Problems / Pathologies
Common problems found in concrete monuments
Reinforced concrete was the material of choice for many architects of the modern era, and they exploited the material in a multitude of creative and innovative ways. Many of the modern era's most extraordinary structures demonstrate the potential of reinforced concrete and illustrate the material's rapid evolution over the twentieth century. Like many modern materials, reinforced concrete has raised new and distinct conservation challenges. These issues relate to the lack of appropriate techniques and materials to meet conservation needs, the current lack of knowledge on the efficacy and durability of existing repair solutions, the shortage of training opportunities, and the lack of technical and guidance resources available to professionals.
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Chloride Induced Corrosion
The most common cause of decay of a reinforced concrete structure is actually not due to a chemical interaction with the concrete but with a major component of a reinforced structure: the steel reinforcement. This reinforcement embedded in the concrete is naturally protected by the surrounding alkalinity.
Chloride Induced Corrosion
Carbonatation
The second most common cause of decay due to chemical origin is the phenomenon of carbonation. Concrete is almost always in contact with the ambient air and therefore subjected according to the location of the structure to more or less strong action of carbonic gas.
Carbonatation
This free-lime is vulnerable to the carbonic gas dissolved in water and creates calcium carbonate based on the following formula:
Ca(OH)2 + CO2 + H2O → CaCO3 + 2H2O
The carbonation has for consequence a reduction of the pH level of the concrete from 13 when non-carbonated down to below 9. If the carbonation front reaches the level of the reinforcement, the steel is no longer naturally protected by the high alkalinity and hence in case of moisture and oxygen at this level, corrosion may occur.
Sulfate Reaction
Sulfate is present in cement and early formation of ettringite (needle crystal) normally occurs in the concrete or mortar. This is due to ground gypsum added at the cement plant which reacts with calcium aluminates of the cement (setting time regulation) during the first hours after addition of water.
Sulfate Reaction
It is also due to sulfate-based expansive agents in shrinkage compensating concretes, mortars.
Concrete decays arise from sulfate reaction when either additional sulfates are penetrating the concrete or late release of sulfate by e.g. gypsum contaminated aggregates. This phenomenon is often called as Delayed Ettringite Formation (DEF) or “Secondary” Ettringite. It occurs heterogeneously and later (after months or even years). The related expansion produces cracking, spalling and strength loss since it occurs in a rigid, stiff, hardened concrete.
Alkali-Silica-Reaction (ASR)
This is the reaction from reactive silicate based aggregate with alkaline elements in presence of moisture. Alkali-aggregate reaction was first identified as a cause of concrete deterioration more than 60 years ago (Stanton, 1940).
Alkali-Silica-Reaction (ASR)
It is widely accepted that the three essential components necessary for ASR-induced damage in concrete structures are:
- Reactive silica (from aggregates)
- Sufficient alkalis (mainly from Portland cement, but also from other constituent materials);
- Sufficient moisture.
In a second stage, this silicate gel reacts with the lime present in the concrete to form a new gel that can absorb a great quantity of water and having the property of swelling. This swelling effect generates an expansion in the harden concrete. Typical patterns of ASR damage are in the form of crazing going deep in the concrete structure.
Other Attacks (Sea water, Pure water)
Sea water, a part of the decay due to the penetration of chloride, is one of the most aggressive media for the concrete. Its attack comes from the reactions more or less simultaneous between sulfates, chloride and the various constituents of the cement.
Other Attacks (Sea water, Pure water)
Pure or soft water attack is indeed less common and may happen in bridges in mountain area where the water from the river is still not so charged with mineral or on the under face of bridges where condensation phenomenon may occur. The pure water attack is due to the leaching away of the Portlandite by the soft water.
Shrinkage
Concrete may suffer from decay due to uncontrolled restrained shrinkage. Shrinkage is a chemical-physical phenomenon that always exist in cement based material and start from early age (actually some sort of shrinkage occurs when the cement has not yet hardened) until very late in the age of the structure.
Shrinkage
Main visual effect due to the shrinkage is cracks formation. These cracks may appear either very early after casting the concrete e.g. settlement cracks, immediately after removing the formworks or few days, weeks or month later due to long term shrinkage.
Freeze and thaw
This is mainly occurring in countries or areas subjected to harsh winter conditions and it is very often accentuated by the presence of de-icing salts used in the bridge to reduce incidence due the frost formation on the road. This is due to improper design of the concrete that does not contain enough properly dispersed air network that provide enough space for the water to expand when freezing.
Freeze and thaw
Main symptoms are the scaling present at the surface of the concrete and swelling of all or part of the structure together with cracking behaviour. When to the extreme, the structure may completely get destroyed as shown in the enclosed picture.
As example, a survey carried out in UK in 1997 has shown that in average 10% of the reinforced concrete structure were affected by incidences of freeze-thaw cycles.
Decay due to other origins
Many other origins may be the causes of concrete decay such as for a bridge structure, accidental chock from a vehicle, abrasion, erosion in bridges over rivers, etc…
Decay due to other origins
Other important sources not detailed in this article but amounting for many failures, are fault of execution during the construction works, e.g. design error, improper spacing, honeycomb due to insufficient vibration or too stiff concrete, damages resulting from fire, etc…