Civil Engineering Reference
In-Depth Information
10.2 Types of inhibitor
10.2.1 Inhibitors mixed into fresh concrete or concrete repair
materials
Calcium nitrite is the main material used for this application and has a long
history of successful use, especially in the United States, with more than 300
structures using this inhibitor in a wide range of different types of structure
(Page et al., 2000), provided the ratio of chloride to nitrite remains below
about 1.8:1 (Broomfield, 2007, Vermani and Clemena, 1998). In practice
this means a dosage of at least 22 kg per cubic metre of calcium nitrite
dissolved in an appropriate amount of water (and that amount included in
the total water content of the concrete when calculating the mix design).
This assumes up to about 2% of cement as contaminating chloride and a
cement content of, for example, 330 kg/m
3
. Other researchers have suggested
a lower nitrite to chloride ratio (Page et al., 2000) but caution needs to be
exercised as too low a ratio can exacerbate pitting corrosion.
Calcium nitrite has been used in a good many structures to inhibit
corrosion, some several decades old and a number of marine structures in
the UK use calcium nitrite at least partly to inhibit corrosion. Information
from Grace, one of the main suppliers, suggests more than 10 million cubic
metres of concrete have been placed with calcium nitrite inhibitor worldwide.
In practice, thousands of structures have undoubtedly been treated with either
mixed-in inhibitors or by migratory systems.
There are other, organic, materials used in admixture with fresh concrete,
such as amines, and esters which, in common with nitrites, do not retard the
setting of the concrete, which can be a problem with some other materials.
Nitrites, in fact, are set accelerators and are used with a retarder to combat
rapid early setting.
The mechanism of the action of calcium nitrite as an anodic inhibitor
depends on reinforcement of the passive film on steel, probably by oxidation
of Fe
2+
ion formed anodically at defects in the film.
Fe
2+
+ 2OH
-
+ 2NO
2
-
→
Fe
2
O
3
+ 2NO + H
2
O
The Fe
3+
ions so formed are stable and re-form the passive layer on the bar
surface.
Since the 1990s, a number of proprietary organic inhibitors for use in
concrete have been introduced, including various amines, alkanolamines, their
salts with organic and inorganic acids and emulsified mixtures of esters, alcohols
and amines (Page et al., 2000). None of these, to the author's knowledge, has
as yet proved as effective as calcium nitrite, and with difficulties in establishing
the chemical composition of these products, it is difficult to comment on their
mechanism of action.
