4. Compaction of Concrete at Prefab Companies
What Happens at Compacting of Concrete?
The uncompacted fresh concrete consists of cement, surcharges of different grain size (sand, split, gravel), and water. Through mixing the necessary uniformity of the compound is ensured. The concrete which is filled out of the mixing unit to the shuttering, because of the many air bubbles has much air pore space, which is equivalent with low concrete strength. Therefore the air has to be brought out of the concrete again.
Vibration effects that the cohesion and the friction of the concrete constituents are strongly decreased. Thus the density of the grain particles increases and the trapped air is pressed to the surface and escapes. The coarse grain of the concrete should only move little to avoid demixing. The coarse grain should only be moved so much that the edges of the grain bodies align to each other; so that thus the remaining interspaces become as small as possible and a high density is reached. The through the vibration created surplus water of the fresh concrete promotes this process (reduced friction) and fills up the remaining spaces between the coarse grain particles. The surplus water is activated by tearing open the surface tension of the water drops which surround the cement grains. For this process high acceleration and high vibration frequencies (100-200Hz) are necessary.