Pavements - Impulse Response (IR)
Target of Investigation
Impulse response methods can be used to evaluate pavements by detecting the following:
- Debonding, delamination, and honeycombing within concrete pavement.
- Voids below concrete slabs and at joints.
Distress at the interface of a portland cement concrete overlay and concrete pavement can induce debonding. Debonding will destroy the integrity of the pavement and cause a loss of structural capacity. Delamination in concrete pavements is caused by inappropriate construction and distress from rebar corrosion.(1) Without proper treatment, delamination may cause further deterioration problems, such as spalling. Honeycombing in the pavement can be caused by a poorly graded concrete mix or insufficient vibration at the time of placement. Honeycombing can provide paths for moisture and cause further deterioration owing to freeze–thaw cycles or rebar corrosion. Voids below the concrete surface course usually occur near cracks and joints or along the pavement edge. Voids can be caused by many factors, such as the expulsion of water and soil through an open joint or shoulder under traffic loads, the compaction of base materials, subgrade failure, and bridge-approach failure. Voids may cause distress and serviceability loss in concrete pavements.
The impulse response method is a nondestructive testing method used in quality control and condition assessment of pavements and deep foundations. The method was first developed in the 1970s and used in performing quality control for drilled shafts. More recent uses of the impulse response method include determining subgrade moduli and detecting the presence of voids or loss of support below rigid pavements and slabs and in reinforced concrete bridges and concrete tunnel linings. The method uses a mobility spectrum to measure the dynamic response of concrete pavement. Damaged areas are detected when the mobility spectrum takes significantly different shapes and amplitudes from those at sound locations.(2) An impulse response testing system is shown in figure 1.
The impulse response method evaluates the dynamic characteristics of a structural element’s response to a given impulse. As shown in figure 2, an impact load is applied on the surface of the pavement with an instrumented hammer; the resulting stress wave propagates in the pavement and is reflected by the interfaces. The dynamic response is measured using a geophone at a nearby location on the surface. The signal of the applied impact force and the response from the geophone are transformed into the frequency domain to obtain the corresponding spectrum. The ratio of the measured velocity response to the impact force in the frequency domain is called the mobility spectrum. An example of a mobility spectrum is shown in figure 3. Compared to the impact echo method, the impulse response method relies more on the structural response under the impact, and the frequency range of interest is also much lower.
ASTM C1740-10, Standard Practice for Evaluating the Condition of Concrete Plates Using the Impulse-Response Method, provides the procedure for using the impulse response method to rapidly evaluate the condition of existing concrete pavements.(4) The equipment manufacturer’s instructions should also be followed to operate the equipment correctly. The general procedures for data collection on pavements are summarized as follows:
- Remove any debris from the survey area to prepare the surface. Grind the surface if it is too rough. Remove loose material before placing the transducer on the surface. Make sure of good contact and proper impact between the hammer and pavement.
- Lay out a grid on the surface of the concrete slab to be tested. Grid spacing normally ranges between 500 and 2,000 mm (20 and 80 inches). Closer spacing can be used for smaller slabs or to locate smaller anomalous regions.
- Impact the concrete surface with a hammer to generate transient stress waves in the concrete.
- Measure the response with a geophone placed adjacent to the impact load.
- Record the force and velocity waveforms with a data acquisition system. The system can synchronize data acquisition of the impact load and velocity response.
The impact force is measured by the load cell, and the velocity is measured by the geophone. These signals are then transformed from the time domain to the frequency domain by using the fast Fourier transform. The mobility spectrum is obtained by dividing the velocity spectrum by the impact force spectrum at each test point. The mobility spectrum summarizes the response of the concrete pavement as a function of frequency. Key parameters, such as the average mobility and dynamic stiffness, are computed from the mobility spectrum for each test point. Average mobility is calculated from the mobility values over the 100–800 Hz frequency range. Dynamic stiffness is calculated as the inverse of the slope of the mobility spectrum up to 40 Hz. A series of regularly spaced, high peaks in the mobility spectrum usually indicate resonant frequencies.
Average mobility and dynamic stiffness are related to density, elastic modulus, thickness, and presence of internal defects. Average mobility and dynamic stiffness can be displayed in the form of contour plots in order to identify likely locations of anomalous regions. An example contour map of the average mobility of a bridge slab is shown in figure 4. The locations of three drilled core samples are labeled.(3)
When debonding or delamination is present within the concrete surface course or voids exist beneath it, average mobility will increase and dynamic stiffness will decrease greatly. The peak mobility below 100 Hz becomes appreciably higher than the average mobility between 100 and 800 Hz. Voids beneath slabs are more likely when the ratio of this peak to average mobility exceeds 2.5.(5) Figure 5 is an example of the mobility spectrum for locations where voids exist beneath the pavement.
When honeycombing exists in a concrete pavement, mobility increases with frequency over the frequency range of 100–800 Hz.(6,7) As shown in figure 6, the mobility slope is determined from the best linear fit to the mobility spectrum between 100 and 800 Hz. A contour plot of the mobility slope can be used to indicate regions where honeycombing is likely owing to poor consolidation.
Advantages of the impulse response method include the following:
- Simple procedures for data collection.
- Automatic data processing and fast output.
- Good repeatability of test results.
Limitations of the impulse response method include the following:
- Small defects might be undetectable.
- Reliable data interpretation is highly dependent on the selection of test points.
- Testing large areas is time consuming.
- Portland Cement Association. (2001). Concrete Information: Concrete Slab Surface Defects: Causes, Prevention, Repair, PCA, Skokie, IL.
- Gucunski, N., Imani, A., Romero, F., Nazarian, S., Yuan, D., Wiggenhauser, H., Shokouhi, P., Taffee, A., and Kutrubes, D. (2013). Nondestructive Testing to Identify Concrete Bridge Deck Deterioration, Report No. S2-R06A-RR-1, Transportation Research Board, Washington, DC.
- Germann Instruments. “s’MASH.” (website) Evanston, IL. Available online: http://germann.org/products-by-application/flaw-detection/smash, last accessed March 7, 2019.
- ASTM C1740-16. (2016). “Standard Practice for Evaluating the Condition of Concrete Plates Using the Impulse-Response Method.” Book of Standards 04.02, ASTM International, West Conshohocken, PA.
- Davis, A.G. and Hertlein, B.H. (1987). “Nondestructive Testing of Concrete Pavement Slabs and Floors with the Transient Dynamic Response Method.” Proc. Int. Conf. Structural Faults & Repair, 2, pp. 429–433, University of London, London, United Kingdom.
- Ottosen, N.S., Ristinmaa, M., and Davis, A.G. (2004). “Theoretical Interpretation of Impulse Response Tests of Embedded Concrete Structures.” Journal of Engineering Mechanics, 130(9), pp. 1062–1071, American Society of Civil Engineers, Reston, VA.
- Nazarian, S. and Reddy, S. (1996). “Study of Parameters Affecting Impulse Response Method.” Journal of Transportation Engineering, 122(4), pp. 308–315, American Society of Civil Engineers, Reston, VA.