How concrete is mixed in different ways

Concrete belongs to a composite material formed by cement, sand, coarse aggregate, water and chemical admixtures (if necessary). To maintain the superior quality of concrete, the materials of the concrete should be blended properly so that the quality of concrete is not impacted.

A well-mixed concrete is formed on the basis of the following conditions:

• The color of the concrete should be consistent.
• Admixture of all concrete materials like cement, fine aggregate, coarse aggregate and water should be uniform.
• Cement paste should wrap all the surface of the aggregate.
• Segregation should not happen as soon as the mixing of concrete is completed.

Mixing concrete is dependent on the following three options:

• Hand mixing (Mixing concrete devoid of a mixer)
• Machine mixing (Mixing concrete with a mixer)
• Ready mix concrete

Hand mixing (Mixing Concrete without a Mixer): Hand mixing stands for the method of mixing the different materials of concrete by hand. Mixing concrete devoid of a mixer is suitable for small works. Mixing of materials is executed on masonry platform or flat iron sheet plates.

The hand mixing concrete is performed as follow:

• Expand the measured quantity of sand on the platform, and then unload the cement on the sand.
• The sand and cement should be blended thoroughly with the help of shovels in the dry state.
• The measured amount of coarse aggregate should be extended, and the mixture of sand & cement should spread on it and mixed in an exact manner.
• Depression is provided at the centre of the mixed materials.

• Include 75% of the required quantity of water in the depression and blend well by the shovels.
• Include the leftover amount of water and the mixing method should be carried on unless a uniform colour and consistency of concrete is procured. Time of mixing concrete should not be in excess of 3 minutes.
• The mixing platform should be washed at the end of the day’s work.

Machine Mixing (Mixing Concrete with a Mixer): Machine mixing is mostly suitable for bigger projects where huge masses of concrete are necessary. The machine mixing can retain the persistent uniformity of concrete. Besides, the machine mixing can significantly reduce the mixing time. In recent times, different types of concrete mixers are available which run with petrol/diesel or electricity.

The machine mixing is performed in the following ways:

• Initially, the concrete mixer should be drenched inside of the drum.
• After that Cement, sand and coarse aggregate should be arranged in the portable concrete mixer in desired ratio.
• The dry materials should be blended in the mixing machine. After this, exact quantity of water should be added slowly when the machine is running.
• The concrete should be blended for minimum two minutes after placing all materials in the drum.
• If segregation occurs, the concrete should be remixed after unloading from the mixer.

Ready Mix Concrete: Ready Mixed Concrete (RMC) is developed in the factory or in a batching plant and supplied in a ready-to-use manner. The quality of the consequential concrete is superior as compared to the site-mixed concrete.

Less time is necessary for ready mix concrete as compared to site mixing (hand or machine mixing) and quality of concrete is also greater than the site mixing.

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Published By

Rajib Dey

www.constructioncost.co

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Some vital properties of concrete for mix design

While designing the reinforced concrete structure, the designer should concentrate on the following properties of concrete in its harden state.

COMPRESSIVE STRENGTH: At the time of making the design of the buildings and other structures, the compressive strength of concrete is applied as the most common performance measurement.

Compressive strength of concrete is affected by several factors like water-cement ratio, cement strength, quality of concrete material, quality control throughout production of concrete etc.

Concrete compressive strength for general construction fluctuates from 15 MPa (2200 psi) to 30 MPa (4400 psi) and elevated in commercial and industrial structures.

Compressive strength is a vital factor to evaluate the performance of the material throughout service conditions. The compressive strength of concrete is decided in batching plant laboratories for each batch to retain the optimal quality of concrete throughout casting. The strength of concrete is essential to work out the strength of the members. Concrete samples are casted and tested under the action of compressive loads to find out the strength of concrete.

TENSILE STRENGTH: Tensile strength is an important property of concrete since concrete structures prone to tensile cracking because of different types of effects and applied loading itself. Tensile strength of concrete is very low with regard to its compressive strength.

Because of the complexity in employing uniaxial tension to a concrete specimen, the tensile strength of the concrete is obtained by indirect test methods like split cylinder test and flexure test.

Tensile strength of concrete = 1/10 times of compressive strength
With adherence to IS: 456, the tensile strength of concrete is measured from the compressive strength with empirical relation provided by:
Flexural strength: fcr=0.7√fck N/mm2

MODULUS ELASTICITY: The modulus of elasticity of concrete belongs to a function of the modulus of elasticity of the aggregates and the cement matrix and their relative proportions. The elastic modulus of the hardened paste may come in the order of 10-30 GPa and aggregates about 45 to 85 GPa.

The strength of concrete is mainly based on the relative proportion and modulus of elasticity of the aggregate.

To determine the exact value of elastic modulus of a concrete batch, laboratory test should be carried out. There also exist some empirical formulas which are arranged with different code to get the elastic modulus of Concrete. These formulas are derived on the basis of the relationship among modulus of elasticity and concrete compressive strength. One can easily get an rough value of modulus of elasticity of concrete by applying 28 days concrete strength (f'c) with these formulas.

It can be measured with following formula for normal density concrete:
E = 57,000 (fc’)^0.5
The result will be produced in psi.
fc’ stands for the 28 days cylinder crushing strength in psi.

Modulus of elasticity of concrete is affected by the various factors like type of the aggregates used, type of cement and Mix proportions.

SHRINKAGE OF CONCRETE: It stands for a physical property of concrete. The volumetric variations of concrete structures occur because of the loss of moisture by evaporation is defined as concrete shrinkage or shrinkage of concrete. It is a time-dependent deformation that decreases the volume of concrete devoid of the influence of external forces.

This shrinkage leads to surge in tensile stress, that results in cracking, internal warping, and external deflection, before the concrete has to undergo any type of loading.

Water content in concrete considerably impacts the shrinkage. The IS: 456-2000 suggests the total shrinkage strain as 0.0003 when there is no test data. Drying shrinkage in plain concrete leads to surface cracks. The deflections of reinforced concrete members is also influenced by the shrinkage of concrete.

CREEP OF CONCRETE: Creep is one of the fundamental property of concrete. It is very crucial for designing of concrete structure because in concrete the microstrains of creep fluctuates from 400 to 1000 x 10 -6

Creep is described as the elastic and long-standing deformation of concrete under a continuous load. Normally, a long term pressure alters the shape of concrete structure and the deformation is found along the direction of the applied load.

Creep stands for the time dependent deformations of concrete under permanent loads (self weight), PT forces and permanent displacement.

The creep is affected by the several factors like creep concrete mix proportion, aggregate properties, age at loading, curing conditions, cement properties, temperature, stress level.

COEFFICIENT OF THERMAL EXPANSION: The coefficient of thermal expansion of concrete is impacted by the type of aggregate applied in concrete and it is necessary for making the design of structures like chimneys, water tanks, silos etc. The values provided in IS:456-2000 are as follow :-

Type of Aggregate - Coefficient of Thermal Expansion for Concrete

Quartzite - 1.2 to 1.3 x 10-5
Sandstone - 0.9 to 1.2 x 10-5
Granite - 0.7 to 0.95 x 10-5
Basalt - 0.8 to 0.95 x 10-5
Lime stone - 0.6 to 0.9 x 10-5

For more: http://constructioncost.co/some-vital-properties-of-concrete-for-mix-design.html

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Published By

Rajib Dey

www.constructioncost.co

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How batching of concrete is done

Concrete is a multipurpose, long-lasting, eco-friendly, and inexpensive material as well as globally recognized construction material. To produce superior quality concrete, the components of concrete should be calculated properly and perfectly. Once mix design is set, the initial task should be batching materials.

Batching of concrete stands for the method of estimating and mixing necessary concrete ingredients both with weight or volume according to the mix design and implanting them into a mixture to form a consistent quality of concrete.

By perfectly computing all the material applied in the concrete formation, it becomes possible to maintain consistency of proportions in all consequent batches.

Why Batching of Concrete is Important?

Usually, following three types of batching are undertaken for concrete components:

01. Random Volumetric Batching: Batching devoid of any control on the size and shapes of applied containers leads to big errors and deviations like utilizing “ghamellas” i.e. – by tallying numbers of ghamellas. This method is totally unscientific, unrefined and should not be recommended even for a small project.

02. Volume Batching of Concrete: Volume batching of concrete is accomplished with the use of measurement boxes, locally named “farmas or gauge boxes”. Concrete components like aggregates ( Kapachi +Sand) and cement is calculated with farmas or gauge boxes and proper precaution should be maintained to ensure that the farmas or gauge boxes are filled excessively. To calculate the quantity of water, the water meter should be utilized when batching or use cans of water contain fixed volume.

03. Weigh Batching of Concrete: Weigh batching of concrete is accomplished with the use of a weigh batch or the weighing system on the batching plant. On big projects, automatic batching plants are set up to facilitate optimizing quality and uniformity.

Water is very vital material and therefore, maximum water should be used for batching. If the quantity of water is less, the functionality of concrete will be hampered whereas surplus water will minimize the strength of concrete. Water should be computed perfectly in litres.

The selection of batching method is based on the size of job, production rate and standards of batching performance.

Usually batching is performed with volume. Precision in batching is very crucial. It is better to perform weigh batching instead of volume batching. Volume batching is mostly effective where weigh batching is not possible. It is always recommended to incur additional cost for it, because in long term, it saves huge costs.

Note: The quantity of cement should also be calculated by maintaining the precision of ± 2 percent and the quantity of aggregates, admixtures and water should be calculated with a precision of ± 3 percent.

Article Source: gharpedia.com

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Published By

Rajib Dey

www.constructioncost.co

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