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Before you read the article, check the meaning of these words and phrases and memorize them.




exploration durability to shrink to swell supplement flexible rigid to glue to cool to solidify film to resist joint crack to enhance fine to cast additive

2. Match the equivalents:

A. 1. flexible pavement a) тщательно укатывать

2. to maintain workability b) бетоноукладчик

3. paving machine c) жёсткость при изгибе

4. to roll thoroughly d) сохранять способность под- вергаться обработке

5. tyre friction e) битумная плёнка

6. light reflectance f) износоустойчивые камни

7. bituminous film g) трение шины

8. to spray coating h) отражение света

9. wear resistant stones i) напылять покрытие

10. flexural stiffness j) нежёсткая дорожная одежда

 

B. 1.rigid pavement a) стальная арматура

2. portland cement concrete b) жёсткая дорожная одежда

3. a poor natural formation c) бетон на портландцементе

4. steel reinforcement d) слабое земляное полотно

5. tensile strength e) поперечные швы

6. fine cracking f) прочность на растяжение

7. transverse joints g) мелкое растрескивание

8. longitudinal joints h) нестандартный материал

9. substandard material i) продольные швы

 

Reading task

 

3. Read the text carefully and do the tasks that follow:

 

Road traffic is carried by the pavement which in engineering terms is a horizontal structure supported by in situ natural material. In order to design this structure subsurface explorations are conducted. The engineering properties of the local rock and soil such as strength, stiffness, durability, susceptibility to moisture, and propensity to shrink and swell over time are established. These properties are determined either by field tests or by empirical estimates based on the soil type or by laboratory measurements. The material is tested in its weakest expected condition, usually at its highest probable moisture content. Soils unsuitable for the final pavement are identified for removal, the maximum slopes of embankments and cuttings are established, the degree of compaction to be achieved during construction is determined and drainage needs are specified.

In a typical rural pavement, the base course provides the required supplement to the strength, stiffness and durability of the natural formation. Its thickness ranges from 4 inches (10 centimeters) for very light traffic and a good natural formation to more than 40 inches (100 centimeters) for heavy traffic and a poor natural formation. The subbase is a protective layer and temporary working platform sometimes placed between the base course and the natural formation.



Pavements are called either flexible or rigid, according to their relative flexural stiffness.

Flexible pavements have base courses of broken stone pieces either compacted into place in the style of McAdam or glued together with bitumen to form asphalt. In order to maintain workability, the stones are usually less than 1.5 inches in size and often less than 1 inch. Initially the bitumen must be heated to temperatures of 300o-400o F (150o-200o C) in order to make it fluid enough to mix with the stone. At the road site a paving machine places the hot mix in layers about twice the thickness of the stone size. The layers are then thoroughly rolled before the mix cools and solidifies.

The surface layer of a flexible pavement protects the underlying base course from traffic and water while also providing tyre friction, generating minimal noise in urban areas, and giving suitable light reflectance for night-time driving. Such surfaces are provided either by a bituminous film coated with stone or by a thin asphalt layer. Bituminous surfacing with stone is relatively cheap, effective and impermeable and lasts for about 10 years. Its main disadvantage is its high noise generation. Maintenance usually involves a further spray coating with bitumen. Asphalt surfacing is used with higher traffic volumes or in urban areas. It commonly contains smaller and more wear-resistant stones than the base course and employs relatively more bitumen. Asphalt surfacing is better resist horizontal forces and produces less noise.

Surface dressing поверхностный слой износа

Surface course верхний слои дорожной одежды



Base course несущий слой

Concrete slab бетонная плита

Subbase подстилающий слой

Formation level уровень грунта; нулевая отметка

Natural formation земляное полотно

 

Rigid pavements are made of portland cement concrete. The concrete slab ranges in thickness from 6 to 14 inches. It is laid by a paving machine, often on a supporting layer that prevents pumping water and natural formation material to the surface through joints and cracks. Concrete shrinks as it hardens, and this shrinkage is resisted by friction from underlying layer, causing cracks to appear in concrete. Cracking is usually controlled by adding steel reinforcement in order to enhance the tensile strength of the pavement and ensure that any cracking is fine and uniformly distributed. Transverse joints are sometimes also used for this purpose. Longitudinal joints are used when the whole carriageway cannot be cast in one pass of the paving machine.

In places where the local natural material is substandard for use as a base course, it can be “stabilized” with portland cement, pozzolana or bitumen. The strength and stiffness of the mix are increased by the surface reactivity of the additive which also reduces its susceptibility to water. Special machines distribute the stabilizer into the upper 8 to 20 inches of soil.

In deciding whether to use a flexible or rigid pavement, engineers take into account service life period, riding characteristics, ease and cost of repair, and the effect of climatic conditions. Often there is little to choose between rigid and flexible pavements.

 


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