CW Milestones Part-5 – Concrete pouring of foundation pad and columns for tower (Greenfield Site)

Once the proper layout, excavation and lean concreting (CW Milestone Part -4) has been done, it comes now the time for steel fixing. The steel for pad/s and columns is then cut and bent according to bar bending schedule. In the meanwhile the size of foundation is marked over the lean layer. The steel for foundation is then fastened with the help of binding wires and placed over spacers to give specified cover in both horizontal and vertical directions. The main reinforcement for columns is also fastened which is extending from top right into the foundation re-bars. 

Workers tying steel ribs for foundation columns



Checking of reiforcement is to be done by qualified engineers

Formwork is then placed and fastened around the in-place reinforcement mesh. At this stage the complete column reinforcement is not essentially required, only those re-bars would be fastened which are going into the foundation steel cage but it is to be ensured that verticality of these columns re-bars is maintained. As if not placed properly these would be difficult to be dealt, after foundation concreting.
Before placing the foundation concrete following must be checked:
• Temporary Bench Mark position.
• Size of foundation.
• Size and length of re-bars.
• Re-bars spacing.
• Re-bars condition. (No signs of rust bend & crack etc.)
• Steel binding.
• Concrete cover (PCC spacers) in both directions and their positioning.
• Placing of chairs (to maintain required distance between main and temperature steel).
• Proper formwork placing.
• Size and quantity of coarse and fine aggregates.
• Availability of required cement bags.
• Availability of concrete vibrators, wheel barrows, trowels and other tools.
• Positioning of columns.
• Application of shuttering oil.
• Verticality of main column reinforcement.
• Extended lengths of column reinforcement into foundation steel mesh.
• Ambient temperature. (Should be below 30 degree Celsius.)
• Chute fixing if height of fall is greater than 6 feet.
• Availability of water for concreting.
• Availability of cubes for testing.
• Availability of slump checking cone (workability checking).
• Condition and working of mixer machine.
• Clear cover to Reinforcement for footing shall be 75mm & for column shall be 50mm.
• W/C ratio = 0.45

Once all the above checks are ensured, concrete (normally having a mix ratio of 1:1.5:3) is ready to be poured.

Ensure that proper size, as described by designer , is utilized

The materials, as per mix ratio, are mixed in the mixing drum with specified water content for a certain period of time (at least for 75 seconds). The mix is then shifted to the wheel barrows and then to the main concreting area. Concrete would be placed with the help of chute if the height of fall is greater than 6’.

Vibraters must be used throughout pouring to avoid any air bubble entrapment

Concrete is spread all over the concreting area in the same fashion with the use of poker vibrator periodically to compact the laid concrete. During placing the workability is checked with the help of slump cone and the cubes are filled randomly from any load.

Pouring must be started from one corner and gradually places till the far ends

On site slump test is a good measure to check if the consistancy of concrete is maintained

Sampling for cube test

   
Every stand-by arrangement (like alternate power arrangement, extra vibrator, availability of proper quantity of material…etc.) is ensured to be available at site to avoid cold joint in concrete and it is ensured that all the concrete should be carried out in one go without providing joint. It is also to be ensured that if joint is inevitable, it must not be located where the columns are placed and other sensitive areas.
The laid concrete is then cured for specified number of days. Meanwhile the remaining steel for columns is fastened and after few days when concrete is sufficiently set, the formwork of columns is fixed.
The formwork should be strong enough to bear concrete weight during concreting and it must be rigidly fixed that it does not get displaced to give the columns an objectionable final shape.
Following checks should be applied before carrying out the columns concrete:
• Temporary Bench Mark position.
• Four back to back distances
• Two diagonal distances.
• Diameter, length and total number of anchor bolts.
• Verticality of anchor bolts.
• Positioning of anchor bolts.
• Leveling of anchor bolts.
• Height of columns.
• Verticality of columns formwork.
• Column reinforcement as per bar bending schedule.
• Re-bars condition. (No signs of rust, bend & crack etc.)
• Concrete cover (PCC spacers) and their positioning.
• Proper formwork placing.
• Application of shuttering oil.
• Alignment of columns formwork.
• Arrangements to prevent leakage/ seepage during concreting.
• Size and quantity of coarse and fine aggregates.
• Availability of required cement bags.
• Availability of concrete vibrators, wheel barrows, trowels and other tools.
• Ambient temperature. (Should be below 30 degree Celsius.)
• Availability of water for concreting.
• Availability of cubes for testing.
• Availability of slump checking cone.
• Condition and working of mixer machine.
After ensuring all the above stated checks, the materials, as per mix ratio, are mixed in the mixing drum with specified water content for a certain period of time (at least for 75 seconds). The mix is then shifted to the wheel barrows and then to the main concreting area. It must be ensured that vibrator is operated periodically to have a dense laid concrete.

Tower achor bolts to be fixed as per manufacturer specifications

Verticality of Anchor bolts is to be checked

Top level of tower anchors is ensured to be maintained before and during concreting

  
During placing the workability is checked with the help of slump cone and the cubes are filled randomly from any load.
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Every stand-by arrangement is ensured to be available at site to ensure uninterrupted concreting.
After concreting of columns, sufficient time is given to concrete to attain ample strength so as to remove the formwork ensuring no damage is done to columns.

Seprate cube test is required for columns concreting

Formwork removal

Farmwork removal 2

  
After formwork removal it is to be checked that honey combing and other defects are not occurred otherwise proper rectification is to done.
The columns are then cured for specified number of days to give such a conducive environment to concrete to move towards the required strength.
Once every step is followed with an intention to be fair it is only then we enjoy a strong foundation for the steel structure above. And this fairness in work can be clearly seen by watching the structure after farm work removal.