Page 151 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

MECHANISM OF CORROSION AND ASSESSMENT TECHNIQUES IN CONCRETE 129

1. Contamination because of (i) deliberate addition of calcium chloride set accel-
erators; (ii) deliberate use of seawater in the mix; (iii) accidental use of inade-
quately washed marine source aggregates.
2. Ingress consisting of (i) deicing salts; (ii) sea salt; (iii) chlorides from chemical
processing.

Until the later 1970s, it was widely thought that the chlorides cast into concrete
existed as chloroaluminates and would not cause corrosion. It was later found that
large numbers of structures were corroded because of the presence of chloride in the
mix, and the binding was not as effective as initially believed.
ACI Report 222 R-01 reviews the national standards and laboratory data. The
consensus is that 0.4 wt% of chloride in cement is a necessary but not sufficient
condition for corrosion and in variable chloride and aggressive conditions, corro-
sion can occur at lower chloride levels, down to ∼0.2% of chloride by weight of
cement.
According to the literature review (5), it is thought that whether chlorides are
bound or not, the chloroaluminates decompose releasing chloride ions, which cause
breakdown of passivity. It is also suggested that the amount of chloride bound in the
cement paste is not very important. The amount of calcium hydroxide available to
maintain the pH has a profound effect on the initiation of corrosion.
Chloride ingress into concrete follows Fick’s second law of diffusion, forming a
chloride profile with depth into the concrete:

−
2
−
d[Cl ] Cl [Cl ]
= D ⋅
t
dt dx 2
−
where [Cl ] is the chloride concentration at depth x and time, t. D is the diffusion
t
2
coefficient (about 10 −8 cm /s).
The solution to the above differential equation is:
(C max − C ) x
x,t
= erf
(C − C ) (4D t) 1∕2
max min c
where C max is surface or near surface concentration; C is the chloride concentration
x,t
at depth x; and time t, C min is the background chloride concentration erf – is error
function.
The parameter C must be a constant as surface measurement is used to avoid
max
fluctuations in surface levels on wet and dry surfaces.
3.5.2 Carbonation of Concrete and Corrosion

Carbonation is a simpler process than chloride attack. Atmospheric carbon dioxide
reacts with pore water to form carbonic acid. Carbonic acid reacts with calcium
hydroxide and forms solid calcium carbonate. The pH drops from about 13