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Common misunderstandings about the use of admixtures in mixing plants

The use of concrete admixtures greatly improves the flow properties of concrete and reduces the amount of cementitious materials in concrete. Therefore, concrete admixtures are widely used. In the long-term production practice, the author found that many mixing plants have misunderstandings in the use of admixtures, resulting in insufficient concrete strength, poor workability, or high concrete mix cost.

Mastering the correct use of admixtures can increase the strength of concrete on the premise of keeping the cost of the mix ratio unchanged; or reduce the cost of the mix ratio on the premise of keeping the strength of the concrete unchanged; on the premise that the water-to-binder ratio remains unchanged , the working performance of concrete.

1. Common misunderstandings in the use of admixtures

The author has inspected a number of batching plants and found that the concrete strength of some batching plants has been difficult to improve. If you want to improve the strength of concrete, you are faced with the confusion of whether to increase the cost and reduce the profit in the cruel market competition environment. Through in-depth investigation and analysis, the author found that most of these batching plants have entered the misunderstanding of the use of concrete admixtures, as follows:

1.1 Purchase additives at low prices

Due to the fierce market competition, the batching plant has strict control over the procurement of raw materials. Mixing plants hope to purchase raw materials at low prices, and so do concrete admixtures.

The mixing station reduces the purchase price of admixtures, which will inevitably lead to the reduction of the quality level of admixture manufacturers. In general, the acceptance criteria of admixtures are rarely specified in the procurement contract of general mixing plants. Even if there is, it is only in accordance with the requirements of the national standard, and the requirements of the national standard are generally low standards. This leads to the admixture manufacturers supplying low-quality admixtures in the case of winning the bid at a low price, generally barely meeting the requirements of the national standard, and it is difficult to meet the functional requirements of the mixing station for the use of admixtures.

1.2 Limit the amount of admixtures

The decision-making level of the batching plant strictly monitors the cost of the mixing ratio, and even has clear requirements for the amount of cement and the amount of additives. This will inevitably lead the technical department to dare not break through the high admixture requirements of the decision-making level when designing the mix ratio.

2. Influence of improper use of admixtures on concrete quality and cost

At present, for the storage testing of admixtures, most batching plants carry out 1-2 tests of technical indicators such as solid content, water reduction rate, density, and slurry fluidity, and few batching plants carry out concrete tests.

In the production practice, we found that even if the technical indicators such as the solid content, water reduction rate, density, and slurry fluidity of the admixture meet the requirements, the concrete test may still fail to achieve the effect of the original trial preparation, that is, the concrete water reduction rate is insufficient. , or poor adaptability.

Due to the low quality level of the admixtures purchased at low prices, in order to achieve sufficient water reduction effect, the technical department often increases the amount of admixtures, resulting in the consequence that the admixtures are low-quality and multi-purpose. On the contrary, some batching plants with stable quality control and better mixing ratio and cost control use admixtures of better quality and higher prices. Due to the low use of high-quality additives, the unilateral cost of admixtures decreases.

Some mixing plants limit the amount of admixtures. In the case of insufficient concrete slump, the technical department either reduces the moisture content of sand and stone, or increases the water consumption of concrete, which directly leads to the decrease of concrete strength. Technical departments with strong quality awareness will increase the amount of cementitious materials appropriately (keep the water-binder ratio unchanged) while indirectly or directly increasing the water consumption of concrete unilaterally, resulting in an increase in the cost of concrete mix ratio.

The batching plant lacks the quality monitoring and trial mix verification of the admixtures. When the quality of the admixture fluctuates (decreases), the technical department still uses the original mix ratio. In order to meet the requirements of concrete slump, the actual water consumption of concrete increases, and the strength of concrete decreases.

3. The basic role of admixtures

(1) Without reducing the unit water consumption, the water-to-binder ratio remains unchanged, the workability of the fresh concrete is improved, and the fluidity is improved; due to the greatly increased contact area between the cement particles and water, the cement is fully hydrated, although the water-binder ratio Invariant, the strength of concrete often has a certain improvement.

(2) Under the condition of maintaining a certain degree of work, reduce the water consumption, reduce the water-binder ratio, and improve the strength of the concrete.

(3) Under the condition of maintaining a certain strength, reduce the amount of cementitious materials, reduce the amount of water, keep the water-binder ratio unchanged, and save cement and other cementitious materials.

4. How to correctly purchase and use admixtures

Properly purchasing and using admixtures can generate enormous economic and technical value. It can not only improve the strength of concrete, but also reduce the cost of concrete mix ratio. The specific method is as follows:

4.1 Test link

The testing and testing of various technical indicators of admixtures is an important link before purchasing negotiation. Through the test, the qualification standards of various technical indicators of the admixture should be determined. Including the solid content of admixtures, water reduction rate, density, slurry fluidity, concrete water reduction rate and other technical indicators. It is suggested that concrete water reduction rate be used as a key indicator to measure the quality level of admixtures.

4.2 Procurement link

After the eligibility criteria for admixtures are clarified, procurement negotiations can begin. It is suggested that the admixture manufacturers should invite bids according to the qualified standards determined by the test. On the premise that the supply quality of the admixture is not lower than the bidding requirements, the supplier shall be determined according to the principle of winning the bid at a low price.

At the same time, in the selection of external additive manufacturers, the manufacturer’s production scale, transportation distance, transportation capacity, supply experience and supply quality level of large-scale mixing plants or large-scale engineering projects, and after-sales service capabilities and levels should be comprehensively considered. Manufacturer screening indicators.

4.3 Acceptance link

The mixing station should test the admixtures before the admixtures are put into storage, and the test results can be put into storage only after the test results are qualified according to the contract signed standards. It is recommended to distinguish between key indicators and reference indicators. Through long-term practice, the author believes that the key indicators of admixtures are water-reducing rate (mortar) and concrete water-reducing rate; reference indicators are density (specific gravity), solid content and cement slurry fluidity. Due to the testing time, the technical indicators that are generally tested in the acceptance link are density, cement slurry fluidity and water reduction rate (mortar).

If the warehousing test shows that the admixture is unqualified, the recommended treatment plan is as follows:

(1) Returns: When formulating the qualification standard of admixtures, there will be an upper and lower limit of allowable fluctuations. If the water reduction rate test result is lower than the lower limit of the qualified standard, it should be returned.

(2) Downgraded use: In emergencies, such as the existing stock of admixtures cannot meet the production needs, the downgrading treatment can be done after consultation between the two parties. Generally, the increase in the amount of admixtures due to the insufficient water reduction rate is caused by the admixtures. The manufacturer is responsible; or it can be downgraded by mutual agreement, and the treatment plan is the same as above.

Regarding the unqualified treatment of admixtures, it is recommended to indicate it in the purchase contract to avoid disputes between the supply and demand sides when the admixtures are unqualified.

4.4 Use link

(1) Under normal circumstances, the quality inspector should strictly implement the established concrete mix ratio, including the amount of admixtures in the mix ratio;

(2) Before and during production every day, the quality inspector should test the water content of sand and stone aggregates to guide the actual production; the use of the measured water content of sand and stone in production can more accurately reflect the unilateral water consumption The actual dosage reflects the fluctuation of the water demand of the cementitious materials and aggregates, as well as the fluctuation of the water reducing rate of the admixture;

(3) When the performance of raw materials fluctuates, the concrete mix ratio should be adjusted in time. The water requirements of cementitious materials and aggregates, as well as the reduction in the water-reducing rate of admixtures, will result in insufficient water consumption per side. The correct way is to appropriately increase the amount of admixtures, so that the unilateral water consumption does not break through the established concrete mix ratio, and the water-binder ratio remains unchanged to ensure the concrete strength.

4.5 Routine testing

The admixtures are tested before storage and are allowed to be put into storage after they meet the standards stipulated in the contract. Due to the discrepancy between the test results of the water reduction rate and the concrete trial mix results, it is recommended that the admixtures be routinely tested after storage. Test items include concrete water reduction rate, solid content and other indicators. Among them, the concrete water reduction rate is a key indicator, which meets the requirements.

After routine testing, if the water-reducing rate of concrete fluctuates greatly, the qualified samples of the past admixtures should be taken out for comparison, and the raw materials that have changed are admixtures or other raw materials. According to the analysis results, the quality inspector will be notified in time to adjust the amount of concrete admixtures to ensure that the water-binder ratio does not exceed the design value, thereby ensuring that the concrete strength is not affected. At the same time, raw material suppliers that have changed should be notified immediately for quality improvement.

4.6 Trial verification

In order to ensure the rationality of the production mix ratio, it is recommended to carry out at least two trial mix verifications for the commonly used concrete mix ratios every month. The author found that most of the batching plants with concrete strength problems or high costs lacked the trial mix verification of commonly used concrete mix ratios.

Concrete trial mix verification is a comprehensive verification of the quality of existing concrete raw materials, concrete workability and strength.

5 Conclusion

(1) The misunderstanding of the use of admixtures in the mixing station is due to the lack of understanding of the mechanism and basic functions of the admixtures, and the lack of a deep understanding of the influence of the water-binder ratio on the strength of concrete;

(2) To correct the misunderstanding of the use of admixtures in the mixing station, it should be systematically controlled from the test link, procurement link, acceptance link, use link, routine testing, trial mix verification, etc.; among them, the key to ensuring the strength of concrete is to control the The actual water consumption in production ensures that the water-to-binder ratio is not greater than the design value;

(3) In the case of low quality level of admixtures, in order to achieve the ideal water-reducing effect, the admixture dosage should be adjusted. Practice has proved that the purchase and use of high-standard admixtures with low dosages are advantageous compared to low-standard admixtures with high dosages, both in terms of strength assurance of concrete and unilateral cost control of admixtures;

(4) Blindly limiting the amount of admixtures is an unscientific practice. This practice often leads to an overall increase in the unilateral water consumption of concrete and the unilateral cementitious material, thereby greatly increasing the cost of mixing ratio. It is recommended to first determine the water-binder ratio and the upper limit of the unilateral water consumption according to the test, so as to control the weighted part (cementing material) in the cost of the mixing ratio; on this basis, find out the amount of admixture that meets the requirements of concrete slump;

(5) Lack of quality monitoring and trial mix verification of admixtures, the quality level of admixtures may continue to decline. In this case, the increase in unilateral water consumption caused by maintaining the original admixture content will have a catastrophic impact on the concrete strength. Therefore, strengthening the quality monitoring of admixtures is directly related to whether the compressive strength of concrete can be guaranteed or stable.

In short, as long as we master the correct use of admixtures, we can increase the strength of concrete on the premise of keeping the cost of the mix ratio unchanged; or reduce the cost of the mix ratio on the premise of keeping the strength of the concrete unchanged, which is a good source for the concrete mixing plant. Create huge technical and economic value.

Post time: Jul-25-2022
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