RENOVATING WITH UHPC PART II: PLATES AND COLUMNS

Renovating with Ultra High Performance Concrete

RENOVATION WITH UHPC

In our latest post, we started a mini-series on renovating with Ultra High Performance Concrete (UHPC). This is the second of the three posts and describes some of the challenges related to adding balconies using slabs and columns - both regular columns and L-columns.

In the previous post, we also provided a general introduction to renovation, so we will skip that this time and jump straight into the theme of slabs and columns.

DIFFERENT STARTING POINTS

The existing conditions vary from project to project: From the old building with limited load-bearing capacity and a facade that the owner wants to preserve, to the newer building with large structural capacity requiring a complete aesthetic makeover.

In connection with solutions involving slabs and columns, the options can be roughly divided into three principles:

Freestanding straight columns (where the placement can vary)
L-columns (with the column standing by the facade, or standing freely)
Bolted columns (attached to the facade, either hanging at each floor, or supported by foundations)

WHY ULTRA HIGH PERFORMANCE CONCRETE?

The above considerations are general and work just as well for steel, regular concrete, or wood. So what does Ultra High Performance Concrete have to offer?

Ultra High Performance Concrete offers durability, fire resistance, and aesthetic freedom to shape and design the appearance of balconies. But first and foremost, Ultra High Performance Concrete offers these properties in combination with limited dimensions - and thus low loads.

By combining the reduced load with columns in Ultra High Performance Concrete and L-columns, it is possible to keep the vertical loads from the balconies on the existing building to an absolute minimum - and keep horizontal loads at a manageable level.

The first principle - freestanding straight columns, makes it possible to add large external balconies to a building without significant vertical loads on the existing structure - which is illustrated in the image from PM in Brønderslev, which you can find at the top of this post. Below are pictures of the original building with niche balconies, which were transformed into indoor spaces during the renovation. New balconies in Ultra High Performance Concrete were placed outside, and insulation and thin cladding panels were added to complete the aesthetic transformation.

The biggest advantage of Ultra High Performance Concrete in this case is the low weight and slender design (the slabs are only 80 to 100 mm with a 110 mm visible edge as a design feature), and the few challenges because the tolerances could be handled in the joints inside the new facade. Columns and slabs in Ultra High Performance Concrete can also be used with brick facades as shown below. Due to the slender design (60 to 70 mm), it was sufficient to remove just 2 layers of bricks to handle the brackets that were cast on-site to attach the balconies to the front wall.

Housing association BM in Brønderslev, Denmark - original building on the left, view from the new CRC i2® balconies on the right.

LOAD-BEARING L-COLUMNS IN UHPC

An alternative to straight columns is load-bearing L-columns in Ultra High Performance Concrete, which support the vertical load. To the left is an example from Lunden in Brøndby, where good horizontal anchoring was possible.

Most of the vertical load is transferred to the L-columns and ends in new point foundations, and the moment in the L-columns is transferred to the building through brackets as a horizontal reaction - the details are shown below. After installation, the facade was insulated and a layer of plaster was added, transforming the building's appearance and almost hiding the L-columns.

In this case, the use of UHPC solved the challenge of fire protection with the limited space available and kept dimensions and thus loads, brackets, etc., within reasonable limits.

The biggest challenges were tolerances, combining the identical prefabricated elements for 305 balconies with the different dimensions of the old buildings, which should have been very similar, but in reality, varied significantly. In this regard, the narrow dimensions of the Ultra High Performance Concrete elements were not a particular advantage.

BOLTED-ON COLUMNS

The third option is bolted-on columns, where the balconies are attached with brackets and stiffeners. The columns - or vertical beams - are usually steel frames but can also be made of Ultra High Performance Concrete. They can either be hidden inside an extra layer on the facade or be visible, as in the example from Oslo below - here with steel frames and CRC i2® balconies. Another example is the Catharina Bridge in the Netherlands.

In this case, the biggest challenge is, of course, attaching the vertical beam sections to the existing wall - once this is done, the rest is quite simple. Both handling tolerances and achieving sufficient strength in the attachment are difficult, and Ultra High Performance Concrete contributes in this case by keeping the loads to a minimum when a concrete slab is needed.