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.
