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Unformatted text preview: e m,ix. shall be capable of so controlling the rate of dec) The equipment livery of each kind and size of materials that the combined inaccuracy in feeding and measuring during normal operations does not exceed 3 percent for all aggregates, and 1 percent for water. mechanism -for measuring the amount of water‘ d) The operating shall be such that no leakage occurs with the valves closed. The filling and discharge valves shall not be opened before the IiUing valve is closed. e) Test scale weights shall be provided and periodic checks made of the accuracy of ail weighing equipment. .1 I. In cases inhere weigh batchers are not available the specification expressing the mix m proportions of weight should be transformed This should be done in the field into one measuring material by volume. *Second revision in 1% 11 IS : 457- 1957 laboratories attached to each construction site. The co-efficients transforming weight into volume should be determined anew each time the ,aggregate i’s obtained from a different source. 4.6.3 Cement Content-Cement in standard packages or sacks need not ordinarily be weighed, but shall be periodically checked in the field. Bulk cement and fractional packages shall be weighed. 4.6.4 Water Content The water-cement ratio established for each class of concrete shall be hell within the limits specified under 4.2. The consistency of the concrete shall be varied only by increasing or by decreasing the amount of cement paste in each batch. Water for mixing may be measured either by volume or by weight, and the measuring device shall be accurate to within one percent. A reliable method of compensating for free water shall be used for maintaining a constant water-cement ratio. Whenever concrete is being conveyed over a long distance during hot or dry wcathcr, an ahowance may be made for probable evaporation during the period concrete is being transported. 4.7 Mixing 4.7.1 Machine iMixing at Sire - The mixing of concrete shall be done in a batch mixer of such approved type as will ensure the homogeneous The plant shall be so designed and operated distribution of all ingredients. that ‘all materials entering the mixer including water, can be accurately The entire batch within the mixer proportioned and readily controlled. The volume of mixed material shall be discharged before recharging. per batch shall not exceed the manufacturer’s rated capacity. Method of charging - 171c proper sequence of operations for the admission of aggregates into any type of mixer shall be ascertained by trial runs conductecl in order to determine the method giving the best results. The following sequcncc of charging the mixer may be adopted: a) Five to ten percent of the total quantity of water required for mixing, adequate to wet the drum thoroughly, shall be introduced before the other ingredients m order to prevent any caking of cement on the blades or sides of the mixers. b) All dry ingredients ( cement and both f...
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This document was uploaded on 10/01/2013.

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