed no effect at dosages of 0.06% and 0.13% CO 2 by weight of cement. Increasing dosages to 0.19 and 0.25% resulted in an increasing reduction in slump flow, immediate increases in yield stress of 62 and 298%, increases in structural buildup by a factor of 5 and 10, respectively, and increased the initial plastic viscosity by 33 and 148%, respectively. The effects are attributable to the growth of reaction products decreasing the interparticle spacing and increasing the surface area as associated with the CaCO 3 and gel phase. CO 2 was demonstrated to be a potential yield stress control agent; an appropriate dose can increase rheological properties as measured immediately after and increase their rate of change with time. This aligns with specialized applications that hinge upon changing rheology such as form pressure control or 3D concrete printing. [ABSTRACT FROM AUTHOR]

Carbon dioxide was investigated for use as a beneficial admixture to concrete as it was truck mixed. The reaction between the CO 2 and the hydrating cement creates finely distributed calcium carbonate reaction products that thereby influence the subsequent hydration. Comparisons of the fresh, hardened and durability properties were made between a reference concrete batch, a batch that contained a conventional accelerating admixture, and three batches subjected to a carbon dioxide addition. The optimum dose of carbon dioxide was found to reduce the time to initial set by 40% and increase the one and three day compressive strengths by 14% and 10% respectively. In comparison to the CO 2 batch, the conventional accelerator provided greater reductions in set time but lower early strength. Concrete durability test results indicated that the carbon dioxide process did not compromise the expected durability performance of the treated concrete. Carbon dioxide is a viable admixture to improve concrete performance. [ABSTRACT FROM AUTHOR]