Page 153 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

MECHANISM OF CORROSION AND ASSESSMENT TECHNIQUES IN CONCRETE 131

Chloride-induced macrocell Expansion of corrosion Tensile stresses in
corrosion initiates on top product produces tensile concrete lead to
rebar stresses in concrete cracking/spalling
Deicing salt/marine Deicing salt/marine Deicing salt/marine
Cl – Cl – Cl – Cl –
Cl – Cl – Cl – Cl –
Cl – Cl – Cl –
Cl – Cl – Cl – Cl – Cl –
Rebar Rebar Rebar
Cl – Cl – –
Cl – Cl
Cl –
Rebar Rebar Rebar

Figure 3.1 Schematic of corrosion damage to rebar (7).

the corroding steel (corrosion products have three to five times greater volume than
the original steel). These high tensile forces cause cracking and spalling of the
concrete at the reinforcement (Figs 3.1 and 3.2). Steel in high-pH concrete in the
absence of chloride is passive and corrosion is negligible, which in theory should
give reinforced concrete structures a long corrosion-free life. However, in practice,
corrosion in concrete is accelerated through two mechanisms:

1. Breakdown of the passive layer on the steel by chloride ions
2. To a lesser extent by carbonation because of carbon dioxide reaction with
cement phase.

In the case of highway bridges, the vast majority of the problems are caused by
chloride migration into the concrete because of deicing salt application and marine
exposure. On reaching the surface, the chloride ions cause the disruption of the
passive film and create conditions favorable for accelerated corrosive attack on the
reinforcing steel.
In addition to chloride ions, oxygen is also necessary for accelerated corrosion.
Chemical, physical, and mechanical properties of concrete can have a significant
effect on concrete deterioration by controlling the following:

1. The chloride and oxygen permeation in the concrete.
2. The sensitivity of passive layer to chloride attack.
3. The rate of corrosion reactions at the steel surface following corrosion
initiation.
4. The rate of cracking and spalling of the concrete when exposed to the expan-
sion forces of the corrosion products. The effect of concrete properties on the
corrosion and concrete deterioration processes of bridge structures has been