One of the way of finding out the surface of concrete in construction are impact hammers in building practise the most widespread resilient impact hammers of Schmidt system. Sometimes there are also used nonnorm impact hammers for exmaple Masek impact hammer and Ciganek prong. By teoretical we can suppose that for same surrfaces will be at same trial place found same values of surfaces, but this presumption wasn't confirmed yet. In case of nonconfirmed tests of concrete it's needed to use only one type of impact hammer. For searching more accurate results is needed to make accurates tests of concrete, everytime for concretes which are more than 1year old and for using different types of impact hammers for classification the surface of construction.
KEYWORDS: concrete, strength, impact hammer.
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