Non-Destructive Testing (NDT) and Non-Destructive Evaluation (NDE) of a concrete structure (bridges, dams, buildings, etc.) are very important components of the life span of an asset.
In this article, we will explore some of the options for equipment needed to give a full and true picture of the structural integrity of your concrete asset.
There are a number of properties of concrete that are required to be tested such as hardness, resistance to penetration by projectiles, rebound capacity, carbonation levels and ability to transmit ultrasonic pulses. These properties can then lead you to identify some of the following major flaws in concrete: Concrete voids, delamination, corrosion of rebar inside concrete (which can cause spalling and cover to break off), cracks, concrete strength.
Concrete testing and NDT is a lot like the Swiss Cheese Model for risk management. Whereby, there is no single test or piece of equipment that is able to provide a full view of the state of your asset. You must use multiple pieces of equipment (or layers of swiss cheese!) to assess multiple properties and give you the best picture to create informed decisions for asset management.
Disclaimer: All of these pieces of equipment should always be used in conjunction with a qualified inspection engineer to interpret the information.
What are the methods and equipment for concrete inspection and testing?
- Penetration method
- Rebound hammer method
- Pull out test method
- Ultrasonic pulse velocity method and Ultrasonic pulse echo technology
- Radar wave technology
- Carbonation testing
- Half-Cell potential testing
- Concrete sounding (also known as hammer test or acoustic test)
So what do these methods do?
- Penetration method
The Windsor Probe is generally considered to be one of the best means of testing penetration depth of concrete. The probe is a powder actuated gun or driver with a hardened probe and a depth gauge.
- Rebound hammer method
A rebound hammer (or a Schmidt Hammer) is a surface hardness tester consisting of a spring controlled hammer which is forced against the surface and the rebound distance is then measured on a scale.
- Pull out test method
The pull out tests involved a specially shaped steel rod which has been pre-cast into the concrete, which then uses a special ram to measure the force required to pull the rod from the concrete. This method can of course cause damage, but typically will be done to reach a threshold of minimum force before damage is caused
- Ultrasonic pulse velocity method and Ultrasonic pulse echo technology
This technology either measures the speed of an ultrasonic pulse through concrete, using a probe on either side of the test specimen to send and receive the signal (in the pulse velocity method) or uses an echo scanner probe which is sent in and received on the same side from echo signals. Typically ultrasonic pulses will either fully reflect when a void, crack or spall is detected or at the very least change in speed considerably enough to highlight defects.
More information can be found from manufacturers such as PROCEQ and OLSON
- Radar wave technology
These methods such as GPR can be great first steps to take as they can be effective in identifying many objects and depths within concrete such as rebar.
- Carbonation testing
During carbonation of concrete, the pH level will fall and can cause the beginning of corrosion of the rebar inside the concrete. The loss of alkalinity will destroy the passive protection film on the rebar and begin to corrode. A carbonation test is looking to determine to what depth has the carbonation taken place. Typically a Phenolphthalein solution in alcohol or water will be sprayed onto the concrete. If carbonation is present, the concrete will turn pink.
Usually, the solution is applied immediately to exposed concrete in one of two methods. Firstly, breaking a piece of concrete off and spraying the underlying area immediately. Secondly, drilling a hole into the concrete and spraying. The hold needs to be flushed with deionised water immediately after the drilling.
- Half-Cell potential testing
The half cell potential test is another way of measuring corrosion of steel rebars, and is typically used in conjunction with the carbonation tests. The half cell potential test measures the electrical resistance of the concrete between a point on the surface (where your half cell connection is made) and the underlying rebar.
8. Concrete Sounding
Concrete sounding (also may be known as a hammer test or acoustic test) is a test to discover delamination within concrete. Delamination occurs where the concrete closest to the surface has debonded from the rebar within. Sounding involves striking or impacting the surface with a hammer, or sometimes a chain, and listening for a chance in sound. Good quality concrete will have a ringing and resonating sound as it moves through the concrete. Where delamination is present, the sound will be dull or ‘thud’ like, as the sound wave is unable to travel through the concrete and is stopped by the air gap where the delamination or debonding has occurred.
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