Non-Destructive Testing: The Industry’s Top 4 NDTs
Non-destructive testing (NDT) is a technique of testing concrete without damaging overall structure. In comparison to destructive testing methods, NDT is more convenient, cost-effective, and less time consuming.
Below we highlight the most common non-destructive testing methods for measuring the compressive strength of concrete.
1. Penetration Tests
The Windsor Probe is a well-known example of a popular non-destructive testing method called penetration tests. This test consists of a hardened steel alloy probe that is propelled at high speed by an explosive charge into the concrete. The compressive strength of the concrete is directly related to the resistance to penetration. This relationship is recognised by determining the Moh’s scale of hardness of the aggregate, and applying a correction factor to the penetration. Although requiring penetration into the outer layer of concrete, only minor patching of holes on exposed faces is necessary, so this method is still considered non-destructive.
The Windsor Probe test is outlined in a number of international standards, including ASTM C803, BS1881.207, and more.
2. Rebound Hammer Tests
Also known as a Schmidt hammer, rebound hammer tests consist of a device which measures concrete strength based on a spring-loaded mass impacting against the concrete surface. The hammer hits the concrete at a defined energy and returns a rebound value. This value can then be converted into compressive strength.
This test method is outlined in ASTM C805.
3. Ultrasonic Pulse Velocity Testing
The ultrasonic pulse velocity test assesses the strength of concrete by measuring the velocity of an ultrasonic pulse passing through it. The device measures the time taken by the pulse to get through the concrete. Higher velocities indicate a better quality, while slower velocities may indicate concrete that is not uniform, consists of cracks, etc. This velocity can also be linked to the compressive strength of the concrete.
This method of testing is outlined in ASTM C597–09.
4. Maturity Testing
Maturity testing is able to calculate the compressive strength of the concrete based on the in-situ temperature. The characteristics required of the apparatus used to measure the temperature of the concrete are outlined in the standard below. For instance, the temperature measured must be accurate within +/- 1 degree celsius, the apparatus must measure the temperature at least once every 30 min, etc.
There are a number of mathematical equations which can translate the in-situ temperature to compressive strength, some of which are more prevalent in different parts of the world. For instance, the Weaver-Sadgrove function is most common in the UK, whereas Nurse-Saul is more prevalent in the USA.
This method of testing is outlined in a number of standards, including ASTM C1074.
While each method has it’s own unique benefits, construction companies are quickly realising that the benefits of NDT methods in general greatly outweigh destructive methods, and are able to give them clear advantages over competitors.
For more, read our post on the limitations of cube crushing.