Page 437 - Forensic Structural Engineering Handbook

12.28 MATERIAL-SPECIFIC FORENSIC ANALYSES

Sound slab - good support Delaminated slab

Mobility Mobility

Frequency (Hz) Frequency (Hz)

High-speed data High-speed data
acquisition acquisition

Impact Impact
hammer Geophone hammer Geophone

FIGURE 12.24 Schematic of the impulse response technique. (Courtesy of CTLGroup.)

this alkalinity produces a thin passivating film on the embedded steel that protects the steel
against corrosion. The passivating film can be compromised by one or more of the follow-
ing: cement paste carbonation, chloride in the concrete, acidic chemical infiltration, and
incomplete consolidation of concrete.
Once a corrosion cell is established, the rate of corrosion depends on the potential dif-
ference between the anode and cathode of the cell. This potential difference is affected by
variations in chemical concentrations, oxygen availability, moisture content, and tempera-
ture. Corrosion of embedded steel in concrete and associated concrete deterioration can
also occur due to stray electric currents and dissimilar metals in contact.
When embedded steel in concrete corrodes, corrosion products (rust) create expansive
pressures within the concrete. As corrosion continues, these expansive pressures cause con-
crete deterioration in the form of cracking, delamination, and spalling.
Half-cell potential testing is a nondestructive test that evaluates the probability of active
corrosion in reinforcing steel at the time of testing. Half-cell electrical potentials of rein-
forcing steel are measured by moving a half-cell probe about on the concrete surface, as
shown in Fig. 12.25. Half-cell potential testing can detect the presence of active corrosion
when no visible or physical evidence of damage is apparent.
Half-cell measurements can be presented on a topographical plot, showing the probabil-
ity of corrosion activity by location. A time series of half-cell topographical plots is helpful
in evaluating the long-term progression of corrosion in a structure.