Tips for a smooth roof-slab laying

Jagvir Goyal

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As the time for laying of roof slab draws near, most house builders get anxiety attacks coupled with the bouts of self doubt. Guidelines for proper laying of RCC slab of the house have been suggested many times yet readers keep asking to provide a concise guideline or chart to pick up and follow whenever the roof slab of their house is to be laid. Here are some guidelines to help our readers tide over this difficult time:

Check shuttering and supports

See that proper shuttering and supports have been provided at the site. For it, first provide bearing plaster over the top of walls, up to the required level and check it with water level. After it dries, provide proper coatings of water proofing material over it. Now, provide shuttering plates for the slab matching the level of bearing plaster and add beam shuttering wherever beams are planned. Always prefer steel shuttering plates. See that all shuttering plates are free of walls and are not resting on walls anywhere. Provide proper side supports to the slab and beams to prevent the overflow of concrete. See that the side supports are fastened well to bear the vibrations caused by vibrator during compaction of concrete. Check the level of shuttering. Affix brown paper tape on all plate joints to prevent any leakage of slurry through them.

Shuttering supports

These days, use of steel girders has become common to support roof slab shuttering. It is a wrong method but it has almost been universally adopted in North India. These girders are inserted in the newly built brick walls at about 4 ft interval as per the width of shuttering plates. To insert the girders in the walls, the masons leave big openings of 9 inch x 9 inch size in the top brick courses of walls. Each of the walls has 3 to 4 such openings. These openings have a detrimental effect on the support system for the roof slab as the top courses of brickwork that act like brick beams to support the roof slab, become weak. Instead, erect vertical pipe supports on the ground and support the I-sections or girders and shuttering on them. Prefer 1.5-inch diameter steel pipes that are truly straight, having not even slight bends and with base plates welded at one end to give better bearing to the pipes and to make them stable and effective. Provide horizontal braces to vertical steel pipes to create a strong support system. Provide diagonal braces also when the height of pipes is more than 15 ft. However, more than 15 ft height is rarely provided these days except in double height lobby.

Laying steel reinforcement

The slab drawing shows the steel to be provided in the beams and the slab. Beams may have steel bars of different diameters at the bottom and top along with stirrups or rings along the length of beams. The steel in slab with cranks at walls or beams and in cantilever portions is also shown. Lengths of steel bars are worked out as per this drawing and bent to shape. Get the steel laid in beams and slab as per drawing. Provide bearing of 6 to 9 inches to the steel in beams and slab over the walls. See that proper cranks have been provided to steel at supports. The steel in cantilevers and projections should be specifically checked to be correct.

Check that binding of steel with binding wire is tight and proper so that steel bars don’t get displaced during concreting work. To keep the steel in upper layer in position, small pieces of steel are cut and bent to special shape, called 'chairs' and inserted below the upper layer. Wherever steel bars are longer, 'laps' are provided in them for proper transfer of stresses. All 'laps' are not kept along one location but are staggered. Just tell the bar binder to provide sufficient length of laps and to stagger them and he'll understand. It is most important to provide proper 'cover blocks' below steel bars to provide concrete cover to them and to save them from exposure. Finally check or get checked all steel reinforcement completely as per drawing.

Providing other inserts

In addition to steel, check that all other inserts have been provided in the roof slab layout. See that all electric conduits and other conduits for phones, LCD cables, AC cables been laid in position for embedment in slab. Further see that fan hook boxes, light boxes, down bends, earthing wires and all lighting fixtures have been placed in position. Check that all vertical pipes for rain water, soil waste and waste water have been erected in position by the plumber. All plumbing specials such as floor traps, elbows, bends, if any, should also be fixed by the plumber and their mouths should be well choked with gunny bags to prevent entry of concrete in them.

Get ready

Next, make all arrangements for pouring concrete in position. Arrange aggregates such as coarse sand, 10 mm down stone aggregate, 20 mm down stone aggregate and required quantity of fresh cement. Have a good and uninterrupted source of potable water supply for preparing concrete. Fill a 200-litre drum with water and keep it ready near the mixer. Arrange a good mixer having baffle plates in it for mixing concrete. Get its brakes, clutch, wire ropes and running checked. Arrange tarpaulins to cover the cement and laid portion of slab during unexpected showers. Arrange jute bags to cover projected portions of concrete after laying it. Keep lighting arrangements ready for use in case concreting continues after dark. See that the staging has been erected in position by the contractor for labour movement and transfer of concrete from the ground to the roof slab area. See that sufficient number of vibrators have been arranged for concrete compaction and extra needles of vibrators are available.

Post pouring care

On completion of concrete work, get the concrete surface roughened with a broom or light wire brush after a few hours of its laying. Create a water pond over the slab for its curing after about 10 hours of laying of concrete. Portions such as beam projections above the slab should be covered with jute bags which should be kept wet through regular sprinkling of water on them. Don't remove the shuttering below the slab for at least 14 days and for 21 days below the beams after the laying of concrete.

Choose the day

Weather predictions are quite accurate these days. Choose a rain-free day. To be safe, there should be no rain prediction on a day prior to pouring of concrete and next day of pouring. Avoid pouring of concrete in extremely hot weather also. Day's temperature should be below 40oC. Similarly, avoid concreting on extremely cold days. Labour's efficiency is down on extremely cold days and concrete takes more time to get set. If temperature is about 10 degree C, setting of concrete slows down to half its speed than at normal temperature. Decide concrete pouring sequence of the slab i.e. the point from where to begin and the direction of proceeding. Begin from the far end. It will avoid labourers walking over the freshly laid concrete. Arrange sufficient number of workers behind the mixer, in front of it, on the scaffold and on the roof slab. Explain the cement, sand and stone aggregate ratio to the workers. Fix and explain the quantity of water per bag to maintain water cement ratio. It is very important to maintain it and keep a check on it as workers tend to add more water to concrete.

Pour the concrete: Make your prayers and start pouring of concrete. Leave a worker below the shuttering plates to keep an all round eye that any shuttering plate is not yielding under the weight of concrete. Give sufficient mixing time to concrete in the mixer. Workers tend to pour concrete out quickly. Add 10% extra cement to the first batch of concrete. See that the vibrator is being applied to concrete continuously for its compaction but over vibration is avoided. Take special care to compact concrete at edges and corners. Allow no stoppage of concreting work during lunch hours and make workers take lunch by rotation. This will help in avoiding cold joints in concrete. Poured concrete surface should be finished side by side with wooden float.