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1.About WORKABILITY OF CONCRETE
Workability of Concrete refers to the ability of fresh concrete to be easily operated during various construction processes (mixing, transportation, pouring, compacting, etc.) and to obtain uniform and dense performance, including fluidity, cohesion, and water retention. It is also known as the workability of concrete.
Workability is an eternal topic for concrete. It is very important, as it is an important parameter for ensuring normal construction of concrete and a touchstone for the technology of water reducers companies. Improving the workability of concrete without affecting other properties and increasing costs is a sign of high technical level and rich work experience. However, in practical operations, it does not seem to be so important, as the state of concrete is often accepted due to fluctuations in sand and gravel materials. If the water-reducing rate can be compared to the performance of the strength of a car's engine, workability is the parameter of brake performance and airbag safety, which is important but not needed all the time. Workability of concrete is a complex and systemic performance.
At the beginning of the development of the concrete industry, the amount of adhesive materials used was relatively conservative, and the abundant natural sand resources made the workability of concrete generally good. In recent years, with the increasing scarcity of natural sand and the trend of reducing adhesive material usage to achieve energy conservation and emission reduction, workability of concrete has become increasingly important. Its importance is also related to the type of concrete. Pumping concrete requires higher workability, while the requirements for workability of prefabricated concrete are not so high.
2.Factors affecting the workability of concrete
There are many reasons why the workability of concrete is poor. The differences in the materials, fineness, and grading continuity of cement, mineral powder, fly ash, sand, stone, water, and admixtures can all lead to changes in workability.
（1）Cement, mineral powder, and fly ash
The material, fineness, and grading continuity of cement, mineral powder, and fly ash are the main causes of changes in workability, although there are other factors that can also affect it. Poor workability caused by the binder material is relatively low, and even if it occurs, it can be improved and resolved by adjusting the formula of the water-reducing agent. Some high-value concrete is usually made with ultra-fine mineral powder and silica fume to improve the compactness and workability of the concrete.
（2）Sand
Sand is a key factor affecting the workability of concrete. In extreme cases, sand can cause concrete to be unable to be constructed normally, and no amount of adjustment can achieve the desired state of the concrete. Natural river sand, which has been screened and polished by the river's erosion over a long period of time, has a better particle size and grading continuity than mechanically crushed sand. Nevertheless, upstream sand tends to be coarse, while downstream sand tends to be too fine, both of which have a negative impact on the workability of concrete. Although theoretically, mechanically crushed sand can achieve the desired particle shape and size, many of the production standards of mechanically crushed sand are relatively low due to limitations in equipment, site, and investment scale, which have a significant impact on the workability of concrete. Common factors include large fineness modulus, poor particle size, excessive needle-shaped particles, too many large particles, and low powder content. Stone powder in mechanically crushed sand is beneficial for improving the workability of concrete, but too much stone powder can also cause the concrete to crack.
(3) Stone
The particle shape, particle size distribution, material, and content of fines and mud are the main factors that affect the workability of stones. If the particle shape is not round enough, it increases the resistance to concrete flow, making it difficult for the stone to flow with the slurry, which leads to concrete exposed stones. High flowable concrete uses shaped stones to improve flowability and workability. Poor particle size distribution continuity results in too many voids that require more slurry to fill, resulting in "lack of slurry". If the stone particle size is too small, it increases the total surface area of the stone, requiring more slurry to wrap it and also leading to decreased workability. Fines and mud, as "inert adhesive materials", can improve the workability of concrete to some extent.
(4) Water
The influence of water on workability is mainly reflected in the case where it does not segregate. The use of more water can improve the workability of concrete when reducing the dosage of water-reducing agents. When the mixing plant is producing at night, it is not conducive to observing the status of the concrete, and the amount of water used will be increased slightly. On the one hand, this can reduce sensitivity to external admixtures, and on the other hand, it can improve the workability of the concrete, making it less prone to segregation, blockage, and pumping problems.
(5) Various Components in Admixtures
The polycarboxylate-based superplasticizer and various small materials all have an impact on workability at some level, making the already complex concrete workability even more challenging. This creates the illusion that there is an admixture that can adjust everything.
First of all, let's talk about the mother liquor. The mother liquor produced using different processes has different workability. Generally speaking, the mother liquor synthesized from carbon five monomers has better workability than those from carbon four and carbon six monomers. However, there is a special case for carbon six, in which the water-reducing rate of the carbon six mother liquor synthesized at high temperatures decreases significantly, but the workability improves significantly. Lowering the amount of acrylic acid can improve the workability of the concrete. Pre-injecting acrylic acid into the reactor before drip-adding is not conducive to workability. The amount and type of oxidant and reductant can also affect workability, but there is too little research done on this, and there is currently no detailed data available. In any case, the amount and type of oxidant, reductant, and chain transfer agent have a significant impact on workability.
The impact of slow-release retarder mother liquor on workability is also quite significant. For poorly designed retarder mother liquor, using a large amount of retarder mother liquor will make the concrete sticky, and workability will deteriorate. This should be due to the expression of the workability becoming worse as the water-reducing rate of the retarder mother liquor expressed macroscopically after the competition for adsorption between the retarder mother liquor and the water-reducing mother liquor. It seems that there is no literature that pays attention to the problem of deteriorated workability caused by excessive retarder mother liquor. If the process design of the retarder mother liquor is reasonable, this problem will not exist.
Without the addition of water-reducing agents, the concrete with a slump of more than 200mm achieved by increasing the amount of water has a muddy feeling, but the workability is not bad, after all, more water was used. The water-reducing agent adsorbed on the surface of the cement particles destroys the flocculation structure of the cement, releasing more free water. The addition of the water-reducing agent is equivalent to reducing the workability of the concrete. However, they balance each other out. It is not so much that the workability of the concrete is improved by changing the molecular structure of the water-reducing agent as it is to minimize the negative impact of the water-reducing agent on workability.
Air-entraining agents are commonly used composite materials to improve workability, with the characteristic of small amount but significant effects. Moreover, different types of air-entraining agents have different effects on improving workability. In our factory's experiments, AOS has the most significant effect on workability, and some literature suggests that K12 has the best effect on workability.
The amount and type of retarder also affect the workability of concrete. Sugar makes the concrete stickier, while sodium gluconate provides a little water-reducing effect, slightly reducing workability. Maltodextrin, as an intermediate state between cellulose and glucose, can improve the workability of concrete. When making ultra-high-performance concrete, using only sodium gluconate or sugar separately will cause the concrete to harden after being left still. Mixing the two in a 1:1 ratio can improve this phenomenon. Therefore, mixing sodium gluconate and sugar in a 1:1 ratio is also a good option.
Water-retaining and thickening agents are a type of material that can significantly improve the workability of concrete. This term is a general term and does not specifically refer to any particular substance. Any material that can improve the workability of concrete and reduce water loss can be referred to as a water-retaining and thickening agent. The most common types include cellulose ethers, gum arabic, and various types of polymer materials in the chemical industry.
Cellulose ethers and modified starch ethers are commonly used as thickening agents in self-leveling cement and grouting materials. However, the amount of thickening agent used in these products is very high, and it is only of reference value for concrete. Moreover, cellulose ethers and water-reducing agents react to produce precipitation, and the reaction rate is relatively slow, often taking several days for floc-like substances to be precipitated. If the speed of the water-reducing agent is slow, the floc-like substance that is precipitated will accumulate in the storage tank for years. Synthesized polymer materials such as acrylic sulfonate (e.g., acrylic acid-2-acrylamido-2-methylpropane sulfonate), acrylamide, and other materials are also used as water-retaining agents.
Like air-entraining agents, the outstanding feature of water-retaining agents is that a small amount can have a significant effect. Using too much can lead to negative effects, such as a loss in water reduction rate, poorer water retention, and lower strength. Workability is the litmus test for the effects of water-retaining agents. If segregation occurs, it means that the water-reducing rate is too high, and if the concrete has low expansion, it means that the water-reducing rate is not enough. If the water retention effect is poor, the amount of water-retaining mother liquor and retarder should be increased.
These technologies are relatively easy to learn compared to mastering workability. Once you have learned the basics, you can optimize the mother liquor formula to improve workability. However, choosing the right water-retaining agent from the numerous options available is a systematic engineering task that requires a lot of concrete adaptation, as well as rigorous observation and analysis skills. Only then can you quickly identify and propose the best solution to adjust the workability.
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