Concrete is one of the most common materials used in construction. Due to the technical and physical characteristics, it can be used in the construction of critical load-bearing structures with sufficient operational properties. But concrete is not without drawbacks, including negative carbonization processes. What it is? Concrete in natural air with carbon dioxide (CO2) inevitably comes into contact with chemical acid elements. The reactions arising against this background can adversely affect the structure of the structure, which leads to the need to control the level of neutralization of the material.
Carbonation Overview
A sharp decrease in the reliability of concrete structures can be due to various factors, most of which are associated with adverse environmental conditions. Carbonization is one of the key reasons why not only the destruction of the outer protective layers of concrete structures occurs, but also the deep processes of deformation. The most dangerous consequences include cracking and peeling, as well as the creation of conditions for the development of corrosion. The latter applies to reinforced concrete structures having in their structure not fiberglass, but traditional steel reinforcement.
Depending on the characteristics of the air environment, destruction processes may occur already in the first months of operation. Therefore, the timely adoption of measures to protect structures is so important. This area of research is regulated by GOST. Concrete carbonation is given attention in document 31384-2008, dedicated to the protection of building structures from corrosion. In particular, a description is given of two interrelated characteristics - durability and permissible depth of structural damage. At the same time, there are diagnostic problems caused by difficulties in accurately predicting the parameters of an aggressive environment. Difficulties arise when analyzing the temperature and humidity conditions and the degree of concentration of CO2.
Chemical carbonization processes
The operation of concrete and reinforced concrete structures in air is accompanied by the influence of acid gases on the material. Since carbon dioxide usually has a higher concentration than other compounds, its effect is especially pronounced. So what is called carbonization of concrete? This is the process of neutralizing the porous material, in which the absorption of carbon dioxide, oxygen and moisture, which is contained in the air. How active the carbonation process will be depends on both the concrete itself and the characteristics of the aggressive environment. Of key importance is the concentration of CO2, which varies on average from 500 to 6000 mg / m3. Structures with the presence of metal reinforcement during carbonization and against the background of tight contact with the acidic medium begin to corrode, as a result of which rust also progresses.
In its pure form, concrete also undergoes negative carbonization processes. Carbon dioxide interacts with clinker minerals and cement stone. Even a small percentage of CO2 in the air triggers neutralization reactions for concrete. Carbonization occurring during the interaction of acid with calcium hydroxide depends on air humidity, porosity and permeability of the material, pressure and temperature. Ultimately, such processes leave hydrated neoplasms with a wide range of by-products in the form of alumina, hydrated silica and iron oxide, when it comes to metal structures.
Carbonization flow rate
The highest carbonization rate indicators are recorded at an air humidity coefficient of 50-60%. Film moisture that fills the pores of the surface of a concrete structure, as it increases, creates the conditions for negative reactions. At the same time, the microcapillary pores of the material at this point should not be filled.
Conversely, a relative humidity of about 25% virtually eliminates the carbonation process due to lack of moisture. The upper limit, close to 100%, is also unfavorable to this chemical reaction. This is due to the fact that micropores undergo capillary condensation processes that reduce the possibility of diffuse permeability.
Are there any features of the reaction indoors? In this case, we can talk about the rapid carbonization of concrete, the determination of which completely eliminates the factor of periodic wetting of the material by precipitation. Another thing is that in the room there may also be factors that slow down carbonation. Firstly, when it comes to places with a normally high level of humidity (kitchen, bathroom). Secondly, a dense textured layer on the surface of the concrete structure physically prevents the intensive passage of carbonization. In street conditions, such concrete is often called non-carbonizable.
The concept of carbonation depth
The degree of absorption of gases and moisture is a determining quantity in terms of assessing carbonation. In air, as already noted, penetrating reactions occur more slowly due to the regular saturation of concrete with moisture. For example, a structure in which the proportion of cement is about 250 g / m3 has a water-cement ratio of 0/60, which allows carbonization to advance by 5-6 mm in the first year of operation. In destroyed structures with exposed reinforcing rods, the depth can reach 30 mm. In this part, it is worth mentioning the concept of local carbonization of concrete, the definition of which is related to the parameters of the mortar aggregate fraction. So, local carbonization exceeds the average absorption of the surface of the structure by the size of the diameter of the coarse grain used cement filler. The unevenness and the very presence of zones with increased absorption capacity is explained by violations of the technology for preparing the cement mixture and its laying. In other words, the solution in some areas was denser than in others.
Determination of concrete carbonization depth
Various methods and tools are used to determine the damaged layer on the surface of a concrete structure. Usually use indicator tests, coupled with carbometric physico-chemical methods. A quantitative indicator will be the degree of transition of the cement stone to the form of carbonate. This characteristic is not directly related to the penetration depth of carbonization, but is more accurate in terms of identifying a damaged and unreliable layer. In the construction sector, the depth of concrete carbonization is recorded as a result of field surveys of target objects. Such an analysis is one of the preventive operations in the overall package of maintenance activities. The terms of reference for the analysis of damage to concrete structures can also contain related tasks aimed at determining the degree of environmental impact, the detection of voids, cracks and air pockets inside.
Concrete Carbonization Assessment Tools
For carbonization tests, vessels like desiccators are used. As for the workpieces, in laboratory conditions, the use of two cylindrical concrete samples with a thickness of 5 cm is practiced. The material is coated with epoxy resin for complete insulation, and then kept in a desiccator under a solution of sodium chloride for two days. Further, with the help of a caliper, the diameters of the cylindrical blanks are re-taken. The results are logged with the designation of the surface area. At the next stage, the samples are installed in vessels to determine the kinetic dependence during carbonization. Concrete after tests is easily exposed to destructive effects. By splitting the sample, one can estimate the penetration depth of the solution. It will testify to the ability of a concrete concrete mixture to undergo carbonization processes. In fact, in this way, the values of the volume of CO2 are derived, which theoretically can be absorbed by a particular brand of concrete in certain conditions.
The use of phenolphthalein to evaluate carbonation
When examining concrete structures to identify damaged areas, marking agents are often used, the most popular of which is phenolphthalein solution. In an acidic medium, it acquires the structure of a lactone and loses its color, however, upon transition to an alkaline chromophore group, a pronounced red or violet color appears. How is the determination of concrete carbonization phenolphthalein? The standard test kit includes a 1000 ml phenolphthalein solution, two 250 g washes and a depth gauge. The surface of the sample must be sprayed with the prepared mixture and, as a result of the decrease in alkalinity that will be caused by the carbonization processes, determine the depth of the damaged layer. In combination with the measurement of protective layers of the structure, this method allows us to analyze the readiness of the material to resist corrosion processes.
When organizing a survey, it will not be out of place to initially think over the analysis tactics. A comparative analysis using an unpainted sample may be required. In the case of serial tests of concrete for carbonization, phenolphthalein samples are removed on a fresh fracture. That is, it is necessary to prepare means for operational physical destruction of the structure of a concrete sample. In addition, destructive testing makes it possible to comprehensively evaluate the design for other important technical and operational qualities by measuring instruments of a different spectrum.
Methods for the restoration of concrete after carbonization
Repair of local areas subject to carbonization processes may be in the nature of a preventive measure or carried out as a major reconstruction with the complete replacement of the problem part of the structure. In both cases, adhere to the principle of restoration and preservation of the passive state of concrete during carbonization. The definition of methods for protecting and restoring the material can be represented as follows: methods and means aimed at reducing the ability of concrete to moisture absorption and oxidation, as well as technical strengthening of the target structure due to physico-chemical treatment. Among the main methods are the following:
- Increasing the thickness of the protective layer by using special primers.
- Replacing the carbonized layer with mechanical removal of signs of chloride ion penetration. After surface cleaning, a protective insulating coating is also applied.
- Recovery of alkalinity of concrete. Use electrochemical effects using conductors with cathodes. Properly selected electrolytes can also provide a passive state of metal reinforcement in concrete.
- Alkalinity recovery due to ion diffusion. High-alkaline solution is laid on the surface, which allows stimulating the necessary chemical balance for the stability of the material.
Carbonization prediction
The need for inspection of structures for their protection against carbonization is due to the desire to prevent negative destruction processes. Comprehensive forecasting is usually performed in relation to engineering reinforced concrete structures experiencing heavy loads. In particular, it can be columns, bridges, floors, etc. The durability and reliability of the design is determined by both internal and external factors. Part of the initial data, which are subsequently used in forecasting, is laid down at the stage of designing the structure, when it remains possible to select the most favorable parameters when creating a mortar. Already during operation, prediction of concrete carbonization is based on the dynamics of changes in material properties, the degree of cement hydration, and external conditions. For example, the methods of non-destructive testing can determine the current state of cement stone, determining its anticorrosive ability.
Conclusion
Along with wood, concrete is one of the most unstable materials in terms of sensitivity to various factors of physical and chemical effects. And this applies not only to negative phenomena. By default, in vivo, the cement structure is gaining strength over many years. The opposite importance will be the carbonization of concrete. What is it from the point of view of influencing the performance of a structure? Carbonization leads to degradation of the surface structure of concrete, reducing its technical quality. But the main danger lies in the creation of conditions for the development of rust. The defeat of the reinforcing cage in a system with dynamic loads can lead to a complete loss of the necessary technical properties of the object in the very first years of operation. That is why it is so important to timely diagnose concrete, while also taking measures to protect and strengthen it.