MATERIALS FOR HOME-READING.
Early development.
During the past half-century concrete and reinforced concrete have been increasingly used in the Russia in preference to other materials for all types of construction. Concrete and reinforced concrete were already being used in Russia at the beginning of the XX century for example, in 1904 a reinforced concrete lighthouse 118ft. high was built at Nikolayev on the Black sea.
In 1904 reinforced concrete began to be developed sore intensively. A four-story warehouse with beamless floors was designed and erected in Moscow. ln 1906 the Ministry of Transport issued the first specifications for reinforced concrete structures. At that time russian engineers submitted proposals which were widely applied in reinforced concrete structures; for example spiral reinforcement in column was suggested, the design of continuous beams was developed and a system of lattice reinforcement was devised.
In 1928-29 a five-story precast dwelling house was erected in Moscow with load-bearing walls and a large number of clinker-concrete blocks and precast floors. Subsequently the use of precast concrete has been widely developed for single-story dwellings and Industrial buildings. On the basic of the experience of precast dwellings, "Instructions on Precast Concrete Construction" were published in 1935.
Reinforcement and principles of reinforced concrete design.
The design of various parts of a concrete structure must be related to all the requirements necessary for satisfactory service. In some structures the only consideration may be that they are capable of carrying the loads imposed. In others there may be a necessity to limit the deflection of certain parts. The necessity for the use of concrete and steel together arises from the nature of the stresses which шау result from loading. These are compressive, tensile and shear stresses, and they may occur in different members singly or all together. Concrete has an adequate compressive strength, but its tensile strength is low. (Tests have shown that the tensile strength is in fact, only one-tenth to one-twentieth of the compressive strength, and for the purposes of rein-forced concrete design it is therefore ignored. On the other hand, steel has a high tensile strength as well . Suitable combination of these two materials, therefore, provide resistance to both compressive and tensile stresses.
Bond and Anchorage.
An efficient bond between the concrete and the reinforcement is very important since the efficient use of combinations of steel and concrete is dependent on the transfer of stresse from the concrete to the steel. The bond stress, or of the effectiveness of the grip between the concrete and the steel, is best defined as the stress at which a very small slip occurs. Bond is due initially to adhesion and frictional resistance , but as soon as slipping begins the adhesion fails and the subsequent bond is due to friction and mechanical resistance. The development of bond strength is most rapid under moist curing conditions. .
Fixing reinforcement.
It is very important that reinforcement should be fixed in the correct position and properly supported to ensure that displacement will not occur when the concrete is placed. Rein-forcemeat is, in some cases, built up on supporting .frames and then hoisted into position in the moulds, the erection being completed after it has been placed in position. Reinforcing rods are usually held together by soft iron wire ties.
Yield Point.
The most commonly used reinforcing material is plain mild steel in the form of round bars of a diameter between 3/16 in. and 12 1/2 in. Besides mild steel, medius tensile and high tensile steel are sometimes used, the common feature of all these steels is that they have a so-called yield point, which can be determined by a tensile test. The characteristic of yielding is that at a certain stress the length of the bar is increased without further increase in the load. During yielding a reorientation of the crystals of the metal takes piece, after which the load can be further increased until the bar fails.