In this work is made a fieldwork about the compressive strength of concrete in real structures in order to establish the correlation between specified strength in project (fck, specified) and estimated strength (fck, estimated). The results from tests of 6.312 molded and extracted specimens were analyzed, as well as hammer rebound tests. The fieldwork lasted a construction period of up to nine months. In the test program was studied ready mixed concrete producers by two concrete plants with the compressive strength specified of 35 MPa in 28 days, provided for four different constructions. Several parameters were analyzed: increasing strength according to concrete age; difference between the specified strength values and concrete strength provided during the constructions; correlation between the strength values obtained in molded and extracted specimens. The results showed that there is a difference between the values of specified strength and estimated strength at the beginning of the construction. The results also showed that the correction coefficient of the drilling effects on core recommended by the NBR 6118:2007 is appropriate. Finally, suggestions are given on the appropriate number of core to be taken when there are differences to be settled.