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Final
Report Inspection
of Post Tension Cables in concrete using High Energy X-rays,
Ramp D at the Ft Lauderdale Airport By Michael
Pinna, Consulting Engineer President
of HESCO April
2002 Final Report Inspection
of Post Tension Cables in concrete using High Energy X-rays,
Ramp D at the Ft Lauderdale Airport By Michael Pinna, Consulting
Engineer President of HESCO Scope To
determine High Energy X-rays capability to inspect and detect flaws in post-tension concrete roadways. Introduction The system chosen was a
Varian Mini-Linatron operated by HESCO Corp. of La Honda, Ca. The equipment
utilizes standard s-band technology common to most industrial linacs, and produces the same high
radiographic quality through
thick sections as can be
expected from these fixed units. The
overall system has been
condensed and repackaged
for field use.
It consists of a power supply
cabled to a small remote control, an
R.F. generator and the small linear accelerator itself. Location
and Description The
Ramp D overpass at the Ft. Lauderdale Airport is a Box style overpass made of
high tensile strength concrete construction. The roadway is segmented or
sectioned and joined together with tongue and groove joints, epoxy, and post tensioning
cable. The ramp is a banked and pitched roadway with an approximately 20° downward pitch running south to north. The
bottom side of the roadway was accessible thru a man hole at the underside of
the box structure. Site
Preparation Sixteen
locations were chosen (8 per side) along a 300 foot long stretch of the
roadway. The locations were marked top and bottom in order to align the x-ray
source with the film and locations were identified by the segment designations.
Locations identified as R for Right were re-identified as S and L for Left was
re-identified as SS. Equipment
Set-up The
HESCO 6MV portable linear accelerator (LINAC) was air freighted and trucked to
the site. FDOT supplied the "Snoop" man lift, night lighting, and
portable generator. The contractor supplied a Gradeall type forklift. The
Gradeall type forklift was chosen because it has an articulating head which
makes possible the angled shots required for this job. The
x-ray and rf-heads were strapped to the forks and covered for whether
protection. The modulator (power unit) and chiller remained in the truck. The
control box was moved to a safe area just below the roadway. Film
Identification The
film locations were identified with lead lettering and one steel 2.7 film side
penetrameter. Source side wire penetrameters were also placed on the roadway.
The locations of the lead lettering were painted on the underside of the
roadway for future reference. X-ray Procedure
Prior
to testing, a radiation survey was performed to determine radiation safety
compliance, (see Radiation Safety). The
testing was conducted by two technicians, one technician positioned and
operated the x-ray machine, while the second technician remained inside the box
roadway to position the film for each location. The film was placed into
position using a telescoping pole and tray which held the film against the
underside of the roadway. Radiation
Safety Survey and Procedure A
radiation safety survey was conducted to determine radiation levels in the test
area. The x-ray source is collimated to a 30° forward primary beam and is
positioned down towards the roadway. A walking survey determined that the
radiation levels directly below the x-ray beam was 5 mr/hr, the highest
recorded level was at 20 mr/hr, approximately
200 ft away at the adjacent freeway off-ramp leading to the Ft. Lauderdale air terminal. The
Florida Highway Patrol provided road control during testing. Prior to each
shot, (x-rays on), the x-ray tech would signal that the roadway be secured.
Personnel on site would move to safe locations and the shot would begin. Shot
times varied from a couple of minutes to 18 minutes for the thickest angled
shot. Closing the roadway was a precautionary measure when taking into consider
the recorded levels. A typical
inspection would allow traffic to flow through the test area making the
following assumptions: Cars traveling at 45 miles per hour or 66 ft/sec would
receive a dosage of less then .02 mR
total. Cars traveling at 35 miles per hour or 51 ft/sec would receive a
dosage of .03 mR total. Even a bicyclist traveling at 10 miles per hour or 14
ft/sec would receive a dosage of .12 mr which is certainly less then the 2
mr/hr requirement. The
x-ray technician inside the box measured readings of less then .15 mr/hr up to
20 ft away from the primary beam. Film
Viewing Results High
energy X-rays has the ability to penetrate and view defects in concrete as
small as 1/16” or less. The table below
lists all visible defects in the grout, strands, and concrete. The defects
listed as ground strands may also be interpreted as voids on top of the strands The
areas that contain the fabricated defects offer an extra layer of
interpretation. The exploration and back filling of an area with grout creates
its own set of defects and creates
density changes due to the difference in material and installation techniques.
As an example, the concrete saw cutting lines are visible as sharp lines
cutting across the cable. Locations that where not excavated and backfilled, for
example segment 79-11B, contain less defects and a uniform film density between
cabling, grout, and concrete. Table 1
X-ray
Films The
x-ray film used was Kodak AA, size 14" x 17". Shot times varied due
to concrete thickness variations. The
x-rays have been sent to Habeeb Saleh for digitization. Habeeb is with the
Federal Highway Administration , NDE Validation Center, 6300 Georgetown Pike,
Mclean, Virginia, 22101, Phone # 202-493-3123, E-Mail Habeeb.Saleh@igate.fwha.dot.gov.
Digitized x-rays should be available early late next week. Conclusion High
Energy X-rays are a very affective method for inspection of post-tensioned
concrete roadways. An x-ray image which is a picture, can be reviewed and
discussed, and held for future reference of the specific area or to develop a
historical record and case studies to help improve the fabrication process or
design of the structure. Project
Notes, Lessons Learned, Improvements 1)
This project was a coordinated effort between HESCO, DMJM-Harris, Florida
Department of Transportation, Federal Highway Administration, and PCL . 2)
An acceptance/rejection criteria must be established for x-ray interpretation
of grouted areas and post tensioned cable. 3)
Positioning the x-ray machine above approximately equals the time to position
and locate the film. 4)
Road or lane closer of the roadway being inspected is essential for inspection.
Closer of adjacent roadways may not be necessary as with this project. Future
inspections may be performed with extra lead shielding to limit scatter
radiation. 5)
The cost for inspection of a single overpass is comparatively high since a high
percentage of the cost is the transportation of the machine and personnel.
Longer-term projects at multiple locations will prove to be more cost
effective. 6)
Road closer coordination took between 25% to 50% of the time for each x-ray
location. 7)
Film review should be closely coordinated with cognizant department of FDOT. 8)
Digitized x-rays can be put into a computerized format and sent to various
locations via E-mail. 9)
Future advancements in machine portability will allow the x-ray machine to be
placed inside the structure, further increasing radiation safety and decreasing
the need for time consuming and costly traffic management. 10)
See Caltrans report attached. |
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