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Saturday, May 5, 2012

Fixing of reinforcement

4.10.2   Fixing of reinforcement

The steel  reinforcement  shall  be  assembled  to  the  exact  shapes  and dimensions   as   approved  by  the  Engineer.  The  rods  shall  have  the approved cross-sectional area and shall be fixed accurately in the moulds. The ends of all tying wires shall be  turned into the main body of the concrete and shall not be allowed to project towards the surface. Spacing blocks shall be used to ensure accurate cover to the reinforcement, where necessary, and these blocks shall be of precast concrete of a strength at least equal to that of the concrete being placed. They shall be as small as possible in view of  practicality and shall be securely fixed in position by means of wires to be cast into them.

No  temporary  supports  for  the  reinforcement  shall  be  allowed  to  be incorporated  into  the  finished  concrete.  At  the  time  of  concreting  all reinforcement shall have been thoroughly cleaned and made free

of all loss rust (crude oil or any other coatings that might destroy or reduce the bond).

Unless otherwise specified or shown on the drawings, minimum cover shall be  determined  in  accordance  with  ACI  318-  77  as  indicated  on  the following table:


Min. cover thickness

Cast against and permanently exposed to earth

75 mm

Exposed to earth and weather

D16 and smaller

40 mm

D19 - D 25

50 mm

Not     Exposed to weather and

Slabs and walls

D25 and smaller

20 mm

not  in  contact with   earth

Beams,     girder, columns

main   bars    ,    ties, stirrups

40 mm

4.11 Form work

This section covers the fabrication, erection and removal of forms and other necessary work thereof, including material and design of forms.  All works, covered by this section shall conform to ACI 347 or relevant B.S. standards unless otherwise specified herein.

4.11.1 Design of forms

a.  Forms shall be constructed complete with centering, sleeves and molds to conform to  the shape, form, line and grade required and shall be maintained  rigid  to  prevent  deformation  under  load  where  required forms shall provide for adequate protection of the precast units placed within the forms, before pouring concrete.

b.  Joints  shall  be  leak  proof  and  arranged  vertically  or  horizontally  to conform to  the  design pattern. Forms shall be placed on successive units for continuous surfaces and fitted to accurate alignment to secure smooth completed surfaces free from irregularities,  In long span, where intermediate supports are not possible, the form deflection due to fresh concrete shall be compensated for. Members shall have true surfaces in accordance with desired lines, planes and elevations. If adequate foundation for shores can not be secured, trussed supports shall be provided.

c.  Form-ties;      in    general,    shall    be     (snap-tie)    type     which    beak predetermined points at least 2.5 mm, behind the finish surface of the concrete, or  type which can be wholly with drawn. Where form ties, bolts, or rods are to be completely removed, they shall be coated with grease,  In  no  event,  either  in   interior,  exterior,  or  below_  grade applications, shall any parts of metal ties be  allowed to remain within
2.5cm, of the finished concrete

d.  Form surfaces shall be coated before placing of reinforcement, when oil is used , surplus oil shall be  removed from form surfaces. All oil stains shall be cleaned from reinforcing steel before pouring of concrete,

e.  Forms shall not be removed without the approval of Engineer. This approval shall not relieve the contractor of responsibility for the safety of the work, the minimum  period of time that must elapse between the pouring of the concrete and the slackening of the form work shall be as follows:

Type of Formwork

Minimum period before removal

more than 16 C°

7- 16 C

Slabs, beams , girders columns, walls

14 days

21 days

24 hours

24 hours

The removal of form works shall in all cases be supervised by an experienced  foreman. All form works shall be removed without such shock or vibration as would damage the concrete, and before the form works and props are removed  the concrete surface should be exposed, where necessary, in order to ascertain that the concrete has hardened sufficiently,  Any  work  showing  signs  of  damage  due  to  premature loading is to be removed and entirely reconstructed at the contractor's expense.

4.12 Expansion joints

Expansion Joints  shall  be  formed  in  walls,  Floors,  roofs,  beams  and foundations where indicated on drawings. Where specified or shown on the drawings, expansion Joint  in Floor shall be covered with specified metal covers, Reinforcement, or their embedded items bonded to the concrete ( except dowels in floors bonded on only one  side of joints),                                                                                                    shall not be permitted to extend completely through any expansion joint,                           Expansion joints in water retaining structures shall incorporate at raid_depth of the concrete a  pressure resisting PVC water stop and joint fillers shall be installed as shown  on the drawing.

4. 13  Construction Joint

Unless otherwise specified here in ,  construction joint shall be formed and treated  in   accordance  with  (  specification  for  structural  concrete  for Building),      ( ACI   301 _ 72). The position of construction joints shall be arranged  to  minimise  the  affect  of   shrinkage  and  construction.  The following Special requirements apply to the position of construction joints:

a.    In beams and slab .

Construction   Joints,   other   than   in   cantilevers,   shall   be   located   at approximate  quarter  span  points  from  the  center  line  of  supports. Construction  joints  shall  not  be  made  in  cantilevers,  and  where  it  is practicable  to  do  so,  the  whole  of  the  internal  span  from  which  the cantilever springs shall be cast at the same time as the cantilever.

b.   In flat slab.

Construction Joints  shall  be  formed  at  the  center  of  supporting  span approximately.

c. In foundation

As far as possible construction joints shall be located midway between the footing or pile cap for column and walls. In isolated footings or pile taps ,

the construction joint shall not be allowed.

At each construction joint, the surface of the existing concrete shall be thoroughly roughened to remove all laitance and other injurious materials and wetted before any further concrete is placed.

All concrete between construction joints shall be placed continuously. In the event of unavoidable delay during concreting, the engineer may require a construction joint to be made, or other appropriate action taken.

Horizontal and vertical joints shall be thoroughly wetted and covered with allayer of mortar immediately before placing new concrete.

4. 14 Finishing

4. 14. 1 General

The finishes described, below must not cause any deviation out sid the specified allowable deviation from plumb or level or from alignment, profile grades or dimensions  specified  in  the paragraph on tolerance. Offsets caused by displaced or misplaced sheathing, lining or form sections, or by knots in forms or by other wise defective items shall be checked by direct measurement.

All other irregularities can be checked by use of straight edges or by use of templates.  Surfaces shall be corrected so as to meet the requirements specified in the paragraph or tolerances.

4.14. 2 Formed surfaces

Formed surfaces to be hidden by backfill, plaster or other finish material shall not require treatment after form removal except for patching defective concrete, the filling of holes and the specified curing, correction of surface irregularities shall be required for depression exceeding a limit not affecting the subsequent finish. Formed surfaces for exposed walls, water conduit, parapets, curbs, stair faces and other areas shall be smooth and regular when completed. Ridges or lips on exposed concrete shall be removed by tooling , grinding or rubbing.

4.14.3 Fair faced concrete

This type of finish is for surfaces which are permanently exposed to view. Forms  to  provide  this  finish  shall  be  faced  with  wrought  tongued  and grooved  boards  or  plywood  or  metal  panels  ,  arranged  in  a  uniform approved  pattern,  free  from  defects  likely  to  detract  from  the  general appearance  of  the  finished  surface.  This  finished  shall  be  such  as  to require no general filling of surface pitting, but fins,  surface discoloration and other minor defects shall be remedied by methods approved  by the Engineer without extra cost to the contract.

All forms shall be removed without damages to the concrete. The use of non- staining mould oil or other material to facilitate this shall not have a deleterious effect on either the strength or appearance of the concrete and

shall be subject to the approval of the Engineer.

4.14.4   Unformed Surfaces

Unformed screeded finish surfaces to be covered by-backfill or concrete and  surfaces  of  sub-floors  to  be  covered  by  concrete  floor  topping: Finishing  operations  shall  include  sufficient  leveling  and  screeding  to produce flat uniform surfaces.

Unformed floated finish surfaces not permanently consealed by backfill or concrete and for which other finishes are not specified, including concrete to be permanently exposed to view such as outside decks, floor of sumps, tope  of walls, surfaces of  gutters, sidewalks and out side enterance slabs, when trowelled finishes are needed,  floating shall be continued until a small amount of mortar without excess water is  brought to the surface, permitting effective trowelling.

Surfaces to receive mastic water proofing or asphalt water proofing shall not  have   any  prominence  likely  to  cause  damage  or  other  harmful irregularity.  Surfaces   irregularities  shall  be  reduced  or  eliminated  by grinding, after concrete has hardened.

4.14.5 Chemical Surface hardening

Whene  shown  on  the  drawings,  a  cement  mortar  trowelled  finish  or concrete trowelled finish surface shall be treated with chemical hardener, it shall be applied to slabs after they are at least 28 days cured, cleaned and free from dust, oil, grease and  other foreign matter, and dry chemical hardener         shall  be     applied    in         accordance                   with     the  manufacturer's instructions.

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