Loads imposed by fresh concrete on wall or column forms are different from the gravity loads on a deck form. The fresh concrete acts temporarily like a fluid, pushing out against the sides of the form, somewhat like water pushing against the walls of a tank. If concrete is placed rapidly in a form, in less time than it takes for the hardening to start, its lateral pressure is like a full liquid head. When a vibrator is inserted in concrete, it can temporarily liquefy all concrete within its radius of action. This increases lateral pressure. Retarding admixtures, fly ash, or cooler weather can also increase lateral pressure by delaying the hardening of the concrete.
Form designers use a modified liquid pressure when planning how strong to make the forms. They know that the amount of the pressure is affected by the weight, rate of placement, and temperature of the concrete mix; the use of retarding admixtures; and the effects of vibration or other consolidation methods.
A controlled placement will result in even loading of formwork members of ties, and make vibration more effective. Concrete will show less segregation, fewer air pockets and honeycombing.