Folker H. Wittmann

Measurement of time-dependent strains of concrete: Prepared by subcommittee 4: Standardized test methods for creep and shrinkage

1359-5997/98

Shrinkage mechanisms, crack formation and service life of reinforced concrete structures

Shrinkage of concrete is most often at the origin of crack formation. Cracks are preferential paths for ingress of aggressive substances such as chlorides or sulfates into the composite structure of concrete. Reduced shrinkage means generally extended service life of reinforced concrete constructions. It is shown that shrinkage is not due to capillary action but it depends essentially on disjoining pressure in the nanopores of hydration products of Portland cement. All reactions, which reduce disjoining pressure, reduce shrinkage. Shrinkage can be minimised by optimisation of the chemical composition of the pore solution.

Influence of Sustained Load on Durability and Service Life of Reinforced Concrete Structures

The influence of an applied high compressive load on strength of concrete has been studied both theoretically and experimentally in great detail in the past. It was observed that load bearing capacity of concrete decreases under the influence of sustained compressive stress. In this contribution the influence of high sustained compressive stress on capillary absorption and on chloride penetration will be described. It was found that under the influence of moderate sustained compressive load the amount of capillary absorbed water decreases. If the sustained compressive load overcomes 50 % of the material’s strength, the rate of capillary absorption increases. This fact can be explained by reduction of the pore space under moderate compressive stress and formation and time-dependent growth of micro-cracks in the composite structure of concrete. Dissolved chloride is filtered out of the aqueous salt solution and remains accumulated in a surface near layer. In this way a high concentration difference of chloride is built up in a short time. This gradient leads to long lasting diffusion of chloride deeper into the pore space. As the diffusion coefficient of dissolved ions in the pore liquid increases with increasing applied compressive load, service life of structural elements under high compressive load will be reduced significantly when exposed to aggressive environment. This fact has to be taken into consideration if service life of reinforced concrete structures is to be predicted in a realistic way.

Influence of Frost Damage on Chloride Penetration into Concrete

In most national and international codes for durability design, service life is estimated after selection of one single and dominant deteriorating process such as carbonation, chloride penetration or frost attack. Application of existing codes has shown, however, that the predicted service life is not reached in practice in most cases. Early damage occurs and as a consequence expensive repair measures become frequently necessary, long before the design service life is reached. One reason for this discrepancy is certainly the fact that in practice each dominant deteriorating process is usually accompanied by other aggravating processes. In this contribution capillary absorption of different types of concrete is studied first. The influence of an increasing number of freeze-thaw cycles on capillary absorption of water is studied first, then chloride penetration before and after exposure to a certain number of freeze-thaw cycles was determined experimentally. It was found that an increasing number of freeze-thaw cycles increases chloride penetration significantly, and hence reduces service life in aggressive environment. It can be concluded that for realistic service life prediction the interaction between frost damage and chloride penetration has to be taken into consideration in regions with low temperatures.