The three main requirements for transporting concrete from the mixing plant to the job site are--
Speed. Fast transportation does not allow concrete
to dry out
or lose workability
mixing and placing.
Minimum material segregation. To produce uniform concrete, you must take steps to reduce segregation
of the aggregates and paste to a minimum and prevent the loss of fine material, cement, or water.
No delays. Organize the transportation to eliminate delays in concrete placement that cause undesirable
fill planes or construction joints.
Importance of Proper Procedures
You cannot obtain the full value of well-designed concrete without using proper placing and curing procedures.
Good concrete placing and compacting techniques produce a tight bond between the paste and coarse aggregate
and fill the forms completely, both of which contribute to the full strength and best appearance.
Preparation before concrete placement includes compacting, trimming, and moistening the subgrade; erecting the
forms; and setting the reinforcing steel. Moistening the subgrade is especially important in hot weather to prevent
water extraction from the concrete.
Just before placement, check the forms for both tightness and cleanliness. Check the bracing to make sure the
forms will not move during placing. Make sure that the forms are coated with suitable form oil or coating
material so the concrete will not stick to them. Remember, in an emergency, you can moisten the forms with
water to prevent concrete from sticking. Forms exposed to the sun for some time dry out and the joint tend to
open up. Saturating the forms with water helps to close the joints.
Depositing Fresh Concrete on Hardened Concrete
To obtain a good bond and a watertight joint when depositing new concrete on hardened concrete, make sure that
the hardened concrete is nearly level, is clean and moist, and that some aggregate particles are partially exposed.
If the surface of the hardened concrete is covered by a soft layer of mortar or laitance (a weak material consisting
mainly of lime), remove it. Wet sandblasting and washing is the best way to prepare old surfaces, if you can
remove the sand deposit easily. Always moisten hardened concrete before placing any new concrete; saturate
dried-out concrete for several hours. Never leave pools of water on the old surface when depositing fresh
concrete on it.