reinforcement Corrosion

From a durability point of view, corrosion of steel reinforcement in concrete is the most frequent cause of damage in reinforced concrete structures. Structures with chloride contamination from de-icing salts or seawater are particularly affected. If the chlorides penetrate into the reinforced concrete up to the reinforcement level, the passivation (i.e. the corrosion protection) of the reinforcing steel is lost. Carbonation of the overlying concrete can also lead to loss of passivation of the reinforcement.

Concerns primarily: parking garages, transportation infrastructure, offshore structures.

 

Alkali-Silica-Reaction (ASR)

ASR can occur if the aggregate used in the concrete contains alkali-reactive constituents. With moisture and alkalis, these constituents react to form an expansive alkali-silica gel. Colloquially, this reaction is known as "concrete cancer." Depending on the reactivity of the aggregate and the external boundary conditions, this reaction may occur already after  a few years or only after several decades.

A certain amount of alkalis are introduced into the concrete via the cement. A cement with a low alkali content can thus reduce the ASR risk. In many infrastructure structures, especially in transport infrastructure, external alkalis act on the concrete in the form of de-icing agents, thus aggravating the attack.

Concerns primarily: Transport infrastructure - especially concrete pavements; Hydraulic structures.

Sulfate attack

Sulfates in groundwater or soils can react with the hardened cement paste in the concrete to form expansive minerals (e.g. ettringite). In extreme cases, the resulting expansion can lead to a loss of the concrete load-bearing capacity. A special case is the formation of thaumasite, which occurs primarily at temperatures <10 °C. In this case, the strength-bearing phases in the hardened cement paste are destroyed and concrete loses its strength.

Concrete can also be damaged by an internal sulfate attack, if high temperatures prevent primary ettringite formation in the first hours after casting. Instead, metastable mineral phases form, which can later lead to expansive reactions in the hardened concrete, when it is exposed to a moist environment.

External sulfate attack: Components in contact with the ground - e.g. foundations, tunnels, base courses.
Internal sulfate attack: precast elements, massive structural elements

Frost and Salt Frost Attack

During the cold season, concrete can be damaged by the combined effects of freeze-thaw cycles and moisture. The damage processes are intensified if de-icing salts additionally act on the concrete. In practice, damage mostly occurs in the form of surface scaling. Particularly for structures with requirements on surface properties, this can lead to premature loss of the desired service properties.

Concretes based on cements rich in clinker usually have good resistance if they are produced as air-entrained concretes. With increasingly clinker-reduced concretes, it will be more challenging to ensure high frost and salt frost scaling resistance in the future.

Concerns primarily: Transportation infrastructure (bridge caps, concrete pavements), hydraulic structures, paving stones.

Construction defects

For the majority of concrete structures, it can be assumed that they are produced in accordance with the requirements and without defects. Despite all this, in some cases imperfections occur that can lead to a restriction in use or a reduction in service life. There is a tendency for construction practices to become more challenging, as the increasing use of clinker substitutes, RC materials and ecologically optimized concretes can reduce the robustness of concrete construction.

 

Typical problems are:

• Air entrained concrete (trowelling, excessive air entrainment, admixture interactions)

• High slump concrete (sedimentation resistance)

• Binder-admixture interactions

• Curing practices