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Materials Engineer, Roorkee
CAPPING THE CYLINDERS
It is required that the cylinders ends must be plane within 0.050 mm. The most common way of achieving this planeness requirement is to cap the ends of the cylinder as per ASTM C6176 with suitable materials. Three different capping materials are permitted (a) A thin layer of stiff Portland cement paste may be used on freshly molded specimens. (b) on hardened cylinders, either high-strength gypsum plaster or sulfur mortar may be used (c) A third method is, an elastomeric pad is placed within a metal retaining ring, and the assembly is then placed over the specimen end. The pad conforms to the shape of the cylinder end, but is prevented from spreading laterally by the metal retaining ring. This provides a uniform load across the specimen ends.
At the present time, this system is usually limited to concrete with strength of 50 MPa or less and is prohibited for concrete with strength in excess of 80 MPa7. However, testing cube have no such restrictions, as no capping is used in the testing of concrete cubes. I.S. specifications specified testing of cubes for quality control of concrete.
CAPPING SPECIMENS, IS: 5161
The ends of all cylindrical test specimens that are not plane within 0.05 mm shall be capped. Capped surfaces shall not depart from a plane by more than 0.05 mm and shall be approximately at right angles to the axis of the specimens. The planeness of the cap shall be checked by means of a straight edge and feeler gauge, making a minimum of three measurements on different diameters. Caps shall be made as thin a practicable and shall not flow or fracture when the specimen is tested. Capping shall be carried out according to one of the following methods.
Neat Cement – Test cylinders may be capped with a thin layer of stiff, neat Portland cement paste after the concrete has ceased settling in the moulds, generally for two to four hours or more after moulding. The cap shall be formed by means of glass plate not less than 6.5 mm in thickness or a machined metal plate not less than 13 mm in thickness and having a minimum surface dimension at least 25 mm larger than the diameter of the mould. It shall be worked on the cement paste until its lower surface rests on the top of the mould. The cement for capping shall be mixed to a stiff paste for about two to four hours before it is to be used in order to avoid the tendency of the cap to shrink. Adhesion of paste to the capping plate may be avoided by coating the plate with a thin coat of oil or grease.
Sulphur – Just prior to testing, the cylindrical specimens may be capped with a sulphur mixture consisting of I part sulphur to 2 or 3 parts of inert filler, such as fire-clay. The specimens shall be securely held in a special jig so that the caps formed have true plane surfaces. Care shall be taken to ensure that the sulphur compound is not over-heated as it will not then develop the required compressive strength. Sulphur caps shall be allowed to harden for at least 2 hours before applying the load.< Hard Plaster – Just prior to testing, specimens may be capped with hard plaster having a compressive strength of at least 420 kg/sq cm in one hour. Such plasters are generally available as proprietary material. The cap shall be formed by means of a glass plate not less than 13 mm in thickness, having a minimum surface dimension at least 25 mm larger than the diameter of the mould. The glass plate shall be lightly coated with oil to avoid sticking.
As soon as possible after the concrete is mixed, a mortar shall be gauged using a cement similar to that used in the concrete and sand which passes IS Sieve 30 but is retained on IS Sieve 15. The mortar shall have a water/cement ratio not higher than that of the concrete of which the specimen is made, and shall be of a stiff consistence. If an excessively wet mix of concrete is being tested any free water which has collected on the surface of the specimen shall be removed with a sponge, blotting paper or other suitable absorbent material before the cap is formed. The mortar shall then be applied firmly and compacted with a trowel to a slightly convex surface above the edges of the mould, after which the capping plate shall be pressed down on the cap with a rotary motion until it makes complete contact with the rim of the mould. The plate shall be left in portion until the specimen is removed from the mould.
Notes – Ordinary plaster of paris will not serve the purpose of the capping material due to its low compressive strength.
Unless the cores ends are prepared by grinding, they should be capped with high alumina cement mortar or sulpfur-sand mixture to provide parallel end surfaces normal to the axis of the core (other materials should not be used as they have been shown to give unreliable results) caps should be kept as thin as possible8.
USE OF GYPSUM CAPPING
However, M/s CHIR-AYU, Vadodara-390023 is marketing Gypsum Material for capping concrete compression testing specimens. This high strength Gypsum mix at room temp and set in 30 minutes. The products have compressive strength range of 280 kg/cm2, 350 kg/cm2, 420 kg/cm2, 560 kg/cm2 and 630 kg/cm2.
IS: 2185 (Part 1) – 19792 and IS: 2185 (Part II)-19833 specified Gypsum Plaster capping for concrete Blocks compression testing. The codes specified that, gypsum plaster, when gauged with water at the capping consistency shall have a compressive strength at 2-hour age of not less than 25 N/mm2, when tested at 20 mm cubes. ASTM C140-034 also specified gypsum capping for testing concrete masonry.
TESTING OF GYPSUM PLASTER5
Normal consistency to be tested by modified Vicat Apparatus. This test method is used to determine the volume of water required for mixing gypsum plaster when performing the setting time and compressive strength tests.
The setting time is to be conducted by Vicat Apparatus. The test method is used to determine the setting time of gypsum plaster and gypsum concrete, and used to determine compliance with gypsum plaster and gypsum concrete specifications.
Compressive strength test is used to determine the compressive strength of gypsum plaster and gypsum concrete. The test is conducted on 50.8 mm split cube molds.
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