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?

Lawrence Berkeley National laboratory has been trying to figure out why Roman cement was so much better than today’s Portland cement: “The ancient Roman recipe is very different than the modern one for concrete, Jackson noted. Most modern concrete is a mix of Portland cement – limestone, sandstone, ash, chalk, iron, and clay, among other ingredients, heated to form a glassy material that is finely ground – mixed with so-called “aggregates.” These are materials such as sand or crushed stone that are not intended to chemically react. If reactions do occur in these aggregates, they can cause unwanted expansions in the concrete.” (Roberts 2017).

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

Vaclav Smil. 2013. Making the Modern World: Materials and Dematerialization.

While this material provides shelter and enables transportation and energy and industrial production, its accumulation also presents considerable risks and immense future burdens. These problems arise from the material’s vulnerability to premature deterioration that results in unsightly appearance, loss of strength, and unsafe conditions that sometimes lead to catastrophic failures, and whose prevention requires expensive periodic renovations and eventually costly dismantling. Concrete, both exposed and buried, is not a highly durable material and it deteriorates for many reasons (AWWS, 2004; Cwalina, 2008; Stuart, 2012). Exposed surfaces are attacked by moisture and freezing in cold climates, bacterial and algal growth in warm humid regions (biofouling recognizable by blackened surfaces), acid deposition in polluted (that is now in most) urban areas, and vibration. Buried concrete structures (water and sewage pipes, storage tanks, missile silos) are subjected to gradual or instant overloading that creates cracks, and to reactions with carbonates, chlorides, and sulfates filtering from above. Poor-quality concrete can show excessive wear and develop visible cracks and surficial staining due to efflorescence in a matter of months. Alternations of freezing and thawing damage both the horizontal surfaces (roads, parking) that collect standing water, as well as vertical layers that collect water in pores and cracks. While concrete’s high alkalinity (pH of about 12.5) limits the corrosion of the reinforcing steel embedded in the material, as soon as that cover is compromised (due to cracks or defoliation of external layers) the expansive corrosion process begins and tends to accelerate. Chloride attack (on structures submerged in seawater, from deicing of roads, in coastal areas from NaCl present in the air in much higher concentrations than inland) and damage by acid deposition (sulfate attack in polluted regions) are other common causes of deterioration, while some concretes exhibit alkali-silica and alkali-carbonate reactions that lead to cracking. Unsightly concrete blackened by growing algae embedded in the material’s pores is a common sight in all humid (especially when also warm) environments. Given the unprecedented rate of post-1990 global concretization, it is inevitable that the post-2030 world will face an unprecedented burden of concrete deterioration.

This challenge will be particularly daunting in China, the country with by far the highest rate of new concrete emplacement, where the combination of poor concrete quality, damaging natural environment, intensive industrial pollutants, and heavy use of concrete structures will lead to premature deterioration of tens of billions of tons of the material that has been poured into buildings, roads, bridges, dams, ports, and other structures during the past generation. Because maintenance and repair of deteriorating concrete have been inadequate, the future replacement costs of the material will run into trillions of dollars. To this should be added the disposal costs of the removed concrete: some concrete structures have been recycled but the separation of the concrete and reinforcing metal is expensive. The latest report card on the quality of American infrastructure gives poor to very poor grades to all sectors where concrete is the dominant structural material: with an estimated investment of at least $3.6 trillion needed by 2020 in order to prevent further deterioration (ASCE, 2013).

Transposed to post-2030 China, this reality implies the need for an unprecedented rehabilitation and replacement of nearly 100 Gt of concrete emplaced during the first decade of the twenty-first century, at a cost of many tens of trillions of dollars.

The construction of the US Interstate Highway System was a major component of this rising demand (USGS, 2006). About 60% of these multi-lane highways are paved in concrete whose standard thickness is 28 cm and hence 1 km of a four lane highway (each lane is 3.7 m wide) requires about 4150 m3. This adds up to roughly 10,000 t of concrete for every kilometer and the entire system of 73,000 km embodies about 730 Mt of concrete in driving lanes, with more emplaced in shoulders, medians, approaches, and overpasses.

Global compilations of CO2 emissions from the cement industry show its contribution almost 5% in 2010 (CDIAC, 2013).

Concrete (particularly its reinforced form) is now by far the most important manmade material both in terms of global annual production and cumulatively emplaced mass.

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 the wood based society we’re returning to 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?

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

Roberts, G. July 3, 2017. New Studies of Ancient Concrete Could Teach Us to Do as the Romans Did. Berkeley Lab, UC Berkeley experiments show how natural chemistry strengthened ancient concrete. newscenter.lbl.gov

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?