1.
Destructive and Non-Destructive Testing of Concrete
2.
When Non-Destructive Testing of Concrete Is Necessary
3.
Non-Destructive Testing Methods for Concrete Structures
3.1.
Ultrasonic Testing
3.2.
Radiographic Testing
3.3.
Acoustic Emission Testing
3.4.
Thermal Imaging
3.5.
Penetration Resistance Test
3.6.
Rebound Hammer Test
4.
Testing Concrete Structures with Voliro Inspection Drones
Globally, over 30 billion tonnes of concrete are produced every year. Versatile and durable, concrete is a reliable material for impact-resilient construction. The lifespan of concrete buildings ranges from 50 to 100 years. So many iconic buildings like the Sydney Opera House and Seattle’s Space Needle were made of this material.
In the industrial sector, concrete is also the material of choice for storage tanks, silos, cooling towers, spillways, wastewater treatment basins, and a large variety of buildings. Although long-lasting by design, concrete assets still require regular inspections for signs of deterioration and structural integrity — and that’s what different destructive and non-destructive concrete testing methods accomplish.
Destructive and non-destructive testing of concrete are the two common inspection methods.
A destructive test determines the limits of the test object’s performance or analyzes how and why it fails. Destructive concrete testing methods like pull-out or flexural tests apply force until the asset gets damaged to determine its strength and durability. Given the outcome, destructive testing is mainly for quality control before beginning the construction or during forensic investigations of failed structures.
Non-destructive testing (NDT) techniques allow evaluation of the assets’ quality, integrity, and strength properties without causing permanent damage. Concrete NDT helps detect flaws, discontinuities, and wear within concrete structures. Common techniques for concrete NDT inspection include ultrasound scans, industrial radiography, magnetic particle inspection, and liquid penetrant testing, among others.
So, NDT is a method of examining a test object without rendering it unusable, whereas destructive testing involves permanent damage or complete ruination. Non-destructive testing methods for concrete allow repeated tests and condition monitoring through the assets’ lifecycle. They provide valuable information about the early signs of mechanical stress, cracking, spalling, and delamination — and allow asset owners to take early action to avoid costly replacements and regulatory fines.
All commercial, residential, and industrial concrete assets must comply with structural safety regulations. Industrial assets, exposed to extra loads, must meet special design requirements (e.g., be resistant to fatigue under frequent footfall).
Although these vary across countries and industries, their general premises are unanimous: Owners must ensure the quality and reliability of core structures.
Regulations exist for a good reason. Inadequate maintenance of concrete structures can lead to deadly accidents. The collapse of the Champlain Towers South condominium in Florida, which claimed 98 lives, was caused by major cracks and concrete spalling due to poor building design, according to a NIST investigation. The UK faces a ‘concrete crisis’ as hundreds of public buildings, including schools and hospitals, are at high risk of structural failure due to the use of cheap but not durable Reinforced Autoclaved Aerated Concrete (RAAC).
At industrial sites, concrete assets like silos, storage tanks, kilns, and other concrete-bearing elements can fail due to deterioration, mechanical stress, abrasion, structural overload, moisture intrusion, and thermal cycling. For instance, moist plant materials in silos can produce lactic and acetic acids. Upon contact with concrete silo walls, these acids react with the cement matrix, causing strength to decrease until the vertical load causes the wall to fail.
Several industry standards and regulations are in place to prevent catastrophic failures of concrete structures, such as:
The recommended frequency of concrete NDT varies depending on the asset type, age, exploitation intensity, and safety regulations. At a minimum, all concrete assets need a comprehensive inspection every 3 to 5 years. Annual NDT testing inspections are recommended for aging assets and those with a history of extensive damage and subsequent conditioning.
Standards like ACI 228.2R-13 and BS 1881-206:1986 suggest a roster of recommended non-destructive testing methods for concrete structures.
Tests must be done by a certified inspector using specialized non-destructive concrete testing equipment. Based on the asset type and its operating requirements, they would recommend the optimal method.
Different inspection methods help discover various defects: visible dents, subsurface cracking, scaling, delamination, and ongoing stress caused by excessive loads. This data can be obtained with ultrasonic, stress wave, nuclear, thermal, magnetic, and electrical readings during the test procedure.
Ultrasonic testing uncovers damage and defects by analyzing reflected high-frequency sound waves. Inspectors can detect the location, size, and severity of debonding, delamination, or tracking by analyzing ultrasonic pulses trajectory and timing.
UT also helps measure the thickness of concrete structures to ensure they meet construction standards. For assets in use, thickness loss can occur due to abrasion and wear. Frequent exposure to freeze-thaw cycles can cause thickness loss due to water penetration into the concrete pores and expansion upon freezing. Fungi and mold can colonize surfaces, degrading protective coatings and causing subsequent asset damage.
Even though UT can deliver high-fidelity data, asset owners often hesitate to run frequent tests as they require contact access to the surface. For large industrial assets, this means commissioning a cherry-picker or building scaffolding. Unless you opt to use an industrial inspection drone.
At Voliro, we’ve developed an omnidirectional drone with mountable payloads for NDT. Its doubled tilt rotors and advanced sensing system allows approaching objects from any angle to perform contact-based work, such as taking wall thickness measurements with an ultrasonic transducer or signs of structural damage with an electromagnetic acoustic transducer (EMAT).
Using Voliro drones, inspectors can collect up to 200 readings per hour and perform 5 to 10 concrete storage tank inspections per day, reaching into the furthest nooks and crannies without lifting a foot from the floor. Because no scaffolding is required, you can effectively collect UT information from assets in use with minimal operational downtime.
Industrial radiography provides an X-ray snapshot of the concrete structure’s condition. Gamma rays can penetrate several feet of material, offering a full view of its density and composition. Defects and irregularities appear as dark spots in the scans, indicating position and size.
Radiographic testing is often used to identify voids, honeycombing, delamination, and irregularities in concrete density, which can weaken the structure and increase the risk of cracking.
This is also a good method for detecting reinforcement bars in concrete structures to ensure compliance with design specs and proper structural integrity. The International Atomic Energy Agency, for example, endorses RT as a good method for ensuring stringent quality control standards for the safe operation of nuclear industrial installations.
On the downside, radiography testing is hard to do in the field because it needs portable scanners, extra safety measures, and protective shielding, rendering the test costs rather high.
Acoustic emission testing detects mechanical vibrations in tested assets, indicative of potential stress. Unlike other NDT methods, AE not only detects defects but also estimates their severity and propagation under different operating conditions (e.g., variable loads, pressure, or abrasion).
AE is a reliable method for measuring the durability of concrete structures under load or in service. Microcracking in concrete detects acoustic emission waves, helping inspectors determine the structure’s load-bearing capacity and flag areas for reinforcement. AE tests, performed before and after repairs, also confirm whether strengthening measures have indeed fixed the problem.
Arguably, the biggest advantage of concrete AE testing is that it delivers reliable results, regardless of cement types, admixtures, hardening conditions, temperature, or the presence of reinforcement, as a recent study found.
Yet, AE has a limited detection range of several meters, so inspectors must have unobstructed access to the surface. Likewise, in large concrete structures, acoustic signals may attenuate before reaching the sensors, limiting data collection.
Industrial thermography tools capture the assets’ thermal signature, showing localized temperature variations on the surface. Heated areas can indicate high mechanical stress or surface damage due to chemical reactions. Low-temperature areas usually signify moisture intrusion, indicating freeze-thaw damage or efflorescence.
Voids, cracks, and delamination can also cause surface temperature variations in concrete due to emerging differences in thermal conductivity and distribution. Thermal cameras capture variations in concrete uniformity (e.g., mix design or curing process), which may affect structural integrity.
A group of South Korean scientists proposed a method for forecasting concrete strength using thermography and deep learning image classification algorithms. The model was trained on historical thermal imaging from construction sites and can predict concrete strength in similar structures with 80% accuracy.
Thermal imaging, however, can only detect apparent surface-level defects, so it’s often combined with ultrasonic or penetration resistance testing to improve assessment accuracy.
Penetration resistance testing measures concrete structure compressive strength, especially for hardened concrete, to determine uniformity and identify poor quality or deterioration.
The test requires specialized non-destructive concrete testing equipment specified by standards, such as ASTM C803 — a steel probe with a conical tip and a powder-actuated driver. The probe is inserted into the concrete structure, and its penetration depth is measured using a gauge. The depth indicates compressive strength, which can vary depending on the concrete’s age, mix design, and curing conditions, among other factors.
Overall, it’s a quick method for measuring how much axial loading, wind loads, or thermal expansion the structure can tolerate. But the results may not always be conclusive, especially in highly porous concrete mixes or reinforced structures.
Rebound hammer test is another popular method to assess the compressive strength and hardness of concrete structures. In this case, however, inspectors measure the rebound velocity of a spring-loaded hammer after striking the test surface. The higher the rebound number (R-number) — the greater the hardness.
The rebound hammer test assesses concrete uniformity, detects deterioration, and helps compare concrete mix strengths. The measurements are useful for quality control and condition monitoring.
Yet, this NDT concrete testing method has several major limitations. Smooth surfaces produce a higher R-number, and rough ones a lower one since the hammer may crush the surface paste. High surface carbonation can also increase the R-number by 50%. Wetting the surface for 24 hours before the test is recommended due to the higher R-number produced by dry surfaces.
Voliro streamlines non-destructive testing of concrete structures with a purpose-built inspection drone. Thanks to omnidirectional mobility and AI-powered autonomy, Voliro T can approach any concrete structure and perform NDT tests with high stability.
With mountable payloads for ultrasonic testing, EMTA, and eddy current testing included in the subscription, Voliro T can fly different inspection missions. Pilot training, all-risk insurance, regular hardware replacement, and access to all new software updates (for navigation and data processing) are also included in the subscription. Collect up to 3x more measurement points in 4X less time with Voliro!
