What is Rebound Hammer ? Applications,Factors that affects the Rebound Hammer Number?

Rebound Hammer

The rebound hammer is one of the most popular non-destructive testing methods used to investigate concrete. Its popularity is due to its relatively low cost and simple operating procedures. The rebound hammer is also one of the easiest pieces of equipment to misuse; thus, many people do not trust the rebound test results. 

Applications- 

The rebound hammer measures the surface hardness of the concrete. This is accomplished by placing the rebound hammer plunger against the concrete surface and releasing a spring loaded weight. The amount the plunger rebounds or bounces back is measured. This rebound number is shown on a scale and will be between 10 and 100. The Impact Hammer is another name for Schmidt Hammer.

 

The surface of concrete gets harder as concrete gains strength; thus, we have a method of estimating the strength of concrete. A low rebound number will indicate that the surface of the concrete is soft and the concrete is weak. A high rebound number will indicate that the concrete is hard and strong. Unfortunately, there is no theoretical relationship between surface hardness and the strength of concrete.

 

“Nondestructive tests of the concrete in place, such as by probe penetration, impact hammer, ultrasonic pulse velocity, or pullout may be useful in determining whether or not a portion of the structure actually contains low-strength concrete. Such tests are of value primarily for comparisons within the same job rather than as quantitative measures of strength.”  

Factors that affects the Rebound Hammer Number-

Since the rebound hammer measures the surface hardness of the concrete, it is important to understand all the items that might affect surface conditions of the concrete and thus, the rebound hammer numbers. These factors include:

 
1. Smoothness of the surface
2.Size and shape of the concrete sample 
3. The rigidity of the test area
4. Age of the concrete
5. Surface moisture
6. Internal moisture (moisture gradient)
7. Coarse aggregates
8. Type of cement
9. Forms used
10. Carbonation 
11. Location of the reinforcement
12. Frozen concrete
 

Standard procedures for Rebound Hammer-
 
Standard Test for Rebound Numbers of Hardened Concrete, provides some standard procedures so that the user can have consistency when using the rebound hammer. Some of these standard procedures are:
 
1. Do not test frozen concrete.
2. The test area must be at least 150 mm (6 inches) in diameter and fixed rigidly within the structure.
3. The surface to be tested must be flat with no loose mortar.
4. The surface to be tested must be free from water.
5. If the layer of carbonated concrete is thick, it should be removed before testing. 
6. The hammer must be held in the same direction — horizontal, upward, downward and it should always  

     be at a right angle to the surface being tested.
7. Do not test over reinforcement with a cover of less than 20 mm (3/4 inch).
8. If estimating concrete strength takes at least two cores from six locations that have different rebound hammer number.
 9. Take 10 rebound hammer readings at each test area. All individual readings should be at least 25 mm (1 inch) apart.
 10. Discard any reading that is over six units from the average and calculate the average of the remaining readings.
11. If two units are over six units from the average, discard the entire set of reading and redo the test.