And other structures may vary in character from mass

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Unformatted text preview: ging from about 150 to 235 kg per iu m ( or 250 to 400 lb per cu yd) to heavily reinforced concrete having a maximum size of aggregate of 2 cm ( or $ in ) and a cement ‘content of approximately 355 kg per cu m ( or 600 lb per cu yd ). To effect the greatest economy, the concrete should preferably contain the maximum size aggregate S?litdh? for the place of use and giving the specified strengths. 4.2 Quality 4.2.1 The proportions of the ingredients in con&ete shaft be detcrmined through preliminary laboratory tests on concrete made from representative samples of ingredients proposed. to be wed in the work. The tests shall be carried out in accordance iith the procedure described in Appendix B of *IS: 456-1957. After the water content has been determined to suit the required consistency, the cement content sl~nll be deteimined in the following manner: A curve representing the relation between water-cement ratio anti the average compressive strength of concrete at 28 days or earth_, at which tt)e concrete is to receive its full working load, shall be established for a range of values at least 15 percent below and 15 percent above 11 times the required strength. The curve shall bc fiscd by at least four pointy, each point rctprescnting the avcragc values of at least three test specimens. The water-cement ratio to ho adoptctl on the work shall correspond to that strength on the curve which is 1) times the strength actually rccpiircd. *Second rrvisior~ in 1964. 9 IS : 457 - 1957 The preliminary tests shall be completed well before the beginning of concreting operations and a complete report on the concrete forming qualities and Gtability of available aggregates as also recommendations for their use at the work shah be made and approved by the engineer-incharge before actual concreting is started. 4.2.2 No substitutions in the materials used on the work or alterations in the established proportions shall be made unless additional tests have been .:onducted to show that the quality and strengths of the resulting concrete are satisfactory. 4.2.3 The engineer-in-charge may carry out check tests and order changes in the mix as may be necessary from time to time to maintain No radical changes, substitutions and the specified quality of the work. additions in the mix, shall be made without such check tests and subsequent approval. 4.3 Mix Proportions -The proportions of cement, water, aggregates and other ingredients, if any, necessary to produce concrete of the desired quality shall be determined as described under 4.2.1. The specified watercement ratio shall be strictly adhered to for each class of concrete. Minor adjustments of the proportions of all ingredients may be made by the engine&-in-charge in accordance with field tests, specified under 7. Both the rate and the sequence of pouring shall be strictly regulated as laid down bv the engineer-in-charge. 4.3.1 Determination of Surface Moisture - In the case of fint? aggregates, the surface moist...
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