AGGREGATES - Materials for Civil and Construction Engineers - page 166

Funnel

Fine
aggregate
sample

Cylinder of
known volume

(a) (b)

FIGURE 5.5 Apparatus used to measure angularity and surface texture of fine

aggregate.

sulfate solution. These sulfates cause crystals to grow in the aggregates, sim-
ulating the effect of freezing. The test starts with an oven-dry sample sepa-
rated into different sized fractions. The sample is subjected to cycles of
soaking in the sulfate for 16 hours followed by drying. Typically, the sam-
ples are subjected to five cycles. Afterwards, the aggregates are washed and
dried, each size is weighed, and the weighted average percentage loss for
the entire sample is computed. This result is compared with allowable lim-
its to determine whether the aggregate is acceptable. This is an empirical
screening procedure for new aggregate sources when no service records are
available.
The soundness by freeze thaw (AASHTO T103) and potential expansion
from hydrated reactions (ASTM D4792) are alternative screening tests for
evaluating soundness. The durability of aggregates in portland cement con-
crete can be tested by rapid freezing and thawing (ASTM C666), critical di-
lation by freezing (ASTM C671), and by frost resistance of coarse aggregates
in air-entrained concrete by critical dilation (ASTM C682).

■ 5.5.3 Toughness, Hardness, and Abrasion Resistance

The ability of aggregates to resist the damaging effect of loads is related to
the hardness of the aggregate particles and is described as the toughness or
abrasion resistance. The aggregate must resist crushing, degradation, and
disintegration when stockpiled, mixed as either portland cement or asphalt
concrete, placed and compacted, and exposed to loads.