A fixed clamp or support with a maximum width of 1 th

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Unformatted text preview: support with a maximum width of 1 ------ th of the length of the specimen. If a variable air-gap exciter unit is 20 used in conjunction with a variable air-gap pick-up, the support shall be metallic and earthed. 10.4 Procedure — Immediately on removal from water, except when the specimen is less than 24 hours old, and while still in a wet condition, the specimen shall be clamped or balanced at its centre on the fixed support. Where a moving-coil type of exciter unit is used, the moving-coil portion shall be fixed to the middle of one end face of the specimen as indicated in Fig. 4. It may be fixed to the specimen by means of resin or other suitable adhesive. The resin can conveniently be applied with an electric soldering iron. A disk of tinfoil, of 5 cm diameter, shall be lightly greased to ensure adhesion and placed on the centre of the other end of the specimen, and the pick-up unit shall be placed with the diaphragm as close as possible to the foil but the pick-up unit shall not be allowed to touch the tinfoil or the specimen. If a variable air-gap exciter is used, this shall be mounted in a similar manner to the pick-up, using a disk of tinfoil. The exciter unit shall be driven by the variable frequency oscillator and the oscillations received by the pick-up unit shall be amplified by the audio-frequency amplifier and applied to the indicator in such a manner as to show the amplitude of the oscillations received. 20 21 ARRANGEMENT OF SPECIMEN FOR THE DETERMINATION OF MODULUS OF ELASTICITY BY ELECTRODYNAMIC METHOD IS : 516 - 1956 FIG. 4 IS : 516 - 1959 The frequency of excitation shall be varied until resonance is obtained in the fundamental mode of longitudinal vibration. The frequency shall be recorded as the natural frequency of the fundamental mode of longitudinal vibration. NOTE — Resonance is indicated by a maximum deflection of the indicator, but several maxima may be obtained, and experience is necessary to know which is the maximum required. Frequencies within ± 10 percent of the expected value, which may be calculated from the formula given in 10.6 should be investigated. Values of the modulus of elasticity range from 1.4 × 105 kg/sq cm for low-quality concretes at early ages to 5 × 105 kg/sq cm for high-quality concretes at greater ages. This represents a frequency range of 125 000/l cycles per second to 225 000/l cycles per second, where l is the length of the specimen in cm. It is usually possible to obtain resonance also at the frequency of the first harmonie which is twice the fundamental frequency and these two conditions are normally the most well defined. 10.5 Measurements — The following measurements shall be made on the specimen: a) Weight — The wet specimen shall be weighed to an accuracy of 0.25 percent. b) Length — The length of the specimen shall be determined to an accuracy of ± 1 mm, the average being taken of at least four symmetrically placed measurements. c) Depth and Breadth — The average depth and...
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