A specialty admixture called a super plasticizer can turn stiff, low-slump concrete into flowing, pourable, easily placed concrete. This dramatically improves workability, speeds finishing, increases strength, conserves cement and helps reduce shrinkage and thermal cracking. It also allows for higher water-to-cement ratios without sacrificing concrete quality, which is why this type of high-range water reducer is essential for self-compacting concrete (SCC) mixes.

The fluidizing effect of superplasticizers in concrete is produced by the interaction between their polycarboxylate chains and the negative charges on the surface of negatively-charged cement particles. Typical polycarboxylate superplasticizers include naphthalenesulfonate-formaldehyde, methyl cellulose ether and aliphatic urethane.

These admixtures can be added to the concrete mix at various points during the concreting process. However, the optimum time to add the superplasticizer is at the point just before placing. This way, the slump loss caused by the hydration of cement is reduced and the desired slump can be achieved with a minimum amount of re-mixing.

To ensure that the superplasticizers remain mixed with the concrete throughout the concreting process, it is recommended to use the mixer on a slow speed setting and only 80% of the water should be added at first. Then, the desired dosage of superplasticizer should be added and mixed for 1 minute. It is also important to keep the temperature of the concrete below 30oC, as this will decrease the hydration reaction and increase the compatibility between the cement and the superplasticizer.

The optimum superplasticizer dosage can be determined by plotting the admixture on a graph with its dosage on the X-axis and the marsh cone time on the Y-axis. The point at which the horizonal line intersects with the marsh cone curve is the optimum superplasticizer dosage for that concrete mix.