Steps involved in cement concrete works

Given below, brief specifications for executing cement concrete works for different objectives :-

1. Materials for Cement Concrete: Different types of materials cement, aggregates and water are required for cement concrete works. The aggregates are categorized as fine aggregates (sand) and coarse aggregates. The aggregates should comprise of inert material and should be clean, dense, hard, robust, long-lasting, non-absorbent. Besides, it should have the ability to make superior bond with the cement mortar.

Cement - Fresh Portland cement or pozzolana Portland cement (PPC) should be used according to requirement or specification and should contain the necessary tensile and compressive strength and fineness.

Fine Aggregates - Course sand with hard, sharp and angular grains should be utilized as fine aggregate or sand and it should get through 5mm (3/16”) square sieves or mesh. It should contain standard quality and does not contain dust, dirt and organic matters. Sea sand is not recommended for concrete works. Fine aggregates should comprise of crushed stone or manufactured sand if indicated.

Coarse Aggregates - These should comprise of hard broken stone of granite or similar stone and does not contain dust, durst and other foreign materials. The size of stone ballast should remain 20mm (0.75 inches) and less and should be arranged on 5mm (0.25 inch) square mesh. These should be well grades to retain voids under 42%.

The size of coarse aggregate is based on the thickness of concrete and nature of work. As for instance, size of coarse aggregates for building works should remain 20mm and 40mm to 60mm sizes are applied for road work and mass concrete works.

Water - The quality of water should be same as drinking water and it does not contain alkaline and acid matters.

2. Proportioning of Cement Concrete: The proportions in cement concrete should be according to the design and strength requirements. The proportion can be 1:2:4 (M15 concrete) or 1:1.5:3 for M20 concrete. The proportions of 1:2:4 concrete include the ratio of cement: sand: coarse aggregates by volume until indicated. Least compressive strength of concrete of 1:2:4 mix proportion should be 140 kg/sq.cm or 2000 lbs/sq.in on 7 days.

3. Measurement of Materials: Sand and coarse aggregates are calculated by volume with boxes. Cement should not be calculated by box, one bad of cement of 50kg weight should be treated as 1/30 cu.m or 1.2 cu.ft volume. Size of measured boxes may be 30 cm x 30 cm x 38 cm or 35 cm x 35 cm x 28 cm similar to the content of one bag of cement.

All materials should be dry and in case of utilizing damp sand, compensation should be done with extra quantity sand to the extent necessary for bulking of sand.

4. Mixing of Cement Concrete: Mixing of concrete should be done with machine to attain superior quality. For small works, hand mixing by batches is suitable.

5. Checking for Concrete Slump: Slump test should be conducted constantly to control the addition of water and to retain the desired consistency. A slump of 7.5cm to 10 cm (3 inches to 4 inches) is perfect for building work and 4 cm to 3 cm (1.5 inch to 2 inches) is ideal for road work.

6. Formwork for Concrete Works: Formwork centering and shuttering should be arranged as per need and the standard specifications prior to place concrete to confine or to support or to retain the concrete in exact location. The inside surface of the concrete should be oiled with formwork oils so that the concrete can’t stick to it.

Before placing concrete, water should be sprinkled over the base and formwork where the concrete will be arranged. Forms should not be detached prior to 14 days in general, side forms may however be detached after 3 days of concreting.

7. Placing of Concrete: It is necessary to place concrete gently in layers not surpassing 15cm or 6 inches and it should be consolidated by pinning with rods and tamping with wooden tampers or with mechanical concrete vibrating machines unless a solid concrete is produced.

Concrete should be placed constantly. If the placing of concrete is postponed for rest of the day or for the following day, the end should be sloped at an angle of 30 degrees and made rough for jointing again.

Curing of Concrete: After about two hours of placing when the concrete starts to become solid gradually, it should be retained moist by covering with wet gunny bags or wet sand for 24 hours and then curing by flooding with water making mud walls of 7.5 cm or 3 inches high or by covering with wet sand or earth and kept damp constantly for 15 days.



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Variations among nominal mix and design mix

Under Concrete mixing method, all the materials of concrete are chosen with exact ratio. The ratios of mix should be maintained in such a manner so that the workability of the fresh concrete remains perfect and when concrete is solidified, it should contain the necessary strength, surface finish and stability.

Generally, two types of methods are utilized for proportioning several materials which range from nominal mix and design mix of concrete. Based on the requirement and type of work, the types of concrete mix are chosen.

The nominal mix is the process in which all the ingredients are prescribed as per specifications and their proportions are specified in the ratio of cement to aggregates for the certain strength achievement. The nominal mix is preferred for simpler, relatively unimportant and small concrete works. In other words, the nominal mix is adopted for ordinary concrete work.

Concrete shall be called ‘Nominal mix concrete’ when it’s adopting by a mix like 1:1.5:3, 1:2:4, 1:3:6 etc. and so without any scientific basis, basis on past empirical studies.

Nominal mix normally includes volumetric batching. Generally, the ratio is determined with weight and for concreting, it is transferred to volumetric proportion, i.e. relating to 1 bag of cement that is 50 kg of cement standard measurement boxes are developed on site that is in the ratio of 1 bag of cement.

Nominal mix concrete is suitable for M20 grade concrete or lower grade like M5, M7.5, M10, M15. Here, M stands for Mix and 20 signifies the compressive strength of concrete cube after 28 days of curing in N/mm2. There are other types of concrete grades like M10, M15, M20, M25, M30 etc.

Ratios as stated by IS for Nominal Mix

The ratios of materials for nominal mix concrete should be compliant with IS 456: 2000.

The ratio of the fine to coarse aggregates should be revised from the upper limit to lower limit gradually since the grading of fine aggregates turns finer, and the maximum size of coarse aggregate turns bigger, graded coarse aggregate should be utilized i.e. for an average grading of fine aggregate (namely Zone II- IS 383- Table 4). The normal ratios are 1:1½, 1:2, 1: 2½ for the maximum size of an aggregate 10mm, 20 mm and 40 mm respectively.

With adherence to the IS 456: 2000, the ratios of the nominal mix are modified when it is required to increase the quantity of water to resolve the issues concerning placement and compaction in order that the water cement ratio is retained as specified.

Benefits:

1. The nominal mix is the prescriptive type concrete; a ratio is settled earlier.
2. It can be simply created at a construction site.
3. Require less time to fix on the ratio as the ratios are already provided with standard code.
4. No skilled persons are required for nominal mix.

Design Mix: Under design mix, the ratios of the materials of concrete are perfectly set with their relevant ratios to attain the required strength of concrete as well as several characteristics of fresh concrete like workability or performance of concrete with particular specifications. Concrete is identified as ‘Design mix concrete’ if the ratio of its materials are settled after trial and error of different option in the laboratory, after conducting the tests from the materials provided for the work.

Guidelines for Mix Design: Mix design is the authentic method for choosing the mix ratios with certain materials containing more or less unique properties. Design mix comprises of weight batching. It allows the concrete to attain required strength. It is also very inexpensive. All the materials are examined properly prior to apply in the design mix.

Properties of Ingredients are verified - Under the design mix method, it is necessary to verify each property of materials and then, the design should be commenced. The following properties of ingredients should be checked :

Cement – Cement grade, consistency, initial setting time and final setting time, specific gravity etc.

Aggregate – Density, bulking of fine aggregate, the specific gravity of fine aggregate and coarse aggregate, grading of aggregate, fineness modulus, particle size, silt content, water absorption, unit weight etc.

Benefits:

• As compared to nominal mix, design mix is more authentic.
• Mix design is suitable for more wide-ranging and vital concrete works.
• Mix design is dependent on the existing actual material to be utilized.
• When locally accessible material can meet the criteria, it should be applied for mix design concrete with the purpose of minimizing the cost of importing material from outside.
• The quantity of the applicable materials should be judicious, i.e. it’s should not be overused nor underused.
• It is made on the basis of the laboratory trial/error experiment method.
• It provides an assurance of strength.
• Mix design concrete is performance-based concrete.
• Design mix concrete is commonly utilized.
• The designer can employ admixtures reasonably to rectify the properties of concrete as per their requirement.
• Slump and strength are co-related that means for strength, several slumps can be utilized by altering water/cement ratio with or without admixtures in order to match concreting of various materials, i.e. for footing it may contain less slump, i.e. less w/c ratio, but for thin materials like chhajja or thin walls, it may require a larger slump, i.e. greater w/c ratio or more quantity of admixture. It is not feasible in the nominal mix.



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Rajib Dey
www.constructioncost.co
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