Mechanized compaction of placed concrete during construction of the Sayano-Shushenskoe hydropower scheme is currently being carried out with a special manipulator equipped with a block of vibrators, types IV-34 and IV-90. The output of such a machine when concreting in 0.5-m layers averages 50-60 mS/h. With a change to concrete delivery with a bucket of 8-m 3 capacity, the concreting rate is increased substantially, and the operation of the manipulator will require further capacity increase. The simplest solution could be to increase the layer thickness. However, the equipment designated to achieve this technology has, at the present time, been issued only in trial quantities, and the new compactors have not yet been proven on a wide scale under production conditions. Therefore, in preparing to change to concretlng in thicker layers, trial runs were conducted during construction of the Sayano-Shushenskoe hydropower scheme, comprising comparative testing of the currently used vibrators IV-90 and new types B-I-691, also horizontal "torpedo" types. The engineering characteristics of these vibrators and other new compactors having similar parameters are presented in Table 1.The characteristic feature of the new vibrators is the comparatively low (2800 osc./m/n) vlbratlonal frequency, whereas the present units have relatlvely higher frequencies. The use of low frequencies increases the useful llfe of the machines, but the question of their effectlveness remains unclear. The tests were conducted in blocks 1.5 x 3 m in area and with a layer thickness ranging from 0.8 to 1.5 m. The concrete mix for all tests was the same, conforming to Standard M200, V8, Mrz (frost resistance) i00, and typified by a content of cement ShPTs M300 of 250 kg/m s of concrete, with maximum aggregate size Dma x of 80 mm. Workabillty of the mix varied from 0.5 to 4.5 cm slump in a standard cone. The compaction regime adopted was as follows: lowering, 15 sec; holding in position, 60 sec; and raising, 30 sec.Test measurements included accelerations of oscillation of the concrete mix; the power used by the compactors; relative concrete density, obtained by electrometry; and the density of the freshly compacted concrete. The results of tests with vibrator IV-90 on concretes with slumps of 0.5 to 4.5 cm showed that the plastlclty of the concrete mix had little effect on the acceleration of the oscillations transmitted to the concrete mlx. Presented in Table 2 are the data for ~wo or three acceleration measurements in mixes of different stiffness by transducers located at equal distances from the vibrator; from these data it is seen that the scatter of readings for any one mix is the same as for mixes of different workability. Fig. la are the averaged distribution graphs of oscillation accelerations in the concrete mix, using vibrator IV-90. As can be seen from the figure, when compacting a layer 80-100 cm thick, acceleration of oscillatlons diminishes rapidly (to 1.5-2 g at a distance of 60 cm) with distance from the vibrator, then the rate of dimin...