RENOVATING WITH UHPC

Renovation with Ultra High Performance Concrete: Part I - Glued Connections

A mock-up balcony on an old building is installed using a glued connection: Ved Volden in Copenhagen, Denmark.

RENOVATION WITH UHPC

In many of the previous blog posts, we have shown various ways to use Ultra High Performance Concrete, but most examples have been new construction projects.

In a series of three blog posts - this being the first, we will illustrate some of the benefits, but also some of the challenges of using Ultra High Performance Concrete (UHPC) in renovation projects.

The first post in this mini-series will address glued connections in balcony renovations, but first, we will start with a brief and general introduction to renovation.

GENERAL CHALLENGES WITH RENOVATION

All renovation projects are difficult, which can be due to:

Uncertainty about the geometry of the existing building (lack of accurate drawings and unknown tolerances, etc.)
Limitations in capacity for (additional) loads (and uncertainty about the actual strength of the structure)
An existing aesthetic appearance that may or may not need to be considered
Tenants/owners and neighbors, who all want to experience as little inconvenience as possible
Often, there are narrow exterior areas available to establish the construction site, store materials, and access the building - including working time restrictions due to traffic, etc.

All this leads to a cost/benefit assessment for the building owner between renovation on one hand, and demolition and new construction on the other.

In some cases, the building is listed for historical or architectural reasons, and in those cases, the only option is often to renovate or rebuild it. It is possible to repair the structure, but in many cases, this will only be a temporary solution that buys time - it does not solve the problem with the structure, indoor climate, or whatever else may be the reason for needing to renovate the building.

RENOVATION PROJECTS INVOLVING BALCONIES

When considering renovating, extending, or adding balconies to an existing structure, the reserve for unused structural capacity is crucial, and in this regard, the building type is important.

Newer buildings made of concrete or steel will often have some surplus capacity and are rarely subject to protection, which leaves more options for attaching balconies.

However, older brick buildings (from the 1930s and 40s or even older) have the general challenge that the walls cannot support too much load - both vertically and especially horizontally. This means that if external columns cannot be used, only the deck is available for attaching balconies to the building.

If the deck is made of concrete, existing balconies will have been made by in-situ casting an extension of the deck that goes out to the balcony slab. This is the building type I will focus on in this blog post.

WHAT IS A GLUED CONNECTION?

In principle, most connections are glued to some extent: Most brackets are attached using 'chemical anchors,' so when using only threaded rods and adhesive without brackets, it is essentially just applying many chemical anchors as the sole means of fixation.

CHALLENGES OF USING GLUED CONNECTIONS

When all force has to be transferred via the deck, the first thing to do is determine the condition and capacity of the deck. The original drawings usually provide you with the intended geometry and reinforcement layout but tell you nothing about the actual structure on-site or the condition of the old concrete and reinforcement. Therefore, the only way you can accurately assess the capacity is by testing it.

The main problem with an old structure like this is that it is very difficult to define in advance where the connections are possible. This is especially challenging when using prefabricated Ultra High Performance Concrete, because it is not possible to make a quick decision to move a support point 5 cm to the right or left on the construction site. The support points are already placed in the prefabricated element. This would be significantly easier if the connection had been cast in-situ, but in-situ casting is not a good option for Ultra High Performance Concrete considering the mixture's very narrow tolerances and sensitivity.

Another challenge is that the existing "old" concrete is often too weak to handle the high stresses from Ultra High Performance Concrete in direct contact in the compression zone, so it may be necessary to use a pressure-distributing layer between the UHPC and the original deck. In the example above, it was even necessary to remove some of the porous layer in advance and use a high-strength mortar casting between the old deck and the Ultra High Performance Concrete balcony to handle tolerances and form a new compression zone.

A third challenge with glued connections is tolerances: To be able to insert stainless steel threaded rods into holes drilled in the deck, the holes must be enlarged, and only a few - and relatively expensive - types of resin compensate for shrinkage in the long term and can be used in this way. These types usually do not harden quickly, meaning temporary support is normally needed for up to several days.

ADVANTAGES OF GLUED CONNECTIONS AND UHPC ELEMENTS

So, with all the above challenges - why use glued connections in Ultra High Performance Concrete?

First and foremost, because in relevant cases, there is no real alternative.

When a concrete element is needed - for aesthetic or other reasons - there are significant challenges associated with replicating the old structure in regular concrete because standards and norms have changed drastically since the building was designed.

Requirements for cover layers, fire resistance, and most importantly, the live loads that the balcony must support have all increased significantly. The live load alone is challenging enough. Therefore, it is not possible to cast a new copy of the original balcony - it would not meet today's requirements - but with Ultra High Performance Concrete, it is possible to create elements with much lower weight than the original, compensating for the increased live loads.

In summary, the combination of glued connections and lightweight balconies in Ultra High Performance Concrete can be the difference that makes renovation possible in some extreme cases - but this is certainly not without challenges.

As far as renovation principles go, bonded connections are by far the most complex and require a lot of planning, documentation, and pre-approvals, risk management, and creative thinking and on-the-fly problem-solving in both the design and construction phases.

So, this is exactly how we like it at Hi-Con: Challenging and complex, and therefore very rewarding when we succeed together with our partners!