How do buildings end up at the end of their life? What are the challenges faced?
Buildings are everywhere around us. They embrace great values that humanity treasures the most, like wealth and social interactions. While we are often willing to spend a fortune to preserve some buildings for their importance according to our values, the majority of structures are built with a certain life span in mind. What’s more, they will end up derelict after many scores of years when they reach the end of their usable life. Until recently, there has been little consideration of what to do with such buildings. So far, the available options were either demolishing them as quickly as possible to make way for new buildings, or retaining them, refurbishing and converting them into new structures.
The latter is clearly the preferred solution, but this only happens in a few cases and only when there is a breeding ground in terms of budget. (e.g. see Figure 1 below). The problem is that when the contents of a building are stripped away, what is left, thus its shell, may be of small value. Nevertheless, most of the embodied carbon is stored in the “leftovers” of the building, i.e. the load-bearing structure, floors, walls, and roof. Furthermore, a detailed investigation may reveal that much monetary value may be extracted also from the shell.
Figure 1: A refurbished building: The University of Manchester Alan Gilbert Learning Commons, United Kingdom: before (left image) and after (right image) £24million investment for refurbishment
So currently, demolition is the only solution in the majority of end-of-life buildings, with typical results as shown in Figure 2. This image shows that the pre-demolition building has been transformed into piles of construction waste. Our best option after the demolition is to manage the construction waste, mainly by recycling. However, due to waste contamination, the recycled materials have many inferior properties to those contained in the original building, even after the time-consuming process of sorting and separating construction waste streams. Furthermore, the remaking of recycled materials from construction and demolition waste is still energy intensive.
Figure 2: A typical demolition site
With the net-zero carbon goal in mind, the current efforts to minimise the release of the embodied carbon of end-of-life buildings are no longer tenable. A better way forward would be to retain the value and the embodied carbon in the building, thus preventing the building from becoming a source of waste in the first place. If this is not possible, another solution would be to demolish the building in such a way that there is minimal waste contamination. In such a way, the need for sorting and separating different waste streams is minimised and the potential to remake high-quality and high-performance materials from construction waste is maximised. For example, after contamination with other waste materials on site, recycled concrete aggregates are used in low-value applications such as road fill. But without contamination, the same recycled aggregates could be used to make high-quality concrete for load-bearing structural members.
There is a massive gap between the current practices and the preferred outcome of dealing with end-of-life buildings. Steps for improvement should and can be made.
What improvements can be made and what are the challenges?
Figure 3 below shows the well-known hierarchy of resource management. The current practice in the construction industry is “recycling”, which is better than the most undesirable options of burn and bury. Not making new buildings is not an option to reduce the use of construction materials. Thus, the next best solution when dealing with end-of-life buildings is either “minimise” (redesign) or “reuse” materials. While the option of “recycle” is still unavoidable, the outcome of “recycle” should and can be aligned with the need to “minimise”.
Figure 3: Hierarchy of managing resources
Thus, to make the necessary changes in dealing with end-of-life buildings, it seems that the following three strategies can be pursued:
A. Reuse of building elements. This is different from “redesign” and “repair/repurpose”, and it enables the entire end-of-life building to be reused, as seen in Figure 1.
B. Elevating the value of recycled materials by preventing contamination of waste streams, and by researching methods of improving material properties.
C. Using conventional “recycled” materials in high-value applications.
Examples are given below to illustrate the above routes respectively, as well as their challenges.
A. Reuse of building elements
The shell of any building construction can be broadly grouped into non-loadbearing elements, typically on the surface of the building, and load-bearing elements. Reuse of many non-loadbearing elements, like ceiling panels, is eminently feasible because they are relatively easy to disassemble/store and have standard dimensions and functions, thus making them interchangeable in many applications. In contrast, the reuse of structural load-bearing elements is much more difficult. Unfortunately, most of the carbon and residual economic value is embodied in the structural load-bearing elements.
Steel: It is one of the most common structural load-bearing materials (steel, concrete, brick, timber). The reuse of steel elements is the easiest one, especially if the steel elements are bolted at the end-of-life building. Any technical barrier to the reuse of steel structural elements is minor, and the main issues are related to developing a viable business model related to storage, re-fabrication, supply chain management, and the confidence of design engineers in selecting the reuse option.
Bricks: Bricks are a material as well as a structural element. Technically, it is possible to reclaim bricks, even bricks with cement mortar (see Figure 4 below), although developing the machinery for reclaiming cement mortar bricks needs investment. However, once this barrier is overcome, the reclaimed bricks can join the market and can be used as new.
Figure 4: Reclaimed bricks by punching
Concrete: Concrete is the most widely used structural material but it is also the most difficult to reuse. The problems with load-bearing concrete elements are twofold. First, they typically are monolithically cast (except for precast elements), and this makes reclaiming very difficult. Second, the reclaiming process involves cutting the steel reinforcement, which makes it exceedingly difficult to restore the continuity of the reinforcement that is necessary for concrete structural elements. Therefore, the reuse of load-bearing concrete elements has to start from the design stage. In the meantime, for the current stream of end-of-life concrete buildings (and many more years to come), the best solutions lie in the next two other options as explained below.
B. Elevating the value of recycling
To elevate the value of recycling, the properties of the recycled materials should meet specific standards and exhaust their potential. Here, extensive research and development studies under the umbrella of low-carbon materials, have to take place. An example is using a tiny amount of graphene in recycled aggregate concrete to increase its mechanical properties so it can imitate completely the use of natural aggregate concrete. There are issues of accepting such material innovations in the construction industry which is by its nature cautious in the absence of accepted standards.
While standards do exist to allow recycled materials to be used, their applications depend on the grades of recycled materials. Therefore, to qualify for the highest value chain of applications, the recycled materials must achieve the highest grade, which requires the highest quality of recycling without contamination.
C. High-value application of recycled materials
In structural engineering, the same material is assumed to be used in a structural member everywhere. Therefore, the material has to meet the demands of the highest-performing part of the structural member. However, a change of mindset can lead to options of using conventional low-grade recycled concrete in high-value applications. For example, it is well-known in structural engineering that the tensile properties of concrete are ignored. If so, why not use low-grade recycled concrete in the tension region of a concrete member? In a typical concrete beam where there is a compression region and a tension region, the latter is typically more than 50% of the volume of the element. Of course, to achieve this objective would need to raise the awareness of structural engineers as well as adapt the existing construction practice.
What is the way forward?
Whatever the best value chain to be pursued for end-of-life buildings, within the constraints of time and money, many challenges (as mentioned in the previous paragraphs) should be overcome. To facilitate this process, it is essential to thoroughly get to “know” the building. This means knowledge of not only the locations and quantities of the different materials and elements of the building but also their history path, which will determine their potential future highest values for recycling and reuse. Such comprehensive knowledge cannot be expected to be possessed by only one person. Also, acquiring this knowledge manually from different sources would take a lot of time and effort, which would be in short supply under the pressure of dealing with end-of-life buildings.
A digital model of the end-of-life building provides the necessary platform to overcome many of the technical challenges. This is where several openBIM standards and other digital tools come to our service and can be used to support the widest audience possible. Such a digital model is being developed in the Horizon Europe-funded project RECONMATIC, an example of which is shown in Figure 5. This digital model can incorporate a comprehensive database of material properties for different types of materials with different histories of use under different conditions and their potential future properties after remanufacturing. Such a database can be independently developed to cover all possible construction materials in end-of-life buildings and then be extracted for the specific end-of-life building under consideration.
The digital model of the end-of-life building can then be used to establish a pre-demolition audit plan to extract the most value from the existing building, by minimising waste contamination during the demolition process, and by reclaiming the largest amount of elements that can be reused.
Figure 5: Digital model of a University of Manchester building
In developing the aforementioned digital model, it must be recognised that the majority of buildings that are coming to the end of their life were designed and constructed before digital CAD became common practice. Therefore, their physical information is most likely to be stored in the form of paper-based drawings. In the RECONMATIC project, a particular challenge is to develop a digital model of as-built buildings with paper drawings. Afterwards, it is necessary to deal with the problem that the original building plan is likely to be different from the actual end-of-life building after many modifications during its lifetime. To overcome the second problem, merging CAD and augmented reality (AR) data for a building inspection is now feasible.
On the other hand, as the need for such a digital model predominantly concerns construction waste management, the level of accuracy of the information, in terms of exact dimensions and connectivity of building elements, can be somewhat moderate compared to digital models of buildings for other purposes such as structural strengthening or refurbishing.
Professor Yong Wang, email@example.com
Dr. Muhammad Omer, Muhammad.firstname.lastname@example.org
1. The Scope of Waste Management in the Construction Industry: Concepts and Definitions, by Mahmoud Alhawamdeh