STUDY ON STRENGTH OF CONCRETE USING VARIOUS COARSE AGGREGATES, PART-6

Values of strength obtained in this way usually range from 2000 to 6000 psi, the most common value being of the order of 3000 psi - a value which is often used for purposes of general discussion. Compressive stress is expressed symbolically as

                                               σ = P / A

in which σ = compressive stress in psi 

P = applied load in lb 

A = cross sectional area in square inch

3.2 Tensile strength 

Concrete is not usually expected to resist direct tensile forces because of its relatively low tensile strength and brittle nature. However, tension is of importance with regard to cracking, which is a tensile failure; most cracking (aside from that due to load or to settlement of parts of the structure) is due to restraint of contraction induced by drying or by lowering of temperature.  Experimental results have shown that the tensile strength ranges from 7 to11 and at usual levels averages about 9 percent of the compressive strength; the higher the compressive strength, the lower the relative tensile strength. The

Bending Test Arrangement for Concrete Specimen

following formula is adopted for tensile strength – 

In which ∆ = tensile stress 

P = compressive load on the cylinder
L = length of the cylinder

D = diameter of the cylinder

3.3 Flexural strength 

When concrete is subjected to bending, tensile and compressive stresses and in many cases direct shearing stresses are developed [3]. The most common plain-concrete structure subjected to flexure is a highway pavement, and the strength of concrete for pavements is commonly evaluated by means of bending tests on 6 by 6 inch beam specimens. Flexural strength is expressed in terms of “modulus of rupture”, which is the maximum tensile (or compressive) stress at rupture computed from the well-known flexure formula

σ = Mc / I

In which σ = stress in the fiber farthest from the neutral axis, psi

M = bending moment at the section, lb-in.

I = moment of inertia of the cross section, in.

c = distance from neutral axis to farthest fiber, in.