Chương I NHỮNG QUY ĐỊNH CHUNG
UNIT 10. UNIT 10. NATURE OF REINFORCED CONCRETE
The main idea in the formation of reinforced concrete is to use the concrete to resist compression but its tensile strength is very low. A plain concrete beam (without reinforcement) resting on two supports, and subjected in bending to tension below the neutral plane and to compression above it (Fig. la), has a very low load-carrying capacity, limited by the low resistance of the concrete to tension. In such a beam the high-compressive strength of the concrete is not utilized.
Reinforced concrete members with reiforcement in the tension zone, giving them a tensile strength many times greater than that of plain concrete, possess a significantly higher load-carrying capacity. For example, a reinforced concrete beam (Fig. lb) in which the tensile stresses are resisted by reinforcement distributed in the lower part mav have a load-carrying capacity nearly 20 times greater than that of a plain concrete beam of the same dimensions.
QUESTIONS
46
O ther reinforced concrete members resisting compression such as columns (Fig. lc), are also reinforced with steel bars. Since steel has high resistance both to tension and compression, the introduction o f a small amount of reinforcement increases the load-carrying capacity of the compression member.
Thus, by combining concrete and steel for joint work qualitatively new material, reinforced concrete, is created whose sphere o f application is broadening without limit.
The basis of the joint behaviour of concrete and steel is the advantageous natural combination of several important physical and mechanical properties o f the two materials.
1. Concrete on hardening forms a strong bond with the steel reinforcement, and in a reinforced concrete member under load both materials are strained or deformed together.
2. Dense concrete (with an adequate cement content) protects the steel in it against coưosion and the direct actic of fire.
3. Steel and concrete have coefficients of linear expansion with very close values -0.00001 -0.000015 per deg.c for concrete and 0.000012 for steel - and so under changes in temperature not exceeding 100°c no noticeable initial stresses appear in the materials, and no harmful strains involving slipping of the reinforcement in the concrete are observed.
Reinforced concrete has found wide application in building because of such positive attributes as durability, fire-resistance, weathering resistance, high resistance to static and dynamic loads, the possibility of using local raw materials in preparation of the concrete (crushed stone, sand), and the low maintenance costs of buildings and structures.
Compared with other building materials, reinforced concrete is distinguished for its very long service life. With proper service conditions, reinforced concrete members can last an indefinite time without reduction of their load-carrying capacity. This arises from the fact that the strength of the concrete does not decrease with time but, on the contrary, increase, and the steel embedded in it is protected against corrosion.
In considering the fire resistance of reinforced concrete it may be noted that at an ambient temperature of around 1000°c it will take an hour to heat reinforcement covered by a layer of concrete 2.5 centimetres thick to about 500°c.
a )
Compression zone I ___ - I-
c)
Neutral plane __ __ _ l -
~ T
Tension zone Crack b)
-Compression zone \ Neutral plane
/ I I 1 1 \ \ — =>
^ 7 w
Tension zone \ Reinforcem ents
Compressed reinforcem ent
Fig. 1. Plain and reinforced concrete
Experience has shown that in fires of average intensity lasting several hours, reinforced concrete structures with the necessary protective coating of concrete sustain only surface damage, but are not destroyed.
The wide distribution of sand and gravel, which are used in great quantities in mixing concrete, make reinforced concrete available for use almost everywhere.
A special feature of the behaviour of reinforced concrete structures under load is the possibility of crack formation in the tension zone of the concrete. The opening of such cracks under service loads is so insignificant in many kinds of members, however, that it does not hinder their normal use.
Where it is necessary to prevent the formation of cracks or to limit their width, the concrete is subjected to intensive preliminary compression, generally by tensioning the reinforcement, before application of the external load. Concrete made in this way is called prestressed.
The drawback of the relatively high weight of reinforced concrete haf been eliminated to a great extent by the introduction of thin-walled and 48
hollow members made from high strength prestressed concrete, and of reinforced concrete made with lightweight aggregates.
The high sound transmission and heat conductivity of reinforced concrete sometimes requires additional outlay on the provision of sound and heat insulation.
The complications in the casting of reinforced concrete, linked with the need to keep components in forms until the concrete acquires the appropriate strength, have largely been eliminated with the introduction of mechanized industrial production and the employment of high strength concrete made from high-early-strength cements.
Because its merits essentially outweigh its shortcomings, reinforced concrete is widely used in building and civil engineering.
TERMINOLOGIES
- Load-caưying capacity - Hardening
- Corrosion
- Compression - tension - Reinforcement - Resistance - Support - Tension zone - Nature
- Reinforced concrete
: Bản chất : Bê tông cốt thép : Nén - kéo : Cốt thép
: Sức chịu, khả năng, cường độ : Gối tựa
: Vùng chịu nén : Khả năng chịu lực : Cứng hóa, đông cứng : Xâm thực, ăn mòn - Coefficients of linear expansion : Hệ số dãn nở luyến tính
- Heat conductivity - Service life
- Prestressed concrete
: Thời gian sử dụng : Bê tông ứng lực trước : Dẫn nhiệt
1. What is reinforced concrete ?
2 What beams among plain concrete and reinforced beams have higher load carrying capacities if they have the same dimensions ?
3 Why can concrete and steel combine in a structural member ? 4. Why has reinforced concrete found wide application in building ? 5. What is prestressed concrete ?