On Mixing, Molding and Curing Equipment

(1)Mixers are the most important item of the precast concrete products industries. Although both batch and continuous mixers are used, the former are in far more general use. A given charge of materials is placed in the batch mixer, mixed and discharged before the next lot of materials is added. Batch mixers are manufactured in a wide range of sizes, and with various combinations of mechanical features. The desirability of extremely strong construction is a good point to keep in mind when selecting a mixer, for a light machine, even though it may be adequate for the mixing job, may prove to be highly vulnerable when some foreign object is by chance introduced. Replaceability of parts subject to wear is another vitally important factor to be considered.

(2) In continuous mixers, the proportioned materials are introduced at intervals, and the mixing goes on continuously during the passage of the materials through the machine. Maintenance of strict uniformity with continuous mixers is somewhat more difficult than with batch machines, and they are for this reason considered to be better suited to very large operations.

(3) Molding of precast plain and reinforced-concrete units is usually accomplished with the aid of vibration. Precast reinforced-concrete pipes are manufactured either by vibrating or by centrifugal process. When reinforced-concrete pipes are to be used in pressure lines the requisite impermeability, density and strength are best obtained by centrifugal casting. Pipes from 12 to 78 inches in diameter and from 8 to12 feet long are made by this process. When the required amount of concrete has been placed into the mould, the latter is rotated at high speed to remove all excess mixing water.

(4) Concrete sets in the presence of moisture and heat. Chemical reactions stop almost completely when the temperature drops below 40 degrees F., and the lack of moisture during the setting period results in incomplete hydration of the available cement. Under either of these conditions precast reinforced-concrete units fail to develop more than a fraction of the strength potentially available in the material. Obviously, then, the maintenance of favorable curing conditions is of prime importance. Although 28-day air curing and drying under cover seems to offer a fair solution to the problem, it creates, especially in high-production plants, some almost equally serious problems, by tying up large amounts of capital in equipment, and because of relatively large storage areas required. That is why high-pressure steam curing is very widely used now.

(5)Although the equipment is more expensive, curing with high-pressure steam is perhaps the most advanced practical form of concrete curing. It reduces the curing period from days to hours, allowing the precast reinforced concrete elements to be delivered to the construction job as early as 15 or 16 hr after they are moulded.

(6)Cylindrical steel autoclaves are employed in this process. The freshly-made units are wheeled into the autoclaves, tightly closed there, and steam at 120 lb per sq. in. gage pressure or more is turned on, the temperature inside reaching 350 to 360 deg. F in about 3 hr. After 7 to 8 hr. of curing the steam is turned off and the pressure is rapidly lowered during a 1/2 hr. period to atmospheric pressure afterwhichthe units are removed. In general units cured in this manner will have strength at least equivalent to that, ob­tained by 28 days of continuous moist curing at 70-80 deg. F.

4. Compose pairs of English and Russian terms

mixing water 1. условия вызревания бетона

high-pressure steam curing 2. схватываться

mixer. 3. складские площади

to set 4. твердение при влажном режиме

molding 5. бетономешалка

curing conditions 6. вода затворения

impermeability 7. формовка

air curing 8. неармированный бетон

storage areas 9. заменяемость

moist curing 10.водонепроницаемость

11.обработка (бетона) паром высокого давления

12.вызревание (бетона) на воздухе

5. Compose word combinations. Make your own sentences on the topic of the text