Polycarboxylate superplasticizer (PCE) is also called polycarboxylate ether superplasticizer, polycarboxylic ether based superplasticizers, polycarboxylate admixture, polycarboxylate type of superplasticizer. As the most widely used concrete admixtures, it is the best superplasticizer for concrete at present. As the third-generation water-reducing agent, Polycarboxylate superplasticizer (PCE)-based water-reducing agent has the characteristics of low dosage, high water-reducing rate, good slump retention performance, strong adaptability to cement, and small concrete shrinkage. It is precisely because of these advantages, coupled with its own environmental friendliness, that it has been universally recognized at home and abroad. At present, almost all domestic key municipal, railway, bridge and other projects have used Polycarboxylate superplasticizer (PCE) water reducing agents, and commercial concrete mixing stations in some areas have also used Polycarboxylate superplasticizer (PCE) water reducing agents. Many projects have achieved satisfactory results. At the same time, some problems were found in the application. Many technicians reported that the slump loss is very fast after using Polycarboxylate superplasticizer (PCE) water-reducing agent, that is, the slump is very good, but it will become almost no fluidity in a short time. Some technicians are confused about the sampling and testing methods of Polycarboxylate superplasticizer (PCE) water-reducing agent. Many products have a high fluidity of the pure slurry, but the expansion of concrete is very small. This article answers how to deal with the problem and the measures taken.
The use of water-reducing agent for commercial concrete not only improves the workability of concrete, but also becomes an influencing factor of fluctuations in the workability of concrete, and the impact on the slump retention performance is particularly obvious. Poor slump retention performance is mostly due to the significant decrease in the fluidity of concrete during transportation and waiting at the construction site, and the construction performance is greatly reduced after pumping. There are many reasons for slump, including environmental factors (high temperature, strong wind, strong sunlight, etc.), high material temperature when mixing concrete (especially cement), poor water reducing agent formulation, water reducing agent and cementitious material adaptability bad. The technical staff of the concrete company should make statistics on the slump protection during the supply process of the mixed concrete of various brands of water-reducing agents, as the basis for the supplier’s re-assessment, the survival of the fittest, and put pressure on the water-reducing agent suppliers, in order to improve the water-reducing agent and different The adaptability of cement, and adjust the water-reducing agent formula with seasonal changes, so as to reduce the poor workability or serious collapse of concrete caused by poor water-reducing agent formula.
In China, the acceptance basis of water reducer is GB8076-2008 "Concrete admixtures", JG/T223-2007 "Polycarboxylate superplasticizer (PCE) series high-performance water reducers", GB50119-2003 "Specifications for the application of concrete admixtures", GB/ T8077-2000 "Test Method for Homogeneity of Concrete Admixtures". Among them, GB8076 and JG/T223 stipulate the eligibility standards for water-reducing agents, GB/T8077 and JG/T223 stipulate the test methods, and GB50119 stipulates the inspection items to be carried out at the places where water-reducing agents enter. If the local standards stipulate the acceptance items and eligibility standards for the local use of water-reducing agents, they should also be the basis for the acceptance of water-reducing agents by the concrete company. The concrete company should learn and be familiar with the international and domestic similar standards and regulations, clarify the acceptance items, test methods, and qualification standards of various water-reducing agents, and then plan to form the company's water-reducing agent acceptance operation instructions. The acceptance of water-reducing agents by concrete companies cannot be “one size fits all”, nor can it avoid the emphasis. The inspection items corresponding to different testing frequencies should be specified for each entry, batch, and periodical. Different types of water-reducing agents should be checked and accepted.
Pure slurry fluidity testing generally cannot replace concrete performance testing, because the pure slurry index of water reducing agent is easier to meet, so the same water reducing agent has high fluidity and good retention performance, but the mixed concrete expands and protects Poor slump performance. It is recommended that the concrete company set a relatively fixed concrete mix ratio for each type of water reducer according to the performance and use requirements of the water reducer, which is close to the production mix ratio (such as sand ratio, fluidity, use of admixtures, etc.), and enter the site every time Mix 15L to test the workability and slump retention performance of concrete. If the test sample can be sampled from the mixing building immediately, the acceptance effect of the water reducing agent will be better. For the fluctuating performance and poor adaptability of the water-reducing agent found during the acceptance, it is best to communicate with the technical personnel of the water-reducing agent manufacturer in time, and then put it into production after the solution.
In recent years, due to the rapid development of Polycarboxylate superplasticizer (PCE)water-reducing agent technology, the production cost has dropped significantly, even lower than naphthalene-based water-reducing agent, resulting in a rapid increase in users of Polycarboxylate superplasticizer (PCE), and the actual problems of using Polycarboxylate superplasticizer (PCE) water-reducing agent have become more and more serious. It's getting more prominent. Use the following thin sections Note:
(1) When the bulk of Polycarboxylate superplasticizer (PCE) water reducer is weighed, it should be sampled and tested immediately. During the test, the production mix ratio must be used (the sand and gravel can be dried in the sun), and the cement paste method must not be used for the test; the storage time of the cement sample must not exceed one week (if the sampling time is too long, the chemical composition in the cement will change, the test result Inconsistent with the actual production, it is not conducive to guide production); mineral powder and fly ash should also be samples taken out recently; the trial mix quantity during testing should not be less than 10L, if a mixer is used, the trial mix quantity should not be less than 15L, And before trial mixing, use the same proportion mortar to paste the inner wall of the mixer and the iron pan to prevent inaccuracy.
(2) During the test, if the workability of the concrete mixture is found to be good and the slump loss does not exceed 50mm over time, the cargo can be unloaded. If it is found that the concrete mixture is bleeding after mixing, and the ground is gripped (the slurry is secreted to the surface, the sand and gravel are sinking and close to the ground, it is difficult to shovel), it depends on the situation after 1h, if not after 1h Re-segregation, when the slump loss does not exceed 50mm over time, it can also be evaluated as qualified and the unloading will be notified; if the concrete mixture still segregates after 1h, it means that the mixing amount of water reducing agent is too large, and it can be mixed according to the slurry flow. The distance of the compound is reduced by 0.1% to 0.3% (0.1% for distances within 50mm, 0.3% for distances above 120mm, and 0.2% for the intermediate value) and then redo, and unload when qualified. If it is found concrete mix over time of the loss thereof is too large, should be returned (some of the ingredients described cement or fly ash and superplasticizer suited), manufacturers please reformulation test again, until after passing by. In order to save costs this year, some cement manufacturers used some industrial waste gypsum or anhydrous gypsum, which caused the mixture to collapse too quickly over time. Therefore, they should communicate with cement manufacturers in time to prohibit the addition of the above-mentioned minerals.
Temperature control of incoming cement
(1) From May to September, the temperature of the factory cement is relatively high, and the temperature of the cement transported to the mixing station can reach up to 83°C. If the maximum temperature of the day exceeds 30°C, the concrete that is mixed with C30 or higher will collapse too quickly; the exit temperature of the concrete mixture reaches 32-34°C, and it reaches 35°C or higher when it is transported to the construction site. When using Polycarboxylate superplasticizer (PCE) water-reducing agent, if the machine temperature is higher (more than 32℃), it will slump too fast. The factory slump is 220-230mm, but only about 150mm after half an hour. When the Polycarboxylate superplasticizer (PCE) water-reducing agent was inspected in the factory, the loss over time was zero, and it only lost 50mm after 3h. Later, hot cement was specially taken for the water-reducing agent test immediately, and the loss over time was still very small. After analyzing the reasons, the following conclusions are drawn: the test volume is too small, and the temperature drops during the mixing process; and during production, due to the large volume, the cement begins to hydrate during the mixing process, generating heat of hydration, and mixing The temperature of the material continues to rise, causing the mixture to collapse too quickly.
(2) During summer production, the temperature of the incoming cement should be controlled not to exceed 60℃, and the temperature of the concrete mixture should not exceed 30℃. If the above conditions can be met, there will be no rapid collapse.
(3) When the cement temperature cannot meet the requirements, ground water can be used to wash the sand and gravel to cool down; the aluminum foil surface above the pool should reflect sunlight to keep the production water temperature below 20°C.
Adaptability of Polycarboxylate superplasticizer (PCE) water reducing agent
Compared with other water-reducing agents, Polycarboxylate superplasticizer (PCE) water-reducing agents are less adaptable to concrete raw materials, and are compared to the temperature of cement, the mud content of sand and gravel, the composition of mineral admixtures, and the amount of admixtures. sensitive. Therefore, before using the Polycarboxylate superplasticizer (PCE) water reducing agent, it is necessary to test with the bulk used in production, and then put it into production when it is suitable.
At the beginning of application, Polycarboxylate superplasticizer (PCE) admixtures are under the banner of "higher than the naphthalene family". From the aspect of publicity, their advantages are too much emphasized and their disadvantages are weakened or even not mentioned. Excessive expectations were placed on the ads and found to have moisture in actual applications. The author thinks that each material has its advantages and disadvantages, and the two aspects of performance exist at the same time. Compared with other series of admixtures, the adaptability of Polycarboxylate superplasticizer (PCE) admixtures to cementing materials and the adaptability to aggregates has no obvious advantages. First of all, the adaptability to cementitious materials and the adaptability to different varieties and different brands of cement are also very different. The adaptability is not very good, and it is often worse in terms of cohesion and water retention of the concrete mixture. The quality requirements for admixtures, such as fly ash, are still relatively high. When using fly ash that meets the grade II or grade III quality, when some indicators are relatively high, the performance of the concrete mixture will be significantly worse, including fluidity, Slump, air content, slump loss, etc. The adaptability to aggregates is also more sensitive. The mud content in sand and the stone powder content in the stone or the mud content and even the particle size of the sand will obviously affect the state of the mixture. Practical application shows that when the content of the admixture is fixed , the slump index can be reduced by 50-60mm when the content of stone powder is 1%, and the mud content of sand differs by 2% to achieve the same slump. , The water consumption is increased by at least 30kg, and the change of the aggregate particle size can cause the whole plate of concrete to segregate, showing its sensitivity. Therefore, when using Polycarboxylate superplasticizer (PCE) admixtures, the mixing ratio for production should be adjusted according to the changes in the raw materials used . The water reduction rate of the admixture itself should not be too high, and the slump retention time should not be too long, otherwise, things will be counterproductive. It is recommended that general concrete use a medium-low water reduction rate, and the water consumption in the design of the mix ratio is not less than 165kg. Monitor the quality of the admixture. If there is a significant change in the detection index, especially when it deteriorates, the material should be reduced in time To ensure the performance of the mixture. Regularly extract raw materials and conduct concrete trial mixing in the laboratory. If there is a significant change in the test indicators, the mix ratio should be redesigned to guide production.
The function of water reducing agent in concrete is to adjust and improve the workability of fresh concrete and the performance of hardened concrete, which plays an important role in the good workability of fresh concrete. At present, the commonly used water reducers for ordinary concrete in China include naphthalene, aliphatic, melamine and Polycarboxylate superplasticizer (PCE) series. In recent years, Polycarboxylate superplasticizer (PCE) superplasticizers have low toxicity, environmental protection and good performance. The application of ordinary concrete has been developed rapidly, but there are also many problems, which greatly restricts its wider application and development.
Polycarboxylate superplasticizer (PCE) water-reducing agent composition material problem; currently there are two major types of esterified macromonomer type and polyether copolymer type. The synthesis process is complicated. Products produced by different manufacturers and different formulations are in molecular structure, functional group type and content. There are certain changes. In addition, many compounding companies have adopted simple product compounding or compounding of admixtures and components, resulting in the compatibility of water-reducing agents with cement, sand, and admixtures in concrete during use. Affect the setting time and strength of fresh concrete.
For the use of Polycarboxylate superplasticizer (PCE) water reducing agent, to use Polycarboxylate superplasticizer (PCE) water reducing agent, the most basic thing is to do the following two aspects: First, learn to adopt standards and specifications, identify and adjust Polycarboxylate superplasticizer (PCE) water reducer Test method for adaptability of agent and cementitious material. The cement paste test alone can no longer be used to determine the compatibility of admixtures with cement. The reason for the incompatibility may be the characteristics and dosage of cement, the constituent materials of concrete, especially the characteristics of admixtures and sand. Such as mud content and the matching problem of the water reducing agent itself. Learn to use pure paste test (such as pure paste fluidity, Marsh time, etc.), mortar test, concrete test; learn to find the saturation point of the water reducing agent, if the dosage is closer to the saturation point, the easier it is to get better adaptation Sex. The second is that the concrete mixing plant and the water reducing agent company should communicate with each other, understand each other, strengthen operation and management, and cannot unrestrainedly pursue low costs and reduce product quality and stability.
Regarding the sampling and testing methods of Polycarboxylate superplasticizer (PCE) water-reducing agents, the current conventional testing method for mixing stations is to measure the density. As long as the density is appropriate, the finished water-reducing agent is basically considered qualified. The test of Polycarboxylate superplasticizer (PCE) water reducing agent is divided into two parts, one is whether it is qualified as a chemical product, and the other is whether it is qualified as a water reducing agent for concrete. As a chemical polymer product, Polycarboxylate superplasticizer (PCE) water-reducing agent, as long as the raw materials, technology, and production process are strictly controlled, the performance and quality of the obtained water-reducing agent mother liquor are basically stable. The product quality after compounding into a finished product is closely related to the quality of the mother liquor. The auxiliary materials added in the compound can affect the performance of the concrete, but the mother liquor is the decisive factor. Therefore, it is recommended that the batch and performance quality of the mother liquor of the water-reducing agent manufacturer should be clear at the mixing station for the inspection of the finished Polycarboxylate superplasticizer (PCE) water-reducing agent. To measure whether it is qualified as a raw material in concrete, it is necessary to consider the issue of mutual adaptation with other raw materials of concrete. The fluidity of cement paste can only show that it has good adaptability to this kind of cement, specifically to the expansion of concrete. The impact of other raw materials that make up the concrete should also be considered, not just the water reducing agent itself.
Concrete slump loss is Yi very comprehensive question, can not simply think that is the reason of Polycarboxylate. When the polycarboxylate water-reducing agent is initially applied to concrete, it is a key project. The requirements for concrete raw materials are quite strict. The water-reducing agent only plays a role in reducing water, strengthening and improving fluidity; After the acid water reducer is widely used, when the raw materials of concrete are no longer and cannot be strictly required, the water reducer is regarded as a "universal agent". As long as there is any inappropriate state of the concrete, it must be adjusted by the water reducing agent, which is a problem in itself.
Factors affecting slump loss：(1) Cement factor: specific surface area (fineness) and particle form; type and content of gypsum, form, grinding temperature, solubility; alkali content; C3A content; admixture; (2) aggregate factor: aggregate Fineness, aggregate mud content, etc.; (3) Additive factors: water reducer, retarder, air-entraining agent, etc.; ( 4) Construction and environmental factors: ambient temperature; cement dosage, construction mix ratio, construction humidity , The method of admixture, concrete mixing and transportation, etc.; the static slump loss of concrete is faster than the dynamic slump.
Slump loss control method: (1) Determine a reasonable mix ratio; (2) Add an appropriate amount of fly ash; (3) Choose cement with low C3A content; (4) Add plastic preservatives; (5) Add retarders and air-entraining agents; (6) Choose new water-reducing agents; high-slump-retaining water-reducing agents, high-slow-release water-reducing agents, etc.; (7) Change the way of adding water-reducing agents: post-admixing method, multiple post-admixing method, etc.
Concrete slump loss is a common problem in the use of Polycarboxylate superplasticizer (PCE)water-reducing agent. The main reason is to consider the following reasons: (1) The structure of Polycarboxylate superplasticizer (PCE) water-reducing agent itself. As we all know, Polycarboxylate superplasticizer (PCE) water-reducing agent relies on the dual effects of electrostatic repulsion and steric hindrance to play a dispersion effect. If more -COOH, -SO3H and other groups are introduced into the main chain of Polycarboxylate superplasticizer (PCE), it is bound to provide stronger charge repulsion and strong dispersibility to cement, but it is easier to be adsorbed by cement, fine powder, etc., it seems Concrete with good fluidity instantly loses fluidity. (2) The "mud" (particles with a particle size of ≤75um) in the aggregate is at work. In the mixing process of concrete with higher mud content, with the presence of Polycarboxylate superplasticizer (PCE) as a surfactant, the inside of the concrete forms large but extremely unstable bubbles. Due to the existence of air bubbles, the concrete has good fluidity, but the air bubbles will burst instantly, the concrete loses the ball effect of the air bubbles, and the fluidity disappears. (3) The composition of cement. In order to pursue higher efficiency, cement manufacturers add various types of cement grinding aids to make the cement grind finer, and also mix different types of mineral admixtures. As a result, the absorption of the Polycarboxylate superplasticizer (PCE) water-reducing agent increases and the hydration is accelerated, causing the Polycarboxylate superplasticizer (PCE) water-reducing agent to instantly lose its dispersion effect.
In order to better solve this problem, it is recommended that the construction party adjust the molecular structure of the Polycarboxylate superplasticizer (PCE) water-reducing agent instead of blindly pursuing the water reduction rate. The user should not deliberately emphasize the initial fluidity of the concrete, and appropriately improve the Polycarboxylate superplasticizer (PCE) water-reducing agent. Slump retention or add a certain amount of anti-adsorption ingredients. In addition, try to control the quality of cement and admixtures used. Polycarboxylate superplasticizer (PCE) water-reducing agent has higher fluidity and less expansion of concrete, mainly due to the longer main chain of Polycarboxylate superplasticizer (PCE) water-reducing agent, stronger charge repulsion, easier adsorption, and strong dispersibility. The loss is caused quickly. It is recommended that the pure mortar should not be used as the only criterion when receiving samples, and the concrete indicators should be used as much as possible when concrete is conditioned; if there are no conditions, mortar water reduction rate and mortar expansion loss test can be done. When testing mortar Don't use the standard sand prescribed by the state, but use the sand on site (Note: the sand on site is dried first , and the small stones are sifted out with a 2.5mm sieve, and then the lower part of the sieve is evenly mixed and reserved).
The application of Polycarboxylate superplasticizer (PCE)water reducer in concrete is more and more extensive, and there are many problems such as slump loss and bleeding in the practical application of Polycarboxylate superplasticizer (PCE) water reducer. The reason is that the technical departments of many commercial concrete companies do not know enough about the mechanism and characteristics of polycarboxylate water-reducing agents, so that they don't know how to carry out inspections and use them correctly and reasonably. Polycarboxylate superplasticizer (PCE) water-reducing agent is a water-reducing agent that is copolymerized with unsaturated monomers containing carboxyl groups, which makes concrete have good performance in water reduction, slump retention, reinforcement, shrinkage and environmental protection. Polycarboxylate superplasticizer (PCE) water-reducing agent belongs to surfactant. The surfactant molecule is composed of two parts: hydrophilic group and hydrophobic group. After being added to water, the hydrophilic group points to the solution, and the hydrophobic group points to the air, solid or non-polar. Liquids are aligned in a directional arrangement to form an oriented adsorption film to reduce the surface tension of water and the interfacial tension between the two phases. Its basic mechanism is that when a water-reducing agent is added to the cement paste, the hydrophobic groups in the water-reducing agent molecules are directionally adsorbed on the surface of the cement particles, reducing the surface energy. The hydrophilic group points to the aqueous solution, forming a single-molecule or multi-molecule adsorption film on the surface of the cement particles, so that the surface of the cement particles is charged with the same charge, showing repulsion, and the steric hindrance between the water-reducing agent molecules adsorbed on the surface of the cement particles The repulsive force disperses the cement particles, destroys the cement flocculation structure, releases a large amount of free water, and increases the fluidity of the concrete mixture.
Polycarboxylate superplasticizer (PCE) water reducer is more sensitive to cement alkali content. The compatibility of cement and Polycarboxylate superplasticizer (PCE) water reducer is considered from the perspective of rheology, and cement has an optimal soluble alkali content. When it is too low or too high, it will adversely affect the compatibility of the water reducing agent with cement. In order to meet the index requirements of low-alkali cement, the soluble alkali content in the cement cannot reach its optimal value. This will not only cause a large loss of slump when the amount of water reducing agent is insufficient, but the amount of Slightly higher than the saturation point, serious segregation and bleeding will occur; high alkali content in cement will shorten the setting time and increase the early strength. The comprehensive effect of the existing form, water-cement ratio, temperature and other factors of gypsum in cement affects its solubility. When C3A reacts with gypsum, less ettringite is formed, which cannot effectively control the hydration rate of C3A, which speeds up the setting time and causes concrete to collapse. The fall loss becomes faster. The adaptability of water-reducing agent to cement depends on the soluble alkali, fineness, C3A content and type of gypsum. The adsorption rate of water-reducing agent on cement particles and the rate of cement hydration are all affected by these parameters. If the cement paste has good fluidity, but the concrete slump is large, it may be due to the change of the concrete mix ratio, such as the water-cement ratio and the smaller water consumption, that the total amount of soluble SO3 will decrease, resulting in rapid loss of concrete slump. In addition, if the amount of water reducing agent is too small, it cannot continue to function, which will inevitably lead to excessive slump loss.
At present, many commercial concrete companies' technical departments often use technical indicators such as solid content, density, and fluidity of the cement in the inspection of admixtures, and rarely conduct concrete tests. It is recommended that commercial concrete technicians, when using admixtures, first determine the reasonable technical indicators of admixtures and formulate appropriate testing indicators and methods according to the raw materials and mixing ratio of the mixing station. Then, admixture suppliers are selected from the procurement process, taking into account factors such as the manufacturer's production scale, transportation capacity, quality stability, technical service capabilities, and price, but price cannot be used as the only indicator for selection. Finally, when the water reducing agent enters the factory, it must be strictly checked. Some mixing station technicians are confused about the sampling method of Polycarboxylate superplasticizer (PCE) water reducing agent. After a lot of practical experience, the author recommends that the mixing station technicians set the necessary test items for the admixture into the plant as: density, concrete water reduction rate or mortar water reduction rate, which can also be verified by the actual use ratio of C30. The reference index items are set as: cement paste fluidity and solid content, pH value, etc.
The slump loss is very fast after using polycarboxylate water reducer, which is a standard and typical phenomenon that the polycarboxylate water reducer is not compatible with cement.
Polycarboxylate superplasticizer (PCE)-based water-reducing agents are listed as high-performance water-reducing agents in national standards, and it is hoped that they can be more adaptable, safer and more effective than other water-reducing agents to cement. However, the actual situation is always contrary to expectations, and there are always problems of this kind and that in engineering applications. Among them, "the performance of Polycarboxylate superplasticizer (PCE)-based water reducing agents is abnormally unsuitable for certain cements" is one example. According to data, 15 cement samples were collected from different manufacturers, different varieties, and different types, and the same Polycarboxylate superplasticizer (PCE) water-reducing agent was used for adaptability tests, and it was found that 13% of the cement was not suitable. Water reducer does have the problem of not adapting to cement. It is not adaptable to any cement as ordinary people say.
Factors affecting the adaptability of cement and polycarboxylate water-reducing agents: (1) Cement: mineral composition and content ratio, specific surface area of cement, alkali content in cement, type, dosage and solubility of cement setting agent, mixed materials Type and dosage, cement grinding aid, etc.That is, the higher the C3A content, the greater the specific surface area, the higher the alkali content, and the higher the SO3 content, which has the most significant impact on the fluidity of concrete. (2) Types and dosage of Polycarboxylate superplasticizer (PCE) water-reducing agents There are currently two types of Polycarboxylate superplasticizer (PCE) water-reducing agents on the market, namely polyesters and polyethers. From the aspect of performance comparison, polyester is better than polyether; from the aspect of production cost (price), the cost (price) of polyether is lower than polyester. The choice of Polycarboxylate superplasticizer (PCE) water reducing agent must be determined after testing and technical and economic comparison. (3) Concrete mix ratio, especially water-binder ratio, variety and mixing amount of mineral admixture. Among them, the accuracy of water and content measurement and the large mud content of sand have the greatest impact on the fluidity of concrete. (4) The mixing time and feeding sequence of concrete, the temperature during mixing, and the type of mixer will also affect the fluidity of concrete.
The adaptability of cement and Polycarboxylate superplasticizer (PCE)-based water-reducing agents is a very complicated issue, which is affected by many aspects and must be improved through experiments to find out the reasons. (1) For fixed cement (that is, the cement has entered the field), choose a commonly used type of Polycarboxylate superplasticizer (PCE) water reducing agent, which is known to have good adaptability, and several different types of Polycarboxylate superplasticizer (PCE) water reducing agents from different manufacturers. The concrete test is carried out to select the adaptability of various Polycarboxylate superplasticizer (PCE) water-reducing agents. (2) Fixed Polycarboxylate superplasticizer (PCE) water-reducing agent (already in the field). Choose cement. Use the same Polycarboxylate superplasticizer (PCE)-based water-reducing agent and several different types and different brands (different manufacturers) of cement for concrete testing to select the the adaptability of carboxylic acid water reducing agent to various cements. (3) Cement and Polycarboxylate superplasticizer (PCE) water reducer have all entered the market. There is no choice but to modify the Polycarboxylate superplasticizer (PCE) water reducer to adapt to cement. This case can be modified by compounding retarding components to delay the setting time of concrete. It must be reminded here that the concrete mix ratio and various raw materials used on the production line must be selected when doing the concrete test, and the method of pure slurry fluidity and mortar water reduction rate should not be used to replace the concrete test.
Polycarboxylate superplasticizer (PCE) type water reducer, the fluidity of the pure slurry is very large, but the expansion of concrete is small. The fluidity of the pure paste is not closely related to the expansion degree of concrete, and it is not related or comparable.
The high fluidity measured according to the "Cement Paste Fluidity Test Method" indicates that the polycarboxylate water-reducing agent has better adaptability to this type of cement, and the water reduction rate is greater. According to the test method of "slump and slump expansion", when the slump is greater than 220mm, the expansion is measured to prevent the segregation of concrete. When the volume of the slump cylinder is constant, the expansion is large when the slump is large. The water reduction rate of concrete is also large, while the water reduction rate of concrete is small. These two test methods are not related and comparable.
The concrete water reduction rate (slump, expansion) is not closely related to the fluidity of the net slurry. For the sake of quick inspection, commercial concrete companies usually use net slurry fluidity to evaluate the performance of pumping agent products. However, the relationship between the pumping agent product, especially the pumping agent compounded by the Polycarboxylate superplasticizer (PCE)-based water-reducing agent, in the concrete water reduction rate (slump, expansion) and the fluidity of the net slurry is not Close. Based on years of experience, there have been many misjudgments in the test of the adaptability of cement and admixtures by the method of pure slurry fluidity, especially for Polycarboxylate superplasticizer (PCE)-based water-reducing agents. The fluidity of the net slurry is not large, but the slump (expansion) of the concrete is large, and the concrete application effect is also very good. Data shows that there is no direct correlation between the fluidity of cement paste and the water reduction rate (slump, expansion) of concrete, and misjudgment is easy to occur when testing additives.
The cement paste added with Polycarboxylate superplasticizer (PCE) water-reducing agent has a high fluidity, but the concrete expansion is small; or the slump after exiting the machine is large, but the loss is very fast. Generally speaking, the compatibility of cement and water-reducing agent is not good, and the problem lies in the following two aspects: (1) The mixing amount of water-reducing agent in concrete is insufficient, or the mixing amount is lower than the saturation point, resulting in the lack of water-reducing agent. Can give full play to water reducing and slump retention performance. When doing the cement paste fluidity test, in addition to testing the initial fluidity of the paste, the fluidity loss over time for half an hour or one hour must also be tested. It is advisable to choose that the fluidity of the pure paste basically no longer increases significantly with the increase in the amount of water-reducing agent, and the amount of water-reducing agent corresponding to the minimum loss over time is the saturation point, which is used to guide the concrete Doping. (2) If the concrete performance is not good without the first problem mentioned above, it means that the problem lies in other raw material quality indicators or mix ratio parameters. Such as aggregate mud content and mud mass content or sand fineness modulus, water requirement of mineral admixture or compatibility with water reducing agent, cementing material dosage, sand ratio, water consumption, etc. Among these indicators or parameters, the most common indicators that have the greatest impact on the performance of concrete with polycarboxylate water-reducer are the mud content and mud content of the sand. Polycarboxylate superplasticizer (PCE) water-reducing agent is very sensitive to mud, and the mud content of sand should not be greater than 2%, otherwise it is difficult for concrete workability to meet the requirements.
There are many testing indicators for water reducing agents, which are divided into two categories: performance indicators and homogeneity indicators of the tested concrete. The performance index of the inspected concrete is under the standard test conditions (the quality of raw materials and the mixing ratio parameters must meet the standard requirements), expressed as the ratio or difference between the inspected concrete and the benchmark concrete performance, and are used to evaluate the quality of different admixtures . The homogeneity index refers to the performance of the admixture itself, which is used to determine or control the product quality stability. In the actual production process, due to the huge difference between the production conditions and the standard test conditions, from the perspective of use, I personally think that it is only necessary to test the homogeneity index of each batch, and it is not necessary to test the performance index of the tested concrete according to the standard requirements. The indicators can refer to the factory inspection report or the type inspection report, and test or verify the actual proportion of concrete according to the production conditions and adjust the admixture amount according to the actual situation.
For the homogeneity index, the most important thing is the solid content and the fluidity of the pure slurry, and the saturation point should be found. As mentioned above, in actual engineering applications, the high fluidity of the net slurry does not mean that the concrete slump is also large. But this at least shows that the main reason for the abnormal slump is not the problem of cement and admixtures. It should be checked in time from other raw material quality indicators and mix ratio parameters. In addition, when the admixture is applied, the admixture amount should not be lower than the saturation point admixture, in order to give full play to the water reduction and slump retention performance of the admixture, which can minimize the amount of cementitious materials and reduce the cost of concrete. Due to the adsorption effect of aggregate containing mud, its actual content in concrete should not be lower than the saturation point.
Introduce a hydraulic concrete project, the engineering situation is as follows: hydraulic concrete with concrete strength grade of C25W4F150, the square volume is 2000m³, the concrete is poured in sections, and it is transported by a mixer truck with a transport distance of 50km (basic road conditions) It is relatively smooth, but it needs to go through the market), all pumped by car pumps. According to the above engineering conditions, it is necessary to transport concrete over long distances, and there will be some other delays in the transportation time. This requires that the concrete must be transported for a long time, and there may be some accidents on the construction site. Delayed concrete pouring time, add up to indicate that the concrete must have a long initial setting time or the slump loss of the concrete should not be too large, according to the existing mixing station pumping agent (naphthalene series) from the slump loss It seems that it cannot meet the requirements. Look freeze-thaw cycle of the project 150, which requires pumping agent based on the need to reintroduce a part of air-entraining agent works to meet the requirements, and the like is also required impermeability order of W4. Combining the above points, if the existing pumping agent (naphthalene series) is used, a series of adjustments are required, and it may even face the pressure of not meeting the actual demand and cost. In view of this, the mixing station decided to use the third generation of new water reducer-Polycarboxylate superplasticizer (PCE) high performance water reducer.
Experiment 1: As Polycarboxylate superplasticizer (PCE) admixtures have very strict requirements on the mud content of sand and gravel, the mixing station strictly controls the total mud content of the aggregate <2% (good local materials), and other materials are also strictly controlled. First, according to the requirements of the Polycarboxylate superplasticizer (PCE) cost performance of the admixture manufacturer, the mixing station has a mixing amount of 1.8% to 2% and a water reduction rate of more than 24%. The technicians first did an adaptability test of the admixture and cement, and the fluidity of the pure slurry was 240-260mm, and the fluidity was 250-260mm in 1h. From the basic test, it basically meets the adaptability requirements. But this is only a basic experiment, which can only reflect part of the situation of admixtures and cement and cannot reflect the performance of the concrete mixture. It is also necessary to strictly follow the experimental requirements to test the polycarboxylate concrete. Only through trial- matching can the workability of concrete and the mechanical properties of concrete be reflected intuitively. It is found through experiments that the initial workability of the concrete is very satisfactory, and the state of the concrete out of the machine meets the requirements. However, after 5-30s of manual mixing, it is found that the concrete suddenly loses its fluidity and the concrete surface slowly dries out after 15 minutes. The concrete is scattered. According to the above experiment, the experiment was repeated once, and it was found that the results of the two experiments were basically the same: the slump was very good, but the slump loss was large in just a few seconds, so that the concrete was basically directly from the large flowing concrete in about 15 minutes. Change to dry hard concrete. Using the concrete to make a test block, it was found that all the slurry flowed away after manual vibrating with a tamping rod, so that the surface of the test block could not be smoothed. So far it shows that the concrete is not compatible with the admixture.
Experiment 2: After coordination with the admixture factory, the technical staff of the admixture manufacturer brought a part of sodium gluconate to adjust the experiment, adding about 7% of sodium gluconate to the experiment, found that the concrete slump and 1 hour slump There is basically no major change in the degree of water, which meets the experimental requirements, but the concrete will have a small amount of bleeding (the slurry flows from the aggregate), that is, the water retention is worse. After several experiments, including a small amount of sodium gluconate, it will have a certain impact on this aspect and also have a certain impact on the setting time. After several experiments, it is decided to add about 5% sodium gluconate, but there is still a small amount of secretion. water. The later concrete test block pressure experiment basically met the requirements. Due to the urgent need for the supply of concrete, the mixing plant decided to use a slightly secreted Polycarboxylate superplasticizer (PCE) admixture for use.
Actual engineering situation
According to the experimental data and the mastery of the performance of Polycarboxylate superplasticizer (PCE), all personnel in the operation of the mixing plant should be technically informed. When it was put into production, it was found that the slump of the concrete was not lost at all when delivered to the construction site, and the workability was very good. During the process, two vehicles had problems. The concrete had waited for 4 hours and the slump was still very good. This actual production is basically consistent with the experiment, which satisfies the long-distance concrete transportation of concrete and meets the construction requirements in terms of time. However, it was found that the concrete poured on the dam bottom plate experienced a lot of bleeding after vibrating, and even the car pump pumped Bleeding will occur during the flow of the floor concrete, and it is estimated that the concrete will solidify after 30 minutes of pouring. This caused some trouble to the later concrete construction. After the last observation post, the appearance of the concrete base present to meet the requirements, the strength basically resilience to meet the requirements.
Through the above experiments and practical engineering applications, it is not difficult to find that the experiments of Polycarboxylate superplasticizer (PCE) admixtures cannot be carried out completely using some of the original experimental methods, and sometimes they cannot even reflect the relationship between the admixture and cement. The experimental work not only requires some basic experiments on some raw materials, but the ultimate experimental purpose is the concrete itself. No matter what kind of material is checked, the final glue material is the concrete itself, which is the ultimate purpose of the experiment as a concrete mixing station. To ensure that the production is to ensure that the state of the concrete itself. After experiments, it is found that Polycarboxylate superplasticizer (PCE) does have some characteristics of its own, such as the hysteresis of mixing, the sensitivity of materials, etc. should be incorporated into some operating specifications and standards , so as to better guide the experimental production of concrete.
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