In this article, we will briefly present and discuss Impact-Echo (IE) method for testing concrete structures. The concept behind the technique will be reviewed. We will also study the applications and limitation of the IE method and How to Test Concrete Using Impact-Echo Method. The test method was adapted as a standard test procedure by the American Society of Testing Materials (ASTM C1383) (1), “Standard Test Method for Measuring the P-Wave Speed and the Thickness of Concrete Plates Using the Impact-Echo Method.”
What is Impact-Echo Method?
Impact-Echo is a nondestructive test method for evaluating concrete and masonry structures. The test utilizes stress waves (sound) that is normally generated through striking concrete by an impactor (Impact), and recording the reflections and refraction from internal flaws and other boundaries (Echo). Generally, a pulse is generated by an impact at one single point. The resulting stress wave propagates in all directions. In this regard, the generated wave lacks the directionality of a pulse that is generated by a large transducer (as in the case of ULTRASONIC PULSE VELOCITY). Since the reflections are arriving in all directions, the test method works best in slender element, such as piles where the external boundary guides the wave through the pile. In this particular application, the test has been standardized as low strain pile integrity test. Read more about PILE INTEGRITY TEST.
The Concept Behind Impact-Echo Method
The concept of the Impact-Echo test is illustrated in the Figure blow. A mechanical impact on the surface of concrete is used to generate the stress wave. The generate stress pulse propagates throughout the concrete along spherical wave fronts as P- and S-waves. Another component is the surface wave (R) which travels along the surface.
As P- and S- waves propagate within concrete element, they get reflected by internal interfaces (concrete-crack, concrete-air, concrete-rebar) or external boundaries. The arrival of these echos on the surface induces displacement. This displacement can be measured by placing a sensitive transducer (which then converts displacement or acceleration into electrical voltage). Data is recorded by a data acquisition and data logging system.
Analyzing Impact-Echo Data
Data analysis can either be done in Time Domain, or the Frequency Domain. While time-domain analysis looks to be a very good an easy way of data interpretation for slender elements (as in the case of pile integrity test), it can be way too complicated in the case of thin slabs. For these components, analysis of the signal in frequency domain is preferred.
Fast Fourier transform (FFT) technique is used to analyze the frequency content of the digitally recorded wave. In this method, a waveform can be represented as a sum of sine curves, each with a particular amplitude, frequency, and phase shift. According to N. J. Carino (2) The method is used to calculate the amplitude spectrum of the waveform, which gives the relative amplitude of the component frequencies in the waveform.
Applications of Impact-Echo Technique
Impact-Echo is a very practical testing solution with wide range of applications in condition assessment of concrete structures. The test can be used to:
1- Measure thickness of concrete slabs and walls
2- Measure thickness of concrete walls and tunnel lining
3- Locate defects such as delamination in concrete bridge decks.
4- Evaluate the condition of concrete retaining walls, dams, tunnel linings, sea walls.
A recent study by researcher at Rutgers University, The University of Texas at El-Paso, and Federal Institute for Materials Research and Testing (BAM), published by the National Academies of Sciences, Engineering, and Medicine (Referred to as SHRP2) (3) summarizes some of the most widely used applications of Impact-Echo method.
1- ASTM C1383-15, Standard Test Method for Measuring the P-Wave Speed and the Thickness of Concrete Plates Using the Impact-Echo Method, ASTM International, West Conshohocken, PA, 2015, www.astm.org DOI: 10.1520/C1383-15
2- M. Malhotra, V & Carino, Nicholas. (2004). CRC Handbook on Nondestructive Testing of Concrete. CRC Press Inc.
3- National Academies of Sciences, Engineering, and Medicine. 2012. Nondestructive Testing to Identify Concrete Bridge Deck Deterioration. Washington, DC: The National Academies Press. https://doi.org/10.17226/22771.