QUESTION: Why does concrete in a bridge structure crack and spall above the rebar?
ANSWER: If all bridge structural members had low-permeability concrete, crack-free concrete, and thick concrete cover, that wouldn’t happen. But on many older bridges (and even some newer ones) that wasn’t the case, although on some older bridges, time simply took its toll.
Here’s what happens in the bridge concrete. When it is new, the very alkaline environment (high pH) in concrete creates a “passivating” layer around the steel reinforcing bars that prevents them from corroding. At the very high pH of the concrete, the steel cannot corrode or rust.
Over time, chloride from deicing salts and carbonation from atmospheric CO2 can penetrate the concrete and destroy the passivating layer. When water and oxygen also get to the reinforcement, the steel begins to corrode or oxidize. The iron in the steel reacts with the water and is oxidized to iron oxide (rust) which has greater volume than the steel. The force of this expansion is greater than the tensile strength of the concrete and so it must crack.
This can also happen where cracks in the concrete are deep enough to allow passage of water and deicing salts to the steel rebar. There are many reasons the concrete can crack but high cement content or thermal gradients in the fresh concrete are the most common. But to be fair to bridge designers of the past, some bridges have simply outlived the concrete’s intended protection of the steel—rust never sleeps!
The solutions to this problem are to make concrete for bridge structures that has very low permeability, is crack-free and has thick enough cover (the distance between the outside face of the concrete and the reinforcing steel) to seal up the steel for a very long time (to maintain the passivating layer).
Another solution is to use reinforcement that is not susceptible, or less susceptible, to corrosion, such as steel that is epoxy-coated or galvanized, or stainless steel, or carbon-fiber-reinforced polymer reinforcement. All of these, of course, have an associated cost but one that is surely much less than the disruption caused by repairing or replacing a bridge.