What Makes Concrete Green?

An important part of the design phase of any construction project is materials selection.  Concrete, steel or wood are typical options for the building frame; for paving applications, the choice is generally either concrete or asphalt. Material choice depends on several factors, including initial cost, life cycle cost and performance for the specific application. Due to the growing interest in sustainable development, engineers and architects are now motivated more than ever to choose sustainable materials.

Operationally, concrete is a very sustainable material with environmental advantages such as long-term durability, high solar reflectivity (lower heat island effect), low Volatile Organic Compounds, (VOCs) and high thermal mass (lowers energy consumption). It is almost entirely recyclable, and can be used in applications such as pervious concrete to reduce storm water runoff and recharge ground water. 

Although many people focus on CO2 emissions as the most critical environmental impact indicator, this forces engineers, architects and product manufacturers to consider reduction of greenhouse gas emissions without regard to other sustainable practices. Life Cycle Assessment (LCA) offers a solution which takes into account the full range of environmental impact indicators over the entire course of material life cycle, including extraction, manufacturing, construction, operations and, finally, reuse/recycling.

A typical cubic yard of concrete contains about 10% by weight of cement, and every one ton of cement produced leads to about 0.9 tons of CO2 emissions. There have been a number of articles written about reducing the CO2 emissions from concrete, primarily through the use of lower amounts of cement and higher amounts of supplementary cementitious materials (SCM) such as fly ash and slag.

However, focusing entirely on CO2 emissions gives you only a part of the “green” equation and can lead one to ignore other important sustainable practices such as using

•  local sources of aggregates for efficient utilization of energy resources

•  recycled or crushed returned concrete aggregates instead of virgin aggregates.

•  water recycled from ready mixed concrete operations (water used for cleaning ready mixed concrete trucks and precipitation at the plant).

•  energy efficient transport of materials (such as by rail) to ready mixed concrete plants

•  additional energy savings from “energy aware” ready mixed concrete plant operations.  

Since LCA considers the environmental impact of materials over the course of their entire life cycle, it should be included in any LEED or “green” project evaluation.  Concrete has substantial sustainability benefits, but a focus on CO2 emissions from cement and concrete production leads to the inaccurate perception that concrete is not sustainable. This can result in a less sustainable material choice (for example a steel frame building or an asphalt roadway). Architects and engineers are wise to consider all these factors in their “green” building designs and material selections.

 References:

 Karthik H. Obla, Ph.D., P.E. – Managing Director, Research & Engineering, NRMCA

 Published in “Concrete in Focus” – May/June 2009