Concreting works :
The following ingredients are necessary for obtaining good concrete in construction field.
- The Cement grade and brand of cement should confirm IS 456: 2000 standards and is recommended by the respective project consultant.
- Each consignment of cement delivered to the project site must obtain Manufacturers test certificate (MTC) and should consumed within 1 month of their manufacturing date.
Storing and stacking of cement:
- Storing of cement in a godown should always be in dry, leak proof and as moisture proof as possible and contains minimum number of windows.
- Stack the cement bags on wooden planks or raised solid block platform of at least 150 mm above the floor.
- Cement should be stacked not more than 10 bags in height, and all round clearance of at least 1 meter distance from all walls.
- It has to be well protected during rainy season.
CEMENT STACKING : Fig 1 and 2: Right way of staking cement at 10 bags height below yellow line
Maintain proper records and see that “first come cement in and first out rule” while issuing cement bags.
Before issuing for work, ensure the cement is tested in-house for consistency, setting time, and strength.
AGGREGATE FOR CONCRETE :
Coarse Aggregates :
- Coarse aggregates confirming to IS 383 and IS 2386 having maximum size of 20 mm and down should be used for all RCC works.
- The aggregate material obtained should be from authorized crushers such as granite, trap and basalt quarry.
- It should be cubical, rounded or angular in shape and free from dust, foreign matter and not thin, porous, laminated or flaky in shape.
- Fine aggregate meeting IS standards, shall be gritty sand with Fineness Modulus FM ranging between 2.6 to 3.2, silt content should be limited to 5% and there should be no traces of salt and clay.
- Aggregate having specific gravity less than 2.6 is giving lesser compression strength, and hence not to recommended its usage in concrete.
- If fine aggregate having more dirt, wash sand before use. Sand should free from organic matters.
- M Sand : Now a days, there is a scarcity, for availability of good quality of river sand. Many state Governments also imposed ban on sand lifting in river beds, which forces to thinking of an alternative to river sand. As a result, Manufactured Sand (M Sand) is used as substitute for river sand, however, M Sand should confirm IS 383 specifications.
- Water used for concrete always should be potable, clean, fresh and free from oil, acid, Alkali and organic matter.
- IS 456:2000 gives permissible limits of solids present in water.
- Importance of water for concrete and its test parameters will be explained shortly in upcoming article.
- Type of admixture and its dosage required for concrete is depends on trial mix done at project site, using available material, and as per design mix by technical consultant.
- Ensure the Shell life of admixture before actually being used at site.
- Stay tuned for : Different types of Admixtures and its uses in future article
Tools required for Concreting:
- Shovels, Trowels, Floating trowel, Masons trowel, Leveling trowels, Line thread, Brushes and buckets, Gum boots, Vibrators, Dumpy level, Spirit levels, Measuring tape, Hammer, Wheelbarrow, Aluminum straightedge (leveler), High pressure water cleaners.
MIXING OF CONCRETE: IN-SITU MIXING (Mixing at site)
- Before mixing of concrete, check all necessary stock and storage of materials, tools, equipment and workers including supervisors – all available and present in working area.
- Calibration : Calibration should be done once in every 6 months for weigh batcher, tower crane, builder hoist, compression testing machine (CTM) etc. and these dates should be in force, during its usage for concrete and other works. Affix due date stickers on all equipment for better and easy monitoring.
Concrete mixer :
- Start and run the concrete mixer with one fourth of required water to rotate 3 – 4 times.
- Normally Water required for each load or each bag of cement is between 20 to 25 liters. But it is highly advisable to use the water cement ratio as per design mix specification. (between 0.4 to 0.5)
- To controlling moisture content of aggregates is very important and to be checked at frequent intervals, it will vary in rainy and summer seasons.
Loading of concrete into the mixer:
- First, Load the hopper with ½ the coarse aggregate of 20 mm down grade, followed by ½ the fine aggregate of 12 mm down grade.
- Over this, the full quantity of cement should spread.
- Then the final quantity of fine aggregate followed by remaining coarse aggregate to be loaded.
- This type of proper mixing is required to avoid falling of cement while discharging into the drum.
- Finally after loading the drum, the balance quantity of water to be added.
- Mixer drum should be rotated for at least 1 ½ to 2 minutes. The rotation of the drum is to be maintained at 15 to 20 revolutions per minute.
- Note that, if any admixture is used, one liter of water should be kept in reserve.
- Add prescribed dosage of admixture to the water and mix it thoroughly.
- This mixture is poured into the mixer after the drum has rotated for about 90 seconds.
Minimum cement content :
Minimum cement content required per cum of concrete is as per the following below:
|Characteristic strength after 7 days (N/mm2)||Characteristic strength after 28 days (N/mm2)||Minimum cement content per Cum|
|M 15||10||15||280 kgs|
|M 20||13.5||20||320 kgs|
|M 25||17||25||360 kgs|
|M 30||20||30||390 kgs|
Points to be remember for concreting :
- RCC works : As Per IS standards, Concrete below M 20 grade should not be used for RCC works.
- See that, Concrete should be reached, placed in position over a good shuttering with steel reinforcement and compacted properly within half an hour of its time of mixing.
- While loading the mixer, follow the design mix criteria for all materials for its correct weight. This has to be monitor by deploying experienced supervisor in absence of computerized panel, or for manual loading into the mixer for correct proportion.
- Measuring boxes : Always use measuring boxes of correct size for loading.
- Use large pans, wheelbarrows, builder hoist o expedite transporting process.
- Adsorption : Wet the surface of the tools carrier to avoid moisture loss due to adsorption (act as a thin film between tool and concrete)
- Care should be taken in order to avoid segregation and wastage during transportation. If segregation occurs, remix the concrete in mixer, before placing.
- Concrete should always made up of well graded aggregate, fluffy and homogenous in colour, if rounded in hands it should form a shape of ball with good consistency.
STAY TUNED FOR MORE UPDATES!!!
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