How to Build a Sound Concrete Slab on the Ground.
Ground slab is basically the ground floor.
There are some important factors worth considering before concreting is done.
It is important to involve the services of qualified persons like engineer, architect or builder when planning for ground slab.
Below are the steps required when laying the ground slab?
1. Planning for materials tools and equipments to be used in concreting.
Assuming the hardcore has already been placed on the foundation area.You should know the quantities of cement, sand and ballast; the type and size of reinforcement and the size and type of dump proof membrane.
Method of mixing the concrete.
Is the concrete mix mixed manually or is it premixed and delivered to site.Concrete mixture is commonly used.In most cases those who rent out concrete mixture also provide the poker vibrator and also comes with a gang of laborers to mix, pour and lay the concrete.It is important to know when giving such a contract what the quoted amount includes.The rate should be for concrete mixture, poker vibrator, fuel, wheelbarrows, and spades and batching containers; transport to and from the site and also complete team of labourers, including machine operators and skilled masons. A standard three bedroom house can cost ksh.15, 000 for all the above mentioned.All materials for concreting should be delivered on site before casting day. Cement can be delivered on the casting day where security on the site is not very good.Availability of water is also very important. To be on the safe side, a storage tank should be on site filled with water a day before concreting as it requires a lot of water.
2. Preparing the ground.
Make sure the ground is level.Note; all entries to the house should be at least one step; 150mm below the general level of the slab.Approved murrum is placed on the leveled hard-core and a roller is used to compact the hardcore together.Anti termite is sprayed on the compacted ground; gladiator or any approved pesticide can be use.
3. Fixing the side formwork of the slab.
The slab formwork is to prevent concrete from spilling over the floor area; it is fixed depending on the thickness of the slab.Most slab thickness is100mm. This means the side form work should be 100mm from the underlay to top of the side formwork.Slab thickness of 100mm can have side formwork of 6×1(inches). Straight and strong timber that is supported with cut poles at 1m interval. The formwork is placed where the floor has a step down; like from entry porch to main house or from main floor to washrooms.
It is important to note that side formwork also determines the horizontal level of the slab; it must be double checked to make sure dimensions are correct and the required level has been achieved.
4. Installing the Sewer pipe.
Drainage pipe must be installed by a qualified and experienced plumber.This sewer waste pipe for the toilet is placed in the hardcore with an outlet passing through the foundation wall.
5. Laying the under lay.
This is commonly known as the Dump proof membrane (D.P.M).Polythene of 1000g will do the task very well. The underlay is placed on the entire floor surface with over lapping joints of 200mm taped together.
The main purpose of dpm is to prevent water from the foundation on to the slab.
6. Place the BRC or Reinforcement.
After the polythene, Brc mesh wire is placed on the under lay. Care should be taken not to create holes on the under lay.Brc mesh wire specification is done by the engineer
7. Laying electrical pipes and waste water pipes.
Place the electrical conduits on the slab and hold them firmly with binding wire on to the Brc mesh wire. These are conduits are the ones that connect to power sockets on the wall of rooms.The main electric conduit for main power supply is placed on the Brc mesh wire.
Drainage pipes placed on Brc mesh wire are those from wash hand basins, from the shower and floor traps. Note the pipes should be heavy gauge and should be held firmly on the Brc with binding wire.
8. Mixing and laying the concrete.
If a concrete mixture is used, there are several things that must be done to make sure the concrete is mixed and laid well.
The cement should be placed on a polythene paper and not directly on the ground to avoid mixing with impurities from ground.
The container for sand, ballast, water and cement should be of equal volume so that the ratio is equal.Care should be taken to make sure that the approved ratio is adhered to; the containers used should be filled to required levels.
There must be a qualified person checking the ratio of materials as they are being poured into the concrete mixture. The concrete mix should also be poured on a well prepared surface without impurities.
The concrete mix is then taken with a wheelbarrow and poured on the prepared wet surface and spread by a skilled mason. The mason will use the trowel or spade to spread it over the surface area.
A poker vibrator is then used to compact the concrete; vibration is done until air bubbles stop rising from the concrete.
Care should be taken not to damage the underlay, Brc mesh or the side formwork when vibrating the concrete. Level the surface of the concrete with a screeding board. It is important to move the screeding board in a sawing and chopping motion as this helps to compact the concrete.
The sawing and chopping motion helps in leveling and gives the final finish of the concrete floor.
9. Curing the Concrete Slab.
Concrete must be protected against loss of moisture as soon as the surface is sufficiently hard to resist spoilage. This process is known as curing.If concrete is not cured well it will dry out too fast, lose strength and the surface may crack.Curing involves pouring and holding water in the concrete to prevent the concrete from drying rapidly –allowing the concrete to gain its full strength.The common way of curing is pouring water on the concrete slab; but to help it retain water, polythene or hessian cloth is spread on the entire concreted surface.Sand can also be spread on the slab to help it retain water.It must be stressed that curing is very very important.If the concrete is not cured then all the previous precaution will go to waste and this will result in poor quality floor slab.
As explained above, all the steps are very important in successful laying of the ground slab.Failure to fallow will result in poor quality concrete, loss of money and materials.Proper planning and consulting qualified and experienced people can help in achieving concrete of sound strength.