Nuclear Plant Journal, May-June 2016 NuclearPlantJournal.com
49
penetrating radar (GPR), utilizing a very
small transducer, was found to be the
most reliable and accurate method for
identifying embedments.
Mockup Specimens
At the completion of preliminary
trials, four full-size laboratory mockup
specimens were fabricated to simulate
the region of the containment structure
where the concrete core excavation
and containment vessel plate exposure
were to occur. Each specimen was
approximately 4 ft. X 6 ft. in plan and
approximately 4 ft. – 6 in. tall. Features
of the specimens include top and bottom
mats of reinforcing steel as shown in
plant construction documents, concrete
with aggregate and compressive strength
to simulate in-situ conditions, and a
pressure vessel-quality steel plate at
the bottom of each specimen to model
the containment vessel plate. The plate
was installed at a slope to simulate the
curvature of the containment vessel. As
part of mockup specimen construction,
a sump hallway mock-up was built to
replicate the working conditions in the
containment structure hallway.
Dry Concrete Removal
Methods
Various dry, controlled concrete
excavation methods were researched and
evaluated. In order to provide controlled
concrete removals in stages so that NDT
could be performed at each stage, it was
concluded that some form of concrete
coring would be the most viable method
of concrete removal over most of the
height of the excavation. Based on this
conclusion, the following tasks were
performed:
1. The possibility of dry coring a
12-in.-diameter core was considered
and eliminated since a manufacturer
of 12-in-diameter dry core bits could
not be found. Six-in.-diameter dry
core bits are manufactured but are
designed for use in coring lower
strength concrete and masonry.
Vacuum attachments to coring
rigs were also found that could
potentially control the dust generated
by dry coring.
2. Experiments with a 6-in.-diameter
dry core bit on specimen concrete
were performed. As part of this
experimentation, a vacuum dust
control device was also procured and
utilized. Results of these trials were
as follows:
a. Dry coring with the 6-in-diameter
bit into relatively high strength
concrete can be accomplished.
b. Drilling rate of dry coring is much
slower than the drilling rate of wet
coring.
c. Vacuum attachment is effective
in controlling dust but it requires
regular and frequent mainte-
nance.
d. In order to provide the approximate
12-in. X 12-in. cross-sectional
dimensions in the completed
excavation, multiple, overlapping
sub-cores, or four, 6-in.-diameter
sub-cores positioned in a grid
pattern would be required.
The use of dry coring with 6-in.-
diameter (nominal) bit and vacuum
attachment was found to be viable with
the following qualifications:
1. Core drilling is performed at a low
speed setting. Understanding the
proper rate of advancement of the bit
to avoid binding and excessive wear
July-August, 2016
Vendor Advertorial Issue
Free Ad Space Offer
Ad Commitment Deadline: July 8, 2016
Paid Space
Free Space
2
2
1/3
1/6
1
1
2/3
2/3
1/2
1/2
1/3
1/4
1/4
1/6
Complimentary
Ad Space
The July-August, 2016 issue is
the annual New Plants and
Vendor Advertorial issue.
All advertisers reserving space
in this issue will receive an
equivalent complimentary ad
space.
Contact:
Michelle Gaylord
(630) 364-4780
Nuclear
Plant
Journal
An International Publication
Published in the United States
