Concrete

Barn Stormer

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Geoff owns a barn. An old barn. A barn so old, the gravel road that once ran next to it has been successively filled and regraded, and now towers a good four feet above. All that extra weight, and the drainage pattern it creates, spell danger for the barn’s foundation wall.

Geoff hired PERCH to ascertain if the barn is safe to keep horses and store 16 tons of hay. He also asked for advice on any low-cost repairs he could make to stabilize the structure. My visual inspection revealed lots of band-aids placed by previous owners. Two tall reinforced concrete blocks prop up the fieldstone foundation wall that’s bowing from the surcharge of the raised-up road. A cable runs from one gable end to a ground anchor, apparently to resist prevailing wind. Extra posts have been added on the lower level to support sagging beams.

Ground-anchored cable stabilizing the south wall..
Concrete wall stabilizing the west wall.

My first suggestion to Geoff was to add more posts to carry vertical load directly to the ground, relieving the burden on the mist critical foundation wall. In particular, I said, the hay should be supported as directly as possible. I also suggested a buried drain pipe between the barn and the road – hydrology is not my area of expertise, but anything to drain water away from that spot will be beneficial. (Geoff believes he already has a drain there, but it’s probably clogged.)

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Beam has migrated right and is no longer supported by the column.

Geoff was pleased to implement my suggestions. Engineers assign a smaller importance factor to a barn than a house because of the low risk to human life, but I want to keep his horses safe too!

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Inside the barn.

Tiny Tuesday: Consider Aerated Concrete

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A recent Boston Globe feature describes a luxury home built with autoclaved aerated concrete (AAC), the first in New England. The material comes in a lightweight masonry block, giving it the flexibility to create unique wall shapes and angles. AAC is a boon for indoor air quality, as it contains no volatile organic compounds (VOCs) and provides excellent air tightness. It’s also fire- and mold-resistant. So why hasn’t AAC caught on in America to match its popularity as a high-performance building material in northern Europe? In short, why have you never heard of it?

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The first New England home built with autoclaved aerated concrete.

Before we answer that question, let’s look at what AAC actually is. AAC is manufactured by mixing sand with a binding agent (such as cement, fly ash, lime, or some combination), water, and a tiny amount of aluminum powder. Unlike regular concrete, it contains no large aggregate like gravel. Instead, the aluminum powder reacts with the other ingredients to form hydrogen bubbles which greatly increases the volume of the mix. That’s the “aerated” part. The mix is still soft at this point, so it’s cut into blocks and placed in an autoclave, or pressure chamber, until it achieves its full strength.

The result is a block that looks like pumice and weighs about the same – much lighter than regular concrete by volume. And unlike a CMU, this block is solid all the way through. In theory this property makes the block self-insulating.

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Building a wall of AAC blocks.

AAC’s insulating properties are disputed, though. A Green Building Advisor article states that the R-value of an 8-inch block is only R-8 to R-11. (Compare 8 inches of dense-packed cellulose which is about R-24.) Other issues include moisture and water vapor – both can readily permeate AAC – and structural integrity – there’s no space for reinforcing bars which could resist extreme wind.

Add that to a 15% cost premium over stick-frame construction, and you can understand why American builders show little interest in the material. But consider AAC for yourself and decide whether the health benefits are worth it.

Retaining Order

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South of Boston, contractor Julian Crane designed a new facility to store and repair their heavy construction equipment. The superstructure is a prefabricated steel frame. They hired me to design the foundation.

I proposed a stem wall with footing below the four exterior walls of the building. Every project is unusual in some way, and for this facility it was a sloping site that dropped off into a wetland. No sitework was allowed on the wetland itself, but my client wanted to build the facility as close as possible on the higher ground. So I proposed that the foundation wall double as a retaining wall on two sides. The wall would have a combined loading of vertical forces from the building itself and horizontal forces from the soil behind it.

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Retaining wall (top) | Frost wall (bottom)

One of the more insidious forces that engineers must consider is live load surcharge. When you stand in a precarious spot, like atop a pile of dirt or a snowy ridgeline, your weight pushes the ground material outward. Too much force and the dirt collapses or the snow avalanches. Your weight is the live load (because it can move around), and surcharge means it’s an extra force that increases the material’s tendency to spread. At the Julian Crane facility, my retaining wall needs to hold back not only the soil itself, but also a surcharge from the weight of any heavy construction equipment inside the building.

Footing formwork.
Footing placed, stem rebar.

I specified an 8-foot retaining wall under the two sides of the facility where the ground slopes down to the wetland, and a 4-foot frost wall with minimum reinforcement under the other two sides for economy. My client decided to go deluxe and had the contractor use the 8-foot wall detail on all four sides, with my approval. I also specified a minimum bearing strength for the soil below the foundation footings, which the contractor achieved by filling and compacting new soil.

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Inside the steel-frame facility.

Construction proceeded over the last year, and (pending some final sitework including guardrails and paving) the facility is substantially complete.

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Outside the new facility.