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Synthesis Study on Load
Capacity of Concrete Slab
Bridges without PlansRichard Miller
Bahram Shahrooz
Paul Gearhart
Problem Statement
• It is estimated there are over 1200 concrete
slab bridges with unknown properties in Ohio.
• Previously, visual inspection was adequate to
assess these bridges.
• FHWA now requires numerical rating.
• In some counties, loads have increased due to
oil and gas exploration.
PARAMETERS NEEDED TO
RATE A SLAB BRIDGE• Geometry
– Span, slab thickness, condition of support
• Concrete strength
• Reinforcing bar
– Size
– Yield strength
– Effective depth
– Spacing
GEOMETRY
Slab geometry is easily determined by
measuring the bridge.
CONCRETE STRENGTHOddly, concrete strength is not a critical parameter
since the bridge is a flexural element. The graph
shows the influence of concrete strength on moment
capacity.
0
1000
2000
3000
4000
5000
6000
0 2 4 6 8 10 12
Mo
men
t ca
pac
ity (
k-i
n.)
f'c (ksi)
Nominal
Design
METHODS OF DETERMINING
CONCRETE STRENGTH• Coring
– Most accurate but does some minor damage.
• NDT (Rebound Hammer, Windsor Probe)
– Not very accurate unless calibrated.
– May be accurate enough for rating purposes.
• Historical Record
– For given time frames, it may be possible to estimate
concrete strength from specifications used at the time.
REBAR SPACING/COVER
• Reinforcing bar spacing and cover can be
determined with Ground Penetrating
Radar or Magnetometers.
– Magnetometers are easy to use, but only
accurate if cover < 3 inches.
– GPR can penetrate large cover distances but
are more expensive and harder to use.
REBAR SIZE• For cover < 3 inches, magnetometers can
determine bar size +/- one bar size.
– Multiple measurements on a bridge may
improve accuracy.
– Accuracy can be improved if it is possible to
determine one bar size through other means
(deteriorated area, core).
– Accuracy of magnetometer verified through
literature, survey of engineers and field tests.
YIELD STRENGTH
• Yield strength of reinforcing bars is a
critical parameter.
• Moment capacity is directly proportional
to yield strength.
• No easy method to measure this in situ.
YIELD STRENGTH –
MEASUREMENT• Historical Records
– CRSI has records of historical bar. During certain
eras, there was a maximum bar strength.
• Prior to 1959– Structural (fy=33ksi)
– Intermediate (fy=40ksi)
– Hard (fy=50ksi)
• Grade 60 did not appear until 1959.
• Most newer bridges will use Grade 60 reinforcing
bars.
YIELD STRENGTH
MEASUREMENT• Historical Record
– CRSI has mill mark data.
– If a mill mark can be found (perhaps in a
deteriorated area), the bar can be identified.
Figure courtesy of CRSI.
YIELD STRENGTH
MEASUREMENT• Tensile testing.
– It is possible that a bar sample could be
removed from a deteriorate area for tensile
testing.
– This test needs an approximately 3 foot long
sample, which is probably impractical.
YIELD STRENGTH
MEASUREMENT• Two possible alternate methods
– Compression testing.
• Data from University of Washington suggests a 3-
4 inch specimen could be tested for yield in
compression.
– Hardness testing
• Literature suggests that there is a relationship
between hardness and strength.
YIELD STRENGTH
MEASUREMENT“Squish” (Compression) Testing
Structural steel sections can be tested in compression
if kL/r < 6.0.
University of Washington has done compression tests
on rebar to find yield strength.
A 3 inch specimen would have kL/r < 6.0 for #4 and
above.
YIELD STRENGTH
MEASUREMENT
Literature suggests that there may be a
relationship between strength and hardness.
Nominal strength is used for rating. Thus the
test has to only be accurate enough to
identify grade.
YIELD STRENGTH
MEASUREMENT• The research team collected samples of
old reinforcing bar.
• Bars are being tested for yield and tensile
strength.
• Rockwell “B” hardness also tested.
YIELD STRENGTH
MEASUREMENT
YS = Yield Strength
YIELD STRENGTH
MEASUREMENT
UTS = Ultimate Tensile Strength
PROPOSED TEST
METHODOLOGY• Determine bridge geometry.
– This is done through simple measurement of
the slab thickness and span length(s).
• Condition of support is more difficult to
determine.
– Literature suggests it is somewhere between
fixed and pinned. Conservative to assume a
pin.
PROPOSED TEST
METHODOLOGY• Determine concrete strength
– Coring is the most accurate.
– NDT techniques like rebound hammer may provide
accurate enough results for rating as concrete
strength is not an important parameter.
• Data suggests NDT techniques without calibration
by coring are +/- 1500 psi on strength.
– Historical data/specifications may provide an estimate
of specified strength.
PROPOSED TEST
METHODOLOGY• Determine bar spacing.
– Magnetometer can find bar spacing for cover < 3 in.
– Accuracy is about +/- 3/8 in, but a large number of
readings allow for reasonable determination of bar
spacing.
– Magnetometer can also locate bar ends.
– GPR needed for large covers.
PROPOSED TEST
METHODOLOGY• Determine rebar cover.
– This can be done with a magnetometer if the cover
is < 3 in.
– Accuracy of magnetometer is about 15% for cover <
1.75 in. and about 23% for larger values. Accuracy
gets worse as cover gets larger.
– For cover > 3 in. ground penetrating radar is needed.
– Cover can be verified with a small core to the rebar
level.
PROPOSED TEST
METHODOLOGY• Determine bar size.
– Magnetometer can only find bar size +/- one
bar size.
– Large number of readings may improve
accuracy.
– Bar size is very important so probably need
to verify bar size in a deteriorated area or
with a core.
PROPOSED TEST
METHODOLOGY• Determine yield strength. Possible methods:
– Get a bar from a deteriorated area for a tensile test.
– A core would yield a 3 inch specimen for a
compression yield test.
– Removing cover to expose bar would allow for a
hardness test (portable hardness test devices
available).
– Historical records might allow an estimate of bar
strength.
FUTURE WORK
• Add to hardness/tensile strength data.
• Verify compression testing protocol.
• Test bridges with unknown properties to
verify methodology.
Questions??