Common Factors in Concrete Surface Cracks
1. Shrinkage. The most common cause of cracks in concrete is the shrinkage, that occurs as the material dries. As concrete cures, the water, that has been mixed with the cement and sand, begins to evaporate. This causes the concrete to shrink slightly. Because the concrete surface is restrained by formwork or surrounding objects, it tries to resist the effects of shrinkage. The additional tension on the material, that occurs when the concrete shrinks, leads to cracks. This is often addressed through the use of control joints, which are joints placed in the concrete, that cause it to crack in a straight, even line. These joints are later filled with grout or caulk to mask their appearance.
2. Temperature Changes. Temperature change is a major cause of cracks in large concrete applications. This is because of the temperature variations between concrete, that is poured first, and subsequent layers of concrete. These temperature variations can cause some portions of the slab to expand, while others are shrinking, which leads to cracks. The more extreme the difference in temperature, the more dramatic and deep the cracking will be.
3. Lack of Support. Some of the most severe cracks in concrete are caused by inadequate structural support, which occurs when the soil beneath a concrete slab shifts position or is eroded by wind and rain. It can also be caused by poor installation, including improper use of rebar or insufficient form work. Concrete, that lacks structural support, can suffer catastrophic failure, which can cause injury to occupants as well as a great deal of property damage.
4. Rusting. Many items made from concrete contain metal reinforcement, which helps increase the material's tensile strength. These reinforcements include wire mesh, rebar or steel beams. If these materials start to rust, the rust will cause the concrete to expand. This expansion can cause hairline cracks and small surface cracks. After the metal has started to rust, the resulting cracks will allow more water into the concrete, which will result in even more rust. To solve this problem, galvanized metals should be used for their ability to resist corrosion and rust.
5. Spalling. Spalling is the term given to concrete that has a rough, flaky surface. This condition is the result of hundreds of tiny cracks, which usually appear only on the surface of the concrete. Spalling typically occurs when too much water is added to the concrete mixture, or when the surface layer dries faster than interior layers. If the water evaporates too quickly, the resulting increase in tension will be too much for the concrete, causing it to crack. This type of surface problem can sometimes be fixed with topping layers or seal coats.
Read and memorize the active vocabulary to the text:
Active Vocabulary
1. crack трещина
2. surface поверхность, внешняя сторона
3. to avoid избегать, отменять, уничтожать
4. quantity количество, величина
5. proper надлежащий, правильный, подходящий
6. flowability текучесть
7. to evaporate испарять(ся)
8. to shrink сокращаться, сжиматься
9. sufficient достаточный
10. requirement требование, потребность
11. random случайный, произвольный
12. to restrain сдерживать, ограничивать
13. surrounding близлежащий, соседний
14. grout жидкий раствор; заливать раствором
15. subsequent последующий
16. reinforcement усиление, арматура
17. to rust ржаветь
18. spalling расслаивание, разрушение
19. flaky слоистый
20. to occur происходить, случаться
21. topping верхний
22. seal coat финишное покрытие
CONTROL TESTS
TEST I
Decide whether these statements are true or false:
1. a). Most of the time cracks could be avoided byincreasing the flowability of the cement concrete.
b). Most of the time cracks could be avoided by using proper concrete mix proportions.
c). Most of the time cracks could be avoided by using structural support.
2. a). Control joints give room for shrinkage and thus control joints help in avoiding random cracks.
b). Control joints give room for shrinkage and thus control joints help in increasing the material's tensile strength.
c). Control joints give room for shrinkage and thus control joints help in resisting the effects of shrinkage.
3. a). Water increases the flowability of the cement concrete and thus decreases the number of cracks, developing in the concrete.
b). Water increases the flowability of the cement concrete and thus increases the ease of its use.
c). Water increases the flowability of the cement concrete and thus can cause some portions of the slab to expand.
4. a). The hydration process gives strength to the concrete but also needs sufficient water.
b). The hydration process gives strength to the concrete but also gives to concrete a rough, flaky surface.
c). The hydration process gives strength to the concrete but also can be a major cause of cracks in large concrete applications.
5. a). Many items made from concrete contain metal reinforcement, which helps resist the effects of shrinkage.
b). Many items made from concrete contain metal reinforcement, which helps the surface layer dry faster than interior layers.
c). Many items made from concrete contain metal reinforcement, which helps increase the material's tensile strength.
TEST II
Match words in columns A and B (synonym match):
A B
1. common a). simple b). wide
c). long d). general
2. civil a). great b). nonmilitary
c). aware d). familiar
3. to shrink a). to add b). to absorb
c). to contract d). to shape
4. to evaporate a). to involve b). to vaporize
c). to estimate d). to mount
5. beneath a). underb). through
c). behind d). above
6. to erode a). to accomplish b). to employ
c). to disrupt d). to record
7. to shift a). to dislocate b). to gather
c). to defense d). to provide
8. to suffer a). to train b). to experience
c). to detach d). to pay
9. expansion a). framing b). function
c). erection d). extension
10. to appear a). to use b). to derive
c). to show up d). to survive
TEST III
Select the best possible answer out of the choices from the list to the statements (multiple choice):
1 Portland cement, being a hydraulic type of cement,. … .
a. evaporates in great extent after curing and shrinks.
b. depends on the temperature variations.
c. needs water for developing strength.
d. can result in hundreds of tiny cracks.
2.In case you have selected wrong or weak cement, ... .
a. the resulting increase in tension will be too much for the concrete.
b.concrete may get dramatic and deep cracking.
c. your concrete won’t have enough strength and will develop cracks.
d. it will obviously cause random cracks in concrete.
3. Spalling typically occurs when … .
a. there are extreme temperature variations.
b. too much water is added to the concrete mixture.
c. the strength requirementof the concrete is violated.
d. the concrete surface is restrained by formwork.
4. Concrete, that lacks structural support, … .
a. can suffer catastrophic failure.
b. can cause some portions of the slab to expand.
c.can obtain increase in tension too much for the concrete.
d. can acquire hairline cracks.
5.After the metal has started to rust, … .
a. concrete won’t have enough strength and will develop cracks.
b. it will give room for shrinkage.
c. the resulting cracks will allow more water into the concrete.
d. it will cause the concrete to crack in a straight, even line.
TEST IV