Concrete tests serve a variety of purposes related to the strength, durability, and estimated viability of the structure over time. These test results can be used to determine whether repairs need to be made to a structure or whether the integrity of the structure is sufficient as it is.
The standard method for assessing the quality of concrete in buildings or structures is to test cast samples for compressive, flexural, and tensile strength at the same time.
The main disadvantages are that the results are not obtained immediately; that the concrete of the test specimen may differ from that of the actual structure due to different hardening and compaction conditions; and that the strength properties of a concrete sample depend on its size and shape.
Although the strength properties of structural concrete cannot be measured directly for the simple reason that the strength determination involves destructive stresses, several non-destructive assessment methods have been developed.
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Popular NDT Tests for Concrete Used in the field are:
- Rebound Hammer Test- RH Test
- Ultrasonic Pulse Velocity- UPV Test
- Combined Method UPV & RH Test
- Core Extraction for Compressive Strength Test
- Penetration method
- Pull out test method
- Radioactive methods
1. Rebound Hammer Testing
This test measures the surface hardness of a concrete surface and requires a tool called a Schmidt hammer.
In 1948, the Swiss engineer Ernst Schmidt from Zurich developed a test hammer to measure the hardness of concrete according to the rebound principle. Since then, the Rebound Hammer (RH) test has established itself on construction sites and in the precast industry.
The hammer has a spring-loaded hammerhead that slides over a piston device. During the test, the hammerhead is pressed into the concrete via the spring mechanism and bounces off when it comes into contact with the concrete surface. This rebound is measured to determine the hardness of the concrete surface. Again, this method is run multiple times to determine and average what will be used to indicate the surface hardness of the area of concrete being tested.
2. Ultrasonic Pulse Velocity-UPV Test
The testing device consists of a means for generating and introducing a wave pulse into the concrete and a means for detecting the arrival of the pulse and for accurately measuring the time it takes for the pulse to traverse the concrete.
Such tests can determine if there are irregularities in the concrete (e.g., a section of concrete is “TRUE” and strong or if repairs are imminent. Note that temperatures are a factor and this should be taken into account when making such a measurement for accurate results to obtain.
3. Combined Method UPV & RH Test
Combined non-destructive techniques refer to techniques in which one test is used to improve the reliability of in-situ concrete strength, which is estimated by another test alone.
The validity of a combined method can be assessed by the degree of improvement that this additional test offers in terms of the accuracy and reproducibility of the predictions, compared to the additional cost and complexity of the combined method and the extent to which it is feasible to perform the additional test will be on site.
Hardness scales are arbitrarily defined measures for the resistance of a material to indentations under static or dynamic loads or to scratch, abrasion, wear, cut or drilling resistance. Concrete test hammers evaluate surface hardness, i.e. H. the hammer’s ability to rebound or bounce back.
The interpretation of the pulse velocity measurements in concrete is made difficult by the heterogeneity of this material. The wave speed is not determined directly but calculated from the time it takes a pulse to cover a measured distance. A piezoelectric transducer, which emits vibrations at its fundamental frequency, is brought into contact with the concrete surface so that the vibrations travel through the concrete and are received by another transducer that is in contact with the opposite side of the test object.
4. Core Extraction for Compressive Strength Test
A core taken up perpendicular to a horizontal surface should, if possible, be positioned with its axis perpendicular to the concrete bed in the original position.
Samples stored in water should be tested immediately after removal from the water and while still wet. Surface water and chips should be wiped off the specimens and any protruding ribs removed. Samples obtained dry should be kept in water for 24 hours prior to collection for testing.
The measured compressive strength of the test specimen is to be calculated by dividing the maximum load exerted on the test specimen during the test by the cross-section calculated from the average cross-sectional dimensions and expressing it in the nearest kg / cm2. The average of three values should be considered representative of the lot, provided that the individual variation does not exceed ± 15% of the average. Otherwise, repeated tests must be carried out.
5. Penetration method
The Windsor probe is widely considered to be the best method of testing for penetration. The equipment consists of a powder gun or pilot, hardened alloy probes, loaded cartridges, a depth gauge for measuring probe penetration, and other related equipment.
Although technically not a destructive test method, the penetration test involves injecting a small probe with a charged load directly into the concrete. Once the probe is embedded in the concrete, the depth reached by the probe is measured and this measurement is used to determine the compressive strength of the structure. Care must be taken to ensure that the tool itself is properly calibrated, and multiple probes are often drawn to get a solid average of depth for a final measurement of compressive strength.
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6. Pull out test method
The pull-out test can be referred to as a “semi-destructive test” because it is a steel jack inserted into the concrete from a distance of 3 inches. Once inside, the rod is removed with a winch device and the relative compressive strength of the concrete structure to be tested is measured. Such debris results in the need for minor structural repairs to the test site.
With the appropriate correlation, the pull-out technique can thus quantitatively measure the stability of concrete. It has been found over a wide strength range that the pull-out strengths have a coefficient of variation that is comparable to that of the compressive strength.
7. Radioactive methods
With radioactive concrete test methods, the position of the reinforcement can be determined, the density measured, and possibly determined whether a honeycomb has occurred in concrete components.
The equipment is expensive to start with, but the cost of testing each test is quite small in comparison. These tests can be used to determine the internal location of the rebar, the overall density of the concrete, and whether the honeycomb has occurred. (Honeycomb is a process in which concrete begins to splinter or crumble due to improper sorting of material or poor mixing prior to pouring.) In addition, gamma radiography is accepted in England and Europe.
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References-
1. https://www.magnumndt.com/5-non-destructive-testing-methods-for-concrete-blog
2. https://theconstructor.org/practical-guide/non-destructive-testing-of-concrete/5553/
3.https://www.nbmcw.com/product-technology/construction-chemicals-waterproofing/concrete-admixtures/popular-non-destructive-testing-of-concrete-structure.html