A. tendency B. durability C. concrete D. distributes
E. corrosive F. cold-weather G. performed H. ingredients
I. admixtures J. sprayed K. stored L. transport
1. The first and most important step in the process is to determine the …….
2. The cost of …… scales rather closely with its strength.
3. Concrete can be poured, pumped, and even …… into place.
4. Workability is usually defined by the slump, which is the …… for the fresh concrete
5. In …… locations the concrete must be able to resist freezing.
6. Weak concrete almost always lacks …….
7. Roads must be able to withstand the …… effects of salt.
8. Concrete mixing trucks can be used to …… already-mixed concrete.
9. The mixing can actually be …… by the truck as it is traveling to the site.
10. Mixing …… the aggregate evenly throughout the cement paste.
UNIT 4 The Concrete Construction Process (Part II)
Read, translate the following text and be ready to fulfil the tests:
Placing. Once the concrete has been adequately mixed, it must be placed into the formwork that defines its final position and shape. If the concrete is to be reinforced, the steel rebar must already be in place so the concrete can flow around it. If the concrete mixing truck can be located close to (and higher than) the site, then the concrete can be poured directly into the forms. In cases where this is not possible, the concrete can be transferred in buckets by a crane or by wheelbarrow. When this is impractical due to the distance required or the size of the job, the fresh concrete can be pumped through a system of pipes or hoses to the site by special concrete pumps. Concrete that is to be pumped has more stringent requirements for workability. If the concrete is too dry, it will not pump well, while if it is too wet it will tend to segregate. Segregation can also occur if the concrete falls into the formwork too quickly, as larger aggregate particles will tend to be driven downward.
Consolidation. Once the concrete is in place, it should be consolidated to remove large air voids developed during placement and to make sure that the concrete has flowed into all of the corners and nooks of the formwork. This process is also called compacting. Overconsolidation can lead to segregation and bleeding, but underconsolidation is more common, resulting in less-than optimal properties. The two most common methods of consolidation are vibration and roller compacting.
Vibration. Vibration is a mechanical process that transfers pulses of shear energy to the concrete, usually by a probe that is inserted several inches into the concrete. Each pulse of shear energy momentarily liquefies the concrete, allowing it to flow very freely. This is the standard consolidating method for general construction projects with the exception of roads. The shear energy will only travel through a limited thickness of concrete, so when a thick concrete structure is being placed, the fresh concrete is poured in layers, with each layer consolidated before the next is poured over it. Vibration is a noisy and labor-intensive step, requiring expensive and specialized equipment. For this reason, there is growing use of self-consolidating concrete which flows so freely (through the use of chemical admixtures) that mechanical consolidation is not needed.
Roller compaction. Roller compaction is a simpler and more cost-effective technique that is suitable for roads and very large mass concrete structures such as dams. A specialized vehicle with a heavy roller on the front is driven over the fresh concrete to drive it into place and remove excess air. The fresh concrete used is very stiff so that it can support the weight of the machine as it passes over. Finishing for concrete floors and pavements, the appearance, smoothness, and durability of the surface is particularly important. Finishing refers to any final treatment of the concrete surface after it has been consolidated to achieve the desired properties. This can be as simple as pushing a wide blade over the fresh concrete surface to make it flat (screeding).
Read and memorize the active vocabulary to the text:
Active Vocabulary
1. formwork опалубка
2. to define определять
3. to reinforce укреплять, армировать
4. site место, стройплощадка
5. to pour наливать, насыпать
6. to transfer перемещать
7. wheelbarrow тачка
8. pump насос; качать
9. stringent строгий, обязательный
10. requirement требование
11. particle частица
12. сonsolidation уплотнение
13. bleeding выпуск, течение
14. segregation расслоение
15. shear сдвиг
16. liquefy превращать в жидкое состояние
17. labor-intensive трудоемкий
18. cost-effective рентабельный
19. to remove удалять
20. treatment обработка, измельчение
21. finishing отделка
22. screeding заливка и выравнивание
CONTROL TESTS
TEST I
Decide whether these statements are true or false:
1. a). If the concrete is to be reinforced, it should be consolidated to remove large air voids.
b). If the concrete is to be reinforced, the fresh concrete should be pumped through a system of pipes.
c). If the concrete is to be reinforced, the steel rebar must already be in place so the concrete can flow around it.
2. a). Concrete, that is to be pumped, must be self-consolidating concrete which flows freely.
b). Concrete, that is to be pumped, has more stringent requirements for workability.
c). Concrete, that is to be pumped, should be dry enough.
3. a). The two most common methods of consolidation are vibration and roller compacting.
b). The two most common methods of consolidation areliquefying and screeding.
c). The two most common methods of consolidation are segregation and bleeding.
4. a). Vibration is a mechanical process that works through a limited thickness of concrete.
b). Vibration is a mechanical process that is widely used in construction of roads.
c). Vibration is a mechanical process that transfers pulses of shearenergy to the concrete.
5. a). Finishing refers to any final treatment of concrete after it has been placed.
b). Finishing refers to any final treatment of concrete surface after it has been consolidated.
c). Finishing refers to any final treatment of concrete after it has been poured.
TEST II
Match words in columns A and B (synonym match):
A B
1. shape a). brick b). form
c). strength d). clinker
2. to require a). to bend b). to weaken
c). disperse d). to demand
3. hose a). sleeve b). clay
c). board d). mould
4. to occur a). to happen b). to avoid
c). to depend d). to concern
5. equipment a). application b). brickwork
c). unit d). facilities
6. admixture a). timber b). additive
c). aggregate d). acid
7. suitable a). successful b). appropriate
c). powerful d). useful
8. vehicle a). automobile b). survey
c). layout d). area
9. layer a). section b). block
c). soil d). stratum
10. treatment a). levelling b). processing
c). measuring d). mapping
TEST III
Select the best possible answer out of the choices from the list to the statements (multiple choice):
1. Segregation can also occur ….
a. if the concrete is poured directly into the forms.
b. if the concrete falls into the formwork too quickly.
c. if the concrete is transferred in buckets by a crane.
d. if the fresh concrete is poured in layers.
2. Once the concrete has been adequately mixed, ….
a. it is important to provide its finishing.
b. thenthe steel rebar must be put in place.
c. it must be placed into the formwork.
d. it should be consolidated to remove large air voids.
3. Overconsolidation can lead to ….
a. liquefying of the concrete.
b. large air voids.
c. durability of the surface.
d. segregation and bleeding.
4. Vibration is a noisy and labor-intensive step, ….
a. being also a simple technique that is suitable for roads.
b. requiring expensive and specialized equipment.
c. whichrefers to final treatment of the concrete surface.
d. used in processing self-consolidating concrete.
5. The two most common methods of consolidation are ….
a. vibration and roller compacting.
b. pumping and placing.
c. pouring and reinforcing.
d. finishing and screeding.
TEST IV
