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.
Electrometric control of the relative density of the concrete mix is based on measuring its electrical parameters, which are related directly to its density. This method, which was first used successfully in the construction of the Dnepr hydroelectric station dam [i], was subsequently used on the Toktogul, Ingurl, and other hydrostatlon projects. However, for the wide industrial use of electrometrlc compaction control a more detailed check of individual problems of the method of its use was required.First of all, the design of electrode-sensors placed in the concrete mix was developed. Single-electrode spear-sensors and double-electrode probe-sensors submerged in the concrete and electrodes left in the concrete mix ~Ig. i) were used in the work. The operating principle of all types of sensors is based on the principle that when an alternating current is supplied to them an electric field is created in the concrete mix whose strength (for constant parameters of the current and composition of the medium) depends on the density of the concrete [2]. Since the electric field potential in the medium decreases with distance from the current source, it was necessary to estimate the zone of action of the sensors and parameters of the electric current, The investigations were carried out under laboratory conditions and directly in the concrete blocks.In the laboratory experiments we used a homogeneous medium (water) and plastic mortar, which were placed in a Plexiglas mold equipped with a probe connected to the measuring instrument, A dielectric plate slmulating nonconducting inclusions, having an area 3-4 times greater than the area of the probe electrodes, was introduced into the mold. The distance between the plate and probe was changed, the resistance of the medium being measured at each position.
The selection of the correct technological scheme of concreting is especially essential and should be regarded as a necessary element of leading technological development in the area of constructing hydraulic structures.A technical of conreting reinforced-concrete elements of the Leningrad flood-control levees, including thick slabs (more than 30 cm thick) of the revetment of its slopes, was examined in [i]. The proposed technology, whlchwas notpresented for coordination to the Design and Construction Materials Office of the Leningrad Hydroelectric Station Administration (PSMO LenGESS), calls for using concrete placing and compacting equipment not being produced by Soviet industry. Not concerning ourselves with the technology of concreting seawalls, which can be solved by various methods, we will examine the characteristics of the placement and compaction of the concrete mix when concreting slope revetment slabs, since this problem is common also for earth dams with reinforced-concrete facings and revetting the slopes of pumped-storage station dikes.For this purpose sliding formwork was proposed [i], which permit placingconcrete mixes with a cone slump of 1-3 cm, which is a more progressive method than the use of removable panel formwork. However, the given design (as far as it can be judged from the schematic diagram) is rather complex and does not consider the main process of concreting--compaction of the mix--which first of all should be mechanSzed, since undercompaction markedly reduces the life and strength of the structure. Withoutdwelling on the technological inconveniences of the proposed scheme of the sliding formwork (the need for sufficiently even lanes along the crest of the slope, complexity of readjusting the equipment upon a change in the length and steepness of the slope, etc.), we note the complete vagueness in the methods of delivering the mix and its compaction. In [i] mention is made of the use of rther stiff mixes (with a slump of 1-3 cm) and at the same time it is suggested to deliver the mix by a concrete pump, whereas Soviet equipment operates with concrete mixes with a slump of 6-8 cm. The increased fluidity of the mix in this case, not saving the need to compact it, slows the rate of rearranging the formwork, i.e., reduces productivity and increases the consumption of cement. Simultaneously, the use of plasticizing and air-entraining additives needed for providing the necessary frost-resistance index delays the setting time.Taking into account the aforesaid, the B. E. Vedeneev All-Union Scientific-Research Institute of Hydraulic Engineering together with the PSMO LenGESS developed a technology of concreting thick reinforced-concrete slope revetment slabs based on experimental data of investigations and experience of world dam construction [2, 3]. The following requirements were taken as the initial ones: all concreting operations should be mechanized; the possibility of concreting reinforced slabs up to 1 m thick of any length with a steepness up to 1:1.3 is necessary; compaction of the...
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