Familiarity breeds contempt, which is why we take for granted a warm ,dry and strong concrete floor under our feet. But take it from me, it calls for great workmanship to get that floor ready for the finishing works, such as sand and cement screed or tiles .Anything short of this level of dexterity will leave you with a damp floor and a failing screed or tiles that needs patching up within a short time. With this in mind, let me unpack below ,at lightning speed, all that goes into concrete floor construction so that you may appreciate the work and get the most out of floors by taking good care of them. This is important because as we all know, prevention care is cheaper than the cost of repair.
Concrete floors can either be constructed using ground bearing slabs or pre-cast concrete. Ground bearing slabs are not supported by an external wall, allowing for deferential movements between them. Construction of the floor is a long seven step process that can take a minimum of one to four weeks as follows:
- First remove the top soil and vegetable matter to avoid the problems associated with slab movements as the ground is compressed down and settles in.
- Then remove debris from the foundation trenches and build the load-bearing walls to the damp proof course(DPC) . Note that damp proof course purpose is to resist the capillary action of water or rising damp .Its cannot be used to resolve high water table problems at the construction site.
- Third step, start to construct the floor. Pour a layer of hard-core stones at least 150 mm thick into the floor areas. The hard-core should consist of a wide range of stone sizes from relatively big stones to dust or Murrum. This ensures it is compressed with ease by a vibrating plate or roller. The hard-core layer helps prevent loose earth from contaminating wet concrete and to spread evenly the load from the concrete slab. If the hard-core is deeper than normal, because of excavated topsoil or the site is on slope ,it’s important to cold compress it in layers. Failure to follow this recommendation will result in problems as the consolidation happens later. In the event that the hard-core is over 500 mm deep, use the more expensive option of suspended concrete floors. This will save you a lot of headache later on.
- Then lay a blinding to the hard-core. Large lumps of stones are crushed to form a blinding. Alternatively you can use Murrum or broken bricks which are easily stamped and compacted. Ensure to keep the top of this blinding below the substructure so that the slabs can be cast inside the walls
- Lay the damp proof membrane (DPM ), to arrest rising damp from the ground. This membrane can either be above or below the concrete slab and the material used will depend on the floor finishing materials . The DPM must overlap with the DPC to isolate the whole superstructure from the substructure.
- Second last step-pour concrete slab. The slab should be between 100 and 125 mm and must of suitable strength. If the ground below the slab contains some soft spots, the concrete can be reinforced with a layer of steel mesh. The slab is then tamped and leveled using a timber beam or a vibrator to remove excess air and water.
- Finally allow the concrete to set and achieve its full strength . In hot weather protect the slab by watering it three times daily for one to two week(s). In addition you can use a polythene sheet to cover it to minimize excessive evaporation of moisture which can reduce the strength of concrete slab and cause cracking. In cold weather, cover the slab with sand to prevent freezing or consider delaying the pouring of slab till when the condition become favourable.
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Sources: The construction of houses by Duncan Marshal and Derek Worthing