United States
Hong KongRebar cover and splices
Concrete cover is 3 inches where cast against and permanently exposed to earth, and 2 inches for formed surfaces exposed to earth or weather with larger bars. Cover protects reinforcement from corrosion and fire.
Cover is the concrete between the steel and the outside world. Too little and the steel corrodes, spalls the concrete and the structure degrades.
The figures
| Condition | Minimum cover |
|---|---|
| Cast against and permanently exposed to earth | 3 inches |
| Formed surfaces exposed to earth or weather, larger bars | 2 inches |
| Interior surfaces not exposed | Less, per the code |
Learn the first two as a pair. The difference is whether concrete was placed directly against the ground, which is rougher and less controlled.
What cover does
- Protects steel from moisture and chlorides that cause corrosion
- Provides fire resistance by insulating the steel
- Develops bond between concrete and steel
- Resists spalling as corrosion products expand
Rusting steel occupies more volume than sound steel. That expansion cracks and spalls the cover from the inside, which exposes more steel, which corrodes faster. Insufficient cover is a self-accelerating failure.
Placement
Bars are supported on chairs or bolsters at the specified height. Steel lying on the ground before the pour has zero cover, and walking it into position during placement is the usual cause.
Splices
A lap splice transfers force between bars through the surrounding concrete over a specified length. The length depends on bar size, concrete strength and location.
That makes splice length a lookup. Tab the table.
Bar identification
Bars are marked with the producing mill, bar size, type of steel and grade. Grade indicates yield strength.
Common questions
What concrete cover is required against earth?
3 inches where cast against and permanently exposed to earth.
What about formed surfaces?
2 inches for surfaces exposed to earth or weather with larger bars.
What does cover do?
Protects steel from corrosion, provides fire resistance, develops bond and resists spalling.
Why is corrosion self-accelerating?
Rusting steel expands, cracking the cover from inside and exposing more steel to moisture.
How is splice length determined?
From a table, by bar size, concrete strength and location. It is a lookup worth tabbing.