When ready mixed concrete trucks are delayed in transit or on the job site, what effect will the additional time of mixing have on the slump of the concrete?
Tests conducted by the Research and Development Laboratory of the Portland Cement Association have demonstrated that the slump of concrete is reduced as mixing time is increased. While loss of water by evaporation is undoubtedly a factor, the phenomenon is probably more specifically related to the increase in fine material resulting from the grinding action of the mixing process. In one test series slump dropped from 9.0 inches after 1 minute of mixing, to 8.4 inches after 5 minutes, to 6.4 inches after 30 minutes and to 2.6 inches after 60 minutes. There were also marked increases in compressive strength with increased mixing time.