International Concrete Abstracts Portal

The greatest threat to the durability of reinforced concrete structures is the reinforcement corrosion. The paper presents the importance of the concrete protective cover and the conditions causing the reinforcement corrosion under the action of chlorides and carbonic acid. Processes of absorption, diffusion and flow, i.e. of transport of media through concrete depend on the pore system and the amount of water in the pores.Physical laws describing the penetration of aggressive agents into concre-te can serve as a basis for engineering calculations of reinforcement durability in the concrete as well as for the designing of the concrete cover. Physical laws and corresponding material parametars are briefly reviewed in the paper. For engineering purposes, in calculating the durability, four typical tasks can be solved. The processes of degradation depend on the pore system in the concrete structure, and the paper indicates some possible technological measures of structure modifications.

Corrosion of reinforcing steel is now recognized as the most critical limit state affecting the durability and long-term stability of concrete structures. Although concrete is inherently alkaline and provides a natural protection to any embedded steel in it, thereby insuring its chemical passivity, concrete structures deteriorate for a variety of reasons. Steel corrosion is the most insidious and destructive form of damage, and once it starts, it is almost impossible to stop the process (unless such remedial measures as cathodic protection systems are applied), until eventually the safety, stability, and design life of the structure are all drastically reduced in time. The primary causes of steel corrosion are inadequate cover to steel, carbonation, neutralization due to atmospheric pollutants, and/or chloride penetration. The quality of concrete, its pore structure, and permeability characteristics are thus major factors controlling steel corrosion. However, it is inherent in the nature of concrete construction that there is no single method of corrosion protection that may be presumed to insure satisfactory serviceability throughout the life of a structure. This paper briefly reviews the factors that influence concrete deterioration and loss of alkalinity. It is shown that long-term durability of concrete structures can only be insured by a dual approach--developing a durable concrete, and providing a protective coating to the steel. Such methods of steel protection as cement-slurry coating, epoxy coating, and galvanizing are discussed. Extensive test results are presented on the corrosion resistance of plain, epoxy-coated, and galvanized bars when exposed to a marine environment, and chloride intrusion in cracked members. Cracking, cover, and concrete quality are identified as the major parameters influencing steel corrosion, but cover to steel is the most critical factor in preserving the electrochemical stability of steel. The paper shows that both epoxy-coated reinforcing bars and galvanized reinforcing bars can provide excellent resistance to chloride-induced corrosion.

As the sole domestic building material in Iceland, concrete is widely used for house construction as well as for other construction, such as dams, bridges, and harbors. In Iceland, conditions are in many ways extreme: the climatic conditions are harsh, the cement is high in alkalies, aggregates are of varying quality (some being reactive), and codes and standards have been sparse. Field surveys have shown that alkali-aggregate reaction (AAR) damage occurs where no preventive measures were taken and other conditions were unfavorable. Preventive measures taken in dam and bridge construction have proven to be effective. No AAR damage has been found in constructions erected after 1979, when several preventive measures were taken. The most important one is 5 to 7+ percent replacement of cement with silica fume Stricter criteria have been enforced to secure freeze-thaw durability, and durability design is improving. Research in repair and maintenance methods has had considerable influence on the construction industry.

Concrete roads have been in use in Denmark since 1923. Many of the old roads are still in use today and exhibit excellent durability. The Danish Technological Institute has investigated some of the old roads from the 1930s. The investigations include thin-section analysis of cores from the roads. This technique was used extensively by G. M. Idorn during the 1950s and 1960s, and in the late 1970s, the technique was further developed by the Danish Technological Institute. The results of the investigation show that the old durable concrete is a dense, strong concrete without signs of internal deterioration.

Five dormitory buildings on the Amherst College campus in Amherst, Massachusetts have essentially identical exposed aggregate precast concrete curtainwall panels. The panels on the three buildings that were constructed in 1963 are severely cracked and spalled; the buildings constructed in 1964 are relatively free of deterioration. The concrete used in the panels of all the buildings is composed of crushed quartz coarse and fine aggregate with strong and hard portland cement paste with low water-cement ratio and low void content. The significant difference between the materials used in the buildings is the amount of alkalies: the alkali content of the portland cement in the 1963 buildings is almost twice as high as in the 1964 buildings. A network of fine cracks developed in the panels due to alkali-silica reaction. These cracks allowed water to enter the panels and freeze during cold weather. The resulting progressive damage has led to disintegration of the cement paste, severe spalling, and corrosion of the reinforcing steel. The phased repair program, which began in the summer of 1989 and is expected to require several years to complete, involves removal and/or replacement of severely damaged panels, repair of damaged panels in place, modification of structural and waterproofing details to reduce exposure, and treating of undamaged panels to prolong their life.