Can Concrete Bend Without Cracking?

Concrete is not usually associated with flexibility, but as this experimental example shows, special formulations of the material can be both strong and pliable. The key is not to completely avoid cracks, but to allow the formation of distributed microcracks that can withstand increasing load. I had an old driveway poured on compacted gravel and riversand (in a river delta). Over the years, the concrete entrance developed cracks, but there were no major ruptures.

Instead of removing the entrance (disposal fees are expensive), I poured another 4" on top of the old driveway with the usual control joints. Now I find that where the cars are parked, the concrete has sunk and when it rains there are puddles in the driveway that was once level. As concrete cures, it shrinks, which can lead to cracking. And as it interacts with water, concrete does something else: it creeps or deforms gradually over time.

This has been known for decades and is included in all concrete-related calculations used in construction projects, so it's not news. But what really causes it to crawl remains a mystery. Joints in concrete can serve both to prevent cracking and as a decorative element. Concrete is not a ductile material, it does not stretch or bend without breaking. That is both its greatest strength and its greatest weakness.

Its hardness and high compressive strength is why we use so much in construction. But concrete does move, contract, expand and different parts of a building move in different ways. This is where joints come into play. It is water that converts calcium silicate in cement into very sticky compounds that can adhere to the aggregate and form concrete. If we have concrete structures that date back to Roman times, how is it that some of the concrete bridges, skyscrapers and other structures built just a few decades ago, at the end of the 20th century, are already collapsing? There are several explanations. The concrete from two mixers (blue) is fed to a tank (red), stirred (green) and then brought by a worm screw (orange) to the top of a huge three-dimensional mold. As we have already seen, concrete is a composite material, a cement matrix with aggregates for reinforcement that works well in compression, but not in tension. However, not all concrete looks as rough as this; I had to look around quite carefully to find this example on a concrete pole near my house.

Modern concrete fails through what is informally known as concrete cancer or concrete disease, which involves three interrelated problems. Because these compounds hold the concrete together, it was discovered that this stress produces a kind of internal flow of C-S-H along the nanometer grains of the concrete, causing creep. Thinking of concrete as a composite material, cement hydrate is the background material, binder (technically called matrix) to which sand and gravel add additional strength (reinforcement). Decorative concrete floors still need joints to prevent cracking, which will be even less acceptable than typical gray concrete. A thorough understanding of creep will help engineers better predict the useful life of a concrete structure and, potentially, design concretes that minimize creep, improving material life and sustainability. Apparently, it's all due to a process called dissolution-precipitation, so called because sticky compounds of C-S-H dissolve in some areas of the concrete while they precipitate (or deposit) in other areas. Dirty brown spots seen on concrete with cancer are usually caused by rusty water running through cracks.

Adding a pigment called titanium dioxide, for example, is a simple way to make concrete shiny and white within a million miles of the monotonous gray matter that gives concrete parking lots a bad name.