Concrete is a construction material, which is a mixture of cement, sand, aggregate and admixtures mixed with water. It hardens with time and gains the strength. Concrete mixing is the process in which all ingredients of concrete are selected with proper proportion. The mix proportions shall be so proportioned to ensure the workability of the fresh concrete, and when concrete is hardened, it shall have the required strength, surface finish and durability. Normally two types of methods are used for proportioning different ingredient, i.e. nominal mix and design mix of concrete. Selection of types of concrete mix depends on the requirement of strength and nature of work.
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 generally contains volumetric batching. Normally the proportion is decided by weight and for concreting, it is converted to volumetric proportion, i.e. with respect to 1 bag of cement, i.e. 50 kg of cement standard measurement boxes are made on site, which is in the proportion of 1 bag of cement. Nominal mix concrete may be used for concrete of M 20 grade or lower grade such as M 5, M 7.5, M 10, M 15. M 20 is identified as the concrete grade in which, M indicates Mix and 20 indicates the compressive strength of concrete cube after 28 days of curing in N/mm2. There are various grades of concrete that can be used like M10, M15, M20, M25, M30 etc.
Also Read: What do you Mean by Batching of Concrete?
Proportions as Per IS for Nominal Mix
The proportions of materials for nominal mix concrete shall be in accordance with IS 456: 2000, which are shown below,
|Grade of concrete||Total quantity of dry aggregate by mass per 50 kg of cement to be taken as the sum of the individual masses of fine and coarse aggregates||Proportion of fine aggregate to coarse aggregate (by mass)||Maximum quantity of water per 50 kg of cement (l)||Proportions|
but it may very from
1:1 to 1:2
The proportion of the fine to coarse aggregates should be adjusted from the upper limit to lower limit progressively as the grading of fine aggregates becomes finer, and the maximum size of coarse aggregate becomes larger, graded coarse aggregate shall be used. i.e. for an Average grading of fine aggregate (that is Zone II- IS 383- Table 4). The proportions are generally 1:1½, 1:2, 1: 2½ for the maximum size of an aggregate 10mm, 20 mm and 40 mm respectively.
As per the IS 456: 2000, in the nominal mix can be proportions changed if the quantity of water is to be increased to overcome the difficulties of placement and compaction so that the water cement ratio can be maintained as specified.
- The nominal mix is the prescriptive type concrete; a proportion are pre-decided
- Easy to make at a construction site
- Does not more time to decide the proportion because proportions are already given by standard code
- No need for skilled persons
- Nominal mix design is based on past experience, have no scientific approach.
- It may or may not give exactly designed strength unless all other factors like compaction, w/c ratio, curing of concrete are strictly followed.
- Water-cement ratio is considered by assumption so, if we do not take care of it, sometimes it leads to bleeding, segregation and may make concrete of poor strength and durability.
- There is no consideration for grading and density of aggregate etc.
- Higher Cement content is used.
- No laboratory tests are conducted.
- This mix does not consider and check specific properties of individual ingredients, i.e. cement- fineness of cement, grade of cement and type of cement, size and grading of aggregate etc.
Design mix is a process in which the proportions of the ingredients of concrete are properly determined with their relative proportions to achieve the concrete of desired strength. Not only the desired strength but according to the properties of fresh concrete like workability or performance of concrete with the certain specifications. Concrete shall be called ‘Design mix concrete’ when the proportion of its ingredients are decided after trial and error of various option in the laboratory, after carrying out tests, from given selected and available material to be used for the work.
Guidelines for Mix Design
There are various standard guidelines available for making a design mix concrete such as, IS method – Concrete mix proportioning guidelines (Bureau of Indian Standards – I.S. 10262- 2009) and Recommended Guidelines for Concrete Mix Design- I.S. 10262- 1982, BS method (BS EN 206- 1 and its complementary standards BS 8500 parts 1& 2- British Standard) and, ACI method (ACI 211, 211- 91, re approved- 2002- American Standard).
Mix design is the most intellectual and rational approach to the selection of mix proportions with specific materials considering with more or less unique characteristics. Design mix contains weight batching. It not only makes concrete of desired strength but also makes it economical too. All the materials are tested before use in the design mix.
Properties of Ingredients are Checked
In the design mix method, we have to check every property of ingredients and after that, we start design. Property of ingredients which we would like to check such as,
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.
- Design mix is more scientific than the nominal mix.
- Mix design is used for more extensive and important concrete works.
- Mix design is based on the actual material to be used which are available.
- If the locally available material can satisfy the criteria, it will be used for mix design concrete, so that it will reduce the cost of importing material from outside.
- The quantity of the ingredient to be used are rational, i.e. its neither overused nor underused.
- It is based on the laboratory trial/error experiment method.
- It gives an assurance of strength.
- Mix design concrete is performance-based concrete.
- Design mix concrete is widely used.
- The designer can use admixtures rationally to modify the properties of concrete according to their requirement.
- Slump and strength can be related, i.e. for strength we can use different slump by changing water/cement ratio with or without or admixtures so as to suit concreting of different elements, i.e. for footing it may have less slump, i.e. less w/c ratio, but for thin elements like chhajja or thin walls, it may need a larger slump, i.e. higher w/c ratio or more quantity of admixture. This is not possible in the nominal mix.
Also Read: What is the Concrete Slump Test?
- It needs more time.
- If the type or quality of ingredient will change during the progress of work, all the proportion will be changed, so be very careful about this or we may have to go for fresh design which may delay the entire project. So that in between the work does not suffer. So, it is better to have mix design with 2/3 possible brands of cement and also aggregate from different sources.
- Need for skill person to prepare the mix design.
Nominal mix and design mix of concrete, both are the important criteria for proportioning different ingredient of concrete. A design mix concrete is preferred over a nominal mix concrete in all important work and which concrete is above M20 grade of concrete and also where there is the requirement of different workability and chances of any deviation. It is used in the large projects’ due assurance of strength, durability and workability.
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