Schmidt Hammer

A H M 531

Schmidt Hammer test

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By: Ahmed Essam Mansour

Measuring the compressive strength of concrete usin g the Schmi Schmi dt hammer.

 A ASTM STM

des ig n ati o n C 805 75T

Testing the compressive strength of c oncrete using the Testing Schmidt h ammer (Also (Also c alled: rebound hammer, impact hammer and sclerometer), is co nsi dered as as a non destr ucti ve test as no destro yed specimens takes place in it.

A H M 531 The Civil Engineering Center

The main prin cipl e of this test is that it measures the rebou rebou nd of an elastic mass when it collides with the concrete surface under the test, this r ebound depends depends o n the hardness of concrete and and on the energy energy it absorbs from t he collision. The tested concr ete has to be smoo th and firml y supp orted. The hammer hammer is pressed against th e concr ete ete,, and and t hen the mass inside the hammer hammer is rebounded from the plung er and and giv es a reading reading on t he scale. This reading reading is called Rebound Rebound Number whi ch is t he distance traveled by the mass expr essed as as a percentage of th e initial extension exte nsion of the spri ng. Note that that the rebound numb er is an arbitrary measure that means different device might give different rebound numbers in t he same test. The The rebound numb er depends depends on energy stor ed in the spring and on t he size of the mass, consequently each each device is c ombined with a graph contains calibrating calibra ting c urves relating relating t he rebound rebound number with the The Civil Engineering Center

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A H M 531

Schmidt Hammer test


compressive s trength. As t his tes t i s sens iti ve to the pr esence of agg regate an d voids, we have to take at least 10 reading in the test area, whic h is 15 cm d iameter c irc le, and there average is taken as the comp ressiv e strength of the conc rete. Readings w here the hammer crashes or breaks into a near sur face air void s shall not be taken. The plun ger shou ld always be normal to the conc rete in test. The hammer posit ion r elative to th e vertic al affects th e reboun d numb er because of the affect of gr avity on th e mass in si de the device. (The rebound nu mber for th e floo r is smaller than it for t he ceilin g) The calibration graph relating the rebound numb er and the compressive strength contains a curv e for each angle (measured fro m the hor izontal to t he hammer axis). Aft er 10 read ing s are tak en, the aver age is calc ulated and al l the readings must not deviate 7 degrees fr om th e average, an allowance is two r eadin g onl y can excess 7 degrees, but if mor e than two reading s existed the test shall be terminated.


The error o f the device is wri tten in th e graph and th e taken values shall be corrected in accord ance with t he erro r value. The Schmidt h ammer test result s can’t be used to reject or accept the conc rete as there are many oth er factors affecting the results includin g surface smoothness and finish , moisture con tent, coarse aggregate type, and the presence of carbonation.

The Civil Engineering Center


A H M 531

Schmidt Hammer test

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Schmidt hammer:

It consis ts of a sprin g loaded steel hammer whic h when r eleased st rik es a steel plunger in c ontact with the concr ete surface. A s cale w hich m easures t he r eboun d numb er takes place on the sur face of the instrument. An d t he cal ib rati on g raph t hat relat es rebound number with comp ressive strength is glued on th e instrument.


 Abr asiv e

sto ne: Consisting of a medium grain texture silicon carbide or equivalent material. It is used to smoot h the concr ete sur face and remove the finishing of it.

1. Preparing th e test area by smo othi ng it and removing mor tar and paint from the sur face. The test area is a cir cle of 15 cm. 2. The hammer is placed in touch w ith th e testin g area and pressed til l the hammer impacts and the release butto n



A H M 531

Schmidt Hammer test


is pr essed to get the reading . 3. The previou s step is h eld for 10 times, and 10 reading s are obtained. The dist ance between any two reading s must not be more that 25 mm.

All the reading where considered in calculations as the deviation of all the points from the average is less than 7 degrees, I think that can be related to the homogeneity of the mix. The gained compressive strength is satisfactory, but errors might be occurred in placing the hammer orthogonally on the concrete surface. The more the rebound number is the more compressive strength of the concrete.




Schmidt Hammer

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