What is concrete?
A mixture of cement, fine aggregate (sand), coarse aggregate and water with a specific ratio & proportioning is called concrete. There’s different types of concrete & grades of concrete.
Unit weight of concrete is:
Plain cement concrete (PCC)= 2400 kg/m3
Reinforcement concrete (RCC)= 2500 kg/m3
Ingredients of concrete:
In manufacturing of conc followings are the ingredients of concrete.
Cement:
Cement is a fine (powder) binding material use to manufacture conc and mortar. Unit weight of Portland cement is 1440 kg/m3.
Fine aggregate:
Sand is used in conc manufacturing as fine aggregate. For a quality product it should be assured that sand is coarser, it has sharp edges, should be angular. To assure all these properties test it on 4.75mm sieve if it passed by the sieve, it is good for use.
Sand should be clean. There shouldn’t be any dust in it nor any organic matter present in it. Sea sand shouldn’t use in anyway. If it fulfill the terms, a stone dust can be used.
Coarse aggregate:
Hard stone pieces used in conc manufacturing are coarse aggregates. Coarse aggregate should be hard, strong, durable and clean. Their shape should be almost cubic.
Coarse aggregate shouldn’t be laminated, flat and elongated. There shouldn’t be dust on them. It should has same size and quality as mentioned describe in design.
It shouldn’t pass from 5mm (0.25 inch) sieve. The coarse aggregate batch should have different sizes particles. Batch should be well graded and shouldn’t have void ratio more than 42% at any cost.
What is water cement ratio:
It is the quantity of water against cement in a conc mix. Water cement is most important in conc mix. For a good quality of concrete WC ratio should must be considered. Increase in water content can result in decrease of conc strength.
For best quality of concrete water cement ratio should be 45% of weight of cement.
How concrete is made?
Proportioning of Concrete:
Cement, sand and aggregate (CSA) should be in defined proportion to insure the healthy conc mix. Bulking of sand should be considered when measuring sand for proportioning.
The defined proportion is considered for dry sand. It should be pre checked if there’s any moisture content present in sand, then extra sand should be added in proportion with calculation.
Mixing of Concrete:
Mixing of concrete ingredient are really important and should be well mixed. There’s two way of mixing. For small construction works it can be done by hand, for major work it should be done with machine.
Hand mixing:
First, make a water tight plate form of steel, wooden or conc. For conc manufacturing firstly pour sand on plate form and then cement on it. Mix it well in dry phase till it becomes consistent.
After this pour coarse aggregate on plate form, and then pour consistent sand & cement mix on it. Mix all three ingredient well in dry phase. After getting well mixed make a circle in center of it and pour water into.
1/3 of total water quantity should be added in first phase, then mix it and pour the remaining water with help of shower. Keep in mind water shouldn’t flow out of conc mix.
Mix it in wet phase until it becomes unicolour. All the concrete manufacture should be used in half hour and the time period shouldn’t exceed.
Machine mixing:
First, wash the machine well before use. Pour aggregates in machine then sand and then cement and mix it in dry form (minimum 4 turns of machine) & then add water in it.
Mix it again in wet foam and turn the machine for 2 minutes till it becomes consistent and unicolor. Conc is ready, use it, and clean the machine after work is done.
Placing of concrete:
After manufacturing of concrete, it should be placed on required place slowly and well mannered. It should be place from height (height shouldn’t exceed 3ft), otherwise ingredient will be segregate.
Concrete should be placed before the initial setting time of cement starts ( which is 30 mins). Initial setting time starts when cement first contact with water.
Before 30 minutes conc should be compacted with vibrator, if vibrator isn’t available or can’t be use at any point then steel rod can be used. Make sure to NOT give any joint in conc work.
Concrete placing shouldn’t be done in 4.5 centigrade in winters & 38 centigrade in summer. Special care should be taken if its compulsory to place.
In case of providing any joint, make the previous conc surface rough, cure it and pour cement slurry on it then place the new conc. It will provide strong joint between.
Protection of freshly placed concrete:
Fresh concrete should be protected from rain and hot weather. It will cause in shrinkage and ultimately cracks in member because of rapid drying. To protect it from hot weather put wet gunny bags on it.
After 24 hour passed cure it every so often or stop 2.5cm (1 inch) water on it. Cure it at least for 14 day. After 48 hours construction upon it can be start but curing of concrete should remains till 14 days.
Source: civilmanage.com
