REDUCED EMBODIED CARBON CONCRETE THROUGH MIX OPTIMIZATION
The most widely used construction material in the world is also a key contributor to carbon dioxide emissions globally; a direct result of the cement production process. For every one (1) pound of cement produced, 0.9 pounds of CO2 is emitted(1). As we move together into the 21st century we must collectively offer solutions that are more environmentally responsible, offer greater durability, reduced greenhouse gas (GHG) and ozone depleting emissions with improved life cycle cost performance that are also widely and readily available.
Accomplishing the above starts with understanding project requirements as well as the properties of local materials ideally sourced within 100 miles of the project location. Transportation of materials is a major contributor to embodied CO2 (eCO2) in materials and would typically be accounted for in a project Life Cycle Analysis (LCA). For the purposes of this discussion, only the manufacturing processes will be considered; this is referred to as Cradle-to-Gate.
Use of high-performance chemical admixtures optimizes concrete mix proportioning and efficiency to provide greater reductions in eCO2 than other commercially available carbon sequestration techniques on the market that inject CO2 into the concrete to offset its carbon footprint.
Mix optimization is easily scalable regardless of geography because local, readily available materials are being used in lieu of specialized equipment and sourcing of liquid CO2. Furthermore, mix optimization can accomplish this typically at a reduced or neutral cost.
Either through conservative performance-based specifications or conservative mix designs, concrete is quite typically an over-engineered material. Accordingly, there is opportunity to optimize the mix proportions through the incorporation of:
Supplemental Cementitious Materials (SCM):
Fly Ash or Slag
- Reduces the cement demand in a concrete mix 5% to +25%
- Improves finishing
- Reduces internal heat gain and cracking potential
- Increased durability
High Range Water Reducer
- Optimizes cement consumption, can reduce cement demand up to 5%
- Yields a more efficient hydration process
- Lowers w/c ratio without sacrificing placing/finishing characteristics
- Eliminates excess free water and reduces shrinkage potential
- Increased durability
Recent examples with optimized mixes compared with standard mixes have shown that achieving a 100 lb reduction in eCO2 per cubic yard is not uncommon at SCM replacement rates of 25 percent and the utilization of a high performance high range water reducer. Additionally, optimized mixes are easily capable of meeting or exceeding minimum target standards set by the Carbon Leadership Forum for various concrete mixes. As an example, a target of 657 lbs/cy (390kg/m3) of CO2 is established for 4,000 psi concrete per the Carbon Leadership Forum EC3 Tool (www.buildingtransparency.com).
Sources: 1 https://www.cement.org/docs/default-source/th-paving-pdfs/sustainability/carbon-foot-print.pdf?sfvrsn=2&sfvrsn=2