Development of Residual Stress Test-Lange

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26 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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Development and Validation of

Residual Stress Test for

Concrete Pavement

David Marks, Daniel Castaneda and David A. Lange

University of Illinois at Urbana
-
Champaign


FAA Worldwide Airport Technology Transfer Conference

Atlantic City, New Jersey

April 2010


Acknowledgments


Sponsored by FAA through Center of
Excellence for Airport Technology


Idea for this new test originated with
Edward Guo


David Marks finished his MS at UIUC


Daniel Castaneda is currently extending
this work for his MS at UIUC

Introduction


What are residual stresses?


Residual stress may diminish capacity to sustain
designed load


Problem:
No standard method to measure residual
stress in concrete


Inspiration:
Residual stress test method for steel


ASTM E837
-
08 Standard Test Method for Determining
Residual Stresses by the Hole
-
Drilling Strain
-
Gage Method


A strain gage measures the change in strain reading as a
small hole (~2mm) is drilled in the vicinity. The change in
strain reading is correlated to a residual stress

TOP VIEW

SIDE VIEW

Overview of test

FAA NAPTF adapted ASTM
E837 to cantilevered concrete
beams with encouraging
results

• Load applied at one end
of concrete beam

• Core drilled adjacent to
mounted strain gage

• Change in strain
corresponds to relieved
stress

Validation Testing by UIUC


Similar setup to NAPTF: Cantilevered concrete
beam with 430 lb (1.94 kN) applied load


Added end deflection measurement


20mm and 30mm strain gages covered with an inert
sealant and aluminum tape to protect from core
-
drill
cooling water


While loaded, a 3” (7.62cm) diameter hole is cored
atop the concrete beam at ¼” (0.635cm) intervals to
a total depth of 1.5” (3.2cm)


Validation Testing by UIUC

Core drill

Validation Testing by UIUC

Validation Testing by UIUC


Practical problems:


Water from drilling is problematic


Heat from drilling is problematic…required 10 min cool time


How deep? Increased isolation of the strain vicinity is
observed as core
-
depths increase…to a point.


Even unloaded beams have some residual stress!

(due to drying stresses)

Validation Testing by UIUC

Microstrain and Load vs. Time for Core Drilled Beam
July 24 -- Beam #10
-60
-40
-20
0
20
40
60
0
10
20
30
40
50
60
70
80
90
Time in minutes
Microstrain
-500
-333
-167
0
167
333
500
Load in pounds
Microstrain
Load
0.6cm cut

1.3cm cut

1.9cm cut

2.5cm cut

3.2cm cut

Validation Testing by UIUC

Strain reading progressively drops from 40
με

to
8
με

for a strain change of 32
με
. This correlates
to a stress reduction of approximately
190
psi.

The applied load produces a theoretical tensile
stress of
230
psi at the location of the strain gage.

Validation Testing by UIUC

Microstrain vs. Load for Core Drilled Beam
July 24 -- Beam #10
y = 0.0842x + 4.0293
y = 0.0422x - 12.471
-20
-10
0
10
20
30
40
50
0
50
100
150
200
250
300
350
400
450
Load in pounds
Microstrain
No Core Ring
1.25 in Core Ring
Validation Testing by UIUC

The load
-
strain response of the concrete beam is
diminished as the depth of the core is increased,
suggesting partial isolation of the applied load.

So, a new approach…sawcut


Testing

was

modified

by

replacing

the

core
-
drill

with

a

circular

saw

fitted

with

a

masonry

blade

and

cutting

linear

notches

on

either

side

of

the

strain

gage


Concrete

beams

were

either

singly

notched

or

doubly

notched


Progressive

depths

of

1
.
27
cm,

2
.
54
cm

and

3
.
56
cm


Wooden

spacers

placed

over

the

strain

gage

to

serve

as

guide

and

to

prevent

contact

with

underside

of

circular

saw


Modified Testing by UIUC

Core drill

Linear Notches

Modified Testing by UIUC

Modified Testing by UIUC

Modified Testing by UIUC


Singly notching the concrete beam on either side
produced similar results to core
-
drilling


Time of strain recovery lessened possibly due to
quick pass of circular saw (~30sec) generating less
heat


Doubly notched concrete beams could achieve full
strain relaxation



Top Microstrain and Load vs. Time for Double Notched Beam
October 26 --Beam #26
-80
-60
-40
-20
0
20
40
60
50
70
90
110
130
150
170
190
210
230
Time in minutes
Microstrain
-800
-600
-400
-200
0
200
400
600
Load in pounds
Top Microstrain
Load
Modified Testing by UIUC

1.27cm 1s cut

2.54cm 1
st

cut

3.56cm 1
st

cut

0.64cm 2
nd

cut

1.27cm 2
nd

cut

0.64cm 1
st

cut

3.56cm 2
nd

cut

Modified Testing by UIUC

The first notch progressively decreases the
strain reading from 25
με

to
-
25
με

for a strain
change of 50
με
. This correlates to a stress
reduction of approximately 340psi.

Modified Testing by UIUC

The second notch progressively decreases the
strain reading from
-
25
με

to
-
40
με

for an
additional strain change of 15
με
. This correlates
to a stress reduction of approximately 102psi.

Modified Testing by UIUC

In combination, the strain reading decreases
from 25
με

to
-
40
με

for a strain change of 65
με
.
This correlates to a stress reduction of
approximately 440psi.

With a theoretical
applied
stress of 230psi,
the estimated residual stress in an
unloaded beam is 210psi.

Top Microstrain and Load vs. Time for Double Notched Beam
October 26 --Beam #26
-80
-60
-40
-20
0
20
40
60
50
70
90
110
130
150
170
190
210
230
Time in minutes
Microstrain
-800
-600
-400
-200
0
200
400
600
Load in pounds
Top Microstrain
Load
Modified Testing by UIUC

As the load is removed and applied, there is no
response in the concrete beam suggesting that
full isolation has been accomplished.

Microstrain vs Load for Double Notched Beam
October 26 -- Beam #26
y = 0.0726x - 6.7854
y = 0.0168x - 35.198
y = -0.0003x - 41.253
-50
-40
-30
-20
-10
0
10
20
30
0
50
100
150
200
250
300
350
400
450
Load in pounds
Microstrain
No Notches
0.95 in Notches
1.37 in Notches
Modified Testing by UIUC

Validation Testing by UIUC

The load
-
strain response of the concrete beam is
diminished as the depth of the notch is increased.
For a doubly notched beam, a flat, non
-
responsive
line is observed indicating full relaxation of stresses.

FEA of Notches in Concrete Beams


Plain

model

(with

no

notches)

was

made

as

a

reference


Singly

notched

beams

were

modeled

with

depths

of

1
.
27
cm,

2
.
54
cm

and

3
.
81
cm


Doubly

notched

beams

were

spaced

8
.
89
cm

apart

and

modeled

with

paired

depths

of

0
.
635
cm,

1
.
27
cm,

2
.
54
cm

and

3
.
81
cm


MatLAB

script

averages

the

stress

readings

over

a

30
mm

length

and

converts

to

a

normalized

strain

reading

for

comparison


FEA of Notches in Concrete Beams


Good

agreement

between

theoretical

strain

values

and

experiment

strain

values


Full

relaxation

of

strains

occurs

when







Higher ratios give worse results with small compressive
stresses created in the strain gage area

Relaxation versus Notch Geometry from Beams 26-28 and Finite Element Analysis
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
0
0.1
0.2
0.3
0.4
0.5
0.6
Ratio of notch depth to spacing between notches
Normalized stress/strain relaxation
Experiment
Finite Element
Small compressive stress occurs

if notch is too deep

Recommended
notch/space ratio

Current work extends to slabs

Conclusions


“An elegant solution”


Measuring residual stress by saw notching is
effective and simple to implement


Full
-
scale slab tests are encouraging


Goal is to develop new ASTM test method