Plain cement concrete for combine footing.

     

General Specification: The plain cement concrete shall be proportion of 1:2:4(M-15)

Detailed Specification:

Material:

1)Coarse aggregate: It shall be crushed or broken from hard stone obtained from approved quarry. It shall be hard,strong,dense, and durable,clean and free from soft friable,thin,flat,elongated or laminated,flaky pieces and shall be roughly cubical in shape. It shall be clean and free from dirt and any other foreign matter. Coarse aggregate may be gravel,may either be river bed shingle or pit gravel. It shall be sound , hard ,clean suitably graded  in size.This  shall be free from  flat particle of shale, powdered clay,slit,loam, and other  impurities. Gravel shall have to washed if it contain soil materials adhering to it. Unless specially mentioned the size of the coarse aggregate shall be 20mm graded down and shall be  retained in a 5mm square mesh so that the  voids  do not exceed 42 % .In the case of road or mass concrete work  size 40 to 60 mm may be specified.

2)Fine aggregate: Aggregate most of which passes from 4.75mm IS sieve is known as fine aggregate. Sand as fine  aggregate shall be coarse ,containing of sharp,angular gains and be of standard specification. It shall be clean and free from dust,dirt, and organic matters. Sea sand shall not be used. Crushed dust stone may also be used as fine aggregate.

3)Cement: Cement shall be fresh Portland cement and confirm to IS:269 up-to-date modification.

4)Water: Water used shall be clean and reasonably free from injurious quantities of deleterious material such as oils,acids,alkalies,salts, and vegetable  growth. Potable water must be used.

b) Proportion: proportion  cement , sand, and coarse aggregate shall be 1:2:4 .Coarse aggregate and sand shall be measured by measuring box of  30cm×30cm×38cm. or suitable size equivalent to the content of one bag of cement of  1\30 cum or .035cum. Cement shall be measured by bag weighing 50kg. Sand shall be measured on the basis of its dry volume. In case of damp sand, the bulkage shall be determined at site and the extra quantity of sand so determined shall be added. No extra quantity shall be added unless bulkage is determined. As the bulking of sand may vary from day to day and  at different part of  the day on account of varying moisture content,frequent tests  for bulking shall be carried out with the sand to be used. While measuring the aggregate , shaking, ramming or hammering shall not be done.

C) Mixing:

a) Hand mixing: Hand mixing by batch shall be permitted fir small work only. Normally all structural concrete shall be mixer machine .Mixing by hand shall be employed only in special cases with the specific prior permission of the engineer-in-charge. The mixing shall be done on a clean water tight masonry or concrete slab or steel plate platform. Measured quantity of sand shall be spread evenly. The cement shall be dumped on the sand and distributed evenly. The sand  and cement shall be mixed thoroughly with spade, turning the mixture over and over again, until it is of even colour through-out and free from streaks. The measured quantity of coarse aggregate shall be spread out  on its top. Alternatively ,the reverse operation may be done. This shall be mixed at least three times by shoveling and turning over by twist from centre to side, then back to the centre and again and again to the sides. A hollow shall be made in the middle of the mix pile. Three quarters of the total quantity of  water required  shall be added while the material is turned in towards the centre with spades. The remaining water shall be added by a water can fitted with rose head ,slowly turning the whole  mixture over again until a uniform colour  and consistency is obtained  throughout  the pile. The mixing platform shall be washed at the end of  the day.

2)Machine mixing: The mixer drum shall be flushed clean with potable water only. Measured quantity shall be placed first in hopper. This shall be followed with measured quantity of fine aggregate followed by cement and sand. Finally add balance quantity of the coarse aggregate . The dry material shall be mixed in mixing drum for at least four turn of drum,after which the correct quantity of water shall be added gradually while drum is in motion, to insure even distribution with the dry materials. The total quantity of  water for mixing shall be it introduced before 25 percent of the mixing time has elapsed and shall be regulated by to achieve specific water cement ratio. The mixing shall be  through to have  a plastic  mix of  uniform colour. The complete contents of  the mixed concrete shall be emptied before recharging. When the mixture is closed down for the day or for any  time exceeding 20 minute ,the drum shall be flushed clean.

Mixing time: The material shall not be mixed in drum for a period of not less than 2 minute and until a uniform colour and consistency are obtained. The time required shall be counted from the all the materials have been put into the drum.

d)Consistency: The quantity of  water to be used for each mix of 50kg cement to give the required consistency shall not be more than 34 litres for 1:3:6 mix , 30 litre for 1:2:4mix, 27 litre for 1:1.5:3 mix and 25 lotre for 1:1:2 mix. In the case of vibrated concrete , the limit specified may be suitably reduced to avoid segregation. The quantity  of water shall be regulated by carrying slump cone test.

e)Laying: The entire concrete used in the work shall be laid gently in layers (not thrown)  not exceeding 15cm and shall be thoroughly vibrated  by means of  mechanical vibrators till a dense concrete is obtained. The engineer-in-charge may however relax this condition at his discretion for certain items  depending upon the thickness of the member and feasibility of vibrating the same and permit hand compaction. Hand compaction shall be done with  the help of  punning rods and tamping with wooden tamper  so that  concrete  is thoroughly  compacted and completely worked into the corners of the formwork. The layers of concrete shall so placed that the bottom layer does not finally set before the top layer is placed. Compaction shall be completed before the initial setting starts i.e. within 30 minutes of additional of water to the dry mixture.

             During cold weather casting of  concrete shall not be done when the temperature falls below 4.5ċ. During  hot weather precaution shall be taken to see that the temperature of  wet concrete does not exceed 38ċ under water,the method  and  equipment  shall not be deposited  under water . Where found necessary to deposited under water. Where found necessary to deposit any  concrete under water , the method and equipment shall first be approved by the engineer in charge. Concrete shall be deposited continuously until it is brought to required height; while depositing,the top surface shall be kept as nearly  level as possible and approved method such as Treamie method. Drop and bottom bucket,bags etc.

             When the placing of concrete is suspended, or the resumed on the following day, necessary roughening of the surface for joining future work shall be done before the concrete sets. When the work is resumed the previous work must be thoroughly cleaned , roughened ,watered and a grout of neat cement slurry of the proportion, 1 kg of cement per 2 litre of water applied  uniformly.

f) Protection and curing: Freshly laid concrete shall be protected from rain by suitable covering, The work should be produced from damage and rain during construction .After the concrete has begun to harden i.e. 1 to 2 hrs after the laying, it shall be protected with moist gunny bags, sand, or any other material approved  by the engineer in charge against quick drying. After 24 hrs of laying f concrete the surface shall be cured  flooding water of about 25mm depth or by covering  with wet absorbent materials.

The curing shall be continued for a minimum period 14 days.

               Over the foundation concrete, the masonry work may be started after 48 hrs of its laying, buy the curing of cement concrete shall be continued along with the masonry work for period of 14 days.

g) Form work:  centering and shuttering are required to be done for this work shall be done in accordance with the size of footing