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.
Materials In Concrete
2.Concrete Aggregates - Concrete Slump.
3.Concrete Placement - Vibration - Hydration.
4.Types of Portland Cement - Concrete Moisture & Cracking.
5.Watertight Concrete - Supplementary Cementing Materials.
6.Supplementary Cementing Materials.
7.Chemicals That Damage Concrete.
8.Concrete Aggregate Properties.
9.Angular & Smooth Aggregate Properties.
12.Air Entraining Admixtures.
13.Air Entraining Admixtures Continued.
14.Conditions Affecting Concrete Air Volume.
15.Superplasticizers & Other Admixtures.
16.Ordering & Mixing Concrete.
17.Concrete Mixing - Subgrade Preparation.
18.Subbase Preparation - Concrete Placement.
22.Hot Weather Concreting.
23.Cold Weather Concreting.
24.Concrete Slump Test.