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Dispensing admixtures

Admixtures are added to concrete in small dosages hence accurate dispensing is important. The dosage of most admixtures is between 0.2 to 1.5% volume by mass on cement (i.e. 200–1500 ml per 100 kg of cement). This means that small changes in actual volume of admixture dispensed can have a large effect on dosage as a proportion of cement and thus significantly affect the quality and properties of the concrete. For this reason, it is important that admixtures are added to the concrete through an accurate, calibrated dispenser. There should be an automatic flushing system to clean the dispenser and ensure all admixture is fully washed into the mixer.

The three main types of admixture dispenser are volumetric measurement, weighing and volumetric metering. All three types have a successful history of use and each has its strengths and weaknesses.

With volumetric measurement, the admixture is measured in a graduated cylinder (the sight-glass) and delivered to the concrete mix via a pump or compressed air. The sight-glass provides the main advantage in that the operator can see the correct amount has been batched.

For volumetric metering, a flow meter is placed in the line taking the admixture from the storage tanks to the mixer.

Most weighing dispensers consist of single- or multi-compartment vessels on one or more load cells. The admixtures are either pumped or gravity-fed to the mixer.

Acknowledgement: The Concrete Society

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TR18 A guide to the selection of admixtures for concrete