What is Concrete Cover?
The concrete cover is the distance between the nearest concrete surface and the reinforcing steel closest to it. It counts links and stirrups as well as main bars.
What it does
Concrete pore water is alkaline. This forms a protective passive film, about 10 nanometers thick, on embedded steel. Corrosion starts after chlorides or carbonation break the film. Carbonation lowers the pore water pH to the 8-9 range, where steel is no longer passive. The cover is the path these agents must cross to reach the steel, so its depth matters.
How Eurocode 2 specifies it
Eurocode 2 sets a nominal cover, which is the minimum cover plus an allowance for deviation during construction. The minimum cover is the largest of the bond value, the durability value and 10 mm. The recommended deviation allowance is 10 mm. Other codes, such as ACI 318 and AASHTO LRFD, have their own tables and are not covered here.
On airfields
Most airfield concrete pavement has no steel to cover. AC 150/5320-6G says airports use reinforced and continuously reinforced pavements only to a limited extent (paragraph 3.17). Where they do, the AC requires at least 3 inches (75 mm) of clear cover on all bars of a reinforced isolation joint (paragraph F.1.7). During construction, AC 150/5370-10H allows non-destructive rebar location devices, such as a pachometer, to check dowel and tie bar position.
Frequently Asked Questions
- What is the difference between minimum cover and nominal cover?
- Eurocode 2 defines nominal cover as the minimum cover plus an allowance for deviation during construction. The minimum cover is the largest of three values: the cover needed for bond, the cover needed for durability, and 10 mm. The recommended deviation allowance is 10 mm.
- How does concrete cover stop rebar from corroding?
- Cover keeps chlorides, carbon dioxide and water from the steel for longer. The alkaline pore water in concrete forms a thin passive film on the steel. Corrosion starts only after chlorides or carbonation break that film down. A thicker cover delays this.
- Is there a minimum concrete cover for airfield pavement?
- AC 150/5320-6G gives one: 3 inches (75 mm) of clear cover on all reinforcing bars in a reinforced isolation joint. The same AC says airports use reinforced pavement only to a limited extent, so most airfield slabs have no rebar to cover.
References
- faa-hrt-04093
- Critical Literature Review of High-Performance Corrosion Reinforcements in Concrete Bridge Applications (FHWA-HRT-04-093), Chapter 1 FHWA
- structville-ec2-cover
- Durability of structures and how to calculate concrete cover, Eurocode 2 Structville
- faa-ac-150-5320-6g
- AC 150/5320-6G, Airport Pavement Design and Evaluation FAA
- faa-ac-150-5370-10h
- AC 150/5370-10H, Standard Specifications for Construction of Airports FAA
This is a reference entry. TarmacView provides drone inspection services and doesn't sell the equipment or materials described in this glossary.
