Additives in concrete: what they are and why they are used?
PLASTMIX - MEDIUM PLASTICIZER
SUPERMIX – SUPER PLASTICIZER
POLIMIX - SUPER PLASTICIZER ON POLYMERIC BASIS
POLIMIX JBI - SUPER PLASTICIZERWITH HIGH LEVEL REDUCTION WATER
AIRMIX-REDOMIX-VODOMIX – SUPPORTING CONCRETE ADDITIVES
Preparing to fill the foundation you need to get maximum information about the concrete - the basis of any monolithic structure. Choosing the optimal ratio of components of the initial mixture (water-cement-sand-crushed stone), do not forget about additives that improve its properties.
The range of construction chemicals today has hundreds of items. Without knowing what concrete additives are, you cannot work in extreme weather conditions and expect high quality.
In this article we will look at the most common types of additives in concrete mixes, examine the mechanism of their action and evaluate the effect of their usage.
Variety of concrete admixtures
The need to change the properties of concrete caused by the following factors:
- Climatic (low and high air temperatures).
- Technological (acceleration of work, improvement of workability, the need for transportation).
- Operational (increase strength, water and heat resistance).
- Financial (cement savings).
- Chemical aggression of the environment (sea salt, groundwater with a high content of substances that destroy cement stone).
The use of additives for concrete allows builders to change its properties in a wide range, improving the quality of structures erected.
Classifier of modifying compositions includes several groups:
- plasticizing agents;
- air-entraining and gas-generating;
- accelerating (slowing) hardening of concrete;
- coloring (pigment).accelerating (slowing) hardening of concrete;
Why do we need plasticizing additives?
It may seem to a construction beginner that the concrete mix does not need to increase plasticity. Pour more water, and the concrete without any additives will be liquid and convenient for installation. In fact, excess water is the enemy of concrete.
There is a strong relationship between the amount of cement and water, at which the solidifying mixture gains maximum strength. Builders call it water-cement ratio. Simply put, if you want to make high-quality concrete, then no more than 7 liters of water can be added to 10 kg of cement. Having poured more liquid, you will sharply reduce a design durability (concrete porosity will increase).
The denser the concrete, the better it resists loads. Therefore, for example, for the production of paving slabs, no more than 4 liters of water are added to 10 kg of cement. How to distribute such a stiff mixture in the formwork evenly, without voids and sinks? Here to help builders come plasticizing additives. They make concrete flow without adding extra water.
Plasticizers for concrete - the most popular types of additives. By reducing the adhesive force of the particles of the initial mixture, they protect it from delamination. Using such additives, you can fill out the casting mold and thin-walled foundation formwork without a vibrator.
On sale this type of additives is found under the names of super and hyperplasticizers. The difference between them lies in the strength of the action. Hyperplasticizing additives reduce the amount of water without reducing the strength of concrete by 20%. In superplasticizers this indicator does not exceed 10%.
Another positive effect from the use of plasticizers is cement saving. By reducing the water content in concrete, it is possible to proportionally reduce the mass fraction of cement without disturbing the “water-cement balance”.
Why cement needs air?
The effect of air-entraining additives is clear from their name. For beginners, the need to use them is not always obvious. Air cavities in concrete reduce its strength. However, in small quantities they are useful because they increase the frost resistance of the monolithic structure.
The mechanism of action of air-entraining substances is very simple. Water, freezing, tears up concrete like paper. Creating air cavities in the material, we give water a place to expand and reduce its destructive force. This task is performed by air-entraining compositions. Их процентное содержание должно быть минимальным, чтобы не снизить прочность конструкции.
The gas-forming additives in action are similar to air-entraining. They also create micro-cavities in concrete, but their main task is different: to reduce the weight of the concrete and increase its heat-insulating properties. Therefore, gas-forming substances are used in the production of wall materials (foam and gas blocks).
Accelerators and retarders of curing
Considering the modifying additives, it is necessary to say a few words about the compositions that increase the rate of hydration of the cement stone. In cold weather, the concrete mix hardens very slowly, but in the frost this process stops. If you do not add a hardening accelerator to the water used for mixing, it will be impossible to work with concrete in sub-zero temperatures. Alternative option - electric heating of the mixture is quite complicated and expensive.
Slowing the rate of maturation of the concrete mix is necessary when transporting it over long distances. In addition, retarders are necessary when concreting massive and extended structures (foundations and floors). In this case, the technology requires that all concrete be in a liquid state until the end of the pouring. Without the use of retarders, portions of the foundation that were filled earlier than others begin to harden, forming joints that weaken the structure.
The denser, the stronger!
There is a direct relationship between the density of concrete and its strength characteristics. Sealer additive reduces the number of pores in concrete, filling the micro voids between its particles. Together with the increase in strength, an increase in the water and frost resistance of the structure occurs (the complex effect of improving the properties).
Consumption of sealing additives is low (200-600 grams per 100 kg of cement), and the achieved effect significantly exceeds the cost of purchasing.
We do not care about frost!
There are two ways to fight the destructive force of frost. We have already said about the first one, considering the hardening accelerators. The second method is antifreeze additives that give water antifreeze properties. When freezing, not at 0 ° C, but at minus temperature (down to -25°C), water has time to fully react with the cement.
It should be noted that many types of hardening accelerators have an antifreeze effect on cement mortar (sodium nitrite, a mixture of sodium chloride with calcium chloride, urea in combination with nitrite and calcium nitrate). This fact must be considered when choosing.