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??