Carbon Group Post 6: Concrete Masonry Units (CMU)

The Carbon Working Group continues our series of blog posts on topics from SEI Sustainability Committee’s newly-released technical report, Building Structure and Global Climate.

As we write this post about the carbon footprint of masonry materials, we see more Environmental Product Declarations use in construction.  Environmental Product Declarations, or EPDs, are documents that quantify a product's embodied carbon footprint and other environmental impacts, and are used to achieve LEEDv4 Materials and Resource credits.  We referenced EPDs throughout Building Structure and Global Climate. Last fall, California's governor signed into law the Buy Clean California Act, which will require some building materials manufacturers for publicly-funded infrastructure projects to report carbon footprints through EPDs.  House Bill 2412 in Washington State, aims to enact something similar.

Concrete masonry is one of the four major structural material systems covered in the committee's technical report, along with wood, concrete and steel.  Concrete masonry unit walls are combined with wood, steel, or concrete floor systems to create many low-rise building types, such as warehouses, shopping centers, offices, and single- and multi-family residences.  While four times as much ready-mix concrete is used in the U.S., concrete masonry units are produced by the billions each year by over 1,000 plants in North America.

Concrete masonry units (CMU) are simply a form of precast concrete with very little water to create zero-slump blocks.  Like ready-mix concrete, the manufacturing of portland cement accounts for more than 90% of the carbon dioxide emitted to produce CMU.  Therefore, as with ready-mix concrete, higher-strength CMU block results in higher global warming potential of the block.

In addition to portland cement, fine aggregate, and water, CMU can be made with a variety of ingredients, including granulated coal ash, expanded blast furnace slag, pumice, shale, slate, clay, and crushed glass. In terms of global warming potential, ingredient variations that use heat to expand aggregates for lightweight block show an increase in global warming potential.  Substituting recycled materials or industrial byproducts for virgin aggregate have a relatively insignificant effect on global warming potential (however beneficial in reducing depletion of finite resources).

Concrete masonry doesn't get built with just CMU, it requires mortar, grout and steel reinforcing.  Like the concrete, mortar and grout is typically made of cement, aggregate, water, and an additional ingredient, hydrated lime.  It turns out that the biggest contribution that engineers can make in reducing the global warming potential of concrete masonry lies in the grout.  This is because grout needs lots water to make it flow into CMU cells, and so as not to dilute its strength, grout needs lots of cement. By adding grout to every cell of CMU, for the same volume of wall assembly, the embodied carbon dioxide can be triple that of an ungrouted CMU wall since the cells are about half the volume.

As environmental product declarations become more common, manufacturers of masonry products will have incentives to reduce global warming potential and other environmental impacts.  We expect to see more manufacturers replacing portland cement with supplementary cementitious materials, such as fly ash and slag cement.  These industrial byproducts have lesser global warming potential because the energy used to create them are attributed to coal-fired power production and iron smelting, respectively.

Structural engineers have at their discretion many aspects of building design that can make a significant difference in global warming potential.  These include right-sizing CMU compressive strength, using the ASTM C476 strength method for proportioning grout, minimizing the extent of grouted cells, considering when lightweight CMU is necessary, and weighing the environmental impacts and thermal envelope performance of CMU walls compared to other wall assemblies.