Construction Cost Estimating

      

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.

Variations among nominal mix and design mix