On every construction project, at some point, there is a question that needs to be answered: solid precast slabs or hollow core? If you select the wrong one, you are spending more money than necessary for weight you didn’t require, or you are specing for loads you actually did require. So neither is a good result. The happy part of this is that while there is a bit of mystery, when you know what each of these slab types does — and what it doesn’t do — the answer isn’t too hard to figure out. With this guide you will have all the facts with real numbers to help you make a decision for your particular project.
What are Solid precast slabs?
Solid precast slabs are as the name suggests — solid concrete panels that have been molded in a factory-like setting. No voids, no cores. Concrete and reinforcement throughout the entire cross-section.
This continuous mass is what makes solid slabs so unique in terms of their characteristics: Impact and vibration resistance, very high fire resistance, and compressive strength. They are the ones you’ll want to use when you really require brute structural reliability — such as in heavy load platforms, foundation elements, industrial floors, or anything else that receives a rough treatment every day.
Other irregular layout and complex geometries are also easily achieved with solid precast slabs. The geometry of internal voids is not a problem for the engineering or architectural changes made to the dimensions, embed connections, or accommodate non-standard layouts, as the entire cross section is concrete.
Hollow Core Slabs are what?
Hollow core slabs are precast, prestressed concrete slabs in which there are continuous longitudinal voids through the entire length of each slab. These voids decrease the quantity of concrete in the cross-section — and thus the weight — without sacrificing much in terms of structural performance.
The cores are made in the interior of the product. The standard widths are approximately 1200 mm and slab thickness varies from 110 mm to 500 mm as per the application. The major engineering trick, however, is called “prestressing,” and it involves putting the slab under tension using very strong steel tendons before they set so that it is compressed and can withstand loads across long spans of material more effectively.
Hollow core slabs are commonly employed in multi-storey residential buildings, commercial offices, education, healthcare buildings, parking structures and industrial warehouses for floors. It is not an empty platitude, their versatility is real.
The Core Difference: Weight and What It Means for Your Project
That’s where things get interesting. The weight of hollow core slabs is about 30-50% less than that of solid slabs of the same size. This isn’t some footnote. It affects your entire project in a cascading manner.
Lighter slabs result in lighter loads on beams, columns and foundations. All of that translates to smaller supporting members, less concrete and steel, and lower material costs, and a leaner overall structure. The savings will add up as levels increase in multi-storey buildings.
Solid slabs, on the other hand, have greater mass. This can be bad for foundation loading and seismic response, but it can also be good for the dead load, impact resistance, and acoustic density. The mass can do useful work in an appropriate context.
The benefits of hollow core with regards to span performance.The advantages of hollow core for span performance
Typical floor loading conditions: Hollow core spans from 5 metres to 20 metres with typical floor loading conditions and can span up to 23 metres with toppings and specific design. In the PCI Manual for the Design of Hollow Core Slabs, the standard span-to-depth ratio used by engineers is 45:1.
However, because of their weight, solid precast slabs will have more self-weight over the span. This reduces the amount of live load supported per span length, thus requiring additional support points or a deeper slab (adding to the cost).
Hollow core spans are ideal for projects with large open floor plate areas with repetitive bays such as parking garages, apartment buildings and office floors. An open, flexible area is difficult to duplicate economically in terms of solid slabs.
Details Matter: Fire Resistance – Both Perform Well
In any occupied building, fire resistance is a must. Both types of slabs offer some measure of fire protection, but there are differences to be aware of.
The fire rating can be up to 4 hours for hollow core slabs according to EN 1992-1-2, depending on the thickness of the slab, type of aggregate and concrete cover. An 8 in. hollow core slab will provide a 2 hour rating. With proper detailing a 12” slab can take 3 hours.
Notably, fire performance of hollow core slabs is correlated with the critical fire temperature of the prestressing strands, published PCI research has determined to be 427°C. The detailing of cover depth is as important as the slab thickness itself because this is what will ensure strands will remain protected in a fire event.
Solid precast slabs have a continuous mass and no internal voids, and are therefore very strong and fire resistant. Their high density results in fewer variables in the equation of fire.
The acoustic performance is a tangible difference-maker.
This is one of the comparisons that is not often brought to the fore in early project discussions but that can be very loud (literally) after the project has been completed.
Hollow core slabs have a Sound Transmission Class (STC) rating of 47 to 57, depending on slab thickness and topping. Hollow core floors have a STC rating of 50 or higher and are suitable for multi-storey apartments, schools, hotels and commercial buildings where sound transmission between storeys is a real concern of the occupants.
A higher mass and density result in higher sound insulation properties of solid precast slabs than hollow core for the same thickness. Among the most reliable sound attenuators in building physics is mass. For applications that require acoustic performance to be a key design objective — for a recording studio, a courtroom, hospital — solid slabs are worth their added weight.
When building projects are repetitive, Hollow Core wins!
The study carried out by the International Journal of Advanced Engineering, Sciences and Applications has shown that precast slab construction requires about 9 working days while cast-in-situ construction requires about 31 working days. This is a huge programme benefit! In particular, hollow core is made for quick installation.
Hollow core slab can be installed by a specialist installation crew in a 1-day period for 5,000 to 7,000 square feet of slab. Slabs are delivered pre-cut, pre-stressed and ready to install. Once they are installed on site, you just raise them in place, fill in the joints with grout, and continue along. No formwork to set up, no waiting for concrete to set, no stripping of shuttering.
Precast slabs also enjoy the many advantages of precast construction over cast-in-situ construction, but are heavier and will necessitate larger lifting equipment and careful sequencing.
Whereas, the cost of a project is a factor that depends on the type of project
It is difficult to determine which slab type is less expensive. It depends on the application.
For long spans, repetitive structures such as a residential building or a parking structure, hollow core slabs tend to be cheaper by the square metre. Small savings can result in big savings – in this case, significant savings in the amount of concrete, plus quicker installation and savings on a smaller supporting structure. Research has documented that the overall cost of the project can be reduced by a significant amount if the project is constructed using precast materials in the proper setting compared to cast-in-situ alternatives.
The price of solid precast slabs is higher because they have more concrete. In cases of short spans and high loading, however, the direct strength of a solid slab may make it easier to design the structural scheme than to reinforce or construct using composite methods and thus compensate for the extra cost of the materials.
The simple answer is “hollow core is almost always the more economical choice if you have a repetitive project, regular bays and need spans over 6 metres. When your project uses irregular layouts, has high point loads or requires high acoustic and impact requirements, solid precast slabs may be worthwhile.
The best locations for each type of slab are indicated:
Consider use of solid precast slabs for industrial floors supporting heavy machinery, high point loads, forklift traffic or when the floor space is not regular enough to utilize the repetitive geometry of hollow core; foundation or ground bearing applications; structures requiring maximum acoustic mass; and layouts that are too irregular in shape to utilize the repetitive geometry of hollow core.
In residential, commercial or educational buildings when frequent bays of floor are to be used, when long clear spans in open plan schemes are required, when parking structures are planned and when programme speed is paramount, or when weight saving is required to reduce foundation and structure costs, then hollow core slabs are to be considered.
The advantages of Hollow Core over Solid Precast Slabs are summarized.A direct summary of the advantages of Hollow Core over Solid Precast Slabs
The key questions regarding solid precast slabs or hollow core are: What loads are you moving? What is the length of your spans? But how repetitious is your arrangement?
For the regular multi-storey buildings, the most prevalent and economical solution is hollow core. It is the most popular floor choice for a relatively regular environment because of its weight property and long span and quick installation. In applications that require maximum structural density, impact resistance or acoustic performance, sound precast slabs stay on the ground.
There is no one that is better than the other. They have different purposes, different functions and different jobs. Know what your project needs and find the right slab for the job, and you’ll never regret it!
