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How to choose admixture for commercial concrete?

April 25, 2023 NFLG 0 Comments

According to the project requirements of the factory to produce to meet the technical requirements of construction, and can be transported to the construction site within a certain period of time, and will not produce changes in the performance of concrete called ready-mixed concrete, and because of its commercial properties, so also known as commercial concrete. At present, there is no certain standard for commercial concrete admixtures used, it is generally believed that can meet the technical needs of commercial concrete special construction, and can bring certain technical and economic benefits of admixtures called commercial concrete admixtures. At present, domestic and foreign admixture factory reference to the current slow-setting water reducing agent or pumping water reducing agent standard detection, while the construction unit is required to add admixture concrete sent to the construction site still has a better compatibility and constructability. Although the long-distance transport still has good water retention, and can make up for some of the defects of the materials used, saving the amount of cement, reduce the cost of use and do not reduce the design strength of concrete admixtures can be used.

1. plasticity of commercial concrete

New concrete liquidity over time and the gradual disappearance of the phenomenon known as slump loss. Concrete mix in the cement gradually hydration, condensation, slump is also gradually reduced, which is a normal phenomenon. However, because of the commercial concrete construction from the production to the site there is a period of transport time, the distance is long transport in order to pour, which will increase the loss of concrete fluidity. If the slump loss exceeds a certain permissible range, the concrete can not be constructed and poured, will seriously affect the quality of concrete construction.

Although the application of additives can greatly improve the fluidity of concrete, but for various reasons, will also increase the loss of concrete slump. Especially high efficiency water reducing agent, most of its products will increase the slump of concrete through time loss rate, so the most important technical needs of commercial concrete admixtures is to reduce slump loss, that is, a good plasticity.

There are many factors affecting the concrete slump loss, mainly cement factors (including cement varieties, specific surface area, mineralization, alkali content and cement conditioning agent species and water solubility, etc.), aggregate factors (including aggregate particle size, fineness, dosage, mud rate, etc.), admixture factors (admixture varieties, molecular structure, polymerization, mixing dosage and NaSO4 content, etc.) and construction environment factors ( Temperature, humidity, wind and transport time, etc.). Construction practice has proved that by adjusting the admixture species and admixture amount, it can improve the slump loss caused by the above factors.

Concrete slump loss is mainly caused by cement hydration and cement particle adsorption. It is not difficult to overcome cement hydration, only a certain amount of retarding agent can be mixed, but it must be pointed out that the mineralization of different cements is very different, different retarding agent retarding mechanism of action is also different, so before using retarding agent must choose the most suitable retarding agent species, and through the test to determine the best amount of mixing, in order to overcome the slump loss caused by cement hydration.

After the cement is mixed with water, due to the adsorption and dispersion of the admixture, the free water wrapped in cement particles in the concrete is released in large quantities, resulting in an increase in concrete slump. However, after a period of time, the admixture is consumed in the cement slurry, and the cement particles will be re-absorbed in one piece, resulting in a rapid reduction in concrete slump, which is often referred to as the second adsorption of cement.

For the second adsorption of cement, the use of retarder has no effect. Increase the amount of water reducing agent or the use of slow-release water reducing agent has a certain effect, but this will increase production costs, from economic considerations is not cost-effective. For the phenomenon of secondary adsorption of cement, the use of air-entraining agent effect is significant. Because the small uniformly dense air bubbles can effectively isolate the cement particles, thus preventing the particles from adsorbing each other, reducing the slump loss. It must be pointed out that the effect of air-entraining agent for plastic preservation, many construction technicians do not have a full understanding. I have investigated nearly 100 mixing plant stations, found that the use of air-entraining agents with other additives compound concrete plasticity is generally better than the concrete not mixed with air-entraining agents.

2. Water retention of commercial concrete

Here the concrete water retention does not mean to reduce the water loss after the concrete is poured, but refers to the commercial concrete in a certain period of time to maintain a certain consistency and reduce the concrete water secretion or layered segregation performance.

As the construction of commercial concrete requires the concrete poured in a certain period of time (sometimes longer) to maintain a plastic state and better pourable, which makes the concrete susceptible to water secretion or delamination, so water retention is another important indicator of the requirements of commercial concrete admixtures.

Fresh concrete before setting suspended solid particles sink under the action of gravity, when the concrete water retention performance is not good, it will make a large amount of water infiltration into the concrete surface. A small amount of water secretion on the quality of concrete has no major impact, but the larger secretion will affect the homogeneity of concrete, reducing the strength of concrete, increasing the concrete settlement shrinkage. As commercial concrete have a certain slow-setting, the initial tensile strength of concrete is insufficient, and the larger tensile stress will cause early shrinkage cracks in concrete. The rise of water secretion, leaving more secretion channels in the concrete, and affect the seepage resistance of concrete.

Mixing a certain amount of high-quality air-entraining agent in the concrete can improve the consistency and cohesion of concrete, effectively reduce or prevent concrete water secretion. Mixing a small amount of thickening agent such as polyacrylamide, polyvinyl alcohol, cellulose, etc. also has a certain effect.

3. The economy of commercial concrete

Commercial concrete production and low sales price, sensitive to the cost of production. An annual output of hundreds of thousands of cubic meters of concrete plant station, each square of concrete to increase the cost of one yuan of admixtures, a year will reduce hundreds of thousands of economic benefits. Therefore, a reasonable formulation, the use of cost-effective admixtures is very important, one-sided requirements of the admixture water reduction rate is not scientific.

According to domestic and foreign experience, when considering the design of commercial concrete admixture formula, the following points can be referred to.

For low-grade concrete, water reduction rate of only 10% to 12% or so, poor concrete gas content can be increased to 4% to 5%. This will not only not affect the strength of concrete, but to improve the water retention of concrete, plasticity is very beneficial. Air-entraining agent is extremely cost-effective, sodium xylose and other ordinary water reducing agent can also be used in large quantities.

For medium-strength concrete C35 ~ C40. water reduction rate of 15% can be, higher water reduction rate is not only economically uneconomical, and the technical benefits are not high, and sometimes have a negative impact.

C50 ~ C60 concrete can be used to reduce the water rate of 18% to 20% of the admixture, and the compound to enhance the effect of ordinary water reducing agent technology and economic benefits are excellent, such as high-efficiency water reducing agent and molasses compound, 28 days compressive strength can be increased by more than 20%, while the use of the cost is not high. A small amount of sodium wood or air-entraining agent in high-strength concrete can still be used, especially for higher fluidity of concrete, but the need to strictly control the amount of gas in 2% or less.

4. The application of retarder in commercial concrete

In order to reduce the slump loss due to hydration, retarding agent is essential. The current problem is that the amount of mixing is too large, bringing many engineering quality problems.

On the application of retarder, because the author has been discussed in detail in another article, here is not repeated. It must be pointed out that the current domestic commercial concrete construction in the amount of retarding agent is large, some mixing stations require the provision of additives retarding time for more than 10 hours, while the concrete mixing to pouring time difference of only about 2 hours, the concrete does not set for a long time not only affect the early strength development, and the concrete is in a plastic state for a long time, a large amount of water evaporation, resulting in serious concrete settlement shrinkage, coupled with A large amount of concrete water secretion increases the early shrinkage of concrete, which is also an important factor in the early shrinkage cracking of commercial concrete found in China. I have repeatedly called in some technical seminars: the amount of retarder should be strictly controlled to reduce the amount of no actual need to retard time, not the longer the retardation time the better the concrete plastic performance.

Many retarders have a very high water-reducing function, such as sodium gluconate, some phosphate, etc., in the low amount of water reduction effect is the same amount of high-efficiency water reducing agent several times. Ignore the above function is not only technically wasteful, and sometimes will bring some adverse effects on the quality of concrete (such as increasing the rate of water secretion, etc.).

5. Finally

To improve the plasticity of commercial concrete, water retention, in addition to the commonly used water reducing agent, retarder, but also a certain amount of air-entraining agent.

Commercial concrete construction requires too high water reduction rate is not scientific.

In commercial concrete, the amount of retarding agent should be strictly controlled, too long retarding time will bring adverse effects on the quality of concrete.

It is not difficult to see, according to the engineering technology requirements to choose the most suitable admixture for the combination, can meet the special construction technology needs of commercial concrete, to ensure the quality of concrete projects, reduce the cost of use and bring better economic and technical benefits.

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