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 Engineering Kansas State University

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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: 1)Set up initial strain measurement device Electrical resistance strain (ERS) gages Laser speckle imaging (LSI) device 2)Core or notch to relieve strain 3)Measure elastic rebound of the concrete 4)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 Relief Laser 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 LengthSpeckle 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 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 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

Method of Determining Average Stress

Finite Element Models Variable Core Depth

Finite Element Models Variable Notch Depth *Spacing between notches 3.5” and length of notch 3”

Finite Element Models Variable Notch Spacing *Depth of notch is 1” and length of notch is 3”

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

Results Surface Strain Results — Cores Beam 1a Beam 1b Avg. = -1.2% Avg. = +4.5% Avg. = +7.6% Avg. = +22.9%

Results Surface Strain Results — Cores Beam 2a Beam 2b Avg. = -2.1% Avg. = +7.8% Avg. = +10.9% Avg. = +16.7%

Results Surface Strain Results — Notches Beam 1b Beam 2a Beam 2b Avg. = +18.3% Avg. = -19.2% Avg. = -19.9% Avg. = +32.1% Avg. = -10.3% Avg. = -6.3%

Results Surface Strain Results — Cores T-beam 1 T-beam 2 Core 3 and 4, Reinforcement present in core Avg. = +1.2%

Results Surface Strain Results — Notches T-beam 2 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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