Pump and conveying concrete construction has become the main construction methods. In the pumping process, it is sometimes found that the concrete slump is normal before pumping, but after pumping, the slump loss is serious, and even does not meet the construction requirements. This phenomenon is called “slump pump loss”.
Concrete pump loss is a phenomenon under the action of pumping pressure. It is difficult to find with the current common detection means. At present, the main means of detecting the concrete mixture in the construction process is the slump method. Slump method is a reflection of the natural flow of concrete mixture, so it is difficult to find whether the concrete mixture will have pump loss. The reason is that the current test method is difficult to reflect the change of concrete work under the state of concrete pressure. Concrete mix to flow, it must be under the impetus of liquid water, that is, the solid material surface with the lubrication of liquid water, promote the flow, tiny gaseous bubbles show the “ball” effect, which is beneficial to the slurry flow.
Concrete is a mixture of solid, liquid and gas. Solid, liquid, gas with different density, state, will show different changes under pressure. The difference in the movement speed of each component of the concrete mixture under the pumping pressure causes the separation of some component of the mixture when encountering the elbow and joint. This separation is a dynamic separation with external force, unlike static separation, the slump method does not reflect this separation state.
Pressure action when the aggregate gap is more, the water absorption rate is too large, the effect of pump pressure can increase the water absorption rate of the aggregate, the free water in the mixture reduced, the concrete slump reduced.
Secondly, under the action of pumping pressure, the small bubbles in the concrete mix will deform or even break, the reduction of the bubbles will cause the disappearance of the “ball” effect, and the concrete mix will also collapse.
The above reasons may be the cause of pump loss. For these reasons, the pump loss may be controlled by the control of free water quantity and gas content of concrete mixture.
Under the pumping pressure, the free water migrates significantly, and the free water is separated from the concrete mixture, resulting in the decrease of the free water of the concrete mixture and the decrease of the fluidity of the mixture. Therefore, the essence of overcoming pump loss is to keep its free water in the pumping process and maintain the thickness of water film on the solid phase surface of concrete mixture. The water of concrete mixture is divided three parts, one part is the water needed for cement hydration, followed by the water adsorbed by aggregate, and finally is the free water in the mixture, free water is the maintenance of the concrete mix. Minimize the consumption of free water during the pumping pressure process to control the collapse loss.
Maintain the amount of free water during pumping, with the following suggestion for reference:
(1) Adjust the compatibility of admixture in concrete mix, improve the water retention and fluidity of concrete mix, should not reduce the amount of mother liquid and the amount of slump retention in the admixture.
(2) improve the quality of concrete aggregate, avoid the use of pores, large water absorption rate of aggregate, especially pay attention to avoid the use of large mud content and the aggregate of flocculant.
(3) Optimize the aggregate grading, reduce the aggregate gap ratio, and use preferentially the coarse and fine aggregate with good grain shape.
(4) Add a certain amount of air guiding agent, bubble stabilizing agent or bubble suppressor agent to the admixture to reduce the bubble rupture loss of the concrete mixture in the pumping process.
(5) In the process of concrete pouring, try to avoid the phenomenon of car rolling, waiting for a long time, easy to cause the loss of concrete slump, poor mobility, and induce the pump damage phenomenon.
(6) according to the state of the concrete mixture to adjust the amount of admixture, try to avoid poor concrete fluidity, usually for the mixture has slump without expansion, dynamic poor, “dead ash”.
(7) cement temperature is high, hydration rate is fast, as far as possible to increase the admixture amount, avoid insufficient admixture amount, high water consumption, concrete water retention is poor.
Other factors causing concrete “pump loss”:
(1) The cement specific surface area is large, the cement particles are fine, under the pumping pressure will accelerate the particle hydration, the formation of flocculent substances will be more and affect the fluidity.
(2) The burning loss of mineral admixture is too large, and the material content with a large water absorption rate is too large. Under the action of pump pressure, the free water quickly enters the material inside.
(3) The water absorption rate of the aggregate is too large, because the open hole or fissure of the surface of the aggregate causes part of the water infiltration under pressure, and even some solid particles of admixture enter the hole and affect the effectiveness of the admixture, which also includes the phenomenon of reduced fluidity.
(4) The quality of the air intake component in the admixture is not good, it should be stable small bubbles, but for large and not stable bubbles will burst under the action of pressure and lose the “lubrication” effect;
(5) Concrete long-distance transport, the larger the horizontal conveying distance, the more likely to appear pump damage phenomenon, especially in the summer mixing temperature, the greater the impact on its fluidity.
Methods to judge the possible occurrence of pump loss in the concrete mixture:
Observe the water retention and cohesion of concrete mixing, test the slump, extension, flow rate slump loss, and judge the pumping ability. For example, during the slump test, the concrete does not flow quickly after lifting the slump cylinder indicates that the fluidity is bad; if the concrete mixture is astringent and divergent and has poor cohesion, it is not suitable for pumping; the concrete mixture has pulp and layered bottom indicates poor water retention, and easy slurry and water separation.
At present, the slump method can not effectively reflect the pump loss phenomenon under the pumping pressure, and the bleeding state of concrete under pressure can reflect the water retention capacity of concrete under pressure. The pressure bleeding performance of concrete mixture is the pressure bleeding rate, which is 100% of the concrete mixture in the specified time. Generally, the relative pressure bleeding rate of pumping concrete for 10s should not be greater than 40%.