1 Definition of concrete segregation and manifestations of concrete segregation
Concrete segregation refers to the amount of pumping agent exceeding The saturation point of the cementitious material is exceeded, resulting in a reduction in the resistance between the cementitious material particles and accelerated displacement. The phenomenon of concrete segregation is: when the concrete mixture is piled into a pile, the fluidity of the mixture is very good. Within one minute or a little longer, the slurry will secrete from the surface of the mixture and flow out to the edge, and the surface will be exposed. Pebbles, and there is no mortar on the surface of the pebbles. The sand and gravel sink close to the bottom plate and are difficult to shovel with a shovel. This phenomenon is also called bottom digging.
2 Hazards of concrete mixture segregation
Hazard 1: If the driver of the transport truck In order to save fuel, the tank is not rotated during transportation. When unloading at the construction site, the tank cannot be rotated due to severe eccentricity, resulting in scrapped concrete and a lot of time required to clear the tank.
Hazard 2: It is easy to cause the pump to be blocked. The segregated material will not be scraped off as long as it is stirred from the bottom of the station to being discharged into the pump hopper. There will be no problem during continuous pumping. When the pump is stopped for one minute, the pipe will be blocked (that is, the slurry will float up, and the sand and gravel will sink close to the pipe wall to form a large resistance, making pumping impossible).
Hazard three: Segregated materials and non-segregated materials in the same mix ratio of concrete will reduce the strength of the concrete by about 10%.
Hazard 4: During the pouring process at the construction site, the floating slurry from the separated concrete mixture will flow out along the formwork joints, polluting the construction environment and causing The demander incurs larger cleanup costs.
3 Cause analysis and preventive measures in terms of raw material quality
3.1 Mud content of sand Analysis of causes of instability
3.1.1 Analysis of causes
Due to the increasing resources of natural sand There are fewer and fewer sands, and the mud content of the sand also fluctuates. This phenomenon sometimes occurs even in sand from the same sand field. When the mud content is large, more pumping agent needs to be added because the mud absorbs water-reducing components; when the mud content in the sand suddenly becomes small, if the amount of pumping agent is not reduced, the concrete mixture will segregate. , because the dosage has exceeded its saturation point.
3.1.2 Preventive measures
To maintain the stability of the mud content in the sand, When the test finds that the mud content in the sand is significantly different (≥0.5%), it should be stored separately and not mixed.
3.2 Cause analysis and preventive measures for unstable fly ash quality
3.2.1 Cause analysis
In April and May every year, large power plants have to arrange maintenance, resulting in a reduction in fly ash production. At this time, if the grade or manufacturer of fly ash is frequently changed, when changing from grade III ash to grade II ash, if the dosage of pumping agent is not changed in time, the concrete mixture will segregate. Because two levels of fly ash are pumped into the same silo, it is difficult to predict when the other level of ash will be used. When the same grade of fly ash from different manufacturers is pumped into the same silo, the concrete mixture will also segregate due to its different ignition loss. Because the carbon content of large loss on ignition is high, it is easy to adsorb the pumping agent.
3.2.2 Preventive measures
Use the same manufacturer and the same grade of fly ash, Prevent fly ash from different manufacturers and grades from being mixed into the same silo. If it is really necessary to replace the silo, wait until the fly ash in the silo is used up before adding new fly ash into it. Fly ash of different models or manufacturers must not be mixed.
3.3 Cause analysis and preventive measures for standard consistency water consumption in cement
3.3.1 Cause analysis
When cement is mixed with poor mixing materials, the standard consistency water consumption will increase significantly. If this kind of cement is being used When production is carried out and the water consumption of the standard consistency of the new cement becomes significantly smaller, it is easy to cause the concrete mixture to segregate. The reason is that the admixture in the cement has become better and the adsorption of water-reducing ingredients has decreased. If the dosage of the pumping agent is not reduced in time, it will lead to overmixing of the pumping agent and segregation of the concrete mixture.
3.3.2 Preventive measures
When doing cement inspection, the cement must first be tested Standard consistency water consumption. When the standard consistency water consumption changes significantly, a mix ratio test of the pumping agent should be conducted to determine the appropriate dosage of the pumping agent. When the water consumption of the standard consistency becomes larger, more pumping agent needs to be added to the concrete, and conversely, less pumping agent needs to be added. To maintain the stability of the quality of the cement entering the plant, communicate frequently with the director of the laboratory of the cement plant and require him to promptly notify the mixing station when there are changes in the mixing materials so that countermeasures can be taken.
3.4 Cause analysis and preventive measures of cementitious material temperature
3.4.1 Cause analysis
The temperature of the cementitious material is also an important factor affecting the segregation of the concrete mixture. In fact, the pumpThe test temperature of the agent is quite different from the temperature of the gelling material used in production. The temperature difference range is about 25 to 50°C. The higher the temperature of the gelling material, the more pumping agent is adsorbed. Therefore, the actual production The dosage was increased compared to the experimental dosage. And if the production volume is not continuous, and the work is stopped for a few days after working for a few days, if the amount of pumping agent is not reduced when the work is restarted, segregation may occur. Because the temperature of the cementitious material is relatively high when it is first unloaded into the silo, the temperature gradually decreases as time goes by. When the temperature of the cementitious material drops a lot before concrete is produced, if the amount of pumping agent is not reduced, overmixing will occur.
3.4.2 Preventive measures
The first truck of concrete newly opened every day must be discharged Test the workability of the concrete and check whether there is segregation in the mixture. If there is slight segregation, it can be shipped out of the factory. If there is serious segregation, cementitious materials must be added for adjustment. Slight segregation is when the slurry flows out of the mixture pile within 3cm, and when it is greater than 3cm, it is severe segregation.
4 Cause analysis and preventive measures in factory inspection
4.1 Cause analysis
When it is found that some raw materials are unstable, every vehicle is not inspected. During the factory inspection, only the slump was visually measured, but no material was discharged to observe whether there was segregation.
4.2 Preventive measures
When there are two batches of the same material that are very different, When entering a silo, the usage time period of the two materials should be estimated. The concrete produced during this period must be discharged from each truck to check whether there is segregation. During the mixing process, the production operator mainly observes the stirring current during unloading. If it is found that the stirring current of the concrete mixture in this plate is significantly smaller than the stirring current in the previous plate, the tester should be reminded to take samples for testing to prevent segregation of concrete. </p
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