GAIL (India) Limited

16Unless otherwise specified in the drawings, the maximum nominal size of coarse aggregates fordifferent grades of concrete shall be as under:a) For concreting in very narrow space or in very small thickness 12 mmb) For all reinforced concrete work except in massive foundations 20 mmc) For all ordinary plain concrete and massive reinforced foundations 10 mmAll mix design grades viz., M10, M15, M20, M25, M30 etc., shall be designed and have a minimumcement content as follows:Grade Qty (in kg)M-10 200M-15 300M-20 400M-25 600M-30 800Minimum content of cement remaining unchanged, as specified above for each type of concrete mix,the proportion and quantities of local sand and aggregate are to be worked out and determined in thefield/laboratory as per Road Research Note No.4, Department of Scientific and Industrial Research,United Kingdom for design of concrete mixes or as per ACI 613 with the approval of the Engineer.Any change in the source of aggregates will require the re-designing of the concrete mix for theEngineer's approval.3.10 Mix DesignGeneralAt the commencement of the contract, the Contractor shall make preliminary tests to determine theproportions by weight of cement, fine aggregates, coarse aggregates and water necessary to producerequired grades of concrete. The mix proportions shall be selected to ensure that workability of thefresh concrete is suitable for the conditions of handling and placing and when concrete hardens, it shallhave the required strength, durability and surface finish. The Contractor shall get approval of theEngineer to such proportions before start of concreting. However, such approval shall not relieve theContractor of his responsibility to produce concrete having compressive strengths as laid down in theforegoing table.No departure from the approved proportions will be permitted during the works unless and until theEngineer gives written authorization for any change in proportion. The Engineer shall have authority atany time to check whether the mixing of concrete is being carried out according to the approvedproportions.For the major and important RC works and for all special works, the design of mixes shall be made bythe Contractor at his own cost, for each grade of concrete as well as for various workability. The designof mixes shall be made according to relevant I.S. codes or to approved standard methods.The concrete made by designing the mix is termed hereinafter as "Design Mix Concrete".The Contractor shall use only design mix and weigh batching for RCC. Only ifspecially permitted by the Engineer, he will be allowed to use nominal mix or volume batching. TheContractor shall make available at least two concrete mixers with weigh batching facility. He shall alsomake available concrete cube, testing machine, cube moulds etc. to test the crushing strength ofconcrete at site. He shall also indicate while submitting the Bid, the methods proposed to carry out thedesigns for the mix.