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| After BRISTAR is poured into holes drilled
in rocks or concrete, the expansive stress gradually increases
with time, and reaches to more than 6000 t/m2 at
room temperature after 24 hours. As the BRISTAR generates its
expansive stress, the material to be cracked undergoes a process
of (1) crack initiation, (2) crack propagation, (3) the increase
of crack width. Therefore, this fracture mechanism is distinguished
from a breakage by blasting.
The mechanism by the expansive stress of BRISTAR
is shown in Fig. 1. Cracks initiate from an inner surface
of the hole, being caused by tensile stress at a right angle
with the compressive stress which occurs by the expansive
stress of BRISTAR. The expansive stress of BRISTAR continues
even after the appearance of cracks, the cracks propagate
and also new cracks initiate during the process. Usually,
for a single hole, 2-4 cracks initiate and propagate. When
a free surface exists, the crack, as shown in Fig. 2, is pushed
apart mainly by the shear stress, and a secondary crack also
arises from the bottom of the hole running toward the free
surface.
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Fig. 1 Fracture
mechanism by the expansive stress of BRISTAR
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Fig. 2 Sectional-view
of the crack formation in the material with two free surfaces
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When multiple numbers of holes are filled with
BRISTAR, that are properly adjacent to each other, the cracks
from the hole propagate to connect with the neighboring holes,
as shown in Fig. 3.
It is therefore possible to determine the directions
of the cracks as planned by appropriately arranging the hole
spacing and its depth and its inclination.
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Fig. 3 Crack propagation
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Establishment of free surface:
In the case of trenching, shafting or tunneling, if
all holes are drilled vertically and filled with BRISTAR,
the crack width can not increase but horizontal cracks
initiate. Therefore, in order to obtain two free surfaces,
inclined holes or pre-splitting must be required.
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