The preparation of reinforced cement concrete (R.C.C.) involves following operations:
Measurement of Materials:
The measurement of materials is very important to ensure the required proportions. In India, cement is available in bags of 50 kg weight. The fine aggregate i.e., sand depends upon its moisture contents.The increase in volume of the sand due to moisture content is called BULKING OF SAND. This bulking be comes 25% by volume at 4% moisture contents by weight. With further increase in water, increase in volume decrease and it becomes zero when quantity of water is increased above 20%. The bulking increases with fineness of sand.
The coarse aggregate can be measured by volume or by weight. There is no bulking of coarse aggregate. Measurement by weight is the best method of accuracy in the work required.
The quantity of water is specified after taking into account the moisture contents present into the aggregate. Sometimes, the aggregate absorb moisture from the concrete. So, necessary allowance for moisture contents and water absorption is kept while specifying the quantity of water.
Proportioning of Concrete
The purpose of proportioning the concrete is to find most economic proportion of cement, aggregates and water which will give the desired strength of concrete, proper work ability and durability. The proportioning of concrete materials will differ with the requirements for which it is to be used. So, to suit different conditions various concrete mixes are used.
Durability of concrete can be achieved by the use of proper quality of cement and aggregate and it can be made water tight by reducing the extent of pores in it. This is achieved by proper compaction. The quantity of water required for chemical action with cement is about 30% by weight. The extra water used is evaporated leaving pores in the concrete.
Methods of Proportioning Concrete
Following methods are used for proportioning the concrete:-
In this method the ratio of coarse to fine aggregate is made to remain between 1 1/2 to 2 1/2, so that the fine aggregate may fill the voids between the coarse aggregate to the maximum. The ratio of cement to aggregate depends upon the strength of concrete desired. For very high strength of concrete mixes of 1:1:2 to 1:12:24 may be used and for general R.C.C. construction of beams, slabs, lintels, columns etc., a mix ratio of 1:1 1/2:3 to 1:2:4 may be adopted.
For mass concrete mix of 1:3:6 and 1:4:8 are generally employed. Adequate quantity of water is then mixed in each case to give the desired consistency and work ability. The proportioning done by this method gives different strengths of concrete for the same mix. The strength of concrete is either too low or too high than the desired value. But this method is very simple and hence widely used.
2. Minimum Void Method
This method is based upon the idea that to obtain a dense concrete, the mix should have minimum percentage of voids. This is achieved by mixing the fine aggregates in sufficient quantity to fill the voids in coarse aggregates and the cement is added in sufficient quantity to fill the voids in fine aggregate before mixing it with coarse aggregate. But the aim of minimum voids is not achieved fully. This method does not give dense and strong concrete.
Mixing and Placing Concrete
After proportioning of mix are fixed for making the concrete, it is desirable to mix them properly. Aggregates are measured in wooden boxes of known dimensions. The proportions of aggregates are measured in whole numbers against one bag of cement. Suitable allowance for moisture content is made before calculating the required quantity of water. This method of measurement is adopted for small works. For big works like dams, mechanically operated batching plants are used. To have good quality of concrete, it is necessary to mix the ingredients thoroughly.
Mixing is done by Hand and Machine.
In hand mixing the usual practice is to have steel pan of 180 x 180 cm in size or a pucca platform. The measured quantities of materials are placed in pan or over the pucca platform. Firstly cement and sand are mixed are dry thoroughly to have a uniform color. The mixture is then spread over the stack of measured quantity of coarse aggregate and then turn over the whole at least for 3 to 4 times in dry state. Water is then added again the whole is turned 3 to 5 times taking utmost care that water or cement are not lost during the process of turning. The water must be as per required quantity.
2. Machine Mixing
Machine mixing is done in concrete mixers which are driven by electricity or oil engines. A mixer is either of rotating type or tilting drum type. These are available in various capacities. The concrete is not allowed to stay for longer period inside as it is liable to stick to the sides of drum. Measured quantities of all the materials are put into the hopper attached to the mixer in dry state and tilt mechanically. The materials fall into the drum which is rotating. Water is poured into the drum through a pipe attached to the mixer and controlled automatically to the requirement. The time of mixing each batch is so adjust that the mixture gives a uniform color. Each batch takes about 2 minutes for proper mixing of ingredients.
After mixing the concrete it should be immediately placed in position so that initial setting of cement may not start. The time of initial setting of cement is about 30 minutes. In dry and hot weather the concrete dries very quickly. So under such cases some more water is remixed with it without increasing the water cement ratio, before the initial setting takes place. While transporting the mixed concrete it should be seen that segregation of materials does not take place, and it is placed within the setting time. A continuous supply of the mixing should be ensured at the place where it is to be deposited. Before actual placing of concrete starts it must be ensured that the forms are rigidly braced true to their position and oiled and cleaned at the bottom. The concrete should be laid in thin and uniform layers and compacted before the subsequent layer is placed over it.
Compacting of Concrete
To get a dense mass without voids the concrete is compacted thoroughly so that it nay fill all the corners and surround all the reinforcements. Compacting is done by pinning with rod, tamping with heavy tamper or by vibrating. In case of plastic and wet concrete, the method of pinning is done while in case of mass concrete work tamping is commonly employed. Excessive tamping brings the cement in the mix at top so it should be avoided.
Vibrating the concrete is done by vibrators which are of three types i.e., Internal, External and Surface vibrator.
Internal vibrators consists of blades which penetrates into the concrete and kept into the position for about 3 minutes for vibrators. Care should be taken that the blades do not touch the reinforcement which may get displaced.
2. External Vibrators
External vibrators are attached to the form work and transmit the vibrations through it with a spread of 6000 to 9000 shocks per minute. Care should be taken that the form work is sufficiently strong to receive vibrations.
3. Surface Vibrators:
Surface vibrators are used to consolidate concrete in flat surface such as roads. The water cement ratio is kept low because thorough compaction can be achieved even for dry concrete.
Curing of Concrete
The concrete, though properly proportioned, carefully laid and compacted cannot give required properties of strength, water tightness etc., if it is not properly cured. So the curing is very important and essential.
When water is added to the cement, it forms some compound as a result of chemical action between cement and water which takes place over a considerable period at a favorable temperature. It is therefore necessary that sufficient quantity of water is available in concrete before it attains its full strength. The process of keeping the concrete wet to gain the sufficient strength is called CURING. The water action with cement causes hydrolysis and hydration which produces gel. This gel ultimately crystallize. Action is rapid during early stages as such hardening and evolution of heat is pronounced. Continuous availability of water in concrete dissolves all the cement thus making it hard and strong. Concrete attains major portion of its strength in about 3 weeks and continues to increase slowly.
The object of curing is to prevent loss of moisture from the concrete due to evaporation and other reasons. The fresh laid concrete should be prevented from direct sun-rays and hot winds to avoid rapid drying. During the period of curing it should be protected from any type of shocks or stresses due to loads or vibrations. 3 weeks time of curing is considered sufficient for wet curing.
Curing is sometimes done by steam or hot water. In this method development of strength is very rapid. This method can be used for precast concrete work. In steam and hot water curing it is most important to control the temperature fully as cracks are formed due to rapid drying and cooling.