Joint-free ceilings are now possible with prestressed slabs

PRECAST CONCRETE ELEMENTS Abeo, 2640 Hedehusene, Denmark

Joint-free ceilings are now possible with prestressed slabs

When prestressed concrete slabs are used in residential building projects, a disadvantage has always been the creation of visible ceiling joints between adjoining slabs.

These joints are often criticized for giving an industrial look and have to a varying degree posed a barrier for precasters looking to supply prestressed slabs to the residential market. In a new development project, the Danish company Abeo has created a highly innovative solution for making “concealed joints� between SL-decks.

Challenges to solve

A natural consequence when using precast/prestressed slabs in residential building projects has long been the creation of visible ceiling joints between adjoining slabs. In a number of countries, especially the Nordics, the visibility of these joints has for decades been accepted by residents, architects and the markets in general. However, in many other countries, such joints have historically inhibited the sale of prestressed slabs to residential projects, where they are often associated with having an industrial and ghetto-like look. As a result, many of these markets have continued to rely on half slab systems with grouted joints or fully cast-in place solutions without visible joints.

2) Even if it is possible to eliminate or reduce such height differences, so the joint between two slabs may be spackled continuously, cracks will likely develop over time breaking the paint on the ceiling surface.

This fact motivated the Danish precast technology company Abeo back in 2017 to start a development project aimed at ďŹ nding solutions for creating at and joint-free ceiling surfaces with its prestressed SL-deck system.

At the start of the project, the team identiďŹ ed two main problems that had to be solved: 1) The use of prestressed strands in concrete slabs naturally produces differential camber, which as a side effect might also create differences in heights between adjoining slabs.

Solution A number of different ideas to solve the ďŹ rst problem were tested during the project, many of which sought to control and/or reduce the differential camber. In the end, however, the best solution was found with inspiration from the gypsum board industry, where the standard method for concealing joints between gypsum boards on walls and ceilings has for decades been the use of a wider joint design.

The team carried out a number of tests on SL-decks and found that by introducing a recess in the bottom surface of

The SL-deck’s joint-free solution has been developed with inspiration from the gypsum board industry. By introducing a recess – similar to that of gypsum boards – in the bottom surface of the slab along the joint, it is possible to visually smoothen out height differences between slabs.

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PRECAST CONCRETE ELEMENTS the slab along the joint, it was indeed possible to visually smoothen out height differences between slabs. Tests with joints of varying dimensions showed the optimal joint recess dimension to be around 5mm deep and 250 mm wide on each side (i.e. 500 mm in total over two slabs), as this made it possible to level out the most typical camber differences, while still being narrow enough for a painter to use a single 600 mm wide scraper for the process.

The second problem was also solved by looking to the gypsum board industry. By adding a self-adhesive fiber mesh to the joint before it is filled with spackling paste, visible cracks did not develop. In practice, what happens on a microlevel is that one large visible crack is replaced with numerous invisible micro cracks that will not break a normal coat of paint.

First reference projects Following the initial tests, the team proceeded to set up several mock-ups as well as applying the solution in a single-family housing project to monitor potential crack formation over a period of 12 months. As tests were progressing successfully, dialogue was initiated with a number of potential customers concerning application on a broader scale in multi-story residential projects and row housing projects.

Alexander Wulff, co-CEO at Abeo, says: “Based on our test results, we decided the best way forward was to do a soft launch to a select number of relevant customers here in Denmark. The reception of the solution was extremely positive, especially from architects and developers, who were very interested in the high-quality aesthetic look that could now be obtained without the costly solution of suspended ceilings. It didn’t take many months before we had the first two orders signed for larger projects, where we could demonstrate the solution in a real environment.”

The first projects utilizing Abeo’s “joint-free” solution are both located in and around the capital city of Copenhagen and were successfully completed during the second half of 2019. Abeo now plans to roll out the solution on a broader scale through its network of SL-deck producers, and based on the initial feedback from market participants, the company feels positively about the market opportunities.

“The development of the joint-free solution is a continuation of our development programmes, in which we are focused on making new solutions that add value to contractors as well as improve the competitiveness of precast concrete producers.

This solution is for some markets solving an old technical problem and could prove a key in opening up the growing residential market. In other markets, it will function as a cost-efficient way for creating joint-free ceilings without the use of suspended ceilings,” says Alexander Wulff.

SL-decks – towards more flexible slab production As a technology- and development-oriented company, Abeo is fast approaching its 10-year anniversary with an impressive list of innovations already being used by the precast industry. The company was formally established in 2010 to commercialize a new technology invented at the Technical University of Denmark making it possible to replace app. 50% of the heavy concrete in a structure with lightweight concrete – all without reducing the load-bearing capabilities. Based on this technology, the company has developed the SL-deck system, which was introduced to the market by the Danish precast group, IBF, in 2014.

In short, SL-decks are a wide and flexible precast slab system with the main feature being its combination of self-compacting concrete and light-aggregate concrete.

Lightweight concrete is casted as arch shaped blocks in the bottom of the slab,

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The SL-deck’s joint-free solution has been developed with inspiration from the gypsum board industry and is based on adding a 5mm deep and 250mm wide recess in each side of the SL-deck creating a 500mm wide recess across two slabs to be spackled.

A self-adhesive fiber mesh is added to the joint before it is filled with spackling paste in order to prevent potential cracks from occurring.

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while self-compacting concrete makes up the rest of the slab, thereby constituting a series of reinforced crossing ribs that are stabilized and protected by the light-aggregate concrete.

high sound and fire insulation properties, while being very cost-efficient to install due to the large slab width of up to 3.6-meter”.

Technical manager at Abeo, Morten Rasmussen, explains:

Cast-in and value-adding solutions

“The SL-deck exploits the flexibility of self-compacting concrete with the low weight and fantastic insulating properties of light-aggregate concrete. This gives a light slab capable of providing the flexibility of cast-in place structures at the speed and cost of a 100% precast slab. In addition, the slabs have

From the start, the key focus point for Abeo has been to develop a flexible slab system capable of incorporating cast-in and other value adding solutions such as the concealed joints feature for better competing with cast-in place solutions and half slabs. And with residential construction becoming an

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The recess is spackled a second time to visually smoothen out camber differences between two SL-decks.

The goal of the SL-deck’s new joint-free solution is to make precast concrete more competitive in multi-level residential construction and help further drive the adoption of total precast solutions.

increasingly important market segment to precasters around the world, the company is seeing a rising demand for such solutions in many markets. Alexander Wulff says: “Precast concrete is generally accepted to be the fastest and cheapest way to build. However, in residential construction, precast systems have in many markets had a difficult time competing with the more flexible half slabs and in-situ systems. With the high flexibility of SL-decks and the possibility of now making joint-free ceilings, we aim to make precast producers more competitive in the residential segment based on total precast building solutions”.

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Today SL-decks are often delivered with various cast-in items that are integrated in the slabs already at the factory. Among the most popular solutions are cast-in bathroom drains, castin hangers, floor heating systems, balcony fixing system etc.

Besides reducing on-site building costs for contractors, a benefit for the precast industry is that these solutions transfer a larger part of the work from the construction site to the factory and thus offer precasters a much larger slice of the economic pie compared to the supply of composite slab systems.

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The SL-deck combines the strength of reinforced concrete with the low weight of light-aggregate concrete resulting in a very wide and flexible slab that can be produced in widths of up to 3.6-meter.

With up to 3.6-meter wide elements and the possibility of making “double-long” SL-decks in projects with shorter spans, installation costs can be significantly reduced.

single element. E.g. instead of having 6-meter long slabs, SLdecks can be delivered as 12-meter long units with continuous span over a supporting wall. In combination with slabs widths of up to 3.6-meter, this means that installation costs and time consumption for precast structures are lower than ever before.

Expanding internationally Another widespread trend is towards wider and larger slabs that reduce the number of expensive crane lifts and joints to be casted on-site. In Denmark, the IBF group is especially seeing an increasing demand for its so-called “double-long” solution, where SL-decks are spanning over 2 or 3 rooms with a 6

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Following the successful market launch in Denmark, Abeo is today building up an international network of SL-deck producers, who share experiences, best practices, solution libraries and development projects such as the “concealed joint” solution through a licensing model.

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PRECAST CONCRETE ELEMENTS In Finland, the market leading producer, Lujabetoni, in 2018 started production of SL-decks under the Finnish brand name “Superlaatta” with great response from the market. Superlaatta production capacity was sold out in just a few months after product introduction, so Lujabetoni already at the beginning of 2019 opened up a new and much bigger SL-deck production facility close to Helsinki. CEO of Lujabetoni, Mikko Isotalo, says: “We presented the SL-deck to the market in December 2017 and our full-year production capacity was already sold out.

We wanted to quickly increase our capacity to our most important target market, the residential market in the Helsinki metropolitan area. With the investment, we now add hundreds of thousands m2 of annual Superlaatta (SL-deck) capacity to the Finnish market.”

In the USA, one of the leading precast groups, Tindall Corporation, is currently in the process of establishing two SL-deck factories in South Carolina and Virginia for production of 3.6meter wide SL-decks. CEO and President of Tindall, Greg Force says: “The SL-deck is a perfect complement to our systems approach to building construction, and I personally believe it will help further drive the adoption of total precast construction in the residential, commercial, and institutional sectors.

This is one of the most well-thought-out products that I’ve encountered during my nearly forty years in this industry. Tindall and Abeo share similar mindsets, placing creativity and progressive thinking at the forefront, and we are very much looking forward to the partnership.”

SL-deck production in the USA is expected to start in the spring of 2020 under Tindall’s own brand name called T-slab.

FURTHER INFORMATION

Abeo Beredskabsvej 12 2640 Hedehusene, Denmark T +45 71990976 info@abeo.dk www.abeo.dk

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