Air—the magical elixir that racks miracles. In its invisibleness, it houses a variety of gasses that include nitrogen (78%), oxygen (21%), argon (93%), and carbon dioxide (0.04%). From it, trees rehabilitate carbon dioxide and transpire oxygen; our lungs separate oxygen from it, which keeps us alive; the oxygen part of air combines with many elements and radicals and is the basis for many compounds; oxygen causes rust; air provides resiliency to tires and adds to our comfort; entrained air protects concrete. Who would think something so visible, whose composition is so invisible, pervades our universe so completely, and has such omnipresence and power, can be both so useful and so harmful?
Oxygen in air combines with many elements and radicals, and is the basis for many compounds. With it, rust occurs, and without it, the oxidation of iron wouldn't happen. It also makes concrete durable in cyclic freezing environments when used as a source for entrained air bubbles.
Air houses carbon dioxide that carbonates portland cement paste and, in part, converts it to calcium carbonate (calcite, CaCO3) while liberating water. Without it, paste wouldn't carbonate. However, when carbon dioxide and carbon monoxide from unvented exhaust fumes contact concrete during its early hardened life, dusty and soft concrete surfaces result, which is a bad thing.
At much later ages, it depresses concrete pH from about 12.4 to 8 or 9. This drop can initiate steel corrosion when steel levels are reached. A nemesis of concrete block is carbonation shrinkage, which if not accommodated by joints, causes cracks in masonry walls. After adequate hardening, carbonation increases strength and hardness because uncarbonated paste is converted to much harder calcium carbonate. In this case, a good thing, except for the shrinkage that accompanies it. Even craze cracks, a result of carbonation and drying shrinkage, are tolerable and accepted when they are virtually invisible and do not extend deeply into concrete.
Air protects concrete from cyclic freezing damage when activated by certain chemicals, such as foaming agents and surfactants, during concrete mixing. In small amounts—4 ounces per 64,000-ounce (4000 pounds) cubic concrete yard accounting for 0.006%—these chemicals generate billions of spherical, microsized air bubbles with a nominal bulk volume of 6% by concrete volume.
The important air-void parameters with respect to cyclic freezing protection are: air volume (percent); specific surface (square inch/cubic inch); and void spacing factor (inches). The 6% air volume is grossly in excess of the theoretical amount needed; however, it is there to help assure the two other parameters, and especially the important void-spacing factor (symbolized as – L), is attained. The void-spacing factor is half the distance between the perimeters of adjacent air voids. When you think about it, this is the greatest distance a particle of water has to travel to get into an air void before disruptive force sufficient to stress concrete is created. Empirical data indicate that distance is 0.008 inch or less—halfway between two adjacent air voids—therefore, the distance between voids is 0.016 inch.
Entrained air also can be bothersome, especially when it is in excess, which creates finishing difficulties along with depressed strengths. Sometimes when penetrating types of surface coatings are applied, air is displaced to rise upward—a phenomenon known as outgassing.
Akin to air is hydrogen gas that can be generated by reactions of alkalies from portland cement and aluminum powder. Thomas A. Edison created foamed concrete for insulating concrete homes he was building using that chemical reaction. The same chemical phenomenon has depressed concrete strengths when aluminum inadvertently scraped from concrete pump hoses generated entrained air-like bubbles.
Zinc also can react to form hydrogen gas bubbles as evidenced by galvanized forms where the generated gas leaves strings of vertically oriented entrained air-like voids in the concrete surface. Dipping the galvanize in chromate solutions or adding chromate to mixing water inhibits the reaction.
Wherever you are air usually will be with you. It pervades the atmosphere, our lives, and concrete livelihoods. All that from something invisible!
