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Petrographic Examinations of Concrete

Petrographic examinations of concrete are typically performed due to some type of problem during a construction project or the premature failure of an existing concrete structure.  The petrographer is retained to determine the cause of the problem and often times to determine ultimately who is responsible.  Petrographic examinations are also often performed as part of a condition study to determine if a concrete structure needs to be replaced or if it can be rehabilitated.

Some common problems requiring petrographic examinations are given below:

  • Low strength - test cylinders did not meet spec.
  • Improper setting time
  • Blistering
  • Delaminations
  • Surface scaling
  • Aggregate popouts
  • Dusting
  • Plastic shrinkage cracking
  • Drying shrinkage cracking
  • Structural cracking
  • D-Cracking
  • Cracking or spalling related to the corrosion of reinforcing steel
  • Cracking due to cement-aggregate reactions
  • Discoloration
  • Sulfate attack

What A Petrographic Examination Can Tell You

Total Air Void Content

  • Size and spacing of air voids
  • Entrained vs. entrapped air
  • Distribution of air voids

 Cement Paste

  • Water-cement ratio
  • Degree of cement hydration
  • Presence and relative quantity of fly ash
  • Cement paste content

Fine and Coarse Aggregates

  • Type of aggregates
  • Quality of aggregates
  • Identification of deleterious aggregates
  • Identification of cement aggregate reactions
  • Presence of dirty aggregates

Coarse Aggregate/Cement Paste Interface

  • Bonding quality

Condition of Wearing Surface Concrete

  • Increased or decreased water-cement ratio
  • Entrained air void content

Extent of Corrosion of Reinforcing Steel

Cause of Cracking Distress

Extent of Carbonation

Presence of Curing Compound

 

Who Is At Fault?

Below is a listing of who would be responsible
for various causes of concrete failures.

Ready Mix Supplier

  • Poor quality air void system
  • High water/cement ratio
  • Improper setting time due to admixture dosage rate
  • Improper mixture proportions

Concrete Contractor - Finisher

  • Adding water at the site
  • Improper finishing techniques
    • Finishing too early
    • Finishing too late
    • Adding water to surface to facilitate finishing
    • Overworking - loss of entrained air at the surface 
  • Improper curing
  • Sawcutting of joints
    • Improper timing
    • Improper spacing
    • Improper depth

General Contractor

  • Protection of concrete from adverse weather

Aggregate Supplier

  • Aggregate quality
    •   Deleterious aggregates
    •   Soft absorptive aggregates - prone to popouts
    •   D-cracking prone aggregates
    •   Alkali-silica reactive aggregates
    •   Dirty aggregates
  • Gradation of aggregates out of specification

Weather Conditions
 

 

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