Measurement and mixing. All cement is to be measured by weight, the 50 kg bag of cement being used as aunit. Specification Concrete Work (2/F/10) F.29 Measurement and mixing (Cont‟d.) The amount of water shall be the minimum required to produce a dense cohesive concrete of adequate workability, to be determined by trial mixes. This amount shall be accurately gauged and adjusted from time to time to compensate for variations in moisture content of the aggregate by an approved method. All concrete shall be mixed in a batch type mechanical mixer of approved type having a drum rotating about a horizontal or inclined axis. The speed of the drum is to be not more than twenty and not less than fourteen revolutions per minute. Each mixer is to be fitted with a water measuring device capable of accurate measurement to five litres for one cubic meter mixers and prorata for smaller sizes and so arranged that the accuracy is not affected by variations in the pressure of the water supply line. The fine and coarse aggregate and the cement shall be mixed for at least four turns, after which the required amount of water shall be added gradually while the mixer is in motion and the concrete mixed for not less than one and a half minutes to uniform colour and consistency. The volume of concrete mixed in any one batch is not to exceed the rated capacity of the mixer. The whole of the mixed batch is to be removed before materials for a fresh batch enter the drum. Concrete as mixed in accordance with the foregoing shall not be modified by the addition of further water or in any other manner. On the cessation of work, including all stoppages exceeding twenty minutes, or any change of type of cement used in the mix, the mixer and all handling plant shall be washed out with clean water. At least one slump test shall be made each day concreting is in progress, under the supervision of the Engineer.
Measurement and mixing. In the case of loose materials such as sand, broken stone, mortar, etc. the proportions demanded by the specifications must be measured in properly constructed measuring boxes, or in such other manner as shall be instructed by the Superintending Engineer. Measurement is not to be done in xxx.xx heaps when intimate mixtures such as mortar, concrete etc. are to be formed. The mixing must always be done on closely constructed platforms so that their will be no leakage of any of the materials through the floor of the platform and also so that no foreign materials can be incorporated during mixing. These platforms must be approved by the Superintending Engineer. The cost of such measuring boxes and platforms and all the work referred to here in shall be borne by the contractor.
