Concrete, Part 3

by Don Friedman on March 7, 2019

Concrete has become something of a scapegoat for carbon emissions in building, but it’s amazingly difficult to pin down how bad it is, as a material, compared with other options. The problem, simply, is that it is difficult to make apples-to-apples comparisons of efficiency and carbon use across different structural systems.

Let’s start with weight. Concrete weighs about 145 pounds per cubic foot (pcf), reinforced concrete weighs about 150 pcf, reinforced lightweight concrete weighs about 115 pcf. Steel weighs 490 pcf, common construction lumber weighs about 35 pcf, and laminated lumber about 45 pcf. These numbers are completely useless as a point of comparison because the amount of material used varies so widely. Despite the fact that steel is ten times (or more) heavier than wood, there are a lot of cases where a steel beam will be lighter than an equivalent wood beam because of the greater strength of steel and the fact that it comes in wide-flange shapes that are more structurally efficient than the rectangular shapes of wood. Reinforced concrete tends to be used in much greater volume than steel, so even though steel is heavier, steel frames tend to be lighter than equivalent concrete frames.

Worse, there’s no such thing as a “concrete building.” Concrete is only useful for building frames when it’s reinforced with steel, and the steel may be an appreciable percentage of the weight of the frame in large buildings. There’s no such thing as a “steel building”, as we don’t construct buildings with steel-plate floors like ships and some bridges. We use reinforced concrete for the floors in a steel-frame building. In other words, both steel and concrete buildings are hybrids. Similarly, buildings with light-gage steel joists and studs have either wood floors or concrete floors. The only “pure” structural system is wood frame, and even wood frame houses typically have concrete foundations.

So we have mixed systems, structural sizes that vary greatly, and carbon impacts that vary greatly. What are the results? Phil Purnell’s paper “The carbon footprint of reinforced concrete” suggests that while concrete is less harmful pound for pound, it loses most of that edge in real designs. He also says that “Reinforced concrete beams designed with optimised strength concrete present significantly lower” carbon per unitized structural value “than comparable steel or timber composite beams” but the most important word in that sentence is “optimized.” Structural optimization in buildings has been an elusive dream for decades. Unlike a jet’s airframe, where the patterns of loading and use tend to be the same all the time, buildings are altered and used in different ways by different occupants. Structural optimization may not be possible. The Structural Engineering Institute of the ASCE has been working on the carbon issue and trying to compare different structural systems. One of the more important sentences there in terms of materials is that concrete shear walls may include “underutilized concrete material.” In other words we don’t consistently stress concrete as highly as we do steel, so our concrete buildings are less efficient in material use, and if the production of cement is a major carbon-producing industry, that matters.

The conclusion: it’s complicated.

Part 1 is here.

Part 2 is here.

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