Page 429 - Forensic Structural Engineering Handbook

12.20 MATERIAL-SPECIFIC FORENSIC ANALYSES

The ingress of CO , and thus rate of carbonation, is influenced by the permeability of
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the paste. Carbonation lowers the typically high pH (12 to 13) of the cementitious paste
between 8 and 9. Use of a pH indicator is an indirect means of assessing regions of lower
pH and thus carbonation.
The depth and pattern of paste carbonation in a concrete sample can be determined by
applying phenolphthalein solution, a pH indicator, to freshly fractured or cut concrete sur-
faces. Phenolphthalein solution indicates a pH less than 9, typical of carbonated paste. The
indicator reacts with high pH, noncarbonated paste, and imparts a deep magenta stain.
Carbonated paste with lower pH does not change color. This rapid method usually provides
a good visual estimation of the depth and pattern of paste carbonation.
The pH value between fully carbonated paste and fully noncarbonated paste is wide, and
results of pH staining may be inconclusive in areas where partial or incipient carbonation
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occur. Other methods can be used to more precisely assess the depth and pattern of car-
bonation. Examining a thin section of the concrete allows a petrographer to see the change
in paste mineralogy from hydration products to calcium carbonate in areas undergoing car-
bonation. Campbell et al. concluded that thin section examination was the most reliable
method of determining depth of carbonation (Ref. 21). Thin sections are prepared by
mounting a portion of the concrete on a glass slide and grinding it thin enough for light to
pass through, allowing microscopical study of the paste mineralogy and microstructure at
magnifications typically up to 400x.
Paste carbonation is a diffusion-type process; it starts at the exposed surface and, as the
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concrete ages, the carbonation front progresses inward. If a crack develops in the concrete,
the carbonation process starts again from the crack face and progresses away from the crack,
provided the crack opening is not obstructed by debris or surface applied coatings. 23
The pH indicator is applied to freshly fractured surfaces, broken perpendicularly to the
exposed surface to determine depth of carbonation. However, problems can arise with this
method if concrete already exhibits surface perpendicular cracks or microcracks. Because air
can penetrate more deeply into the concrete along cracks, paste along the crack surfaces can car-
bonate to a greater depth than at distances away from the crack. Using phenolphthalein on a frac-
tured surface in these cases may give a falsely deep measurement of carbonation due to
potential interferences from preexisting cracks. Application of phenolphthalein on a saw-cut
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surface will reveal carbonation depth and pattern relating to the cracks. The carbonated zone
of paste along a crack is usually V-shaped, wider at the top, and narrowing with depth.
Fick’s second law establishes a relationship between depth of carbonation and time,
through which depth of carbonation can be quantified by using the formula 22

D = K t

where, D = depth of carbonation
t = time
K = rate of carbonation, mm/y 0.5
Based on this premise, and holding the carbonation rate the same, a relationship of
depth of carbonation to time can be drawn when evaluating cracks. In comparable con-
crete (of similar quality, with similar water-to-cementitious-materials ratio, and with the
same exposure conditions), cracks that show different depths of carbonation and differ-
ent carbonation distances from the crack face occurred at different times. Though many
factors influence the rate of carbonation in a concrete, the rate can be calibrated in the
sample by the depth of paste carbonation from the exposed surface. By comparing depth
from the surface with distances of carbonation from the crack face, relative crack ages
can be established. If a crack propagates for any reason, the carbonation process starts
again from the newly generated crack surface. In this way, the relative age difference