Environmental Engineering Reference
In-Depth Information
Total hardness H T is defined as
H T
H R
H A g 1/2
(3.1)
where H R is the Schmidt hardness and H A the abrasion test hardness.
Schmidt rebound hardness test : An L-type concrete test hammer (Figure 3.1a), with
a spring in tension, impels a known mass onto a plunger held against the speci-
men (energy
0.54 ft lb. or 0.075 m kg). The amount of energy reflected from the
rock-hammer interface is measured by the amount that the hammer mass is
caused to rebound (ASTM C805).
Shore (C-2) sclerescope is also used to measure rebound hardness. The rebound
height of a small diamond-tipped weight falling vertically down a glass tube is
measured and compared with the manufacturer's calibration.
Abrasion hardness test is performed on a thin disk specimen which is rotated a
specific number of times against an abrading wheel, and the weight loss
recorded.
Durability
General
Durability is the ability of a material to resist degradation by mechanical or chemical
agents. It is the factor controlling the suitability of rock material used as aggregate for road-
way base course, or in asphalt or concrete. The predominant mineral in the specimen, the
microfabric (fractures or fissures), and the decomposition degree are controlling factors.
Test Methods
Los Angeles abrasion test (ASTM C535-03 and C131-03): specimen particles of a specified size
are placed in a rotating steel drum with 12 steel balls (1 7/8 in. in diameter). After rotation
for a specific period, the aggregate particles are weighed and the weight loss compared
with the original weight to arrive at the LA abrasion value. The maximum acceptable
weight loss is usually about 40% for bituminous pavements and 50% for concrete.
British crushing test : specimen particles of a specified size are placed in a 4-in.-diameter
steel mold and subjected to crushing under a specified static force applied hydraulically.
The weight loss during testing is compared with the original weight to arrive at the British
crushing value. Examples of acceptable value ranges, which may vary with rock type and
specifying agency, are as follows: particle size (maximum weight loss), 3/4-1 in. (32%),
1/2-3/4 in. (30%), 3/8-1/2 in. (28%); and 1/8-3/16 in. (26%).
Slake durability test (ASTM D4644): determines the weight loss after alternate cycles of
wetting and drying shale specimens. High values for weight loss indicate that the shale is
susceptible to degradation in the field when exposed to weathering processes.
Reactivity: Cement-Aggregate
Description
Crushed rock is used as aggregate to manufacture concrete. A reaction between soluble sil-
ica in the aggregate and the alkali hydroxides derived from portland cement can produce
abnormal expansion and cracking of mortar and concrete, often with severely detrimental
effects to pavements, foundations, and concrete dams. There is often a time delay of about
2 to 3 years after construction, depending upon the aggregate type used.
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