When stationary mixers are used, the first cubic metre of concrete
should be mixed for 1 minute and each additional cubic metre should be
mixed for 20 seconds. Mixing time starts when all cementing materials
and aggregates are added to the mixer provided all of the water is
added before 1/4 of the mixing time has elapsed. Concrete that is kept
agitated does not get very stiff in the first 1/2 hour and usually
can be placed and compacted up to 1 1/2 hours after mixing.
Moisten the subgrade when
concrete is placed in warm or hot weather, and keep it unfrozen in cold
weather. The subgrade should have even bearing, be drained,
have no plant matter or frost, and should have the correct level
or sloped contour. Undisturbed soil is generally better at
supporting
footings and slabs than compacted soil.
