The use of concrete admixtures, greatly improved the flow performance of concrete, while reducing the amount of cementitious materials in concrete. Therefore, concrete admixtures have been widely used. In the long-term production practice, the author found that there are a lot of mixing stations have errors in the use of additives, resulting in lack of concrete strength, poor workability, or high cost of concrete mix.
To master the correct use of admixture, the premise can keep the mix cost at the same time, improve the strength of concrete; or in the premise of maintaining the same concrete strength, reduce the mix cost; ensure in the case of a constant ratio of water cement, improve the workability of concrete working performance.
1, the common use of additive errors
In this paper, a number of mixing stations in Guangdong were investigated, and it was found in some batching stations that it was difficult to improve the strength of concrete. If you want to improve the strength of concrete, in the brutal market competition environment, but also faced with the confusion that whether to raise costs and reduce profits. Through in-depth investigation and analysis, the author found that the majority of such mixing stations get into the errors of using concrete admixtures, as follows:
1.1 low price procurement admixture
Due to the fierce competition in the market, mixing station for the procurement of raw materials strictly controlled. Mixing stations are expected to purchase the raw materials at the lowest price, concrete admixture is also the case.
Mixing station reduce the purchasing price of admixture, will inevitably lead to admixture manufacturers to reduce the quality level. The general mixing station in the procurement contract is very unclear about the acceptance criteria of admixtures. Even if there is only in accordance with the requirements of the national standard, and the national standard is generally the lowest standard. This leads to admixture manufacturers in the case of low price, the supply of low quality additives, generally barely meet the national standard requirements, it is difficult to meet the use of external mixing agent functional requirements.
1.2 limit admixture dosage
The mixing station decision layer is strict in monitoring the mixture ratio, and it also makes a clear request to the dosage of cement and admixture. This will inevitably lead to the technical department in the mix design, dare not break through the admixture of the highest decision-making level requirements.
1.3 lack of quality control and test verification of external additives
At present, most of the mixing station is about 1 to 2 items in the technical indexes of the solid content, the water reducing rate, but few of them conduct a concrete test.
In practice we found that even if the additive solid content, water reducing rate, density, fluidity and other technical indicators meet the requirements of concrete test may still not up to the original test results, the water reducing rate of concrete is insufficient, or not good adaptability.
2, the effection of using admixtures incorrectly on concrete quality and cost
Due to the low quality of the low quality of the low level of procurement, in order to achieve adequate water reduction effect, the technical departments tend to increase the amount of admixture, resulting in low quality multi-purpose additives consequences. On the contrary, some of the quality control stability, mixing cost control better mixing station, the use of admixture quality is good, the price is higher, due to the high quality and less use, but the cost of admixture.
Some mixing stations limit admixture dosage. In the concrete slump under the condition of insufficient technical departments or to reduce the moisture content in sand and stone, improve the concrete unilateral water or directly lead to concrete strength. Strong quality awareness technology sector will be in directly or indirectly improve water use and unilateral concrete, appropriate to increase the amount of cementitious material (to maintain constant water cement ratio), resulting in increased cost of concrete mix proportion.
The mixing station is lack of quality control and test verification. In addition, the quality of the admixture is fluctuating (down), the technical department still uses the original mix proportion. In order to meet the requirements of concrete concrete slump, the actual increase in the water content, water binder ratio increases, the strength of concrete decreased.
3, the action mechanism of admixture
At present, the commonly used naphthalene additives and polycarboxylate admixtures are organic compounds with relatively high molecular weight (1500 ~ 10000), which belong to the category of surfactant.
The molecule of the surfactant has a bipolar structure, one end is a nonpolar lipophilic group (or a nonpolar hydrophobic group), and the other end is a polar hydrophilic group. When the surfactant is dissolved in water, it can play a variety of roles, such as dispersion, wetting, emulsification, foaming, washing and so on.
3.1 adsorption-dispersion
The flow depends on the concrete mixture in the concrete free water amount. After the addition of additives in concrete, the cement particles are dispersed due to the adsorption of additives on the surface of cement particles. In order to destroy the flocculated cement structure, the release of large amounts of free water, greatly increasing the fluidity of concrete mixture.
3.2 wetting effect
As a result of the orientation of the admixture molecules on the surface of cement particles, a single molecule solvation water film is formed. On the one hand, the water film increases the contact area between the cement particles and water, on the other hand, it has a certain wetting effect. Therefore, the cement can be fully hydrated, cement strength increased rapidly.
4, the basic role of admixture
(1) in reducing the unit water use under the condition of constant ratio of water binder, improve the fresh concrete workability, improve the fluidity; since the contact area of cement particles and water increased greatly, cement hydration is sufficient, although the strength of concrete water cement ratio unchanged, but there is often some improvement.
(2) in order to maintain a certain degree of work, reduce water consumption, water cement ratio decreased, improve the strength of concrete.
(3) in the case of maintaining a certain strength, reduce the amount of cementitious materials, reduce water consumption, water cement ratio unchanged, saving cement and other cementitious materials.
5, how to purchase and use of additives
The right way to purchase and use of additives, can produce enormous economic and technological value. Not only can improve the strength of concrete, but also reduce the cost of concrete mix. Specific methods are as follows:
5.1 test link
The testing of various technical indicators of the admixture is an important part of the procurement negotiations. Through the experiment, we should confirm the qualified standard of the technical indexes of the admixture. It includes the technical indexes such as the solid content, the water reducing rate, the density, the fluidity of the slurry, the water reducing rate of concrete, and so on. It is suggested that the water reducing rate of concrete is the key index to measure the quality of admixture.
5.2 purchasing link
After the qualified standard of the additives is clear, the purchasing negotiation can be started. Recommendations in accordance with the test to determine the eligibility criteria, external agent manufacturers to tender. Suppliers in the supply of quality level is not lower than the requirements of the premise, according to the principle of low price to determine the supplier.
At the same time, additive manufacturers, manufacturers should consider the size, distance, transport capacity, large-scale mixing station or large project delivery experience and quality level, and customer service capacity and level of service, the price is not the only indicator screening of manufacturers.
5.3 acceptance link
Mixing station should be tested before admixture can be stored, and the test results can be stored in the warehouse before the contract is signed. Key indicators and reference indicators. Through long-term practice, the author thinks that the key indicators of admixture are water reducing ratio (sand) and concrete water reducing rate, and the reference index is density (specific gravity), solid content and cement paste fluidity. As a result of the detection time, generally in the acceptance of the technical indicators of the link is the density, cement paste fluidity and water reducing rate (sand).
If the storage test shows that the additive is not qualified, the proposal is as follows:
(1) Returns: there is an upper and lower limit of allowable fluctuations in the formulation of the eligibility criteria for admixtures. If the test results are lower than the lower limit of qualified standards, should be returned.
(2) downgrade: in case of emergency, such as additive existing inventory can not meet the production requirements, agreed by the parties to the downgrade, generally due to insufficient water reducing rate leads to an increase in the amount of admixture admixture by some manufacturers responsible; or as agreed by the parties, as well as downgrade treatment scheme.
With regard to the unqualified treatment of additives, it is recommended to give a note in the procurement contract, to avoid the supply and demand sides in the absence of additives when there is no chapter and disputes.
5.4 use link
(1) in general, inspectors should strictly enforce the established concrete mix proportion, including the dosage of additives;
(2) a day before production and production process, detect the inspector coping stone and sand moisture content of aggregate, used to guide the actual production; use of the measured water content of sand and stone production, can accurately reflect the actual amount of water used unilaterally, which reflects the demand of cementitious materials and aggregate fluctuations. And the admixture of the water reducing rate;
(3) when the fluctuation of raw material performance, the concrete mix proportion should be adjusted in time. The increase of water demand of cementitious materials and aggregate, as well as the decrease of the water reducing rate of admixtures, will result in insufficient water consumption. The correct approach is to appropriately increase the amount of admixture, so that the water consumption does not break through the established concrete mix ratio, keep the water cement ratio unchanged to ensure the strength of concrete.
5.5 routine detection
Admixture before storage detection, in accordance with the contract agreed to grant the standard. Because of the reduction of water rate and the results of the test results will have some differences, it is recommended that the external agent after storage for routine testing. Test items include concrete water reducing rate, solid content and other indicators. Among them, the concrete water reducing rate is the key index, must meet the requirements.
After the conventional test, such as the concrete water reducing rate fluctuations, should be removed from the previous qualified qualified samples for comparison, analysis of the raw material is changed or other raw materials. And notify the inspector adjusting concrete admixture timely according to the analysis results, to ensure the water binder ratio does not exceed the design value, and to ensure that the concrete strength is not affected. At the same time, the raw material suppliers should be notified immediately to improve the quality.
5.6 test verification
In order to ensure the rationality of the production mix ratio, it is recommended that at least 2 times a month to test the mix proportion of concrete. The author found that the concrete strength problems or high cost of most of the mixing station, the lack of common concrete mixture ratio test verification.
The test and verification of concrete is a comprehensive verification of the existing raw material quality, workability and strength of concrete.
6, conclusion
(1) the misunderstanding of the admixture used in the mixing station is due to the lack of understanding of the mechanism and the basic role of the additive, and the influence of water cement ratio on the strength of concrete;
(2) station errors admixtures use corrective stirring should test process and purchase process, acceptance and use of links, each with conventional detection, test verification and other aspects of the control system; the key to ensure the strength of concrete is to control the actual water consumption in production, ensure the water binder ratio is not greater than the design value;
(3) in order to achieve the ideal water reducing effect, the admixture content must be increased in order to achieve the ideal water reducing effect. Practice has proved that the procurement and use of low dosage of high standard admixture, compared to the high content of low standard admixture, both from the strength of the concrete guarantee, or the admixture of unilateral cost control, there is an advantage;
(4) it is not a scientific way to restrict the admixture dosage blindly. This approach often leads to an increase in the total amount of water used in concrete and the monolithic cementitious material, thereby greatly increasing the cost of mix. First proposed according to the test, determine the water binder ratio and water consumption limit unilateral, so as to control the cost of the maximum weight part ratio (cementitious materials); on this basis, find the optimum dosage of admixture concrete slump requirements;
(5) there is a lack of quality control and test verification of external additives, and the quality level of additives may decline. In this case, the increase in the amount of water used to maintain the original admixture will have a disastrous impact on the strength of concrete. Therefore, it is directly related to whether the compressive strength of concrete can be guaranteed or whether it is stable or not.
In short, as long as we grasp the correct use of additives, the premise can stay in the same proportion cost, improve the strength of concrete; or in the premise of maintaining the same concrete strength, reduce the proportion of cost, station to create a huge economic value for concrete mixing technology.
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