Hollow Core Slab Section

[Amalia Antill]

Asystem of floors, roofs and terraces, amongst other things, made of slabs with continuous hollow cells, is an alternative to that of concrete poured in place. The use of hollow-core slabs is not restricted to the commercial market. It applies to most types of situations in which larges clearances and heavy design loads are needed.

he slab section has continuous cells and a concrete area paired with minimal overlap of the framework, guaranteeing fire resistance of at least two hours. In addition, the cells allow a 30% reduction in weight compared to a solid section.

I have brought in a Hollowcore beam family (Insert > Load Family > Structural Framing > Precast Concrete > Precast-Hollowcore Slab) and created a simple beam system using the imported family. I then tried to cut an opening through the beam system (Structure > Opening - By Face) and tried to have the opening span several slabs. Unfortunately it only cuts through one slab only and not the rest of them. I even tried using the Shaft option but no luck.

Currently we model HC slabs as a floor in Revit and that makes it easy to create any openings on it. Is there a better way to cut voids in the HC planks if it is modelled as a beam system using the built in HC family?

I am fairly new to Revit and have used it only for architectural modeling so far. I'm trying to model the construction of a project that contains hollow-core slabs. I have managed to model and split the floor (with the precast extension) but the voids and profiles are not showing in the sections (screenshot added).

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Choosing the right materials is key in the construction world, especially for parking buildings where durability and efficiency matter. Precast concrete is becoming the go-to for these projects because it's quick to install and lasts a long time. We sat down with Mangesh Dhumal , a pro in precast concrete, to compare two top choices: double-tee and hollow-core slabs. This chat digs into their costs, upkeep, and how to pick the best one for your needs. Dive in for essential tips straight from an expert and find out why precast could be perfect for your next parking project!

Mangesh: Absolutely. Precast concrete technology offers significant benefits, such as speed of construction, dimensional accuracy, and reduced labor requirements on-site. These advantages make it an excellent choice for quickly developing robust and durable parking structures.

Mangesh: Double-tee slabs are incredibly strong and durable, thanks to their T-shaped cross-section. This makes them ideal for supporting heavy vehicle loads, a common requirement in parking structures. They also offer excellent fire resistance. When prestressed, they can achieve greater load capacity and help reduce slab thickness, which is a huge plus.

Mangesh: Their solid construction means they are quite heavy, which might require heavier support structures and foundations. Also, their design offers less flexibility in terms of span lengths and configurations compared to hollow-core slabs.

Mangesh: Certainly. Hollow-core slabs are lighter, thanks to the voids in their construction, reducing loads on support structures and potentially saving costs. They offer more design flexibility, improved acoustics between parking levels, and similar to double-tees, benefit significantly from prestressing.

Mangesh: Yes, the presence of voids means they generally have a relatively lower load capacity and require be thicker than double-tees for equivalent loads. Fire resistance is also lower. Plus, the voids can increase the potential for cracking under certain conditions.

Mangesh: The choice really hinges on your project's specific needs. You need to decide based on load requirements, fire safety, budget constraints, and the benefits of prestressing. Consulting with experienced structural engineers is always the key to making the best decision for your project's success.

The idea to reduce the self-weight of concrete slabs by putting voids in the centre of the cross-section, dates to the beginning of the previous century. Several inventors from different countries applied for patents on various systems. The present article is mainly based on an analysis of patents published during the first half of the 20th century, and personal experiences from 1960 on. Patents usually offer a complex description of inventions (claims). Reconstructing the history of hollow core slabs based on these patents is a laborious but fascinating exercise. This article aims to give a general overview and is not meant to be exhaustive.

In general, these manufacturing methods can be used for the production of reinforced slabs as well as for the production of pre-stressed slabs. They are mostly in normal dense concrete, but there are also examples of structural light weight concrete.

In the early days, hollow core slabs were manufactured eitherin a plant, or on site. Often individual moulds were used and sometimes even long line beds, but in a discontinuous way. The compaction of the concrete was mostly carried out by tamping the fresh concrete. Here also a patent study could bring more insight, but it is not the main subject of the present article.

The most characteristic feature in the development of hollow core slabs was that they deviated strongly from the at that time existing design principles of reinforced and prestressed concrete, by which compression is taken up by concrete and tension by reinforcement. Indeed, in most cases, the developed manufacturing technique was only possible under the following conditions:

As a consequence, the tensile capacity of the concrete had to be taken into account in the design and new techniques for connections had to be developed. This was new especially with regard to transfer of forces at the support, shear capacity of the units, diaphragm action of the floors, transversal load distribution among adjacent units, non-rigid supports, floor openings, fire resistance, etc.

With regard to prestressed hollow core slabs, the fib Commission on Prefabrication played a crucial role in the development of the design. Extensive research and intensive field experience gathered from all over the world, learned that hollow core floors are perfectly able to fulfil all the needed structural functions, on condition that some elementary design principles are met. In 1988 the FIP Commission on Prefabrication published Recommendations for the design of prestressed hollow core floors. They have been used as a basis for national and international standards, for example the Eurocode 2 and the European CEN Product Standard EN 1168. An updated version of the FIB Recommendations 1988 will be published this year.

Wilhelm Siegler (Germany, 1906) can probably claim the first application of longitudinal void formers in concrete slabs [3]. His system to realize cores was based on prefabricated short moulding tubes in hardened mortar or another material, which were positioned on a scaffolding (Fig. 2). The length of the slabs was arbitrary. The tubes had lateral lugs at the bottom, serving as mould for the webs. They were placed either continuously in the longitudinal direction, or with short inter-distances at certain places to form transversal ribs. The longitudinal and transversal webs were reinforced in the classical way.

The question could be raised about the distinction between hollow core elements and box elements. The above variants still correspond to the definition of hollow core slabs given afore, but from a certain thickness on, they are to be classified as box slabs or beams. By the way, the inventors of the solutions of Table 1 are in the first place claiming for floor slabs, although they do not exclude in the patent description the applicability for box beams or even walls.

Today, this production technique is rather rare but is still used. After pouring a bottom layer, prismatic void formers, usually in polystyrene, are installed. Afterwards, a second layer of concrete is poured to shape the webs and the top layer.

In 1930, a patent is granted to the Belgian inventor Jules Heyneman for a precast floor slab with longitudinal voids [10]. These voids are formed by means of elastic moulds made of e.g. steel and held in place by wedges. When these wedges are removed, the cross-section of this mould is reduced, and the mould can be removed from the hollows in the beam without difficulty. Unfortunately, the drawings of the patent contain no details about these void formers. The number of voids in the cross-section could be modified. The floor units were in reinforced concrete. The patent describes mainly the product itself, without any detail about the manufacturing. The longitudinal joints between the units are indented and provided with transversal reinforcing stirrups. They were filled on site with mortar.

The inconvenience of the solution was of course the weakness of the flexible steel pipes. In 1939 a solution with pneumatic expansible and collapsible rubber core forms was patented by Walter H. Cobi (US) [11]. Fig. 4 shows a longitudinal

and transversal section of the system.

Charles Lethbridge (GB) [12] presented in 1940 an improved method with removable steel tubes of uniform cross-section extending longitudinally through the whole mould and conforming in shape to the cross-sectional form of the hollow core unit. After positioning of the desired reinforcement bars, the concrete was cast, and the mould vibrated as a whole. At the same time the core tubes were slightly moved relatively to the mould. When the concrete was sufficiently compacted to maintain its shape, the tubes were withdrawn via the end of the mould and the concrete was left to harden. By the employment of metal core members with a smooth surface and maintaining them in motion, the concrete was prevented from adhering to the tubes and the latter could be removed without difficulty. Preferably and for simplicity the core tubes were of circular cross-section which allowed for a rotary motion during casting.

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