The mould using a trowel and covered with a glass or

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Unformatted text preview: finished level with the top of the mould, using a trowel, and covered with a glass or metal plate to prevent evaporation. 2.10.1 Compacting by Hand — When compacting by hand, the standard tamping bar shall be used and the strokes of the bar shall be distributed in a uniform manner over the cross-section of the mould. The number of strokes per layer required to produce specified conditions will vary according to the type of concrete. For cubical specimens, in no case shall the concrete be subjected to less than 35 strokes per layer for 15 cm cubes or 25 strokes per layer for 10 cm cubes. For cylindrical specimens, the number of strokes shall not be less than thirty per layer. The strokes shall penetrate into the underlying layer and the bottom layer shall be rodded throughout its depth. Where voids are left by the tamping bar, the sides of the mould shall be tapped to close the voids. 2.10.2 Compacting by Vibration — When compacting by vibration, each layer shall be vibrated by means of an electric or pneumatic hammer or vibrator or by means of a suitable vibrating table until the specified condition is attained. NOTE — The mode and quantum of vibration of the laboratory specimen shall be as nearly the same as those adopted in actual concreting operations. 2.11 Capping Specimens — 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 as practicable and shall not flow or fracture when the specimen is tested. Capping shall be carried out according to one of the following methods. *Specification for moulds for use in tests of cement and concrete. 7 IS : 516 - 1959 2.11.1 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. 2.11.2 Sulphur — Just prior to testing, the cylindrical specimens may be capped with a sulphur mixture consisting of 2 or 3 parts sulphur to 1 part of inert filler, such as fire-clay. The specimens shall be securely held in a special j...
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This document was uploaded on 10/01/2013.

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