3.2. Prism strength

Reinforced concrete structures in form from the cubes, so concrete cube strength can not be directly used in the calculations but the strength of the structure. Oc basic characteristics of the strength of concrete struts are etsya-prism strength. Under the prism strength understand temporary resistance Rb axial compression ratio of prism-em prism height h to the size of a square base, equal to 4 (Fig. 3).

In real structures of concrete compressive stress state of the zone close to the tense of the prisms. Samples of prismatic form, for which the influence of friction forces less than cubes, for the same cross-section shows a lower compressive strength. With respect to the height of the prism side of the base h / a> 4 the influence of friction forces practically disappears, and the strength becomes constant and equal to ≈ 0,75 R.

Fig. 3 Dependence prizmeiioy concrete strength of the aspect ratio of the test specimen.

3.3. Axial tensile strength

Strength of concrete in axial tension Rbt depends strongly STI tensile cement stone and its adhesion to the grains of coarse aggregate.

Empirically, it is determined by tensile tests on samples in the form eights (Fig. 4a) for splitting samples in the form of cylinders (Fig. 4b), cubes (Fig. 4c) or flexural concrete beams (Figure . 4d). Here M bending moment; b - width of the beam section.

Fig. 4. Circuit test samples to determine the tensile strength of concrete.

Strength of concrete in axial tension is of relatively little importance.

Rb, = 0.lRb ... 0.05Rb.

Approximate value Rbt can determine the empirical formula Feret:

3.4 Strength of concrete under shear and shearing

Under clear cut understanding of the separation element on the cross section, which are attached to the shear forces.

Under pure shear fracture understand mutual displacement (shift) between the parts of an element is under the influence of shear (shift would presuppose) effort.

Reinforced concrete structures rarely work on a clean cut and spalling. Usually accompanied by a slice of the action of the longitudinal forces and shearing - the action of shear forces.

Shear resistance may occur in keyways, connections and supports beams, and resistance to chipping - flexural intended conjugate beams until they inclined cracks, if not provided a reliable connection between the upper and lower parts of the concrete pillars.

The time allowance shear resistance and chipping is not given, and it is taken approximately equal to:

3.5 Strength of concrete under long-term load

Limit prolonged resistance of concrete called most large static constant of the voltage that it can withstand an unlimited time without breaking.

Under long-term load concrete sample failed is at voltages lower than in short-term load. This is due to the influence of developing inelastic deformation:

If the operation of the design under conditions favorable for the growth of the concrete strength, the stress level gradually

decreases, the duration of application of the load factor can not affect the bearing capacity of elements. Prof. BG Skramtaev offers to predict the strength of concrete with prolonged dependence on uploading

where t - time in days.