Aggregate Moisture and Absorption: Correcting Concrete Mix Water
Surface moisture in sand changes the water-cement ratio. Learn the moisture states, saturated surface-dry condition and how to correct batch weights and mix water.
Quality and technical information
Alkali-Silica Reaction in Concrete: How to Assess Aggregate
Some siliceous aggregates react with cement alkalis and crack concrete. Learn the signs, the ASTM tests and the usual ways of managing this risk.
Some concrete damage appears years after construction and starts in the aggregate. Alkali-silica reaction is one such form of damage, and identifying it before a material is used costs far less than repairing a structure.
How the reaction happens
Reactive silica in some rocks reacts with alkalis in the concrete’s pore solution and forms a gel. The gel takes up water, expands and creates internal stress. The reaction needs three things at once: reactive silica, sufficient alkali and moisture. Removing any one reduces the risk.
Signs in a structure
Map cracking, gel seeping from cracks and deformation of structural members are common signs. They are not unique to this reaction, and a firm diagnosis needs laboratory examination.
Ways of assessing aggregate
| Method | Reference | Approximate duration |
|---|---|---|
| Petrographic examination | ASTM C295 | Short |
| Accelerated mortar bar | ASTM C1260 | About two weeks |
| Concrete prism | ASTM C1293 | One year or more |
ASTM C1778 summarises the route for assessment and choice of measures. The accelerated method is fast but may give a conservative result for some rocks.
Common measures
- Choosing aggregate with low reactivity or a good performance record
- Using low-alkali cement
- Using supplementary cementitious materials such as fly ash, slag or silica fume
- Reducing the concrete’s access to moisture through suitable detailing
The right route for each project is set by its technical specification and the design engineer. For other requirements, see the Concrete page.
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