SCHMIDT HAMMER 



General Description and Operation 



The Schmidt hammer utilizes the rebound method for determining the 

 compressive strength of concrete. This is accomplished by correlating 

 the rebound height of a spring-driven mass after it impacts the surface 

 of the concrete with the compressive strength of the concrete under 

 test. A Schmidt hammer, Model RM 710, Type L, modified for underwater 

 use, is shown in Figure 8. A cutaway view of the hammer, illustrating 

 the internal mechanisms, is shown in Figure 9. 



The Schmidt hammer is principally a surface hardness tester. It 

 consists of a spring-driven mass that slides on a guide bar within the 

 tubular housing as shown in Figure 9. To carry out a test, the impact 

 plunger is pressed strongly against the concrete surface under test. 

 This releases the spring-loaded mass from its locked position causing an 

 impact. The mass then rebounds, taking the rider with it along the 

 guide scale. By pushing a button, the operator can hold the rider in 

 position while the index is read to the nearest whole number. This 

 value is referred to as the rebound number and can vary over the range 

 of 10 - 100 with higher numbers indicating stronger concrete. It is 

 recommended that a minimum of 12 readings be taken per test site and 

 averaged after discarding the minimum and maximum values (Ref 9) . A 

 general calibration chart (provided by Soiltest, Inc., Evanston, 

 Illinois) that relates the rebound number to cylinder compressive 

 strength for the Model RM 710 Schmidt hammer is shown in Figure 10. 



The Schmidt hammer has numerous limitations that should be 

 recognized when using this instrument to obtain surface hardness data. 

 For example, the test results obtained with the hammer are effected by 

 the following: 



1. The surface of the concrete under test has an important effect 



on the accuracy of the test results. Higher rebound numbers were obtained 

 from smoother surfaces and the scatter in the data was less. Minimizing 

 the data scatter increases the confidence in the test results. Thus, 

 underwater concrete surfaces must be thoroughly cleaned and smoothed 

 with something like a carborundum stone before measurements are taken. 



2. Surface and internal moisture conditions of the concrete will 

 also affect the results. Saturated concrete tends to show rebound 

 readings five points lower than when tested dry. This will affect the 

 comparison of data taken above and below the waterline. 



3. The type of coarse aggregate and cement significantly effects 

 the correlation of the rebound numbers with actual compressive strength 

 of the concrete under test. A calibration curve is required for each 

 particular concrete mix to assure accuracy. This is not practical for 

 most situations. 



4. Size, shape, rigidity, and age of the concrete become important 

 when testing small concrete samples or recently poured concrete. This 

 should not be a concern for the underwater inspection application. 



