The difference in mechanism between delayed bleeding and ordinary bleeding
The difference between delayed bleeding and ordinary bleeding There is a big difference in the mechanism. Ordinary bleeding is caused by the sinking of concrete aggregates under the action of gravity, and excess free moisture is separated from the slurry and precipitated to the surface of the concrete. The way to solve ordinary bleeding is to increase the viscosity of concrete. , increase the sand rate to increase the slurry concentration. The causes of “lag bleeding” can be attributed to two types according to relevant literature. One is the “water absorption balance” of the powder. That is, in concrete with a large amount of mineral admixtures, due to the porous structure of the minerals, the initial bleeding begins. It absorbs water. When the water is adsorbed to a certain extent and reaches saturation, it begins to release excess water, resulting in delayed bleeding; another reason is the hysteresis effect of the retarder.
Examples of solutions to delayed concrete bleeding projects
Discovering problems
The Jintang Bridge of Zhoushan mainland-island project is the third largest cross-sea bridge in China. It has a designed service life of 100 years and uses high-performance marine concrete. As the substructure, the cap is located in the splash zone. It needs a high resistance to chloride ion penetration and a dense concrete surface to block the erosion channels of chlorine salt and delay the erosion time. Therefore, the appearance of the concrete should be uniform, without bubbles and Defects such as sand lines. After the first cap was poured in the project, many sand line defects appeared. The sand lines started from 1m above the bottom of the cap. They were distributed irregularly along the periphery of the cap, with varying widths and lengths until the edge of the top surface of the cap. .
Analyzing the problem
From the perspective of concrete Observation and analysis of the pouring process showed that bleeding began to occur in large quantities 2 to 3 hours after the concrete was poured and formed, and the entire bleeding process continued until 1 hour before the initial setting. From a phenomenon analysis, this is “hysteresis bleeding” that rarely occurs in projects. Solving “hysteresis bleeding” has become the only technical way to eliminate sand line defects.
Find the reason
(1) Initial use mix ratio
The design mix ratio and materials used for the first capping platform are shown in Table 1.
This mix ratio is a mature mix ratio used in the later stage of the Hangzhou Bay Bridge cap. The two places are very similar in terms of regional environment and engineering characteristics. The only biggest difference is Rust inhibitors vary greatly. A migratory complex aminoalcohol rust inhibitor is incorporated into the design mix ratio, which is quite different from the previous design of marine concrete in China. Both Hangzhou Bay and Donghai Bridge use calcium nitrite rust inhibitor, but this type of rust inhibitor is The biggest disadvantage of the agent is that it has a coagulation-promoting effect and pollutes the environment, so it is no longer used in this project. This project uses a migratory compound aminoalcohol organic rust inhibitor. The organic rust inhibitor can migrate into the hardened concrete through diffusion and form a single-molecule protective layer after contact with the steel bars in the concrete. It not only protects the cathode area of the steel bars in the concrete, It also protects the anode area, has dual functions, and is non-toxic.
Due to the short preparation time in the early stage of the project, the above mature mix ratios used in Hangzhou Bay have not been used in this project. More verification tests were conducted, but only the working performance and mechanical properties of the concrete were tested, and the problem of delayed bleeding was not observed.
(2) Indoor verification test of initial mix ratio
When using different rust inhibitors Under the condition, various properties of concrete have changed significantly, especially the setting time of concrete and 2h pressure bleeding. The 28d compressive strength is also reduced by about 5MPa compared with the blank group due to the incorporation of complex aminoalcohol organic rust inhibitor. The initial plasticity of each group of concrete is similar, indicating that the concrete can ensure better workability when mixed with different rust inhibitors.
JT/T537-2004 “Reinforced Concrete Rust Resistance Regulations” has an impact on the initial setting time in the range of -60~+120min, but it can be seen from Table 2 , due to the use of large amounts of mineral admixtures, the actual impact of the rust inhibitor on the initial setting time is beyond the specified range. Compound amino alcohols delay the initial setting by 3 hours, while those using calcium nitrite advance the initial setting by 1 hour.
From the analysis of the phenomenon that the initial setting time is delayed, the compound aminoalcohol rust inhibitor plays a certain role in dispersing the cement particles and delays the hydration. This is one of the causes of “hysteresis”, but it is not the root cause.
From the gas content, it can be analyzed that the compound amino alcohol rust inhibitor introduces a large number of bubbles, and the gas content is significantly increased compared with blank concrete and calcium nitrite. . Introducing 4% to 6% of tiny bubbles into concrete is beneficial to improving the workability, impermeability and freeze-thaw resistance of concrete, but introducing too many bubbles will directly lead to disadvantages such as a decrease in strength. It can be seen from the results in Table 2 that under the condition of 7.5% gas content, the pressure bleeding after 2 hours increased significantly compared with the blank and calcium nitrite groups, reaching 18.7g, which was more than the initial pressure bleeding. This It is contrary to the conventional concrete bleeding law and fundamentally conforms to the current situation of “lag bleeding”. And emptyThe pressure secretion in both the �� group and the calcium nitrite group showed a decreasing and stopping trend after 1 hour. At the same time, the decrease in its 28d strength also shows that excessive bubbles are introduced into the complex amino alcohol rust inhibitor.
The experimental phenomenon explains that the compound amino alcohol rust inhibitor is the fundamental cause of “delayed bleeding”, but the mechanism can only be explained empirically. In the early stage of concrete pouring, a large number of air bubbles introduced will partially disappear under the force of mechanical vibration, but a certain amount of air bubbles will still stay inside the concrete forever. After the air bubbles stabilize, they will play a role in blocking the bleeding channel. However, due to the excessive introduction of bubbles, the initial free water cannot be discharged even under mechanical vibration force. As time goes by, due to the covering force, gravity and disturbance force of the back layer of concrete, the excess bubbles will still burst and form bleeding. channel leading to “delayed secretion”. These channels increase the internal voids of the concrete and reduce the strength of the concrete, thus explaining the real reason why the strength of 28d is lower than that of blank concrete.
Solution
The root cause of “hysteresis bleeding” is the introduction of excessive air bubbles. However, the common bleeding problem can be further studied to optimize the mix ratio so that the concrete itself has less bleeding, so that delayed bleeding is also relatively reduced.
Marine concrete needs to be mixed with various mineral admixtures. Since the microscopic particle structure of each powder is very different, the properties are different, especially for water. Adsorption, reaction time, etc. Therefore, the proportion of powder has become one of the factors affecting bleeding. The different proportions of each component of marine concrete lead to different slurry settling rates of different components under the action of gravity. Choosing the appropriate slurry rate can ensure the uniformity of the concrete and reduce segregation. Therefore, the selection of the sand rate has become another key factor affecting bleeding. In addition, the dosage of additives is also one of the influencing factors.
Based on the above considerations, the proportion of fly ash, the proportion of mineral powder, the sand rate, and the amount of admixture are used as four factors to form a four-factor, three-level The orthogonal test of L9 (34) is used to analyze and determine the optimal mix ratio with the least bleeding.
1) Optimization of mix ratio and materials
2) Adjustment of admixtures
Due to the air-entraining effect of complex aminoalcohol, the air-entraining effect of the concrete itself increases. Therefore, the air-entrainment must be adjusted in the water-reducing agent. /10000 air-entraining agent reduced to 0.3/10000.
3) Test results of optimized mix ratio
After further optimization and adjustment of the mix ratio Basically, the amount of air-entraining agent in the water-reducing agent is reduced. The test results are shown in Table 4. The pressure bleeding in 140s is 7.5g, and the pressure bleeding in 2h is only 2.3g, which greatly reduces the bleeding and fundamentally eliminates the phenomenon of “lag bleeding”.
Conclusion
Bleeding occurred when the cap was poured using the mix ratio in Table 3 Fundamental improvements have been made. After all 160 caps were poured, there was no delayed bleeding. In the later stage, the sand line defects no longer appeared in the caps poured with this mix ratio, thus fundamentally solving the “delayed bleeding” problem caused by the new rust inhibitor.
Suggestions
“Delayed bleeding” in projects is rare and needs to be solved The approach should be considered from the following aspects:
First, by optimizing the mix ratio and selecting high-quality materials to ensure the water retention performance of the concrete itself, the optimized mix ratio should start from the powder ratio, sand Factors such as rate and admixture dosage should be considered to select the optimal ratio. At the same time, more importantly, the performance of the concrete should be analyzed and judged together with the admixture manufacturer, so that adjustments can be made from the components of the admixture to achieve the purpose of cost-effectiveness.
The second is to fully understand the performance and chemical mechanism of new materials. In the future development of high-performance concrete, it is inevitable to adopt more and more new materials. materials to adapt to different usage requirements. For problems arising in concrete construction, the impact of new materials should first be considered. The simplest method is to extract the material for testing to compare and judge the impact of new materials.
Third, the slump of concrete needs to be strictly controlled during construction. When the slump exceeds 20cm, excess free water will appear and precipitate quickly, from the surface It seems that it is due to the poor water retention performance of concrete. In fact, it is because there are no free bubbles inside the concrete to block the bleeding channel, and the free water cannot stay and precipitate. In this case, secondary vibration should be strengthened to eliminate the bleeding caused by bleeding. aisle.
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