Original concrete quality
Concrete undergoes long-term chemical, and micro-structural changes and these processes begin as soon as the concrete is placed. Some aspects of these processes may be beneficial such as long-term curing in a damp environment whilst others are damaging. Other than where concrete is substantially deteriorated it is generally possible to evaluate the original quality of the concrete by looking at the concrete at depth petrographically using thin sections. Some of the most important factors affecting the durability of concrete include:
- Original water/cement ratio
- Compaction
- Cement content (note this can be too high as well as too low)
- Quality of the aggregate
All of these factors can be investigated and where required quantified using petrographic techniques.
Current condition
Over a period of time, concrete can undergo long-term weakening as a result of a wide range of processes that include:
- Moisture penetration and leaching
- Fire damage
- Sulphate attack
- Alkali - aggregate reaction
- The thaumasite form of sulphate attack (TSA)
- Delayed ettringite formation
- Chemical attack by compounds including silage, dairy products, beer, organic acids, sewage, fluorine compounds and sulphuric and hydrochloric acid
Petrographic techniques are employed at Geomaterials to evaluate the not only the causes of concrete deterioration but also the severity of deterioration and depth of deterioration. Quantitative petrographic assessments of the residual compressive strength of deteriorated concrete are generally made and where applicable a comparison is made of the quality of the concrete at depth with any deteriorated concrete at the surfaces. As an aid to the interpretation of the results of the petrographic examination, concrete condition is expressed in terms of its level of deterioration with respect to an in-house Grades of Deterioration Scale.