METHOD OF CONSTRUCTION FOR FORMWORK,REINFORCEMENT & CONCRETE

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METHOD OF CONSTRUCTION FOR FORM WORK/REINFORCEMENT/CONCRETE
(Acceptance criteria: Ref. Sec. 1500, 1600, 1700 of MORT&H)

Scope:

Form work:
This work shall consist of furnishing and fixing of form work (shuttering) of approved quality for cast-in-situ / precast/pre-stressed plain or reinforced concrete with necessary props, staging, entering, scaffolding and any temporary construction required for their support.
Reinforcement:
This work shall consist of furnishing and placing of high yield strength deformed bars of the shape and dimensions as shown in approved drawings.
Concrete:
This work shall consist of furnishing and placing of plain or reinforced concrete of all grades with 20mm downgraded aggregates as defined in MOST specifications, machine mixed and mechanically vibrated to the required level, slope etc. as per approved drawings.
Materials:
The source of materials (for cement, coarse aggregate, fine aggregate, water, admixture, reinforcement steel shall be identified and the sources shall be approved by the Engineer.
Cement:
The Cement to be used in the works shall be conforming to IS. The manufacturer test certificate shall be furnished, and cement shall be stored as per IS 4082.
Water:
The water to be used shall be free from oils, acids, alkalis, salts, sugar, organic materials or other substances that may be deleterious to concrete and steel. Potable water is generally considered satisfactory. The water to be used shall satisfy the requirements as per clause 1010 of MOST specification.

Coarse aggregate:
Coarse aggregate shall consist of clean, hard, strong, dense, non-porous and durable pieces of crushed stone. All coarse aggregates shall confirm to IS 383, when tested in accordance to IS 2386 parts 1 to 8. The grading of coarse aggregate shall confirm to table 1000-1of MOST specification.
Fine aggregate:
Fine aggregate shall consist of clean, hard, strong, dense, and durable pieces of crushed stone or natural sand or a combination of both. All fine aggregates shall confirm to IS 383, when tested in accordance to IS 2386 parts 1 to 8. The fineness modulus of fine aggregate shall not be less than 2.0 or greater than 3.5. The grading shall confirm to table 1000-2 of MOST specification.
Admixtures:
Admixtures shall confirm to the requirements of IS 9103. The manufacturer test certificate shall be furnished, and shall be approved by the Engineer.
Reinforcement:
For plain or reinforced cement concrete the reinforcement shall be of grade Fe-500 confirming to IS 1786- HYSD bars. The steel supplied with test certificate shall be assembled in accordance with approved drawings and shall ensure storing to avoid deterioration and corrosion.

Procedure:

FORM WORK:

  1. The form work shall provide a smooth finish of uniform texture and appearance. This material shall leave no stains on the concrete and so fixed to its backing as not to impart any blemishes.
  2. Ensuring necessary care will be taken, the form work in true to line, levels, plumb, etc., rigid, adequately braced both horizontally and vertically. Form joints shall be as much as minimized shall be tight and shall not permit any leakage of slurry from concrete.
  3. Required form work shall be made out of plywood / steel shutters to suit the site conditions. Form work materials shall be rigid, even surface, and sufficiently strong to prevent deformation during concrete.
  4. Releasing agents such as diesel mixed with grease, shuttering oil, etc. shall be applied before fixing form work, and shall not allowed to come into contact with any reinforcement.
  5. The form work shall be cleaned carefully, dried and releasing agents shall be applied before re-use.
  6. The form work shall be so removed as not to cause any damage to the concrete. Centring shall be gradually and uniformly so as the concrete to take stresses due to its own weight uniformly and gradually to avoid any shock or vibration.
  7. Before re-use all components of form work shall be examined for damage and damage pieces shall be rectified. Before re-use all components shall be cleaned of deposits of soils, concrete or other unwanted materials. Threaded parts shall be oiled after cleaning.

REINFORCEMENT STEEL:

  1. Bar bending schedule shall be strictly as per approved drawings, and reinforcement steel shall be cut and bend as per approved bar bending schedule. Bars shall be bent cold to the specified shape and dimensions, using a proper bar bender, operated by hand or power to obtain correct radii of bends and shape.
  2. Reinforcement bars shall be-placed accurately in position as shown in the drawings. The bars, crossing one another shall be tied together at every intersection with binding wire (dia. of binding wire shall not be less than 1mm), to make the skeleton of reinforcement rigid such that it does not get displaced during placing of concrete or any other operation.
  3. Cover blocks of thickness equal to the specified cover shall be placed between formwork and bars made of concrete/ cement mortar of specified grade of concrete/strength as those of the member.
  4. Necessary stays, blocks, chairs, spacers, hangers etc. or other subsidiary reinforcement shall be provided to fix the reinforcement firmly in its correct position. Laps shall be staggered.

CONCRETE:

The grades of concrete shall be as specified in the drawings/contract. Necessary mix designs shall be submitted well in advance for approval from the Engineer. The mix design of concrete shall be as per clause 1700 of MOST specifications.
Mixing:
The mixing of concrete shall be done in batch type concrete mixer or by concrete batching and mixing plant. All measuring devices shall be maintained in a clean and serviceable condition. Its accuracy shall be checked periodically.

Mixing of concrete shall be continued till the individual particle of coarse aggregate shows complete coating of mortar containing its proportionate amount of cement. In no case the mixing done for less than 2 minutes. Corrections in water and aggregate with respect to moisture present in the aggregates shall be done on day-to-day basis.

Transportation and placing

  • Concrete shall be transported and placed as near as practice able to its final position, so that no contamination, segregation or loss of its constituent materials takes place. Concrete from batching plants shall be transported by scoop tippers or transit mixers.
  • Concrete shall be placed in horizontal layers to a compacted layer not exceeding 450mm. Fresh concrete shall not be placed against concrete, which has been in position for more than 30 minutes unless proper construction joint is formed. Concrete when deposited shall have a temperature not less than 5 °C, not more than 40 °C.
  • All formwork and reinforcement contained in it shall be cleaned and made free from Standing water, dust, etc., immediately before placing concrete.

Compaction:

  • The concrete shall be compacted within 30minutes of its discharge from the mixer, unless carried by properly designed agitators, operating continuously, when this time shall be 1 hour. When trials show that the periods stated above are unacceptable, it is necessary to add retarding / plasticising admixtures. This shall be controlled by taking slump at batching plant and site.
  • Concrete shall be thoroughly compacted with vibrators or other means during placing and worked around reinforcement and embedded figures and into corners of formwork to produce a dense homogeneous void-free mass having a required surface finish.
  • When vibrators are used, vibration shall be done continuously during the placing of each batch of concrete until expulsion air has practically ceased and in a manner does not promote segregation. Where immersion vibrators are used contact with reinforcement and all inserts shall be avoided. Standby vibrators in serviceable conditions shall be kept at site so that they can be used in the event of breakdowns.

Construction joints:

  • Construction joints shall be avoided as far as possible, when unavoidable shall be provided in a direction perpendicular to the axis of the member. The old surface shall be thoroughly cleaned and soaked with water. Standing water shall be removed and surface shallbe wetted with neat cement slurry shortly before placing of fresh concrete and shall be thoroughly compacted. Concreting shall be carried out continuously up to the construction joints.
  • For a vertical construction joint, a supporting board shall be fixed previously at predetermined position and shall be properly stayed to prevent displacement or bulging when concrete is compacted against it. Concreting shall be continued right up to the board.

Curing:

I. Exposed surfaces of concrete shall be kept continuously in a damp orwet condition by ponding or by covering with a layer of sacks, canvas, Hessian or by similar materials and shall be kept constantly wet.

2.Curing compounds shall be permitted in special circumstances like PQC, etc. and will require specific approval from the Engineer. Curing compounds shall be continuously agitated before use. All concrete cured by this method shall receive two applications; the first coat immediately after acceptance of surface finish, and the second coat shall be after first coat has set, on the saturated surface before the surface water film disappears.

Finishing:

Immediately after removal of forms, exposed bars or bolts, if any, as far as possible shall be removed, or if necessary, shall be cut inside the concrete member to a depth at least 50mm below surface of concrete and the resulting hole shall be filled by fresh cement mortar. All fins caused by form joints, all cavities produced by removal of form ties all other holes and depressions, honeycomb spots, broken edges or corners, and other defects, shall be thoroughly cleaned, saturated with water, and carefully pointed and rendered true with mortar of cement and fine aggregate mixed in the proportions used in the grade of concrete. A surface, which has been thus rendered, shall be kept moist for a period of 24hours.

Quality control tests:

  1. Physical tests on cement at a frequency of 1 test per lot (manufacturing week) MTC (manufacturer’s test certificate) shall be maintained.
  2. Gradation test for coarse/ fine aggregates at a frequency of 1 test per month.
  3. Tests on water at a frequency of once per source.
  4. Physical and chemical requirements of admixtures MTC (manufacturer’s test certificate) shall be maintained.
  5. Tests on reinforcement at a frequency of once per source per dia. Test certificate to be maintained.
  6. Workability of concrete by slump cone test at a frequency of once per session at concrete placing area.
  7. The natural origin materials of construction (Aggregate & Sand) and purchased material like cement, admixture will be tested by site laboratory as per routine frequencies as established in ITP.
  8. The concrete will be prepared by batching plant / mixer machine. In case the mixer machine is used the required quantity of material as per the design mix will be established by calibrated weigh batcher on the site of mixing.
  9. The weigh batcher will be checked prior to concreting, to establish that it is in order and is ready for concreting.
  10. The pour card will be prepared after ensuring the pre-check for concrete is completed (formats enclosed). The component is ready to receive concrete.
  11. After completing of above said process the concrete will be started. During the concreting the representative samples of concrete shall be taken and cubes will be cast.
  12. The cubes will be marked with date of pour, grade and identification of structure.
  13. The de-moulding cast cubes will be cured in curing tank and will be tested for 7 days & 28 days strength.

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