Stages of corrosion development in structures 1. Neutralization of the cover layer or contamination with aggressive chemical substances – decrease of… [626963]

DURABILITY
OF THE
REINFORCED CONCRETE
STRUCTURES

Stages of corrosion development
in structures
1. Neutralization of the cover layer or
contamination with aggressive chemical
substances – decrease of protective
properties
2. destruction of the passive layer of the
rebar
3. separation of the cover layer from the
rebar

Steel corrosion
Ferrum oxidation – Fe2O3 (rust)
Concrete corrosion
1.Carbonation ( starts when pH less than 12)

2.Leaching of calcium compounds
3.Interaction of chlorids (airborn, sea water,
industrial water, spray)
4.Interaction of sulfur compounds
 OH2 CaCO COH OHCaCOH OH CO
2 3 3 2 23 2 2 2
 

Corrosion protection
of steel reinforcement
•Adequate concrete tightness and c/w ratio,
•Concrete resistant to environmental conditions,
•Adequate class of concrete,
•Low reinforcement ratio (economic),
•Adequate cover depth,
•Low reinforcement diameter,
•Water and chemicals insulations,

Exposure classes of the environment

Minimum classes of concrete

(The recommended structural class is S4)

Concrete cover
•The nominal cover

•The minimum concrete cover
shall be provided in order to ensure:
– the safe transmission of bond forces,
– the protection of the steel against corrosion,
– the fire resistance.
dev min nom c c c 
 mm10; c c c c; c max c add,dur st,dur ,dur dur min,b min, min   

•nb – the number of bars in the bundle
– vertical bars in compression and for
bars in a lapped joints
– other cases
mm55 nb n 
4 nb
3 nb

The recommended modification of the
structural class
The limit may be reduced by one strength class if air entrainment
of more than 4% is applied

cdur, – additive safety element,
Δcdur,st – reduction of minimum cover for use of
stainless steel ,
Δcdur,add – reduction of minimum cover for use of
additional protection

Recommended values : 0

Cover deviation
•Recommended value: 10mm

In certain situations, the accepted deviation and
hence allowance, Δcdev, may be reduced.

The reduction in Δ cdev in such circumstances for use in
a Country may be found in its National Annex.

The recommended values are:

-where fabrication is subjected to a quality assurance
system, in which the monitoring includes measurements of
the concrete cover, the allowance in design for deviation
Δcdev may be reduced:

10 mm ≥ Δcdev ≥ 5 mm

-where it can be assured that a very accurate measurement
device is used for monitoring and non conforming members
are rejected (e.g. precast elements), the allowance in
design for deviation Δcdev may be reduced:

10 mm ≥ Δcdev ≥ 0 mm

Special cases
•If in-situ concrete is placed against other concrete
elements, than the minimum cover to the interface
may be reduced to a value corresponding to the
requirement for bond provided that:
– the concrete class is at least C25/30,
– the exposure time of the concrete surface to an
outdoor environment is short (< 28 days),
– the interface has been roughened.

•Uneven surfaces: minimum cover should be
increased by at least 5 mm.
•For concrete cast against uneven surfaces, the
minimum cover should generally be increased by
allowing larger deviations in design. The increase
should comply with the difference caused by the
unevenness, but the cover should be at least k1 mm
for concrete cast against prepared ground (including
blinding) and k2 mm for concrete cast directly
against soil.

•The cover to the reinforcement for any surface
feature, such as ribbed finishes or exposed
aggregate, should also be increased to take account
of the uneven surface.
The values of k1 and k2 for use in a Country may be
found in its National Annex. The recommended
values are 40 mm and 75 mm.