Disappearing Sand

Disappearing Sand

This story isn’t exactly about composites. But in the end, it is.

The dodo bird became extinct because nobody realized it was threatened.  It lived exclusively on the island of Mauritius, near Madagascar, in the Indian Ocean. Until European settlers arrived in the late 1500’s, the island was not inhabited by humans, and the dodo had no natural predators. The name dodo is thought to derive either from the Dutch “dodoor” meaning sluggard, or possibly the Dutch “dodaars” meaning fat-assed or knot-assed, or possibly from the Portugese “doudo” meaning crazy or fool. Contemporary literature suggests that the dodo was fearless of humans: you could walk right up to it and it wouldn’t think to run away from you, so you could hit it on the head, kill it, and eat it. It was about 3 feet tall, and provided food for many hungry sailors at a sitting.

It was extinct within 100 years of encountering humans, partially because of hunting, but also due to competition for resources with other species that humans brought to the island, especially pigs.

It is one of the most extinct species, too.  There are no known complete skeletons or fossils, nothing but a dried head, a leg, and a handful of bones. Stuffed dodos are said to have been trashed by museums before the 20th century. They seemed abundant, so each museum probably thought they could afford to ditch theirs, since there would be others elsewhere.

We recently learned about another resource, one generally considered so abundant as to be inexhaustible, that is actually disappearing and becoming scarce: sand.

Sand is largely silica (silicon dioxide), a mineral most commonly found in the form of quartz. Sand is little tiny bits of it, broken apart by wind and water movement.   Silica is said to be the most abundant mineral in the Earth’s crust. Small wonder it is everybody’s favorite metaphor for abundance. In the Old Testament, God tells Abraham, “I will multiply thy seed as the stars of the heaven, and as the sand which is upon the sea shore.” A lot of other people have utilized this metaphor since then.

plaster-sand_0715-cropSand is widely used in construction, especially in combination with portland cement. Sand, and its big brother, gravel, are referred to collectively as aggregate. Both are used in landscape and infrastructure construction for road substrates, in concrete and asphalt pavements, as retaining wall backfills, for drainage, and other large-scale projects. In structural applications, aggregate is a major component of concrete, which is the most widely used construction material in the world. Concrete uses six to seven times as much aggregate as it does portland cement. Sand is also in mortar for brick and block construction, and tile installation; and in asphalt for roads and roofs. It is a component of building cladding materials, too, including plaster (stucco), fiber cement siding, fiber-reinforced concrete, and glass.

One of the reasons sand became a standard component of these materials is that, during the past century when standardization took hold, sand was not only abundant, but ubiquitous. Sand and gravel could be sourced locally just about anywhere, minimizing the shipping cost for the heaviest component of concrete and asphalt.

But good aggregate is becoming harder to find. It is formed by erosive processes over thousands of years, and we are currently extracting it and using it faster than it is being formed. The worldwide rate of extraction is monumental, estimated at about 40 Billion metric tons (88,000,000,000,000 lbs) annually worldwide.

Desert sand, which is formed largely by wind erosion, is considered bad for concrete because it is too rounded, so it doesn’t lock up well to form a strong structure. Inland sources of usable sand are being used up. Riverbeds and beaches are being stripped and sand is being dredged out of the ocean.

The costs of this activity are many and they are steep.

There’s a cost in money: roads and buildings made of concrete are getting more expensive as sand has to be dredged up, trucked longer distances, and sometimes treated to remove unwanted components (like salt).

There are many costs to the environment.  More mining operations and transportation means more fossil fuels burned, adding CO2 to the atmosphere and accelerating climate change. Rising sea levels caused by climate change threaten the existence of low-lying islands. In the Maldives, the main island Male is building a wall to protect the land from rising seas and provide a future safe haven for citizens from the surrounding smaller islands that are threatened by the tides. They’re using sand from neighboring small sand islands… ironically hastening the disappearance of those small islands and the need for their denizens to seek refuge in Male.

Riverbeds are being lowered. This has increased erosion surrounding the rivers, and even lowered the water tables in surrounding areas.  Combined with rising temperatures, that’s a double-whammy on fresh water supplies.

Ocean ecosystems are being changed and in some cases destroyed by dredging and by the clouds of disturbed particles that filter out sunlight. In places where the humans rely on fishing for food or commerce, their way of life is being threatened along with that ecosystem.

There is even a black market in sand.  People have been killed in battles with law enforcement over illegal sand mining.

Amazingly our sand addiction has become a significant social, economic, and environmental issue. But how do you use less sand?

One of the things we can do to reverse this trend is to use less concrete. Coincidentally, one of the major uses of composites in buildings is to replace concrete as a cladding material, and in some cases, as a structural material. The most common composite is based on glass fiber, which is also silica, but the amounts used are orders of magnitude smaller than in concrete.

Since composites are among the lightest materials in weight, they save more sand beyond the concrete they replace directly. Lighter cladding does not need as heavy a structure to support it (and the structure is often concrete). Composite structural elements replace concrete, wood, or steel directly. The lighter the building, the less foundation needed, and that’s a lot less concrete. More composite buildings means less concrete, which means less sand used.

The new expansion of the San Francisco Museum of Modern Art (SFMOMA) is a good example. The composite East façade is about 54,000 sf. It was originally conceived as fiber-reinforced concrete panels, which would have been about 6 inches thick and weighed about 70 lbs/sf, or about 3.8 million pounds. Of that, over 3 million pounds would be aggregate. The composite façade is less than ¼-inch thick and weighs less than 5 lbs/sf. While it does include some natural aggregate, the amount (compared to the original concrete proposal) is miniscule. The switch to composite cladding saved all that concrete, plus an additional 1 million lbs of steel that would have been needed to support the concrete. Undoubtedly, this reduced the load requirements right down to the foundation, saving money and concrete from top to bottom.

Sand isn’t going to become “extinct,” but our assumptions about its abundance are driving behaviors that could have far more catastrophic consequences than the loss of the dodo. There are strategies to reduce damaging sand extraction, such as mixing in a proportion of desert sand along with the “good” sand, to keep feeding our concrete habit. But it is time we started looking beyond concrete altogether. It requires a change of thinking: concrete is familiar, and that makes it easy to use for everybody in the construction chain. But the cost of concrete is high, the monetary cost being the least of it. It’s time to venture beyond our comfort zone and look at new materials that will better sustain the planet and our life on it.

SFMOMA - rendering - image via Snohetta

The facade of the new expansion of the San Francisco Museum of Modern Art (SFMOMA) is composed of 700 composite panels weighing about 270,000 lbs, replacing about 3.8 million pounds of concrete.