How does concrete deteriorate over time?

Concrete deterioration can occur through scaling, disintegration, erosion, reinforcement corrosion, delamination, spalling, alkali-aggregate reactions and concrete cracking. Concrete degradation can have several causes. Concrete can be damaged by fire, aggregate expansion, seawater effects, bacterial corrosion, calcium leaching, physical damage and chemical damage (from carbonation, chlorides, sulphates and non-distilled water). This process adversely affects concrete exposed to these damaging stimuli.

But it is not a wonder material. As concrete cures, it shrinks, which can lead to cracking. And when reacting with water, concrete does something else: it creeps, or deforms progressively over time. This has been known for decades and is included in all concrete-related calculations used in construction projects, so it is nothing new.

But the actual cause of creep remains a mystery. Corrosion is the deterioration of steel reinforcement in concrete. Corrosion can be induced by chloride or carbonation. Corrosion can lead to cracks in the concrete cover, delamination in concrete decks, etc.

Different defects can be involved in the deterioration of a given structure. The following is a brief summary of the most common defects observed in existing structures. Usually, one or more of these defects can be observed in structures; therefore, it is necessary to identify them properly. A good understanding of these different defects is necessary to obtain a more realistic assessment of the structure.

Disintegration is the physical deterioration (such as flaking) or breakdown of concrete into small fragments or particles. Erosion is the deterioration of the concrete surface as a result of moving water particles rubbing against the surface. Future deterioration of concrete structures can result in part from errors in the production, pouring or placement of concrete and compaction. For example, inadequate curing can lead to the development of micro-cracks perpendicular to the surface of the concrete, due to drying shrinkage.

Mixing and segregation can be assessed by searching for domains in the cement paste with less or no (fine) aggregate at the microscopic level, accompanied by the assessment of the presence of cement or binder agglomeration, the distribution of coarse aggregate at the mesoscale level. In fluorescence mode, the effects of processes affecting capillary porosity, such as micro-bleaching, can be identified. Inadequate compaction can reveal local areas of poor adhesion of the cement paste to the aggregate particles and excessive voids (Fig. Most of the deterioration of concrete resulting from environmental exposure occurs through water, either by the expansion of water in the freeze-thaw cycle, by contaminants carried in the water into the specimen, or by a combination of both effects.

If the concrete is exposed too quickly to very high temperatures, explosive spalling of the concrete can occur. In projects where all areas of corrosion-induced deterioration have been addressed and the repair has been carried out, the long-term benefits can be enhanced by protecting the deck against increased levels of chemicals, typically chlorides, which can be prevented from penetrating the concrete surface by using a sealer, polymer coating or latex-modified concrete coating. Patching localised areas of concrete deterioration has proven to be an effective means of increasing the life expectancy of a structural element. All known concrete corrosion and deterioration mechanisms (and there are 11 mechanisms) have a common denominator, and that is WATER.

Léger et al. have developed finite element models of structural behaviour, including the expansive behaviour of concrete gravity dams due to alkali-aggregate reactions. Concrete has enjoyed a reputation as a "set-it-and-forget-it" building material since it became popular in the mid-20th century. Due to its low thermal conductivity, a layer of concrete is often used for fire protection of steel structures.

In addition, a review of the following two documents is highly recommended to learn about concrete defects and deterioration (Reference 1 and Reference 2). For months, or years, after cooling of the young concrete, AFt crystallises very slowly as small acicular needles and can exert considerable crystallisation pressure on the surrounding hardened cement paste (HCP). This chapter investigates the increase in shear and flexural strength of a beam by carbon fibre cast in concrete using the VARTM method. Concrete from buildings that have suffered a fire and remained standing for several years shows an extensive degree of carbonation due to carbon dioxide being reabsorbed.

The researchers used a combination of experimental and theoretical techniques to determine this mechanism: they used microindentation (which involves pressing a tiny, hard point into concrete samples and observing their response) and vertical scanning interferometry (which involves imaging 3D C-S-H patches and measuring them as they shrink or grow). Figg308 and Eglinton provide good summaries of sulphate attack; 309 Swenson310 and a publication by the American Concrete Institute provide more extensive treatments. They found that when one area of the concrete was stressed (crushed, stretched or bent), C-S-H compounds dissolved in the concrete and were deposited in a nearby non-stressed area. To maximise the benefits of repairs incorporating patches that are the result of corrosion, the reinforcing steel that has been found should have the surrounding concrete completely removed.

Chloe Robinson
Chloe Robinson

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