Technical Questions

What factors affect the mixing time?

Q. What factors affect the mixing time?

A. The importance of thorough mixing for the development of strength and uniformity throughout the batch has long been recognized. With a proper loading sequence, truck mixers should adequately mix the concrete in 70 to 100 revolutions. If high-range water-reducing admixtures are added to the batch at the job site to produce flowing or high-slump concrete, an additional 30 revolutions of mixing should be used to provide adequate incorporation of the admixture. Mixing should be done in the range of mixing speed recommended by the mixer manufacturer, generally in the order of 12 to 18 rpm. The mixer should have a visible manufacturer’s rating plate attached to the frame that specifies mixing and agitating speeds. For safety reasons, revolving the mixer drum at mixing speed during travel is not recommended. Agitating speed during travel after loading and mixing is in the range of 2 to 4 rpm.

An excessively long period of mixing after adequate blending has been attained may be harmful in any type of mixer. Slump is reduced and, if water is added to restore the original slump, strength will be reduced. If concrete is maintained at a constant slump by retempering with water, concrete will lose strength at a rate of 8 to 20% per hour, depending on the concrete temperature and the rate at which it loses slump. With some softer aggregates, long continued mixing in a stationary mixer will cause additional slump loss as additional fines are generated due to the grinding of ingredients that takes place. With truck mixers, after 100 revolutions at mixing speed, the drum should be rotated at an agitating speed less than 6 rpm, and there should be no more agitation than is necessary to prevent settlement and difficulty in starting again.

References: SP-1(02); ACI 304R-00

Topics in Concrete: Concrete Fundamentals