Of the concrctc after it has been placed by the one

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Unformatted text preview: urfaces. The method shall be by means of jets o 4”air and water applied at high velocity with such additional [email protected] of the 18 IS : 457 - 1957 surfact? means of stiff-wire brushes as may be required. Brushing shall by be done by jabbing and digging into the surface rather than by merely sweeping. The whole process shall be conducted in such manner as not to loosen the coarse aggregates but vigorously enough to erp&- a fresh clean-cut concrete surface. Should the next lift be delayed, the contact surface shall be kept wet and covered so as tr3 minimize the evalmration of curing water which may cause an injurious coating on the joint. Where necessary all defective and undesirable concrete shall be removed by chipping and picking by hand or, if so required, by wet sand blasting the top to a depth just sufficient to expose a fresh, clean-cut surface over the entire area, which shall then be thoroughly flushed with water. Every precaution shall be taken to afford suitable bond for the succeeding lift. Immediately before depositing fresh concrete, the contact surface shall again be gone over and thoroughly washed to remove all debris and loose material. The final pick-up of loose materials shall be made near the centre of the joint and away from the outside edges of the masonry. Dry contact surfaces shall be kept saturated with water for not less than 24 hours, but all standing water shall be removed from deprcssions before-spreading the mortar layer. The joint shall then be coveicd with about 1.5 cm ( or ) in ) of mortar (in the same proportion as in the original concrete but not lower than 1: 2 ) and the concrete immediately -deposited thereon according to the manner speciticd under 4.9.2 for rock foundation. Hardened surfac1.s of old ITI:IW~W on which new. concrete is to be placed shall, unless otherwise or&red, be prepared in the same manner as provided for rock foundations under 4.9.2. 4.10.6 P94eumatic Plmitg - Tht: pneumatic method of placing concrete or mortar may IN: used to: sucl1 sj)ecial purposes as tulintl linings, under-pinning foundations, prcssurc grouting, etc, provided special permission has been granted and the method approved. 4.10.7 Rule 0.fPltrcing --- Concr&ng shall be continued without avoidable intcrruptioii until the structure or ycction is complctcd or until satisfactory construction joints can bc made. Concrctc slinll not bc placed faster than the placing crew ~a.11 compact it properly. In placing concrete in thin mcmbcrs and columns, prccautious shall he taken against too rapid a placement which may result in movomcnt or fsilurr of the form due to excessive internal pressure. An interval of at least 4 and prcferably 24 hours should elapse bctwecn the completion of columns alid walls and the placing of slabs, beams _orgirders supported by them in order All concrete shall be placed in apto avoid cracking due to settlement. proximately horizont‘al lifts not exceeding 150 cm ( or 5 ft ) in th...
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This document was uploaded on 10/01/2013.

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