Impulse Response (IR)Impulse Response (IR)

Applications:Applications: • Pile integrity testingPile integrity testing

• Floor slabs, walls and pavementsFloor slabs, walls and pavements

• Concrete consolidation Concrete consolidation

• Cylindrical structures (silos, chimney stacks and Cylindrical structures (silos, chimney stacks and tanks)tanks)

• Concrete bridges Concrete bridges

• Cladding on high-rise buildingsCladding on high-rise buildings

Impulse Response MethodImpulse Response Method

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S1

S2

S3

S4

S5

S6

S7

S8

S9

Impulse Response in use on Impulse Response in use on Concrete TankConcrete Tank

Impulse Response (IR): Impulse Response (IR): Fundamentals in Pile TestingFundamentals in Pile Testing

• Low strain impact on pile head generates Low strain impact on pile head generates axial bar wave (compression stress)axial bar wave (compression stress)

• Impact force measured by load cellImpact force measured by load cell

• Response to impact measured by velocity Response to impact measured by velocity transducer (geophone) in time domaintransducer (geophone) in time domain

• FFT on force and velocity, combined to give FFT on force and velocity, combined to give transfer function: Mobility (v/F) v. frequencytransfer function: Mobility (v/F) v. frequency

• Theory well establishedTheory well established

Idealized Pile Response CurveIdealized Pile Response Curve

Impulse Response (IR): Impulse Response (IR): Basics in Structure TestingBasics in Structure Testing• Low strain impact on structural element Low strain impact on structural element

generates compression stress wave (as in generates compression stress wave (as in Impact-Echo method, but lower frequency band Impact-Echo method, but lower frequency band [0-800 Hz] & much greater force input)[0-800 Hz] & much greater force input)

• Impact force measured by load cellImpact force measured by load cell

• Response to impact measured by velocity Response to impact measured by velocity transducer (geophone) in time domaintransducer (geophone) in time domain

• FFT on force and velocity, combined to give FFT on force and velocity, combined to give transfer function: Mobility (v/F) v. frequencytransfer function: Mobility (v/F) v. frequency

IR Parameters – Structure TestingIR Parameters – Structure Testing

• Dynamic stiffness in MN/mm from mobility Dynamic stiffness in MN/mm from mobility slope between 0-50 Hzslope between 0-50 Hz

• Average mobility from 100-800 Hz (function Average mobility from 100-800 Hz (function of element thickness and concrete quality)of element thickness and concrete quality)

• Slope of average mobility (function of Slope of average mobility (function of concrete consolidation concrete consolidation andand proximity of proximity of structural shape changes)structural shape changes)

• Peak/average mobility ratio (support beneath Peak/average mobility ratio (support beneath slabs on grade, voiding)slabs on grade, voiding)

Stiffness Average Mobility

Example of IR Slab TestExample of IR Slab Test

IR from Highway PavementIR from Highway Pavement

Voiding beneath slab

IR from Cooling TowerIR from Cooling Tower

Honeycomb

Mobility Slope

Test point indicating low thickness and good support (Black)

Test point indicating good thickness and good support (Blue)

Mobility Plots

Impulse Response Test OutputImpulse Response Test Output

Debonded Bridge Deck OverlayDebonded Bridge Deck Overlay

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S2

S3

S4

S5

S6

S7

Column

Ro

w

Silica fume concrete bridge deck overlay Average Mobility

10-18 18-26 26-34