Oversite concrete or german floor avoidable problems

oversite concrete

Membrane

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.
Brace

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.

An architect with decades of experience

26 Comments

  1. Thanks a lot.I hope building engineers will take note of this so as to save lives and properties.

  2. Thanks alot to the host of this web side. This is a job well done by the article writer(s). Atleast even a novis can take precaution when building his house. What a self explanatary article. With this expo, u’ve saved lives and properties. Keep it up. Nagode kwarai.

  3. What is the difference between the oversite done by iron rod and the ones done by mare stones as being seen nowadays in some filling stations. Most filling stations dont use iron rod now but stone from quary. Pls educate me

    1. Quarry stones have very high bearing capacity thats why you see them been used in filling station. Because of the effect of expansion and contraction in concrete You need to have a mass that can easily give room for those effects.
      Much fixed loads are on oversite concrete in a building which is not so in an open space there is no much transfer of weight like what you have in buildings.The iron rods stands as tensile strenght for a compressive weight of concrete. However filling station is open to the earth which only needs the forces applied on concrete to be transfered under.

  4. Your doing a wonderful job…please keep it up. I have a question…I’d like to know if creating blocks on the site is a better option than buying from professional block moulders. I feel it would be cheaper, but the question (which should be the determining factor) is that is it stronger/better?

    1. Depends on you buying may be faster, but if you are thinking of a very large project it could be cheaper to mould yours.

  5. Thank you for the job you are doing on this forumn. Please in the setting of an oversite concrete, at what stage will putting a membrane (the polythine cover) come in. Thank you.

  6. do i need to put sand in a claye foundation before pouring foundation concrete?,,,generally how do i treat a claye foundation

  7. Hi
    Practice in Africa are totally different from Europe.

    build a house is an expensive project, money should be spent on the foundations, weak foundation you have problems.

    Here the mason will say the ground is to hard or it’s difficult to dig, wants more money. your footings should be a min of 36″ or .900 from the ground level, the concrete should be a min of 9″ or .225mm.
    Using steps must be planned, ideal the floor level should be level, no silly little steps in it,(these are in because the mason is lazy can’t be bothers to bring extra fill in, the tiler should not leave little steps, a step should be a min of 6″ or .150mm high to 7″ or .180 mm high finished level.

    The best practices for build here are after you have dug the footing and build the footing blocks is compact in layers the fill, starting with the space between the footings and the block work, DO NOT fill more then a block high, then compact using 2″x4″.
    Then when you get to the soil hight, again spread the fill evenly over the area and compact again no more than ta block in hight, compact preferable with a compactor plate,or use hard wood as a rammer, do this until you fill the fill to the top of the block work.

    the floor level should be at min of .300mm or 12″ above the finished ground level.

    Because of the low density of the sand-create block it’s advisable to put a layer of plastic sheet on top of the fill.

    Shutter the sides of the building get the top level, there are many ways to do this, water level laser, dummpy level, a spirit level is not accurate (you must know how to check a spirit level to make sure it is reading true, New ones are not true, It’s not level simply because the bubble is in the middle).

    Depending on the thickness of concrete, you have 2 options, 6″ or .150mm thick concrete can be laid at the same level across the floor, if you wanted to save a little money and use 4″ or .100mm thickness, you can dig around the inside of the block to a depth of 3″ deep and 9″ wide, chamfer the edge, (lay your plastic sheet) and pour your concrete, this give extra strength to the edges. (like a pudding raft design)

    It’s not necessary to use re-bars for the floor, depends on a Number of things why you use re-bar, if you do use re-bar specification are required for the use of re-bar, basically it must be a min of 2″ or .50mm in from the out side of the concrete, whether on the floor or beam that is exposed to the outside, on an internal wall a min of 1″ or .25 is acceptable
    Not like in the pic where the re-bar is on the ground unsupported.

    Stone: if your using cracked stone it is worth paying a little bit more to get cracked stone that is about 3/4″ .019mm to 1″ or .025mm, using larger stone with weaken the concrete as it doesn’t compact will crack.

    Mixing concrete DO NOT USE TO MUCH WATER it weakens the concrete. the masons like it because it’s easier to handle, remember your paying for it.

    Do not give the mason all the money, buy your materials, then give him small amount if he doesn’t comply with your requests, get someone else, especially if the mason starts to argue. (I say to anybody who I get to work for me, I’ll pay them providing they do a 95% correct job, if it’s not then I don’t pay them they should then redo it as there expense then I’ll pay them. (that way you find out who are good at their job, those who don’t agree beware)

    for the inside of the house get the mason to steel float the floor, this seal it better than tamped finish, if you can afford buy don’t get the dust. if you want to paint the floor use oil base paint, not as good as proper floor paint it’s works.

  8. Oversite concrete is not a structural member of a building structure. It is just a “mass of concrete”. However, it serves the purposes of 1)keeping in place the layer of a Damp Proof Course laid on compacted soil and 2) providing a hard surface for ground loads that include finishes and furniture. It neither provides any stability to the structure nor any form of ties to the columns.
    To convert this mass concrete to a stabilising member, reinforcement must be at least of 10mm mesh. This way, it can now be referred to as “reinforced ground slab”.

  9. I have taken time to go through your blog, I think you are doing a great job. Your posts are most useful to environmentalists and dangerous to a larger part of the public who are not conversant with the basic knowledge your are sharing and would want to venture into doing these things themselves believing you have given them enough exposure to this. By experience people tend to be inquisitive in things that have to do with building. I would advice that this blog be made a specialised one where people who are into the field will subscribe and get these information rather than how it is been shared. Remember this is a rough world where any thing is possible. Another thing I notice is you feed the public with secret of contractor but never advise them to seek professional help when doing these. Thanks, your post remains a helpful one.

  10. MY NAME IS ALI GEORGE,YOU MY ROLE MODEL IN BUILDING AND CONSTRUCTIONS WORK.GOD BLESS YOU IN JESUS NAME...BECAUSE YOU MADE UNDERSTAND THAT BUILDING IS ALSO AN ASPECT ANOTHER KIND OF LIFE.(e.g;family needs good foundation for their children's upbringing and says:

    WHAT QUANTITY OF CEMENT CAN BE USE FOR A GOOD CONCRETE RATIO OF BUNGARLO FLAT OF 1BEDROOM-PALOR AND KITCHEN,TOILET.

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  12. You have been a source of blessing to millions that Include myself as a Pastor I pronounce Gods mercy into your life am about to start a project this is an eye opener I will surely use a professional engineer but will supervised it myself once more I will like to say a Biggggggg thank you may the presence of God never leaves you ijn. Amen

  13. Morning, pls want to know if it’s normal to cast a floor for more than 3days. If so, what difficulties can be experience. I mean will it cracked on the long run.

  14. Do I need to place 50mm thick sharp sand on the compacted laterite before spreading the water proof membrane, blind with 50mm thick 1:3:6 concrete (leave for a day to set), spread BRC and cast fully (1:2:4)to a thickness of 200mm?. In the sequence stated. The area is uphill, dry and hard. Abaranje, Ikotun Lagos

  15. Good One Buh I Think U Should Make Dis For Only D Expertries They Can Even Subcribe To Get D Info To Avoid Quacks Diving Into It Thinking They Knw Wat U R Saying And By So Doing Putting Pple’s Lives On D Line.

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