콘크리트 강성 결함 Impulse Response Method IR 방법

영신컨설턴트 (02) 529 8803 ystcha@naver.com 2020 11

 

에너지를 측정할 수 있는 헤머와 속도 측정 센서를 콘크리트에 부착하여 콘크리트의 강성과 결함 발견을 위한 IR 기법에서 공진주파수보다 낮은 저주파수에서 나타나는 작은 피크 구간이 공동에 의한 반응으로 보이며, IR 기법에서는 주파수에 따른 운동성(mobility)과 동적 강성도 (dynamic stiffness)의 변화를 통해 공동의 유무를 확인할 수 있었다.

 

시험방법

현장타설말뚝

ASTM D5882 – 16 Standard Test Method for Low Strain Impact Integrity Testing of Deep Foundations

 

 

콘크리트

ASTM C1740, “Standard Practice for Evaluating the Condition of Concrete Plates Using the Impulse-Response Method.”

 

Detecting voids beneath concrete slabs in highways, spillways, and floors

Detecting curling of slabs on ground

Evaluating the integrity of anchoring systems of wall panels

Locating delaminations and honeycombing in bridge decks, slabs, walls and large structures such as dams, chimney stacks, and silos

Detecting the presence of damage due to freezing and thawing

Detecting the presence of alkali-silica reaction (ASR)

Detecting debonding of asphalt or concrete overlays and repair patches from concrete substrates

Evaluating the effectiveness of the load transfer system in transmitting forces across joints in concrete structures

 

 

운동성(mobility)과 동적 강성 (dynamic stiffness)

 

 

운동성(mobility) unit (m/s)/N

 

M= fft V/ fft F V(f) – Velocity spectrum. F(f) – Force spectrum.

강성도 (dynamic stiffness)

 

Kd = 2πfM / M

 

fM – any frequency where the mobility curve is close to straight line from origin

M – mobility value corresponding to fM.

 

 

 

Velocity and Force in Time Domain

 

 

Spectrum of Velocity and Force

 

 

Mobility

 

 

Nc = 1 / ρ c A = 1 / Z

 

ρ - density of pile material (kg/m3);

c – wave speed (m/s)

A – pile cross sectional area (m2)

Z – pile impedance (N/m/s).

 

The mean value of measured mobility

 

Nm = root (P x Q)

 

P – maximum peak mobility value (m/s/N)

Q – Minimum valley mobility value (m/s/N)

 

 

현장타설말뚝

Nm is also called the characteristic mobility and a measure of relative pile mobility. Theoretically, Nm, theoretically computed mobility Nc and the dynamic stiffness kd indicate the relative quality of the pile.

 

The dynamic stiffness kd relates to a low load, elastically recoverable spring stiffness of the pile and may be compared to the quantity EA/L (E = elastic modulus and L = pile length).

A lower value may indicate a defective pile or lower resistance. Higher shaft resistance or a bulge will result in a higher dynamic stiffness. In this way, both integrity and soil resistance are to some degree quantified by the dynamic stiffness value. However, application of high/low pass filtering may significantly affect this value.

 

If force and velocity are proportional then the ratio of velocity to force should be around Nc in the steady state region, i.e. Nm is close to Nc for a uniform pile. Typically Nm falls between 0.5 and 2.0 of Nc. If Nm is larger than Nc and kd is lower, the pile might be defective. If Nm is less than Nc and kd is higher, the pile might have bulges. It seems the existance of soil resistance only affects the difference between P and Q.

 

It is noted that for large diameter piles, the force and velocity are not proportional due to limited spherical point contact of impact. Thus direct comparison between Nm and Nc or kd and EA/L is meaningless. However, comparison of Nm and kd among same piles or same soil conditions may also help identify the group of piles that have potential problem with integrity or soil resistance (e.g. relatively higher mobility or lower stiffness).

 

L = c x Δt / 2 = c / (2 x Δf )

 

L – distance from the gage to a defect or pile toe (m)

Δt – time different from the rise (peak) time of input pulse and therise (peak) time of reflection pulse from the defect or pile toe(s).

c – wave speed (m/s);

 

Δf – frequency difference between peaks of mobility curve orvelocity spectrum (Hz). Usually, this is the frequency difference between adjacent peaks (smaller frequency differences translate to longer lengths (L).

 

But it is also important to look for the peak pattern of mobility curve. For example, the highest peaks may not be adjacent and the difference between them may indicate defects (e.g. defects along the shaft – at reduced length – will have a higher Δf. Thus it is often difficult to judge defects solely in the frequency domain. Reviewing also the time domain measurements is usually helpful when evaluating defects. Similarly defects usually result in higher mobility and lower dynamic stiffness values. All tools should be used when assessing potential defects.

 

콘크리트

 

The parameters from the mobility plot that are used for integrity evaluation are:

 

The dynamic stiffness (the inverse of initial the slope of the mobility plot, the blue line in previous figure;

The average mobility (dotted blue line in previous figure);

The mobility slope between 100 to 800 Hz from fitting a straight line; and

The voids ratio (the ratio of the amplitude of the low frequency peak to the average mobility)

 

공극결함(좌) 곰보결함(우)

Examples of mobility plots for different types of flaws in plate-like structures are shown above.

 

The figure on the left shows the mobility plot for a slab-on-ground with a void below the slab at the test location and the mobility plot for a slab with uniform support at the test point.

The voids ratio is the ratio of the amplitude of the low frequency peak to the average mobility of the slab with good support.

 

The figure on the right figure compares the mobility plot of a honeycombed region in a silo wall with the mobility plot of properly consolidated concrete. Honeycombed concrete is typically associated with a high mobility slope (the dashed lines).