Strains observed on the second and third readings

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Unformatted text preview: ed on the second and third readings differ by more than 5 percent, the loading cycle shall be repeated until the difference in strain between consecutive readings at ( C + 1.5 ) kg/sq cm does not exceed 5 percent. 9.9 Calculation — The strains at the various loads in the last two cycles shall be calculated separately for each extensometer and the results shall be plotted graphically against the stress. Straight lines shall be drawn through the points for each extensometer; the slopes of these two lines shall be determined and from them the average value shall be found. If the difference between the individual values is less than 15 percent of the average value, this average value, expressed in kg/sq cm to the nearest 1 000 kg/sq cm shall be recorded as the modulus of elasticity of the concrete. If the difference is greater than 15 percent, the specimen shall be re-centred in the testing machine and the test repeated. If the difference after re-centring and testing is still greater than 15 percent of the average value, the results of the test shall be discarded. 9.10 Report — The following information shall be included in the report: a) identification mark; b) date of test; c) age of specimen; d) shape and nominal dimensions of specimen; e) modulus of elasticity; and f) remarks, such as number of loading cycles. 10. DETERMINATION OF THE MODULUS OF ELASTICITY BY ELECTRODYNAMIC METHOD 10.1 This clause specifies the procedure for determining the modulus of elasticity of concrete by an electrodynamic method, namely by measuring the natural frequency of the fundamental mode of longitudinal vibration of concrete prisms. This is a non-destructive test 19 IS : 516 - 1959 and therefore the same specimens may subsequently be used for the flexural strength test specified in 8. 10.2 Specimens — The specimens shall in all respects comply with the requirements of 9.2. 10.3 Apparatus — The apparatus shall consist of the following. 10.3.1 A variable frequency oscillator with a working range up to 10 000 cycles per second, reading to an accuracy of ± 0.5 percent. A means of varying the output power shall be incorporated, and the output power shall be not less than 2 watts. 10.3.2 An electro-magnetic exciter unit of the moving-coil or variable air-gap type. If the moving-coil type is used, the former and coil should have as low an inertia as possible, the mass being not more than 0.2 percent of the mass of the specimen (the former should be of light card or paper). A suitable type of instrument is shown in Fig. 4. If a variable air-gap exciter unit is used, this shall be of a similar type to the pick-up unit. 10.3.3 An electro-magnetic pick-up unit of the ordinary telephone type. Alternatively, a piezo-electric gauge may be used, provided that its mass is not more than 0.2 percent of the mass of the specimen. 10.3.4 An audio-frequency amplifier. 10.3.5 An amplitude indicator consisting of a voltmeter, milliammeter or cathode ray oscilioscope. 10.3.6 A fixed clamp or...
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This document was uploaded on 10/01/2013.

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