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

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

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

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

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

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

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

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)

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

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

Dowels Placed in a Basket

Basket cutBasket not cut

Typical JointProblem Joint

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)

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

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

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

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)

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

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

Project location, ACPA Study

Basket

DBI

Basket &

DBI

WA

NV

MOKS

IN

GA

NC

Retrofit

SC

CA

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

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

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 (tyu@ctlgroup.com)

� Coordinate equipment loan through Mobile Concrete Lab

Gary Crawford

gary.crawford@fhwa.dot.gov

(202) 366-1286

The real secret