Aggregates for concrete - Concrete Mix Design, Quality Control and Specification - page 54

3.1.6 Weak particles and high water absorption
Weak particles and high water absorption are not common in river sands
but can be encountered in pit sands. Except in very high strength concrete,
or concrete required to have wear resistance or frost resistance, the direct
effect on concrete strength is not likely to be a problem. Degrading during
mixing, increasing fines content, and therefore increasing water require-
ment, is possible (but more likely in a coarse aggregate). A high water
absorption may indicate an increased drying shrinkage and could also
indicate a reduced freeze-thaw resistance.
3.1.7 Mica content
Except possibly in very high strength concrete, there does not appear to be
a problem with moderate amounts (less than 5%) of mica directly weak-
ening the mortar. Rather the problem appears to be an increased water
requirement. Probably mica that can be seen does not do much harm, but it
may indicate the presence of finer mica particles that will have much more
influence on water requirement and possibly significantly increase the mois-
ture movement tendency of the mortar.
Mica is usually detected visually but can be extracted by the use of a
liquid heavier than mica but lighter than sand. However, its effect on the
water requirement of mortar and therefore its strength , this time at fixed
workability, is probably easier to determine and more relevant.
3.1.8 Common tests and their pitfalls
Sieve analysis/particle size distribution or grading
• Aggregates are passed through a screen with square openings. The
dimension of a particle that determines the screen that it is retained
on is its median dimension. However, depending on the shape of the
particle, the square aperture may allow a larger particle to pass.
• Problem: Shape and size influence the result of the sieve analysis.
There is also a potential error with the change in material specific
gravity as particle size changes.
Fine aggregate angularity/sand flow—voids
• Aggregates flow through an orifice, into a receiving container, the
volume of the container is known, and the percentage voids are cal-
culated. This voids number is supposed to represent particle shape,
surface texture, and angularity characteristics.
• Problem: The critical dimensions of the device interfere with the
material flow and subsequent compaction, interfering with the voids
result. Also, many combinations of the three characteristics will yield