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The properties of concrete such as durability, strength, workability, and setting time are greatly influenced by way of handling, transporting, laying, compacting, and curing of concrete. Therefore, it is very vital to follow the specifications laid down by the Engineer-in-charge in the process of concreting.
The steps involved in the process of concreting are:
- Selecting quantities of materials for selected mix proportion
- Checking workability
- Pouring in formwork for casting
- Vibrating for proper compaction
- Removal of formwork after a suitable time
- Curing members with suitable methods and required time
In this article, we discuss how to place, compact, finish, and cure concrete properly.
1. Placing of Concrete
- The concreting shall be commenced only after the inspection of centering, shuttering, and reinforcement is completed.
- The shuttering shall be clean and free from shavings, sawdust, pieces of wood, or other foreign material.
- In case of concreting of slabs and beams, wooden plank or catwalks of chequered MS plates supported directly on the centering with the use of wooden blocks or lugs should be provided to convey the concrete to the desired location without disturbing the reinforcement.
- The workers shall not be allowed to walk over the reinforcement.
- In the case of columns and walls, it is desirable to place concrete without construction joints.
- The progress of concreting in the vertical direction shall be restricted to one meter per hour.
- The concrete shall be deposited in its final position in a manner to preclude segregation of ingredients.
- In deep trenches and footings, concrete shall be placed through chutes.
- In the case of columns and walls, the shuttering shall be so adjusted that the vertical drop of concrete is not more than 1.5 meters at a time.
- During cold weather, concreting shall not be done when the temperature falls below 4.5°C.
- The concrete shall be protected against frost action by suitable methods. The concrete, if damaged by frost, shall be removed and placed again.
- During hot conditions, precaution shall be taken to see that the temperature of wet concrete does not exceed 38°C.
- The time taken between mixing and placing of concrete shall not exceed 30 minutes of the initial setting process, else retarders must be used in the concrete to keep it green for the desired period.
2. Compaction of Concrete
- The concrete shall be compacted into dense mass immediately after placing it by means of mechanical vibrators designed for continuous operations.
- Hand compaction shall be carried out with the help of tamping rods to thoroughly compact the concrete around the reinforcement, embedded fixtures, and into corners of the form.
- The concrete layers shall be placed so that the bottom layer is not completely set before the top layer of the concrete is laid.
- The vibration shall keep the concrete in an agitated state until the concrete is entirely de-aerated and compacted.
- The concrete shall be compacted adequately until the mortar fills the spaces between the coarse aggregates and starts to cream up to form an even and finished surface.
- When this condition is achieved, the vibrator shall be stopped and removed from the concrete.
- The needle vibrators shall be withdrawn so as to prevent the action of loose pockets in case of internal vibrators.
- In case both external and internal vibrators are being employed for compaction, the internal vibrator shall be withdrawn first, after which the external vibrators shall be removed so that no loose pocket or air gaps are left in the concrete.
- Shaking of reinforcement for the purpose of compaction should be avoided.
- The whole process of compaction shall be completed before the initial setting begins, i.e., within 30 minutes of mixing of water into the dry mix of concrete.
3. Curing of Concrete
- Once the concrete has begun to harden, i.e., about 1 to 2 hours after it is laid, it shall be protected from quick-drying by covering with moist gunny bags, sand, canvass Hessian, or any other material.
- The surface of concrete shall be cured after 24 hours of laying of concrete by ponding method for a minimum period of seven days from the date of placing of concrete.
4. Rectification of Defects after Concreting
In the case of roof slabs, the top surface shall be finished even and smooth with a wooden trowel before the concrete begins to set. Immediately after the removal of forms, the R.C.C work shall be examined before any defects are made good.
- The concrete that has sagged or contains honeycombing to an extent that is detrimental to structural safety or architectural concept shall be rejected after a visual inspection test.
- Surface defects of a minor nature may be accepted. On acceptance of such a work, the same shall be rectified as follows -
1) Surface defects which require repair when forms are removed, usually consist of the following-
- Bulges formed due to movement of forms, ridges at form joints.
- Honeycombed areas
- Damage resulting from the stripping of forms and bolt holes.
The bulges and ridges are rectified by careful chipping or tooling, and the surface is then rubbed with a grinding stone. Honey-combed and other defective areas must be chipped out, the edges being cut as straight as possible and perpendicularly to the surface, or preferable slightly undercut to provide a key at the edge of the path.
2) Shallow patches are first treated with a coat of thin grout composed of one part of cement and one part of fine sand and then filled with mortar similar to that used in the concrete. The mortar is placed in layers not more than 10 mm thick, and each layer is given a scratch finish to secure bonds with the succeeding layer.
The last layer is finished to match the surrounding concrete by floating, rubbing, or tooling on formed surfaces by pressing the form material against the patch while the mortar is still plastic.
3) The large and deep patches are reinforced and carefully dowelled to the hardened concrete and filled with concrete held in place by forms.
4) The holes left by bolts are filled with mortar carefully packed into places in small amounts. The mortar is mixed as dry as possible, with just enough water so that it will be tightly compacted when forced into place.
5) The tiered holes extending deep into the concrete may be filled with mortar with a pressure gun similar to the gun used for greasing motor cars.
6) Normally, patches appear darker than the surrounding concrete, possibly owing to the presence on their surface of less cement laitance. Where uniform surface color is important, this defect shall be remedied by adding 10 to 20 percent of white Portland cement to the patching mortar, the exact quantity being determined by trial.
7) The patches shall be cured with the same care as the whole structure. The curing shall be started as soon as possible after the patch is completed to prevent early drying.
FAQs on Concrete Placing, Compacting and Curing
Concrete is a construction material composed of cement, fine aggregates (sand) and coarse aggregates mixed with water which hardens with time.
The steps involved in the concrete construction are-
1. Selecting quantities of materials for selected mix proportion
3. Checking of workability
5. Pouring in formwork for casting
6. Vibrating for proper compaction
7. Removal of formwork after a suitable time
8. Curing members with suitable methods and required time.
During cold weather, concreting shall not be done when the temperature falls below 4.5°c.
During hot conditions, precaution shall be taken to see that the temperature of wet concrete does not exceed 38°C.
The surface of the concrete shall be cured after 24 hours of laying of concrete by ponding method for a minimum period of seven days from the date of placing of concrete.