1. Field of the Invention
The present invention relates to a method of instrumenting by means of a concrete inclusion an already erected structure made of material including concrete and/or rock in order to detect deformations of that structure. The invention also extends to the inclusion itself and to the so instrumented structure.
2. Brief Description of the Prior Art
In order to prevent dramatic ruptures of concrete works such as the dam of an hydroelectric complex, the pillars of a bridge or skyscraper, a concrete beam, etc. . . , these works are systematically scrutinized to detect any deformation thereof. For that purpose, strains gauges are conventionally embedded in these engineering structures during pouring of the concrete. Such strain gauges are capable of detecting deformations as small as one micrometer/meter (10.sup.-6 m/m) in amplitude, and are positioned at critical points in the structures. As an example, U.S. Pat. No. 3,286,513 (WASIUTYNSKI) issued on Nov. 22, 1966, describes a probe to be embedded in concrete and comprising a set of strain gauges for simultaneously detecting six strain components at a selected spot in the concrete.
Before 1950, the engineering works were not so instrumented during pouring of the concrete. Also, it has been observed that a non negligible proportion of the embedded strain gauges become faulty after some years of service.
Accordingly, need has arisen for an efficient method of instrumenting already erected concrete structures.
Two prior art techniques have been developed to instrument such structures. The first one consists of a plastic cylinder on which strain gauges are adhered. This plastic cylinder is disposed in a hole of small diameter and any empty space between the cylinder and the concrete is sealed with epoxy. The second technique is concerned with steel inclusions placed in holes and subjected to the stresses occuring in the so instrumented structure. These two prior art techniques however present the disadvantage of being unsuitable for long-term monitoring for the following reasons:
the electric strain gauges adhered on the plastic cylinder are designed for short-term tests (some weeks); drift of the zero setting is very important after some months of service;
sealing with epoxy is unreliable in about 50% of the cases in bodies containing water;
the elastic properties of the involved materials change in time to make impossible the interpretation of the measurements; for example the steel corrodes and the plastic becomes fragile as it ages; and
the composition of the inclusions is very different from that of the concrete of the structure, and accordingly the physico-chemical lifetime of the inclusions is likely to shorten.