Non-destructive Concrete Tests (NDT) for Structure Strength

Non-destructive concrete tests are performed without causing any destruction to the concrete component of the existing structure to determine the strength. Non-destructive concrete tests are also known as concrete NDT. Nowadays, non-destructive concrete tests have become popular to assess the strength of concrete, before you carry out repairs work of the structures.

Non-destructive concrete tests help in the assessment of the in-place strength of RCC Structures in various contexts:

  • For quality control of recently built structures or under construction structures.
  • For checking the performance of an existing structure.
  • When a structure needs retrofitting due to ageing and or change of use.
  • To assess the residual strength of structure when it has lost certain strength due to aging, wear and tear, corrosion. It may also lose certain strength due to natural disasters or man-made calamities like earthquake, fire, cyclone, etc.

Commonly Used Non-destructive Concrete Tests:

01. Rebound Hammer Test:

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Rebound hammer test is one of the non-destructive concrete tests for the evaluation of the structure strength. Rebound hammer is known as Schmidt’s Hammer. It is also known as Swiss Hammer because it is invented by Ernst Schmidt, a Swiss Engineer.
Rebound hammer test is conducted to assess the relative compressive strength of concrete. It is based on the hardness at or near its exposed surface.
IS 13311 (Part 2): 1992, BS 6089-81and BS 1881-202 explains the standard procedure for test. It also clarify the correlation between concrete cube crushing strength and rebound number.

What is Rebound Number in Rebound Hammer Test?

In rebound hammer test, the spring controlled plunger of rebound hammer is pressed on the surface of concrete and the surface hardness is measured on graduated scale. This measured value is designated as Rebound Number. It is also known as Rebound Index. Concrete with low compressive strength and low stiffness has a lower rebound Number or Rebound Index.

02. Ultrasonic Pulse Velocity Test or UPV Test:

Ultrasonic pulse velocity test is also known as UPV test. Ultrasonic pulse velocity tests are recognised methods of non-destructive concrete tests. It is used to determine the strength by evaluating the homogeneity and integrity of the concrete. it is performed with the help of ultrasonic pulse velocity tester.

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Use of UPV Test  of Concrete:

  • Qualitative assessment of the strength of concrete. Its gradation in different locations of structural members and plotting the same.
  • Determine any discontinuity in cross section like cracks, concrete cover, delamination, etc
  • The depth of surface cracks in house.

This test consists of measuring the travel time of an ultrasonic pulse of 50-54 kHz. The pulse is produced by an electro-acoustical transducer system. The higher pulse velocity indicates higher elastic modulus, density and integrity of the concrete.

03. Penetration Resistance Test:

Penetration resistance test is one of the non-destructive concrete tests. It is used for determining the relative strength of concrete in the same or the different structure. Because of the nature of the equipment, it should not be expected to yield absolute values of strength.
Windsor Probe is famous penetration resistance measurement equipment.

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The penetration resistance test for compressive strength depends on the type of aggregates (soft or hard). It is also depends on the size of aggregates and nature of equipment.
It is observed that approximately 20% accuracy may be possible in strength determination of concrete.

04. Pullout Test and Pull-off Test:

The pull out testing is one of the non-destructive concrete tests and it can be done with the LOK test and CAPO test. The test equipment designed to a specific geometry will produce pull out forces. The pull out forces are correlated to the compressive strength of concrete. This Correlation is achieved by measuring the force required to pull a steel probe of the disc.

Use of Pull Out Test of Concrete:

  • Measure the compressive strength of in-situ concrete.
  • Determine the strength of concrete for carrying out post- tensioning operations.
  • Measure the time for removal of forms and shores based on the actual strength of the in-situ concrete.
  • To terminate curing based on the strength of in-situ concrete strength.

The Pull-off test is one of the non-destructive concrete tests to evaluate the existing structure strength. In Pull-off Test, the traction force (pull-off force) required to pull off a metallic disc bonded to a surface of the concrete is related to the compressive strength of the material.

05. Concrete Core Testing:

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Cores are usually cut by means of the rotary cutting tool with diamond bits. A cylindrical core specimen is obtained with its ends being uneven, parallel, and square and embedded pieces of reinforcement with concrete core cutting machine.
It is then tested in compression testing machine in a moist condition as per BS 1881: Part 4: 1970 or ASTM 42-77.

Use of Concrete Core Sample:

  • Strength and density determination.
  • Depth of carbonation of concrete.
  • Chemical analysis.
  • Water or Gas permeability.
  • Petrographic analysis.
  • ASHTO chloride permeability test.

The rational use of the various available Non-destructive Concrete Tests, as well as a possible combination of more than one test, can be particularly interesting from the viewpoint of the validity of results. The core test is more reliable, but during repairs or rehabilitation of existing structures, it is not always possible to do numbers of core tests. Again, it is costly also.
Hence the experienced engineers first carry out Rebound hammer test, which is suggestive of comparatively stronger and weaker areas. The doubtful areas than can be tested with Ultrasonic test. The core test would become a must where even the Ultrasonic tests suggest poor concrete. Thus the engineer has to use his common sense and judgment for carrying various tests in combination and make the judgement before determining any strengthening proposals.

Must Read:
Signs of Foundation Problems in Your House
Know the Causes of your Foundation Cracks
How to Repair Concrete Cracks in RCC Structure or House?

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