Floor PCC(plain cement concrete) is a very important stage during grey structure construction of a house. In most of the cases floor pcc is done at the ending stages of grey structure after completing plasters, however it can also be done before plasters provided that all required technicalities are followed and checked. Plots are normally in depth as compared to the roads before construction of house, hence after making foundations, backfilling is required to obtain the finish floor level of house. Backfilling is done by sand or a mixture of sand and silt which in local language is called “گھسو”. This backfill is required to be concreted during grey structure and the process is called floor pcc or in local language it is termed as “فرش کچا”.
Floor PCC is a critical step during which a minor negligence can cause a lifetime distress and trouble. Most common defect that arrises after house construction due to this negligence is the sinking, sagging and depressed floors(فرش کا بیٹھنا). Before starting the process of floor pcc, contractors in order to cut the cost might try to skip some very mandatory steps and as a layman if you are constructing the house yourself or even if you have hired a construction company, should be aware and have basic knowledge of technicalities that must be taken care of.
Floor pcc is a procedure that has following stages:
Stage 1 Compaction
This is the very first step of preparing the floor which includes compaction of backfill. As discussed earlier that plots are depth as compared to road level and sand/silt mixture “گھسو” is required to obtain finish floor level, it should be made sure that backfilling is done with pure sand silt mixture “گھسو”, mud which is slightly cheaper than sand silt mixture can also be used for backfilling but it is not recommended, as when it gets mixed with excess water it becomes a slushy matter “دلدل”. Backfilling’s best method is in layers, however if you are constructing a house, due to certain constraints it becomes difficult to perform the task in layers as it is to be done manually by labor. Convenient method for labor is to start filling from one end, complete it and move to next end.
Compaction of backfill is done using hammer compactor as shown in video below. Before this hammering a step that is in normal practice in market is making of small ponds of water and refilling it with water as soon as sand absorbs the water. Though there is no harm in this but excessive water can be damaging for foundations and become a major cause of seepage in house.
Stage 2 Brick Ballast
Brick ballast is made by crushing the clay bricks into small pieces of approximately 40mm to 50mm in size. Bricks used for crushing are slightly over burnt or left over half cut bricks on site during brickwork masonry. It is used under the floor PCC that is laid on compacted earth-fill. Brick blast should be made from “Awwal” A class bricks, as the strength of floors is heavily dependent on quality of brick ballast. Maximum 20% mix of over burnt bricks (Khinger) is acceptable. Brick ballast should be clean, any kind of dirt, leaves, straw or soil materials indicate that the ballast is old or has been collected from some other site. Size of brick ballast should not exceed 50mm so that the material can make bond when laid and compacted. Larger size means more open spaces will be left between pieces that can affect the strength of floors.
One common mistake that people make is that they do not place brick blast on upper floors as they are already concrete slabs. Brick ballast should always be placed on these surface too before pcc. Person in lower portion whose upper floor is prepared with brick ballast will bear and hear much lesser vibration and sound whenever someone walks on that floor as compared to the one whose pcc was carried out without brick ballast.
Brick ballast should always be laid after marking the water level throughout the walls of house and its volume should be strictly as mentioned by the architect in architectural drawings. Mostly the architects mention 3 to 4 inches of brick ballast.
Stage 3 Level compactor
Once brick ballast is laid, another compactor is used which is called level compactor. As its name suggests, it levels and evens up the uneven brick ballast surface making it ready for concreting.
Stage 4 Anti termite spray
At this stage anti termite medicine is sprayed on ground floor brick ballast only, quantity of which is kept strictly as prescribed on medicine being used.
Stage 4 Conduit
Any conduit and piping if required underground for electric, water or gas supply is completed at this stage. Incase floor that is to be concreted is external, it requires much more pipes as shown in picture below. In this case conduit is completed before laying brick ballast. Manholes for sewerage are also made before concreting and placing brick ballast.
Stage 5 Concrete
After completing all above steps, floor is finally concreted using Cement, sand(fine aggregate) and crush (course aggregate) in the ratio of 1:3:6 or as advised by the architect or engineer. Thickness of this concrete is kept in between 1.” to 2″ and placed with proper level marks. For protection against dampness and seepage from ground, make sure floor pcc is ready at bottom or the mid/centre line of DPC (damp proof course).