How are in-situ cast suspended concrete slabs made?

First, supporting brickwork has to be built up to the slab’s soffit level. Then we install temporary propping to support the deck. Props are placed between 1.2m and 1.5m apart, and are all connected by ledgers. This will give us a secure structure to build the deck on.
After the props are erected, we start to build the deck. The deck mostly consists of steel decking panels / shutters, and this will hold the wet concrete in place while it is curing.
On top of the deck, we then install a mat of steel reinforcing bars. The spacing and diameter of the reinforcing is calculated by a structural engineer, and differs on every project due to various factors.
This is such an important part of concrete slabs that the engineer has to inspect and approve the reinforcing after it is installed, before any concrete is cast.
After the reinforcing is in place, the plumber and electrician has an opportunity to install pipes and conduit that will be cast inside the slab.
This eliminates the need for ceilings under the slab, and helps reduce the construction costs.
When everything is in place, and the engineer approves of the reinforcing, it is time to place the concrete. Concrete consists of specific quantities of small stones, sand, cement and water, depending on the required strength. Ready-mix concrete is mixed at a batch plant near the site and is delivered to site in concrete mixers mounted on trucks. The plant has precise mixing recipes than ensures the final strength much more accurately that conventional mixing, and it’s all done much faster than as well.
On most sites, the level at which the concrete is discharged from the mixer trucks will be lower than the deck. When this is the case, the concrete has to be pumped to where it is needed.
The concrete will be placed on the deck with exact thicknesses that was also calculated by the engineer that designed the reinforcing. The top surface of the concrete is normally leveled to a rough finish, which is ideal for a floor covering such as tiles.
When the floor finish will be carpet or something that requires a smoother finish, the installer or main contractor can easily screed the top surface. The top surface can also be powerfloated, which will give it a very smooth finish. This is generally done in areas that will have exposed concrete with no other floor finish, such as in garages or where the client prefer to have an exposed concrete finish.
After the propping and the deck is removed, the slab will remain in place, supported on the walls underneath it.
 

What else do you do other than slabs?

We do all types of formwork for all types of concrete structures, such as columns, beams, staircases, ground beams, raft foundations and surface beds. Please feel free to contact any of our consultants for assistance or queries regarding this.
 

How long does it take to construct a slab?

This obviously greatly depends on the size of the slab. Other factors that can influence the construction time are the shape of the slab (rounded edges takes longer than straight edges); the size, shape and amount of beams (if required); and the site location and access. On a small to average size slab, the propping and deck will generally take three or four days, the reinforcing a day or two, and then the engineers inspection, electrical and plumbing will generally happen in the following day or two. This means most of these slabs will be ready to pour within a week to ten days from starting with the project. However: all projects differ, and we recommend you consult your agent about an estimated construction time when a quotation is issued.
 

How long does the propping have to remain in place to support the slab?

The props has to remain in place until the slab can support it’s own weight. Concrete curing times may vary and will be influenced by the amount of water in the concrete, the strength required, and weather conditions, such as continues rain or extremely cold or hot conditions. Generally the temporary support will be removed 10 to 14 days after the concrete is cast, but this can be increased by the structural engineer if the design requires.
 

In which areas do you work?

Although we are based in Gauteng, we have hubs covering most of Southern Africa. Feel free to contact one of our consultants if you are unsure about your area.
 

How long does it take before you can start with my slab?

This depends on how many projects we are busy with at the time, and can vary from one week up to as long as six weeks. Thus we recommend you start getting quotations and decide on a slab contractor as soon as you start with construction. Also, having an engineer appointed from the start of the project could save thousands by optimizing the layout to save costs on the slab, as well as prevent having to break down parts of walls to accommodate the slab.
 

Should I appoint an engineer myself?

We have a team of engineers that will design and inspect the slab and supply an engineer’s certificate. The engineer will design the structure to be safe, and the lowest construction cost. All municipalities/councils require an engineer to oversee all projects. It is thus ideal to have one engineer to assist with the entire project to design all structural components, from the foundations to the slab. This is why more and more owners appoint an independent engineer themselves, or involve the slab contractor early in the project to benefit from the engineer’s assistance. If you already have an engineer appointed, the engineer’s cost will not be included in your quotation, so please advise your consultant in advance.
 

What will I need to do during the whole process, and who do I need to appoint?

We require all supporting brickwork to be level and built up to the slab’s soffit level, so the client will probably require a bricklayer. If the project involves electrical work and plumbing, as most projects do, the client will also need to appoint these specialists. As explained above, pipes and electrical conduit can be hidden inside the slab, if they are in place before concrete is cast, so it is ideal to have these specialists appointed early in the project. Due to our construction method, the slab soffit will not have a perfectly smooth finish. If the client prefer, it can be plastered and painted. The top surface may also be screeded, as stated above. Curing of the concrete is a very easy process, but is very important, and is also the responsibility of the client. This can all probably be done by the bricklayer/main contractor. Roof slabs and open balconies may require to be waterproofed. This should be done by a specialist. Because of the extent of these items, many owners that aren’t familiar with the construction process decide to appoint an experienced main contractor to handle all these items.
 

What should we consider then planning our house or building?

The factor that mostly influence the cost of a slab, is the floor layout. The slab thickness can be reduced and the need for beams can be eliminated by limiting the distance between loadbearing walls on the ground floor. Large open spaces without supporting walls will increase the slab thickness, and large openings in walls, such as for double garage doors, may require beams. All of these increase costs. Ask your architect or draftsman to contact any of our consultants for more information or assistance.
 

So what does the slab cost?

The cost of the slab can vary from R300/m² to R1000/m² or more, depending on the design, the size of the slab, and distance from any of our storage yards. The only way we can give a competitive, accurate quotation, is to do a preliminary design of the slab and to price it accordingly. To do this, we will require a copy of the drawings, or at least a copy of the ground floor and first floor plan. We do quotations free of charge, and they normally are ready within a day or two. Large projects might take a bit longer, enquire when you submit the drawings.
Drawings can be sent to quotes@firstfloorslabs.co.za, or faxed to 086 558 3819. If you are unable to send the drawings to either of these contacts, please contact one of our consultants for assistance.
 

What if I don’t have drawings?

Don’t give up hope, we can still assist you. Please contact one of our consultants for assistance.
 

What if I’m building on top of an existing floor or extending an existing building? Does this change things?

When building on top of an existing ground floor, it is important to check that the existing walls will be able to carry the extra weight. This includes the foundations, and depending on the circumstances, the engineer may require an inspection of the existing foundations or additional support may be required. One of our consultants should be able to assist you with this.
Keep in mind that erecting a slab involves wet concrete and heavy equipment, and although we take great care to protect existing finishes, we cannot guarantee this. The best is to remove items such as built in cupboards and other fittings, before any work starts. Some floor finishes may also need to be replaced after the slab is finished.
Extending a building outside its existing footprint is a much easier process. Slabs can easily be joined to existing structures, and it can then be done before any finishes are installed, preventing the risk of damaging them. When requesting your quotation, please inform your consultant if any parts of the building are existing.
 

If you have any other queries not covered above, please feel free to contact our office for assistance.

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