Reinforced concrete has among the highest upfront carbon emissions of all building materials primarily from production of portland cement and steel for rebar. There are, of course, ways to reduce these emissions such as substituting fly ash for cement and fiberglass for steel rebar. While it is not as emissions dense as, say, aluminum, its weight when used as a building element makes it a significant contributor of emissions. Designed today, given the original criteria and a new concern for reducing upfront carbon, cross-laminated timber construction combined with glulam columns and beams, would be a serious consideration. This system, which is significantly lighter than concrete, would pair well with the helical piles proposed as the substitute for concrete piles. This would offer additional benefits to the well-being of the building’s occupants through enhanced biophilic connections to the natural world, supporting the high priority given to daylighting and views. Other benefits of mass timber construction include improved indoor air quality, enhanced sound absorption, and thermal stability. And perhaps most importantly, mass timber sequesters carbon in its cells, rather than emitting it to the atmosphere. There are other issues to consider in substituting cross- laminated timber or CLT construction for concrete, such as fire resistance and vibration. There are now UL assemblies and details that address the fire resistance issues, so this concern can be overcome by careful detailing and construction methods. Concerns about vibration arise particularly in science buildings with precision instrumentation in use. There are two possible approaches: dampening or stiffening. Dampening involves placing a lightweight concrete topping on the CLT floor deck. Stiffening involves adding beams to reduce the span of the deck. The potential for deconstruction and recycling, rather than disposal, is enhanced by using dry construction processes such as bolting. Deconstruction and recycling reduces end-of-life emissions and keeps waste out of landfills.
Because New Orleans is low-lying, most of it below sea level, the building has no basement. The main mechanical and electrical equipment are in a penthouse, so while not explicitly done for reasons of sustainability, a reduction in the amount of concrete used was built-in by virtue of local topography. This was offset by the poor bearing capacity of the soils, necessitating deep piles and a structural (spanning) ground floor slab, rather than a conventional slab on grade. Considered today, there is likely little that could be done about the structural ground floor slab. Removing soil and replacing it with structural fill would have been considered at the time of design and judged too costly compared with a structural slab. It is likely, however, that using lighter weight steel helical piles would reduce the upfront emissions. This would probably only be feasible if there were a lighter superstructure. The superstructure of the Environmental Science Building is in situ concrete, and was selected primarily for its potential 4 aesthetic quality. It is exposed and there was particular architectural interest in the floor slab system, formed using a modular pan system to create a primary (beams) and secondary (joists) scale. This decision was also based on establishing a kinship with the existing Stern Hall, which has a similar structural system. Lastly, there was a functional rationale for the exposed structure: it is not unusual to expose the mechanical, plumbing, and electrical systems in a laboratory environment, as there is often the need to access those systems to maintain or add to them. Exposing them eases those processes and makes the building more adaptable over time.
TSW Design
Global warming potentials of common building materials
4 ‘Potential’ because exposed site-cast concrete carries with it a certain amount of risk, despite a design and construction team’s best efforts. Formwork can be deformed once the concrete is placed and honeycombing on the surface can be caused by poorly vibrated concrete, just two possible undesirable outcomes. A stringent specification is no guarantee, particularly in a construction market that lacks experience with architectural concrete. One must be prepared to accept the unexpected.
23
on site review 48 :: building materials
Made with FlippingBook interactive PDF creator