The principle

instruments and test results to measure concrete surface density by

γ

-rаy backscattering are described. A radioactive source

137

Cs 2.59 MBq was used

the penetration depth in concrete is about 10 cm

and the measuring precision is 0.1g/cm

3

. The penetration area and depth in concrete

the calibration method and the measurement sensitivity are discussed. A suggestion for in-situ calibration with aluminium as standard specimen is put forward. Finally

some successful examples of in-situ practice are introduced.