Concrete, acoustics and sustainability

When working in acoustics it can be difficult to make decisions that reduce our impact on the planet, purely because no-one can alter the laws of physics. The dense materials that generally provide the best acoustic performance are typically more carbon-intensive materials and therefore detrimental to the environment. Here, Tom Van Dongen, senior project engineer at Mason UK, unpacks this dilemma by focusing on the specific example of a concrete floating floor.

Above: Bearings supporting CLT structure in auditoria

At a global level, the built environment accounts for 39% of gross annual carbon emissions ¹ . This includes operational carbon, the emissions produced during the day-to-day operation of buildings and infrastructure, and embodied carbon — the emissions associated with the production and transportation and construction of building materials. A key element in reducing embodied carbon is the choice of materials we use for our buildings. Materials like steel and cement have high levels of embodied carbon due to the energy-intensive processes involved in their production, while renewable materials like wood may have lower embodied carbon. Cross-laminated timber (CLT) is a good example of a material that has become increasingly popular in part due to its sustainability credentials. CLT is typically made from sustainably managed materials and the manufacturing processes that produce it require less energy and generate fewer emissions when compared with traditional building materials like concrete and steel.

Swapping in materials like CLT to replace energy-intensive materials would certainly help the built environment sector reduce the quantity of embodied emissions it generates. Yet while many businesses wanting to promote themselves as ‘green’ are openly embracing CLT, things are not always as straightforward as they appear. In the world of acoustics, ditching dense materials and replacing them with wood produces complex engineering trade-offs that we need to be honest in recognising.

Above: Concrete pour in studio 2

Example: concrete floating floors
Concrete floating floors are used in acoustics and vibration control engineering to create an air gap between a floor and the structural slab beneath, hence the name ‘floating’. This breaks the transmission path for vibration, therefore mitigating vibration caused by activities like weight drops in gymnasia or from tube trains that are close to a hotel basement, to give two common examples. The floor is raised (or jacked) using rubber or spring isolators, depending on the application. The floor itself, however, is made from concrete. In addition, it is reinforced with layers of steel mesh. While these material choices have obvious drawbacks from a sustainability perspective, the choice is driven by engineering considerations.

In the realm of acoustics, greater mass equates to better acoustic performance. Materials that are less dense simply will not provide the same level of acoustic performance and, ultimately, would not be suitable for a floating floor in the most critical applications. If we are going to discuss sustainability and acoustics, we need to acknowledge some uncomfortable realities. To skirt around this issue would be like an oil company spending all their time discussing how many trees they are planting.

Above: Research hub exterior render (https://www.bristol.ac.uk/bristol-digital-futures-institute/ facilities/)

Greener considerations
At a fundamental level we need to accept that dense materials like concrete are not going to disappear as they will always be necessary for acoustics. Mason UK recently worked on a project at Bristol University, where an historic building was being renovated using CLT panels. Both the reuse of an historic building and the choice of CLT as a building material were beneficial from a sustainability point of view. However, although 90% of the building was made from CLT, we still needed to install concrete floating floors within this structure to meet the acoustic specification.

However, recognising the necessity of concrete in acoustics does not mean we should give up on improving how we do things, or dismiss the possibility of more environmentally-conscious engineering choices. The use of steel reinforcing mesh could be reconsidered. In the Bristol University project cited above, by taking into consideration the specific loading conditions on this project, we were able to reduce the layers of mesh from two to one. We are currently exploring the implications of having a single layer of mesh as standard in our concrete floating floors, rather than two or more layers which is common practice.

Another thing to consider is the importance of accurate calculations about the volume of materials being used. Less dense materials might appear to be greener alternatives if you ignore the increased quantity that is required to achieve the correct level of acoustic performance. Finding ways to accurately estimate and account for all materials that are used in a project, including taking into consideration the quantity of each material, will be important in allowing us to better understand the trade-offs at stake. There might be instances where concrete is being swapped out for ‘greener’ materials, yet the overall increase in quantity of material means there is zero reduction in the overall amount of embodied carbon being used.

Using materials like fly ash or blast furnace slag to partially replace the cement in the concrete floating floor is also worth considering. Currently, there are some challenges in using blended cement in concrete floating floors. One is that you would need to allow for a 50% increase in curing time for a floor made using these materials. In principle, this is not a major obstacle provided it is correctly programmed into the project from the outset and the environmental benefits are clearly communicated to the contractor and other stakeholders.

Although these suggested changes might seem minor in the grand scheme of things, the cumulative impact of lots of small changes can be significant. Some applications demand the use of concrete due to a need for robustness, for example, absorbing impact from heavy weights or a need to support high loading from vehicle traffic or mechanical equipment. In these cases, it is still possible with a good specification to greatly lower the carbon cost of the concrete. Accepting the continued necessity of concrete as a key material in acoustics should not be an excuse for doing nothing. With the right engineering choices, we can achieve tangible reductions in embodied emissions without compromising on acoustic performance.

Below: Render of studio 2 (https://www.bristol.ac.uk/bristol-digital-futures-institute/facilities/)

Reference
1 https://www.deloitte.com/global/en/Industries/energy/perspectives/sustainable-construction.html