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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Why water cement ratio is vital in concrete mix?

In concrete there are different types of materials like cement, sand, Coarse aggregate and water which are combined in exact ratios. This ratio is provided at either as a nominal mix or by design mix.

All the materials contain definite properties and the strength of concrete is mostly dependent on these properties.

Cement is employed as a binding material in concrete because of the best adhesive and cohesive property whereas water is the cheapest and most vital of concrete. Once these two materials are blended, a chemical reaction alias hydration is produced with a sticky and thick gel that contains a binding property to tie all aggregate in form of hard and strong material. So, cement and water both can be used for binding all the materials of concrete jointly.

When cement and water are applied according to their ratio, it is known as water-cement ratio. Water-Cement Ratio means the ratio of the weight of water to the weight of cement.

Water/cement ratio with adherence to IS 10262 (2009) for the concrete mix differs from 0.4 to 0.6. 0.7 w/c ratio is also applicable for pumped concrete. 0.5 w/c ratio signifies that for each 100 kg of cement, 50 lit. of water is added. When w/c ratio is raised, the amount of water is reduced.

In technical terms, the cement should contain 23% of water for the hydration process and 15% for development of pores and volume increment. So, minimum 38% of the water by weight of cement should be maintained.

There should be adequate amount of water in the concrete mix to maintain full hydration of cement paste and eliminate capillary pores there. The minimum quantity of this water should be similar to 0.38 w/c ratio. Accordingly, if water-cement ratio is under 0.38, complete hydration can’t be performed. If the ratio is surpassed, this concrete mass turns out to be porous and penetrable and if it is less, concrete turns out to be dry and harsh and may not bond and attain full strength.

For work-ability purpose, this water-cement ratio should remain as 0.4, but its selection is based on the requirement of work-ability in various exposure conditions and nature of work. At a water-cement ratio of 0.4, the mix contains adequate water for hydration and simplifies the concreting process for mixing, placing, transporting and compacting since water functions as a lubricant in the concrete mix and decreases the internal friction among particles of concrete.

At this water-cement ratio, the ‘gel’ that is produced remains in good physical structure and solid state. At greater water/cement ratio the ‘gel’ that is produced remains in the poor physical structure. It produces voids throughout subsequent evaporation and develops into more porous. As a result, the strength gets lower in due course of time.

Computation of water quantity for concrete

While measuring the quantity of water for concrete mix, initially determine the cement content for the volume and w/c ratio. Suppose, there is 80 kg of cement for concrete mix and w/c ratio is 0.45.

Required amount of water = Cement volume X w/c ratio
So, essential water quantity is 80 kg X 0.45= 36 lit.

To get further information, go through the following link gharpedia.com




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www.constructioncost.co
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How to check quality of concrete in jobs site

The following precautions should be taken during preparation of good quality concrete:

To build up a quality, durable and strong structure, the concrete that is used should contain standard quality. A good quality concrete should be stable, condensed, workable and cost-effective.

The following properties should be maintained to produce a good quality concrete :-

1. The quality of aggregate should have been rigid and long-lasting.

2. The grading of the aggregates should be done perfectly and consistently.

3. Adequate quantities of cement should included in concrete to maintain necessary strength and water-resistance capability.

4. The water does not contain any organic or deleterious materials.

5. The quantity of water should be maintained in such a manner so that the uniformity is retained properly.

6. The proportion of water-cement should keep as minimum as possible.

7. To keep up conformity, mixing of concrete should be performed systematically.

8. To eliminate air bubbles and voids, the freshly arranged concrete should be compacted properly.

9. While transmitting the concrete to the job site, no segregation of or separation of materials should happen in concrete.

10. Concrete that is provided at the point of placing should have been homogeneous & contain right symmetry.

11. Ensure that the concrete is not thrown from a distance of high point to prevent segregation.

12. Curing of concrete should be done perfectly for minimum 28 days.

13. The temperature of concrete should be retained over the freezing point unless it gets rigid enough.

14. It should be stored in even horizontal layer having invariable thickness. Concrete should fill each section in this form.

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Detail lists of useful construction tools

Given below, the detailed lists of various useful construction tools and their applications :-

Hoe – It is effective for digging and arranging concrete, cement mortar in head pan. Hoe is also useful for excavating the soil but here the metal plate is set with acute angle to the wooden handle.

Head Pan – It is utilized to transmit materials. Head pan is built with iron to uprise the excavated soil or cement or concrete to the construction site etc. it is mostly found in construction sites.

Masonry trowel – The objective of this tool is to organize cement mortar. It refers to a hand trowel that is applied in brickwork or stonework for the purpose of leveling, dispersion and shaping mortar or concrete.

Measurement Tape – It is employed to examine the thickness, length, widths of masonry walls, foundation beds, excavated trenches etc.

Plumb Bob - It is applicable for verifying the vertical alignment of the structures. It comprises of a solid metal bob attached with the end of a thread. It can also be applied in surveying to level the instrument position.

Wheel Barrow – It is useful for carrying out cement mortar or any materials. Often, it is utilized to estimate the quantities of materials for site level concrete mixing.

Concrete Mixer – It is a machine that is used for blending the concrete perfectly with water, fine aggregate, coarse aggregate and cement at construction site.

Vibrator - It is utilized to vibrate the concrete when pouring is started. For the purpose of workability, water is added to concrete. To get rid of that, vibrators are applied.

Bump Cutter/Screed – The objective of this tool is to level fresh concrete surface particularly in slab concrete.

Wooden Float/wooden rendering float - This tool is useful for providing a flat finish to the plastered area.

Crow Bar - This tool is found in formwork to eliminate nails from boards. Crow bar is also applied for digging the ground and taking out the roots of trees in the ground, nails etc.

Framing Square - This tool is mostly found in Brickwork, Plastering to verify exact angle.

Line Level - This tool is required to verify horizontal level in brickwork, plastering , flooring and tile works.

Flat Pry Bar - This tool is found in shuttering and often utilized to modify the column formwork to align.

Digging bar - This tool is used to divide and unloose the compacted / hard surface area. Digging bar refers to a solid metal rod having pin shape at the bottom. It is also utilized to dig the hard surfaces of ground.

For more information, go through the following link www.civilology.com



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www.constructioncost.co
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