Even within the specified limits on slump and water-cementitious materials ratio, excess water must be avoided. In this context, excess water is present for the conditions of placing if evidence of water rise (vertical segregation) or water flow (horizontal segregation) occurs. Excess water also tends to aggravate surface defects by increased leakage through form openings. The result may be honeycomb, sandstreaks, variations in color, or soft spots at the surface.
In vertical formwork, water rise causes weak planes between each layer deposited.
In addition to the deleterious structural effect, such planes, when hardened, contain voids through which water may pass.
In horizontal elements, such as floor slabs, excess water rises and causes a weak laitance layer at the top. This layer suffers from low strength, low abrasion resistance, high shrinkage, and generally poor quality.