Pavements – FHWA InfoTechnology

Pavements - Impact Echo (IE)

Target of Investigation

Primary applications of the impact echo (IE) method include the following:

Estimating the thickness of concrete slabs.
Detecting and characterizing the delamination of concrete slabs and debonding of overlays.

These applications can also be used on asphalt pavement if the asphalt is cool and stiff.

Description

The IE method uses seismic or stress waves to estimate the thickness of intact concrete members and detect defects, primarily delamination, within concrete members. The objective of an IE survey is to detect and characterize wave reflectors, or resonators, in concrete elements. IE devices can have multiple probes (figure 1) or a single IE probe (figure 2). Each probe consists of an impactor and a sensor.

This photo shows testing with automated multiprobe impact echo (IE) equipment. The equipment is attached to a horizontal steel bar, which is perpendicular to the traffic direction and connected to the rear bumper of a truck. Behind the steel bar are three pairs of rollers on which IE probes are mounted. The pairs are located from the center to the right end of the steel bar with equal spacing. — Figure 1. Photos. Automated IE equipment with multiple probes.⁽¹⁾

This photo is a closeup view of two impact echo sensors. The two rollers are connected by a steel bar. Each roller has a sensor, which is a bronze circle with a black dot in the center.

This photo shows an operator walking a probe along the paved surface of a road; the probe is in one hand, and a readout panel is in the other. The probe is a sticklike instrument with sensors on the bottom. The probe’s tip is in contact with the road surface. A closeup inset shows the impactor and sensor in more detail. — Figure 2. Photo. Manual IE testing using a single IE probe and a closeup of the probe.

Physical Principle

An IE test is conducted using an impactor and sensor. The impact generates waves that propagate within solid material. Waves are reflected by boundaries or by internal defects, which create a difference in acoustic impedance. The reflected waves, or echoes, return to the surface and are measured by the receiver. In frequency domain analysis, multiple reflected waves monitored by the sensor manifest as resonance conditions. Defects, such as delaminations and big voids, can be detected by differences in the resonant frequency compared to sound areas. The principle of IE is illustrated in figure 3. The resonant frequency of areas with deep delaminations is higher than the sound areas. The resonant frequency of areas with large, shallow delaminations is lower than the sound areas owing to excitation of the flexural mode.

This illustration is a schematic of impact echo (IE) testing and typical spectrums for different levels of bridge deck delamination. The upper part of the figure provides a schematic of the IE test being performed at different locations on a concrete slab. A rectangular area represents the concrete slab. There are impact hammers and sensors at four locations on the concrete surface. A set of arrows represents the propagation and reflection of the stress waves in the concrete after the impacts. A set of curves indicate the locations of the delamination in the concrete. The lower part of the figure contains four graphs showing corresponding spectrums obtained from the IE tests at the different locations. The x-axis of each graph is frequency, ranging from 0 to 40,000 hertz. The y-axis of each graph is normalized amplitude, ranging from 0 to 1.2. The first graph on the left has a high peak at approximately 10,000 hertz and is labeled “Good.” The second graph has a high peak at approximately 10,000 hertz and a smaller peak at approximately 20,000 hertz and is labeled “Fair.” The third graph, labeled “Poor,” has a high peak at approximately 20,000 hertz and two smaller peaks, one close to the origin, and one at approximately 7,000 hertz. The fourth graph has a peak at approximately 4,000 hertz and is labeled “Serious.” — Figure 3. Illustration. Physical principle of IE.⁽²⁾

Data Acquisition

For a complete data collection procedure, refer to ASTM C1383-15, Test Method for Measurement P-Wave Speed and the Thickness of Concrete Plates Using the Impact-Echo Method.⁽³⁾

Before IE testing, the testing surface must be cleaned of debris. A grid should be drawn with washable paint or chalk. Data collection can be conducted using a variety of devices that provide different levels of accuracy, speed, and automation.⁽⁴⁾

The IE impactor should be selected carefully to provide enough energy in the frequency range of interest. Some impactors are equipped with a sensor to measure the characteristics of the impact. Many IE systems utilize steel balls of different sizes as impact sources, with the center and maximum frequencies being inversely proportional to the diameter of the ball. However, computer-controlled impactors are also available.

Data Processing

An amplitude spectrum obtained from a fast Fourier transform analysis of the time-domain signal will show dominant peaks at certain frequencies, which can be interpreted to assess the slab thickness or potential delamination or debonding.

The thickness mode normally dominates the spectral response of a platelike structure that does not contain any near-surface defects.⁽⁵⁾ The frequency of the thickness mode can be related to the thickness of the member (figure 4).

Figure 4. Equation. Calculation of thickness.

Where:
- T = thickness or depth of a defect.
- β = correction factor.
- V_p = Pressure wave velocity of concrete.
- f_t = IE frequency acquired from the delamination thickness or the concrete slab thickness.

With the IE method, the accuracy or uncertainty in thickness estimates depends on a pooled error of the return-frequency measurement and the P-wave velocity determination. Research has shown that the typical uncertainty of thickness measurements is 5–10 percent. (See references 2, 3, 6, and 7.) Other factors that may impact uncertainty include the stiffness of the underlying layer, closeness to boundaries, and texture of the concrete surface.⁽³⁾

Data Interpretation

IE results are presented in various forms, some describing the position of reflectors, some interpreting the condition with respect to the state of delamination.⁽⁸⁾ Figure 5 presents the IE resonant frequencies from an asphalt pavement. The colors refer to different dominant frequencies identified in the response spectrum. Blue and purple indicate low frequencies of flexural resonance, which is caused by shallow delaminations. Light green and yellow indicate resonant frequencies of the normal thickness of the asphalt pavement. Figure 6 presents results in terms of different condition grades related to the progression of delamination development. For example, fair and poor grades refer to incipient or progressed delamination, and a serious grade corresponds to either shallow or wide delamination.

This contour map depicts the impact echo resonant frequencies from an asphalt pavement. The x-axis is the station number from 15 to 65, and the y-axis is lane width from 0 to 12 feet. The pavement is divided into two parts by a vertical line in the center. Section 1 has station numbers less than 40. This section is shown in scattered high and low frequencies. Areas with high frequencies are labeled “H,” and areas with low frequencies are labeled “L.” Five arrows point to different locations on section 1; these are labeled “Delaminated at 5-inch depth.” Section 2 has station numbers greater than 40. Most of section 2 has dominant frequencies corresponding to the thickness of the asphalt concrete, indicating sound pavement. — Figure 5. Contour map. Resonant frequency indicating thickness.⁽¹⁾

This contour map depicts a condition assessment of a pavement. The x-axis of the contour map is longitudinal distance in feet, ranging from 0 to 84. The y-axis is lateral distance in feet, ranging from 2 to 12. A color-coded scale at the bottom of the figure shows ratings of sound, fair, poor, or serious. The contour map contains irregular areas of different conditions. Most of the map shows sound and fair conditions. Large areas of serious condition are located at longitudinal distances of 0 to 4, 18 to 24, 28 to 34, 48 to 52, and 76 to 80 feet, and are labeled S on the map. — Figure 6. Contour map. Condition assessment with respect to delamination progression.⁽⁸⁾

Advantages

Advantages of IE technology include the following:

Fast data analysis.
Insensitive to traffic-induced vibrations.
Reliable and repeatable results.
Early delamination detection.

Limitations

Limitations of IE technology include the following:

Requires moderate to significant expertise and training for equipment setup and data collection, processing, and interpretation.
Necessitates lane closure for slow data collection with traditional single-probe equipment.
Entails complex evaluation of the condition of an overlaid member.
Requires consideration of geometrical and boundary effects.
Cannot estimate thicknesses for members resting on materials of similar stiffnesses.
Has a limited ability to provide the degree of severity.
Cannot measure pavement condition below the top of the discontinuity.⁽⁸⁾

References

Heitzman, M., Maser, K., Tran, N.H., Brown, T., Bell, H., Holland, S., Ceylan, H., Belli, K., and Hiltunen, D. (2013). Nondestructive Testing to Identify Delaminations Between HMA Layers, Report No. S2-R06D-RR-1, Transportation Research Board, Washington, DC.
Gucunski, N., Imani, A., Romero, F., Nazarian, S., Yuan, D., Wiggenhauser, H., Shokouhi, P., et al. (2013). Nondestructive Testing to Identify Concrete Bridge Deck Deterioration, Report No. S2-R06A-RR-1, Transportation Research Board, Washington, DC.
ASTM C1383-15. (2015). “Standard Test Method for Measuring the P-Wave Speed and the Thickness of Concrete Plates Using the Impact-Echo Method.” Book of Standards 04.02, ASTM International, West Conshohocken, PA.
Nazarian, S., Yuan, D., Smith, K., Ansari, F., and Gonzalez, C. (2006). Acceptance Criteria of Airfield Concrete Pavement Using Seismic and Maturity Concepts, Report No. IPRF‑01‑G‑002-02-2, Innovative Pavement Research Foundation, Skokie, IL.
Sansalone, M. and Carino, N. (1989). “Detecting Delaminations in Concrete Slabs with and without Overlays Using the Impact-Echo Method.” ACI Materials Journal, 86(2),
pp. 175–184, American Concrete Institute, Farmington Hills, MI.
Maser, K.R, Holland, T.J., Roberts, R., Popovics, J., and Heinz, A. (2003), “Technology for Quality Assurance of New Pavement Thickness.” Proceedings from the 82nd Annual Meeting of the Transportation Research Board, Washington, DC.
Azari, H., Yuan, D., Nazarian, S., and Gucunski, N. (2012). “Sonic Methods to Detect Delamination in Concrete Bridge Decks: Impact of Testing Configuration and Data Analysis Approach.” Transportation Research Record, 2292, pp. 113–124, Transportation Research Board, Washington, DC.
Strategic Highway Research Program. “SHRP2 NDToolbox.” (website) Washington, DC. Available online: http://www.trb.org/StrategicHighwayResearchProgram2SHRP2/Blank2.aspx, last accessed March 6, 2019.