2. Moisture content of substrate
Continuity of materials in the makeup of the
concrete
Thickness of nonconductive lining
Voltage level
DC constant or DC pulse
3. Moisture content is directly related to the dielectric value
of the concrete substrate. The DC voltage of high voltage
testing units use the continuity of the moisture in the
concrete to complete the circuit and identify the
discontinuity's in a non conductive coating system. If the
concrete has a low moisture content(0 to 3 %) the high
voltage test will be inconclusive at best. A moisture
content of 3% to 7% is better. A moisture content od 7% to
12% is optimal . Moisture contents over 12% can inhibit
the resin migration of a topical coating and produce a
lower bond strength.
Throughout the United States we have a wide variety of
climate zones with varying relative humidity and soil
moisture contents. With buried concrete structures. These
soil moisture contents play a key role in hydration of the
concrete and the effectiveness and accuracy of the high
voltage test being performed. For concrete structures
above ground relative humidity plays the most important
role for hydration of the concrete because of the porous
nature of concrete it will absorb and retain moisture and
keep a fairly steady moisture content relative to the
atmospheric humidity.
Where this is most affected is in the Southwest United
States do to low atmospheric humidity and deep ground
water tables. Stable conditions around buried concrete
structures that have been in place for more than 2 months
will have an equalized moisture content matching the soils
around it with normal transpiration . Newly excavated
soils will has an inconsistent moisture content do to the
fact that normal movement of moisture thru transpiration
has not been reestablished . This is most prevalent in new
construction. A waiting period after installation of the
concrete structures and back fill of the sub grade materials
should be observed (2 months) so the moisture contents
can stabilize.
Case point: Sierra Morado sub division.
In the first two weeks of November 2014, 18 new
construction manholes were coated and tested prior to
back fill and topical treatment (asphalt) . High voltage
tests were done on all manholes and discontinuity's were
repaired. Manholes were retested and no further
discontinuity's were found . The municipality’s
representative was present for a final high voltage test and
no discontinuity's were found.
In the last 2 weeks of March 2015 the topical treatments
were complete and the contractor was called in to patch
the adjustment rings. Asphalt had been down 3 weeks.
Upon completion of the patching ,the adjustments were
tested and patched. A field superintendent retested one of
the previously tested and passed manhole completely and
found discontinuity's throughout . All manholes were
retested and found to have 30% to 50% more previously
unidentified discontinuity's . This is believed to be do to
the raised moisture content and increased dielectric value
of the substrate concrete.
Instrument used to test moisture contents Delmhorst BD-
7 serial # 008167
4. Usually the materials that concrete are
comprised of has a very low dielectric value.
Random materials usually in the aggregate have
a higher than normal dielectric value. For
instance a piece of aggregate with a high iron
content will be more conductive than a piece of
quartz. This is a very rare thing that should not
greatly affect the out come of a high voltage
spark test. Test Equipment that will give you a
dielectric value of the concrete substrate should
be used to verify the value. One such piece of
equipment is a Prostat PRS-801 Resistance
system. These units will cost approximately
$2500 to $3000 dollars. As you test the substrate
you will notice a variation in values from one
location to another ( sometimes as small as 2
inches difference). You will also see variations in
values from vertical to horizontal .
Horizontal surfaces have a much more
consistent values do to much more consistent
moisture contents.
Vertical surfaces have many more variable like
back fill of soil around the opposite side ,
moisture content of the soil, transpiration rates
of the soil, free standing walls that have a very
low moisture content do to evaporation.
5. The thickness of a no conductive
coating is a very important factor in
your high voltage test. The high
voltage test unit has a set voltage for
each thickness of coating/lining. If
the voltage used is insufficient to
jump the distance of the coating
thickness (spark over) you will not
get a valid test. Irregularity's in
substrate and over application of
products can impact the test results.
It is always wise to do adhesion
tests (D-7234-05) along with spark
testing of new coatings. After the
adhesion test are complete, check
the profile of the puck removed
from the adhesion test to verify
thickness of coating for proper
voltage selection.
The voltage selection is important to
the accuracy of the test results. If
voltage is to low you will not be able
to achieve the proper spark over. If
voltage is to high you will get
incorrect voltage back to your high
voltage test unit and an audible
alarm will sound with no
identifiable discontinuity. This is
also true when the coating/ lining is
to thin . The high voltage test unit
will sense greater than expected
voltage return and will sound the
audible alarm with no discontinuity
and at proper voltage for specified
thickness.
6. CONCRETE STEEL
We used one Tinker and Rasor APW-
GSE pulse high voltage test unit 1298 ,
one Tinker and Rasor AP/W pulse
high voltage test unit13436 and one
Elcometer T26620033-3 direct voltage
tester HKA934 to test a liner at 170
mills on concrete. Both the APW-GSE
and the Elcometer T26620033-3 high
voltage testers worked well and picked
up all the discontinuity's but two do to
the lack of moisture in the concrete
substrate making it hard to determine
accuracy of the test. Each unit picked
up two different discontinuity's that
the other would not. This makes it
impossible for me to say whether one
system works better than the other.
Both the APW-GSE and the T26620033-
3 ricked up 30% more discontinuity's
than the older AP/W unit 13436
We used one Tinker and Rasor
APW-GSE pulse high voltage test
unit 1298 , one Tinker and Rasor
AP/W pulse high voltage test unit
13436 and one Elcometer
T26620033-3 direct voltage tester
HKA934 to test lining at 85 mills
on steel. All three test units
performed identical. This leads
me to believe that that the
substrate continuity of concrete
for identifying discontinuity's
plays a key role in the accuracy of
the test results.
