Oversite concrete or german floor avoidable problems

Oversite concrete is often reffered to as german floor by the laymen, others mistook it for dpc,however oversite concrete is the mass concrete that you pour to cover the entire lenght and breadth of a building on the ground floor to serve as a source of additional support to the structural stability of a building especially storey buildings.

Is the mass concrete which is cast after the dpc or damp proof course level has been established.
Going by the definitions above it is clear that this portion as part of a building foundation is one of the most important parts of the building.
Not long ago one of my subscribers asked me on how to salvage an oversite concrete that was sinking/collapsing, before then I had also experienced a poorly done job too that resulted in parts of the concrete collapsing, so with the first hand experience and for the benefits of those that haven't gotten to that stage of your building project is good you read on.

To have a good oversite concrete these are the steps you must take-

(1) Make sure the soil underneath/ or the soil used to fill the dpc is a laterite and not any other kinds of soil especially loose and peat soil. The filling requires sand that can be compacted to have a good sustainability and the ability to carry the compressive load of concrete coming on top of it. A loose soil in this case will not allow the soil to be easily rammed or beat to compaction hence won't be good for the task.

(2)If it is recommended to use wire mesh or brc by your Engineer then save yourself of future unending agonies with a few thousands of naira and apply it. The wire mesh forms a ring that helps the concrete to be able to carry its weight and even when the soil underneath is getting more compacted, because of the tensile strength of this wire mesh the concrete is most unlikely to cave in, so make sure you spread it round the building as specified.

(3)Also if it was specified in the drawing that you must use a membrane- just call it a polythenelike material that can be able to block water from rising from the soil to the ground floor or slab, then it is important you also do it. this is usually specified where the ground is waterlogged.

(4)Ensure you follow the right mixture ratio of the concrete, hence you may be faced with a not so strong concrete.

(5) Before the day you will cast, make sure you get all the materials you need in place human,materials and otherwise. If the numbers of people you have is not adequate then you may run into problem , some portion will be left till the next day and you may not have a cohesive single bonded foundation.

(6) Take a day get two bricklayers, let them take the proper level of the dpc round and make sure they apply plumb ( an instrument used in determining the level) round the building with the level established and traced with ropes round, when this is done you are most unlikely to have a badly built house. Foundation is the root of the beauty of any house just as it applies to every facet of life.

(7)When you have casted don't be in a hurry to remove the sideboard or wooden/iron materials that were used to support the concrete from running off the edge, it is good you give it a day to dry before removal else you will notice cracks around the edges when the boards are removed especially when the concrete mixture ratio is weak.
Make sure the iron rods of the columns/pillars coming from the base of the foundation is aligned and shoots out or longer than the top of the concrete by atleast 450mm or 0.45m or 9 inches.

Causes of Caving in /Collapsing oversite/german concrete
(i) When your contractor wants to make profit and save cost, the mixture ratio of the concrete may likely be weak

(ii)When your project has no capable professional directing the affairs on site

(iii)If you are careless or take no cognisance of little details when executing the project

(iv)Soil underneath not well rammed and when it is not a compacting soil that has ability to bond.

(v)During ramming water must be poured again and again until the soil can no longer compact before you stop.

(vi)When the sides of your structure are not well supported by wood or iron and brace properly.

(vii)Damages on site by workers

(viii)When inadequate or substandard materials are used

(ix)The hollow blocks under the concrete if thats what was recommended must be filled with concrete to give the concrete additional support and help to easily transfer the weight to the ground.

Above all, caution! caution! CAUTION!
hope this have helped.

How To Calculate Materials In Concrete Foundation.

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image 1


image 2





Using images 1 and 2 above and in a bid to also demystify the raft foundation cost, this article will deal extensively on how to calculate your concrete materials like cement, sand and granite that you will use for the foundation.
Often time concrete are mostly mixed in 1:2:4 ratio, when you see such specificationit simply means one bag of cement to four headpans of sand and eight headpans of granite.
Notice that first number represent a bag of cement which is equivalent to two headpans, hence the sand is taken as four headpanss and the granite represented by 4 in the ratio is taken as eight headpans.
Going by this knowledge it is easy for us to do a very fast calculation at least to be close to what we might need on site.
To do this calculation, add the sum of ratio 1+2+4=7
Calculate the cubic volume of concrete for foundation.
Going by the previous article the total lenght of trench was taken as 65.6m
Height of concrete beam is 1.1m or 1100m from image 2
And width is 0.225m or 225mm
therefore total volume of concrete beams =65.6m x 1.1 x 0.225 =16.236
Now divide 16.236 by the sum of the ratio which is 7
16.236/7=2.31943m3
Taking a constant, the volume of a bag of cement is 0.035m3
Divide 2.31943 by 0.035
=66 bags of cement to do all the concrete beams , however don't forget that cement and sand will also be used for the blinding before the concrete beams are placed .
For the sand reqd.,multiply 2.31943m3 by 2 to know the volume since it is in ratio of 1:2:4
Therefore volume of sand requires =4.63886m3
If we take volume of a tipper as 3.8m3 then
4.63886/3.8=1 1/2 trip of 5 tons tipper for the concrete beam.
For the granite use the same ratio of 1 bag of cement to 4 headpans of sand to 8 headpans of granite
So for granite reqd, multiply 2.31943m3 by 4 to know the volume since it is in the ratio of 1:2:4
therefore volume of granite requires=9.27772
If we take volume of a tipper as 3.8m3 then,
9.27772/3.8=2 1/2 trips of granite (Disregarding water and and other attending expenses
If one bag cost N1,800, then 66 bags =N118,800
And if one trip of sand is N15,000
then 1 1/2 trips =N22,500
And if one trip of 5 tons of granite=N30,000
then granite cost needed =2 1/2 x N30,000
=N75,000
Total=N118,800 + N22,500 + N75,000
=N216,300
For the german floor or oversite concrete-
Multiply the lenght by breadth by the thickness or width of the plan from image.
Add 3.825 +3.95 +1.375 +1.25+4.975+1.25=16.625
Add 3.825+2.910+3.825=10.560
So multiply the sum for the volume=16.625 x 10.560 x 0.2
=35.112m3
To know the number of cement required divide 35.112 by the sum of the ratio which is 7
=35.112 / 7
=5.016m3
Taking a constant, the volume of a bag of cement which is 0.035m3,
divide 5.016m3 by 0.035
=143 bags of cement for the german floor or oversite concrete.
Apply the steps above for the sand and granite,
For sand,
Multiply 5.016 by 2=10.032, now divide by 3.8m3
=3 trips of 5tons sharp sand
For granite,
multiply 5.016 by 4=20.064, now divide by 3.8m3
=5 1/2 trips of 5 tons granite
So if one bag of cement cost N1800
then 143 bags of cement will cost N1800 x 143 =N257,400
And if one trip of sand cost N15,000
then 3 trips of 5 tons sharp sand =N45,000
And if one trip of 5 tons granite cost =N30,000
then 5 1/2 trips of 5 tons granite =N165,000
So total for concrete excluding other attending materials
=N216,000+257,000+45,000+165,000
TOTAL=N683,400
This is hard to understand I believe but you can still study it by bits.