in-situ concrete slab casting Cost: do you get the value for your money

How much does it cost to do the concrete slab of your house? This is one of the most commonly nagging issues among present and future homeowners and developers in Kenya. Whether or not you are using a contractor and the level of finish for your slab should be up to par, which brings us to the amount of concrete you need for your project.

Determining the amount of concrete needed for your slab project should always be the first thing to do. The amount is determined in terms of volume, also known as dry volume. The dry volume occupies a smaller volume when compared to the mixed wet concrete again bringing us to calculating the number of materials needed for your slab.

calculating the number of materials needed for your slab.

Calculation of the materials such as the aggregate, sand, cement and water needed for your slab construction is crucial. Assume you need a flat slab for 100 square feet, based on your project dimensions. To calculate the number of materials you will need, we need to calculate the volume of your slab.

For instance, an area of 100 square feet is equal to 9.2903 square meters. Predominantly, the grade required for most floors is M 15, and the mixing ratio provided for this type of grade is 1:2:4 (Cement, Sand, Aggregate) and your slab thickness should be 150mm.

To know the number of materials needed, you need to find the dry volume of your slab. Your contract on-site should be able to give you the exact accurate quantity of materials for your project. There are also other things that you need to consider, such as material for shuttering, formwork, reinforcement, and labour.

Although preferred by many, the conventional slab construction method can be very challenging, especially when it comes to the cost estimation of your slab. The quotation procedure can be very tricky, plus do you get the value for your money? Not forgetting the logistics involved in the materials and general site constraints, including labour, these aspects can affect the quality of your structure, which means increased risk, headache, and issues.

So, let us break down three aspects that may affect the performance and cost of your slab:

Omitting or swapping materials

Your contractor knows you won’t be able to spot your slab grade differences, leave alone the construction material differences.

The ratio for conventional slab casting for M15 concrete is 1:3:6 for cement, sand, and ballast, respectively.

The recommendation ratio for class 20 concrete for columns, beams, and house floors, is 1:2:4 for cement, sand, and ballast, respectively as proposed by everything about concrete and a4architect.

Sadly, since most clients are oblivious of these factors, some contractors usually end up omitting materials or buying low-grade alternatives to cut on cost. Rather than using the high-grade alternatives that help reach the targeted slab strength on your building floor and slabs.

Another commonly swapped or omitted element is formwork. Instead of acquiring the precise types defined within their works specification document, some contractors reuse low-quality formwork remnants that may affect your slab curing, hence performance.

Labour Force

Many clients and developers in Kenya only deal with the architect, quantity surveyor, and principal contractor. Most rarely review their qualifications. With the workforce on-site, the matter is even worse. No client checks the credentials of the corresponding site workforce.

So instead of using skilled labourers, especially when it matters the most, they choose to hire cheap, unskilled labourers.

In the end, this affects the ultimate results of your slab, and you end up with a poor-quality slab.

But what if there was a way to know what you are paying for your project. Every coin is accounted for and escape further headaches and inconvenience down the road?

Inadequate mixing and curing

Poor concrete quality results in lower concrete strength, specifically, tensile strength. Incorrect water to cement ratio, inadequate concrete mixing, improper placement of concrete, and insufficient consolidation are factors that can jeopardize the concrete slab quality.

Precast beam and block flooring system

You might get a good idea of why a more innovative method might be a better alternative. It comes as a Precast beam and blocks flooring system. Unlike in situ, this newer flooring system is constructed uniquely. Think of it as like the railways, but only this time instead of waiting for a train, you use lightweight infill solid or hollow pot blocks.

The system is manufactured entirely off-site in a factory environment, with constant monitoring, strict moulding instructions, and quality control.

It’s an agreeable solution for many projects, especially if time is of the essence to your project. It can also be built to high tolerances and lowers long-term maintenance costs.


Still, weighing up your options? Most people are sceptical of new technologies until they’ve seen the product and how it works. 

It may be a good idea to seek a quantity surveyor engineer and contractor. So, they can take you through the other aspects of your project and give you a better idea of what’s possible. 

Still curious about how beam and block systems compare to the conventional slab method in terms of cost? Read more on beam and block flooring system and if it is cheaper than the traditional slab?

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