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H. Thomas YuCTL Group
RealRealRealReal----Time Nondestructive Time Nondestructive Time Nondestructive Time Nondestructive Testing of Dowel Alignment Testing of Dowel Alignment Testing of Dowel Alignment Testing of Dowel Alignment
Using MIT ScanUsing MIT ScanUsing MIT ScanUsing MIT Scan----2222
AASHTO Construction Subcommittee Meeting
Louisville, Kentucky � August 2, 2005
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Concrete Pavement Technology Program
� This presentation is a part of FHWA’s CPTP
implementation efforts
� CPTP
� Promote best practices for concrete pavement
design, construction, repair, and rehabilitation
� Goal – safe, smooth and durable concrete
pavements for the Federal-Aid highway system
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MIT Scan-2
� Developed by Magnetic Imaging Tools, GmbH� Based on the principles of pulse induction
� Prototype in 1999 (Scan-1)
� Commercial version in 2000 (Scan-2)
� Designed specifically for the measurement of dowel bar position and alignment
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Advantages
� Works on fresh or hardened concrete
� Real-time, automated data analysis
� Very accurate and reliable
� Efficient (1-2 min per joint)
� 200 or more joints can be tested in an 8-hr
workday
� Up to 3 lanes can be tested in a single pass
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Evaluation of MIT Scan-2
� Conducted under FHWA’s Concrete Pavement Technology Program (CPTP)� ARA
� University of Minnesota
� South Carolina DOT
� Objectives� Determine applicability
� Verify accuracy
� Conduct demonstrations
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Horizontal Alignment Results
-8
-6
-4
-2
0
2
4
6
8
-150 -100 -50 0 50 100 150
Side shift, mm
Err
or,
mm
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Vertical Alignment Results
-8
-6
-4
-2
0
2
4
6
8
-150 -100 -50 0 50 100 150
Side shift, mm
Err
or,
mm
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Evaluation Results
� MIT specified accuracy was verified
� Overall standard deviation of measurement
errors is estimated to be about 3 mm
� Accurate to about +5 mm at 95% confidence level
on rotation
� Depth +5 mm (0.2 in)
� Side shift +8 mm (0.3 in)
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Factors affecting accuracy
� Dowel placement – more error with greater placement error� Bar depth
� Side shift
� Amount of misalignment
� Presence of foreign metal� Results are invalid if metal objects are within the
influence region (within 1 m [3 ft])
� Influence of foreign metal can usually be detected on graphical output and numerical results
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Dowels placed in baskets
� Good results can be obtained if the basket is
cut and the bars are coated (insulated)
� Approximate results with general calibration
� More accurate results with calibration to specific
basket type
� Basket type is defined by
� Bar type – bar diameter, length, and type of metal
� Basket construction – geometry and wire diameter
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Dowels Placed in a Basket
Basket cutBasket not cut
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Typical JointProblem Joint
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State experience
� Caltrans – first to evaluate and purchase the device (December 2002)
� South Carolina DOT – first to use MIT Scan-2 on a construction project (I-95 reconstruction; May, 2003)
� Nevada DOT – first use of MIT Scan-2 on dowel bars placed in baskets (August 2003)
� North Carolina DOT – specified the documentation of dowel alignment as a condition for allowing the use of DBI (April 2004)
� North Carolina Constructors – first contractor to use MIT Scan-2 to comply with the State requirement (May 2004)
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Conclusions
� Dowel bar alignment can be determined very
accurately and efficiently using MIT Scan
� MIT Scan is an effective tool for monitoring
and fine-tuning the dowel placement process
� There is a critical need to improve dowel
placement tolerance specifications
� NCHRP Project 10-69 – University of Minnesota, ARA, CTL Group, and Dr. Mark Snyder
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State of the Practice
� Many agencies specify strict alignment
tolerances
� Tolerance on rotation ranges from 3 mm (1/8 in) to 13 mm (1/2 in) for 457-mm (18-in) dowels
� Typical tolerance is 10 mm (3/8 in)
� Acceptance is based on limited number of
cores
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MIT Scan-2 Stirs Up All Kinds of Trouble
� MIT Scan-2 measures dowel alignment with
unparalleled efficiency and accuracy
� Problems that went unnoticed in the past are being detected
� How bad is bad?
� Limitations of existing specifications:
� Based on limited laboratory testing and
analytical evaluation
� Not verified with field performance
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Example Results
82.1%
12.3%
2.4%0.7%
2.4%
93.6%
5.8%
0.0% 0.2% 0.4%0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
d ≤ 10 10<d≤15 15<d≤20 20<d≤25 d > 25
Range of misalignment, mm
Pe
rce
nt
of
ba
rs
IN1 (Basket)
IN2 (Basket)
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Joint Score� Developed to assess the risk of joint locking – the
higher the Joint Score, the higher the risk
� Determined as a sum of product of number of bars at
each level of misalignment and weighting factors that
reflect the relative adverse effect
� Further research is needed to refine Joint Score
Range of misalignment, mm Weight
10 < d < 15 0
15 < d < 20 2
20 < d < 25 4
25 < d < 38 5
38 < d 10
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A Joint-by-Joint Look
0
5
10
15
20
25
30
35
40
45
50
1 3 5 7 9 11
13
15
17
19
21
23
25
27
29
31
33
35
37
39
41
43
45
47
49
Joint
Jo
int
Sc
ore
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Project location, ACPA Study
Basket
DBI
Basket &
DBI
WA
NV
MOKS
IN
GA
NC
Retrofit
SC
CA
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Findings from the ACPA Study
� Dowel alignment in typical in-service pavements is generally very good
� All projects contained at least a few misaligned bars
� Several projects contained many severely misaligned
dowel bars, but none exhibited any distresses
� Poor dowel alignment may cause looseness around
dowel bars, which could greatly undermine the effectiveness of dowel bars
� Dowel alignment achieved using baskets and DBI are comparable
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78.8%
14.2%
3.6%1.6% 1.8%
72.7%
19.2%
5.7%1.7% 0.6%
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
d ≤ 10 10<d≤15 15<d≤20 20<d≤25 d > 25
Range of misalignment, mm
Pe
rce
nt
of
ba
rs
Basket
DBI
Comparison of DBI and Basket
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An equipment-loan program is planned under CPTP
� Several devices will be available to the States on a loan basis for field trials
� Training and support through CPTPTom Yu ([email protected])
� Coordinate equipment loan through Mobile Concrete Lab
Gary Crawford
(202) 366-1286
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The real secret