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.

Concrete, Part 2

March 6, 2019

The first article I read in the Guardian’s Concrete Week series was last Monday’s piece “Concrete: the most destructive material on Earth.” It’s an odd piece in terms of its organization, jumping around a bit. Eventually I realized that the problem I was having is that the article is discussing several quite different topics, linked […]

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It Depends On How You Look At It

February 23, 2019

At first glance, this article on how to reduce New York’s energy use seems fairly grim. But there are hidden bright spots. For example, if the cost of an optimal retrofit is twelve percent of a building’s value, that means that retrofitting is a lot less expensive than replacement. For example, if the problem is […]

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Green Roofs

January 27, 2019

We’ll see how this goes in the city council. A lot of NYC buildings are too small to have much available roof area for green roofs: once you subtract a stair (and sometimes elevator) bulkhead, rooftop mechanical, and a cross-roof path for the fire department, there’s not much room left on a townhouse or tenement […]

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The Message Only Becomes More Important Over Time

January 26, 2019

I feel like I read an article on this topic every month or so, but the repetition is okay. The greenest building is the one that already exists and is reused. Or, as Mark Alan Hewitt puts it in Common Edge: Why Reusing Buildings Should Be the Next Big Thing. Existing buildings vastly outnumber new ones, […]

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Low-Tech Solutions

January 8, 2019

This is a view of the north facade of the Alexander Hamilton U.S. Custom House on Bowling Green, a Cass Gilbert building completed in 1907. I have a fondness for this building, aside from its architecture, due to an early project I worked on involving the Africa statue by Daniel Chester French on the northwest […]

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An Analysis of Rooftop Farming

December 29, 2018

The construction of buildings does not remove or destroy land acreage, it simply converts some of it to rooftops. People have had plantings on roofs for a very long time – the Hanging Gardens of Babylon, one of the seven ancient wonders of the world, were planted terrace roofs – and one of the reasons […]

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What Green Means

December 12, 2018

I don’t have a great deal to add to Building Consensus by Amy Howden-Chapman. LEED and other programs to green new buildings are fine, but most of our built environment already exists. If we want to use less energy we have to address all of those buildings. Fortunately, there are some built-in ways to address the issues. […]

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Worthy of Repetition

November 18, 2018

There’s nothing new in this article in the Guardian – Preserving historical buildings: the most sustainable thing is not to build new stuff – but that’s okay. The points in it are important enough that having them repeated every so often in a general publication like a newspaper is worth it. As a reminder: Existing […]

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Structural Engineering Is Everywhere

November 16, 2018

Breaking news, displacing today’s scheduled blog post: it snowed in New York yesterday. Heavy, wet snow, maybe three inches accumulation in Manhattan before the rain started. Despite the small total, there were a surprising number of downed trees. The picture above, taken before most of the snow fell, shows why: we haven’t got deep enough into […]

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