Why is modern concrete falling apart?

Here’s more information from Courland’s book “Concrete Planet” and other information I found on the web since I wrote Enough energy after Peak Oil to rebuild and repair concrete infrastructure?

It appears there’s very little testing of projects later on to see how they stood up over time to wear and tear.

It wasn’t until 1987 that engineers discovered the so-called “high strength” concrete used since 1930 was far worse than the concrete before then — buildings, roads, and other structures were falling apart over twice as fast as pre-1930 concrete structures.

This happened because “High Strength” (Portland) concrete gets strong much faster than pre-1930 concrete, greatly speeding up the time it takes to build a structure.

Why and how does “high strength” modern concrete crack and erode (which lets water in, eventually rusting out the rebar inside, ruining the structure)?

  • Annual freeze and thaw cycle, freezing of trapped water
  • Expansion of aggregates
  • Erosion by fast-flowing water
  • Vibrations and loads on bridges
  • Wind pressure sways and oscillates concrete buildings – cracks result
  • Deterioration by surface wear: Abrasion, Erosion, and Cavitation
  • Cracking:from crystallization of salts in pores, drying shrinkage, thermal contraction
  • Radiant heat
  • Deterioration by Frost Action
  • Fire
  • Chemical: carbonatation, chlorides, sulfates Corrosion of steel, Alkali-silica reaction, Sulfate attack, Delayed ettringite formation, Acid attack

Concrete statistics

  • 40 tons of concrete for every person on the planet, plus 1 ton per person per year added (7.5 billion cubic meters of concrete made per year)
  • 100 million years from now, crushed and recrystallized concrete will leave a rust-colored layer of sediment
  • First skyscraper 1891 Monadnock Building in Chicago, the tallest brick masonry structure and commercial building, the first to use aluminum for staircases. 17 stories, 214 feet high.  The word skyscraper comes from the name for the tallest sail on clipper ships (page 228).
  • 1891 also first concrete street (in Bellefontaine, Ohio).
  • American Interstate Highway System (1956-1992) largest use of concrete in any civil engineering project until then.
  • Then steel frames possible, the end of brick masonry buildings.
  • We might all be living in concrete homes now if Edison hadn’t messed up so badly (chapter 7)
  • US dams are on average 52 years old, it could cost over $52 billion to rehabilitate them

Concrete and Earthquakes

  • “Concrete lobbyists twisted the data [after the 1906 San Francisco earthquake and fire] to prove that reinforced concrete had stood up well…because of this deception, many people around the world would die in the course of the following century to buildings that they thought were immune to collapse from the violent movements of the earth”. Page 305, more details on pages 313-317.
  • Brick on the other hand, had a bad reputation, but recent research has shown that well-built brick structures did well in the 1906 earthquake (page 315).

Concrete and Fire

  • Concrete is not fireproof, but you’re less likely to be injured than in a wood structure, and have more time to escape
  • Brick, on the other hand, is born in fire, and immune to all but “insanely high temperatures”, this is why bred and pizza ovens are made of brick – if they were concrete they’d fall apart.

A world without concrete: Smaller and shorter buildings, more brick buildings, dams made of earth or huge blocks of stone, road surfaces rough except after recently applied newt layer of asphalt, lots of potholes

Roman concrete from Swift’s Big Roads

Like modern concrete, the Roman variety consisted of cement, water, and filler. Mixed, the first two ingredients form a binding paste; the filler, usually sand, gravel, or shale, is added for volume. The only complex part of the mix is the cement, which is derived, in part, from calcium carbonate, a compound found the world over in limestone; heating it in a kiln burns away the compound’s carbon and much of its oxygen, leaving behind calcium oxide, also known as quicklime. Adding quicklime to water sparks a chemical reaction—heat, gas, and a sticky gunk called slaked lime, which the Romans stored wet, in jars, until they were ready to mix it with sand to create mortar. If the job called for a denser, harder, less porous material, they held back on the sand and substituted pozzolan, or volcanic ash, which they possessed in abundance; the result was a gray concrete of such exceptional strength and durability that it wasn’t matched until modern times. Over centuries of trial and error, the Romans came to understand that concrete has great compressive strength, meaning it can bear weight placed on top of it, but little tensile strength—it can’t be pulled or twisted. They learned that it is susceptible to cracking because it shrinks as it hardens, and does so faster near its surface than in its depths, and that cracks exposed to the elements can spell its end; water seeping into a fissure expands when it freezes, scouring the crack, forcing it open, and over time reducing the concrete to rubble. Ancient engineers found that by adding horsehair to the mix they could better regulate its shrinkage, and that a dab of blood or animal fat helped it weather the freeze-thaw cycle; combined with calcium oxide, the fats created a primitive soap, and its bubbles formed microscopic air pockets that enabled the mass to withstand temperature shifts. The ancients used their expertise to build monuments, libraries and public baths, shops and houses, and roads and aqueducts traversing leagues of rolling countryside.

What set it apart from the competition was its mixture of slaked lime and clay—the latter replaced the Roman pozzolan—which together were fired in a kiln, then ground into a powder. Mixed with water, it proved fast-setting and strong. Years later, Aspdin’s son William used more limestone in the mix and cooked it in much hotter ovens. This yielded hard, dry nodules called “clinker,” which he then ground. The resulting powder was what goes by the Portland name today. By the close of the 19th century, concrete was in use around the world. Spurred by demand for fireproof buildings and a cheap alternative to stone and brick, reinforced concrete—poured around steel dowels, or “rebar,” to increase its tensile strength—had been fashioned into thousands of hotels, offices, and factories. But much was still unknown about the stuff. Engineers understood that adding filler to the mix in the form of aggregate—crushed rock, gravel, whatever— didn’t compromise strength. That because aggregate was cheaper than cement, it made sense to add a lot of it. But the specifics were sketchy. Was coarse aggregate stronger than fine? What made the stronger mix—more cement or less water? Should cement be measured by weight or volume? Measuring its strength eluded them, too.

1918 paper sharing insights he’d gleaned from “about fifty thousand tests” on concrete mixtures. The most important: water, more than any other ingredient, determined concrete’s strength. “One pint more water than necessary,” he wrote, “… reduces the strength to the same extent as if we should omit two to 3 pounds of cement from a one-bag batch.” He concluded that “the following rule is a safe one to follow: Use the smallest quantity of water that will produce a plastic or workable concrete.”

Research on roads from Swift’s Big Roads

It behooved the bureau to nail down what mixes and thicknesses of pavement lasted longest, and at the same time to establish the maximum loads that pavement should bear, a number on which the states had never achieved consensus.

On the plains west of Chicago, the bureau and its partners built a chain of six looping test tracks, each a quilt-work of paving types, thicknesses, and base layers, 836 test sections in all.

Then they moved a company of army Transportation Corps soldiers into a barracks at the complex, put them behind the wheels of 126 trucks—everything from pickups to big semi rigs, all loaded with concrete blocks—and sent them around the loops. Nineteen hours a day they drove, every day for two years, maintaining a steady thirty-five miles per hour on the straightaways, thirty in the curves. They racked up more than seventeen million miles. Along the way, the strain the trucks caused was measured by electric gauges, until three hundred million pieces of data had been recorded on punched paper tape.

What they learned filled six volumes and came down to this: The thicker the pavement and subgrade, the better. And: Trucks wear out roads in a predictable fashion.

A small piece of road survives — Loop 1– a mile or so west of Ottawa, Illinois. This was the only loop on which trucks didn’t roll; Loop 1 was intended merely as a venue to study the effects of weather. More than 50 years later, some of its test sections have devolved to loose gravel, and waist-high weeds sprout from the joints in its concrete. But here and there, the pavement looked almost new.

Further reading

If you’re interested in what a wood based society is like and what it’s capable of, read John Perlin’s outstanding “A Forest Journey: The Role of Wood in the Development of Civilization”.

To see how fast the world would crumble if we weren’t around (or there were far fewer of us): Alan Weisman “The World Without Us” and How long will concrete last if it isn’t maintained?

Andrea Hamilton. 6 march 2014. Concrete conservator. Nature.

Swift, Earl. 2012. The Big Roads: The Untold Story of the Engineers, Visionaries, and Trailblazers Who Created the American Superhighways.

 

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One Response to Why is modern concrete falling apart?

  1. Jan Steinman says:

    It would be interesting to see natural building materials — cob, straw bale, light clay, etc. — compared to brick and concrete.

    Some cob-and-thatch houses in England have been continuously occupied for 500 years. Of course, that implies they had maintenance, which is the point, no?