Lesson l/Learning Event 2
materials may be included in some gravel deposits as pebbles or as coatings on gravels. Potential alkali-
aggregate reactions may be anticipated in the field by identifying the rock and comparing it to known
reactive types or by investigating structures in which the aggregate has been used. Generally, light-
colored or glassy volcanic rocks, chert, flints, and clayey rocks should be considered reactive unless
proven otherwise.
An additional property of rock is gradation. This property is also important for evaluating rock as
possible construction material. Gradation is the distribution and range of particle sizes which are present
in, or can be obtained from, a deposit. The gradation of pit materials can be readily determined from
simple tests. Quarry materials may be more difficult to evaluate. The upper limit of particle sizes
available is normally controlled by the thickness of rock layers and the spacing of cracks or fractures in
the rock. The amount of fine particles produced during rock crushing operations can be high variable.
Generally, hard, tough rocks produce few fines while soft, weak rocks produce large quantities of fines,
Weak sandstones and granites usually produce large amounts of sand-size material.
Types of Quarry Material. Natural sand and gravel are not always available and it is sometimes
necessary to produce aggregate by quarrying and processing rock. Quarrying normally is done only
where other materials of adequate quality and size cannot be obtained economically.
Many rock types suitable for construction exist throughout the world. The quality and durability of the
rock type selected will therefore depend on local conditions. The following rock types are generally
easy to quarry, durable, and resistant to weathering. When these are not available, it may be necessary
to use softer rocks for base courses and surfacing on a temporary basis. The softer rocks will usually
require little or no blasting.
Granite. As a dimension stone, granite is fairly durable and has a texture and color desirable
for polishing. As a construction material for base courses and aggregate, it is not as desirable
as some of the more dense, fine-grained igneous rocks.
Felsite-Rhyolite. This is durable and makes a good aggregate for base courses. It is not
suitable for concrete aggregate.
Gabbro-Diorite. Gabbro and diorite both have good strength and durability. The mineral
crystals of both rocks are deeply intermeshed, making them very tough and excellent for
construction aggregate.
Basalt. The dense, massive variety of basalt is excellent for crushed rock for base course or
bituminous aggregate. It is very strong and durable. Due to the high compressive strength of
basalt, production may be more difficult than for other rocks.
Sandstone. Few sedimentary rocks are desirable for construction due to their variable physical
properties. However, sandstone is
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