Seek the perfect precast concrete flat slab supplier? One of the only things a flat slab is supposed to do is carry the load of whatever is on it: a floor, roof or platform on the ground — and do it flat, level, without deflection problems the entire life of the structure.
Otherwise it’s not noticed.
If it doesn’t, then the issue is not the concept of precast slabs, its the precast slab supplier who designed and cast them.
This guide outlines the steps to take when assessing a precast concrete flat slab supplier to ensure that you receive slabs that perform structurally for many years, not slabs that cause deflection, cracking, or installation problems.
More than Thickness and Span Charts, Choosing a Supplier is a Complex Process
Slab thickness, standard span capacity and cost per square metre are usually the first factors that most buyers look at. That’s a good starting point, but it leaves out how the supplier reinforces, prestresses (if applicable), and how he controls quality so that all of the slabs in an order perform exactly as the design specs require them to do.
Flat slabs share a load that ceases to cease. They are subject to dead loading the structure provides and any live loading for the space, day in, day out, year after year, building life. A supplier who works slab production as a precision structural process will not just “make a flat piece of precast” but will have good control over the placement of the reinforcement, concrete strength, and camber, so it can be guaranteed. One that doesn’t will deliver slabs that will follow a spec sheet on paper and then show a mismatch in deflection, cracking, or camber when installed side by side.
If You Have a Project Where You Need a Flat Slab, You Will Need to Know Exactly What Type of Flat Slab You Are Looking For
It is helpful to understand the type of flat slab your project requires before you can begin comparing suppliers because not all precast manufacturers stock the entire range offered at the tolerance level that meets the code requirements for structural applications.
Solid Flat Slabs
Where maximum strength and minimal thickness are more important than weight, a single uniform concrete slab is cast as a solid flat slab, with no voids. Do not mistake these for hollow-core – they may be heavier per sqm, so be mindful of the lifting/transportation capacity from the outset.
Hollow-Core Flat Slabs
Hollow-core slabs have longitudinal voids to decrease weight as well as provide depth, typically used in floor and roof applications where weight and span are both of concern. They are very dependent on consistent void formation during casting, with inconsistent voids impacting strength and weight predictability.
Prestressed Flat Slabs
Prestressed slabs provide longer spans, smaller amounts of material than the conventional spans of the same material. These must be controlled in terms of how much of the prestress force is applied and when it is released, and these errors directly influence the camber and deflection performance over time.
Composite Flat Slabs
Combining precast speed and in-situ finish and added structural capacity, composite slabs are designed to work together with an in-situ topping poured on site following installation. They must be prepared for a good bond with the topping and detail the shear connection. They must be prepared so that they can bond well with the topping because a poor bond will compromise the composite action that the design is based on.
The knowledge of which category is suitable for your project’s span, load and weight requirements you can reduce the list of suppliers to those who have the experience of doing real structural slabs, not just flat panels which are a catalogue item.
Inquire about the Structural Design and Load Checking of Their Structures
Any supplier who is not just playing a game with flat slab manufacturing will be able to supply a full set of structural design data for every slab type in their product line, and not merely a span table without any accompanying calculation. This should include the design load capacity, reinforcement or prestress schedule behind that capacity and verification confirmation of the design verification.
Interrogate directly what the load capacity of a given slab is intended, how this is attained and if the published figures are backed up by independent structural verification or testing. You’re talking a supplier’s language when you get specific, documented information. If they only show a general span chart with no calculations, it would be prudent to ask them more questions especially if it is not the low load, simple application.
Discuss Their Reinforcement Placement and Concrete Cover Control
In particular, for flat slabs the accuracy of placing reinforcement will have a direct and measurable impact on the structural performance because even minor errors in the position of reinforcement (bars and cover) will have a significant influence on the actual capacity of the slab, as compared to the designed capacity. Inquire about the manner in which the supplier maintains the placement of reinforcement in the cast; the tolerance they have on concrete cover; and how they check for placement before and after the pour.
In order to maintain tight tolerances consistently throughout the production run, some suppliers employ fixed jigs for in-process checks. Others depend more on the experience of the operator and not formally verified. Both can yield satisfactory results, but you do want to know what the process is and whether there is one. This is one of the more difficult quality problems to spot visually once the slab has hardened.
Check on Their Manufacturing Quality Control and Camber Consistency
If the quality control in the manufacturing is not close, two slabs can be designed to the same specification and still differ in service. Questions include the unit’s casting method, curing process and length of time, and the inspection procedures that are followed prior to approval for dispatch, such as camber measurement for prestressed slabs.
The consistency of camber is more important for flat slabs than generally realized. Even if each slab maintains its design camber to specification, if it is installed side by side with a different camber, it will result in a visually uneven soffit or floor surface. Inquire about the camber tolerance the supplier has for the batch, not just for an individual unit.
Request Project-Specific Design Verification, Rather than a Catalogue Span
A competent supplier will be able to tell you that a slab type and thickness requested is appropriate for your project’s loading, span and support requirements, not the closest catalogue item. This should include checking for compliance with your actual design loads, confirmation of bearing and support detailing at both ends, and advice of any propping requirements before the structure is up to full design capacity during construction.
Suppliers dealing with the conditions of your project will detect the differences between catalogue conditions and what you are applying and avoid them before casting your slabs. If you don’t get a span table that the supplier is willing to test against your particular project they are only testing for the standard span table.
Compare Handling, Transport and Installation Requirements
Usually a heavy, large slab, installation requirements differ based on size and type. Inquire about the lifting and handling equipment the supplier’s slabs require, the temporary propping and/or support required during and after installation, and whether they offer guidance on how to install the product or work with contractors who have experience in installing their product.
Don’t forget to ask any questions regarding load application restrictions before the composite topping cures or when the design strength is achieved; one of the most frequent installation mistakes with flat slab systems is applying load prematurely.
Verify Their Track Record and References
Design data and reinforcement specifications are used to indicate what a supplier promises their slabs to do. Completed buildings are a reflection of their actual deliveries. Request case studies of flat slabs provided for other spans and load conditions, and their performance post installation.
For a supplier with a long history of slabs that perform as designed, with no deflection and cracking problems that have developed over time, a span chart tells much more about manufacturing discipline than it does about performance.
Warning Signs to Watch Out For
Less reliable flat slab suppliers will have a few basic warning signs found throughout:
- Vague answers to questions about tolerances in reinforcement placement.
- Unclear information about concrete cover tolerances.
- Reluctance to compare a slab to your project specific conditions.
- Price significantly lower than other companies with no explanation for the difference.
Each by itself is not an automatic disqualification of a supplier, but two or three of these together are a good argument to take a moment to reconsider the supplier and to engage your structural engineer directly before signing a contract.
Connect and Organize It All Together
The key to the right precast concrete flat slab supplier is confirming the precast supplier provides actual structural data with each slab, reinforcement and camber control, and is willing to develop a structural profile for your project to verify their precast flat slab against the actual loads and spans, not just a catalogue span table.
If you get it right, your slabs do what they are designed to do: They carry the load; They sit flat and level; They perform quietly throughout the life of the structure.
