CONCRETE. Concrete cannot be placed during very cold weather because freezing water will destroy a concrete which has set partly

PART II

Concrete cannot be placed during very cold weather because freezing water will destroy a concrete which has set partly. An approximate limit is that no wet concrete, after it is placed in its final position, should be at a temperature less than 5°C or 40°F. Even then insulation of the concrete will be necessary and the curing period, which is required to gain strength, will be long. In hot weather the problem during placement is to prevent a premature set and, after placement, in order to ensure a proper cure. The concrete must not dry out too fast because this may cause cracking.

One of the principal disadvantages of the use of cast-in-place concrete is the time that is required before the removal of the formwork. This period of time is necessary so as to allow the concrete to gain sufficient strength before it has to support itself; therefore the period before the removal of formwork must be longer for horizontal elements, such as beams and slabs, than for vertical elements, such as columns. Because of the cost of the formwork and in order to allow as rapid re-use as possible, cements which gain strength quickly are preferred by contractors. However, these cements produce such heat after they set that they may cause thermal cracking. In addition, the high temperature at which curing takes place leads to losses in the long term properties of the concrete. Slag cements and other low heat cements are better from this point of view. The formwork to beams and slabs is usually removed after a reasonable period of time but the props remain for a further period.

Typical minimum periods before removal of formwork for concrete with a surface temperature of 16°C (60°F) are from 9 hours to 15 days.

Notes:

freezing water замерзающая вода

premature adj. преждевременный

prop n. стойка

slag n. шлак

Ответьте на вопросы к тексту.

1. Why can’t concrete be placed during very cold weather?

2. What is the minimum temperature allowable for wet concrete?

3. What is the problem during placement of concrete in hot whether?

4. Why must concrete not dry out too fast?

5. What are the principal disadvantages of using cast-in-place concrete?

6. Why can’t formwork be quickly removed?

7. How does it influence the choice of cements?

8. What are the disadvantages of the cements which gain strength quickly?

9. What cements are better from this point of view?

10. What are typical minimum periods before removal of formwork?

Date: 2016-02-19; view: 665; Нарушение авторских прав