Horizontal construction joints for massive high-head structures

Abstract

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.