Voided Biaxial Concrete Slab System Method Statement
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Voided Biaxial Concrete Slab System Method Statement

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Voided Biaxial Concrete Slab System Method Statement

This method statement for one of the most voided biaxial concrete slab systems used which is Cobiax light concrete slab.


- PURPOSE and SCOPE.

The scope and purpose of this method statement is to provide as guideline to ensure the fundamental checks and good working practices during reinforced concrete work for Cobiax system on site with respect to approved drawings and material submittal.

Cobiax voided slab system is like solid slab, it consists of bottom and top steel mesh with Cobiax modules in between. Forming voids of spherical shape inside which slab will reduce about 35% of the slab’s self-weight, while keeping almost the same static behavior structural elements including columns, shear walls and footings.

- PROCEDURES.

Prior to the commencement of this activity or work, necessary work permits shall be obtained as required in the contract condition.

All HSE activities related to the Project, will be based on the Company issued HSE Management System guidelines.

All quality assurance and quality control activities will be in line with the specified requirements of the contract.


- PRECONSTRUCTION ACTIVITY.

The QA/QC Representative will ensure that the briefing for this method statement to the personnel was conducted.

A toolbox talk will be initiated by the HSE Team to the personnel who will perform this activity.

Review the relevant documents to verify whether the preceded activity was completed and approved by the consultant.

Allocate a dedicated Site Engineer to supervise this activity that will ensure that this Method Statement, ITP, and other specific documents related to this activity.

Delivered materials should be verified by the QA/QC Representative and must be inspected by the Engineer.

To shift the material to the relevant slabs, the Cobiax Modules should be arranged in bundles, and each bundle will be gathered by U shape steel or wide rope.



Store materials in dry area out of direct sun light and the steel pieces shall be stacked on wooden pieces.


- EXECUTION.

- Scaffolding & Form Work.

The site engineer shall follow the approved scaffolding layout for slab and beams.

All level for the scaffolding will be checked by the surveyor.

All Plywood to be used will be in good condition.

The Surveyor shall mark the locations of slab edge, beams, and the opening as per approved concrete layout.

Ensure enough numbers for the tie rods to be provided for the beams.

Ensure all bracing for the scaffolding before pouring the slab.

Ensure that scaffolding & form work done as per recommended by form work manufacture (GHI or other approved system).

- Preparation& Installation of Cobiax Slab.

Prior to works commencing, the Project Engineer shall ensure that the following activities have been completed and signed-off by the Consultant:

The approved method statement is understood by all relevant personnel.

The concrete design mix has been approved.

The trial mix report has been approved.

Engineer must count number of void formers around the solid area around the solid area and between it.

The engineer at site shall mark the solid area using spray on the formwork to control placement of Cobiax.

The first layer of the steel shall be installed at the short direction of the building and this direction must be the direction of the Cobiax steel cage.



After installing the second layer of the bottom steel mesh the labours must install the electrical fixtures (sockets) and vertical mechanical pipes without the horizonal electrical tubes and pipes.

A straight line with the right angle should be built beside known right angle wall at the short direction of the slab using rod, which must be perpendicular to the vertical structural member axis and at the middle of the Cobiax steel cage.

The first Cobiax steel cage line shall be installed at the middle of the slab by installing a right angle rod with the axis of the vertical structural member and the location of the first line which is at the middle of the Cobiax steel cage must be checked from both sides using the dimension on the layout. The process shall be used to increase the productivity of the distribution of Cobiax since the labor can install cobiax on both sides of the rod, and it is better where the edge of the building is not perpendicular to each other.

In addition, the straightness of the steel cage installation can be monitored by the lines of the formworks if they are parallel to the steel cage.

Start with adjacent steel cage line after finishing the first one to ensure the straightness of the steel cages.

Fixing the steel cage of Cobiax with the bottom second layer of the steel mesh on the slab shall be after the first void former from the beginning of the steel cage of the Cobiax, after the first void former from the end of the steel cage and at the middle of the steel cage from the other side.




During installation, wood, or aluminum shall be used to ensure equal distance between the adjacent steel cages so that the identical distance shall be achieved between the void formers.

In addition, the steel cage shall be fixed with the next front one to keep 5 cm space between each other.

Insulation of the electrical pipes shall be through 5cm gap between the void formers and shall be completed at site before placement of the top steel mesh.

In case it is required to connect pipes with socket at the location of the void former then the labor shall remove one void former from the region of 5cm and the steel cage shall be fixed again.

During the insulation of the steel mesh and electromechanical works, labor must walk on the squares (5cmX5cm) between the void former, and it is preferred to place wooden boards for easy walking and to avoid the void formers from any sort of damage.

If there are congested services coming at the same area, then it is recommended to remove the void former from that location.

Bends for the electrical conduits as a change in direction shall be in solid area around the vertical structural members



Installation of the top steel reinforcement shall be reverse of the Cobiax Steel cage direction.

The Project Engineer shall ensure that the necessary Statutory Authorities approvals have been received.

Ensure that the existing concrete surface at junction points to the new column locations is roughened to form a key.

Ensure that all existing reinforcement bars to be used as starter bars are cleaned.

The Surveyor will mark two perpendicular offsets (3 survey points) at fixed distances from grid lines for each column/ wall. Ensure that the survey points and coordinates layout is approved by the Consultant.

Nail 18 mm thick plywood strips to the concrete footing/slab at the exact shutter location (as per the approved shop drawings). 

Safe Scaffolding will be built at columns/ walls locations to allow labors to fix the steel, the scaffolding will be removed after completing the steel erection. 


- Reinforcement.

The site engineer shall check that the vertical reinforcement bars along with the horizontal /stirrups have been fixed correctly as per the approved shop drawings.

The nominal cover over all reinforcement slab shall be as per approved shop drawings and project specification (20 mm for slab and beams).

Ensure the required concrete cover to the reinforcement bars is provided using approved cover block material.

Mechanical splices shall be fixed as per approved shop drawings.

Welding not allowed without The Engineer approval.

Each intersection of bars shall be tied securely to prevent displacement during concrete placing and vibrating. The ends of ties shall be turned towards the interior of the concrete.

Splicing of bars, except where shown on the Drawings will be submitted for The Engineer approval.

Ensure that all embedded items (plates, starter bars, couplers, water bars etc.) are fixed properly to the reinforcement (where required).

Check that all additional reinforcement specified around penetrations, openings or sleeves are correct (as per the approved shop drawings).

All electrical conduits and MEP services will be fixed as per the approved MEP drawings.

Ensure that all reinforcement is correctly tied using wire in order to maintain their location and stability during formwork fixing and concrete casting.

One stirrup will be fixed at the top of the columns to maintain the distance between bars and cover distance.

Clean the column area by compressed air before closing the shutter.

MEP clearance shall be received before closing the shutter.

The site Engineer shall check that the reinforcement is as per the approved shop drawings.

The Site Engineer shall ensure that the cut and bend reinforcement is as per the approved shop drawings.

All reinforcement shall be free of rust.

A check shall be made to ensure the additional reinforcement layers are as per the

approved shop drawings and appropriate details.

The site engineer will be ensuring all stirrups for columns necks are fixed properly. 

Ensure that all reinforcement is correctly tied using wire to maintain their location and stability during concrete casting.

The Site Engineer shall check the levels, fixings for the chairs to ensure that the slab top level and safe working loads for top layers is not exceeded.

Ensure the required concrete cover to the reinforcement bars is provided using approved cover block material.

One stirrup will be fixed at the top of the starter bars for vertical members to maintain the distance between bars and cover distance for the next floor.

A comprehensive check shall be made by the Site Engineer to ensure that the reinforcement works for the slab are as per the approved shop drawings (number of bars, spacing, lap length, etc.).

Overlap length will not less than 50 times of smaller dia.

Cleaning of the area shall be done regularly by air compressor and hand tools during all working stages.




Formwork.

Formwork shall be constructed to achieve the quality of concrete surface specified and readily removed without damage or shock to the concrete edges.

Approved formwork system shall be used for columns and walls.

Ensure face of formwork is free from defects.

Ensure that the formwork is free from old concrete and cement slurry.

Replace any damaged plywood.

The site engineer will ensure that the formwork is assembled correctly (as per the approved shop drawings).

Only approved shutter release agents will be applied to the internal side of the formwork.

Formwork will be shifted by crane to the column/ wall location.

The site engineer will ensure the shutter is fixed in its correct position (as per the approved shop drawings).

All dimensions, bracing rods, tie rod locations etc. will be checked as per the approved shop drawings).

All formwork shall be signed-off prior to concrete pouring.

Formwork shall be monitored to indicate movement of the formwork during concrete.

Water bars shall be provided for water tanks walls and around temporary openings (if required) asper details shown on shop drawing. 

All the above-mentioned procedure will be follow/repeat for the concrete protection for the columns inside the water tanks.

- Concrete Pouring.

The quality control engineer will ensure all design mixes and trial mix reports are approved prior to commencement of concrete works.

Before casting concrete, it should be ensured that all void formers area closed and if there are any opened void formers it should be closed using foam or adhesive.

The properties of the concrete for the Cobiax slab shall be the same as of any normal slab as per the approved design mix by the Engineer.

However, for small sizes S-100, S-120, S-140 the max aggregate size of concrete shall not be more than 16 mm.

Max acceptable fresh concrete temp is 30 deg. for thin elements ~ elements lesser than 500 mm thick. & 25 deg. for elements equal or thicker than 500 mm.  and ambient temperature is not above 40 ⁰C.

Check that the ambient temperature does not exceed 40oC during concrete pouring.

Ensure that all concrete pouring is via a mobile pump, placing boom and bucket or other method as approved by Engineer.

Ensure that the concrete trucks are fully checked to ensure approved design mix.

Ensure no concrete more than 2 hours after stated batching time will be use.

Concrete will be poured in layers to avoid excessive pressure on the shutter. Max layer height will be as per approved calculation

Maximum free-falling height for the concrete will be1.5mtrs.

Where possible, ensure that the flexible concrete supply hose is inserted inside the to prevent excessive free fall height.

The pumping force should be ¼ of the normal force, and concrete should be casted between the void formers and pumping spout should be moved frequently between the void formers.

Concrete vibrating and compaction shall be via electrical vibrators to avoid honeycombed areas. Excessive vibrating will not be allowed.

There should be at least four vibrators with 50 mm dia lead working on four parallel lines. Therefore, that vibration of the first layer shall not be less than 4 seconds between every void former.

In case of casting in two layers the following points shall be considered at the time of casting the concrete:

  • The first layer of the concrete shall be casted on the Cobiax area only and its height would be 1/3 of the full height of the slab. It will be noticed that the void formers will arise while casting and vibrating the first layer.
  • To cast the second layer, the first one should be hard enough to avoid floating.
  • The second layer of the concrete, which is the main concrete layer of the slab, shall be for all zone including solid area and beams and it shall be casted without entering the vibrator into the solid first layer. 

In case of casting in one layer the following points shall be considered at the time of casting the concrete:

Steel wire shall be installed 1 m by 1 m above the top steel mesh and it get through the formwork, or steel wire is tied between the top and bottom steel, then bottom steel shall be tied with formwork support as illustrated in the photo.





Steel wire shall be fixed with steel or wooden bar from the bottom.

The effective depth of the slab should not differ while casting concrete.

After casting the first layer of the concrete and vibrating between the void formers, the effective depth is measured at different locations on the slab to ensure consistency.

After casting the second layer of concrete, before the first layer become solid, the effective depth is measured again at different locations to ensure the same depth is maintained throughout.


Concrete site test for temperature and slump shall be recorded.

The Concrete slump tests/ temperature shall be on a random basis

The Site Engineer shall ensure that all required authority approvals have been received prior to concrete pouring.

The Site Engineer shall ensure that MEP clearance has been received prior to concrete pouring.

Enough lighting (if required) will be provided during the casting process.

The required number of concrete test cubes shall be taken. These shall be individually marked and passed to the checkpoint.

Concrete trucks will be checked by the QA/QC representative prior to delivery of concrete to ensure the mix is as per the approved design mix.

All surplus waste concrete shall be cleared away.

Pouring concrete will be carried out using mobile pumps or other method as approved by Engineer.

The concrete shall be vibrated. Spare vibrators shall be kept in case of emergency.

Concrete casting will proceed to avoid any vertical or horizontal cold joints. 

The surface level of concrete shall be marked by pegs or tape over exposed reinforcement bars. The Site Engineer shall ensure that the top surface is level and does not exceed the required depth.

The concrete finishing shall be undertaken using power float machines.

The approved curing compound will be applied to concrete surface just after power float process.

Whenever using the curing compound material, this shall be as per manufacturing instruction.

Concrete cubes will be sent to the third-party testing authority for testing at 7 & 28 days.

- Formwork dismantling.

All safety precautions shall be maintained during the shutter dismantling process.

Forms shall be removed in such a manner as will not damage the concrete.

No forms shall be removed until the concrete has gained enough strength to support itself.

The minimum period before striking formwork for Vertical formwork to columns, walls and large beams will not be less than 12 hours.

All shutter and plywood will be clean/maintain properly and keep in safe location for next casting.

- Concrete Sampling and Testing.

Slump of concrete will be done on random basis.

Temperature will be monitored and recorded.

 1 sample (six cubes) per 50 cu. Meters. 2 cubes tested at 7 days, 2 tested at 28 days and 2 spares for compressive strength test.

Testing of cubes will be done in approved third party laboratory

For every250-cu. Meter of concrete delivered for slab and beams, 1 set of 12cubes to be taken for durability test, 3 cubes for ISA, 3 cubes for WA, 3 cubes for WP and 3 Cubes for RCP

The slump, temperature of arriving concrete and cubes will be taken by Technician.

The cubes will be kept in containers protected from direct atmosphere.
















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