Concrete

PART I

A property that is particularly connected with concrete is shrinkage, which is the permanent, contraction of concrete. It is caused by loss of water from the cement paste after hardening. The rate of shrinkage is rapid in the period which immediately follows the set of the concrete but becomes much slower with time. Because of the low tensile strength of concrete, shrinkage when it is limited leads to cracking.

Cast-in-place reinforced concrete as a method of construction is able to be applied to non-standard areas and shapes. It forms heavy, rigid, fireproof construction with relative ease.

Because all the necessary materials are easily available, construction work in cast-in-place concrete may start almost immediately, which sometimes gives faster completion times than if shop fabricated elements are used, such as those in steel or precast concrete.

About 50% of the cost of cast-in-place construction is that of the formwork. Hence, standard dimensions, that enable constant re-use of formwork, will significantly reduce the cost of cast-in-place concrete work.

In order to obtain good strength and durability, concrete must be allowed to cure, in conditions of temperature and humidity that are reasonably controlled after the initial set of the concrete. In general, moderate temperatures, between 20° C and 40° C (68°F and 86°F), and moist or wet conditions are the best. The total curing period is extremely long. However, curing is often considered to be over after the concrete has gained a reasonable strength, although curing and strength are not related. A minimum curing period is usually at least seven days.

Notes:

completion n. завершение, окончание

contraction n. сжатие

moist adj. влажный

Ответьте на вопросы к тексту.

1. What property is associated with concrete?

2. What is shrinkage caused by?

3. What is the rate of shrinkage?

4. Why does shrinkage lead to cracking?

5. What is the advantage of “cast-in-place reinforced concrete”?

6. Why is the use of cast-in-place concrete sometimes better than the use of shop fabricated elements?

7. What is the cost of formwork?

8. Why is it useful to re-use the formwork?

9. What should be done to obtain good properties of concrete?

10. What conditions are these?

11. What is the total curing period?

12. What is a minimum curing period?