International Concrete Abstracts Portal

Steel reinforcement embedded in concrete is normally passivated in its alkaline environment. In the presence of chlorides, however, the passivating layer is destroyed and the reinforcing bar corrodes, leading to concrete cracking and spalling. Several concrete rehabilitation techniques are available, of which only the cathodic protection (CP) of reinforcing bars was found to stop corrosion regardless of the chloride content of concrete. The CP system consists of reinforcing bars (cathode) connected to the negative terminal of a DC power supply, the positive output of which goes to an anode usually embedded in concrete. This paper describes some properties and applications of a proprietary activated titanium anode mesh and ribbon to a variety of old and new concrete structures: bridges, tunnels, buildings, wharves, piers, offshore platforms. Since its commercial introduction in 1985, 290,000 mý of this anode mesh has been or is being installed all over the world, whereas from 1987 the anode ribbon has been or is being applied to 29,000 mý of concrete surface. The paper addresses also the design and various installation techniques of anodes, as well as the testing, commissioning, and cost of the CP system.

Gunite (dry-mix shotcrete) was introduced to the construction industry in the early days of concrete technology (1911) when many new ideas and concepts were just developing. The literature indicates that what we know as wet-mix shotcrete had its start about the same time but did not receive positive acceptance until 1950. The evolution of the shotcrete process to its present status in the construction industry is tracked by definitions of the process from Gunite in 1912 to the American Railway Engineers Association (AREA) definitions of Shotcrete in early the 1930s. By 1951, the adopted the term shotcrete, making it the official generic name of the process. The definitions by ACI from that time on reflect the introduction of new ideas and equipment into the market place. The author questions whether some low velocity shotcretes are truly shotcrete or just an extension of the concrete pumping process. A brief description and evaluation, from the past to present, of shotcrete materials, applications, and testing is presented. The future of the process is discussed and two major problems posed: the improvement of applicator workmanship and the development of a rational design for most shotcrete applications.

The assessment of reinforcement corrosion in concrete structures before visible cracking and rust stains occur will enable the engineer to take cost-effective and timely remedial action. While half-cell potential mapping techniques have been used very successfully to indicate the location of active corrosion, this method provides no information on the severity of the corrosion rate and hence the scale of the problem. The linear polarization technique provides a means of assessing directly the rate of active corrosion by measuring the response of the reinforcement to a small perturbative signal. The method has generally been restricted to laboratory studies, due to difficulties in evaluating the region of influence of the test by allowing for a relatively high concrete resistance and in the availability of field instrumentation. This paper reports on the development of unique programmable polarization equipment, enabling corrosion rate measurements to be easily collected and stored on site. The results of laboratory studies on control specimens are compared with those taken from structures in aggressive corrosion environments. Guidance is given on the interpretation of linear polarization results from durability studies for those unfamiliar with the technique.

An investigation was carried out to evaluate the effectiveness of epoxy systems injected into cracks in concrete in order to repair damaged concrete structures. It was found that the usual parameters (viscosity, elastic modulus) characterizing epoxy system are not sufficient for such an evaluation, since concrete microstructure and crack width are determinant factors. Successful1 injection into larger macrocracks (>0,8 mm) is independent of the epoxy system viscosity. However, for narrower cracks (< O,3 mm), the effectiveness of the injection strongly depends on the viscosity of the epoxy system, so that a threshold value of viscosity can be determined for each microcrack width. The injection of epoxy systems into cracks of porous concretes causes, in addition to the crack filling, an impregnation of the material surrounding the crack area. This impregnation causes a strength increase, which is higher for more porous concretes and higher epoxy system viscosity. The mechanical behaviour of injected concretes do not depend significantly on the mechanical properties of the epoxy system, whereas the rheological properties of the epoxy system can affect the performances of injected concretes.

Recently, the quality of the surface layer of concrete has been recognized as an important index to relate the durability of concrete structures. However, an appropriate assessing method has not yet been achieved. The assessment of the change in the quality of the surface layer should be made from the viewpoint of not only the mechanical property but also other physical and chemical properties, such as pore structure, carbonation and so on. Attaching importance to the simplicity of tests on site, the authors have proposed that the pull-off tensile strength by the pull-off method is applicable to the former and the recovering speed in the rapid air permeability test to the latter. This study describes the results of tests on the surface layer of concrete using specimens exposed in both inland and seashore locations in the cold district of Japan. Then, the capabilities of two specific tests are examined to assess the properties of the surface layer of concrete due to weathering and other environmental attacks.