Acoustic Emission Testing of Concrete Structures

Reinforced concrete structures are the building blocks for a variety of infrastructure including towering skyscrapers, concrete parking decks, and concrete bridges. Ensuring these structures remain safe over time is a requires periodic non-destructive testing (NDT) and long-term monitoring with structural health monitoring instrumentation.  monumental task. Acoustic Emission (AE) testing is an advanced non-destructive testing method that listens to the sounds of concrete failure mechanisms to enable screening of large areas. Acoustic emission may be used to target defects in concrete structures such as concrete cracking, rebar corrosion and failure, and interface conditions between concrete and rebar. This article explores the important aspects of concrete structure acoustic emission testing.

Figure 1-1 Typical Acoustic Emission (AE) non-destructive testing setup in concrete reinforced structures – in this case parking deck spandrels. Two acoustic emission sensors have been mounted where ground penetrating radar has identified possible structural defects.

Review of Acoustic Emission Testing of Structure

Acoustic emission testing is used to detect active flaws in concrete, composite, and steel structures. Active flaws include concrete crack growth, rebar corrosion activity, active rebar – concrete interface deterioration. When a damage mechanism propagates inside a concrete structure, it generates stress waves, or acoustic emission, that travel through the material. These waves are detected by AE sensors, which act like the ears of the system, capturing these high-frequency signals [1]. These AE signals are then amplified, filtered, and recorded, revealing a detailed picture of the internal state of the reinforced concrete, and identifying failure modes, and their approximate location.

AE capitalizes on the fact that transient waves can identify active damage processes within concrete, such as crack initiation, crack growth, and steel corrosion. These waves, also known as acoustic emissions, are picked up by piezoelectric sensors attached to the surface of the reinforced concrete structure. The sensors convert these mechanical waves into electrical signals, which are then analyzed to determine the nature and severity of the damage. These damage modes such as rebar failure, corrosion, cracking. The severity of the damage is assessed by the rate at which acoustic emission activity is released and the intensity at which it is emitted.

The Acoustic Emission Process in Concrete Structures  

Acoustic emission is emitted in concrete when the stress applied to the asset propagates the damage mechanism. Therefore, the concrete structure is the source of the AE, and this is classified as a passive non-destructive testing method. AE sensors are installed at strategic location based on documented attenuation studies. These sensors are strategically placed on the concrete surface to maximize coverage and accuracy. AE sensors detect the transient elastic waves generated by internal damage processes specific to concrete bridges, concrete parking decks, and other concrete structural systems. The captured AE signals are then amplified, filtered to remove mechanical and electrical noise, and enhance the AE signal-to-noise ratio (SNR). Figure 1-2 below shows a typical AE waveform detected from a concrete structure using a 30 kHz  sensor. 

Figure 1-2 Example of acoustic emission sampled signal in concrete.

Acoustic emission waveform features such as AE amplitude, duration, rise time, energy and counts are then extracted to start the damage assessment process. These parameters provide insights into the characteristics of acoustic emissions like the type of source, severity of the source and to help discriminate between failure modes, mechanical noise, and electrical noise. Additional analyses include pinpointing the location via 2-D source location algorithms and damage progression over time. Figure 1-3 shows a typical frequency response from a concrete structure in the 30 to 100 kHz range. Note that this frequency range is significantly lower than the 100 to 500 kHz range of acoustic emission fatigue crack sources in steel.

Figure 1-3 Different concrete configurations can create different frequency responses which is intuitive in identifying the correct sensor usage for the type of concrete and the scope of the inspection.

Understanding the appropriate frequency range is crucial for effective AE testing in concrete. Engineers ensure that AE testing processes utilize the optimal frequency range to detect and analyze damage within concrete structures accurately. The frequency ranges for acoustic emission testing of concrete structures vary, but most damage mechanisms occur in the 30 – 100 kHz range [2-3].

Acoustic emission is unique in that it can be applied to long term monitoring applications – or structural health monitoring applications. By instrumenting a concrete bridge, parking deck or hydraulic dam structure, AE is used to continuously monitor for damage in real-time providing asset owners with an important diagnostic tool to track active damage. This real-time capability is particularly valuable for critical structures such as bridges and dams, where early detection of damage can prevent catastrophic failures.

AE Testing in Concrete

Acoustic emission testing of concrete structures is incredibly versatile and can be applied in a variety of scenarios. For instance, it is a powerful tool for crack detection and monitoring. Cracks are one of the most common and potentially serious forms of damage in concrete structures. AE testing can identify the onset and growth of cracks, enabling timely maintenance and repair to prevent further deterioration.

In addition to crack detection, AE testing is invaluable for structural health monitoring. Continuous or periodic monitoring of critical structures—such as concrete bridges, concrete dams, concrete parking decks and concrete high-rise buildings—enables early damage detection and informed maintenance planning. By tracking AE activity over time, engineers can assess the effectiveness of repair measures and ensure the long-term safety and integrity of the structure.

Another important application of AE testing is corrosion detection. Corrosion of reinforcing concrete steel is a major concern in concrete structures, as it can lead to significant weakening and potential failure. Acoustic emission testing of concrete structures can detect the acoustic signals generated by the corrosion process, providing early warnings of potential structural compromise. This allows for timely intervention and remediation to prevent further damage.

In post-tensioned concrete structures, AE testing can detect issues such as wire breaks or debonding. These issues can compromise the integrity of the prestressing tendons, leading to structural instability. By monitoring AE activity, engineers can identify and address these problems before they become critical.

Navigating the Challenges of Acoustic Emission Testing on Concrete

Despite its numerous advantages, AE testing does present some challenges for economic long-term integration in concrete structures. Acoustic emission (AE) attenuates more in concrete than in steel due to several key factors related to the material properties of concrete and steel. While steel is a homogeneous, dense, and elastic material with a high modulus of elasticity, concrete is a heterogeneous material composed of cement, aggregates, and voids. Its lower density and complex structure lead to greater scattering and absorption of acoustic waves, resulting in higher attenuation [4-5]. Figure 1-4 shows an example attenuation profile of an acoustic emission source in a concrete parking deck spandrel. The attenuation was calculated at 4.2 dB/ft. The acoustic emission attenuation profile will be unique for each concrete structure and must be established prior to final sensor instrumentation.

Figure 1-4: Acoustic emission attenuation in concrete to ensure correct sensor placement is imperative to the inspection process.

Additionally, distinguishing AE signals from background noise and other vibrations requires advanced signal processing techniques. Background noise from environmental sources, machinery, and other activities can interfere with the detection of AE signals. Sophisticated algorithms and filtering techniques are needed to separate the relevant signals from the noise.

Interpreting AE data also demands an elevated level of expertise. Numerous factors, such as material properties, sensor placement, and environmental conditions, can influence the signals. Engineers have the deep understanding needed to accurately interpret the data and make informed decisions.

Ensuring accurate acoustic emission testing of concrete structures involves careful calibration of sensors and validation of results with other NDT methods or destructive testing. Calibration ensures that the sensors are correctly tuned to detect AE signals with the desired sensitivity and accuracy. Validation with other testing methods provides additional confidence in the AE results and helps to identify any discrepancies or limitations.

Summary of Acoustic Emission Testing in Concrete

Acoustic Emission testing of concrete bridges, parking decks, and other concrete structures is a useful non-destructive testing methods meaningful change for the continued safe operation of concrete structures. Its ability to provide early detection of damage and real-time monitoring concrete damage progression makes it a valuable tool for engineers for predictive maintenance.

References

1. "Nondestructive Testing of Concrete: Materials and Structures" edited by Ravindra K. Dhir and John G.M. Wood (ISBN: 978-0203087607)

2. Chen, G., & Spencer Jr, B. F. (Eds.). (2007). Acoustic Emission and Related Non-Destructive Evaluation Techniques in the Fracture Mechanics of Concrete: Fundamentals and Applications. Springer Science & Business Media.
   

3. Meo M (2014) Acoustic emission sensors for assessing and monitoring civil infrastructures. In: Sensor technologies for civil infrastructures, vol 1. Woodhead Publishing, pp 159–178
   

4. Xu, S., Zhang, X.: Determination of fracture parameters for crack propagation in concrete using an energy approach. Eng. Fract. Mech. 75, 4292–4308 (2008)
   

5. D.G. Aggelis Classification of cracking mode in concrete by acoustic emission parameters