The use of concrete admixtures greatly improves the flow properties of concrete while reducing the amount of cementitious material used in concrete. As a result, concrete admixtures have been widely used. In the long-term production practice, I found that many mixing stations are the use of admixture misunderstanding, resulting in insufficient strength of concrete, poor work, or concrete mix cost is too high.
Master the proper use of admixture method, you can keep the cost of mixing the same premise, to improve the strength of concrete; or to maintain the strength of concrete under the premise of reducing the cost of mixing ratio; in ensuring the same water-cement ratio , To improve the performance of concrete.
Common admixture use errors
Now a lot of mixing station concrete strength has been difficult to improve. If you want to improve the strength of concrete, in a brutal competition in the market environment, but also faced with whether to raise costs and reduce profits confused. Through in-depth investigation, the analysis found that most of these mixing stations into the concrete admixture of the use of errors, as follows:
1. Low-cost procurement of admixtures
As the market competition is fierce, the mixing station for the strict control of procurement of raw materials. Mixing stations are hoping to purchase raw materials at the lowest price, as well as concrete admixtures.
Mixing station will reduce the purchase price of admixtures, will inevitably lead to lower quality of admixture manufacturers. While the general mixing station in the procurement contract is rarely clear admixture acceptance criteria. Even if there is only clear in accordance with the national standard requirements, and the national standard is generally the lowest standard. Which led to the admixture manufacturers in the case of low bidder, the supply of admixture quality is low, generally barely meet the national standard requirements, it is difficult to meet the mixing station on the use of admixture functional requirements.
2. Limit the admixture dosage
Mixing station decision-making level of the cost of monitoring with strict control, and even the amount of cement, admixture content also carried out a clear request. This will inevitably lead to technical departments in the mix design, not to break the decision-making layer of the highest admixture requirements.
3. The lack of quality control of the admixture and test with the verification
At present, for the admixture of the admixture, most of the mixing station is carried out with solid content, water reduction, density, slurry fluidity and other technical indicators in the 1 to 2 detection, few mixing stations for concrete testing.
In the production practice, we found that even if the admixture solid content, water reduction, density, slurry fluidity and other technical indicators to meet the requirements of concrete testing is still possible to achieve the effect of the original test, that is, concrete water reduction , Or poor adaptability.
Effect of improper use of admixture on concrete quality and cost
As the low-cost procurement of admixture quality level is low, in order to achieve adequate water reduction effect, the technical department tends to increase the amount of admixture, resulting in high-quality admixture of the consequences. On the contrary, some of the quality control is stable, with better cost control than the mixing station, the use of admixture quality is better, higher prices, due to high quality and less use, the agent of the unilateral cost but decreased.
Some mixing stations limit the admixture dosage. In the case of insufficient concrete slump, the technical department either reduces the sand and stone moisture content, or increases the unilateral water consumption of the concrete, which directly leads to the decrease of the concrete strength. A technical department with a strong sense of quality will increase the amount of cementitious material (keeping the ratio of water to cement) at the same time as indirect or direct increase of unilateral water consumption of concrete, resulting in an increase in the cost of concrete mix.
The mixing station lacks the quality control and test verification of the admixture. In the admixture of mass fluctuations (decline), the technical department is still using the original mix. In order to meet the requirements of concrete slump, the actual water consumption of concrete increases, the water-cement ratio increases and the concrete strength decreases.
The mechanism of action of admixtures
Commonly used naphthalene admixtures and polycarboxylate admixtures, are relatively high molecular weight (usually 1500 ~ 10000) of organic compounds, belonging to the scope of surfactants.
The molecules of the surfactants have a bipolar structure with a non-polar lipophilic group at one end (or a nonpolar hydrophobic group) and a polar hydrophilic group at the other end. Surfactant dissolved in water, while reducing the surface tension at the same time, can play a dispersion, wetting, emulsification, blistering, washing and other effects.
Adsorption - dispersion
The fluidity of the concrete mix depends on the amount of free water in the concrete. When the admixture is added to the concrete, the cement particles are dispersed with each other due to the electrostatic adsorption of the molecules on the surface of the cement particles. Thus destroying the cement flocculation structure, releasing a large amount of free water, greatly increasing the fluidity of the concrete mixture.
2. Wetting effect
As a result of the orientation of the admixture molecules on the surface of the cement particles, a single molecule solvated water film is formed. This water film on the one hand increased the cement particles and water contact area, on the other hand has a certain wetting effect. As a result, the cement is fully hydrated and the cement strength is growing rapidly.
The basic role of admixtures
(1) without reducing the unit of water consumption in the case of water and plastic ratio of the same, to improve the work of fresh concrete and improve the mobility of cement particles and water contact area greatly increased, cement hydration fully, although the water glue Than the same, the strength of concrete is often a certain increase.
(2) to maintain a certain degree of work, reduce water consumption, water to reduce the ratio of concrete to improve the strength of concrete.
(3) to maintain a certain strength in the case, to reduce the amount of cementing material, reduce water consumption, water and plastic ratio unchanged, saving cement and other cementitious materials.
How to properly purchase and use admixtures
Proper procurement and use of admixtures can produce enormous economic and technical value. Not only can improve the strength of concrete, but also reduce the cost of concrete mix. The specific method is as follows:
1. Test link
Adhesives of the technical indicators of the test test, procurement is an important part of the negotiations before. Through the test, should determine the technical indicators of the admixture of eligibility criteria. Including the solid content of the admixture, water reduction, density, slurry fluidity, concrete water reduction and other technical indicators. It is suggested that the concrete water reduction rate should be used as a key indicator of the quality of admixtures.
2. Procurement links
Clear the admixture of qualified standards, you can start procurement negotiations. It is recommended in accordance with the test to determine the eligibility criteria for the admixture manufacturers tender. In the admixture quality level of not less than the premise of the tender, according to the principle of low price to determine the supply manufacturers.
At the same time, the choice of admixture manufacturers should take into account the manufacturers of production scale, distance, transport capacity, large-scale mixing station or large-scale project supply experience and supply quality level, as well as after-sales service capabilities and level, The only indicator of the manufacturer is screening.
3. Acceptance link
Mixing station in the admixture before the admixture should be tested, the test results after the signing of the contract signed the standard before the storage. It is recommended to distinguish between key indicators and reference indicators. Through the long-term practice, the author believes that the key indicators of the additive for the water reduction rate (mortar) and concrete water reduction; reference indicators for the 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 detection of density, cement paste fluidity and water reduction (mortar).
Such as storage inspection shows that the admixture failed, the proposed treatment program is as follows:
(1) return: in the development of admixture of qualified standards, there will be a fluctuation of the upper and lower limits. Such as water reduction test results lower than the minimum standard of qualified, should be returned to deal with.
(2) downgraded use: in emergency situations, such as the existing stock of admixtures can not meet the production needs, by the two sides agreed to do the downgrade, generally due to the lack of water content increase in the amount of admixture by the admixture The manufacturer is responsible for; or by the two sides agreed, can also be downgraded, the same process.
On the admixture of nonconforming treatment, it is recommended in the procurement contract to indicate, to avoid both sides of the supply and demand in the admixture of non-compliance can lead to disputes.
4. use links
(1) Under normal circumstances, quality inspectors should strictly implement the established concrete mix ratio, including the ratio of the amount of admixture;
(2) daily production and production process, the quality inspection staff should be sand, stone aggregate moisture content for testing, to guide the actual production; production in sand, stone measured the use of water content, can be more accurate reflection of unilateral water Of the actual amount, that reflects the cementitious materials and aggregate water demand fluctuations, as well as the admixture of the water reduction rate fluctuations;
(3) when the raw material performance fluctuations, should be timely adjustment of concrete mix. The increase in the water requirement of the gelling material and the aggregate, and the decrease in the water reduction rate of the admixture will result in unilateral water shortage. The correct approach is to properly increase the amount of admixture, making the amount of water does not break through the established concrete mix ratio, to maintain the same water to ensure the strength of concrete.
5. Routine detection
Admixture through the storage before the test, in line with the contract agreed to allow admission after the standard. Due to the results of water reduction and concrete test results will be a certain amount of access, it is recommended after storage of conventional additives for testing. Test items include, concrete water reduction, solid content and other indicators. Among them, the concrete water reduction rate as a key indicator, must meet the requirements.
After routine testing, such as concrete water reduction rate fluctuations, should be removed from the past admixture of qualified to compare, analysis of changes in raw materials is an admixture or other raw materials. And in accordance with the results of the analysis promptly notify the quality inspector to adjust the amount of concrete admixture to ensure that the water-cement ratio does not exceed the design value, and thus ensure that the concrete strength is not affected. At the same time should immediately notify the changes in the raw material suppliers to improve the quality.
6. Trial allocation verification
In order to ensure the rationality of the production mix, it is recommended that at least twice a month the common concrete mix ratio is not less than 2 times the test with the verification. I found that the emergence of concrete strength problems or the cost of the vast majority of mixing stations, the lack of common use of concrete with the test with the test.
Concrete test and verification, is the quality of existing concrete raw materials, concrete work and strength of the comprehensive verification.
(1) the mixing station to produce the use of admixtures of misunderstanding, because the mechanism of action on the admixture and the basic role is not enough understanding, and water and cement than the impact of concrete strength is not a profound understanding;
(2) To correct the misunderstanding of the admixture of the mixing station should be controlled from the aspects of the test link, the procurement link, the acceptance link, the use link, the routine inspection, the test and verification, and so on. Among them, the key to ensure the strength of the concrete is Production in the actual water consumption, to ensure that the water is not greater than the design value;
(3) In the case of low quality of admixture, in order to achieve the desired water reduction effect, it is necessary to increase the admixture content. Practice has proved that the procurement and use of low-volume high-standard admixtures, compared to the high content of low-standard admixtures, both from the strength of concrete to ensure that the admixture of unilateral cost control, are advantageous;
(4) blind restrictions on the admixture of admixture is an unscientific approach. This practice will often lead to unilateral water consumption of concrete and unilateral cementitious material as a whole to improve, thereby greatly improving the cost of mix. It is suggested that the maximum weight part (cementitious material) of the mixing ratio cost should be controlled according to the test, the water-cement ratio and the unilateral water consumption limit should be determined. On this basis, the optimum dosage of the admixture satisfying the concrete slump requirement ;
(5) the lack of quality control of admixtures and test with verification, the quality of admixtures may continue to decline. In this case, the increase in the amount of unilateral water used to maintain the original admixture content will have a disastrous effect on the strength of the concrete. Therefore, to strengthen the quality of the admixture monitoring, directly related to the compressive strength of concrete can be guaranteed or whether the stability.
In short, as long as we have mastered the proper use of admixtures, you can keep the cost of mixing with the same premise, to improve the strength of concrete; or to maintain the strength of concrete under the premise of reducing the cost of mixing for the concrete mixing station To create a huge technical and economic value.
From:mobile concrete mixing plant