Civil Engineering Reference
In-Depth Information
fact is the immense wetting of the concrete caused by the ECE. This might
reduce the inner friction of the concrete (and the concrete-steel interface)
and may also be responsible for a degradation of bond. As concrete dries out
after finishing the ECE, this is a reversible effect which does not harm the
concrete structure.
8.9 Use of ECE on pre-stressed concrete
A third possible result of the cathodic reaction is the evolution of hydrogen
from water reduction (see equation (8.1c)). It can be triggered according
to the Nernst equation at potentials (IR free) more negative than -770
mV vs. NHE (normal hydrogen electrode) or -1,070 mV vs. CSE (copper/
copper-sulphate electrode) at a pH of 13 (which can be assumed on the steel
surface). If evolved in larger quantities, hydrogen may migrate into micro-
joints of pre-stressing steel and lead to hydrogen embrittlement and failure
of pre-stressing steel. That is why ECE must not be applied on pre-stressed
structures with immediate bond.
In post-tensioned structures using metal ducts, the pre-stressing steel is
protected by tendon ducts that act as a Faraday cage. Unpublished studies by
the author's company (Gruenzig, 2002) have shown that tendon ducts shield
the duct inside even at larger defects (holes up to 3 cm²) safely, so that in an
aqueous solution an outside voltage of 40 V did not shift the potential inside
the tendon duct more than - 50 mV. On the other hand, chloride cannot be
removed with the help of electrochemical methods if it has entered the inside
the duct. In reinforced concrete structures it is assumed that hydrogen cannot
cause dangerous gas pressure on the steel surface, but diffuses through the
capillary pores. A temporary reduction of breaking elongation of reinforcing
steel has been observed (Bennett et al., 1993a) which recovered soon after
stopping the ECE.
8.10 Initial condition survey
A solid, qualified condition survey is essential for any successful concrete
repair. It provides the necessary information for the diagnosis and the
understanding of the problems of a structure. The decision for corrosion
rehabilitation by ECE has to be a result of all technological, financial and
lifetime considerations and can be met in combination with other repair
methods. The condition survey shall provide the following information:
•
concrete cover and reinforcement layout (for evaluating the cathode
surface)
•
reinforcement continuity, possible short circuits to the anode
•
state of the electrical connection between reinforcement and metallic
parts within the possible treatment area (e.g. pipeline supporters,
handrail fixings)
