Civil Engineering Reference
In-Depth Information
of failure after 57 years is ca. 50%. Uncertainties in the critical carbon
mass, the oxidation efficiency factor and the (local) current density may
cause earlier failure of anodes. Non-uniform current distribution would
lead to earlier failure of high current density spots, which would increase
the current density of the remaining active surface, which in turn would
accelerate degradation. Increased local current density due to local lower
electrical concrete resistance (for example due to water leakage) would
definitely reduce anode service life, as can be observed in practice (see
below).
6.2.3 Loss of anode/(overlay)/concrete adhesion
A conductive coating or a titanium/overlay system may fail due to acid
attack of the adhesion with the concrete. Acid is formed at the anode by
reactions (6.1) and (6.2). Acid formation and related failures, in particular
due to local high current densities, have been reported (Brown & Tinnea,
1991). Adhesion is determined by the tensile strength of the concrete
substrate, its surface preparation (cleaning, pre-wetting), the application
method, the overlay materials' properties (shrinkage, E-modulus) and
environmental actions (temperature and wetting/drying cycles), see for
example, Julio et al., 2005. The requirements in concrete repair and/or
CP standards are usually an average and/or minimum bond strength at 28
days age. Here it is assumed that materials and execution have been up
to the requirements. During CP operation, the coating or overlay must
be able to resist mechanical forces and environmental actions. Experience
with cementitious overlays has shown that a bond strength of ca. 1 MPa
is sufficient for a long overlay life. For CP overlays, EN 12696 requires
an average of at least 1.5 MPa (minimum 1.0 MPa). For cementitious
overlays in general, Dutch regulations require an average of at least 1.3
MPa (minimum 0.6 MPa). Such values can be obtained with proper surface
preparation, materials and application. For coatings, a minimum of 1.5
MPa seems appropriate. These requirements are probably conservative and
contain some reserve, even in the minimum values.
Here it is assumed that a bond strength of 0.5 MPa is sufficient to resist
external action. So it is acceptable if some bond strength is lost from the
original level of say 1.5 MPa. The difference between 1.5 MPa and 0.5 MPa
is the capacity that can be consumed by CP-specific acid attack. A model is
proposed for bond degradation due to CP-related acid formation. The acid
dissolves parts of the cement matrix in the bond plane and the adhesion
decreases. This model needs a description of the bond loss due to acid
dissolution of the cement matrix and of the amount of acid produced for a
unit of electrical charge.
