Freshly placed unconsolidatedconcrete contains excessiveand detrimental entrapped air. If allowed to harden in thiscondition, the concrete will be porous and poorly bonded tothe reinforcement. It will have low strength, high permeability,and poor resistance to deterioration.It mayalso have a poorappearance.
The mixture should be consolidated if it is tohave the properties desired and expected of concrete.Consolidation is the process of inducing a closer arrangementof the solid particles in freshly mixed concrete or mortarduring placement by the reduction of voids, usually by vibra-tion, centrifugation (spinning), rodding, spading, tamping, orsome combination of these actions.Stiffer mixtures require greater effort to achieve properconsolidation.
By using certain chemical admixtures (ACI212.3R),consistencies requiring reduced consolidationeffort can be achieved at lower water content. As the watercontent of the concrete is reduced, concrete strength, perme-ability,and other desirable properties improve, provided thatthe concrete is properly consolidated. Alternatively, the ementitiousmaterials content can be lowered, reducing thecost while maintaining the same strength.
If adequateconsolidation is not provided for these stiffer mixtures, thestrength of the in-place concrete decreases rapidly.Equipment and methods are now available for fast andefficient consolidation of concrete over a wide range ofplacing conditions. Concrete with a relatively low watercontent can be readily molded into an unlimited variety ofshapes, making it a highly versatile and economicalconstruction material. When good consolidation practicesare combined with good formwork and good form releaseagents, concrete surfaces have a highly pleasing appearance(Fig. 1.1(a) through (c)