Repairing a concrete floor (continued)
Lay the infill (rubble)material to the required depth in layers of no more than 225mm at a time, compacting each layer thoroughly and breaking up the larger pieces with a sledge hammer. You can use brick and tile rubble or, better still, gravel rejects, which are coarse stones from quarry waste. If you are using second hand rubble you should remove from it any fragments of plaster, which can react unfavourably with cement, and any pieces of wood. Bring the surface of finely broken rubble up to within 25mm of the chalk line for the concrete and ‘cover’ the surface with a layer of sand, tamped or rolled flat.
Spread a sheet polythene damp-proof membrane of 1000-gauge minimum thickness over the surface of the sand turning its edges up all round and lapping it up the walls to form a tray. Make neat folds at the corners and hold them temporarily in place with tape. If the floor needs more than one sheet to cover it the sheets must overlap by at least 200mm and the joints be sealed with a special waterproof tape available from builders hardware store.
Laying the concrete
Mix a medium-strength concrete of 1 part – cement: 2.5 parts sand: 4 parts aggregate. Do not add too much water; the mix should be a relatively stiff one. Lay the concrete progressively in bands about 600mm wide. The direction of the bands will depend on the door because you will have to work in such a way as to finish at the doorway. Tamp the concrete with a length of 100mm x 50mm timber to compact it and finish level with the chalked line. As you go along, check the overall surface with a spirit level and straight edge, and fill in any hollows, though slight unevennesses will be taken up by the screed.
When the concrete has set firmly enough to support a board to walk on, brush the surface with a stiff broom to make a key for the screed. Leave it to cure for at least three days under a sheet of polythene to prevent shrinkage caused by rapid drying. Alternatively, call for readymix concrete using a 25 MPA mix which you then have to place and screed yourself using pegs driven into the soil to make sure that an even level is achieved.
Laying the screed
Mix the screed material from 3 parts sharp sand : 1 part Portland cement. Dampen the floor and prime it with a cement grout mixed to a creamy consistency with water and bonding agent in equal parts. Working from one wall, apply a 600mm band of grout with a stiff brush.
Apply a bedding of mortar at each end of the grouted area to take 38mm x 38mm ‘screed battens’. True them with a spirit level and straight edge so that they are flush with the surface level line on the walls. Lay mortar between the battens, tamp it well down, level it with the straight edge laid across the battens and smooth it with a wooden float. Lift the battens out carefully. fill the hollows left with mortar and level with the float. Repeat the procedure, working across the floor in bands 600mm wide. Cover the finished floor with a sheet of polythene and leave it to cure for about a week.
The floor will be hard enough to walk on in two weeks, but not fully dry for about six months in winter, this is weather dependent. A rule of thumb would be tp allow a month for every 25mm of thickness and meanwhile do not lay an impermeable floor covering.
Trim the damp-proof membrane to within 25mm of the floor and fit the skirtings to cover its edges.
In the next post we will talk about cleaning Brick and Stone.
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