AGGREGATES - Materials for Civil and Construction Engineers - page 192

to cycles of freeze-thaw conditioning. The strength or modulus of the spec-
imens is measured and compared with the values of unconditioned speci-
mens (ASTM D1075).

5.6 Handling Aggregates

Aggregates must be handled and stockpiled in such a way as to minimize
segregation, degradation, and contamination. If aggregates roll down the
slope of the stockpile, the different sizes will segregate, with large stones at
the bottom and small ones at the top. Building stockpiles in thin layers cir-
cumvents this problem. The drop height should be limited to avoid break-
age, especially for large aggregates. Vibration and jiggling on a conveyor belt
tends to work fine material downward while coarse particles rise. Segrega-
tion can be minimized by moving the material on the belt frequently (up and
down, side to side, in and out) or by installing a baffle plate, rubber sleeve,
or paddle wheel at the end of the belt to remix coarse and fine particles.
Rounded aggregates segregate more than crushed aggregates. Also, large ag-
gregates segregate more readily than smaller aggregates. Therefore, different
sizes should be stockpiled and batched separately. Stockpiles should be
separated by dividers or placed in bins to avoid mixing and contamination
(Figure 5.16) (Meininger and Nichols 1990).

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Sampling Aggregates
In order for any of the tests described in this chapter to be valid, the sam-
ple of material being tested must represent the whole population of materi-
als that is being quantified with the test. This is a particularly difficult
problem with aggregates due to potential segregation problems. Samples of
aggregates can be collected from any location in the production process,
that is, from the stockpile, conveyor belts, or from bins within the mixing
machinery (ASTM D75). Usually, the best location for sampling the ag-
gregate is on the conveyor belt that feeds the mixing plant. However, since
the aggregate segregates on the belt, the entire width of the belt should be
sampled at several locations or times throughout the production process.
The samples would then be mixed to represent the entire lot of material.

5.6.1

FIGURE 5.16 Aggregate bins used to
stockpile aggregates with different sizes.