What Are The Manufacturing Process Of Concrete | Process Of Manufacture Of Concrete

MANUFACTURING OF CONCRETE

While manufacturing concrete, it should be ensured that every batch of concrete has the same proportions. This is a mandatory requirement so as to satisfy two aspects, viz., same workability and uniform strength. In the manufacturing of concrete the following steps are followed:

1. Proportioning of concrete

2. Batching of materials

3. Mixing of concrete

4. Conveyance of concrete

5. Placing of concrete

6. Compaction of concrete

7. Curing of concrete

 

PROPORTIONING OF CONCRETE

Selection of the proper quantity of cement, coarse aggregate, sand and water to obtain the desired quality is known as proportioning of concrete. Concrete is formed by successive filling of voids in aggregate by sand, the voids in the sand by cement and voids in cement by water and undergoing a chemical reaction.

The concrete formed by proper proportioning of ingredients should satisfy the following properties:

(i) The fresh concrete should have adequate workability for uniform placement.

(ii) The hardened concrete after setting should have the desired strength and durability.

(iii) The concrete should be cheap considering the materials and labour.

There are two approaches to proportioning concrete. In the first method, no preliminary tests are conducted. But based on experience, arbitrary ratios such as 1:2:4; 1:1½:3; 1:1:2, etc., are used. This method of proportioning by adopting an arbitrary ratio is called the mixing method, and the concrete formed by this method is called ordinary concrete. This type of concrete is used for ordinary or common works such as columns and members subjected to medium loads, all general building RCC works, mass concrete work in culverts, retaining walls, compound walls, and ordinary machine bases. Ordinary concrete can also be used for long-span arches with a mix of 1:1:2 and for heavily stressed members with a mix of 1:2:2.

In the second method, preliminary tests are conducted, the mix being designed by any one of the mix design methods to get the desired strength and durability. The concrete formed by this method is called controlled concrete. This type of concrete is used for all plain and reinforced concrete structures. The concrete mixes for controlled concrete are designed to have an average strength in the preliminary strength test as shown in the Table below

Strength Of Concrete In Preliminary Strength Test

Sl. No.	Desired strength (kg/cm2)	Preliminary test value (kg/cm2)
1.	100	135
2.	150	200
3.	200	260
4.	250	320
5.	300	380
6.	350	440
7.	400	500

BATCHING OF MATERIALS

After fixing, the desired proportion of the number of required ingredients, viz., cement, coarse- aggregate, fine aggregate, cement and sand, has to be measured out in batches for mixing. This process of measuring out ingredients is called batching. Batching may be done by weight or by volume. Volume batching is inferior to weight batching as using the former is liable to change the volume of sand in bulking or aggregate constant void feasibility.

1. Weight Batching

In this batching method, all the ingredients of concrete are directly weighed in kilograms. As the weight of cement bag is 50 kg, 20 bags are needed for 1 tonne of cement. For all important works, the batching method should be used. This is a slow process.

2. Volume Batching

In this batching method, two units of measurements are employed: liquids are measured in litres and solids in cubic metres. That is all ingredients, viz., water, cement, sand and coarse aggregates are measured in litres, while the end-product concrete is measured in cubic metres. In volume batching, other quantities are measured keeping cement as the base. Considering that 1 litre of cement equals 1.44 kg, a bag of 50 kg cement has a volume of 3.5 litres. Hence, for measuring aggregates wooden boxes of an inner volume of 3.5 litres have to be used. Size of a box of 40 cm × 35 cm × 25 cm satisfies this 3.5-litre volume requirement. Handles are provided on the sides for handling.

As the density of water is 1 g/ml, it can be measured by weight or by volume. The quantity of water required depends on the water-cement ratio. Thus, for a water-cement ratio of 0.50, the quantity of water required is 25 litres (0.50 × 50 = 25 litres). For accurate batching, the moisture content and absorption of aggregates and bulking of sand have to be ascertained.

 

MIXING OF CONCRETE

Mixing of concrete may be done by hand or by a machine. Mixing should be done thoroughly so that the ingredients are uniformly distributed, and this can be judged by the uniform colour and the consistency of concrete.

On a clean, hard and water-tight platform cement and sand are mixed dry using shovels until the mixture shows a uniform colour. Then aggregates are added and the ingredients are thoroughly mixed. Based on the water-cement ratio, the quantity of water required is calculated and added to the dry mix. The mass is then turned to obtain a workable mass and placed in the required area within 30 minutes. Hand mixing can be used for small quantities of concrete, or due to the non-availability of a machine or where the noise of the machine should be avoided. In general, extra cement of 10% is used to compensate for the possible inadequacy.

Mixing by machine is always preferred. Concrete mixers are used for mixing concrete and are of two types, viz., (i) continuous mixers and (ii) batch mixers.

Continuous mixers are used for purposes where large quantities of concrete are needed such as dams, bridges, etc. Batch mixers are also called drum mixers, which consist of drums with blades or baffles inside them, and they are rotated. In the batch mixer, all required materials are fed into the hopper of the revolving drum incorrect quantity. When the mix has attained the desired consistency, the mix is discharged from the drum and conveyed to the concreting yard.

 

CONVEYANCE OF CONCRETE

The mixed concrete should be conveyed to the concreting yard as early as possible but within the initial setting time of the cement. The choice of conveyance depends on several factors, viz., nature of work, distance from the mixing place to the construction site, height to be lifted, type of cement, etc.

During the transit from the point of mixing to the point of placement, the following factors have to be borne in mind:

1. Care should be taken not to allow segregation of aggregates.

2. The containers of the drums should be tight such that there is a minimum loss of water.

3. The mixed concrete should be brought to the site before the initial setting time of the cement.

For ordinary simple works, a temporary ladder is erected to convey the concrete using baskets, or it is passed from hand to hand, i.e., by manual labour. For larger and important works, various mechanical devices such as vertical hoists, lift wells for tall structures, wheelbarrows, etc., are used.

 

COMPACTION OF CONCRETE

Compaction of concrete has to be done as early as possible after placing the concrete in place. The purpose of compaction is to expel air and bring the particles closer so as to reduce the void and make the concrete denser. This increased density will give higher strength and make the concrete impermeable. Over-compaction leads to segregation while under-compaction makes the concrete lean. To check for correct compaction, the compaction should be stopped as soon as the cement paste starts appearing on the upper surface of the concrete.

 

Compaction by hand may be performed by rodding, tamping, ramming or hammering. Wooden or steel hammers are used for ramming massive concrete works. Tampering is adopted for compacting slabs or other horizontal surfaces. Rodding is followed for compacting vertical sections. In all cases of hand compaction high water-cement ratio should be adopted.

Compaction by machines is performed using mechanical vibrators. This method of compaction has several advantages as detailed below:

(i) The concrete produced is dense and impermeable.

(ii) A lesser water-cement ratio results in about 15% reduction in the use of cement.

(iii) A better bond exists between steel and concrete.

(iv) The surface of the concrete is uniform because of machine compaction.

(v) Because of a high aggregate–cement ratio, there is a possibility for the reduction in creep and shrinkage.

(vi) Filling small openings is feasible because of good consistency in concrete.

(vii) It is relatively fast in placing concrete.

(viii) It consumes comparatively less time, materials and labour and is hence economical.

 

CURING OF CONCRETE

The following methods of curing are adopted depending on the type of work:

1. Direct Curing

In this method, water is directly applied to the surface of curing. In this process, the surface is continuously cured by stagnating water or using moist gunny bags, straws, etc. These methods are used for horizontal surfaces. Vertical surfaces can be cured by covering moist gunny bags or straws.

 

2. Membrane Curing

In this method, steps are taken to prevent water evaporation from finished concrete sur- faces. This is done by covering the surfaces with water-proof papers, polythene papers or by spraying with patented compounds or covering with a bituminous layer to form an impervious film on the concrete surface.

 

3. Steam Curing

This approach is widely used in precast concrete units. Here the precast units are kept under the warm and damp atmosphere of a steam chamber.

 

4. Surface Application by Chemicals

Chemicals such as calcium chloride are spread as a layer on the finished concrete. The chemical absorbs moisture from the atmosphere and prevents evaporation of the moisture from the concrete surface.

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