|
Emerging Corrosion Control
Technologies for Repair and Rehabilitation of Concrete Structures
Dr.
Jianhai Qiu
School
of Materials Engineering,
Nanyang Technological University
Nanyang
Avenue,Singapore 639798
http://come.to/corrosion
INTRODUCTION
Technology always advances faster than the development of codes,
specifications, and standards. Recent innovations in materials,
processes and corrosion control technologies have enabled designers
and architects to meet performance-based specifications at lower
life cycle cost. This paper focuses on the recent developments and
applications of state-of-the-art emerging technologies for repair
and rehabilitation of reinforced concrete structures. These technologies
include: press-on zinc hydrogel anode CP system, snap-on zinc mesh
anode CP system, pre-packaged zinc sacrificial anode system, conductive
concrete, electrochemical chloride extraction (CE) or electrochemical
chloride removal (ECR), electrochemical realkalisation (ER), duplex/stainless
steels and alloys reinforcements. Most of these emerging technologies
are also increasingly used for corrosion prevention in new concrete
structures. Some application examples are also cited to demonstrate
the potentials of these promising technologies in the new millennium.
PRIMER
ON CONCRETE CORROSION
General
Reinforced
concrete is the most versatile and potentially one of the most durable
materials that a designer can choose to build almost any type of
structures.Under normal conditions, the reinforcing steel is in
a passive state – it is protected from corrosion by a rather inert
oxide film (passive film) on its surface. The formation and the
subsequent breakdown of such an oxide film are mainly determined
by the pH and the chloride content of the concrete. When the local
environment at the rebar/concrete interface cannot maintain the
passive state of reinforcing steel, active corrosion in either uniform
or localized form (pitting) will occur. Because of the widespread
use of reinforced concrete, today corrosion of reinforcing steel
is rapidly becoming a major problem throughout the world. Bridges,
marine structures, buildings and other concrete structures are being
severely damaged by corrosion.
Carbonation
Carbonation
refers to the interaction of carbon dioxide in atmosphere with the
hydroxides in the concrete. Carbonation is detrimental to the corrosion
resistance of reinforcing steel as it destroys the passive film.
Ca(OH)2+CO2=CaCO3+H2O
CaCO3+CO2+H2O=Ca(HCO3)2
soluble
bicarbonate
Leaching of Ca(OH)2
due to carbonation causes the pH of the concrete to fall below
9 and this leads to the depassivation of the reinforcing steel
surface.
Chlorides
Chlorides
are well known for their ability to penetrate and destroy passive
films on steels and alloys.
Fe => Fe2+
+ 2e-
FeCl2+2H2O=2HCl
+ Fe(OH)
Chlorides
may come from an external source such as seawater or exist as
mixed-in. The local acidification due to the hydrolysis of metal
chloride creates an auto-catalyzing situation for the corrosion
of steel. Chlorides either alone or combined with carbonation
are the primary cause of concrete corrosion.
PAGE 1 2 3
4 5
6 7
8 9
next
| 
