Assessing the Damage Potential in Pretensioned Bridges Caused by Increased

Truck Loads Due to Freight Movements

Robert J. Peterman, Ph.D., P.E.

Martin K. Eby Distinguished Professor in EngineeringKansas State University

Disclaimer

The contents of this report reflect the views of the authors, who are responsible for the facts and the accuracy of the information presented herein. This document is disseminated under the sponsorship of the U.S. Department of Transportation’s University Transportation Centers Program, in the interest of information exchange. The U.S. Government assumes no liability for the contents or use thereof.

Other Contributors

Steven F. Hammerschmidt, CE Dept.Dr. Weixin Zhao, MNE Dept.Dr. B. Terry Beck, MNE Dept.Dr. John Wu, Ph.D., IMSE Dept.

Overview

•Introduction

•Surface Strain Relief Method

•Test Specimens

•Finite-Element Models

•Results

•Conclusions

Introduction•Many bridges are approaching their design life expectancy and/or exposed to larger demands (10-15% are currently deficient).

•In order to accurately assess the condition of a prestressed concrete bridge (highway or railroad), the remaining prestress force level must be known.

•Time dependent losses decrease the prestress force in a member.

•The project’s goal was to develop an efficient, and inexpensive way to determine the existing stress in a prestressed concrete bridge member, thus the condition of these bridges can be accurately assessed.

Surface Strain Relief

Major Steps:2) Set up initial strain measurement device

•Electrical resistance strain (ERS) gages

•Laser speckle imaging (LSI) device

3) Core or notch to relieve strain

4) Measure elastic rebound of the concrete

5) Relate rebound of the concrete to the average prestress force

Surface Strain Relief

•Gage length of 2”

•Epoxy used to mount gage to surface

•Gages protected with polyurethane coating and microcrystalline wax

•Four pin terminal block was connected to the lead wires attached to the strain gage with silicone

Electrical Resistance Strain (ERS) Gages

Surface Strain ReliefLaser Speckle Imaging (LSI) Device•Device developed at Kansas State University

•Images the speckle pattern produced by a laser reflection off the surface which serves as the “fingerprint” of the location

•Subsequent images are related to the reference images and the amount of displacement is calculated

LSI Device with a 2” Gage Length Speckle Pattern

Coring/Notching Procedure

•Used a 3” outside diameter dry coring diamond bit

•Used a 4.5” diameter dry diamond cutting wheel

•Core and notch temperature was monitored using a non-contact thermometer

Procedure•Locations were marked on the beam and gages attached

•All gages were initially set to zero microstrain or the LSI device was used to take initial readings

•Coring guide was clamped into position on the surface of the beam or layout lines were drawn on the beam with a distance of 3.5” between notches

•Core locations were cored to an initial depth of ¾” and then 1”

•Notch locations were cut to an initial depth of 1” and then 1¼”

•There was a 10 minute delay between any increase in depth to allow the entire location to reach equilibrium with the surrounding area

Coring Procedure

Notching Procedure

Calculating the Average Prestress Force•The relief strain is a positive or tensile strain so a sign change is needed

•Relief stress related to the relief strain through Hooke’s Law

•The modulus of elasticity was determined in accordance with ASTM C469 and by the load deflection response of the beam

σ = ε · E

Calculating the Average Prestress Force

Test Specimens

Beam 1

Beam 2

Rectangle Beams•Cast in 2010

•Strands initially stressed to 202.5 ksi

•Average 28-day compressive strength: 7,440 psi

Test Specimens

T-Beams•Cast in March of 2002

•Lightly reinforced in longitudinal direction

•Strands initially stressed to 202.5 ksi

•Average 28-day compressive strength: 7,040 psi

Finite Element Models•Models created:

•Varying depth of cores: 0.75”, 1”, and 1.25”

•Varying notch depths, spacing, and lengths:

•Depths of 1”, 1.125”, 1.25”

•Spacing of 2.5”, 3”, and 3.5”

•Lengths of 2”, 3”, and 4”

•Beams restraint as a pinned, roller

Roller Support

Pin Support

Length

Depth

Finite Element Models

Finite Element ModelsMethod of Determining Average Stress

Finite Element ModelsVariable Core Depth

Core Depth (in)Simpson's Rule

Calculated Stress (psi)% Relieved

Stress 0.75 -266 82%

1 15 101%1.25 174 112%

Finite Element ModelsVariable Notch Depth

*Spacing between notches 3.5” and length of notch 3”

Notch Depth (in)Simpson's Rule

Calculated Stress (psi)% Relieved

Stress 1 -352.85 76%

1.125 3.97 100%1.25 317.93 121%

Finite Element ModelsVariable Notch Spacing

*Depth of notch is 1” and length of notch is 3”

Notch Spacing (in)Simpson's Rule

Calculated Stress (psi)% Relieved

Stress 2.5 283.18 119%3 81.45 105%

3.5 -352.85 76%

Finite Element Models

Notches on T-Beams

Core parallel to bottom of beam

Core perpendicular to web

Notch

*T-beam properties were the same as the rectangle beam models

Finite Element Models

MethodSimpson's Rule

Calculated Stress (psi)% Relieved

Stress Core Parallel 20.53 103%

Core Perpendicular -23.22 97%Notch 64.85 109%

ResultsSurface Strain Results — CoresBeam 1a

Core Theoretical (με) 3/4" Depth (με) 3/4" Percent Error 1" Depth (με) 1" Percent Error1 330 301 8.8% 314 4.8%2 331 298 10.0% 328 0.9%3 329 325 1.2% 361 -9.7%4 331 297 10.3% 332 -0.3%

Beam 1bCore Theoretical (με) 3/4" Depth (με) 3/4" Percent Error 1" Depth (με) 1" Percent Error

1 381 279 26.8% 351 7.9%2 381 309 18.9% 390 -2.4%

Avg. = -1.2%

Avg. = +4.5%

Avg. = +7.6%

Avg. = +22.9%

ResultsSurface Strain Results — Cores

Beam 2aCore Theoretical (με) 3/4" Depth (με) 3/4" Percent Error 1" Depth (με) 1" Percent Error

1 269 240 10.8% 290 -7.8%2 268 220 17.9% 254 5.2%3 268 250 6.7% 300 -11.9%4 268 261 2.6% 290 -8.2%5 269 224 16.7% 247 8.2%

Beam 2bCore Theoretical (με) 3/4" Depth (με) 3/4" Percent Error 1" Depth (με) 1" Percent Error

1 266 184 30.8% 235 11.7%2 268 236 11.9% - -3 266 197 25.9% 219 17.7%4 268 273 -1.9% 278 -3.7%

Avg. = -2.1%

Avg. = +7.8%

Avg. = +10.9%

Avg. = +16.7%

ResultsSurface Strain Results — NotchesBeam 1b

Beam 2a

Beam 2b

Notch Theoretical (με) 1" Depth (με) 1" Percent Error 1.25" Depth (με) 1.25" Percent Error*1 268 288 -7.5% 316 -17.9%*2 268 303 -13.1% 323 -20.5%

* Used LSI device to measure strain

Notch Theoretical (με) 1" Depth (με) 1" Percent Error 1.25" Depth (με) 1.25" Percent Error1 383 236 38.4% 323 15.7%2 383 200 47.8% 248 35.2%*3 382 310 18.8% 369 3.4%*4 384 293 23.7% 311 19.0%

* Used LSI device to measure strain

Notch Theoretical (με) 1" Depth (με) 1" Percent Error 1.25" Depth (με) 1.25" Percent Error1 266 283 -6.4% 305 -14.7%2 266 327 -22.9% 352 -32.3%*3 268 240 10.4% 302 -12.7%

* Used LSI device to measure strain

Avg. = +18.3%

Avg. = -19.2%

Avg. = -19.9%

Avg. = +32.1%

Avg. = -10.3%

Avg. = -6.3%

ResultsSurface Strain Results — CoresT-beam 1

T-beam 2

Core Theoretical (με) 3/4" Depth (με) 3/4" Percent Error 1" Depth (με) 1" Percent Error1 118 104 11.9% 123 -4.2%2 118 101 14.4% 126 -6.8%3 122 103 15.6% 108 11.5%4 118 111 5.9% 113 4.2%

Core Theoretical (με) 3/4" Depth (με) 3/4" Percent Error 1" Depth (με) 1" Percent Error1 184 171 7.1% 180 2.2%2 186 117 37.1% 149 19.9%3 189 58 69.3% 71 62.4%4 188 61 67.6% 78 58.5%

Core 3 and 4, Reinforcement present in core

Avg. = +1.2%Avg. = +1.2%

ResultsSurface Strain Results — Notches

T-beam 2Notch Theoretical (με) 1" Depth (με) 1" Percent Error 1.25" Depth (με) 1.25" Percent Error

1 182 162 11.2% 206 -13.0%*2 182 144 20.7% 157 13.6%*3 188 142 24.5% 163 13.4%*4 183 162 11.5% 328 -79.2%*5 183 131 28.4% 321 -75.4%

* Used LSI device to measure strain Avg. = +18.3%Avg. = +19.3%

Conclusions•A 3” core bit used with a 2” strain gage resulted in an almost complete rebound of the surface strain when coring to a depth of 1”, with an average error of less than 8%

•A notch depth of 1”, spacing of 3.5” and length of 3” provides more varied results, with an average error of around 20%

•The Laser Speckle Imaging device provided a quick and accurate way to measure the strain.

•Multiple locations can be tested to reduce the overall error, and taking the average of 4 cores is recommended.

Conclusions•Strain drift due to temperature change can be mostly eliminated by allowing 10 minutes after coring and 5 minutes after notching.

•Finite element models successfully predicted the amount of relieved strain similar to the experimental results, and could be used to determine the optimal method for other geometries and strand configurations.

•Reinforcement around the core area significantly affects the measured relief strain, and steps should be taken to prevent coring in the immediate vicinity of stirrups.

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A special thanks to our Associate Director, Dr. Mustaque Hossain.

CREDITS