13 Non-destructive testing of concrete

Non-destructive concrete testing (NDT) is more common in the construction industry due to the need to check several parameters of hardened concrete.

Depending on the type of test, different devices must be used according to their respective specifications.

The different occasions on which these tests are carried out can be identified below.

1. Quality control in prefabricated and on-site concrete construction
2. As a measure against uncertainty regarding acceptance of materials due to non-compliance with specification
3. As confirmation of any doubts regarding the quality of the craftsmanship in dosing, mixing, placing, compacting or curing the concrete
4. monitor strength development; early demolding, early application of loads
5. Determination of the extent of cracks, cavities, honeycombs
6. Determination of the position and quantity of reinforcement
7. Determining the appropriate location for destructive testing
8. Assessing the Potential Durability of Concrete
9. Monitoring long-term changes in concrete

As mentioned above, non-destructive testing is very useful in the construction industry due to its numerous advantages.

Furthermore, these tests can be carried out very cheaply compared to destructive tests.

Non-destructive testing

1. Visual inspection : A visual inspection can detect many things and this is the first test to be done before any type of testing when access is available.
2. Half-cell electronic potential method : Recognize the corrosion potential of reinforcing bars in concrete
3. Rebound hammer test : to determine the surface hardness of concrete
4. Test to measure depth of carbonation : is used to determine whether moisture has penetrated to the depth of reinforcing bars and therefore corrosion may occur
5. Permeability test : Measure the ability of water to flow through concrete.
6. Penetration resistance or Windsor probe test : Measure the surface hardness of the concrete to evaluate the strength of the concrete
7. Checking the coverage gauge : Reinforcement coverage and bar diameter can be measured
8. X-ray examinations : used to detect cavities in the concrete and the position of stress channels
9. Ultrasonic Pulse Velocity Test : It is used to determine the compressive strength of concrete by measuring the speed of sound.
10. Tomographic modeling : uses data from ultrasonic transmission tests in two or more directions to detect voids in concrete
11. Effects of eco-test : used to detect voids, delamination and other anomalies in concrete
12. Ground Radar or Pulse Radar Test : Used to detect the position of reinforcing bars or tension channels.
13. Infrared thermography : Used to detect voids, determinations and other anomalies in concrete and also detect water entry points in buildings

VISUAL TESTING (most reliable concrete non-destructive testing)

An assessment can be made based on the condition of the structure and known failure criteria.

For example, a visual inspection can determine whether the crack pattern in the beam is a shear crack, a flexural crack, or a torsional crack.

Judgments can also be made based on the inspection results.

Rebound Hammer Test

Rebound hammer testing is one of the most commonly used non-destructive testing methods for concrete and is used to check the compressive strength of concrete.

Based on the cam number determined for a concrete surface, the compressive strength can be derived from the empirical relationship developed for the cam hammer.

Advantages of the rebound hammer test

• Easy to deal with. No special knowledge is required to take the test
• Establishes uniformity of properties
• Equipment is cheap and easily available
• A large selection of concrete testing hammers is available with an application range from M10 to M70

Disadvantages of the rebound hammer test

• Evaluate only the local point and the plane to which it is applied
• No direct connection to strength or deformation properties
• Unreliable for detecting currents
• Cleaning and maintenance of the probe and spring mechanism

Testing carbonation depth as a non-destructive test of concrete

Carbonation of concrete occurs when carbon dioxide in the atmosphere reacts with hydrated cement mineral in the presence of moisture and produces carbonates, e.g. B. calcium carbonate.

The carbonation process is also called depassivation.

Carbonation penetrates extremely slowly beneath the surface of exposed concrete.

The time required for carbonation can be estimated using the following equation and knowledge of the quality of the concrete.
t = (d/k) ²

Where,

t – time for carbonation

d – concrete cover

k – permeability

Typical values ​​for concrete permeability can be found in the following table.

The importance of carbonation is that the protection of the reinforcing steel normally present in concrete is neutralized by the alkaline conditions of the cement stone through carbonation.

Therefore, if the entire concrete covering over the reinforcing steel was carbonated, the steel would corrode if moisture and oxygen were allowed to reach the steel.

The depth of carbonation can be easily measured using the phenolphthalein solution used.

Spray the newly exposed concrete surface with a 1% phenolphthalein solution.

Calcium hydroxide is pink, while the carbonated portion is colorless. There is another formula that can be used to determine carbonated depth. This equation can be used to determine the age of the concrete or building.

y = 7.2C ² / (R ² (4.6x-1.76) ² )

Where,

y – age of the building in years

x – water-cement ratio

C – carbonation depth

R – constant (R=αβ)

“R” varies depending on the surface coating of the concrete (β) and whether the concrete was used outdoors or indoors (α).

This formula is contained in the Japanese Ministry of Construction publication “Engineering to Improve the Durability of Reinforced Concrete Structures”.

α is 1.7 for internal concrete and 1.0 for external concrete.

The β values ​​can be found in the following table.

The phenolphthalein test is a simple and economical method to determine the depth of carbonation in concrete and provides information on the risk of corrosion of reinforcement.

The only limitation is that drilling or drilling causes only minor damage to the concrete surface.

Source: Internet and reference books