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Non-destructive testing (NDT) methods can evaluate concrete structures to determine their strength and durability, without affecting the concrete to the point that it fails. As part of conventional quality control processes for concrete and masonry construction, NDT methods can investigate the severity of cracks, the existence of microcracks and overall damage to concrete. These tests are based on the fact that observable physical qualities of a concrete sample can be associated with its strength and can be assessed by NDT processes. Such qualities include solidity, the ability to resist penetration, density, and composition.
Most NDT procedures are quite straightforward, but they do call for skilled technicians with specialized knowledge that allows them to understand and evaluate test outcomes.
The predominant disadvantages with NDT methods include the inability to get immediate results, specimens or sample areas that differ significantly from the whole structure and the fact that many strength qualities of concrete or masonry structures depend on that scale and shape.
The following tests are standard NDT methods in use for evaluating concrete and masonry materials.
Penetration Method
Penetrations tests are used to assess the compressive strength of a concrete sample. Typically thought to be the best device for penetration tests, the Windsor probe is a powder-actuated gun with strong alloy probes and a gauge for gauging penetration depth.
A penetration test involves a probe about 6.5 mm across and 8 cm in length being fired into the concrete test sample with a powder charge. The probe's depth of penetration gives an indication of the structure's compressive strength.
The test is considered non-destructive because structures can be tested in situ, creating holes on exposed faces that can be fixed with minor patching.
Rebound Hammer Method
Using a quantifiable scientific relationship between strength and the rebound caused by a concrete surface, the rebound hammer technique is used to assess exterior hardness.
Weighing about 1.8 kg, the Schmidt hammer is the standard tool for this test. It is made up of spring-controlled mass that glides on a plunger inside a tubular enclosure.
In the test, the hammer mass is sent into the concrete surface via spring action and the length of rebound is assessed based on an established scale. The surface to be tested can be at any angle. However, the instrument has to be calibrated at the proper angle before the test can commence.
The Schmidt hammer offers a cost-effective, straightforward and fast technique for determining concrete strength, but the experimental error of 15%-20% is possible for some specimens.
Pull-Out Methods
Pull-out resistance procedures assess the force necessary to extract a standardized, embedded probe from a concrete structure. Using established scientific relationships, the force necessary to take out the inserts can be used to determine strength qualities of the sample. The two kinds of inserts, cast-in and fixed-in-place, are each related to a different kind of pull-out technique. Cast-in tests involve an insert to be put into fresh concrete, while fixed-in-place tests involve putting an insert into a hole that has been drilled into a structure.
Pull-out resistance procedures are non-destructive yet involve invasive procedures that are typically used to determine compressive strength qualities of a structure.
Dynamic Methods
Using sonic and ultrasonic methods, these types of NDT processes are used to assess resilience and uniformity of a structure and to determine its strength and elastic qualities.
At the moment, the ultrasonic pulse velocity technique is the sole type of test capable of evaluating concrete strength in situ. The test is based on the time it takes for an ultrasonic pulse to travel through the structure.
The pulse velocity technique is useful for establishing if the concrete is uniform. The test can be used on both completely built structures and those still under construction. If large inconsistencies in pulse velocity are discovered inside a structure for no obvious reason, it is generally determined the concrete has deteriorated significantly.
Radioactive methods
Used to measure density, find the location of any reinforcements and establish if honeycombing has taken place, radioactive techniques pass x-rays and gamma radiation through a test structure.
The equipment is relatively easy to operate, and operational costs are minimal, while the initial purchase price can be high. These tests can evaluate concrete up to 45 cm without difficulty.
References and Further Reading
https://civilblog.org/2015/03/27/what-is-non-destructive-testing-of-concrete-various-ndt-test-methods/
https://theconstructor.org/practical-guide/non-destructive-testing-of-concrete/5553/
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