Concrete carbon emissions: Real challenges, real opportunities

Composite rebar reinforcement is an alternative to steel for reinforcing concrete. Basalt and fibreglass rebar are stronger than steel and enjoy higher tensile strengths. Fibreglass reinforcement weighs 25 per cent less when compared to steel and basalt rebar is is almost 90 per cent lighter. This allows for lighter concrete panels, less fuel usage during transport, and a smaller carbon footprint. Basalt and fibreglass rebar information can be found in ACI 440.1R-06 and can be used in 2018 and 2021 International Building Code and International Residential Code reinforced designs. Photo courtesy CompleteCrete Systems

Building strategies

In addition to making sure what building are made with results in a lower carbon footprint, how they are built must also guide design and construction decisions. Transporting a component product great distances may reduce or even eliminate its original low carbon benefit. Simple changes to execution of the exposed concrete finishes on millions of acres of concrete slabs worldwide can potentially improve carbon expenditure at installation, as well as improved life-cycles. Industry experience understands how micro fracturing of concrete can reduce matrix life-cycles and increase the need for repairs. Employing a laser screed to initially create flatter floors in these scenarios can reduce or eliminate the need to mechanically flatten a floor once it has hardened and plasticity is lost. Once mechanical grinding or scarifying has begun, the top layer of concrete loses an amount of integrity, often leading to additional mobilizations for patching, repairing, or petroleum formulated grout coats during initial construction and certainly throughout the slab’s lifetime.

Slag and recycled glass, while excellent choices in most cases for reducing carbon in concrete construction, may produce a similar effect in exposed concrete finishes. Because both materials are brittle, they will commonly fracture along with the portions of the concrete around them as grinders and trowel machines move over the surface of the concrete, creating voids. These, too, will not only require attention during initial construction, but may create additional material and energy demands throughout the concrete system’s life-cycle.

Curing is one of the simplest ways to reduce carbon expenditures, as cured concrete is good concrete. With cured concrete curling, shrinkage, cracking, and all manner of negative effects can be reduced. Wet curing is commonly recognized as one of the better ways of curing, but often not approached due to schedule and cost increases. Instead, common approaches will include cure and seal style applications, which are generally not as effective and in many cases contractors will not apply them when cutting corners. With this comes many physical defects and increased probability of short- and long-term concrete failure and increased energy requirements to keep the concrete matrix functional. Internal curing with nano silica admixture technologies like NIC maintains additional internal water to reduce early shrinkage and do so without need of cure and seals, membranes, and curing compounds. Increased hydration also means that a slab can be ready for framing or polishing a few days after placement, making it more cost and time effective approach than standard concrete construction.

Rethinking rebar

Rebar is a necessary tension tool to reinforce concrete structures and aid the concrete system when it is under tension. However, when chloride ions migrate to steel inside of concrete, corrosion occurs and affects the surrounding concrete with detrimental results, lowering life-span and increasing the need for spending time, materials, and energy on maintaining the concrete structure.

Newer fibreglass and basalt rebar products provide an alternative. Both types of rebar offer the benefits of reduction in rust and corrosion. They can even be demolished along with the original concrete system and turned into aggregate for future projects. Fibreglass is more readily available in North America with basalt supply occupying a lower share of the market. However, the transportation weight of basalt can be lower and could alter availability in the future as the demand for lower carbon rebar solutions increase.

Centre Hospitalier Universitaire Sainte-Justine utilized Portland-limestone cement to lower its embodied carbon footprint. Photo courtesy CHU Sainte-Justine

Lastly, engineers must rethink of when and where to use rebar at all. While some structural reinforcement demands will never change there are areas of concrete construction where reinforcement is unnecessary. Many topping slab specifications will include rebar when perhaps the most sustainable approach would be to let the topping slab float unbounded to the structural slab. As the concrete moves and flexes with changes in thermal and volume pressures the slab is more likely to have cracking and reduced integrity. Engineering and design teams will need to work together with builders to determine the need to include or exclude rebar with topping slabs, but in many cases project teams will find a simple way to prolong the life of the slab while eliminating the carbon cost of rebar.

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