Described in 2 94 preparation of test specimens the

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Unformatted text preview: n 2. 9.4 Preparation of Test Specimens — The test specimens shall be prepared in accordance with 2 and shall be stored in water at a temperature of 24° to 30°C for at least 48 hours before testing. At least three specimens shall be made and tested. 9.5 Age at Test — Normally test shall be made when the specimens reach the age of 28 days. 9.6 In order to determine the compressive strength of the concrete, three test specimens for compressive strength shall be made together with each set of cylinders or prisms in accordance with 9.2. The test specimens for compressive strength shall be made from the same sample of concrete as the cylinders or prisms and shall be cured and stored under identical conditions. 9.7 Apparatus 9.7.1 Testing Machine — The testing machine shall comply in all respects with the requirements of 5.2.1 and in addition shall be capable of maintaining the load at any desired value. 9.7.2 Extensometers — Two extensometers are required each having a gauge length of not less than 10.2 cm and not more than half the length of the specimen. They shall be capable of measuring strains to an accuracy of 2 × 10–6. Mirror extensometers of the roller or rocker type are suitable. 9.8 Procedure — The three test specimens for compressive strength shall first be tested in accordance with 5 and the average compressive strength shall be recorded. Immediately on removing the cylinder or prism from the water and while it is still in a wet condition, the extensometers shall be attached at the ends, or on opposite sides of the specimen and parallel to its axis, in such a way that the gauge points are symmetrical about the centre of the specimen and in no case are nearer to either end of the specimen than a distance equal to half the diameter or half the width of the specimen. The extensometers shall be fixed with the recording points at the same end. The specimen shall be immediately placed in the testing machine and accurately centred. The load shall be applied continuously and without shock at a rate 18 IS : 516 - 1959 of 140 kg/sq cm/min until an average stress of ( C + 5 ) kg/sq cm is reached, where C is one-third of the average compressive strength of the cubes calculated to the nearest 5 kg/sq cm. The load shall be maintained at this stress for at least one minute and shall then be reduced gradually to an average stress of 1.5 kg/sq cm when extensometer readings shall be taken. The load shall be applied a second time at the same rate until an average stress of ( C + 1.5 ) kg/sq cm is reached. The load shall be maintained at this figure while extensometer readings are taken. The load shall again be reduced gradually and readings again taken at 1.5 kg/sq cm. The load shall then be applied a third time and extensometer readings taken at ten approximately equal increments of stress up to an average stress of ( C + 1.5 ) kg/sq cm. Readings shall be taken at each stage of loading with as little delay as possible. If the overall strains observ...
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