Consideration of moisture effects on the upstream face of concrete dams

Abstract

The specifications SNiP 11-54-77 and SNiP 11-56-77 when designing concrete dams recommend, along with other factors, taking into account swelling of concrete in the zone of the upstream face. However, these recommendationsare seldom used in dam designs in connection with an insufficient study of the givenproblem. Heretofore swelling of concrete was investigated only under laboratory conditions. The investigations conducted at the B. E. Vedeneev All-Union Scientlfic-Research Institute o~ Hydraulic Engineering (VNIIG) showed that additional compressive stresses reaching S ~MPa can occur from swelling of concrete of the upstream face of the dam, which can have a substantial effect on the static work of the entire structure [I], Observations of the stress--strain state of gravity dams shows that during filling of the reservoir, compressive stresses exceeding by 1.5-2 MPa the stresses calculated with consideration of the weight of the concrete, hydrostatic load, and temperature effects are noted at the upstream face [2~ In the author's opinion these additional compressive stresses in the structure occurred due to swelling of concrete. However, in connection with the fact that several factors affect the work of a dam, it has not been possible so far to isolate and estimate quantitatively molsture-lnduced stresses, which is related to the absence of data on a change in the moisture content of concrete in a structure.The Siberian Branch of VNIIG has developed apparatus for remote measurements of the moisture content of concrete in structures based on a thermophysical method. Tests of the apparatus conducted on concrete of various compositions showed that the thermophysical method can be used for determining moisture changes in concrete caused by external effects of moisture, in which case the maximum error of measurements was 0.3% [3].The apparatus based on the thermophysical method is rather simple, but, as investigations showed, the method does not permit determining changes in the moisture content of concrete during its intense structural formation. When dams are put into temporary operation at intermediate heads the concrete is often under water soon afterlts placement in the block. The use in this case of the thermophysical method becomes difficult, and therefore investigations of the relation between moisture content and the dielectric constant of concrete were conducte@ and apparatus based on the "dielcometric" method was developed. Laboratory tests showed that it is expedient to calibrate a moisture gauge on concrete of the investigated structure~ Thus, for the concrete being placed in the Sayano-Shushenskoe dam the maximum error of measuring the moisture content was 0.32% and the standard deviation was 0.04%[41.Moisture-induced stresses in a dam can be determined from the results of on-site investigations of the moisture content of concrete.For approximate calculations a concrete dam can be regarded as a free wall, one side face of which is in contact with water. In this case, at any point located at distance x ...