With the development of modern concrete technology, it has good construction performance, high volume stability (less shrinkage, no cracking during hardening), and compressive strength that meets actual requirements (generally ≥50MPa), and high-performance concrete (HPC) with excellent durability have increasingly become a hot topic in research and application in the engineering community. The essential difference between HPC and ordinary concrete (NC) or high-strength concrete (HSC) is “the premise of ensuring durability and good working performance.” When preparing HPC, the measures adopted are generally to add mineral admixtures and admixtures mainly high-efficiency water-reducing admixtures at the same time to achieve the purpose of reducing the water-binder ratio, improving fluidity, strength and durability, and meeting the high-performance requirements of concrete. Require.
However, due to the diversity of concrete admixtures and the adaptability of concrete admixtures and cement/admixtures, the preparation of HPC is not as simple as theoretically .
1. Correct selection of admixtures
Various admixtures have different properties and functions Similarly, appropriate admixtures should be selected based on concrete performance requirements. Superplasticizer is the most important and essential admixture in HPC, so it must be chosen carefully. Other types of admixtures, such as air-entraining agents, retarders, early strength agents, expansion agents and antifreeze agents, are also required by HPC in some cases.
1. High-efficiency water-reducing admixture
The dosage and mixing of high-efficiency water-reducing admixture Adding methods, adaptability to cement and other issues restrict the performance of the prepared HPC. Since polycarboxylate-based high-efficiency water-reducing agents account for a relatively low share of the market (less than 2%), the most commonly used high-efficiency water-reducing agents currently include melamine-based and naphthalene-based high-efficiency water-reducing agents. Melamine-based superplasticizers are mainly sodium salts and are colorless; naphthalene-based superplasticizers are mainly sodium salts and calcium salts and are brown.
When selecting high-efficiency water-reducing admixtures in concrete, the type of cement and the characteristics of other ingredients, such as the characteristics of admixtures, should also be considered. When the introduction of chloride ions is not allowed or potential alkali-aggregate reaction hazards are expected, sodium salt superplasticizer should be selected carefully. When selecting, you must consider both economy and the quality stability of the water-reducing agent.
It should be noted that you must not select the high-efficiency water-reducing agent and determine the dosage based solely on the product instructions. You must select the appropriate type through experiments and determine the dosage. Appropriate dosage. When two or more admixtures are selected for compound addition, their matching proportions must be determined through experiments to avoid precipitation failure.
Melamine-based high-efficiency water-reducing agent itself is colorless and will not bring in light brown color when combined with white cement, but its price is higher than that of naphthalene-based high-efficiency water-reducing agent. The dosage is high and is rarely used in HPC preparation.
In order to control the slump loss of concrete, it is often necessary to use naphthalene series high-efficiency water reducing agent in combination with retarders and other components. However, the causes of concrete slump loss are very complex, and a suitable method has not yet been found to solve the slump loss.
Although sulfamate and polycarboxylate superplasticizers have good effects in controlling slump loss, sometimes it is inevitable that Unexpected phenomena.
2. Air-entraining agent
Incorporating air-entraining agent can make the concrete mix better during the mixing process. It introduces a large number of evenly distributed tiny bubbles into the concrete to prevent segregation and bleeding. More importantly, it effectively improves the freeze-thaw resistance and deicing salt resistance of the concrete. It is an effective measure to increase the service life of concrete. The air-entraining agents currently available on the market mainly include rosins and alkyl sulfonates. In addition, there are also products processed from saponins. Rosin air-entraining agents are easy to prepare and relatively cheap, but the bubble structure introduced is poor; alkyl sulfonates are typical surfactants and have better air-entraining effects.
The air-entraining amount in concrete is not only related to the type and dosage of air-entraining agent, but also to the cement type, cement dosage, and admixture type and dosage. , water-binder ratio, mixing method and time, slump, parking time, vibration method, time and temperature are related to many factors. The rules must be found through experiments, and construction parameters should not be easily changed during actual construction. . In projects, admixtures such as air-entraining agents and high-efficiency water-reducing admixtures are often used in combination. The appropriate ratio must be determined through experiments to maximize the synergistic effect.
3. Retarder
Adding retarder can extend the setting time of concrete . The compound addition of retarder and water-reducing agent can also delay the slump loss of concrete, reduce the hydration heat release rate, and prevent large-volume concrete from cracking. There are two main types of retarder: inorganic salts and organic substances. When selecting a retarder, the type and dosage should also be determined through experiments. It must be noted that the setting time of concrete mixed with retarder is not a simple linear relationship with the amount of retarder. For some retarder, when the dosage exceeds a certain value, the retarding effect will increase dramatically, which can easily lead to serious engineering accidents. When the retarder is combined with other types of admixtures, such as water-reducing agents, waterproofing agents or swelling agents, tests must be conducted in advance.
2. The dosage of admixture
To determine the appropriate dosage of admixture, it should be achieved on the premise of ensuring the technical performance requirements of concrete. The most economical effect. When some additives are added in excessive amounts, they will not only fail to achieve the expected results, but will also bring serious negative effects.
Solid doping In terms of quantity, the common dosage of the following admixtures is: 0.5% to 1.0% of naphthalene-based high-efficiency water-reducing agent (accounting for the weight percentage of cement, the same below); 0.5%-1.0% of melamine-based high-efficiency water-reducing agent; sulfamate High-efficiency water-reducing agent 0.4% ~ 0.8%; polycarboxylate high-efficiency water-reducing agent 0.1% ~ 0.4%; sodium lignosulfonate (calcium, magnesium) 0.2% ~ 0.3%; the amount of air-entraining agent is generally Very small (one thousandth to one hundred thousandth); the retarder is special, and its dosage has a great relationship with its type.
Three , The way of adding admixtures
The way of adding admixtures has a great influence on their use effects, especially high-efficiency water reducing agents, air-entraining agents and pumps Delivery of additives and other admixtures. There are usually three methods for adding admixtures: co-mixing, post-mixing and batch addition. They each have their own advantages and disadvantages, but one thing is undeniable: post-mixing and batch addition. It can improve the adaptability of cement and admixtures, enhance the use effect, and even reduce the dosage of admixtures. However, when the high-efficiency superplasticizer is post-mixed, it is easy to cause bleeding.
4. Correctly treat the adaptability of admixtures and cement/admixtures
Different admixtures have different functions and can Modify one or more aspects of concrete. According to the technical specifications for the application of concrete admixtures, a certain admixture that is tested to meet the relevant standards is added to the concrete prepared with cement that can use this type of admixture according to regulations. If the desired effect can be produced, the cement and the additive are compatible; on the contrary, if the desired effect cannot be produced, there is incompatibility between the cement and the additive.
Admixtures that are prone to incompatibility with cement include: lignosulfonate water-reducing agent, naphthalene-based high-efficiency water-reducing agent, air-entraining agent, retarder and quick-setting agent. Coagulant, etc.
There are many factors that affect the adaptability of admixtures and cement in HPC, such as: cement type, cement mineral composition, gypsum form and dosage in cement , cement alkali content, cement fineness, cement freshness, admixture type and dosage, water-binder ratio, etc. Before formulating HPC, several cement and admixture samples must be selected and cross-tested to find the most adaptable admixtures and cement varieties, so as to maximize the performance of HPC and meet construction requirements.
After encountering the problem that the admixture is not suitable for the cement, it can be solved by changing the admixture or cement type, using the post-mixing method of water-reducing agent, etc. </p
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