WATER JET CONFERENCE - Waterjet Technology Association

psig. Figure 7 shows these units ready for test. The building at the left in the photograph
was 45-feet long. An open end was provided because the Percussive Jet eventually strikes
the ground 125 feet from the nozzle.
The firings were always static because the project as originally defined and
funded did not provide for traverse facilities. The runs were 5-seconds in duration
because this was about the shortest time interval that was required to turn the Jet on and
off in a consistent and reproducible manner. Usually, the concrete blocks were moved
away from the nozzle until the ordinary jet could no longer make an appreciable cut on
the concrete but could only roughen the surface. This was the point at which the ordinary
jet was considered to be unable to cut the block. The Percussive Jet was then fired on the
same concrete face of the block and the relative effects photographed and measured.
Then the block was moved further away from the nozzle until the Percussive Jet could no
longer cut the block in 5-second duration runs.
Several concrete mixes were poured and evaluated as test specimen targets.
Concrete typically consists of a mixture of sand (and gravel), cement, and water. For our
purposes, a homogeneous mixture was desired, so no aggregate or gravel was used.
Curing time and the water/cement ratio are the main considerations which determine the
strength of concrete. In the construction industry, the water/cement ratio often is in the
range of 0.6 to 0.9. Since rather weak cement was desired in order to simulate bituminous
coal, the concrete mixes which were evaluated for use in this program ranged between
1.3 and 2.6.
However, the mixture that was finally chosen was suggested by Barker (1980).
This mixture seemed to function satisfactorily and was also chosen so the results of this
program could be more easily compared to those of others. The mixture used was
prepared on a volume basis as
5 parts washed masonry sand,
1 part masonry cement
2 parts water
For Jet impact tests, the blocks were cast in cubic molds with 21-inch sides. Care
was taken to eliminate any voids or air spaces. The insides of the mold were lined with
polyethylene film to produce a smooth surface texture. The resulting water/cement ratio
was 1.3. The blocks were allowed to cure at ambient temperatures for a period of only
two weeks before cutting tests were performed. Additional curing,, would have increased
the strength. During this program, 18 tons of sand were consumed making concrete block
test specimens.
Sample cylindrical molds, commonly used for concrete testing in the construction
industry, were poured and taken to a testing laboratory. The molds were allowed to cure
alongside the test blocks for the same period of time. The compressive strength of these
samples was about 440 psi..
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