UNIT 20 Concrete Curing Compounds
Read, translate the following text and be ready to fulfil the tests:
Need for curing. The necessity for curing arises from the fact that hydration of cement can take place only in water-filled capillaries. That is why a loss of water by evaporation from the capillaries must be prevented. Evaporation of water from concrete, soon after placing, depends on the temperature and, relatively, humidity of the surrounding air and on the velocity of wind over the surface of the concrete. Curing is essential in the production of concrete to have the desired properties. The strength and durability of concrete will be fully developed only if it is properly cured. The amount of mixing water in the concrete at the time of placement is normally more than required for hydration & that must be retained for curing. However, excessive loss of water by evaporation may reduce the amount of retained water below what necessary for development of desired properties. The potentially harmful effects of evaporation must be prevented either by applying water or preventing excessive evaporation.
Curing method. The two systems of maintaining a satisfactory moisture content are: (1) continuous or frequent application of water through ponding, sprays, steams, or saturated cover materials such as burlap or cotton mats, rugs, earth, sand, sawdust and straw. (2) prevention of excessive loss of water, from the concrete, by the application of a membrane forming curing compound to the freshly placed concrete.
Water curing .Curing by water is one of the most efficient ways for curing concrete. The curing should begin as soon as possible after the casting of concrete. Any delay in curing will lead to evaporation of mixing water, and the early drying may lead to shrinkage and cracking of concrete. However, in practice, on some construction sites regular supply of potable water for curing may not be available or it may be inconvenient and expensive. To such sites, concrete curing compound is recommended in place of water curing.Concrete curing compound consists essentially of waxes, natural and synthetic resins, and solvents of high volatility at atmospheric temperatures. The compound forms a moisture retentive film shortly after being applied on fresh concrete surface. White or gray pigments are often incorporated to provide heat reflectance, and to make the compound visible on the structure for inspection purpose. Curing compound should not be used on surfaces that are to receive additional concrete, paint, or tile which require a positive bond, unless it has been demonstrated that the membrane can be satisfactorily removed before the subsequent application is made, or that the membrane can serve satisfactorily as a base for the later application.Curing compound can be used with advantage where wet curing is not possible. It is very suitable for large areas of concrete which are directly exposed to sunlight, heavy winds and other environmental influences. It can be used for curing of:
•Concrete pavements, airport runways, bridge decks, industrial floors.
•Canal linings, dams and other irrigation related structures.
•Sport arenas and ice rings.
•Precast concrete components.
•Roof slabs, columns and beams.
•Chimneys, cooling towers and other tall structures.
Read and memorize the active vocabulary to the text:
Active Vocabulary
1. curing уход за бетоном, выдерживание бетона, отверждение
2. to prevent предотвращать, предупреждать; предохранять
3. relatively относительно, сравнительно, довольно
4. velocity скорость, вектор скорости, быстродействие
5. essential (a) обязательный, необходимый
6. humidity влажность, содержание влаги (в материале)
7. excessive чрезмерный; излишний; избыточный
8. to maintain сохранять, обслуживать, эксплуатировать
9. moisture влага, влажность, влагоёмкость
10. burlap брезент; пеньковая или джутовая мешочная ткань
11. casting литьё; разливка, отливка
12. delay задержка; запаздывание, время задержки
13. shrinkage усадка, сжатие, сокращение, сморщивание, образование усадочной трещины
14. potable годный для питья, питьевой
15. compound смесь, состав, соединение
16. solvent растворитель; растворяющий
17. volatility изменчивость, неустойчивость, непостоянство, нестабильность
18. moisture retentive гигроскопический, удерживающий влагу
19. reflectance коэффициент отражения; отражательная способность
20. subsequent последующий, более поздний, являющийся результатом (чего-л.)
21. influence влияние, действие, воздействие
22. lining облицовка; обшивка; футеровка; вкладыш (в опалубку), крепление (траншеи, котлована)
CONTROL TESTS
TEST I
Decide whether these statements are true or false:
1. a). The necessity for curing arises from the fact that the strength and durability of concrete will be fully developed only if it is properly cured.
b). The necessity for curing arises from the fact that any delay in curing will lead to evaporation of mixing water.
c). The necessity for curing arises from the fact that hydration of cement can take place only in water-filled capillaries.
2. a). Evaporation of water from concrete depends on the temperature, humidityof the surrounding air and on the velocity of wind.
b). Evaporation of water from concrete depends on the moisture retentive film applied on fresh concrete surface.
c). Evaporation of water from concrete depends on the extent of areas of concrete which are directly exposed to sunlight.
3. a). Curing is essential in the production of concrete as the early drying may lead to shrinkageand cracking of concrete.
b). Curing is essential in the production of concrete to have the desired properties.
c). Curing is essential in the production of concrete for making precast concrete components.
4. a). The amount of mixing water in the concrete at the time of placement is provided by the use of a moisture retentive film.
b). The amount of mixing water in the concrete at the time of placement should be enough for developing strength and durability of concrete.
c). The amount of mixing water in the concrete at the time of placement is normally more than required for hydration.
5. a). White or gray pigments are often incorporated to provide heat reflectance.
b). White or gray pigments are often incorporated to provide aesthetic properties of concrete.
c). White or gray pigments are often incorporated to provide a base for the later applications.
TEST II
Match words in columns A and B (synonym match):
A B
1. evaporation a). foundation b). vaporization
c). differentiation d). supportiveness
2. surrounding a). twisting b). covering
c). burning d). neighboring
3. property a). feature b). masonry
c). space d). readiness
4. harmful a). prevalent b). transparent
c). optional d). destructive
5. to retain a). to paint b). to preserve
c). to match d). to contain
6. delay a). assembly b). purpose
c). retention d). choice
7. available a). disposable b). institutional
c). adhesive d). primitive
8. satisfactorily a). practically b). impressively
c). entirely d). adequately
9. to expose a). to compose b). to exhibit
c). to crack d). to strengthen
10. application a). resolution b). improvement
c). accumulation d). use
TEST III
Select the best possible answer out of the choices from the list to the statements (multiple choice):
1. The strength and durability of concrete will be fully developed only … .
a. if loss of water by evaporation is prevented.
b. if the amount of mixing water in the concrete at the time of placement is more than required for hydration.
c. if it is properly cured.
d. if curing begins as soon as possible after the casting of concrete.
2.Excessive loss of water by evaporation may … .
a. lead to possible adverse effects on concrete.
b. preclude from obtainingthe necessary properties of the concrete.
c. depend on the temperature and humidityof the surrounding air.
d. reduce the amount of retained water below what necessary for development of desired properties.
3. Curing compound should not be used on surfaces … .
a. that are to receive additional concrete, paint, or tile which require a positive bond.
b. covered with such materials as burlap, cotton mats, rugs, etc.
c. which are laid open to strong winds and bright sun's rays.
d. where wet curing is not possible.
4. Curing compound is very suitable for large areas of concrete … .
a. in case of continuous or frequent application of water.
b. when regular supply of potable water for curing is available.
c. if the membrane can be satisfactorily removed before the subsequent application.
d. which are exposed to sunlight, heavy winds and other environmental influences.
5.The potentially harmful effects of evaporation … .
a. will hinder the development of the desired properties of concrete.
b. depend on the temperature, humidity of the surrounding air and on the velocity of wind.
c. must be prevented either by applying water or preventing excessive evaporation.
d.. will lead to the reduction of the strength and durability of concrete.
TEST IV