The effect of curing on the strength of concretes.
It is important to stress that good curing is an essential factor in producing good concrete. In addition to precaution against frost it is also necessary to prevent too rapid drying by sun or wind, as this inevitably increases shrinkage and cracking.
The effect of temperature on the strength of concretes:
Concrete mixed and placed at temperatures below freezing point will not set. At temperature nearing freezing the rate of strength gain will be much lower than under warmer conditions. It will be seen that for ordinary Portland cement the strength gain up to 28 days is greatly reduced at low temperatures. This must be allowed for if the concrete has to carry load soon after setting, and it will seriously affect the time at which it is safe to remove shuttering. With high alumina cement the reduction is much less. In the case, when work is done at low temperatures, must be carefully covered up and must not bе stressed until it has had time to gain strength. Where urgent work must be continued in cold weather, special precautions can bе taken. These include the use of a richer cement mix, warming of aggregates and mixing water, special protection of the completed work and sometimes heating of the building. It is also possible to accelerate hardening by mixing a small proportion of calcium chloride with the cement, but it must be done under careful control and reference should be made to special publications for more detailed information.
REINFORCED CONCRETE
The combination of two materials, steel and concrete, has led to the discovery of a very strong material - reinforced concrete. Concrete offers good resistance to loads which tend to compress it. Steel, on the other side has a very high tensile strength and is able to offer considerable resistance to forces tending to pull it. When steel and concrete are used together as in reinforced concrete, a building material is produced which possesses both, compressive and tensile strength. The introduction of steel reinforcing bars would not have been possible unless both materials were able to act аs one medium in supporting any forces imposed on them. Early research, on the problem brought out the following important points :
1. the adhesion between the steel and concrete is very strong.
2. the coefficients of expansion of the steel and concrete are almost identical. Large spans of reinforced concrete can therefore be constructed without setting up any 
dangerous forces in the material. Reinforced concrete has all the hard wearing properties of stone, and can be moulded and fashioned easily. It has been used in the construction of many types of buildings: concert halls, scholls,etc. The fire resisting properties of reinforced concrete are very great. Exhaustive tests have been carried out on the resisting properties of building materials, and specimens of reinforced concrete withstood a temperature of 2300 F. for eight hours without showing any signs of breaking down. Specimens of iron, steel and brickwork collapsed in less than two hours. During the air-raids in 1941-45 many reinforced concrete buildings withstood fire and direct bits from large bombs.. The preceding brief introduction to the properties of reinforced .concrete, indicates its wide applications in the .building industry. Its discovery is one of the most important events in the history of building.
EXERCISES.
I Find out the "ing forms" in different functions.
II. Form derivatives from the following words:
reinforce , strength, resist, support, insulate, introduce, impose
III. Answer the following questions:
1. What compressive strength has reinforced concrete ?
2. What tensile strength does reinforced concrete possess?
3. Does reinforced concrete possess good wearing properties?
4. What temperatures does reinforced concrete withstand?
5. Why is the combination of concrete and steel called reinforced concrete?
6. What properties of reinforced concrete indicate its wide application?
7. What has the discovery of reinforced concrete led to?