Page 426 - Forensic Structural Engineering Handbook

CONCRETE STRUCTURES 12.17

• Extent of corrosion of reinforcing steel
• Identification of evidence of harmful alkali-aggregate reaction, sulfate attack, or other
chemical attack
• Identification of potentially reactive aggregates
• Evidence of improper finishing
• Estimation of air content
• Evidence of early freezing
• Amount and pattern of microcracking
• Identification of causes of materials-related cracking
• Relative ages of cracks or fractures
Examples of observations and several forms of distress are shown in Figs. 12.13 through 12.17.
Although petrographic examination is extremely useful, it can provide only approxi-
mate values of the water-to-cementitious-materials ratio. When more accurate values are
needed, chemical analysis or x-ray fluorescence analysis may be required. Scanning elec-
tron microscopy is also an important tool for examination and analysis of microstructural
and microchemical characteristics.

Paste Carbonation as a Tool to Evaluate Relative Age of Cracks

The extent of cementitious paste carbonation can be used to determine relative ages of
cracks. Paste carbonation in concrete results from a reaction of carbon dioxide gas (CO ) from
2
the air with calcium hydroxide and other cement hydrates in the paste to form principally
calcium carbonate (CaCO ), some alkali carbonates, various silicate hydrates, hydrated alu-
3
20
mina, iron oxides, and water. The calcium hydroxide reaction is as follows:
Ca(OH) 2 + CO → CaCO + H O
2 3 2

FIGURE. 12.13 Photomicrograph of a thin section of concrete shows paste microstructure of
a ternary concrete mix that contains portland cement, fly ash and ground slag (arrows from left to
right). Field width from left to right is approximately 0.45 mm. (Courtesy of CTLGroup.)