Tiny Tuesday: Consider Aerated Concrete

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?

AAC3

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.

AAC2

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.

Tiny Tuesday: The Cargominium

We’ve seen shipping containers transformed into single homes, hotels, stores, and farms. How about high-density housing? That’s the premise of the Cargominium, a complex in Columbus, Ohio that when complete will likely be America’s largest shipping container residence.

This Columbus Dispatch article describes the 25-apartment structure, designed by Columbus architect Moody Nolan. It’s three stories tall and consists of 54 8-foot-by-40-foot steel containers. General contractor Chelsi Technologies stacked the modules on-site in one week, over 10 times faster than stick-frame construction would have taken. Each two-bedroom, 640-square-foot unit consists of two containers side by side. Exterior stucco will give the building an appealing facade and hide the containers from view. (Although if that’s the goal, the developer might also consider changing the name.)

Probably the biggest advantage to using shipping containers as structure is the reduction in construction cost. The developer claims the project cost 30% less than a similar-size building built conventionally, mainly thanks to a reduction in labor with the faster installation schedule. Containers also withstand wind loads and earthquakes exceptionally well – they’re built to cross oceans, of course – and have a low embodied energy since very little virgin material is required to make them habitable.

I remain wary of the insulation and airflow systems required for a shipping container interior to provide adequate comfort. But increasing numbers of designers in recent years are making it work, functionally and financially.

The building expects to house people in transition, moving from homeless shelters or rehab facilities. (The purpose is less groundbreaking than the “Housing First” initiative in some cities, but it fills a similar void in that making more housing available to those who need it most.) Thus, it is not just the containers but also the residents who will find a new life for themselves within the Cargominium. There’s something poetic in that.

Monthly Mechanics: Simple Machines, Part 2

Please read last month’s article for the first three simple machines: The lever, the pulley, and the wedge.

4. The Inclined Plane

Basically a fancy name for a ramp, an inclined plane makes up-and-down motion easier by spreading it over a distance. The most obvious use of an inclined plane is for transportation by pedestrians (stairs are difficult for a mobility-impaired person or a wheelchair) and vehicles (stairs are basically impossible for a car). The slope, or pitch, of the plane determines the mechanical advantage. It takes only half as much force to push an object up a 30-degree plane as it does to lift it straight up, but you’ll need to move the object twice as far.

Another use of an inclined plane is to slow down something that’s falling. A roller coaster doesn’t drop straight to the ground (that would kill you); it rolls down a ramp, which also gives it forward momentum and enables it to climb the next hill smoothly. When Isaac Newton developed his laws of motion, he slid various objects down an inclined plane to see how quickly they accelerated. Freefall happened too fast for him to observe with his 17th-century equipment.

SimpleMachines_InclinedPlane

5. The Screw

A screw converts circular force to linear force. You twist your screwdriver, or wrench, or drill… and it moves your screw, or bolt, or drill bit in a completely different direction. Which direction? Screw threads usually follow the right hand rule: if you twist in the direction your fingers curl on your right hand, the screw moves in the direction your thumb points. (Righty tighty, lefty loosey.) If you ever see a screw with left-hand threads, there is a good reason for it. For example, you’ll find a left hand screw inside your toilet that keeps the flusher from falling off with repeated presses.

As with most of the simple machines, a screw trades force for distance. Twisting a screwdriver might be 100 times easier than pushing a screw straight into a piece of wood, but your hand will need to move 100 times as far as the screw actually goes. The total, force x distance, is always preserved no matter what machine you use – a concept known as conservation of energy.

SimpleMachines_Screw

6. The Wheel

A wheel (with an axle) converts sliding movement to rolling movement. Think of how hard it is to drag a file cabinet across the floor, and how much easier it is if that cabinet is on a dolly. Sliding usually creates lots of friction because there’s a huge amount of surface area in contact. A wheel reduces that contact zone to a tiny area.

The irony is that even though a wheel reduces friction, it depends on friction to work in the first place. On a frictionless surface, a wheel can’t roll forward; it just spins in place. You know this if you’ve ever tried to drive on a wet or icy road. The wheels spin without moving forward, and the car doesn’t roll – it slides.

SimpleMachines_Wheel

Wheels are commonly found in more complex machines. A gear is a series of wedges spaced equally around the outside of a wheel. A crank is a lever that turns a wheel at the fulcrum. And technically, most pulleys are actually a combination of a pulley and a wheel.

Here’s a fun question to ponder: if your car is moving forward at 5 feet per second, how fast is your tire tread moving? Place your bets now, and we’ll answer this question next time on Monthly Mechanics.