by Abstract: Based on the analysis of carbonation depth and rebound strength detection of concrete components, this paper determines the influencing factors of components, so as to deeply explore the impact of concrete carbonation on the detection results, so as to provide an effective reference for more scientific detection.
1 concrete carbonization and testing mechanism 1.1 mechanism of concrete carbonization concrete carbonization is actually a physical process.
When the cement stone in the concrete interacts with the carbon dioxide in the environment, it is a very complex process.
In the process of cement hydration, due to the influence of free water vapor and chemical shrinkage, the cement stone will become another substance, which has pores, solid phase and liquid phase mixed substances.
As we all know, concrete is a porous body.
There will be bubbles, capillaries and some gaps in the concrete, and even obvious defects.
In the environment full of carbon dioxide, the gas will penetrate into the concrete from these cracks, and there will be a lot of air inside.
The pores and capillaries will be filled with carbon dioxide gas.
Under liquid erosion, it will dissolve in pores and eventually form carbonic acid.
Finally, carbonization reaction occurs with various chemical products in cement stone, and new substances are generated in the carbonization process.
1.2 discoloration mechanism of phenolphthalein concrete carbonation depth test is mainly based on the fact that phenolphthalein will show different colors in different pH environment.
Phenolphthalein is a common weak organic acid.
When it is in acidic and neutral aqueous solution, it generally exists in colorless lactone structure.
When it belongs to weak alkaline solution, it generally exists in purplish red quinone structure.
In strong alkaline solution, the purplish red of phenolphthalein will fade and appear colorless; Orange in strong acidic solution.
The quinone type or quinone acid salt of phenolphthalein is very unstable in alkaline medium, and it will slowly transform into colorless carboxylate type, so the red color will slowly fade after concrete carbonization detection; Phenolphthalein will immediately change into colorless carboxylate formula when encountering concentrated alkali solution.
Therefore, when phenolphthalein reagent drops into the concentrated alkali solution, phenolphthalein will have new changes, phenolphthalein will gradually turn red, and soon the red will gradually fade into colorless.
Generally speaking, phenolphthalein solution will have new changes and color changes with different pH.
1.3 cement clinker the main components of cement clinker are tricalcium silicate and dicalcium silicate, which will produce a large amount of calcium hydroxide during hydration.
Therefore, when the concrete is not carbonized, there will be a large amount of calcium hydroxide in the concrete components.
After testing, the pH of concrete without carbonation is as high as 12.23.
In general, the pH is between 8 and 10 before the concrete is carbonized.
When the environment is natural, the carbonation of concrete gradually spreads from the inside to the outside.
With the aggravation of carbonation reaction and the extension of time, the pH of concrete components will show different phenomena from inside to inside.
According to the change principle of phenolphthalein, when phenolphthalein has changed to a certain extent, once the carbonated concrete is met, the pH at this time will be less than 8, and it will be colorless at this time.
When the concrete is not carbonized or not completely carbonized, the pH at this time will be between 8 and 10, and it will appear purplish red at this time.
When the pH of concrete increases gradually and is greater than 10, phenolphthalein will be colorless.
2 Effect of concrete carbonization on rebound of concrete members 2.1 effect of concrete carbonization on durability of reinforcement concrete carbonization will gradually reduce the alkalinity of concrete.
After carbonization, the pH that has been completely carbonized will gradually decrease from 12 to below 9.
Generally, the passivation film covered on the surface of reinforcement will be damaged more seriously.
When the film is damaged, it will lose its protective effect, resulting in the loss of protective effect of reinforcement and corrosion.
The interaction between carbon dioxide gas in the atmosphere and alkaline substances in concrete is a very complex and changeable chemical reaction.
It is based on continuous in liquid phase, gas phase and solid phase.
When concrete carbonization is in progress, carbon dioxide in the air will gradually diffuse into the capillary holes in the concrete from inside to outside.
Under the influence of cement mixture, it will interact to form calcium carbonate, which will reduce the alkalinity of concrete and gradually reduce the alkalinity value.
When the carbonization layer gradually carbonizes to the reinforcement, the passivation film of the reinforcement will be destroyed.
Once there are corrosion conditions in the surrounding environment, the corrosion of reinforcement will be more obvious, and it will be found that the volume will gradually expand, a large number of cavities will appear, and the surface will become loose.
Such a surface will easily absorb water molecules and have strong air permeability.
This time will aggravate the corrosion of reinforcement and cement.
Once the corrosion of reinforcement increases gradually, the volume will expand.
Once the internal stress exists, the concrete will crack, and the surface layer will collapse over time.
2.2 excessive carbonation will sharply reduce the rebound value of concrete components.
Carbonation will lead to expansion, cracking and shrinkage of concrete.
When concrete is carbonated, a large amount of CaCO3 will be generated, which will lead to the brittleness of concrete.
Both shrinkage and expansion of concrete will affect the surface structure.
When springback occurs, the springback will absorb part of the projectile impact energy, which will reduce the springback value.
In addition, the shrinkage rate of concrete will be increased during carbonation.
When the shrinkage rate is relatively high, cracks on the concrete surface will be accelerated, which provides favorable conditions for a large number of corrosive media to enter the concrete internal component structure.
Once the carbonation of concrete is accelerated, the rebound value will decrease rapidly.
Once it decreases, the carbonation depth will increase, which will affect the concrete rebound effect and greatly reduce the setting value..
