Role of contractor and tips for construction and precautions

Role of contractor and tips for construction and precautions

Role of contractor and tips for construction and precautions

Construction with the help of Labour Contractor
Its generally observed that more than 90% of the construction of residences, commercial buildings and Industries at the small scale level are done directly by the Owner with the help of Labour Contractor and a Architect / Engineer. Here we try to list the activities to be undertaken by the Civil Labour Contractors, the work involved and precautions to be taken during construction since the Owner is dierectly incharge here. This article shall be periodically reviewed and additions made periodically.
Basement     

CONSTRUCTION ACTIVITIES UNDERTAKEN FROM FOUNDATION TO RCC ROOF:

The Contractor shall Mark the Layout of Building on the Ground in the form of centre lines of walls and coloumns. These centre lines shall by guided by brick bhurjis made along the centre line at a distance of 4' from the outer walls and coloumns with the centre marked on the bhurjis woth fresh plaster. Excavation to be done to the prescribed basement floor lvl at the cost of the owner. Balance excavation to be done at contractors cost after remarking the position of coloumns on excavated ground and also making temporary markings of centre lines on excavated sides.Marking the foundation/ beams sizes and then doing the balance excavation giving shape to the raft foundation / coloumn foundations.


      Laying of Plain Cement Concrete as per Architects directions to the required thickness.


      In case external waterproofing is to be done the its to be done on the PCC if internal the that shall be done after completing the RCC of thr basement. Now Contractor is to do the final Layout of walls, coloumns and beams on the PCC and get it verified by the architect / engineer for accuracy.


      Contractor is to guide the bar binders ( Lohar) with respect to laying of steel in foundations and has to position the coloumn reinforcement. Payment to bar binders to be done by owner usually around Rs 1/- per Kg. Cover blocks to be made by contractor in PCC at the time of laying PCC in the thicknesses stipulated and placed below or around reinforcement so as to provide proper cover. After laying of the Steel reinforcement it is to be checked by the Architect for accuracy and cover to reinforcement.


      Contractor is to make provisions for shuttering material required to give shape to the foundation elements and to position it at his cost.


      Contractor is to pour concrete of prescribed mix in foundation which is usually M 150 ie ( 1: 2: 4) - ( 1 cement : 2 coarse sand : 4 coarse aggregate) by volume or M 200 ie.( 1: 1.5: 3) in small residential projects. In case of bigger projects the mix is designed for giving the required strength stipulated by the structural designer. It has been observed that owners instead of using Cement of grade 43 use cement of grade 53 in RCC on the behest of suppliers and in their belief that this will be more strong, this should not be done since cement of grade 53 sets very early and also has more shrinkage which can cause surface cracks to appear with in a few hours of laying if not cured with water. Its also observed that the time lapse between mixing, laying, vibrating the concrete and then smoothing it to shape / level required in normal circumstances is at certain points of time more than the setting time of Grade 53 Cement. Thus Cement of grade 43 should be used for concreting unless other grades are specified by the architect / structural designer. Additives to cement may be added on advice.
      

Contractor is to do shuttering of Coloumns and RCC walls as per details provided by architect in conjunction with the steel binder and ensure positioning of spacers to provide proper cover to reinforcement. Care should be taken by him to ensure that the shuttering is tight so that it does not bulge at time of concreting and that the wall does not go out of plumb. The architect / engineer should be requested to check the accuracy of shuttering and whether its in plumb ie: ( Its exactly vertical and not bending outwards or inwards.)


      Contractor is to pour concrete as per mix adviced in walls and coloumns and is to vibrate it properly for ensuring proper laying, cement additives can be added for generating proper flow and compaction of concrete as per advice of architect. Casting of walls is generally done in two lifts causing a joint to be formed. Care should be taken to provide a water key at this point. For this at the time of laying the lower wall a depression of aproximately 1" x 1" x 1" with any wooden batton or plank should be made in centre of the top of RCC wall. When concrete is layed in the upper half it fills in to this groove forming a water seal and reducing chances of seepage from joint. This precaution should also be taken at the time of laying of foundation and groove should be provided along the centre line of RCC wall position.


      Contractor is to cast the balance height of coloumn after proper shuttering upto roof beam bottom.


      During all the above activities of concreting right from the foundation to roof and above its generally seen that the masons at times are careless in keeping the reinforcement of coloumn vertical or in plumb or in providing proper cover to it thus causing it to drift to one side due to its own weight and shifting from the centre line by an inch or more. This aspect should be specially checked by contracter, owner and architect at every stage and where its height above the shuttering is more and its swaying due to its own weight it should be tied in various directions to keep it in plumb.


      Contracter is to provide and do shuttering of the roof as per structural drawings and check its level. He has to ensure that proper amount of supports are provided and also that the shuttering is not uneven or done with very old planks or bent plates. Since generally the shuttering is taken on rent by the contractor he does not provide shuttering of wareproofply or of steel sections made to the size required since this increases the cost per sqft. But steel shuttering of various coloumn sizes is generally available for rent and also steel plates for roof shuttering are also available for rent and these should be used. Architect / engineer should check the shuttering for levels and design aspect he should also check the supports for any loose ends.


      Contractor should guide the steel binder w.r.t laying of steel reinforcement in roof and his masons should provide the proper cover blocks where required. Overhere its generally seen that at the time of positioning of beem main bars at the junction with coloumn they passed from outside of one side if vertical coloumn reinforcement by the binder this causes it to either press the coloumn bars inwards thus disturbing there plumb in the evertical or reducing finishing the cover to the beam bars since they are pushed outwards by the coloumn bars this also at times causes the shuttering to bulge or reduced space blocks proper flow of concrete in to positon required. This can be avoided by gently bending the beam bars inwards before the junction causing them to pass on the inside of the coloumn reinforcement without causing any of the above problems.


      Its also seen at times that some architects / engineers do not provide the distaces of curtailment of bars or the distance for which cranked bars are to travel into the next slab or the distance from edge of beam from which bars are to be cranked by simply giving a general rule or leaving it to the knowledge of the steel binder. This practice should be avoided since the binder is not a engineer and very few working in the general residential sector are educated enough do make percentage calculations thus all distances should be mentioned by architect / engineers in there drawings and shourtcuts should be avoided. This should be looked into by the owners and he should request them to provide detailed drawings.


      Time should be given to the electrical contractor and the plumbing contractor for laying there pipes, fan and light boxes properly. Thus a gap of one day should be provided after the binding of steel reinforcement so that both the electrical and sanitory contractors can execute there work properly and also for the architect / engineer to check the reinforcement, shuttering, electrical and sanitory work. This at itmes is avoided by the contracter thus forcing the electrical and sanitory person to do work on the morning of pouring of concrete and the engineer to check incomplete reinforcement which is an unsafe practice. Thus while finalizing the contracter make this as one of the conditions that one day gap shall be given after laying of reinforcement.


      Contractor has to lay the concrete in roof slab as per specified mix and design. Care should be taken to provide the specified thickness of slab concrete and also to vibrate it properly so as to remove air gaps and make concrete fill into all edges. Masons have the tendency of adding water to the layed concrete to make it flow in the beams this should be avoided and be done in rare occasions where some time has elapsed and the concrete is dry otherwise it drains out the cement and the sand through gaps in shuttering reducing strength. Again here in small works its observed that the labour doing the mixing of concrete is not so carefull in ensuring the proper mix thus owner should depute one of his own persons to keep a check at the point of mixing for volumes of cement, coarse sand, coarse aggregate. The labour and the mason also have the habit of increasing the quantum of coarse sand for getting easy workability this causes reduction in strength of laid mix and less consumption of cement. We advice that today we have many additives for concrete available in the market for reasonable price for increasing the workability of concrete and these should be used instead of disturbing the mix ratio. Adding too much of water by labour for getting smooth flow of Concrete should also be avoided.

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