Alkali-silica reaction starts when alkali hydroxides in the pore solution attack reactive silica in some aggregates, forming a gel that absorbs water and swells. Over years it produces map cracking, closed-up joints, popouts and white gel at crack edges, mostly in damp exposures such as pavements, dams and bridges. Prevention relies on testing aggregates, limiting cement alkalis and adding Class F fly ash, slag or silica fume.
Cores show gel rims around the chert, so it's ASR, and that whole apron is going to keep growing.
Also heard as
- alkali-silica reaction
Do not mix it up with
- Crazing: Crazing is a shallow surface pattern from finishing and drying; ASR cracks run deep and keep growing.
Related terms
Fly ash
Fine powder captured from coal-fired power plant exhaust, used to replace part of the cement in concrete.
SCM
Supplementary cementitious material: fly ash, slag cement or silica fume used alongside portland cement in a mix.
Aggregate
The sand, gravel and crushed stone that make up 60 to 75 percent of a concrete mix by volume.
Popout
A small conical crater where a swelling stone just under the surface pushed out a chip of concrete.
Core
A cylinder of hardened concrete drilled out of a structure to test its strength or inspect its condition.