The water reducing rate of water reducing agent test method

                          The water reducing rate of water reducing agent test method

Water reducing agent is an additive that can reduce the necessary unit water consumption in concrete, meet the specified consistency requirements and improve the workability of concrete. The main functions of water reducing agent are as follows: increasing hydration efficiency, reducing unit water consumption, increasing strength and saving cement consumption; Improve the workability of the concrete that has not yet solidified and prevent the segregation of concrete components; Improve impermeability, reduce water permeability, avoid water leakage of concrete building structure, increase durability and chemical corrosion resistance; Reduce the shrinkage rate of concrete solidification and prevent cracks in concrete components; Improve the frost resistance, which is conducive to winter construction. During the preparation of concrete, mortar and grout, a product that can change the performance of concrete, mortar or grout by adding a small amount (not exceeding 5% of the cement dosage) is called concrete admixture. Adding an appropriate amount of admixture into the concrete can improve the concrete quality, improve the concrete performance, reduce the concrete water consumption, save cement, reduce the cost and speed up the construction progress. With the progress of technology, admixtures have become the fifth necessary material except cement, coarse and fine aggregates, admixtures and water. Admixtures are an important measure to optimize the design of concrete mix proportion and improve the durability of concrete. Therefore, the newly revised code for construction of hydraulic concrete (dl/t5144-2001) emphasizes that an appropriate amount of admixture must be added to hydraulic concrete. 1. Classification of admixtures concrete admixtures can be divided into the following four categories according to their main functions: (1) admixtures to improve the rheological properties of concrete mixtures. Including common water reducing agent, high-efficiency water reducing agent, air entraining agent and pumping agent, etc. (2) Admixtures to adjust the setting time and hardening performance of concrete. Including retarder, early strength agent and accelerator, etc. (3) Admixtures to improve the durability of concrete. Including air entraining agent, waterproof agent, rust inhibitor, etc. (4) Admixtures to improve other properties of concrete. Including expansion agent, antifreeze, colorant, etc. The commonly used admixtures for hydraulic concrete mainly include water reducing agent, retarder, air entraining agent and various composite admixtures, such as retarding water reducing agent or retarding high-efficiency water reducing agent, early strength water reducing agent and air entraining water reducing agent. According to special needs, other types of admixtures, such as pumping agent, waterproof agent, antifreeze, etc., are also used. National and industrial standards such as GB8076-1997 concrete admixtures and dl/t5100-1999 technical specification for hydraulic concrete admixtures have strict requirements on the performance indicators and technical specifications of these admixtures, which can be selected according to the different needs of concrete. 2. Water reducing agent water reducing agent is also called plasticizer or dispersant. When mixing concrete, adding an appropriate amount of water reducing agent can make the cement particles disperse evenly, and release the water content wrapped by the cement particles, which can significantly reduce the water consumption of concrete. The function of water reducing agent is to improve its workability while keeping the mix proportion of concrete unchanged; Or reduce the water consumption and improve the concrete strength while keeping the workability unchanged; Or reduce the cement consumption while maintaining the same strength, so as to save cement and reduce cost. At the same time, the concrete is more uniform and dense after adding water reducing agent, which improves a series of physical and chemical properties, such as impermeability, frost resistance, erosion resistance, etc., and improves the durability of concrete. In the past, the water reducing agents used in hydraulic concrete are generally common water reducing agents such as pulp waste liquid, wood calcium and molasses. The water reducing rate is not high, generally 5% ~ 10%. With the improvement of the quality requirements of hydraulic concrete, the quality requirements of water reducing agent are becoming higher and higher. Naphthalene series superplasticizer is widely used in Ertan, Three Gorges and other large-scale hydropower projects. Its water reduction rate is as high as 20% ~ 30%. It is mainly used to prepare high-strength and high flow concrete or concrete that needs to greatly reduce water consumption. Superplasticizer has obvious effect on improving the use efficiency of cement, and can save about 20% of cement consumption. When the ordinary water reducing agent is used in the granite artificial aggregate concrete of the Three Gorges project, the water consumption is up to about 110 kg/m3. After the optimized zb-1a high-efficiency water reducing agent is used and combined with dh9 air entraining agent and grade I fly ash, the water consumption is reduced to about 85 kg/m3, reaching the advanced level at home and abroad. The Three Gorges project also uses acrylic high-efficiency water reducing agent X404 with a higher water reducing rate of more than 30%. However, this kind of water reducing agent is expensive and is mainly used in high-strength concrete. 3. Retarder retarder can delay the setting and hardening time of concrete and facilitate construction; It can slow down the hydration speed of concrete paste, prolong the hydration heat release process, and is conducive to the temperature control of mass concrete. Retarder will reduce the early strength of concrete in 1 ~ 3 days, but it has no effect on the normal development of late strength. General retarders can prolong the initial setting time of concrete by 1 ~ 4 h, but this is not enough for the concrete construction of large warehouse surface under high temperature. In order to meet the needs of mass concrete construction in high-temperature areas and high-temperature seasons, the national "Eighth Five Year Plan" key scientific and technological research project has developed a high-temperature retarder, which can take 6 ~ 8 hours when the temperature is (35 + 2) ℃ and the relative humidity is (60 + 5)%. At present, the retarding superplasticizer widely used in the Three Gorges Project and other projects can make the initial setting time of concrete reach more than 6 ~ 8 h by appropriately increasing the dosage, which can meet the needs of large warehouse surface concrete construction in the high temperature season of about 35 ℃. 4. Air entraining agent air entraining agent is a kind of surface active substance and one of the commonly used admixtures for concrete. It can introduce a large number of uniformly closed small bubbles from the atmosphere during the mixing process of concrete, so that a certain amount of air is contained in the concrete. A good air entraining agent can introduce as many as 1billion bubbles into the concrete. The pore diameter is mostly 0.05 ~ 0.2 mm. It is generally a discontinuous closed sphere with uniform distribution and good stability, which can significantly improve the frost resistance and durability of the concrete (the internal concrete of the Three Gorges project has more than 150 freeze-thaw cycles, and the external concrete has more than 300 cycles)