Inspection and Non-destructive Testing of Concrete Tanks at regular intervals will ensure the safety and reliability of these assets. Application of NDT methods for inspection and monitoring will help detect and quantify potential defects and deterioration mechanisms at early stage. Test results will help the owners in prioritizing repair and maintenance needs, and prevent excessive damages which can have great financial and environmental impacts. In this article, we will briefly review existing challenges in the inspection and monitoring of concrete tanks, and NDT methods than can be used to address these challenges.
Deterioration of concrete can occur over time. Environmental conditions such as weather (extreme hot/cold) can cause degradation. The contents stored in the tanks, on the other hand, can lead to deterioration of concrete (i.e. corrosion of steel rebar). It is very common to find cracks on concrete surface. While most cracks may not pose great risk to the integrity of the concrete tank, others can create shortcuts for aggressive agents to get inside concrete, and kick start the deterioration. These cracks can make a leakage path on the concrete tank.
Inspection techniques and equipment should therefore be able to identify degradation and potential total failure, as well as the more likely corrosion that leads to leakage. The main advantage of NDT methods is that these methods are minimally intrusive, and testing can be done with minimum intervention. NDT tests are repeatable, which make them ideal for regular inspection and monitoring.
Challenges in Concrete Tanks
Concrete tanks are used for water storage, wastewater treatment centres, and storage of chemical agents. The internal surface (exposed to chemicals or water) is often covered with epoxy coatings to prevent leakage of materials. However, cracks (either structural or durability related) could impact the integrity of these structures. Excessive cracks, or through the thickness cracks can open up direct path for chemicals to leak out of the storage tank. Poor quality patches of concrete can negatively impact the safety and reliability of the tank structure. Internal chemicals can also contribute to the deterioration of these facilities (i.e. corrosion).
Why Non-Destructive Testing of Concrete Tanks?
Non-destructive testing of concrete tanks can help identify potential surface and sub-surface defects in concrete tanks. NDT methods help engineers obtain a realistic picture of existing condition of the concrete tank, and design/plan accordingly. NDT methods help
1- identify potential location of defects,
2- study the state of certain damages over the life-cycle,
3- provide information about durability properties of concrete (permeability, surface electrical resistivity),
4- shortening the inspection timeline, to ensure maximum productivity.
Different NDT methods can be used to obtain critical information about the existing condition of concrete tanks. The following section describes some of these methods and their applications.
Non-Destructive Testing of Concrete Tanks
1- Ground Penetrating Radar
Ground penetrating radar (GPR) is a very useful technique for nondestructive evaluation of concrete. GPR uses pulsed electromagnetic radiation to scan concrete. GPR consists of a transmitter antenna and a receiver antenna, and a signal processing unit. GPR emits electromagnetic pulses (radar pulses) with specific central frequency to scan the subsurface medium. The reflected waves from subsurface layers, and objects are captured by the receiver antenna. The scanning apparatus can be mounted on a truck or a special vehicle and perform the scan at the traffic speed. This will eliminate the need for extended road closures. The practice has been standardized by ASTM D6087, 2008.
The main advantage of GPR method is the speed of test. Large areas can be scanned in a limited period. GPR can be used for scanning the location of steel reinforcement. The device can be used over existing coatings. While GPR can not directly detect delaminated areas on the concrete, it can indirectly identify potential delamination where moisture content varies. It is noted that GPR can not provide information about the mechanical properties of concrete.
2- Cover Meter
When accurate as-built drawings are not available, cover meter can be used to determine the effective cover thickness of the reinforcement. If the remaining service life is a concern for the owner, the cover thickness can be used as a key input in mathematical models used for service life prediction.
3- Rebound Hammer
Rebound Hammer can be used to evaluate the quality of concrete near the surface. While test results do not directly correlate to strength of concrete, a site-specific calibration on concrete cores can be used to predict the concrete strength on-site. Rebound hammer is easy and fast to perform in the field. Test results can be used for comparative assessment of concrete quality at different locations.
4- Ultrasonic Tasting
The Ultrasonic Pulse-Echo (UPE) method is used for thickness measurements, flaw detection, detecting delamination, and evaluation the integrity of concrete. The concept behind this method relies on the propagation of stress waves through materials. A transmitter introduces a stress pulse into the object at an accessible surface. The pulse propagates into the test object and is reflected by flaws or interfaces. The emitted impulse and the reflected acoustic waves are monitored at the receiving transducer. The signals are analyzed in the time domain, to calculate the wave travel time. UPE can be used to effectively identify voids and poor quality patches within concrete tank wall.
5- Surface Electrical Resistivity
When durability of concrete is a concern to the owner or the engineer, surface electrical resistivity of concrete can provide useful information about the resistance of concrete material to penetration of chloride ion. This method is adapted by several Department of Transportation (DOTs) to replace the labour-intensive and time-consuming Rapid Chloride Permeability Test (RCPT). Test results are in good agreement with the RCPT method, and can be used to study the permeability properties of concrete. However, engineers should consider that the variation in moisture content and presence of steel rebar can affect the test results.
Read More: Electrical Resistivity of Concrete
6- Half-Cell Corrosion Mapping
Corrosion mapping is a widely used test procedure to identify the areas with active corrosion activity. The test can reveal the locations with high likelihood of corrosion; it can also be used to evaluate the quality of repair. Engineers should consider that any coating or paint residue should be removed before conducting the test.
Read More: Half-Cell Corrosion Mapping