Superficial asphalt seals in joints of hydraulic structures

Abstract

Asphalt seals are a reliable method of waterproofing contraction joints of hydraulic structures, especiallywhen subjected to temperature-settlement deformations [1,2]. However, owing to the thermoplasticity of the asphalt filler, it is necessary to have a reliable protection against sharp temperature variations, since under high temperatures the asphalt mastic may flow, and under low temperatures it may lose its plasticity; leading to cracking when the joint is opened or to excess pressure in the seal cavity when the joint is closed. According to a procedure developed for the analysis of asphalt seals of the shaft type [1, p. 81], in order to prevent tension in the asphalt filler under a design temperature of 0~ in a 50-m high dam, it is necessary to place a seal 180x 260 cm in cross section; with a seal measuring 30 x 30 cm, the pressure produced during closing of the joint at a rate of 10 -s cm/sec may reach 45 kg/cm 2. The first attempts to place superficial seals on the structures of the Moscow canal and the Uglichsk hydraulic development by using ordinary asphalt mastic consisting of 30% of BN-III asphalt and 70a/o cement led to destruction of the water-retaining elements of these seals [4, p. 102]. For this reason, the norms SN 123-69 prohibited the use of superficial asphalt seals and permitted the use only of internal, shaft asphalt seals. They have substantial disadvantages: the complex formwork and the difficult concreting operations, the high labor consumption involved, and the impossibility of verifying the quality of the seal preparation before placing the mastic and just after the electric heating system is installed. As a result, in most structures small-section asphalt seals do not operate satisfactorily. (At the Lenin Dnepr and Kama hydroelectric plants, costly and complex repair work was carried out.) At most hydraulic developments built on soft compressible foundations an established practice at the present time is the waterproofing of construction joints with asphalt seals having a cavity measuring 1.5 x 1.2 m (seals used at the Volgostroi). The waterproofing of the joints with asphalt seals 80 x 100 cm in cross section at the Bratsk hydroelectric plant and 60 x 80 cm in cross section at the Plyaviusk hydroelectric plant [3], although sufficiently reliable, was so complicated that at the Krasnoyarsk and Ust'-Ilimsk dams asphalt seals were not used, and the joints were waterproofed with metal compensators.The development, in recent years, of fundamentally new asphalt-polymer waterproofing materials [3] opens up new prospects for the use of superficial asphalt seals under a sharply continental climate [2].At the Sarrans darn in France (1936, height 114.5 m), the joints were waterproofed by means of a reinforcedconcrete beam 1 x 1 m in cross section with an adjacent asphalt seal 7 x 21 m in cross section. Although this seal