Page 425 - Forensic Structural Engineering Handbook

12.16 MATERIAL-SPECIFIC FORENSIC ANALYSES

FIGURE 12.12 Dial gage used to measure slip of prestressing strand. (Courtesy of CTLGroup.)

Petrography

One of the fastest and most cost-effective methods used to evaluate concrete quality, diag-
nose causes of deterioration, and determine the extent of internal damage is petrographic
(microscopical) examination. Petrography is applicable to aggregates, portland cement
mixtures of concrete, mortar, grout, plaster, stucco, terrazzo, and similar hardened
hydraulic cement mixtures.
Petrographic methods combine inspection with the unaided eye and microscopical exam-
ination using stereo and petrographic (transmitted) microscopes. Interpretation of findings
by professional petrographers has proved valuable in developing practical solutions to prob-
lems. At least one petrographic examination is desirable on every job involving concrete.
When visual examination suggests deleterious behavior of concrete, petrographic examina-
tion should be done on each type of concrete and on each apparently different condition.
Among the information developed during a petrographic examination performed
according to the guidelines given in ASTM C856, Standard Recommended Practice for
19
Petrographic Examination of Hardened Concrete, are
• Condition of material
• Causes of inferior quality, distress, or deterioration
• Compliance with project specifications
• Description of concrete and cement matrix, providing:
• Degree of cement hydration
• Estimation of water-cement ratio
• Extent and pattern of paste carbonation
• Presence and estimate of residual amounts of fly ash, ground slag, and other supple-
mentary cementitious materials