At some point, all construction projects have to make the following choice: solid precast or hollow core? Do you get it wrong, you end up spending on weight that you didn’t need or specing on loads that you did have. Both of these are not desirable. The bad news is that the only way to make that decision is to know exactly what each of the slab types does, and where they fail. This guide provides you with an overview of the real numbers to make the appropriate decision for your project.
So What Are Solid Precast Slabs?
Solid precast slabs are as the name implies solid concrete slabs made in a factory with no cores or voids inside the slab. The concrete and reinforcement are continuous along the whole cross section.
The continuous mass is the source of the slabs’ unique features: high resistance to impact and vibration, high fire resistance and high compressive strength. They’re the correct choice when a work project truly requires sheer structural strength, such as heavy-load platforms, foundation elements, industrial floors, or any surface that is subjected to harsh daily wear and tear.
An irregular layout or a complex geometry are also no problem with solid precast slabs. No internal void geometry to work around, so engineers and architects can modify dimensions, embed connections, or fit a nonstandard layout.
Hollow Core Slabs Are What?
Precast, prestressed concrete slabs with continuous longitudinal voids throughout the slab are hollow core slabs. Those voids not only reduce the amount of concrete in the cross section (and thus the overall weight) but without sacrificing much in the way of structural performance.
The cores are located on the inside of the slab. Standard widths are approximately 1200mm and thickness are usually in the range 110mm to 500mm, depending on the application. The secret of the engineering is to “pre” stress it: high-tensile steel tendons are pulled taut before the concrete hardens, and thereby give the resulting slab a much greater load carrying capability over long spans than an unstressed slab.
Hollow core slabs are a common sight in multi-story residential structures, offices, schools and hospitals, parking garages and industrial warehouses, etc. It’s not a sales claim, it is true versatility.
This Is the Most Important Difference When It Comes to Your Project
The true difference is here. Hollow core slabs are lighter than solid slabs of the same size, ranging from 30-50%. It’s not a trivial detail, but it trickles through the entire project.
Lighter slabs result in lighter loads being transferred to beams, columns and foundations. This means smaller supporting members, less concrete, lower material costs and a smaller, end to end structure. If the building is a multi-storey one, those savings are cumulative with each additional storey.
While heavier slabs may not be desirable for foundation loading and seismic response, they can be beneficial when the effects of dead load, impact resistance, or acoustic density are more important. There is no negative side to mass; it all depends.
The Key Is That Hollow Core Has a Head Up in the Trenches When It Comes to Span Performance
Hollow core slabs will easily support spans of 5 to 20 metres under standard loading conditions, and can support up to 23 metres with toppings and special engineering. An industry standard span-to-depth ratio is 45:1, according to the PCI Manual for the Design of Hollow Core Slabs.
The higher the self weight on the same span, the more the load capacity is absorbed by the precast slabs’ own weight, which means typically that the precast slabs require additional support points or a deeper (and more expensive) slab to obtain similar spans.
When considering the cost-economics, hollow core is a real contender for large open floor space projects and repetitive structural bays, such as parking garages, apartment buildings and office floors. The open, flexible floor plate is difficult to cost-effectively replicate in solid slabs.
Fire Resistance: Both Perform Well, With Different Considerations
In a building that is occupied, there is no choice of slab type and both types provide meaningful protection, but in different ways.
According to EN 1992-1-2, hollow core slabs can be used to provide up to 4 hours fire resistance depending upon the thickness, type of aggregate and the concrete cover. An 8 inch hollow core slab usually provides 2 hours and a 12 inch slab properly detailed can provide 3 hours.
Importantly, the critical temperature of the prestressing strands is closely related to the cover depth detailing, and this is just as important as the slab thickness requirements as it is what will save the strands in a fire event; PCI published research indicates the critical value is 427°C.
Precast slabs are solid slabs with no internal voids, and inherently provide strong, direct fire resistance — there are fewer variables in a fire event, because of the high density.
Acoustic Performance: A Real World Advantage
This comparison is not usually heard at the outset of project conversations, but it’s very evident — quite tangible — once it is a reality to be lived in.
The Sound Transmission Class (STC) rating for hollow core slabs is usually between 47 and 57, depending on the thickness and topping material. The rating of 50 or above is generally acceptable in multi-storey apartment, school, hotel, and commercial building applications where sound transmission between floors is a genuine concern of the occupier.
The high mass and density of solid precast slabs, in general, will provide superior sound insulation performance compared to hollow core given the equivalent thickness — mass is one of the most effective means of sound insulation in building physics. Where acoustics are a true design concern, such as recording studios, courtrooms, hospitals, the added weight of a solid slab is more than compensated for.
When It Comes to Repetitive Projects, Hollow Core Really Shines
When it comes to repetitive projects, Hollow Core really shines.
The research work published in International Journal of Advanced Engineering, Sciences and Applications indicated that the precast slab construction took about 9 working days on average, while the equivalent cast in situ construction took about 31 working days, which is a significant programme benefit. Hollow core is especially designed to be installed quickly.
The typical installation crew can install 5,000-7,000 square feet of hollow core slab per day. Slabs are pre-cut and pre-stressed and ready to lift into place, grout the joints and move on. No formwork to build, no curing time for the concrete to dry, no stripping of the shuttering after it’s set.
Precast slabs typically share the same popular benefits of precast construction as cast-in-situ, but they are heavier and thus can require heavier lifting equipment and installation sequencing than the lighter hollow core slabs.
It’s a Cost That Truly Depends on the Application
The answer for which type of slab is lower cost depends solely on the application.
However, for long span, repetitive structures such as residential buildings, parking structures, hollow core slabs are more competitive per m2. Less concrete, quicker installation, and smaller structure support systems are significant savings throughout the project. Satisfactory cost reduction using precast building elements, relative to cast-in-situ methods, has been demonstrated in a variety of studies over the years when they are used in appropriate circumstances.
Problems with cost of the precast slabs exist because they are more expensive initially due to a higher volume of concrete used. However, for short spans that are heavily loaded the intrinsic strength of a solid slab can simplify the design considerably, eliminating the need for extra reinforcement or composite construction techniques which can be used, which could counterbalance the increased price of the slab.
The compact form: hollow core is nearly always more cost effective when bays and spans are regular and the project is repetitive, and where the span exceeds 6 metres. The cost premium for solid precast slabs is justified in situations where layouts are irregular, point loads are high, or acoustic and impact performance are really important.
This Table Shows the Best Location for Each Slab Type
Use solid precast slabs for industrial floors with heavy machinery, high point loads or forklift traffic; for foundation or ground-bearing applications; for structures where maximum acoustic mass is required; and for layouts that are too irregular to utilize the repeated geometry of hollow core.
Use hollow core slabs for repetitive floor bays in residential, commercial or educational structures, for long clear spans in open plan layouts, for parking structures, and on any project where programme speed or reducing foundation and structural loading are key factors.
The Bottom Line on Solid Precast Slabs
The important questions are: What kinds of loads are you putting on, what is the length of your spans, and how often are your runs repetitive?
The most commonly used and often the most cost-effective solution for typical multi-storey buildings is hollow core, as this material is well suited to relatively regular building environments due to its weight advantage, long span capabilities and quick installation. Solid precast slabs are the superior selection for applications requiring the highest structural density, impact resistance or acoustic performance.
There is no clear winner or loser regarding these two choices. They are designed for various work tasks. Know what your project requires and match your slab to it and you’ll thank us later.