土木工程专业英语翻译Curing ConcreteWhen a batch of concrete is first m

土木工程专业英语翻译
Curing Concrete
When a batch of concrete is first mixed, it forms aplastic mass that can be poured into prepared forms with little effort. Inabout an hour the concrete sets into a rigid mass that weighs about 150 lb percu ft. The cement paste is said to hydrate, that is, the individual particlesof cement absorb the surrounding molecules of water into their molecularstructures. As they do, the cement crystallizes into a kind of rigid gel,something like gelatine, that gradually changes with the passing of time into asolid mass of minute, interlocked crystals. The longer hydration continues, thestronger the concrete will become. It is standard engineering practice,therefore, to calculate the final design strength of concrete on the basis of a28-day curing period, although under exceptional conditions concrete has beenknown to continue increasing in strength for a quarter of a century and longer.
The time during which concrete hydrates and increasesin strength is its curing period. Concrete cannot hydrate, or cure, unlessthere is water present within the concrete. Throughout the curing period,therefore, all the exposed surfaces of the concrete must be kept moist. As longas the concrete is kept moist, curing will continue and the concrete will becomeincreasingly stronger, denser, and more impervious to water.
Once concrete is allowed to dry out completely,however, hydration stops. Usually it is the surface of concrete that isadversely affected by a too-short curing period. For example, the surface of asample of concrete that has been kept moist for 28 days will be twice as strongas a surface that has been kept moist for only 3 days.
The outside air temperature is an extremely importantfactor in proper curing. If the air temperature is too high, over 90°F, say, the water in the concrete may evaporate awaybefore hydration can be completed. An excessive internal temperature may alsointerfere with proper hydration. Concrete that has been mixed and placed duringvery hot weather is never as strong as concrete that has been placed when theair temperatures are 70°F and below,mainly because of the difficulty of keeping the concrete properly moist. Furthermore,concrete that sets too quickly during hot weather is more likely to crackafterward because it will have shrunk an excessive amount during is curing, andit will not thereafter be able to withstand the stresses imposed on it by largechanges in temperature.
Nor can concrete be placed during freezing weatherunless special precautions are taken to keep the concrete above 50°F, for at least 4 days after it has been placed. Ifthe temperature of the concrete should fall below 50°F, it will never harden properly. And if the concreteis allowed to freeze before it has set, it will be permanently damaged.