Concrete Non-Failure

Concrete, as a composite material, has more potential modes of failure than steel. A steel beam can be overstressed (and yield or rupture), rust, or buckle sideways for lack of bracing. Barring some really esoteric failure modes, that’s about it and that’s plenty. A concrete beam (or, as pictured above, a slab) can be overstressed, can buckle for lack of bracing (although that’s quite rare), can have concrete spalling, can have steel rebar rusting, can have loss of bond between the concrete and the rebar, can fail in serviceability because of the presence of tension cracks, and can have some rare esoteric problems.

What’s interesting is how bad a failure can look without much actually being wrong. In the case of the balcony above, the concrete was badly cracked and spalled before demolition; removal of the damaged concrete showed minimal steel damage. The terrible state of the concrete may have been secondary damage (freeze-thaw cycle in unprotected surface cracks, for example), or maybe poor quality of the original concrete (concrete quality follows a bell curve, and if the weak and porous tail of that curve happens to occur at a balcony where the concrete will be exposed to the weather, oops).

It can be more difficult to diagnose a problem when not much is wrong. If the rebar were rusted, then we’d know immediately what the problem was. Fortunately, we’re engineers and not scientists: we don’t need to know the exact cause if we know what to do now. In this case, what to do was coating the rebar and repairing the concrete. Problem solved, even if the cause of the problem was unknown.