C+S May 2022 Vol. 8 Issue 5 (web)

in the world. While concrete is often characterized as our biggest problem, it may be our biggest opportunity. Concrete is durable and long-lasting, and its thermal stability can lead to more energy- efficient buildings and facades. Low carbon solutions for concrete exist today and some have been used successfully for over 40 years. These solutions make use of materials called Supplementary Cementitious Materials (SCMs), which are often post-industrial waste products such as flyash (a by-product of the coal industry) and slag (a by-product of the steel industry). These materials, which can replace a portion of the cement in concrete, are not burdened with high amounts of CO2 and improve the concrete’s performance. The only downside is that SCMs can slow down the curing process, causing the concrete to gain strength more slowly. Historically, the construction industry has refined building design with only a single driving variable - cost. In efforts to simplify formwork, structurally inefficient systems emerged that save labor but use more material. Efforts to accelerate construction led to the development of mixes that can achieve high strengths in only a few days. These prac- tices combined result in buildings that use more concrete and concrete mixes that use more cement. Today, the industry must consider a second variable in building de- sign, CO2, and can’t simply take a traditional design, toss in some SCMs and call it sustainable. In 2018, Clark Pacific evaluated dozens of potential designs for cost and carbon. Concrete volume was carved out by thinning slabs and adding ribs to achieve the same strength with less. Ribs were made wider to reduce the early strength require- ments of the concrete and enable higher SCM mixes. Offsite fabrica- tion allows for the use of a form heating system that accelerates early strength gain. This allows for the removal of up to 70 percent of the cement from mixes. The heating system removes roughly 20 times the carbon from the mix than it created while heating the forms. This aggressive approach removes roughly 25 percent of concrete from a building and the remaining concrete has half the carbon when com- pared to 2019 National Ready Mix Concrete Association Baseline mixes for similar strengths. Whether building with concrete, steel, timber or precast, projects need to be addressed with both cost and carbon in mind. Advances in technology, materials, systems and processes captured in this article are encouraging first steps. As we look ahead, there’s no doubt that future advances will take construction projects further down the path to Net-Zero.

Working with Clark Pacific, the project was delivered six months ahead of schedule. Energy Codes Drive Demand Driving demand for high-performing building envelope systems are building energy codes, such as ASHRAE 90.1 and California’s Title 24, that have significantly progressed to focus on reducing carbon emissions from the built environment. Prefabricated systems, such as composite precast panels, are not only energy efficient during the con - struction process, but also create buildings that are more sustainable throughout their entire lifecycle. This is done in part by incorporating barrier systems into premanufactured frames that include continuous insulation and caulked in punched windows which are proven to be more successful in resisting heat flow. A recent Glumac study that compared 2 inches of a composite precast exterior envelope to baseline curtain wall and metal stud systems, found concrete precast panels performed better than all other envelope options and demonstrated a ~30 percent reduction in the amount of heating energy used. According to the same study, a precast composite envelope system can reduce HVAC energy use and cut costs compared to conventional envelope designs. With these advances in building materials, prefabricated buildings will hold temperatures better and decrease the need for heating and air conditioning use. This is also ben- eficial to avoid the inflated cost of energy during peak demand hours when the thermal mass within a structure can retain a desired interior temperature. With each additional building following this practice, the demand during peak hours is reallocated across the entire day and en- vironmental strain is reduced. These systems routinely reduce energy costs by 10-20 percent. This marvel in energy efficiency is on the cut - ting edge of innovation in more environmentally responsible buildings. Accelerating the Path to Zero Industry conversations around embodied carbon have increased tenfold in recent years and for good reasons. Building materials rep - resent nearly a third of the construction industry’s embodied carbon. When it comes to materials, concrete isn’t always viewed favorably. In fact, Portland Cement, the glue that holds concrete together, is alone responsible for 8 percent of the world’s CO2 emissions. At the same time, concrete is the most widely used man-made material

JON MOHLE senior product manager at Clark Pacific.


May 2022 csengineermag.com

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