Have true plane surfaces care shall be taken to

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Unformatted text preview: ig 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. 2.11.3 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 proprietry 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. NOTE — Ordinary plaster of paris will not serve the purpose of the capping material due to its low compressive strength. 2.11.4 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 position until the specimen is removed from the mould. 2.12 Curing — The test specimens shall be stored in a place, free from vibration, in moist air of at least 90 percent relative humidity and at a temperature of 27° ± 2°C for 24 hours ± ½ hour from the time of addition of water to the dry ingredients. After this period, the specimens shall be 8 IS : 516 - 1959 marked and removed from the moulds and, unless required for test within 24 hours, immediately submerged in clean, fresh water or saturated lime solution and kept there until taken out just prior to test. The water or solution in which the specimens are submerged shall be renewed every seven days and shall be maintained at a temperature of 27° ± 2°C. The specimens shall not be allowed to become dry at any time until they have been tested. 3. MAKING AND CURING COMPRESSION TEST SPECIMEN IN THE FIELD 3.1 This clause deals with the procedure for making and curing compression test specimens of concrete sampled during the progress of construction where the nominal maximum size of the aggregate does not exceed 38 mm. 3.2 Size of test specimen, apparatus to be used and procedure to prepare specimens shall be the same as specified in 2. 3.3 Curing — The test specimens shall be stored on th...
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This document was uploaded on 10/01/2013.

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