Soundness & Alkali-Aggregate Reactivity — Parts 5, 7 & 8
Soundness (Part 5, sodium/magnesium sulphate accelerated weathering) screens an aggregate's resistance to weathering/disintegration; alkali-aggregate reactivity (Part 7) and petrographic examination (Part 8) screen for the slow, untreatable expansion of reactive aggregate with cement alkalis. These are long-term durability tests — critical for marine/aggressive exposure and where cement alkalis are high.
•Alkali-aggregate reactivity (Part 7): mortar-bar/chemical screening for reactive silica/carbonate aggregate
•Petrographic examination (Part 8): mineralogical identification of potentially reactive/unsound constituents
•Where AAR risk: combine with low-alkali cement / SCMs (PPC/slag) and the cement alkali content (IS 12813 AAS)
•Judge against IS 383 / project durability requirements for the exposure
Practical Notes
✓Alkali-aggregate reaction is a slow, essentially untreatable concrete disease — it must be screened out at the aggregate stage (Part 7/8) and managed with low-alkali cement/SCMs, not fixed later.
✓Soundness matters for marine/freeze-thaw exposure: an unsound aggregate disintegrates in service even in otherwise good concrete.
Common Mistakes
⚠Skipping AAR screening where reactive aggregate + high-alkali cement coincide.
⚠Ignoring soundness for marine/aggressive/freeze-thaw exposure.
⚠Treating petrographic examination as optional for unknown aggregate sources.