TESTING HIGH-STRENGTH CONCRETE COMPRESSIVE STRENGTH

Testing high-strength concrete (HSC) for compressive strength puts int o doubt a certain number of concrete testing practices. First, most co ncrete testing laboratories are equipped with loading capacity machine s inadequate for testing 150 x 300-mm (6 x 12-in.) specimens, thus lim iting HSC specimens to 100 x 200 mm (4 x 8 in.) in size. As most desig ners rely on compressive strengths derived from larger specimens, they must be given relations between the different values obtained when sm aller specimens are used. Comparative testing reveals that HSC compres sive strength values are greater by 5 percent when measured on 100 x 2 00-mm specimens. Moreover, the coefficient of variation on one set of three 100 x 200-mm specimens is as small as, or smaller than, that for 150 x 300-mm specimens. Furthermore, a high-performance capping compo und testing 60 to 70 MPa using 50-mm cubes has proved to be effective when used for testing HSC up to 120 MPa due to the confinement of the capping compound between the platen and the specimen, but the capping layer has to be less than 3 mm thick. Under these conditions, the resu lts are similar to those obtained when the ends are faced by grinding. Nevertheless, it is safer to grind the specimen ends when the strengt h of the concrete exceeds 100 MPa. Finally, it is shown that eccentric ity of less then 4 mm between the testing machine and specimen axis do es not affect the compressive strength values.