Concrete can be exposed to many harmful environmental effects, from ground water to sea water, and can receive chemical attacks. These environmental effects can lead to the concrete’s performance decrease in time and lose its function and durability completely before its service life. Transfer of water and harmful substances into the concrete and the interaction of these with the concrete is very important in terms of durability of the concrete. Such environments are described as “harmful environments” for concrete. In our article, we have compiled everything you need to know about chloride ion permeability and carbonation of concrete.
How Does Chlorine Ion Permeability Occur in Concrete?
Chloride ions found in the environment can enter into the concrete in various ways. Sea water or salty ground waters or defroster salts especially in contact with concrete are important chlorine sources.
In sea structures exposed to repetitive wetting-drying cycles, chloride ions which leaks into the concrete through sea water remains in the concrete after the water evaporates, as the repetition number increases, ion concentration also increases.
In addition, due to very think sea water droplets rising from the sea, chloride ions may settle on the surface of the concrete by being carried away significant distances.
In case of corrosion, chloride ions act as catalysts and accelerate reaction significantly. As a result of the reaction, as the chloride ion renews itself constantly, the destruction becomes constant in the reinforcement and the reinforcement finally breaks off. This event can even occur in low chlorine concentrations.
How Does Carbonation of Concrete Occur?
Certain components are required to be present in the environment for carbonation of concrete to occur. These are;
- Calcium hydroxide occurring as a result of hydration of cement (CH),
- Carbon dioxide, which is free in the atmosphere and,
- The necessary moisture for these two substances to enter into reaction. Concrete will carbonate constantly as long as these 3 parameters are present.
Carbonation starts on the surface of the concrete and advances towards the inner parts with decreasing rate. Generally, there is 0.03% CO2 in the clean air of nature. This ratio can go up to 0.3% in the polluted air of big cities and 1% in the tunnels where air circulation is not good. This increases carbonation rate of concrete almost by 30 times.
If humidity rate in the air is less than 25% or more than 90%, CO2 cannot penetrate into the concrete. Carbonation does not occur in such cases.
One of the main carbonation factors of concrete is high porosity of concrete. If the voids inside the concrete are reduced to a minimum, it will be more impermeable and more durable against environmental effects that will come from outside.
How To Ensure Resistance Against Environmental Effects According to the Standards?
EN 206 (2017) standard proposes limit values for mixture ratios for a concrete design which can be durable against these environmental factors. These limit values are maximum water/equivalent binder ratio, lowest concrete class, minimum equivalent binder content.
Considering differences in climate, geographical conditions, safety levels, using national applications is allowed in many parts of EN 206 standard. National application rules that applies to Türkiye are given in TS 13515 (2019) which is the complementary standard for the application of EN 206.
Table 1. Limit Values Suggested by EN 206 (2017) and TS 13515 (2019) Standards for Concrete Mixture According to Effect Classes
When TS EN 206 and TS 13515 are analyzed in terms of minimum binder content, it is seen that the limitation related to the minimum equivalent binder content is found in both standards, and that a limitation related to the minimum cement content is an additional requirement of TS 13515. Countries’ own experiences are observed to be effective in determining the minimum cement content.
Experimental Study: Performance Against Chlorine Ion Permeability and Carbonation
In the study, concrete’s chlorine ion permeability, capillary absorption and carbonation resistance properties have been studied according to the limit values ofXC, XD and XS environmental effect classes (TS EN 206 and TS 13515).
Table 2. A total of 12 concrete mixtures which do not include mineral additives, and which include 50% SLAG, 25% FLY ASH and 30% FLY ASH in proportion to binder mass have been prepared.
Concrete mixtures have been prepared as per the maximum water/equivalent binder ratio, lowest concrete class, minimum equivalent binder content as per TS EN 206 and TS 13515 standards in 21±1 cm slump fixed consistency, however when mineral additive is used as in the additional condition of TS 13515, they were kept independent of minimum cement content.
Equivalent binder amount is calculated as (cement + k × mineral additive) (TS EN 206). K value for FLY ASH is taken as 0.4, k value for SLAG is taken as 0.8.
Concrete mixtures have been subjected to the tests of capillary absorption (TS EN 772-11), chlorine ion permeability (ASTM C 1202) and carbonation resistance (EN 13295) determination.
Figure 1. 28-Day Capillary Absorption Coefficients (TS EN 772-11)
Figure 2. Chlorine Ion Permeability According to Concrete Mixture Ratios (ASTM C 1202)
Figure.1 and Figure.2 show that as equivalent binder amount increases, capillary absorption coefficient and chlorine ion permeability values drop.
Compared to the mixture, which does not include mineral additive, instead of the cement with the same equivalent binder amount, capillary absorption coefficient and chlorine ion permeability values drop in 50% SLAG, 25% FLY ASH and 30% FLY ASH containing mixtures.
While mixtures which do not contain mineral additives and which contain FLY ASH have high chlorine ion permeability, mixtures with 50% SLAG have very low ion permeability (Table.3). The fact that mineral additive usage decreases chlorine ion permeability value can be due to the decrease in permeable voids, this situation is quite effective in the mixture which contains 50% SLAG.
Table 3. Assessment of Chlorine Ion Permeability of Concrete as per ASTM C 1202
| Passing Electrical Load Amount (Coulomb) | Chlorine Ion Permeability |
| >4000 | High |
| 2000-4000 | Moderate |
| 1000-2000 | Low |
| 100-1000 | Very Low |
| <100 | Negligible |
Image 1. Color After 1% Phenolphthalein Solution On the Splitting Inner Surface in Carbonation Test and Measurement of Carbonation Depth
Figure 3. Carbonation Depth Values (EN 13295)
When carbonation is considered, mixtures containing 30% FLY ASH have lower carbonation depth compared to concrete which do not contain mineral additives, mixtures containing 50% SLAG show similar performance with the concrete which does not contain mineral additives (Figure.3).
When mineral additive consumes Ca(OH)2 in the concrete void solution, it causes CO2 in the environment to penetrate deeper and increases the depth of carbonation rate. And mineral additives effect of decreasing permeability creates a reverse effect which is decreasing carbonation rate (Sulapha, 2003). The reason why literature has contradictory results related to the effects of mineral additives on carbonation rate’s depth may be interpreted as the dominant effect being reflected on the experiment results.
Mixtures containing 50% SLAG and 30% FLY ASH do not fulfill the minimum cement content requirement, i.e., the condition related to XC, XD, XS environmental effect classes when mineral additive is used as per TS 13515.
However, when the chlorine ion permeability performances of mixtures are compared, although it does not fulfill the conditions as per TS 13515, compared to concretes with the same equivalent binder which does not contain mineral additives, 30% FLY ASH containing mixtures are seen to have lower and 50% SLAG containing mixture are seen to have much lower chlorine ion permeability.
References:
- TS EN 772-11 (2011) Masonry units – Experiment methods – Section 11: Capillary absorption in masonry units made of concrete, gas concrete, artificial and natural stone and determination of first water absorption rate in clay masonry units.
- ASTM C 1202 (2012) Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration.
