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The bubbles in concrete above 100μm are called large harmful bubbles, 100 ~ 50μm are called harmful bubbles, 50 ~ 20μm are called low harmful bubbles or harmless bubbles, and those below 20μm are called beneficial bubbles.

When the air content in concrete is appropriate, micro-bubbles have a certain stability in the process of concrete construction under the condition of uniform distribution and airtight independence. In terms of concrete structure theory, the voids formed by these tiny bubbles belong to the range of capillary pores or harmless pores or less harmful pores, which not only will not reduce the strength, but also greatly improve the durability of concrete.

Causes of bubbles:

1. Raw materials

Effect of cement on Bubble formation

When grinding aids are used in cement production, there are usually too many bubbles, and the alkali content in cement is too high, the fineness of cement is too fine, and the air content will increase. The amount of cement and water consumption is also an important cause of bubbles. In the trial blending of concrete, the amount of cement is mainly considered for the strength of concrete. if the amount of cement is increased on the premise of meeting the strength of concrete, thus the water-cement ratio is reduced, the amount of air bubbles will be greatly reduced. Its function is to use excess mortar to fill the remaining space, so as to block the bubble formation space.

Effect of coarse and fine aggregate on bubble formation

According to the grain gradation compaction principle, in the process of construction, the material gradation is unreasonable, the coarse aggregate is too much and the size is inappropriate, the content of needle-like particles is too much, and the actual sand rate used in the production process is smaller than that provided by the laboratory. Such fine particles are not enough to fill the coarse material voids, resulting in uncompacted particles and forming free voids, which provides conditions for the generation of air bubbles.

2. Construction technology

Effect of stirring time on Bubble formation

The mixing time also has different effects on the bubbles produced in the concrete. In the process of concrete mixing, if the concrete is mixed unevenly, the same water-cement ratio will make the admixture not be uniformly distributed, and there will be more air bubbles in the surplus part of the admixture, while the part that has not been mixed to the admixture will appear the phenomena of uneven slump, large slump, segregation and so on, but excessive mixing will make more and more air bubbles formed in the mixing process of concrete, thus producing negative effects.

Effect of release Agent on Bubble formation

The pure oil release agent will still be used by some construction units, often using the waste oil recovered from the machinery factory, which has a strong adsorption to air bubbles. Once there are air bubbles in the concrete, it will be adsorbed on the formwork and formed on the surface of the concrete structure. There is to pay attention to the selection of release agent should be combined with the type of template for adaptive selection, so that the use of combination can reduce the amount of bubbles.

Effect of Vibration on Bubble formation

Due to the different vibration environment in the construction, the operation of the vibrator has different effects on the number of air bubbles on the concrete surface. As a concrete structure, the better the vibration, the denser the internal structure of the concrete. from the two aspects of the height and time of the layered vibration, the higher the height of the layered vibration, the less likely the air bubbles inside the concrete will be discharged. The insertion speed of the vibrator will also affect the discharge of air bubbles, requiring "quick insertion and slow pull-out", that is, the insertion speed should be fast, so that the upper and lower concrete will be vibrated almost at the same time, and it will be slow when pulled out, otherwise the position of the vibrator is not easy to be filled and form a gap, which is not conducive to the discharge of air bubbles. However, with the emergence of overvibration, undervibration and leakage vibration, there will be more and more air bubble defects on the surface of concrete. Supervibration will cause the tiny air bubbles in the concrete to break up and recombine under the mechanical action, from small to large. Both undervibration and leakage vibration will lead to uncompacted concrete natural voids or air-type irregular bubbles.

How to deal with the bubbles?

1. It is suggested that priority should be given to the use of cement with few bubbles and low alkali content.

2. For aggregates, the aggregate size gradation and needle-like particle content should be strictly controlled, and the unqualified materials should be carefully screened when preparing materials. Choose reasonable aggregate gradation and sand ratio.

3. Pay attention to the mixing time, so that the admixture can be fully and evenly mixed with other raw materials.

4 It is recommended to use high quality mold release agent, and check whether the template and release agent match. The template should be kept smooth and clean, and the release agent should be applied evenly but not too much or too thick.

Concrete Additives Supplier

TRUNNANO is a reliable concrete additives manufacturer with over 12-year experience in nano-building energy conservation and nanotechnology development.

If you are looking for high-quality concrete additives, please feel free to contact us and send an inquiry. (sales@cabr-concrete.com)

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Polycarboxylate superplasticizer has more advantages than traditional naphthalene and aliphatic superplasticizer in mixing high strength grade concrete, but its advantages of high water reduction cannot be brought into full play when the strength is not greater than C30.

In order to make polycarboxylate superplasticizer play a better role in low strength grade concrete, attention should be paid to:

Reduce the solid content of polycarboxylic acid superplasticizer

The higher the solid content of polycarboxylic acid superplasticizer is, the lower the content of polycarboxylic acid superplasticizer is. In the preparation of medium and low strength concrete, if polycarboxylic acid with high solid content is used, the admixture content is often too low, the weight of admixture increases 0.5kg/m or water consumption increases 5kg/m, concrete mixture will change greatly, sometimes segregation, bleeding, consolidation, and even pipe plugging during construction, on the contrary, reduce 0.5kg/m superplasticizer or water consumption reduce 5kg/m. The workability of concrete mixture is also difficult to be guaranteed. Therefore, when using polycarboxylic acid superplasticizer to prepare medium and low strength concrete, it is appropriate to use polycarboxylic acid superplasticizer with solid content of 8% ~ 10% (when the content is 1.8%, the water reducing rate is controlled at about 20%). Low solid content polycarboxylic acid reduces the sensitivity of mixing amount and water consumption. When the moisture content of sand and gravel fluctuates, it can effectively prevent the workability of concrete from fluctuating, which is convenient for production operators to control.

Select the appropriate water consumption to ensure the necessary slurry volume

Polycarboxylic acid superplasticizer has low content (0.15%-0.35%) and high water reduction rate (water reduction can be up to 35% or even higher at high content). In terms of reducing water consumption, polycarboxylate superplasticizer has more advantages than traditional naphthalene and aliphatic superplasticizer. The water-binder ratio of medium and low strength concrete is often on the high side, for example, the water-binder ratio of C30 is 0.42-0.45 and that of C20 is 0.5-0.55. If the water consumption is reduced excessively through high water reduction rate, the amount of concrete cementitious material will often be reduced, and the amount of concrete mixture paste will be insufficient, so it is difficult to maintain the good workability of concrete mixture and bring difficulties to concrete construction. Therefore, when using polycarboxylate superplasticizer, appropriate water consumption should be selected to ensure a certain amount of pulp, such as: C30 water consumption 165-175 kg C20 C20 water consumption 175-185 kg C15 C15 water consumption 180~190kg/m. When using polycarboxylate superplasticizer to prepare medium and low strength concrete, attention should be paid to selecting the matching amount according to the water consumption, so as to make the concrete mixture have good workability. At the same time, attention should be paid to properly adjust the sand ratio to maintain the water retention and fluidity of low strength grade concrete.

Pay attention to the adjustment of aggregate gradation and voidage

The paste content of low strength concrete whose strength grade is lower than C30 is generally low. in order to make the concrete mixture have good workability, attention should be paid to the adjustment of aggregate gradation so as to reduce the aggregate voidage. The lower the voidage of the aggregate, the less the paste used to fill the gap between the aggregate, and the larger the paste wrapped on the surface of the aggregate, the more conducive to the improvement of the workability of the concrete mixture. For example, the voidage of pebbles is reduced from 43% to 38%. Due to the reduction of 50L/m in the paste filling the coarse aggregate voids of concrete, the concrete mixture paste is equivalent to increasing 50L/m, which greatly improves the workability of concrete.

Pay attention to the change of sand fineness modulus

Due to the small amount of low strength concrete paste, when polycarboxylic acid superplasticizer is used, the inclusiveness of concrete paste is poor, the sand fineness modulus should be reduced appropriately, the medium sand with sand fineness modulus less than 0.315 should be selected, and the particle content of 20% should be guaranteed. When the voidage of coarse aggregate of concrete is large, the concrete is prone to the phenomenon of slurry running and stone leakage, and the concrete has no cohesion. At this time, the content of fine sand should be increased appropriately, and when there is no fine sand to adjust the fineness modulus of mixed sand, the sand ratio can be increased by about 2%. To improve workability. However, the amount of fine sand with small fineness modulus should not be too large to prevent the increase of aggregate specific surface area, the increase of water demand and the deterioration of concrete fluidity. In addition, when preparing medium and low strength concrete, it is not suitable to use coarse sand with high particle size > 1.18 because of the small amount of paste. This kind of sand should cause divergence of concrete mixture, poor cohesion and water retention, and the production is difficult to control.

Polycarboxylic acid superplasticizer should pay attention to compound water retention and thickening components

The slump-retaining effect of polycarboxylate superplasticizer is greatly affected by the content of polycarboxylic acid superplasticizer in the medium and low strength concrete whose cementitious material is lower than 350kg/m, but a large amount of polycarboxylate superplasticizer is easy to make the mixture segregate and bleed water. Therefore, when the concrete mixture has the tendency of segregation, air entraining agent or thickener can be added to the pumping agent to improve the water retention and cohesion of concrete, and polycarboxylic acid superplasticizer containing viscosity modifier can also be used.

Concrete Additives Supplier

TRUNNANO is a reliable concrete additives manufacturer with over 12-year experience in nano-building energy conservation and nanotechnology development.

If you are looking for high-quality concrete additives, please feel free to contact us and send an inquiry. (sales@cabr-concrete.com)

We accept payment via Credit Card, T/T, West Union, and Paypal. TRUNNANO will ship the goods to customers overseas through FedEx, DHL, by air, or by sea.

Natural river sand has been exhausted after decades of large-scale mining, concrete production enterprises began to use machine-made sand as fine aggregate on a large scale. Some limestone powder will inevitably be produced in the process of machine-made sand preparation, which will not only affect the performance of concrete, but also affect the effect of polycarboxylic acid admixture.

The results show that stone powder has different effects on the working properties, mechanical properties, chloride penetration resistance, sulfate resistance and freeze-thaw resistance of concrete. Stone powder can play a physical filling effect in the cementitious material system, which can reduce the water secretion of cement paste, improve the dispersion ability of cement paste, and can also be used as the crystal nucleus of calcium hydroxide to accelerate the hydration of cement particles and affect the pore structure of hydration products. Stone powder can adsorb polycarboxylic acid superplasticizer molecules on the surface, which can reduce the saturation point of superplasticizer under certain conditions, increase the initial fluidity of cement mortar, reduce the time loss of fluidity of cement mortar, and improve the compatibility between superplasticizer and cement.

Effect mechanism of limestone powder on water-reducing effect of pce powder

The hydrophilicity of limestone powder particles is low, that is, the adhesion of water molecules on the surface of limestone particles is small, which makes limestone powder have a certain physical water-reducing effect. In addition, limestone powder has a larger specific surface area than cement and has the filling effect of micro-aggregate, that is, small limestone particles can fill the pores between cement particles and replace the free water between cement particles. This is the aspect that limestone powder is beneficial to the water-reducing effect of polycarboxylic acid.

At the same time, polycarboxylic acid superplasticizer can be adsorbed on the surface of cement particles and on the surface of limestone powder, that is, there is competitive adsorption of polycarboxylic acid molecules between limestone powder and cement in the paste. this will reduce the density of polycarboxylic acid adsorbed on the surface of cement particles, and then reduce the water-reducing effect of polycarboxylic acid. In addition, the limestone powder with larger specific surface area needs more water, which will reduce the fluidity of the slurry. This is the aspect that limestone powder is not conducive to polycarboxylic acid to play a water-reducing effect.

The above analysis shows that limestone powder has both positive and negative effects on the water-reducing effect of polycarboxylic acid. In the case of low content of limestone powder, because the hydrophilicity of limestone powder is low, and the filling effect of fine particles is easy to play, the favorable effect of limestone powder on the water-reducing effect of polycarboxylic acid can effectively counteract the adverse effect, which shows that the effect of low content of limestone powder on the water-reducing effect of polycarboxylic acid is not obvious. In the case of high content of limestone powder, because limestone powder has larger specific surface area and higher water demand, and limestone powder and cement have competitive adsorption to polycarboxylic acid molecules, limestone will obviously reduce the water-reducing effect of polycarboxylic acid.

Concrete Additives Supplier

TRUNNANO is a reliable concrete additives manufacturer with over 12-year experience in nano-building energy conservation and nanotechnology development.

If you are looking for high-quality concrete additives, please feel free to contact us and send an inquiry. (sales@cabr-concrete.com)

We accept payment via Credit Card, T/T, West Union, and Paypal. TRUNNANO will ship the goods to customers overseas through FedEx, DHL, by air, or by sea.

In order to improve the early strength of concrete, some manufacturers will add triethanolamine, calcium chloride and industrial salt to the superplasticizer, especially in winter or prefabricated component factories and cement products factories that require high early strength.

The chloride salt brought into the admixture will do great harm to the volume stability and durability of concrete, including:

1. The invasion of chloride-based corrosion medium into concrete will reduce the alkalinity of concrete, destroy the passivation film on the surface of steel bar, and there will be a large potential difference in different parts of the surface of steel bar, resulting in electrochemical reaction, forming anode and cathode.

2. The coarse volume expansion of crystal particles precipitated by repeated hydrolysis and crystallization of chloride salt in concrete makes the concrete crack and reduce the durability of concrete.

3. It is generally believed that the amount of chloride in ordinary reinforced concrete will not necessarily rust if the amount of chloride in ordinary reinforced concrete is less than 0.5%. However, when sodium chloride reduces the freezing point of the solution, when the concrete gradually freezes from the outside to the inside, the chloride ion content in the unfrozen solution becomes higher and higher, that is, the chloride ion concentration of the remaining solution will be more than 0.5%, causing local rust. Therefore, in the actual production, although a small amount of chlorine salt may form a higher concentration, corrosion may occur. And sodium chloride and calcium chloride will not enter the molecular structure of hydration products, but exist in a free state in the void, which is easy to absorb moisture and increase the shrinkage of hardened concrete, which is also disadvantageous to the durability.

Effect of alkali content on concrete additives

When the alkali content in cement is high, the effect of superplasticizer is reduced. The setting time is shortened and the early strength is increased. Too high or too low alkali content in cement has an adverse effect on the compatibility of superplasticizer and cement. This may be due to the fact that it changes the solubility of gypsum and thus affects the hydration of C3A. The best soluble alkali content is 0.4%-0.6% (equivalent Na2O). When the adaptability between admixture and cement is serious, some aminosulfonate superplasticizer should be added to replace nano superplasticizer (or polycarboxylic acid system), and the total amount of water reducing components should be appropriately increased to improve the adaptability of the product.

Concrete Additives Supplier

TRUNNANO is a reliable concrete additives manufacturer with over 12-year experience in nano-building energy conservation and nanotechnology development.

If you are looking for high-quality concrete additives, please feel free to contact us and send an inquiry. (sales@cabr-concrete.com)

We accept payment via Credit Card, T/T, West Union, and Paypal. TRUNNANO will ship the goods to customers overseas through FedEx, DHL, by air, or by sea.