Analysis of the cost of construction of a concrete slab in Kenya. Comparison between prefabricated and in situ.

Analysis of the cost of construction of a concrete slab in Kenya. Comparison between prefabricated and in situ.

Concrete slabs can either be constructed as prefabricated, meaning, they are fabricated off the site and transported to the site for erection.
Prefab concrete

They can also be cast in situ, meaning that a form work to retain the concrete mixture is set up and concrete is poured into the shape to form the slab.

Lets check out the costs for both situation.

In Situ concrete.


A concrete ratio of 1 to 3 to 6 for cement, sand and ballast produces class 15 concrete, mostly used in mass concrete to level out foundation footings.

A concrete ratio of 1 to 2 to 4 for cement, sand and ballast produces a much stronger class 20 concrete , mostly used for suspended slabs, beams and columns, which are self supporting.

In Kenya, the fundis and contractors mix 1 bag of cement to 3 wheelbarrows of sand to 4 wheelbarrows of ballast.

This is a ratio of 1 to 6 to 8.This is a weaker concrete strength than the recommended class 15 hence not suitable for high-rise buildings.

This can hold for light structures such as maisonettes. In the case where fundis and contractors don’t have engineering consultancy to guide them, they may mistakenly use the same ratio for a building with more than 2 storey levels, leading to building collapse.

In buildings higher than 2 storey levels, the fundis and contractors use buckets to measure.
The ratio is for 1.5 buckets of cement to 5 buckets of sand to 8 buckets of ballast for slabs and beams, which carry lesser weight than columns.
This makes out a ratio of 1 to 3.3 to 5.3, which is closer to class 15 concrete strength.

For columns, the cement ratio is increased to 2 buckets of cement , 5 buckets of sand and 8 buckets of ballast.
This makes out a ratio of 1 to 2.5 to 4, close to class 20 concrete strength recommended.

In this case, lets work out the costs using the recommended ratio of 1 to 2 to 4 for high rise structures.

We will assume a 1m2 surface of concrete slab, 150mm thick.

This surface has a volume of 1m x 1m x 0.15m=0.15m3.

This volume has the below volume of sand, cement and ballast.

Cement.

Cement volume=0.15m3 x 1/7=0.021 m3 of cement.
Weight of 0.021m3 of cement.
Cement density is assumed to be 1,500 kg per m3.
0.021m3 of cement will weight 32kg.

A 50kg bag of cement costs kes 700.
32kg of cement will cost kes 448.

Sand.
Sand volume=0.15m3 x 2/7=0.043 m3 of cement.
Weight of 0.043m3 of cement.
Sand density is assumed to be 2,000 kg per m3 when wet.
0.043m3 of sand will weight 86kg.

A tonne of sand costs kes 2000.
86kg of cement will cost kes 172.

Ballast.

Ballast volume=0.15m3 x 4/7=0.086 m3 of cement.
Weight of 0.086m3 of cement.
Ballast density is assumed to be 2,000 kg per m3 when wet.
0.086m3 of ballast will weight 172kg.

A tonne of ballast costs kes 2000.
172kg of cement will cost kes 344.

Total cost.
Total concrete cost will be cost of cement,sand ballast =kes 964 per m2 pf concrete.
Add cost of water and labor, say 30%=kes 1,250 per m2.

Add cost of steel reinforcement.
Assuming a bottom reinforcement of Y10 at 200 mm centre to centre and top reinforcement of Y8 at 200mm centre to centre, this will be as follows

Y10
12 Meters long @ kes 600 per 12 meter length.=kes 600

Y8
12 meters long at kes 400 per 12 meter lenght=kes 400

Cost of steel per m2 of slab=kes 1,000.

Cost of steel plus concrete=kes 2,250 per m2.

Cost of formwork.

Formwork can be hired or purchased.
In case of purchase, Grivellia timber is mostly used.

This is because the wood from this tree tends to warp and crack hence cheap compared to cypruss which is more expensive and more long lasting.

Grivellia wood 6 x 1 foot costs kes 21 compared to cypress at kes 34 per foot for the same length.

Grivellia can only be used for formwork once.

In a 1m2 slab, this will cost as below.
6 inches convert to 0.25m
1 foot converts to 0.3m
6 X 1 Timber has a surface of 0.075m2, costing kes 21.

1m2 surface will cost kes 280.
Add labour and nails, say 30%=kes 360 per m2.

The total cost of in situ concrete slab will now become kes 2,250 plus kes 360=kes 2,610 per m2.

Cost of prefabricated concrete slabs.

The selling price for eco slab prefabricated slabs is kes 1,600 per m2.Add cost of cement screed to level the surfaces and plaster at the bottom part of the slab. This estimates to kes 2000k per m2.

Conclusion.

Casting the slab in situ is at a higher cost than buying prefabricated slabs.

Francis Gichuhi Kamau, Architect.


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40 responses to “Analysis of the cost of construction of a concrete slab in Kenya. Comparison between prefabricated and in situ.”

  1. LESEDI RESOURCES Avatar
    LESEDI RESOURCES

    Morning George,
    Am looking for suppliers of precast slab in Kenya.
    Any leads?

    Regards,

    1. Gathoni Kimani Avatar
      Gathoni Kimani

      Hi

      Would you by any chance know where I could purchase bulk slubs for foundation and what could be the price range per slub?

      Am also looking for cheaper building leads

      Thanks & regards
      Gathoni

    2. Ahmed Avatar
      Ahmed

      Pre cast slabs also available all pre cast products culverts road kerbs, channel’s, idb ,fencing posts welcome.

      1. karuga Elizaphan Avatar
        karuga Elizaphan

        how much is one concrete fencing post?

        Kind regards,

        Karuga Elizaphan

        1. Bernard Nyaga Avatar
          Bernard Nyaga

          Hi,
          I manufacture concrete posts, concrete wall panels and other prefabricated concrete products.
          The prices for concrete fencing posts are as listed below:
          7ft @ 1450
          8ft @ 1588
          9ft @ 1661
          10ft @ 1260
          11ft @ 2256
          12ft @ 2277

          1. Heyzoal Avatar
            Heyzoal

            Hi please i nid more info on prefab wall

          2. Virginia Avatar
            Virginia

            what about concrete wall panels

          3. Nduta Avatar
            Nduta

            How much does a pre-fab concrete panel cost

          4. sam Avatar
            sam

            kindly WhatsApp me on 0713943000 we talk businesses

      2. Naim Avatar
        Naim

        Hi I’m interested can you send me more information please.

      3. Al Avatar
        Al

        Ahmed,

        I am doing some research prior to starting my project. My pre-fab slab will be 10m X 10m. Can you estimate a price?

    3. Parmenas Avatar

      Hello,
      At Silk Road International Trading Company, we import and sell precast slabs.

    4. Eng. Muriuki JM Avatar
      Eng. Muriuki JM

      In situ concreting is easier and simpler than precast coz, we don’t have machinery to do the work, so we adopt and adapt manual way

  2. Michael Muturi Avatar
    Michael Muturi

    Contact engineers ltd they have offices at ruiru

  3. sam Avatar

    How much u selling y8 n y 10

  4. Waringa kamau Avatar
    Waringa kamau

    Hi, what are the different types of masonry designs and and approximate cost?

  5. Edward Waboer Avatar
    Edward Waboer

    Can the prefabricated Slabs be used for internal walling?

  6. Waga Avatar
    Waga

    I want to put in situ slab 150mm. The area is 15ft by 25ft. What would be the tentative total cost including timber for shuttering. Is it cheaper to contract a fundi to deliver me a finished slab of the above dimensions because they normally have their timber for shuttering which they carry away after curing.

    1. Sammy Avatar
      Sammy

      You need approximately Kshs 1 Million for that.

  7. niRobinson Mog Avatar
    niRobinson Mog

    where can I buy the precast slab you have pictured above? and at hoe much.

  8. joseph kimari Avatar
    joseph kimari

    please can you advance where to interlock bricks in nairobi area their details and prices thank you

  9. Esau Kabui Avatar
    Esau Kabui

    Good advice architect. Such posts especially from Kenyans rare but much needed.
    I trained in Building Construction (Diploma) but over the years I have learnt my gifting and passion is in Architecture. I am looking for a place I can study in the evenings but no success yet. I want to join you in this practice. Any help or advice? Desperately needed.

    1. Rooney Muteti Avatar
      Rooney Muteti

      I’m studying Dip. Arch at N.I.T westlands. They offer two diploma options: one examined by KNEC which is a little lengthy and the other is offered by JKUAT. The mode of learning is FULL TIME due to the a lot of studio work involved.

  10. Wicliff Avatar
    Wicliff

    I really like you posts about construction as they really help me since I am also in the Architectural department but just a technician. I would really like for you to show on how one can get the cost for columns and beams per m3

  11. Geofrey Ndegwa Avatar

    we offer commercial and industrial scaffolding services in Kenya and throughout Eastern Africa. Whether you require a simple tower and access platform for a small building job or a complex scaffold system for a major building project, refinery, or mine site, we can create a scaffolding solution to meet your requirements, at very reasonable rates. As one of our scaffolding specialities, we have a vast inventory of scaffolding products and equipment available for projects of all sizes. We can also supply experienced scaffolders and project managers to ensure your scaffold is designed, erected, maintained, and dismantled safely and in a timely manner.

  12. Brian Kibiwot Avatar
    Brian Kibiwot

    Your calculation for the ratios are wrong. I think cement do noy contribute to volume….. so use 1/6 instead of 1/7

  13. DONALD Avatar
    DONALD

    Hi Francis,

    I like your article, it is very informative. Kindly advice where i can get prefabricated slabs within Nairobi at a good cost because i have a construction going on somewhere a long Kangundo road and my wish is to move in that house by 1st Sept,2016 which i think is too soon considering the fact that i have not even done a slab. Therefore my only hope to enable me meet this deadline is by using prefabricated slab which i do not know where to get a good quality one at a good price. Could you please advise .

    Thank you in advance

    Regards

    Donald

  14. Mary Avatar
    Mary

    I supply at the best prices:

    Y8 – Ksh. 450
    Y10 – Ksh. 650
    Y12 – Ksh. 900
    Y16 – Ksh. 1,500

  15. Roselyne Juma Avatar

    I would like to visit the place where the slabs are sold ,please direct me

  16. Antony Macharia Avatar
    Antony Macharia

    I think your analysis is great commendable job; however abit misleading heres why:
    1. The total cost of in situ concrete slab will now become kes 2,250 plus kes 360=kes 2,610 per m2.
    2. The selling price for eco slab prefabricated slabs is kes 1,600 per m2. Add cost of cement screed to level the surfaces and plaster at the bottom part of the slab. This estimates to kes 2000k per m2. (a 30% difference)

    You have not factored in form work mainly props at 600 spacing, 100x50mm timber bearers; beam-rib steelwork; concrete cover 75mm over precast and 150mm beam ribs and labour-the main items we save on is boarding/prapers) Noting also that new technology is usually expensive due to the slower learning curve and that the finished precast would cost abit more than concrete insitu) the comparative would tilt infavour of onsite concrete-ing for smaller projects/single slabs and the real benefits acrrue on larger sites due to reuse of materials.

    However with many more people venturing to alternative construction methods then the client can benefit from value addition based on competitive pricing.

    Tony
    Design-Build Contractor

  17. brooks nandwa Avatar
    brooks nandwa

    where can i get them

  18. CATHERINE Avatar
    CATHERINE

    HI,
    AM LOOKING FOR A PLACE TO SUPPLY THEM WITH CONSTRUCTION MATERIALS..IE, SAND, BALLAST,
    REGARDS
    CATHERINE

    1. Andree Avatar
      Andree

      How much cost Your materials supply within ruiru and kerugoya .. sand, hardcore, stone, ballast(kagoto).

  19. kingsley etafo Avatar
    kingsley etafo

    in construction site where to determine price of concrete 0 m level 15$ per m3 to 30 m level height of a building, what is price or rate? what % to apply.

  20. isaac Avatar
    isaac

    whats the current price of high beam 254*102?

  21. VINCENT KOECH Avatar
    VINCENT KOECH

    WHATS THE STANDARD LENGTH OF Y12 IN FEET. I SEE YOU USE METERS

  22. Arturo Petrizzo Avatar
    Arturo Petrizzo

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    informations a whole lot and I realised that
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  23. christine Nzelu Avatar

    Hi i would love to have your no.
    I have been cheated many times n i have lost alot of money.please

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