What affects the slump loss so quickly?

Concrete slump mainly refers to the plasticizing performance and pumpability of concrete. The factors that affect concrete slump mainly include gradation changes, moisture content, and weighing of scales. Deviation, the amount of admixtures, and the temperature of the cement are easily overlooked. Slump refers to the workability of concrete, specifically to ensure the normal progress of construction, including the water retention, fluidity and cohesion of concrete.

Causes of slump loss

There are many reasons for slump loss, mainly including the following aspects:

Influence of raw materials

Whether the cement used and the pumping agent match and adapt must be determined through adaptability testing. The dosage of the pumping agent must be tested through the suitability of the cement cementitious material to determine the optimal dosage. The amount of air-entraining and retarding components in the pumping agent has a greater impact on the slump loss of concrete. If there are more air-entraining and retarding components, the slump loss of concrete will be slow, otherwise the loss will be fast. The slump loss of concrete formulated with naphthalene-based high-efficiency superplasticizer is rapid, and the loss is slower when the temperature is below +5°C.

If anhydrite is used as the setting agent in cement, it will cause the slump loss of concrete to accelerate. C3A content in cement is high, so use “R” “type cement, very fine cement fineness, fast cement setting time, etc. will cause the slump loss of concrete to accelerate. The speed of concrete slump loss is related to the quality and amount of mixed materials in the cement. The C3A content in cement should be within 4% to 6%. When the content is lower than 4%, the air-entraining and retardant components should be reduced, otherwise the concrete will not solidify for a long time. When the C3A content is higher than 7%, the content of C3A should be increased. Air entrainment retarding component, otherwise it will cause rapid loss of concrete slump or false setting.

The mud and mud content of the coarse and fine aggregate used in concrete exceeds the standard, and the content of gravel needle and flake particles exceeds the standard, etc., which will cause the slump loss of concrete to accelerate. If the coarse aggregate has a high water absorption rate, especially the gravel used, after being exposed to high temperatures during the high temperature season in summer, it will absorb a large amount of water in a short time once it is put into the mixer, resulting in accelerated slump loss of the concrete in a short period of time (30 minutes). .

Influence of mixing process

The concrete mixing process also has an impact on the slump loss of concrete. The model of the mixer is related to the mixing efficiency. Therefore, the mixer is required to be regularly inspected and the mixing blades are to be replaced regularly. The concrete mixing time cannot be less than 30 seconds. If it is less than 30 seconds, the slump of the concrete will be unstable, resulting in a relatively accelerated slump loss.

Temperature effect

The effect of temperature on the slump loss of concrete should be paid special attention to. When the temperature is above 25°C or 30°C in hot summer, the slump loss of concrete is accelerated by more than 50% compared to 20°C. When the temperature is lower than +5°C, the slump loss of concrete is very small or no loss. . Therefore, when pumping concrete production and construction, we must pay close attention to the effect of temperature on concrete slump.

The high use temperature of raw materials will cause the temperature of concrete to increase and the slump loss to accelerate. It is generally required that the concrete exit temperature should be within 5 to 35°C. If it exceeds this temperature range, corresponding technical measures must be taken, such as adding cold water, ice water, and groundwater to cool down and heat the water and raw material usage temperatures, etc.

Generally, it is required that the maximum operating temperature of cement and admixtures should not be higher than 50℃, and the operating temperature of pumped concrete heating water in winter should not be higher than 40℃, otherwise , not only causes the concrete slump loss to accelerate, but also causes the concrete to set quickly, causing a false setting state in the mixer, making it impossible to get out of the mixer or difficult to transport to the site for unloading.

The higher the temperature of the cementitious material used, the worse the water-reducing component in the pumping agent will be at plasticizing the concrete, and the concrete slump loss will be accelerated. The temperature of concrete is directly proportional to the slump loss. Every time the concrete increases by 5 to 10°C, the slump loss can reach about 20 to 30mm.

Strength grade

The slump loss of concrete is related to the strength grade of concrete. Concrete High-grade concrete loses slump faster than low-grade concrete, and gravel concrete loses faster than pebble concrete. The main reason is related to the unit cement dosage.

Concrete state

Concrete loses slump faster statically than dynamically. When it is dynamic, the concrete is constantly stirred, so that the water-reducing component in the pumping agent cannot fully react with the cement, hindering the hydration progress of the cement, thereby making the slump loss small; when static, the water-reducing component is in full contact with the cement, The cement hydration process is accelerated, so the concrete slump loss is accelerated.

Transportation machinery

The longer the transportation distance and time of the concrete mixer truck, the higher the cost of concrete clinker Due to various reasons such as chemical reactions, water evaporation, and aggregate water absorption, the free moisture is reduced, resulting in a loss of concrete slump over time. Concrete belt conveyors and stringers will also causeMortar loss is also an important cause of concrete slump loss.

Pouring speed and time

During the concrete pouring process, the concrete clinker reaches the silo surface The longer the time, the free moisture in the concrete clinker will rapidly decrease due to chemical reactions, water evaporation, aggregate water absorption and other reasons, resulting in slump loss. Especially when the concrete is exposed to the belt conveyor, the surface The contact area with the external environment is large, and water evaporates quickly, which has the greatest impact on the slump loss of concrete. According to actual measurements, when the temperature is around 25°C, the on-site slump of concrete clinker can be lost up to 4cm within half an hour.

The difference in concrete pouring time is also an important reason for the loss of concrete slump. The impact is smaller in the morning and evening, and the impact is greater at noon and afternoon. The temperature is low in the morning and evening, and water evaporates slowly. The temperature is high at noon and afternoon, and water evaporates quickly. The faster the water loss, the greater the slump loss of concrete, and the strength of concrete. The worse the fluidity, cohesiveness, etc., the harder it is to ensure quality.

Slump loss control

Loss is inevitable, the important thing is how to reduce the loss value Minimize the loss speed to the slowest and minimum to facilitate smooth construction.

Technical measures to control concrete slump loss are multi-channel and multi-faceted. Targeted measures should be taken according to the causes of concrete slump loss. , in order to achieve immediate results, the following measures should be taken.

When the slump loss of commercial concrete is too fast due to unpredictable circumstances and cannot be pumped, the method of adding the agent after pumping can be used to solve the problem. After adding the pumping agent, the concrete transport truck must run quickly for 2 minutes and measure the slump of the concrete to meet the requirements before it can be used. The dosage of the post-adding method should be determined in advance through tests, but it cannot be used arbitrarily multiple times, such as the dosage. Once the limit is exceeded, the concrete will not solidify for several days.

The finer the cement particles, the more water is required. The increase in water demand will inevitably aggravate the slump loss of concrete. To alleviate the slump loss, it is necessary to Pumping agent adaptability test to determine the optimal dosage of pumping agent.

The raw materials used for pumping concrete must be selected. Cement, admixtures, aggregates, pumping agents, etc. must meet the quality required by the standards and be The quality and performance are understood, the frequency of re-inspection is increased, the manufacturers used are relatively stable, the concrete mix ratio is determined through experiments and continuously optimized and summarized through production practice, so as to achieve the optimal concrete mix ratio design and optimal aggregate gradation. , with the least mud content and the smallest slump loss.

The slump of pumped concrete will cause faster loss over time and increase as the temperature increases. The concrete should be poured into the mold within 60 to 90 minutes after it comes out of the machine. The slump loss of concrete will accelerate after 60 minutes, especially during construction in high-temperature seasons. When the daily average temperature is 25°C, it should be put into the mold within 60 minutes. Both parties of concrete production and construction should strengthen communication, scheduling and command. The truck should be started according to the transportation distance and pouring speed. Do not press too much, too long, or exceed the initial setting time of concrete. Concrete should be discarded and cannot be used again. Even if it is diluted with water-reducing agent, it cannot be used. Generally, concrete without retarder cannot be used for more than 5 to 8 hours. Concrete with retarder should be used according to the retarder. Time determined.

Concrete mixing time must be strictly controlled. Controlling the concrete mixing time can effectively reduce concrete slump loss. The mixing time of pumped commercial concrete should be controlled according to the concrete grade. The mixing time of C10~15 concrete shall not be less than 30s; the mixing time of C20~25 concrete shall not be less than 40s; the mixing time of C30~40 concrete shall not be less than 50s; the mixing time of C45 and above concrete The stirring time should be no less than 60 seconds. Concrete with special requirements must be controlled for no less than 60 seconds.

Incorporating air-entraining agents or air-entraining retarder into concrete can effectively reduce concrete slump loss, and the amount of addition is determined through experiments. After adding the air-entraining agent, a large number of fine bubbles are generated, which can isolate a large number of dispersed cement particles, which is an effective way to reduce the secondary adsorption of cement and reduce the slump loss of concrete. The appropriate air content of concrete can also improve the workability and workability of concrete. An effective way to improve cohesiveness, water retention and pumpability.

In the high temperature season of summer, the amount of admixture can be increased through experiments, which can effectively reduce the slump loss of concrete.

Gravel concrete generally has a slump 20 to 30mm higher than that of pebble concrete; the slump of concrete that does not rely on pumping agents increases by one grade ( 20~30mm), the sand rate needs to be increased by more than 1%, the water consumption needs to be increased accordingly by 5~10kg/m3, and the cement needs to be increased by about 10kg/m3.

Some people think that the greater the slump of concrete, the better and the easier it is to pump, but this is wrong. As we all know, increasing slump will inevitably require more water consumption. Increased water consumption will increase W/B, which will inevitably lead to a decrease in concrete strength. If water consumption is increased without limit, concrete segregation, bleeding, pipe blockage, and pump blockage will inevitably occur. . Generally, the maximum slump of C15~C45 concrete should not exceed 220mm, otherwise concrete segregation will inevitably occur and construction will not be suitable.

�The maximum slump of concrete should not exceed 220mm, otherwise concrete segregation will inevitably occur and construction will not be suitable. </p