Reinventing the green wall
The presence of green walls and roofs in our densest urban environments is no longer a rare sight. This quirky way of “greening” our cities has captured the imagination of architects the world over. Beyond the aesthetic value of green walls, which has positive connotations of nature, health and wellbeing, data has shown that as a result of the evapotranspiration process that occurs, a cooling effect on the immediate area is created.
However, a main criticism of this secondary skin of plants often fixed to the sides of buildings, is that it has proven to be expensive to implement and maintain. Green walls rely on mechanical irrigation systems to keep the plants hydrated and alive, and if the watering systems fail, the plants die.
This ongoing maintenance is considered expensive and intensive, a fact that hasn’t escaped the notice of Marcos Cruz, professor of innovative environments at the Bartlett School of Architecture and Richard Beckett, lecturer in bio-digital materials also at the Bartlett.
“Even small areas of plants dying on such walls is seen as a failure and perhaps relates to the principle of green walls which aim to flourish all year round with greenery,” says Beckett. “We know this doesn’t happen in nature, and there are times in the seasonal cycles where things aren’t green, but do go brown and recede, ultimately coming back to flourish later in the year. This is not really accepted for green wall systems, adding to the need and cost for on-going maintenance”.
Together with a team of architects, engineers and biologists, and industrial partner Laing O’Rourke, Beckett and Cruz have formed the Bartlett’s BiotA Lab, and developed a new bio-receptive concrete panelling system. This isn’t a bolt-on extra, but is ingrained in the architectural fabric. Beckett and Cruz use the metaphor of tree bark when describing the material, where the facade acts as a host to promote the growth of living micro-organisms and cryptogams (plants that reproduce by spores, including algae, lichen and moss).
“There’s no need for artificial irrigation or maintenance, the material relies on rainfall,” says Cruz. “We’re creating a setting, like a stage set, from which the vegetation can develop naturally and grow in a self-regulating way”.
Even though the technology could be adapted to other building materials, Beckett and Cruz have used concrete for a number of reasons. A chance meeting with a Spanish biologist, Sandra Manso Blanco (now part of the BiotA Lab team) who was doing her PhD on bio-receptive concrete was instrumental, but also, concrete enabled the team to work at an architectural scale that wasn’t possible with other materials.
“Architects love concrete because of its plasticity and expression, it can be cast into all forms and is very durable. It is also the most used material in the world today,” says Cruz.
However, it should be pointed out that Beckett and Cruz are not using traditional, Portland cement. Instead, a magnesium phosphate concrete is being employed, which has a low pH of around seven, allowing mosses, lichen and algae to grow.
“We make it ourselves and it’s a very specific process so that we can control the aggregate sizes to achieve porosity, to achieve surface roughness and to achieve moisture absorption,” says Beckett. “It still counts as a cementitious material, but it’s a very dry mix and is more like mortar and tends to be packaged into moulds rather than poured. It’s probably quite similar to an old type of concrete that might have been used before the arrival of Portland cement.”
Traditional Portland cement is typically too alkaline for living systems to survive and has a pH of 11 or higher, and therefore couldn’t be used. Although as Beckett highlights, after many years of weathering and as traditional concrete becomes less alkaline, older buildings made of this material will sometimes develop plant growth known as bio-colonisation. This accumulation of micro-organisms, plants and algae is often considered unsightly and cleaned off or painted over to stop it occurring.
“The difference with what we’re doing is that we’re designing a concrete type of material that already has these optimal conditions for plant growth, we’re not allowing this to happen randomly,” says Beckett. “We’re providing panels that grow photosynthetic organisms that absorb water, preventing storm water run-off, provide solar absorption, absorb CO2 and other pollutants from the environment and produce oxygen.”
Beckett cites research by the Max Planck Institute in Germany that assessed the importance of cryptogams in fixing carbon dioxide and nitrogen from the atmosphere and how this is influencing the global and regional biogeochemical recycling of these vital chemicals.
“The role these tiny species of plants play in our climate is huge and they are rapidly being lost from our urban environment because we’re using very high pH concrete,” says Beckett.
From May until April next year, Sandra Manso, Bill Watts and Chris Leung will lead extensive environmental tests via the outdoor exposure of the panels in a 12-month cycle. Currently, Cruz and Beckett are exploring how three-dimensional geometries can increase the biological growth and improve the panel performance. By observing tree bark, they have used sophisticated computational tools to design a wide range of geometry types to show where areas of growth can occur on the panel.
“When you look at tree bark there are spots where biological growth is thick and dominant and other areas where it’s not,” says Cruz. “Features such as fissures and depressions are designed on to the facing surface of the panels to emulate tree bark and channel rainwater to specific growth areas”.
By directing water to parts of the panel where it can collect, areas of shade and protection can be created while other sections are left exposed. The panels will also be designed with nort-west facing orientation and the team are looking at fabricating them using a layered concrete casting method and CNC-milled moulds. The panels will then be seeded with a mix of algae cells and moss spores and located outside to undergo environmental monitoring and measurements over a year.
“Inclination plays a very important role, the more inclined the surface, the greater the possibility of growth,” says Cruz. “When the panel is totally flat and vertical, growth will happen but it will probably be slower and will need more water. Absorption, retention and distribution of water becomes a quintessential dynamic that we are working on with the material.”
Beckett suggests that the panel sizes at first will be around 1.5m x 0.5m. However they anticipate that these will gradually scale up to 2m x 4m. He also suggests that the thickness of each panel will vary from 9cm to 15cm due to the typography created on the surface. For example, in section, the top part of the panel might be the thinnest which will gradually thicken further down the panel where an inclined surface is created to allow growth to occur.
However, the thickness of the panels presents the team with a huge challenge. The panels need bulk because they work like a sponge in order to retain and store the water during periods of drought. But this makes the panels cumbersome and very heavy.
“The panels will only take on a certain amount of water depending on their size,” says Beckett. “However, they won’t be heavier than concrete and any excess water will run down the outer surface.”
There is still a way to go before the panels are available commercially. A considerable amount of testing is required to check aspects such as the ageing of the material, and Cruz is anticipating that the bio-receptive concrete panels will be used initially on low-risk, small-scale projects. But eventually, he says, the material could either be integrated into walls of infrastructural projects or used as a rainscreen.
Cost-wise Beckett says there is no reason why the material would be more expensive than existing green wall systems.
“At the moment we’re using laboratory grade materials to ensure the reliability of the research,” says Beckett. “If the material is used commercially, more affordable phosphates would be found.”
Beckett and Cruz both add that given magnesium oxide is a waste product there is great potential to use it together with other unwanted materials if the panels are manufactured on a larger scale. “The idea that bio-receptive concrete is made of recycled materials is of great value not only in terms of sustainability, but also in terms of cost and is something we will be looking at in more detail,” says Cruz.
When the bio-receptive concrete panels are available commercially they will offer architects a new aesthetic.
“It is now possible with these cementitious mixes, new technologies and manufacturing to produce very different work. Architecture always moves forwards when technology shifts,” says Cruz. “There’s a demand from society for this shift and a technological capability for this to happen and that’s what we’re facing.”