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Water absorption limits on aggregates

Limits on aggregate water absorption are rare though they may be useful to link to an undesirable concrete property, such as freeze-thaw susceptibility, as water absorption is easily determined. Historically it is not known the source of the limits but the following may be of interest.

According to BS 8007, aggregates should comply with either BS 882 or BS 1047 and have an absorption, as measured in accordance with BS 812-2, generally not greater than 3%. A paper by Anchor et al discusses the then new BS 8007. Regarding aggregate absorption it says “Water absorption of aggregates is retained at 3% maximum. Here again considerable time was spent in committee discussing comments that it was unnecessary to keep this restriction. From practical experience it has been found that some aggregates are unsatisfactory, and one of the characteristics is high absorption. To ensure these aggregates are not used the limit is retained.”

The Civil Engineering Specification for the water industry (CESWI) follows BS 8007 and water absorption should not exceed 3%.

The maritime code BS 6349 specifies that water absorption should not exceed 3%, or 2% in critical conditions such as highly aggressive chloride or freeze–thaw exposure, when tested in accordance with BS 812-2.

The drying shrinkage test in BS 812 Part 120 is limited to aggregates with water absorption <3.5%.

CIRIA Report C559 says that air entrainment may not provide freeze-thaw resistance if the aggregates are not of suitable quality. However, high water absorption is only an indicator of possible aggregate susceptibility. For example lightweight aggregate concrete is often made with aggregates of higher absorption but has a long track record of being able to resist freeze-thaw conditions.

Aggregates tested for water absorption to BS EN 1097-6 are considered in BS EN 12620 to be freeze-thaw resistant if less than 1% but the Standard provides no guidance on whether the aggregate is suitable for all exposure classifications or just one. For example, some Jurassic limestones and sandstones frequently have absorption values in excess of 4% whilst blastfurnace slags, Permian limestones, dolomites and Carboniferous sandstones frequently have absorption values in excess of 2% but these materials can still have adequate freeze-thaw resistance.

Notes for guidance in Specification for Highway Works have a limit for paving, when using flint aggregates, of 3.5% for separate nominal aggregate sizes and 2% for combined coarse aggregates, as a precaution against freeze-thaw.

The limits on water absorption effectively rule out the use of recycled aggregates as values of 6-7% are typical and would therefore exclude them from water retaining structures. Similarly this applies to many lightweight aggregates.

Note that British Standards assume that 1% means measured values up to 1.49% whereas 1.0% is 1.049%.

References
BS 8007:1987, Code of practice for the design of concrete structures for retaining aqueous liquids
BS 882:1992, Specification for aggregates from natural sources for concrete
BS 1047:1983, Specification for air-cooled blast furnace slag aggregate for use in construction
BS 812-2:1995, Testing aggregates. Methods for determination of density
Anchor, RD et al. BS 8007: the new code, The Structural Engineer, Vol 66 No. 3, 2 Feb 1988
The Civil Engineering Specification for the water industry (CESWI)
BS 6349-1:2000, Maritime structures. Code of practice for general criteria
BS 812-120:1989, Testing aggregates. Method for testing and classifying drying shrinkage of aggregates in concrete
CIRIA Report C559, Freeze-thaw resisting concrete, London, 2001
BS EN 1097-6:2000, Tests for mechanical and physical properties of aggregates. Determination of particle density and water absorption
BS EN 12620: 2002, Aggregates for concrete
NATIONAL HIGHWAYS, Specification for highway works

Acknowledgement: The Concrete Society