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PROCEDURE FOR WALL PRECAST CONCRETE OF MANHOLE
1) All precast concrete manhole (Casting, Curing, Stripping, Handling and Storage etc.) shall be in accordance with applicable Specification and Standard.
2) Complies with the dimensional tolerance as note in the drawing & design of concrete should specify minimum compressive strength.
3) Precast concrete manhole is a construction product produced by casting concrete in a reusable mold or “Form” which is then cured in a controlled environment, transported to the construction site.
4) All precast concrete manhole shall be cast in watertight forms approved plywood, to provide a standard finish with a dense texture, free of honeycombing and excess air holes surface texture shall be uniform.
Work Procedure FOR WALL PRECAST CONCRETE OF MANHOLE
1) Execution & Setting Out
a. The precast concrete shall meet the requirements of these specifications and those indicated in the construction drawings and the following relating to precast concrete manhole work.
b. Prior to commence the concrete on any structure, all pertinent documents shall be acquired, such as work permit and other relevant QC inspection to be done.
c. Proper coordination with the batching plant plays a great role for the success of the work, access for the transit mixers and concrete pump shall be planned carefully to have proper flow of traffic during casting of concrete.
d. Traffic control man/flagman shall be assigned as indicated on the traffic route plan, in rder to have a proper, safe and organized traffic flow. Communication for coordination with concrete batching plant must be followed.
2) Formworks & Support
a. Formworks shall be prepared in accordance with approved project drawings under the supervision of a Civil Supervisor.
b. Form oils or forms releasing agents shall be used on the surface of the forms for easy stripping, prior to the closure of the formworks, reinforcing bars shall be placed and rigidly fixed and verified to its size and concrete covering is verified.
c. Formworks shall be designed in such a manner that it can be easily dismantled without causing damage to the concrete surface or disturbing the remaining parts.
d. Formwork shall not be removed or stripped before the concrete has set sufficiently to withstand safely any loadings and stresses to which the structures may then be subjected.
3) Rebar Setting
a. All reinforcing bar shall be hot rolled high tensile deformed steel with maximum yield strength not exceeding 422 MPA (60 ksi).
b. Reinforcing bar shall be fusion bonded epoxy coated bar. Manufacture, fabrication, handling and placing of bars shall be as per ACI 318 Chapter 7.
c. Reinforcing bars shall be pre-assembled and placed to avoid contact with soil.
d. Reinforcements shall be free of loose rust, loose mill scale and shall be covered with tarpaulin or polyethylene sheets until required to prevent weather damage.
e. Prior to the installation of reinforcing bars, ensure that all dimensioning and rebar requirement is intact and all relevant drawings are available. Skilled steel fixers shall perform the rebar laying.
4) Placing of Concrete
a. Prior to the commencement of concrete pouring, verify RFI issued by QC Inspection team indicating scheduled date and time of concrete casting.
b. Ensure that all areas schedule to receive finishes have been inspected and are acceptable prior to pouring.
c. Concrete shall be batched in a batching plant and shall be transported to site Jay down yard by concrete transit mixers and will be adjusted accordingly depending on the rate of discharge to its final deposit. Pump shall be used for pouring concrete.
d. Concrete shall be thoroughly consolidated by concrete vibrator during placement and thoroughly work around the reinforcement, embedded items and into corner of forms.
e. Concrete shall not be re-vibrated to remove or reduce cracks. Over vibration will cause segregation, surface laitance or leakage through formwork shall be avoided. Immersion vibrators shall be withdrawn slowly to prevent the formation of voids.
Please read also: CIVIL ENGINEERING QUESTION AND ANSWERS INTERNATIONAL STANDARDS
f. Concrete shall be covered with protective plastic sheeting immediately following completion of casting to prevent premature drying, excessive hot temperature and mechanical disturbance. Water curing (wet curing) using burlaps shall be used after consolidation of concrete covered with plasticised vapour barrier (Polyethylene Sheets).
5) Concrete Curing
a. Curing shall be continuous until compressive strength has reached 70% of the specified strength, but not less than seven (7) days after placement.
b. Saturated burlaps shall be covered with plasticized vapour barrier and continuously water sprinkled.
c. Visual inspection of the curing process shall be carried out on a daily basis by JGC/DEC and Subcontractor QC Inspector. Any deficiency found on the concrete shall be rectified immediately.
6) Transport of Precast Concrete Manhole
a. Prior to transport/delivery of the newly precast concrete manhole, make sure that the location and final resting place for the said manhole has been previously prepared and ready.
b. Cranes with adequate capacity shall be used to lift the precast concrete manhole, precast concrete manhole shall be labelled and marked where it is to be installed.
c. Lifting hooks shall be placed prior to concreting in a manner that will not affect the strength and stability of the precast concrete manhole likewise, cranes shall be used to unload the precast concrete manhole directly into its final location.
7) Installation of Precast Concrete Manhole
a. Precast installation shall be checked and verified to its dimension and elevation prior to placemen Uinstallation of the precast concrete manhole .
b. Survey team should have established prior to placement of the precast concrete manhole true to its coordinates and elevation.
c. Cranes shall lift the precast concrete manhole from the trailer directly to its final location and coordinates already established by a competent surveyor.
d. Once completed, verify the alignment and correctness of the elevation of each newly installed/placed precast concrete manhole.
e. After the completion of placement is done and have been verified, checked and confirmed, all lifting devices shall be recessed. Lifting devices shall be removed and patched to match the interior surface of precast concrete manhole after installation is complete. For acceptance of Saudi ARAMCO representative, proper housekeeping is observed at all times.
Safety Control
1) HSE Manager and safety supervisor shall carry out the safety site inspections to ensure that safety requirements laid out in the Safety Management Plan are followed during the execution of precast concrete manhole works.
2) Persons involved in precast concrete manhole works must be provided with and use the necessary PPE and as a standard each person shall wear a Safety Helmet and Safety footwear etc. The requirement for personal protection equipment shall be approved by client company.
a) All personnel will be issued with the following personal Protective equipment and must be worn at all times.
• Safety helmets
• Safety boots
• Safety Spectacles
• Hand Glove
• Dust Masks
• Ear Defenders
• Respirators as Required
• Wellington Boots
• Cotton coveralls with full sleeves shall be worn on construction site
• Disposable coveralls
3) Confined Space Entry:
a) No one may enter in any deep excavation and enclosed space without specific approval and a current work permit prior to start of work. The safety equipment, gas test and instruments are required.
b) No new activity shall be introduced into a confined space without the permission and signed approval of the permit to work issuing authority.
c) Subcontractors shall not enter or commence work in any deep excavation and enclosed space, until a valid permit to work has been issued. Where subcontractor operations may result in a dangerous atmosphere arising during the work activity.
4) All personnel involved in the precast concrete manhole works shall have to undergo Safety training being conducted by HSE Department.