AGGREGATES - Materials for Civil and Construction Engineers - page 191

FIGURE 5.15 Example of cracking in concrete due to alkali-silica reactivity.

sand-gravel aggregate with crushed limestone (limestone sweetening) can
minimize the alkali reactivity (Kosmatka et al. 2002).

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5.5.11 Affinity for Asphalt

Stripping, or moisture-induced damage, is a separation of the asphalt film
from the aggregate through the action of water, reducing the durability of the
asphalt concrete and resulting in pavement failure. The mechanisms caus-
ing stripping are complex and not fully understood. One important factor is
the relative affinity of the aggregate for either water or asphalt. Hydrophilic
(water-loving) aggregates, such as silicates, have a greater affinity for water
than for asphalt. They are usually acidic in nature and have a negative surface
charge. Conversely, hydrophobic (water-repelling) aggregates have a greater
affinity for asphalt than for water. These aggregates, such as limestone, are
basic in nature and have a positive surface charge. Hydrophilic aggregates
are more susceptible to stripping than hydrophobic aggregates. Other strip-
ping factors include porosity, absorption, and the existence of coatings and
other deleterious substances.
Since stripping is the result of a compatibility problem between the as-
phalt and the aggregate, tests for stripping potential are performed on the as-
phalt concrete mix. Early compatibility tests submerged the sample in either
room-temperature water (ASTM D1664) or boiling water (ASTM D3625);
after a period of time, the technician observed the percentage of particles
stripped from the asphalt. More recent procedures subject asphalt concrete