Because of the wide development at the present time of highly mechanized methods of compaction, the problem of the quality of horizontal construction joints arouses great interest. The current -Technical Rules for Hydraulic Engineering Work" stipulate the placing of an underlying layer of cement mortar or of fine-grained concrete on the surface of the hardened block prior to placing fresh concrete. This technique, developed from long experience with construction of hydraulic structures in which the concrete was compacted by hand-held vibrators, makes it necessary to increase the quantity of cement and complicates and raises the cost of the construction work. As shown by previous experience, the use of intense vibration permits concreting hydraulic structure block without the need for placing an underlying layer of new concrete. By this means the construction of the dam at the Toktogul hydroelectric plant [12] is being suceessfuUy carried out. Experience with cons=uction of the Dworshak dam in the USA [13] also supports the advantages of this method. However, these techniques have not yet been formalized in normative documents.In this connection, at the Laboratory of Vibration Techniques of the 13. E. Vedeneev All-Union ScientificResearch Institute of Hydraulic Engineering (VNIIG) and at the Sayano-Shushenskoe hydroelectric plant, analyses were made of the available experimental material from studies of the quality of the bond between new and old concrete, and special tests were designed for investigating the quality of the joints obtained by applying different concreting techniques. Table 1 presents the bond strength indices for new and old concrete according to published data. They indicate that the results of tests on the relative strength of joints, obtained by many investigators over a long period of time for different compositions of concretes, agree sufficiently satisfactorily on the whole.The mean value of the relative bond strength (with respect to monolithic concrete) was 0.63 without an intermediate layer of mortar and 0.68 with it. In addition, the uniformity of the relative strength, characterized by the coefficient of variation, was higher for the samples prepared by placing an intermediate layer. In this case, the coefficient of variation was 16.4%, whereas for samples made without intermediate layers it was 23%. One of the possible causes of such nonuniformity is the insufficiently intense compaction of the mix in the contact layer, which was not considered in the tests. In order to take into account this important construction fact, which affects the formation of the contact joint, material was gathered from full-scale tests, and investigations were carried out with Fig. i. General view of stand for water-absorption tests. 1) Tank of compressed air; 2) reducing valve; 3) flexible highpressure hoses; 4) tank of water; 5) pressure gauge; 6) weights; q) tube with seal; 8) test block.
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...
Concrete work in the trench opened for the basic structures of the Sayano-Shushenskoe hydroelectric plant (Fig. 1) was begun in 1972 with the raising of the spillway section of the dam. Concrete work in the trench was first carried out simultaneously with drilling and blasting operations and rock work for preparation of the dam foundation, as well as with structural complications encountered with the dam's spillway section.
The danger of segregation of a concrete mix is the reason why the Construction Specifications and Regulations (SNIP) limit its free drop height to 2 m. The limitation of the free drop height of the mix into blocks being concreted stipulated bythe standards serves to prevent segregation of the mix being placed, which can cause increased inhomogeneity of the physical and mechanical properties of the concrete in the structure (Fig. 1). At the same time, for example when delivering concrete to the blocks of hydraulic structures in the winter, the drop height limitation does not permit placing the mix without uncovering the protective tent, which complicates and increases the cost of construction.The main means for controlling segregation is to reduce the falling speed of the concrete mix by using "snouts" and to reduce the free fall height of the mix during discharge. Since the limitation of the mix drop height specified by SNiP is based on expert evaluations and since it is extended to all types of structures and construction conditions, it was necessary to investigate an increase in the drop height of concrete mixes for the specific conditions of the Sayano-Shushenskoe hydroelectric station. The design of the main blocks in the dam differs from the designs of industrial buildings, to which SNiP III.G-1.70 is mainly oriented, by a smaller amount of reinforcement, which actively influences segregation of the mix x,,hen it falls. The concrete is placed in the blocks by buckets with a greater capacity than those used in industrial construction, as a result of which the mix leaves the buckets as a heavy, dense flo~,, ~, kich can prevent segregation of the coarse aggregate from the mix.Experimental work was carried out under construction conditions on blocks of the main structures of the Sayano-Shushenskoe station without any changes in the general technology of placing the mix (with the exception of its drop height) or additional requirements on the concrete design. The working compositions of the mix (3-4-cm slump) used during the experiments are presented in Table 1. The concrete mix was delivered to the blocks by buckets with a capacity of 3.2 m s loaded from dump trucks.The experimental methodology called for taking samples and wet screen sizing of the mix after its discharge from various heights (3 and 6 m) with complete opening of the bucket flap, leveling, and compaction by deep vibrators. The samples were taken in the center and along the edges of the discharged portion of the mix. The holes for taking the samples were formed by vibratory driving of hollow steel cylinders into the freshly placed layer of concrete to a depth equal to the layer of placed mix (0.5 m). In all. seven experiments were conducted~ in six of which the mix was discharged from a height of 6 m and in one, the control, from 3 m. Samples from 21 holes in four blocks with different compositions of the mix were tested. The mobility of the mix was 1.5-2 cm and 5-6 cm with respect to the slumps of a standard cone.The sampling schematics are shown...
The necessity frequently arises during the construction of large hydraulic structures to transport concrete ag= gregates over long distances. The quality of these materials and of the mixed concrete depends on a well=thoughtout method of transport from the source of the material to the place of use.The sand=gravel quarry at the construction site of the Krasnoyarsk hydroelectric plant is 70 km downstream of the dam. The gravel screening plant ( Fig. 1) is located at the quarry. The plant has an output of 600 tons/h of screened gravel (washed in summer) in four sizes: 0=5, 5=20, 20=40, and 40=80 mm.The gravel is discharged into intake hoppers. Aggregates of different sizes are delivered by conveyors from a height reaching 15 m to aggregate storage cones 9.. An underground shaft 3 with two conveyor lines is used for the delivery of materials to wharves for loading into barges. Additionally, the material may be delivered by the con= veyors to stock piles in standby storage 4, and further along shaft 5, beneath the stockpiles, to wharves 6. 9 In summer the aggregates are conveyed in barges. The barges are unloaded by means of gantry-cran e clam= shells to handling storage 7 (at the wharf), then through shaft 8 the aggregates are carried by conveyors into pile= stackers of main storage 9, and the aggregates are stockpiled to a height of 10 m. The materials drop from a height of 5 to 10 m into the barges during loading operations. The aggregates are delivered from the main storage through underground shafts 10 and 11 to delivery bins 12 of the concrete mixing plant, where the material is stockpiled to a height of 10 m. From the batching bins the aggregates are transported by conveyors to delivery bins 13 of the concrete mixing plant, and further, through hatching conveyor 14 to the continuous concrete mixer. Thus, the mated= al, while moving from the screening facility to the concrete plant is dumped 6 to 7 times into high heaps of coni= cal shape. When moving along the conveyors the material is handled 28 to 46 times with a total length of conveyor lines of 4714 to 4980 m.
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