Elevating utility of buildings with hollow-core precast concrete slabs

Photo © Steve Evans

By Jack Laken, P.Eng., B.Sc.
With the increasing concerns of climate change, natural disasters, and energy disruptions, buildings need to be resilient. Architects and decision-makers are often on the lookout for new ways of enhancing building projects, particularly when it comes to comfort, energy efficiency, and safety. Concrete hollow-core slabs help establish value-added groundwork for a resilient building, as the material is well-known for its durability, resiliency, and thermal mass. Opting for factory-made precast concrete also ensures a consistent product throughout the entire structure. Historically used for multi-storey buildings, hollow cores are now being seen in an increased variety of projects.

These products are prestressed slabs of concrete, with circular openings forming hollow silos within. This opening runs the length of the slab, making the unit ideal for equipping electrical wiring or mechanical ventilation. Highly versatile, hollow-core slabs can also be altered to include plumbing and even sprinkler systems within the building—useful for high-rise apartment buildings, classrooms, and institutions requiring one- to two-hour fire ratings between floors. Typically 1.2 m (4 ft) in width and 9.1 m (30 ft) in length, these 203- to 254-mm (8- to 10-in.) thick units are ideal for schools, industrial buildings, offices, hotels, and multi-storey apartments.

Why use hollow-core slabs?
Precast hollow-core concrete planks require transportation to deliver and cranes to install, which can be a roadblock for some builders. However, traditionally, poured concrete and other flooring systems come with their own sets of challenges, such as interruptions and delays due to poor weather conditions, increased labour time, and inconsistent forms. As hollow-core slabs are manufactured indoors in a factory-controlled environment, they increase efficiency, providing a consistent form for builders to use. They can be delivered and installed in any weather.

Dr. Frank J. Hayden School (Burlington, Ont.) benefits from increased ventilation and energy storage through use of hollow-core slabs.
Photo courtesy Svedas Architects

As an additional benefit, the voids in hollow concrete slabs not only reduce the amount of material used, but also make it lighter than non-hollowed precast forms. Using less raw material, precast hollow-core slabs make a positive addition to a project’s sustainability.

Furthermore, integrating hollow-core planks with a building’s conventional HVAC system can leverage the naturally occurring thermal properties that are in dormant concrete floors. This means more sustainable and efficient heating and cooling methods can be incorporated with no need for new components. Simply combining hollow-core precast and pre-existing or readily available HVAC equipment will suffice.

These preformed units also enable builders to respond to tight deadlines, as their congruency makes them easy to place. Given the fact these slabs are formed in a controlled environment, builders can be assured high-quality products are delivered to the construction site. This means hollow-core concrete slabs can evade some of the challenges common with poured concrete—for example, their consistent quality allows for quick installation, with some builders installing up to 929 m2 (10,000 sf) a day.

While many buildings implementing hollow-core slabs are designed to be rectangular (e.g. schools, institutions, and multi-level complexes), the units can also be adapted onsite to unconventional forms such as curves and sloped roofs if they are saw-cut with diamond blades.

Activating thermal mass of concrete
Using concrete in commercial, institutional, and residential buildings is ideal for low-energy design, and for allowing a building to absorb and store energy, thanks to the material’s high thermal mass. This quality also means a large amount of energy is required to alter concrete’s temperature, making it ideal for a building where the goal is to maintain a consistent interior environment. Concrete works well in passive structures that use an alternative energy source (e.g. solar or geothermal), as well as in active buildings integrating hollow-core slabs with the HVAC system to store energy.

A hollow-core concrete building’s thermal mass also makes it more resilient than lighter structures, allowing it to withstand extreme temperature changes and maintain comfort even during failure of heating or cooling systems. The large surface area of these slabs helps preserve the building’s core temperature for an extended period of time. The volumetric thermal capacity of concrete with a density of 2300 kg/m3 (143 lb/cf) is 2.07 MJ/m3 per K
(30.89 Btu/cf per F). In other words, if a building requires the temperature of 1 m3 (35 cf) concrete to be raised to 1 K (1.8 F), it requires approximately 575 W-h (1962 Btu). The benefit of this is if the HVAC system fails, the building can still function and remain comfortable.

In fact, the heat capacity of precast hollow-core slabs is estimated to be about 100 W-h/m2 per K (17.6 Btu/sf per F). In a conventional system (e.g. steel studs and drywall), the thermal exchange between a room’s air and walls has a heat-transfer value of only about 10 W/m2 per K(1.76 Btu/hr/sf per F). (It is important to note the first units are energy or heat capacity of the hollow-core slab, shown in Btu or W-h. The second is the energy transfer rate, shown in Btu/hr or W.)

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