Rebound Hammer Test

Rebound hammer test is also known as Schmidt Hammer Test is used for assessing the strength of Concrete. The test is done as per IS: 13311 (Part 2) – 1992in order to analyze the compressive strength of concrete by using rebound hammer. This method is used to study the elastic properties, Uniformity of any concrete. It also helps in determination of variation in strength of similar structures.
Since it is a Non-Destructive Testing Method, it can be applied for finished as well as unfinished concrete structures.

Principle

The rebound of an elastic mass depends on the hardness of the surface against which its mass strikes. When the plunger of the rebound hammer is pressed against the surface of the concrete, the Spring-controlled mass rebounds and the extent of such a rebound depends upon the surface hardness of the concrete. The surface hardness and therefore the rebound is taken to be related to the compressive strength of the concrete. The rebound value is read from a graduated scale and is designated as the rebound number or rebound index. The compressive strength can be read directly from the graph provided on the body of the hammer.

Procedure

Before commencement of a test, the rebound hammer should be tested against the test anvil, to get reliable results, for which the manufacturer of the rebound hammer indicates the range of readings on the anvil suitable for different types of rebound hammer

1. Apply light pressure on the plunger - It will release it from the locked position and allow it to extend to the ready position for the test. 
2. Press the plunger against the surface of the concrete, keeping the instrument perpendicular to the test surface. Apply a gradual increase in pressure until the hammer impacts. (Do not touch the button while depressing the plunger. Press the button after impact, in case it is not convenient to note the rebound reading in that position.) 
3. Take the average of about 5 readings.

Interpretation of Results

The rebound reading on the indicator scale has been calibrated by the manufacturer of the rebound hammer for horizontal impact, that is, on a vertical surface, to indicate the compressive strength. When used in any other position, appropriate correction as given by the manufacturer is to be taken into account.

Table:- Average Rebound number and quality of concrete

Average Rebound Number	Quality of Concrete
>40	Very good hard layer
30 to 40	Good layer
20 to 30	Fair
<20	Poor concrete
0	Delaminated

Table: - Average Rebound number and quality of concrete

 

Using the Humboldt Concrete Rebound Hammer

Always hold the Concrete Rebound Hammer in both hands while in use. Also, when in use, keep the hammer perpendiculars to the surface you are testing with the scale pointing up, making it easier to read.

Figure 1: The Concrete Rebound Hammer

Before the Hammer can be used for testing, the piston must be released out of the hammer into the testing position. Typically during storage and transportation, the Hammer piston will be locked inside the Rebound Hammer and will need to be released from the storage position. If the piston is not extended into the test position, place the end of the piston against a stiff surface and gently press the Rebound Hammer firmly against the surface. You will hear a click, and the piston will extend into the test position.

Figure 2: Always hold the Rebound Hammer with both hands perpendicular to the surface

To perform a test, make sure the piston is extended and gently press the   Rebound Hammer against the concrete surface to be tested. When the piston is pressed all the way into the Rebound Hammer, continue to push harder until you hear a rattling sound. Keep the Rebound Hammer firmly pressed against the concrete surface and read the rebound number on the scale.

Figure 3: Push the piston in as far as it will go

As long as you hold the Rebound Hammer firmly against the concrete, the re- bound number will remain on the scale. Record your reading. When you pull the hammer away and allow the piston to re-extend, the scale will return to zero. The Rebound Hammer is now ready for another test. If desired, once you get a reading, you may press the button on the handle to lock the piston in place. This makes your test results easier to read, as it will also lock the indicator. Do not touch the button until you get a reading. To release the piston again for the next test, press against a hard surface. When you are finished using the Rebound Hammer, lock the piston inside the Rebound Hammer for storage, as described above

Taking a Measurement

Selecting a Surface for Testing Select a concrete surface that is:

1. Smooth, dry, and fixed in place 
2. At least 4 inches (or 102 mm) thick 
3.  Mature— at least 14 days old 
4. Free of any decay or scaling

If necessary grind a section of rough concrete smooth before taking your   readings. A grind stone in a plastic case is provided with the hammer for this purpose.

Surfaces to Avoid

Avoid testing concrete that is:

1.  Frozen 
2. Honeycombed or excessively porous 
3. Known to have rebar 2 cm or less below the surface 
4. Carbonated on the surface

If you must test a mature concrete surface that is damp from rain or flowing water, or if you must test new concrete, keep in mind that you will probably get. Rebound numbers that are lower than they should be. Using the Rebound Hammer on new concrete may damage it if the strength is less than 1,000 PSI. If you are testing concrete with a lot of surface carbonation, the rebound readings will be significantly higher than normal. The thicker the layer of carbonated concrete, the higher the rebound numbers.

Holding the Rebound Hammer against Walls, Ceilings, and Floors

The Rebound Hammer will work against any concrete surface as long as you keep it perpendicular to that surface. The Rebound Hammer can be used against a ceiling (upward), a floor (downward), or a wall (forward). You will need to consider gravity when estimating the compressive strength of concrete. The rebound number found when pressing up against a ceiling will differ from the number that appears if you are pressing down against a floor. Please review the strike angles A, B, and C and the correlation curves in “Reading your Results” below.

Specialty Concrete

The Rebound Hammer is designed to estimate the compressive strength of standard concrete. The correlation diagram for standard concrete is printed on the side of your Concrete Rebound Hammer. It shows the correlation curves deter- mined when the compressive strength of standard concrete, as measured with your Rebound Hammer, is directly compared to the compressive strength from laboratory tests of concrete cylinders or cubes.

This diagram is not designed for use with:

• Lightweight concrete 
• Concrete using artificial aggregates 
• High performance concrete

In order to gain an accurate rebound number for these specialty materials, you will need to create your own correlation curves diagram intended for this use.  See the standards ACI 228.1R-2010 and ASTM C-805-08.

Reading your Results

Make at least ten readings from a concrete surface and discard the highest and lowest rebound numbers. Average the remaining eight numbers. If desired, take a few test readings before you complete your series of ten regular tests. Use the average rebound number to estimate the strength of the concrete. Compare your average rebound number to the chart shown on your Concrete Rebound Hammer. Consider the diagram shown here:

Figure 4: Concrete Rebound Hammer correlation curves

Rebound Number

Correlation curves. This diagram shows the relationship between the compressive strength for concrete, measured directly in a laboratory using concrete cylinders or concrete cubes, and the rebound numbers you receive   using your Rebound Hammer.

• Refers to using the Rebound Hammer against a concrete floor (downward).
• Refers to using it against a concrete wall (forward) 
• Refers to using it against a concrete ceiling (upward).

The strength values shown in this diagram are directly based on compressive strength testing in a laboratory, using concrete 14 to 56 days old. The tests use concrete cylinders, 6 inches wide and 12 inches high (15cm x 20cm), or concrete cubes six inches high.

According to ASTM C-805-08, you should discard any readings that differ   from the average value rebound number by more than six units. Then, take an average for the remaining readings. If more than two readings differ by more than six units from the average value, throw out all of your readings and perform the test over again. Take ten new readings with your Rebound Hammer in a   different section of the concrete but in the same test area.

Note that some countries have set their own national standards for calculating test results. Therefore, follow the standards of the country where you are using your Rebound Hammer.

Rebound Hammer Test Equipmet

A simple equipment known as Rebound Hammer or Schmidt Hammer is used for this purpose. Surface hardness measured during the test give an idea about the soundness and quality of cover concrete. Locations having very low rebound numbers indicate weak surface concrete and may be affected by corrosion. The quality of concrete may be interpreted as shown in the above table

Advantage

• Simple to use. No special experience is needed to conduct the test 
• Establishes uniformity of properties. 
• Equipment is inexpensive and is readily available. 
• A wide variety of concrete test hammers is available with an operational range of M10 to M70.For rehabilitation of old Monuments

Disadvantage

1. Evaluates only the local point and layer of masonry to which it is applied 
2. No direct relationship to strength or deformation properties.
3. Unreliable for the detection of flaws 
4. Cleaning maintenance of probe and spring mechanism     

Maintenance

We recommend that you calibrate your Rebound Hammer regularly using   the Humboldt Calibration Anvil, H-2972, ordered separately. For a Rebound Hammer, you should start verifying the readings after approximately 2,000   uses (or completed tests). You should also verify the readings if you have   reason to suspect that your Rebound Hammer is giving you false data.

Calibrating your Concrete Rebound Hammer

When tested against the Anvil, the impact energy level of the Rebound Hammer must be 1.64 foot-pounds or .227 kilograms-meter and the rebound number must be a value of 80 ±2. As the Rebound Hammer approaches 2,000 uses, it may start to read outside of the range of 80 ±2 on the test Anvil. At this point you should calibrate it.

1. With the piston fully extended, twist the cap on the top of the Rebound   Hammer to open it. Keep your hand positioned on the top of the cap, as you will feel some slight spring compression pushing against the cap. Set the Re- bound Hammer aside and examine the cap. The cap has an adjustment screw and nut, which are used to calibrate the Rebound Hammer.

 

Figure 5: Rebound Hammer cap

2. using a 10 mm wrench, loosen the adjustment nut away from the cap. If the Rebound Hammer is reading low, tighten the adjustment screw (clockwise) into the cap. If the Rebound Hammer is reading high, loosen the adjustment screw (counterclockwise) away from the cap. When you are finished, tighten the adjustment nut to lock the screw in this new position.

3. Put the Rebound Hammer back together, and test it on the Anvil again to get a calibrated reading. If it reads 80 ±2, the Rebound Hammer is calibrated. If the readings fall outside of this range, take the Rebound Hammer apart and try to adjust it again. Keep adjusting and testing the Rebound Hammer until it reads 80 ±2.

Clean the Rebound Hammer with a dry, clean cloth. Do not use solvents, abrasives, or water for cleaning. The springs in the Rebound Hammer may become worn and ineffective after more than 8,000 tests. Therefore, if you have an older Rebound Hammer that is no longer providing accurate measurements, and you tried to recalibrate it but it still doesn’t work properly, send it back to Humboldt for an evaluation and/or repair.

Safety:

• When using this device wear safety glasses
• Do not fire the Rebound Hammer against any body part  

1.  Impact Plunger 
2. Housing, complete 
3. Rider with Guide Rod
4. Push-button, complete 
5. Hammer Guide Bar 
6. Guide Disk 
7. Cap 
8. Two-part Ring 
9. Rear Cover 
10. Compression Spring 
11. Pawl 
12. Hammer Mass
13. Retaining Spring 
14.  Impact Spring 
15. Guide Sleeve 
16. Felt Washer 
17. Plexiglass Window
18. Screw 
19.  Lock Nut
20. Pin 
21. Pawl Spring