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Unformatted text preview: ine and coarse aggregates ) shall he simultaneously ribboned into the mixer in such a manner that the pcrioci of flow for each ingredient is about the same. 12 IS : 457 - 1957 4 4 Eighty to ninety percent of the total quantity of water required for mixing shall be added uniformly along with the dry ingrcdicnts. The remaining quantity of water shall be added after all the other ingredients are in the mixer. Cobbles or a portion of the coarwst aggregate, howcvcr, may be added last; tllis facilitates the clcarancc of the chutes and removes any fine aggrcgrzte or cement adhering to the sides. Time af mixircg 4.7.1.2 a) Unless othcr\vise permitted, machine mixing of each batch shall Durlilg continue for not less than the period indicated in Tnljle I. this period the drum shall be rotated at a speed recommended by the manufacturer. The mixing period shall be timed after all materials, including water, art in the drum. The efficiency of the performance of the mixer shall be periodically checked. For any one mix, the variation in the air-frw wit weights of three samples taken from the front, centrc and back of a batch of concrete in the mixer, shall not exceed the following: For :one Average Average Average batch of 9 batches of 2h, batches of 90 batches TABLE CAPACITY 37.2 25.9 19.4 14.6 I kg kg kg kg TIME per per per per cu cu cu cu OF m m m m ( or 2.3 ( or I.6 ( or 1.2 ( or 0.9 lb lb lb lb per per per per cu ft cu ft cu’ft cu ft ! ) ) MIXING OF MIXER M~~IMIJM 'l‘inm 01' MIXING .____*_...- . . r--- ..~_..._ Natural Aggregates Manufactured f\g:gregatcs 2 minutes 11 minutes 1 f minutes 3 cu m ( or 3 cu yd ) or larger 2 cu m ( or 2 cu yd ) 1 cu m (or 1 cu yd ) 0~ smaller ) 2! minutes 2 minutes 1 .j miuutes b) Each mixer shall have a mechanically oIwratc~t1 timmg device for signalling tlrc completion of the rccIuired mixing Iwriod. The actual time of mix+ shall bc checked at least twice during. each shift and the timing tlcvicc shall be adjusted if in error. 4 The timing device shall be so interlocked wit11 the discharge gate of the batch hopper that timing does not start until the discharge gate is fully closed.and all ingredients are in the drum. .4 suitable record shall be kept of the average time consumed in charging, mixing aud discharging a batch during each run. 13 IS : 457 - 1957 d) Excessive mixing, necessitating workability shall be avoided. the addition 4.7.1.3 Dischargiltg - The full contents charged quickly so as to avoid segregation of water of the drum ko provide shall be dis- 4.7.1.4 When the mixer is stopped, before placing again any inedients in the mixer all hardened concrete or mortar shall be removed !r rom the inner surface of the mixer. 4.7.2 Central Plant Mixing -- Concrete mixing at the central plant shall be done in accordance with the requirements for machine mixing at sate ( see 4.7.1 ) before being loaded into the transporting vehicle. 4.7.3 Truck Mixing - Truck mixers, unless otherwise approved, shall be of the revolving drum ty...
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This document was uploaded on 10/01/2013.

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