Batching Mixing Transporting Placing Compacting Curing Finishing.
The measurement of materials for making concrete is known as batching. There are two methods of batching: a. Volume batching b. Weight batching. Volume batching: Volume batching is not a good method for proportioning the material because of the difficulty it offers to measure granular material in terms of volume. The volume of moist sand in a loose condition weighs much less than the same volume of dry compacted sand. Cement is always measured by weight. It is never measured in volume.
Weight batching:- Weight batching is the correct method of measuring the materials. For important concrete, invariably, weigh batching system should be adopted. The use of a weight system in batching facilitates accuracy, flexibility, and simplicity.
Read More————————————- Test on Cement
Thorough mixing of the materials is essential for the production of uniform concrete. The mixing should ensure that the mass becomes homogeneous, uniform in color, and consistent. There are two methods adopted for mixing concrete.
Hand Mixing: Hand mixing is practiced for small scale unimportant concrete works. As the mixing cannot be thorough and efficient, 10 percent more cement may be added in comparison to machine mixing to cater to the inferior concrete produced by hand mixing.
Machine Mixing: Mixing of concrete is almost invariably carried out by machine, for reinforced concrete work, and for medium or large scale mass concrete work, machine mixing is not only efficient but also economical when the quantity of concrete to be produced is large. The mixing time for concrete drums is generally 2 minutes.
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Transporting of concrete
Concrete can be transported by a variety of methods and equipment. The precaution to be taken while transporting concrete is that the homogeneity obtained at the time of mixing should be maintained while being transported to the final place of deposition. The methods adopted for the transportation of concrete are
a) Mortar Pan
b) Crane, Bucket, and Ropeway
c) Belt Conveyors
d) Skip and Hoist
e) Pump and Pipeline
f) Wheel Barrow, Hand Cart
g) Truck mixer and Dumpers
i) Transit Mixer
Placing Concrete: It is not enough that a concrete mix correctly designed, batched, mixed, and transported; it is of utmost importance that the concrete must be placed in a systematic manner before its setting time. The maximum free fall of concrete is 1.5meter.
Watch Video ———————— Types of Mortar
Formwork shall be designed and constructed so as to remain sufficiently rigid during the placing and compaction of concrete. The joints are plugged to prevent the loss of slurry from concrete. Stripping Time: It is the time up to which formwork should not be removed until the concrete has developed strength of at least twice the stress (due to loading) to which concrete may be subjected at the time of removal of formwork.
Specification as per IS code 456 (11.3.1)
|Type of Form Work||Stripping Time|
|Vertical formwork to columns, walls, beams||16-24 hour|
|Soffit formwork to slabs (Props to be refixed|
immediately after removal of formwork)
|Soffit formwork to beams (Props to be refixed|
immediately after removal of formwork)
|Props to slabs:|
|Spanning up to 4.5 m||7 days|
|Spanning over 4.5 m||14 days|
|Props to beams and arches|
|Spanning up to 6 m||14 days|
|Spanning over 6 m||21 days|
Read More————————————- Formwork
Compaction of Concrete
Compaction of concrete is the process adopted for expelling the entrapped air from the concrete. In the process of mixing, transporting, and placing concrete air is likely to get entrapped in the concrete. The lower the workability, the higher is the amount of air entrapped.
The following methods are adopted for compacting the concrete:
a. Hand Compaction
b. Compaction by Vibration
i. Internal vibrator (Needle vibrator)
ii. Formwork vibrator (External vibrator)
iii. Table vibrator
iv. Platform vibrator
v. Surface vibrator (Screed vibrator)
vi. Vibratory Roller.
c. Compaction by Pressure and Jolting
d. Compaction by Spinning.
Curing of Concrete
Concrete derives its strength from the hydration of cement particles. The hydration of cement is not a momentary action but a process continuing for a long time. Of course, the rate of hydration is fast to start with but continues over a very long time at a decreasing rate. cement requires a water/cement ratio of about 0.25 for hydration and a water/cement ratio of 0.15 for filling the voids in the gel pores. In other words, a water/cement ratio of about 0.38 would be required to hydrate all the particles of cement and also to occupy the space in the gel pores. Theoretically, for concrete made and contained in a sealed container a water-cement ratio of 0.38 would satisfy the requirement of water for hydration and at the same time, no capillary cavities would be left. However; it is seen that practically a water/cement ratio of 0.5 will be required for complete hydration in a sealed container for keeping up the desired relative humidity level. Curing can also be described as keeping the concrete moist and warm enough so that the hydration of cement can continue. More elaborately, it can be described as the process of maintaining satisfactory moisture content and a favorable temperature in concrete during the period immediately following placement, so that hydration of cement may continue until the desired properties are developed to a sufficient degree to meet the requirement of service.
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