There are three kinds of voids in concrete surfaces: water, air, and attached. Each has distinguishing characteristics by which it can be recognized. All can be minimized in number and total volume by changes in the concrete mix and the casting and consolidation methods. Water voids occur in the concrete surface when drops of water become entrapped between the concrete and the form during vibration. The water later evaporates to leave voids. Most of these water drops come form the mixing water, but some are introduced from the cavities of the moist aggregate particles form which they are forced during vibration. Air voids are formed when air bubbles become entrapped between the concrete and the form. Although this happens in all concrete, the voids in stiff concrete are much more difficult to eliminate. Attached voids consist of air bubbles or water drips that become attached to the sides or bottom of an aggregate particles or reinforcement where the shape is irregular or cavitated enough to hold them. They are common in all concrete made with crushed or odd-shaped aggregates. To help reduce surface voids an aggregate should be selected that is smaller than standard size, has smooth surfaces, round edges, and a shape as nearly spherical as possible. The spherical shape provides the smallest surface area possible per unit volume and minimizes the frictional resistance to movement during vibration. The low resistance to movement from vibration increases the likelihood that aggregate will come to occupy the most suitable final position in existing voids and cavities and that cavities themselves will move up out of the concrete. The paste of fine cement particles in water should surround every particle of aggregate and act as a lubricant. This facilitates movement of the mortar between the form and the gravel and forces free water to the top of the concrete. If the quantity of cement paste is increased the flow behavior of the fresh concrete is improved. A water-reducing agent can be used to increase slump without changing the water content. When used in this way, it facilitates the movement of air bubbles, water drops and solid particles.
