Civil Engineering Reference
In-Depth Information
pH Value vs. Depth in Concrete
12
11.5
11
pH threshold for
corrosion
10.5
10
limit of fully alkaline area
9.5
Neutralisation
front measured
by phenolphthalein
9
8.5
8
7.5
7
0
10
20
30
40
50
60
Depth (mm)
Carbonation-induced corrosion damage occurs most rapidly when there
is little concrete cover over the reinforcing steel. Carbonation can occur
even when the concrete cover depth to the reinforcing steel is high. This
may be due to a very open pore structure where pores are well connected
together and allow rapid CO
2
ingress. It may also happen when the alkaline
'reserves' in the pores are low. These problems occur when there is a low
cement content, high water to cement ratio and poor curing of the concrete.
A simple rule for carbonation is that the depth of carbonation (in mm)
should roughly equal the square root of the age in years. There are numerous
models available for the estimation of carbonation depth, see (Broomfield,
2007).
Carbonation depth (mm) =
k
(age in years)
1/2
where
k
is a constant and roughly equal to 1 for good-quality concrete
However, the value of
k
depends on humidity, type of surface finish and
a number of other factors.
At the carbonation front there is a sharp drop in alkalinity (
Figure 1.10)
although it can be seen that a small zone exists where corrosion is possible
though phenolphthalein would remain pink. Once the pH drops, the passive
oxide layer, created by the alkalinity, is no longer stable so corrosion can
commence in the presence of sufficient air and moisture.
