Concrete thickness measurement is performed by using various non destructive tests such as ultrasonic and radiography, GPS (ground penetrating radar), electromagnetic test, etc.
The most popular test is using low frequency ultrasonic energy. LMATS uses Olsen thickness gauge which is predominantly used for most of the applications in the range of 60mm to 600mm and occasionally up to 1.8m. There are several other equivalent instruments available in the industry to perform this test. 
LMATS have upgraded the line of Concrete testing instruments and nondestructive systems using the Impact Echo principle, for measuring the thickness and integrity of concrete slabs, pavements, tunnel linings, walls and other plate-like structures and other structural elements with the added benifit of detecting internal voids, internal cracking, honeycombe
LMATS can reliabaly test thickness to 1.8m and in some conditions 2.0m
Faster non destructive ie no core drilling etc.
LMATS offers concrete thickness test from Melbourne and Sydney laboratories.
ISO 17025 & ISO 17020 accredited