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Unformatted text preview: pacting. 4.11.3.2 The USC of flexiblr shaft vibrators, if permitted, closely follow special instructions issud for tt!c purpose. shall 4.11.3.3 Internal vibrators shall be allowed to penetr;ltcB as deeply as possible under their own weight and shall so consolidate fhe succcssivc layers as to break up effectually all- strata or seams. The vibrators shall be inserted and withdrawn slowly in such manner as not to leave voids in the plastic concrete. The entire operation shall be condncted in a systrmatic manner and each course or layer vibrated uniformly. The method of dumping or depositing the loads shall be so arranged as to keep the vibrators working continuously during placing operations. The courses shall be kept approximately l+el, and the concrete. even when depdsited in thin layers, shall be as st;iI as can be satisfactorily worked. However. concrete for which a slump greater than 10 cm ( or 4 in ) is specified shall not be vibrated unless otherwise ordered. Care shall be taken ndt to disturb a set of partially set layer. The vibrators shall be held vertical as far as possible. 4.11.3.4 Under no conditions shall internal vibrators strike the face of the forms, nor shall reinforcement steel or embedded metal be jarred with sufficient force to impair the bond between the concrete gnd the metal. 4.11.4 Ex&c7nul Tybe Vibmtors 4.11.4.1 Whereyer so required. platform vibrators shall be used to embed all large stone or cobblblc projecting above the top of the lift, but such vibrators shall be used with caution and operated only in such manner as to avoid pulling exccjs mortar to the surface. The entire top of the lift for a depth c;i about 30 cm ( or 1 ft ) shall be thoroughly vibrated. Particular care s.lall be taken in making keyways and shear grooves. Where a raised key is required, the form shall be filled to overflowing and a platform vibratox used to compact the concrete and bond the key to the body of the lift. 22 IS : 457 - 1957 4.11.4.2 Form vibrators shall be permitted-only for special purpose specified under 4.11.1.3 and extreme care shall be exercised to avoid pumping air into the concrete. 4.11.5 Surface Voids - Large voids or air pockets, which may be left in the permanently exposed faces of the structure by vibration, shall be removed systematically spading the face in the following manner. Wherever practicable;-a motor-driven, slowly revolving, square steel rod shall be held in a vertical position and moved slowly back and forth in short intervals along the entire face. Care shall be taken to avoid prolonging such spading to the point of leaving excess mortar in the face. 4.12 Depositing Concrete Under Water 4.12.1 Concrete shall not be deposited under water, if it is practicable to unwater the area and place the concrete in the regular manner. If it is found necessary to deposit any concrete under water, the method, equipment, materials and mix shall first be approved by the engineer-incharge. Concrete shall not be placed in runni...
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This document was uploaded on 10/01/2013.

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