Why does pumping concrete have ＂pump loss＂ phenomenon?
Pumping and conveying coagulation construction has become one of the main construction methods. During the pumping process, it is sometimes found that the slump of the concrete entering the pump is normal, but the slump loss after pumping is serious, and even does not meet the construction requirements. This phenomenon is called "slump pumping loss".
Concrete pump loss is a phenomenon produced by concrete under the action of pumping pressure, which is difficult to find with the current commonly used detection methods.
At present, the method of detecting concrete mixture in the construction process is mainly the slump method. The slump method is a reflection of the natural flow of the concrete mixture. It is difficult to find out whether the concrete mixture will have pump loss. The reason is that the current test methods are difficult to reflect the change of concrete workability under the concrete pressure state.
If the concrete mixture wants to flow, it must be driven by liquid water, and the mortar drags the stones to flow, that is, the surface of the solid material is lubricated by liquid water and pushed to flow, and the tiny gaseous bubbles show a "ball" effect, which is beneficial to slurry flow.
Concrete is a three-phase mixture of solid, liquid, and gas. Due to the difference in density and state, the solid, liquid, and gas will show different changes under pressure.
The difference in the movement speed of each component of the concrete mixture under the pumping pressure will cause a certain component of the mixture to separate when it encounters an elbow or a joint. This kind of separation is a kind of dynamic separation with external force, which is different from static separation, and the slump method cannot reflect this separation state.
Pressure effect When the aggregate has many voids and the water absorption rate is too large, the pump pressure can increase and accelerate the water absorption rate of the aggregate, causing free water in the mixture to enter the inside of the aggregate, and the free water in the mixture is reduced. Concrete slump is reduced.
Secondly, under the action of pumping pressure, the tiny air bubbles in the concrete mixture will be deformed or even burst. The reduction of the air bubbles will cause the "ball" effect to disappear, and the concrete mixture will also collapse.
The above reasons may be the cause of pump loss. For these reasons, the pump loss phenomenon may be controlled by controlling the free water content and air content of the concrete mixture.
Under the pumping pressure of the concrete mixture, the free water migrates obviously, and the free water is separated from the concrete mixture, resulting in a decrease in the free water of the concrete mixture and a decrease in the fluidity of the mixture.
Therefore, the essence of overcoming the pump loss phenomenon is to keep its free water during the pumping process and maintain the thickness of the water film on the surface of the solid phase of the concrete mixture.
The water in the concrete mixture is divided into three parts, one part is the water required for cement hydration, the second is the water absorbed by the aggregate, and the last is the free free water in the mixture. Free water is the driving force for the maintenance of the concrete mixture. .
Minimize the consumption of free water during pumping pressure to control slump.
During the pumping process, to maintain the amount of free water, the following suggestions can be used as a reference:
1. Adjust the compatibility of the admixture in the concrete mixture, improve the water retention and fluidity of the concrete mixture, and it is not appropriate to reduce the amount of water reducer mother liquor and slump retaining agent in the admixture.
2. Improve the quality of concrete aggregates, avoid the use of aggregates with many pores and high water absorption, and pay special attention to avoiding the use of aggregates with large mud content and flocculants.
3. Optimize the aggregate gradation, reduce the void ratio of the aggregate, and give priority to the use of coarse and fine aggregates with better particle shape.
4. Add a certain amount of air-entraining agent, foam stabilizer or foam inhibitor to the admixture to reduce the loss of air bubbles in the concrete mixture during pumping.
5. During the concrete pouring process, try to avoid the phenomenon of rolling, waiting for a long time, it is easy to cause the loss of concrete slump, the deterioration of fluidity, and the induction of pump loss.
6. Adjust the amount of admixture according to the state of the concrete mixture, and try to avoid the poor fluidity of the concrete, which usually shows that the mixture has slump but no expansion, poor dynamic feeling, and "dead gray".
7. The temperature of cement is high and the hydration rate is fast. Increase the amount of admixture as much as possible to avoid insufficient admixture, high water consumption, and poor water retention of concrete.
8. Pay attention to check the sealing condition of the sealing ring at the pump pipe joint to ensure no air leakage or slurry leakage.
Other factors that cause concrete "pump loss":
1. The specific surface area of cement is large, and the cement particles are fine. Under the pumping pressure, the hydration of the particles will be accelerated, and more flocculent substances will be formed, which will affect the fluidity.
2. The loss on ignition of the mineral admixture is too large, and the substance with a high water absorption rate has a large content. Under the action of the pump pressure, free water quickly enters the substance
3. The water absorption rate of the aggregate is too high, because the opening pores or cracks on the surface of the aggregate cause some water to infiltrate under pressure, and even some solid particles of the admixture enter the pores and affect the effectiveness of the admixture. Here It also includes the phenomenon of reduced mobility that occurs.
4. The quality of the air-entraining component in the admixture is not good. It should be stable and small bubbles, but the bubbles that are too large and not stable will burst under the action of pressure and lose the "lubricating" effect;
5. Concrete is transported over a long distance, and the greater the horizontal transport distance, the more prone to pump loss, especially in summer, when the temperature of the mixture is high, the impact on its fluidity is greater.
The method of judging that the concrete mixture may have pump loss:
Observe the water retention and cohesion of concrete mixing, test the slump, expansion, and flow rate slump, and judge the pumpability.
For example, during the slump test, the concrete does not flow quickly after the slump cylinder is lifted, indicating that the fluidity is not good;
If the concrete mixture is astringent, divergent, and poor in cohesion, it means that it is not suitable for pumping;
Slurry bleeding and layered bottoming of the concrete mixture indicate poor water retention, and the slurry and water are easily separated by pumping.
The current use of the slump method cannot effectively reflect the pump loss phenomenon under the pumping pressure. The bleeding state of the concrete under the pressure can reflect the water retention capacity of the concrete under the pressure.
The pressure bleeding performance of the concrete mixture is the pressure bleeding rate, which is the percentage of the concrete mixture bleeding within a specified time under a certain pressure. Generally, the relative pressure bleeding rate of the pumped concrete for 10 seconds should not be greater than 40 %.