Concrete is a mixture of cement, sand and aggregate in a fixed proportion used in the construction work. Steps in the Preparation of concrete are as follows:
1)Storing of concrete materials
5)Placing of concrete
Steps in the Preparation of concrete
1)Storing of concrete material
The coarse aggregate and fine aggregate should be stored in a bin watertight store. The bins may be opened or closed but it should be constructed in such a way that loading and unloading take a minimum amount of work.
2)Batching in Preparation of concrete
The measurement of materials for making concrete is known as batching.
The following are two methods of batching is practised:-
In this method cement, sand and concrete are batched by volume. The gauge box is made with wooden plates and its volume is equal to that of one bag of cement. One bag of cement has 35 litres of volumes. The required amount of coarse aggregate and sand is added by measuring into the gauge box. After deciding the water-cement ratio, the quantity of water which is required for making concrete is found.
For example:– If the water-cement ratio is 0.5, for one bag of cement (50 kg), water required is 0.5 *50 =25 kg, which is equal to 25 litres.
Suitable measurement is used to select the required quantity of water. Volume batching is not an ideal method of batching. Wet sand has a higher volume as compared to the same weight of dry sand. It is called bulking of sand. Hence it upsets the calculated volume required.
This is the recommended method of batching. A weighing platform is used in the field to pick up the correct proportion of coarse aggregates and sand. Large weight batching plants have automatic weighing equipment. Weighing batching is more accurate than volume batching.
3)Mixing in Preparation of concrete
To produce uniform and good concrete, it is necessary to mix ingredients like cement, sand and coarse aggregate, first in dry condition and then in wet condition after adding water.
The following methods are practised:-
The required amount of coarse aggregate for a batch is weighed and it is spread on an impervious platform. Then the sand required for the batch is spread over coarse aggregate, then they are mixed in dry condition by overturning the mix with shovels. Then the cement required for the batch is spread over the dry mix and mixed by shovels. After the uniform texture is obtained, water is added gradually and mixing is continued. The full amount of water is added and mixing is completed when uniform colour and consistency is observed. The process of mixing is completed in 6-8 minutes after adding water. Hand mixing is not very good but for small works, it is commonly adopted.
In large and important works machine is preferred. The figure given below shows a typical concrete mixer. Required quantities of coarse aggregate and sand are placed in the drum of mixer 4 to 5 rotations are made for dry mixing and then the required the quantity of cement is added and the drum is rotated for 2 to 3 minutes during which period it makes about 50 rotations. At this uniform and the homogeneous mix is obtained.
4)Transporting of concrete
After completing mixing concrete should be transported to the final position. In small works, it is transported in iron pans from hand to hand of a set of workers. Wheelbarrow and hand carts also may be employed. In large scale concreting chutes and belt conveyors or pipes with pumps are employed. In transporting care should be taken to see that segregation of aggregate from the matrix of cement does not take place.
5)Placing of concrete
Concrete is placed on form works. The form works should be properly cleaned and oiled. If concrete is to be placed for foundation, the soil bed should be compacted well and it should be made free from loose soil. Concrete should be dropped on its final position as closely as it is possible. If it is dropped from a height, the coarse aggregate falls early and then the mortar matrix. This segregation results in weaker concrete.
6)Compaction of concrete
In this process of placing concrete, the air is entrapped. The entrapped air reduces the strength of concrete up to 30%. Hence it is necessary to remove this entrapped air. This is achieved by compacting the concrete after placing it in its final position. Compaction can be carried out either by hand or with the help of vibration.
In this method concrete is compacted by tamping, ramming, spading or by slicing with tools. In intricate portions, a pointed steel rod of 16 mm diameter and about a meter long is used for poking the concrete
b)Compaction by vibrators
Concrete can be compacted by using high-frequency vibrators. Vibration reduces the friction between the particles and set the motion of particles. As a result, entrapped air is removed and the concrete is compacted. The use of vibrators reduces compaction time. When vibrators are used for compaction, the water-cement ratio can be less, which also helps in improving the strength of concrete. Vibration is not good for the concrete. The following types of vibrators are commonly used in concrete:-
a)Needle or immersion vibrators
c)Form or shutter vibrators
Needle vibrators are used in concrete beams or columns. Surface vibrators and form vibrators are useful in concrete slabs. Vibrating tables are useful in preparing precast concrete elements.
7)Curing of concrete
Simply curing can be defined as the process of maintaining satisfactory moisture and temperature for freshly placed concrete for some time for the hardening of concrete. Curing in the early ages of concrete is most important. Curing for 14 days is very important. It is better to continue it for 7 to 14 days more. If curing is not done properly, the strength of concrete reduces, Cracks develop due shrinkage, the durability of the concrete structure reduces.
The following are the methods of curing:-
a)Spraying of water
b)Covering the surface with straw, wet gunny bags, etc
d)steam curing and
e)Application of curing compounds