Cover protects steel from corrosion and fire and surrounds each bar with enough concrete to develop bond. Building codes set minimums by exposure: 3 in for concrete cast against earth, 1.5 to 2 in for formed surfaces exposed to weather and 3/4 in for interior slabs. Inspectors check it before the pour, and shortfalls show up years later as rust stains and spalls.
You've got maybe an inch of cover on the bottom mat where it's cast on dirt, it needs three, get more dobies under it.
Too much cover is a problem as well: a top mat sitting low in a slab loses its lever arm and leaves the surface free to crack.
Also heard as
- concrete cover
- clear cover
- clear
Related terms
Chair
A wire or plastic stand that holds rebar or mesh at the right height inside the form.
Dobie
A small precast concrete block, often with tie wire cast in, used to hold rebar up off the ground.
Spalling
Chunks or flakes breaking away from a concrete surface or edge, often from corroding rebar beneath.
Rebar
Deformed steel bar cast into concrete to carry the tension that plain concrete cannot.