🏗️ Rebound number of hardened concrete — the sclerometric test with a Schmidt hammer

Construction PN-EN 12504-2

Overview

The standard describes a method of determining the rebound number of an area of hardened concrete by means of a spring-driven hammer, commonly called a Schmidt hammer or a sclerometer. It is a non-destructive test carried out on the structure, without taking and destroying specimens.

The standard has to be read with its own provisos, because they are crucial for the correct use of the result. A rebound number determined by this method serves to assess the uniformity of the concrete in a structure and to identify zones and areas of poor-quality or damaged concrete. The method is NOT intended as a substitute for the determination of the compressive strength of concrete according to PN-EN 12390-3 — with a suitable correlation it may, however, give an estimate of the compressive strength in the structure; the assessment of the in-situ strength is described separately by PN-EN 13791. The hammer may also be used for comparative tests, relating the result to concrete of known strength or to concrete from a defined volume checked as conforming to a given strength class.

The standard contains no definitions of terms. It does, however, refer to PN-EN ISO 6508-1 — the Rockwell hardness measurement serves to check the hardness of the reference anvil used to verify the hammer.

In Polish practice the test is a standing item in the lists of the GDDKiA road laboratories. The list of the Bydgoszcz Division names it under the item „Badanie nieniszczące, Badanie liczby odbicia" [Non-destructive testing, Test of the rebound number] with a reference to PN-EN 12504-2:2021-12, and the list of the Lublin Division under the name „Badanie betonu w konstrukcjach. Badania nieniszczące. Oznaczanie liczby odbicia" [Testing concrete in structures. Non-destructive testing. Determination of the rebound number], permitting alongside the European standard the old national standard PN-B-06262:1974. Note: the Lublin list cites at the same time the older European edition PN-EN 12504-2:2013-03 and not the 2021-12 edition given in the heading of this entry.

Method principle

A mass set in motion by a spring strikes a plunger held against the surface of the structure or specimen under test. The result of the test is expressed by a number denoting the distance of the rebound of that mass. The number can also be obtained as the difference of the energy or the velocity of the mass before and after the impact — depending on the type of hammer used, the result is denoted by the symbol R (rebound distance) or Q (measurement of energy or velocity).

Applications

• Assessment of the uniformity of concrete in a structure
• Identification of zones and areas of poor-quality or damaged concrete
• Estimation of the strength of concrete in a structure — only with an established correlation, in accordance with PN-EN 13791
• Comparative tests against concrete of known strength
• Diagnosis of bridge structures and of cement concrete pavements

Key parameters

Standard

Standard number

PN-EN 12504-2:2021-12

Title (PL)

Badania betonu w konstrukcjach — Część 2: Badania nieniszczące — Oznaczanie liczby odbicia

Title (EN)

Testing concrete in structures — Part 2: Non-destructive testing — Determination of rebound number

Step-by-step procedure

1. Choosing the test location

   The concrete members tested are to have a thickness of at least 100 mm and be fixed in the structure; smaller members or specimens may be tested provided they are rigidly supported. Avoid areas with honeycombing, scaling, a rough texture and high porosity. In making the choice take into account the strength of the concrete, the kind of surface (formed or unformed), the kind of concrete (normal or lightweight), the state of moisture of the surface, carbonation, the direction of testing and other relevant factors. The test location is to be about 300 mm × 300 mm.

2. Preparing the surface

   Grind surfaces with a pronounced texture, soft surfaces or those with loose mortar with an abrasive stone until they are smooth and free from loose material. Surfaces cast smooth against formwork or trowelled smooth may be tested without grinding. Remove any water present on the surface of the concrete.

3. Checking the hammer before the test

   Use the hammer in accordance with the manufacturer’s instructions. Before a series of tests clean the impact surfaces of the reference anvil and of the plunger, make at least five impacts on the anvil and record the readings from the next five impacts. If those readings do not lie within ±3 of the value given by the manufacturer, clean or adjust the hammer in accordance with the instructions and repeat the check.

4. Working conditions

   The hammer may be used only at a temperature from 0 °C to 50 °C. At the moment of the test the hammer is to meet the requirements of the check in step 3.

   🌡 Temperature: 0–50 °C

5. Making the impacts

   Hold the hammer firmly in a position allowing the plunger to strike perpendicularly to the surface under test. Gradually increase the pressure on the plunger until the impact occurs. After the impact record the rebound number determined from the rebound distance or from the measurement of energy or velocity.

6. Rejecting invalid readings

   Examine each impression left on the surface after an impact; if the impact has crushed a near-surface void or broken through it, reject the reading.

7. Number and distribution of the points

   Make at least nine valid readings in order to obtain a reliable estimate of the rebound number of the test location. Record the position and the orientation of the hammer for each set of readings. Ensure that no two impact points lie closer than 25 mm to each other or closer than 25 mm to an edge. It is recommended to draw a regular grid of lines every 25–50 mm and to take its intersections as the test points.

8. Checking the hammer after the test

   After the tests have been made, take five readings on the steel reference anvil. If they do not lie within ±3 of the value given by the manufacturer, clean or adjust the hammer in accordance with the manufacturer’s instructions and repeat the test.

9. Result

   Take as the rebound number of the test location the median of all the readings, corrected where necessary for the orientation of the hammer in accordance with the manufacturer’s instructions, and give it as a whole number. If more than 20 % of all the readings differ from the median by more than 25 %, reject the whole set of readings.

10. Test report

    In the test report give: a reference to the standard with the year of publication, the identification of the structure or member, the identification of the test locations, the type and identification of the hammer, a description of the preparation of the test locations, data on the concrete and its condition, the date and time of the test, the rebound number for each test location corrected for the orientation of the hammer, any deviations from the method (for example water on the surface or a temperature outside the permissible range) and a statement by the person technically responsible that the test was carried out in accordance with the standard, except for the deviations indicated. Optionally the individual readings and the specification of the hammer are appended.

Required equipment and apparatus

Health and safety (OHS)

• The hammer releases the energy of a spring — hold it firmly with both hands, the plunger directed only at the surface under test, never towards people
• Chips of concrete and of loose material at the impact — protective goggles obligatory for the operator and for those nearby
• Silica dust when grinding with the abrasive stone — respiratory protection, wet grinding or extraction
• Testing on a structure is often carried out at a height and from scaffolding — fall protection, the hammer on a lanyard
• The reference anvil weighs 16 kg — a stable base, care when carrying it
• Damaged or carbonated concrete is sometimes unstable — before the test assess whether the member is not in danger of spalling

Frequently asked questions