Civil Engineering Reference
In-Depth Information
6.2.5 Loss of adhesion due to acid formation
The starting point for our model for loss of bond due to acid formation is a
reduction of the bond strength from the required 1.5 MPa to 1/3, so 0.5 MPa,
as a failure criterion. A macroscopically flat bond plane and homogeneous
acid attack over the complete surface are assumed. The acid will dissolve
an equivalent amount of cement hydration products. The bond capacity is
expressed as a three-dimensional quantity, namely the amount of cement
paste in a bond layer with finite thickness, see
Figure 6.3.
By definition,
the cement hydrates outside the bond layer do not contribute to the bond.
Considering the scale of roughness (interlocking) from microscopy, the bond
layer may have a thickness of 50 μm. For 300 kg cement per m
3
, a square
metre of bond plane of 50 μm thickness contains cement hydrates equivalent
to about 15 g unhydrated cement. The acid neutralisation capacity of cement
is assumed to be equal to its CaO content, about 65% by mass for Portland
cement. So the amount of acid that reduces the bond to 1/3 of its original
value by dissolving 2/3 of the hydrated cement in a square metre of bond
plane is equivalent to ca. 10 g of CaO, or 0.24 mole of acid (H+). This
constitutes the resistance to acid-induced bond loss in the model.
6.2.6 Limit state function for CP anode adhesion loss due to acid
production
For surface covering anodes (conductive coatings), the limit state function
for current related acid formation induced bond loss is described as follows.
The resistance R is the acid neutralisation capacity of a critical bond layer
that may be dissolved before bond loss will occur. The load S is the acid
produced as a function of time and CP current density, NADF * I(A) * t /
F. For NADF = 0.07, a cement content of 300 kg/m
3
and a critical bond
+
+
Conductive
coating
Bond layer
Acid dissolution
front moving in
Concrete
