When we look at each other we see the patina of years gone by on our faces and hands. That patina is a nice way to describe the spots, wrinkles of time, and weathered look. And along with them comes aging, of course. Bones harden and shorten, skin thins and loosens, eye lenses harden, and endurance shortens, along with other aging necessities.
So too does concrete, brick and stone masonry, plaster, stucco, and the like, change with time. Some changes are for the better—others are for the worse. But many of them can be prolonged or accommodated. Portland cement-based products have a projected service life, or at least we think we can make that projection using formulas and data stuffed into computers. But they are only intelligent guesses, like using background data on a thoroughbred to assess its potential for winning. The odds of winning are increased, but never to 100 percent. We recall one service-life formula that projected the concrete would fail before it was even put in service!
One of our major goals for concrete performance at acceptable maintenance costs is the projected longevity. Some architects and engineers are now asking for service lives of 100 years—and think they will get it. Maybe so, but who will be around to celebrate the event?
With time, concrete develops a patina, the wrinkles of craze cracks, color changes, the topsides of sand particles protruding from surfaces accompanied by light sandpaper-textured surfaces. We all know that weathered look and accept it just as you accept the patina of man. And, just like us, there are internal changes. The concrete skin carbonates (from the day it is exposed to atmospheric air); that slows with time, but keeps progressing. The carbonation depth is usually shallow for reasonable quality concrete, but may extend to ¾ inch or more. The carbonation process strengthens the surface region—a good thing—but increases shrinkage. Along with normal drying, the shrinkage results in enhanced and slightly deeper craze cracks caused by the carbonation, not necessarily a good or bad thing. With respect to wear, carbonation is a good thing because of the surface hardening. But with respect to reinforcing steel that carbonation may touch, the reduced pH accompanying carbonation will allow the steel to rust, a bad thing.
The Europeans—who designed shallow cover over reinforcing bars—invented the re-alkalinization process where, using electrochemical methods, alkalies move to the reinforcement, raising the low pH levels resulting from carbonation to levels that passivate corrosion. A twist of that method is used to remove chlorides from concrete.
Alkali-silica-affected concrete too has a champion—lithium salts that mitigate the reactions. It is easier to add them during concrete manufacture than after the concrete is hard and the alkali silica reactions have occurred. So, work is underway to develop means and methods for getting the lithium salts into hardened concrete.
Cracks that form as a consequence of aging (shrinkage) or physical forces that overtax concrete strength, or delaminations, can be repaired by epoxy injection, and deteriorated joints can be repaired a variety of ways.
We recall the time when warehouse slab curling was a pain to forklift trucks. The slabs couldn't pass the corn flakes test. That is, corn flakes cartons are piled on the forks of the trucks and driven at a given speed over the joints. The cartons are piled higher and higher until the stack topples. The joint allowing the greatest number of stacked cartons wins? Or loses? In any event, based upon someone's advice, the owner elected to have the joints ground flat, level with the rest of the slabs, contrary to what others told him. The repair overwhelmingly passed the corn flakes test. However, sometime later, the joints became shallow swales as the concrete progressively shrank in depth from continued drying shrinkage—and then failed the corn flakes test.
We must live with aging and accommodate and repair the effects of aging as may be appropriate. We now have the bionic man with heart repairs and replacement, new knees and hips, teeth implants, and more. The age of bionic concrete may now be with us, for better and not for worse.