Author: A4KENYA

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.

Building Materials in Kenya.

1.What are the most common building materials used in Kenya?

For walls, we have machine cut quarry stones and cement.
For roofs, we have gauge 30 and gauge 32 steel roofing sheets, concrete and clay tiles and stone coated steel roofing tiles.
For windows, we have steel casement frames with glass infill. Aluminium framing is also increasingly becoming common.
For doors, we have timber doors on wooden frames for internal doors. For external doors we have steel doors on steel frames and hardwood doors on hardwood frames.
For internal finishes, we have cement plaster and paint.
For ceilings, we have chipboard, uPVC.

2.What causes buildings to collapse with reference to building materials?

Collapsing of buildings is usually a structural integrity issue. Structural integrity has the building materials and labour/engineering consultancy aspect.
Building can collapse if the labour and or engineering consultancy is not certified to offer such service hence poor quality.
Buildings can collapse also if the materials used , mainly cement, sand, ballast and steel, are not of the specified quality.
In case of cement, the cement could be weak, hence not strong enough when contractors use the usual mix ratios to make the building structure.

In case of sand, the sand could be flaky, as in the case of the dark Naivasha sand, hence not offering the much needed grip as in the case of the granulated yellow river sand.
In the case of ballast, the size could be too big to offer the much needed ratio strength.
In the case of steel, the quality of steel may be too malleable or too brittle, hence not offering the needed strength requirements.
All these are to be checked by registered structural engineers and Kenya Bureau of Standards, in the case of cement and steel, which require tests to ascertain quality.

Most cement sold in Kenya has the mark of 32.5 Kn/mm2 as the strength. Its up to Kenya Bureau of Standards to carry out random periodic test strengths to ensure these companies actually sell cement of the said strength.

In the case of steel, I have never seen any tests done by Kenya Bureau of Standards or if KBS have ever carried or do carry out random periodic steel malleability tests, they don’t advertise to the public. Same case with cement strength. In this case, the public will therefore need to carry out their private tests to confirm this,or KBS carries these tests out publicly for all the public to see and benefit.
In my hands on experience on day to day workings with structural steel, I come across some steel which is too brittle and breaks easily when fundis are working on it .

3.How do local building materials promote the structure of a building?

Some local building materials, eg quarry stone is structurally strong, hence increasing the structural strength of the building.

4.What types of local building materials can be used for construction?

Local building materials are machine cut masonry stone, timber doors, clay roofing tiles, locally manufactured paints, locally manufactured ceramic and concrete wall and floor tiles, locally manufactured ceiling boards etc…

5.Why are local building materials not often used in Kenya?

In some instances, some people believe imported materials , which tend to be more expensive due to high cost of shipping and high tax, are better quality. For example, stone coated roofing sheets are imported from America, Europe ans Asia into Kenya. These are imitations of clay roof tiles. These costs kes 2000 per m2 while natural clay tiles cost kes 600 per m2. The reason for the high cost is because of import tax, high transport costs and cost of factory manufacturing. This high cost is not necessarily a translation of higher quality.

Another example is mazeras slate stone compared to ceramic tile look alike. Mazeras slate stone is better quality at a much lower cost. An immitation of this sells for twice as much as the real tile since some people tend to equate quality with price.

Mazeras tile imitation.

Real Mazeras tile

6.What are the advantages and disadvantages of using local materials for building

Advantages of using local materials is that they cost less, are of higher quality and help to improve economy and create jobs.
Disadvantages are that in some instances, some clients are willing to pay high rents or buy apartment units only if buildings are made of imported materials such as mdf boards, textured paints etc. hence imported materials results to higher rents.

7.Are local building materials expensive to obtain?

Local building materials are not expensive to obtain.For example, cost of steel Structural Insulated Panels SIP used to construct prefab units is the cheapest from of housing in China.

For this cheapest method of constructing in China to be brought to Kenya, if you add the cost of shipping, import duty and transport/storage to Nairobi, the SIP prefab panels will actually be more expensive than local masonry stone.
This means that the local materials for a particular region are only cheap if used there. If transported to another region, they cease to become cheap.

Francis Gichuhi Kamau, Architect.
info@a4architect.com

RETURN ON INVESTMENT FOR KASARANI ,SANTON AREA 1 BEDROOMED AND BEDSITTER UNITS.

1 bdm	area	number	rent	rent for 3 units	annual rent	construction cost	ROI
	29	3	10000	30000	360,000.00	2,175,000.00	6.041667
Bedsitter	area	number	rent	rent for 3 units	annual rent	construction cost	ROI
	19	3	6000	18000	216,000.00	1,425,000.00	6.597222
http://muthaiga.olx.co.ke/clean-spacious-1-brm-flat-15-000-iid-562086153
http://muthaiga.olx.co.ke/spacious-bed-sitter-to-let-iid-561988705

 

Conclusion,
The Return on Investment at an average of 6 years is impresive. This means a lowering of rent to enable competitiveness can be put in place if need and demand/supply arises.

Francis Gichuhi Kamau, Architect.

In Kenya, particularly in Nairobi, the cost of housing is high mainly due to high bank interest rates and high land value appreciation.
See below for details on this

http://www.a4architect.com/2013/10/18/housing-costs-in-kenya-why-cost-of-acquiring-a-home-in-kenya-is-high/”
“http://www.a4architect.com/2013/07/12/7cost-of-housing-in-kenya-kitengela-vs-south-africa-594/”

Employees in formal employment can have the advantage of huge bank interest rate discounts.

With the huge bank interest rate discount, they can then stand to gain from lower housing costs than average.

On the issue of land, ingenuine methods to resolve this problem have been sought by a4architect.com in terms of pooling resources and sharing land.
The minimum land size permitted for subdivision under Kenyan laws is 1/8th acre. This land size is quite large for a single family considering many families are currently living in flats with no outdoor space at all.

With 2 or more people coming together to share a 1/8th acre plot, the spiralling land cost issue can be resolved since this makes land more affordable.

Employees in formal employment need to be as close to urban centres as possible so as to make it in time to and from work.

The solution to majority of employees is to rent houses close to urban centres e.g. Kikuyu,Kitengela, Ngong, Kiambu etc.

With their savings, they buy land in inaccessible places where they cant use it to live and effectively make it to and from work in time. This means the plots bought in far away places like Isinya, Kamulu,Joska, Juja farm, Kiserian pipeline, Limuru,Thika etc. are unusable for lving purposes hence the money used to purchase the plot not put into good use.

To resolve this, a4architect.com assists employees to buy expensive land near tows eg Kitengela, Syokimau, whereby an average of 2 people share a 1/8th plot and construct 2 separate houses for each owner. This gives alot of privacy and room for future expansion.

Ownership.
To enable 2 people to legally won a 1/8th acre title, these 2 will need to jointly purchase the land whereby both their names appear on the title deed or they both register a limited liability company which has 2 shares then each owns a share of the company.

A4architect.com will design 2 separate houses to the design requirement of each owner then this is taken to the local authority for approval. The ownership document then comprises of the house designs , describing how both owners should live together, similar to the sectional properties act which allows people to buy apartments , common around Nairobi neighbourhoods eg Lavington.
Estimated cost of construction and initial designs can be viewed here http://www.a4architect.com/house-designs/”

Forum.

Someone can start a discussion forum topic here

Someone can put their preferred area and preferred land budget eg Kitengela 1/8th acre selling for kes 2m.
Other interested people can then also register and reply to the posting saying they are interested. Once a quorum is reached, if its 2 people sharing a 1/8th plot, they can then go ahead and purchase the plot after due diligence.
a4arhitect.com will then design the 2 houses and submit the drawings to the local authority for approval.
After approval, a4architect will then advice the 2 land owners on how to choose a contractor to build.

After selection of a contractor, the bank laid procedures of disbursement fo construction funds to the contractor shall be used and a4architect.com will supervise the construction till completion.

Francis Gichuhi Kamau, Architect.
info@a4architect.com