Determination Of The Uniaxial Compressive Strength Of The Given Rock Materials.

Scope:-

This method of test is intended to measure the uniaxial compressive strength of a rock sample in the form of specimens of regular geometry. The test is mainly intended for strength classification and characterization of intact rock.

Apparatus:-

• A suitable machine shall be used for applying and measuring axial load to the specimen. It shall be of sufficient capacity and capable of applying load at a rate conforming to the requirements set in Section 3.
• A spherical seat, if any, of the testing machine, if not complying with specification 2(d) below, shall be removed or placed in a locked position, the two loading faces of the machine being parallel to each other.
• Steel platens in the form of discs and having a Rockwell hardness of not less than HRC58 shall be placed at the specimen ends. The diameter of the platens shall be between D and D + 2 mm where D is the diameter of the specimen. The thickness of the platens shall be at least 15 mm or D/3. Surfaces of the discs should be ground and their flatness should be better than 0.005 mm.
• One of the two platens shall incorporate a spherical seat. The spherical seat should be placed on the upper end of the specimen. It should be lightly lubricated with mineral oil so that it locks after the dead weight of the cross head has been picked up. The specimen, the platens and spherical seat shall be accurately centered with respect to one another and to the loading machine. The curvature centre of the seat surface should coincide with the centre of the top end of the specimen.

Theory

Compressive Strength:-

Compressive strength is the capacity of a material or structure to withstand axially directed pushing forces. When the limit of compressive strength is reached, materials are crushed.

By definition, the uniaxial compressive strength of a material is that value of stress reached when the material fails completely. The compressive strength is usually obtained experimentally by means of a compressive test. The compressive strength of the material would correspond to the stress at the red point shown on the curve. Even in a compression test, there is a linear region where the material follows Hooke's Law

Compressive Strength=LoadArea

Comparison Of Compressive And Tensile Strengths:-

An example of a material with a much higher compressive strength than tensile strength is concrete. Ceramics typically have a much higher compressive strength than tensile strength. Composite materials tend to have higher tensile strengths than compressive strengths. One such example is glass fiber epoxy matrix composite.

Procedure:-

• Test specimens shall be right circular cylinders having a height to diameter ratio of 2.5-3.0 and a diameter preferably of not less than NX core size approximately 54 mm. The diameter of the specimen should be related to the size of the largest grain in the rock by the ratio of at least 10:1.
• The ends of the specimen shall be flat to 0.02 mm and shall not depart from perpendicularity to the axis of the specimen by more than 0.001 radian (about 3.5 min.) or 0.05 mm in 50 mm.
• The sides of the specimen shall be smooth and free of abrupt irregularities and straight to within 0.3 mm over the full length of the specimen.
• The use of capping materials or end surface treatments other than machining is not permitted.
• The diameter of the test specimen shall be measured to the nearest 0.1 mm by averaging two diameters measured at right angles to each other at about the upper height, the mid height and the lower height of the specimen. The average diameter shall be used for calculating the cross-sectional area. The height of the specimen shall be determined to the nearest 0.1 mm.
• Samples shall be stored, for no longer than 30 days, in such a way as to preserve the natural water content, as far as possible, and test in that condition. This moisture condition shall be reported in accordance with “Suggested method for determination of the water content of a rock sample”.
• Load on the specimen shall be applied continuously at a constant stress rate such that the failure will occur within 5-10 min of loading, alternatively the stress rate shall be within the limits of 0.5-1 M Pa/s.
• The maximum load on the specimen shall be recorded in Newtons to within 1%.
• The number of the specimens tested should be determined from practical considerations but at least five are preferred.