The tensile strength of concrete is from 1/8 do 1/12 compressive strength of concrete. Due to the low tensile strength of concrete, unreinforced concrete can be practically used in these elements, in which there are compressive stresses, or very low tensile stress. Some types of foundations can be mentioned among the most common concrete structures, retaining walls, water dams, etc..
Concrete reinforced with steel bars is predominantly used in building structures. Reinforced concrete according to the relevant rules is called reinforced concrete. In reinforced concrete structures, concrete and steel cooperate with each other both in the transfer of compressive stresses, and stretching.
Schemes of work of concrete beams: a) unreinforced concrete beam and stress diagram, b) bending moment diagram, c) destruction of the concrete beam, d) cracked reinforced concrete beam and stress diagram, e) stress diagram due to beam stress, f) stress diagram due to bending moment; 1 - scratch, 2 - steel rod, 3 - steel bar stretched, 4 - stress diagram in an RC beam, 5 - final diagram in a prestressed beam.
In bending elements, reinforcement bars are placed in this zone, in which tensile stresses occur. The cross-section of the reinforcement is 1-3% concrete element cross-section.
In a simply supported beam loaded with vertical forces, tensile stresses occur in the lower fibers, and the compressive in the upper (rys.b). If the tensile stresses due to external loads exceed the tensile strength of the concrete, then the beam will crack and fail under small compressive stresses in the upper zone of the beam. To better use the concrete in the compression zone, steel bars should be used in the tension zone, which will transfer the tensile stresses after cracking the concrete.
In prestressed structures, the concrete in the tension zone is not allowed to be scratched.
The general principle of the work of a compressed element is this, that in this zone where tensile stresses occur, compressive stresses are introduced corresponding to the compressive strength of concrete (rys.e). Under the influence of external load, tensile stresses arise in this zone, and in the upper compression zone. Summing up the stresses caused by prestressing and external load, we get the stress diagram shown in Fig. e. In the tension zone, the stresses σs are usually assumed;≥ 0.
Concrete is used to make monolithic and prefabricated structures as well as small-sized prefabricated elements, how: blocks, pavement tiles, wall and ceiling blocks, floor beams and lintels, etc..