Destructive factors of concrete

Today concretes with correspondent characteristics have not been designed in some projects due to different factors including, not having sufficient knowledge. Despite the fact that concrete structures have sufficient durability during their lifetime, they damage in aggressive environmental conditions. The effective factors in concrete damage and destruction are:   

1. Chloride ion penetration into the concrete or concrete carbonation and therefore, the corrosion of the rebar buried inside the concrete.
2. Placing concrete in freezing and thawing cycles
3. Sulfate attacks
4. The alkaline reaction of the aggregates
5. Acid corrosion
6. …….

In Iran, most reports of concrete damage are about rebar corrosion, since the volume of the rusted part of the rebar increases once corrosion starts and leads to cracking and concrete spalling if there is high corrosion intensity.  

The most important reasons for rebar corrosion (Figure.1) are chloride ion penetration and concrete carbonation. On the other hand, the projects that either have been repaired currently or in the past have not been satisfactory; since the most important note in repair operation is the connection and interface between the repair material and base concrete which is not generally complied with.  

Brief explanations regarding some destructive factors of concrete are given in the following.

Effective factors in concrete damage

1. Rebar corrosion

Concrete damage due to the corrosion of the buried rebar in the concrete reveals in the form of expansion, cracking, and finally the spalling of the concrete cover on the rebar. The corrosion has two phases or steps:

First step: the start of corrosion is a step that the protective cover or layer on the rebar due to carbonation or the chloride ion penetration damages and the corrosion begins.

Second step: the second step is the extension of corrosion that continues over time so that the structure is out of serviceability.  

Regarding the aforementioned explanations, one of the factors that protect the rebar from corrosion is the concrete cover on the rebar. The corrosion can be prevented to some extent during the lifetime of a structure by increasing the thickness and reducing the permeability of the concrete cover.

Today, impermeable concrete or concrete with low permeability can be designed by suitable design of concrete and using suitable chemical concrete admixtures.

CapcoGel D (Figure.2) is one of the materials that increase concrete durability. The CapcoGel D product by having strong pozzolanic properties reacts with the Calcium Hydroxide from hydration and reduces the secondary products from the pozzolanic reaction, concrete permeability, and finally improves concrete durability.    

Carbonation of concrete

Regarding the development of urban space and different industries, the amount of pollutants in the air has increased. Among which this carbon dioxide plays an important role in the destruction of concrete structures. 

The moisture of concrete highly affects carbonation. The more moisture is, on the other hand, the more water in the concrete pores, the less CO₂ gas release speed will be.

The greatest effect of carbonation on concrete forms is in the relative moisture of 50% to 70%. Also, the corrosion increases significantly if the structure is subjected to drying-wetting cycles.

To distinguish the carbonation of the concrete in a structure, phenolphthalein is sprayed on the concrete.

Chloride ions penetration

The chloride ion penetration is one of the important factors that destroy the protective cover on the rebar and eventually leads to rebar corrosion and concrete destruction.

The Chloride ion penetration is of significant importance in the southern shores of our country. If the concrete has a cover with a suitable thickness and adequate durability, the Chloride ion diffusion proceeds slowly therefore, the amount of chloride adjacent to the rebar during the lifetime of a structure does not reach the critical threshold. 

CapcoGel D is one of the admixtures that is used to increase the durability of concrete structures. Based on Code428, the use of Silica fume should be a minimum of 5%. Regarding the minimum of 45 silica fume in the CapcoGel D product, 11% of this product must be used to make concrete to increase the durability of the structure.

• Freezing and thawing cycles

The freezing and thawing cycles are the main reasons for damage to the structure in cold regions.

The damage due to freezing the water in the pores exists in some different forms some of the most important of which are cracking, flaking, and spalling in some cases. 

The damage happens when there is already water in the pores or water penetrates inside the concrete due to rainfall.

 The volume of the water inside the pores increases when it starts freezing and imposes pressure on its surrounding space. When the pressure is more than the tensile strength of the concrete, the concrete starts cracking. Also, the concretes adjacent to freezing and thawing cycles and deicer chemicals spall over time (Figure.4). 

Sulfate attack

When the concrete is in contact with water and the soil containing sulfate, the sulfate ions penetrate inside the concrete.

They react with the cement paste after penetration and appear in the form of concrete expansion. Therefore, the concrete cracks and increases its permeability by which the corrosive water simply penetrates inside the concrete.

Finally, the rate of damage increases. Researchers believe that the expansions related to sulfate attack in concrete are related to Ettringite (C₃A.3CS.H₃₂). 

Meanwhile, the sulfate attack can also happen in a non-sulfate environment which is because of the high amount of sulfate in the used cement or aggregates.

Generally, the sulfate attack reduces strength slowly and continuously, by damaging the cohesion properties among hydration products.

The alkali reaction of the aggregates

The alkaline reactions of the aggregates one of which is the Alkali-Silica reaction will happen due to chemical reactions between alkali ions in Portland cement, hydroxyl ions, and some of the silica minerals in the aggregates. It should be noted that the occurrence of the reactions is conditional on the simultaneous presence of 3 factors reactive potential aggregates (Silica), moisture, and alkali (Figure.6).  

Concrete destruction due to acid corrosion

When the concrete is adjacent to acid solutions, it may be disintegrated depending on concrete permeability, acid density, and its type, hydrated components such as Portlandite, Sulfoaluminates, and calcium silicate hydrate gel. 

The greater the solubility of the produced salts, the faster and more severe the concrete damage will be.

The acids damaging concrete include sulfuric acid, hydrochloric acid, nitric acid, acetic acid, Humic acids, and solutions containing carbon dioxide.

Based on EN 206-1 Standard, the acid attacks in terms of pH or the amount of aggressive CO₂ due to underground water and soil are divided into 3 categories and their properties are provided in Table.1.

Table.1 Categorization of acid attack based on EN 206-1