And into the coricrs of the forms every precaution

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Unformatted text preview: s except finishing, as are necessary to consolidate and mould the concrete properly. The rate of placing mass concrete or reinforced concrete in thin sections. whether mechanically or by manual labour, shall bc cle,uly defined. Accumulation of water on the surface due to bleeding, or other causes taking place during compacting shall be stopped as much as possible by adjustments in the mix. All free water on the surface shall be removed by sponging or mopping. Under no circumstances shall such accumulation of water be covered up with concrete, or dry concrete used to soak up escess water. rJnless otherwise permitted, all concrete shall be compacted by mechanical vibration. The number and type of vibrators shall be subject to the approval of the engineer-in-charge. In general, only vibrators of the internal type shall be used. However, in inaccessible places in the forms where spading, rodding, or forking is impracticabie, the concrete may be gently Lvorked into place and compacted by light vibrating or hammering the forms. 4.11.2 Vibrating Wherever practicable, concrete shall be internally vibrated within the forms, or in the mass, in order to increase the plasticity as to compact effectively to improve the surface texture and appearance, and to facilitate placing of the concrete. The intensity and duration of vibration shall be sufficient to ‘cause complete settlement and compaction without anv stratification Prehminary experiof the successive layers or separation of ingredients. ments in vibrating shall be ‘conducted under actual conditions of mix and placement in order to determine the optimum duration and method of vibration, as \vc!l as to deveiop the necessary skill. Vibration shall be continued until the entire batch melts to a uniform appearance and the surface just starts to glisten. -1 minute film of cement paste shall be discernible between the concrete and the Over vibration causing segregation, forms and around the reinforcement. unnecessary bleeding or formation of laitance shall be avoided. 4.11.3 Internal Type Vibrators M,ass concrete sl:a!l be thoroughly compacted with the aid of high frequency, mechanical vibrators of the internal type, having 21 IS : 457 - 1957 not less than 3 600 and preferably more than 5 000 impulses per minute. Immediately after depositing the concrete, the vibrators shall be inserted into each pile, operated from 10 to 20 seconds in one spot and then moved to another not over 90 cm ( or 3 ft ) away. The operation shall be repeated over until the entire mass is thoroughly compacted and the pile levelled down. Equal attention shall be paid to the edge of the pile and to the centre. A sufficient number of two-man vibrators shall be used to compact each batch properly before placing the next one. A sufficient number of reserve vibrators in good condition shall be kept on hand at all times so as to assure that there is no slackening or interruption in com...
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