Page 433 - Forensic Structural Engineering Handbook

12.24 MATERIAL-SPECIFIC FORENSIC ANALYSES

Strictly speaking, Fick’s law applies only to transport by diffusion in materials that are
homogeneous, do not change over time, and do not interact with the diffusing substance. Even
though it does not directly apply to chloride ion transport in concrete, Fick’s second law does
provide a convenient way to compare concretes that have been exposed to ponding under the
same conditions for the same length of time. (Note that very different values may be obtained
for the same concrete under different exposure conditions.) An apparent diffusion coefficient
27
can be calculated and used for this purpose. Some values obtained for D for high-quality
ce
concretes ponded with a 3% NaCl solution for 6 months are shown in Table 12.3.
TABLE 12.3 Apparent Diffusion Coefficients for Concretes Ponded with
3% NaCl Solution for 6 Months

Water-to- Apparent
Silica cementitious- diffusion
2
fume (%) materials ratio coefficient (m /s)
Effect of 0 0.40 4.11E-12
Silica Fume 6 0.40 1.66E-12
Content 12 0.40 1.08E-12
Effect of 6 0.35 0.96E-12
w/cm Ratio 6 0.40 1.66E-12
6 0.45 2.47E-12

Some practitioners use the effective diffusion coefficient in models that predict the ser-
vice life of concrete. These models make a series of assumptions about the behavior of the
structure. While any one assumption may be reasonable, one must use the results with cau-
tion, if at all. For example, one might calculate the service life under a given set of condi-
tions in order to compare the life-cycle costs of several alternate construction methods.
Even in this type of application, it would be helpful to calculate the service life based on
different assumptions in order to see the sensitivity of the result.
Under field conditions, several transport mechanisms are at work which cannot be
described or predicted by Fick’s second law. Nevertheless, chloride profiles based on field
specimens still provide useful information about the past performance of the concrete. They
are also a reasonable basis for comparing different concretes that have been exposed to the
same field environment.

Nondestructive Testing

Nondestructive testing has always been an effective way to gain information about hard-
ened concrete. Some of the simplest methods can give useful information. A rebound ham-
mer (Schmidt hammer) provides a qualitative measure of the surface condition of concrete.
This instrument has a spring-loaded weight that impacts and rebounds from the concrete
surface. The amount of rebound, read as a number directly from the instrument, is used as
a measure of the quality of concrete. Although not reliable as a measure of strength, this
procedure can be used to identify surface damage. By using the rebound hammer method
at many locations over a large area, surface damage caused by fire or other attack can be
mapped. When calibrated with core tests, rebound hammers can be used to give an approx-
imate value of concrete strength.
Chain drags and surface tapping with a mason’s hammer or other metallic device are
useful in finding delaminations. When a surface is lightly tapped with a piece of metal, a
“ringing sound” will be heard when the concrete is sound. If delaminations are present