Page 432 - Forensic Structural Engineering Handbook

CONCRETE STRUCTURES 12.23

One useful tool for predicting durability is to determine precise chloride profiles of con-
cretes exposed to salt solutions. These profiles are particularly suited for evaluating high-
quality concretes, evaluating new materials or mix designs, and for litigation.
A precise chloride profiling method affords several advantages over other test methods,
including the following:

• Either field or laboratory concretes can be tested
• No artificial transport mechanisms are introduced
• Core sampling prevents loss or contamination of field powder specimens
• Large sample size gives representative, repeatable results
• Suspect data points are easily identified and reanalyzed
• The results are sensitive enough to distinguish among high-performance concretes
• The sampling technique can be used for substances other than chlorides

More traditional methods of evaluating the ability of concrete to resist the penetration
of chloride ions are not sensitive enough to make comparisons of high-performance con-
cretes, or to make fine distinctions among concretes. Accelerated test methods, while con-
venient for quality control purposes, have been the subject of intense debate and may be
problematic if introduced as evidence in litigation. Precise chloride profiles can show fine
distinctions among concretes and have built-in checks to ensure high-quality data. Thus
they provide a high degree of confidence in the results.
Steel reinforcement is normally well protected from corrosion by the surrounding con-
crete. Because of the high calcium hydroxide and alkali content of cement paste, the pH of
the pore water is usually greater than 12.5, high enough to maintain the passive layer of iron
oxides and hydroxides on the surface of the steel. This passive layer restricts further corro-
sion of the steel. The passive layer may be broken down either by reduction of the pH (due
to carbonation or to leaching of the calcium hydroxide) or by the ingress of chloride ions.
Chloride ions migrate toward the surface of the steel, polarize, and enter the passive layer.
2+
The negative chloride ions aid in the dissolution of the ferrous (Fe ) ions. Additional chlo-
ride ions are attracted to the location where the passive layer has been broken. Because the
chloride ions act as catalysts in this process, they are not consumed. Consequently, the
25
process of corrosion continues. Generally accepted threshold values range from 0.1 to 0.4
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percent by mass of cement. The actual concentration of chloride ions will depend on the
ability of the cement paste to bind the chlorides, the moisture content of the concrete, the
ambient temperature, the electrical resistivity of the concrete, the pore solution chemistry,
and the availability of oxygen.
Fick’s second law is commonly used to describe the transport of chloride ions through
concrete. In the following form, the equation is used to calculate the effective diffusion
coefficient from chloride profile data using a curve-fitting program.

⎛ x ⎞
=
yC ⎜ 1 − erf ⎟
s ⎝ 2 Dt⎠
ec
where x = distance from surface, m
y = chloride content (% by mass of concrete) at depth x
C = notional surface chloride level (constant; set at concentration obtained for
s
outermost layer)
t = time of exposure, s
2
D = effective diffusion coefficient, m /s
ec x
erf = error function/erf(x) = 2/ ττ ∫ 0 e t − 2 dt