Concrete

Curious Cracks

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Beth is buying a 100-year-old house in Barre, and hired PERCH to investigate some cracks in a concrete foundation wall. The cracks were discovered by the current owners 10 years ago when they removed a wall covering in the basement. Beth needed a structural engineer to assess the cracks’ severity before proceeding with the sale.

The terrain outside the wall slopes at a steep angle, up to 45 degrees, with a window above the high point of the grade. Several roof lines direct water into the corner; gutters were added at some point to divert the water. The outside face of the wall jogs inward at two locations: a stair-step jog near the southwest corner, and a vertical jog near the bottom corner of the window.

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Exterior view of the foundation wall.

On the interior side, there are several vertical and diagonal cracks in the wall between the floor slab and the window. The widest cracks extend from the top east to the bottom west, and vary from ¾ inch to 2 inches wide. (Other cracks are smaller.) There’s also a crack in the floor slab itself, which has a slight downward slope toward the corner. All the cracks were filled with spray foam by the current owners and have remained stable over the last 10 years.

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Wall crack – filled with spray foam. Floor slab crack – slopes down into corner.

PERCH determined that the culprit for the cracks was a familiar one: differential settlement. Soils under a heavy load may compress at different rates, causing one location to sink lower than another. This is especially true of old houses built on whatever soil happened to be on site, with no fill or compaction. Here it’s clear that the corner of the floor slab has settled lower than the middle of the slab. The stress concentration in that corner, along with the lack of damage elsewhere, makes differential settlement a better explanation than frost heave.

As for the wall, concrete is most likely to crack at stress points, where the forces in the concrete change rapidly. The bottom corner of a window, in which the wall next to the window supports two floors and a roof while the wall under the window supports only the window itself, is a textbook example of a stress point. Here one can imagine the wall sliding along the diagonal crack, down to the lower point as a result of differential settlement.

Cracks emanate from corners of window.
Wall is plumb.

So should Beth be concerned? In this case, no. The cracks have not widened in the last 10 years, and gutters divert water to prevent further impact on the soil. Also, the wall is plumb and the cracks don’t extend all the way through. (There’s a simple explanation for the oddities on the outside of the wall: imperfect formwork when the concrete was originally placed.) PERCH delivered a report to Beth with some long-term suggestions for keeping the cracks insulated and the soil dry. The sale proceeded on schedule.

Monthly Mechanics: Reinforced Concrete

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This is Part 2 of a series about building materials. Read Part 1 here.

If you measure the #1 building material in the world by weight, steel and concrete are neck and neck. If you measure by volume, concrete wins hands down. The World Steel Association estimates that 1630 million metric tons of steel were produced worldwide in 2016 – a bit over 7000 million cubic feet. Meanwhile, the Royal Society of Chemistry estimates that 2000 million metric tons of concrete are placed each year – over 30,000 million cubic feet.

These are staggering numbers. If you put a year’s worth of concrete all together you could build a solid cube measuring over half a mile on each side. Obviously, concrete has the edge over steel by volume because it’s much less dense. Steel is over 90% iron, and its density is basically the same as pure iron. (Every structural engineer knows this density by heart: it’s 490 pounds per cubic foot.) But concrete is a complicated mixture of different sizes of rocks and sand, Portland cement, water, and various additives, and depending on how it’s mixed a builder can trap a significant amount of air inside or just a little bit. Thus the density of concrete varies.

Sand and gravel (aka aggregate).
Portland Cement.
Water.

Rocks might seem heavy, but in fact they don’t get much denser than 150 pounds per cubic foot, so this is the upper limit on the density of concrete. Lightweight mixtures get down to 90 pounds per cubic foot with more entrained air; they’re often specified when concrete doubles as the architectural finish and needs to look sleek. Specialty products, such as autoclaved aerated concrete, are actually so light that they float. But such lightness comes at a big financial cost and reduces the strength as well.

Speaking of strength, concrete as a building material has one serious shortcoming. It works wonderfully in compression, but has almost no capacity at all in tension. That’s why you will never see a concrete chandelier.

Engineers address this shortcoming by putting a little bit of steel inside the concrete. The result is a composite material called reinforced concrete, in which the concrete provides the compressive strength and the steel provides the tensile strength. If the concrete doesn’t need much tensile strength (a slab on grade for example), then a steel mesh or welded wire fabric may be sufficient. In other situations, reinforcing bars (aka rebar) are used. They come in sizes ranging from 3/8 inch diameter to over 1 inch diameter, and they always have bumps on their surface to prevent them from sliding through the hardened concrete.

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Concrete beam cross section analysis. The striped region at the top is the effective concrete area. The dots at the bottom are the steel reinforcing.

In designing a reinforced concrete beam, engineers try to balance tension and compression. Recall that when a beam bends downward, the bottom is in tension and the top is in an equal amount of compression. Reinforcing steel goes in the tension zone. It takes a certain amount of concrete in the compression zone to balance the strength of the steel, and that amount is surprisingly small. (Most reinforced beams are tension controlled, which means the steel fails before the concrete does.) In other words, most of the concrete in a reinforced beam never feels any stress! Its only purpose is to hold the beam together.

UNreinforced concrete structures are rare today, but early builders took advantage of the material’s pure compressive strength to produce spectacular arches and shells. The Pantheon in Rome, finished in the year 128, remains the world’s largest unreinforced concrete dome. In the early 20th century, engineers like Félix Candela and Robert Maillart took unreinforced concrete to its artistic apex; many of their structures still survive.

Pantheon, Rome.
Los Manantiales, Mexico. (designed by Felix Candela)

Tiny Tuesday: Recycled Plastic for Stronger Concrete

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Concrete production is responsible for about 4.5% of the world’s carbon emissions. As such, materials engineers are interested in reformulating the material to reduce its impact. One technique is to replace the most production-intensive ingredient, Portland cement (concrete’s binding agent), with fly ash (a byproduct of burning coal) – emissions reduction with no loss of strength. Another technique previously explored in this blog is autoclaved aerated concrete, which eliminates the large aggregate and adds hydrogen bubbles for a lighter block, resulting in a greater volume of concrete from the same amount of cement.

Recycled plastic is enticing as a material to replace even more Portland cement, but every formulation with plastic has produced a weaker concrete… until now. A team of MIT students has discovered that if polyethylene flakes are exposed to gamma radiation and then ground to a powder, the resulting crystal structure is actually beneficial to concrete. In fact, compression tests on concrete samples made with fly ash and 1.5% irradiated plastic showed a 15% higher strength than concrete made with Portland cement and no plastic. Gamma ray radiation leaves no radiation residue behind; it’s used commercially to decontaminate food. All this and more is explained in this MIT News article.

The article does not indicate when irradiated plastic concrete will hit the market, or how much more it will cost than regular concrete. Radiation uses specialized equipment and has some PR issues, so it may be difficult to achieve mass production. The MIT project team included an old professor of mine, affectionately known as Oral B.