Slump loss in concrete is inevitable. The important thing is how to reduce the loss value to the minimum and the loss rate to the slowest and smallest in order to facilitate smooth construction.
Technical measures to control slump loss in concrete are multi-path and multi-faceted. According to the causes of slump loss in concrete, targeted measures should be taken in order to achieve immediate results. The following measures should be taken mainly.
1. In the unpredictable situation causing the slump loss in concrete too fast and can not be pumped, you can take the method of post-adding pumping agent to solve. After adding the pumping agent, the concrete truck must be fast running 2min and then measure the concrete slump to meet the requirements before use. The amount of post-adding pumping agent should be determined in advance through the test, but can not be used arbitrarily many times. If the amount of admixture exceeds the limit, it will cause the concrete not to be set for several days.
2. The finer the cement particles, the more water is required. Increased water demand will certainly aggravate the slump loss in concrete. To alleviate slump loss, the pumping agent must be adapted to the test to determine the optimal amount of the pumping agent.
3. The raw materials for pumping concrete must be preferably selected. Cement, admixtures, aggregates, pumping agents, etc. must meet the quality required by the standard. Increase the frequency of re-inspection. The concrete mix ratio is determined by tests and constantly selected and summarized through production practice so that the concrete mix ratio design is optimal, aggregate gradation is optimal, mud content is minimal, and slump loss is minimal.
4. The slump of pumped concrete will be accelerated with time and increased with temperature. Concrete should be poured into the mold within 60-90min after it comes out of the machine. Slump loss in concrete will be accelerated after 60min, especially in the high-temperature season construction is more obvious.
When the average daily temperature <25℃, it is appropriate to enter the mold within 90min after leaving the machine. When >25℃, it is advisable to enter the mold within 60min. Both sides of concrete production and construction should strengthen liaison, scheduling, and command. The trucks should be sent according to the distance and pouring speed. Concrete that exceeds the initial setting time of concrete should be discarded, and can not be used, even after the addition of water-reducing agent thinning can not be used. Generally concrete without retarder can no longer be used beyond 5 to 8h. Concrete mixed with retarding agent, according to the retarding time to determine.
5. Concrete mixing time should be strictly controlled. Control of concrete mixing time can effectively reduce the slump loss in concrete. The mixing time of pumped commercial concrete should be controlled according to the concrete grade. C10 ~ 15 concrete mixing time is not less than 30s. C20 ~ 25 concrete mixing time is not less than 40s. C30 ~ 40 concrete mixing time is not less than 50s. C45 concrete mixing time is not less than 60s. The mixing time of concrete with special requirements should be controlled to not less than 60s.
6. Concrete mixed with an air-entraining agent or air-entraining retarder can effectively reduce slump loss in concrete. The amount of admixture is determined by the test. After mixing with the air-entraining agent, a large number of fine bubbles can be generated to isolate a large number of dispersed cement particles, which is an effective way to reduce the secondary adsorption of cement to reduce the slump loss in concrete. The appropriate air content of concrete is also an effective way to improve concrete consistency, cohesiveness, water retention, and pumpability.
7. In the high-temperature summer, you can test to increase the amount of admixture, which can effectively reduce the loss of concrete slump
8. Crushed concrete generally increases the slump of 20 ~ 30mm more than pebble concrete. Concrete slump without pumping agent each increase in grade (20 ~ 30mm) needs to increase the concrete sand ratio by more than 1%, water consumption needs to be correspondingly increased by 5 ~ 10kg / m3, and cement increased by about 10kg / m3.
Some people believe that the greater the slump of concrete the better, the easier to pump, which is wrong. As we all know, increasing the slump is bound to increase the amount of water used. With more water, W/B increases, which inevitably leads to a decrease in concrete strength. If the unlimited increase in water consumption, it is bound to cause concrete segregation, water secretion, blocking the pipe, and blocking the pump. Generally, C15 ~ C45 concrete slump should not exceed a maximum of 220mm, otherwise, it is bound to appear concrete segregation, unsuitable for construction.
Therefore, the amount of water used for pumping concrete should be strictly controlled. When the concrete mix is not added with water, there is no connection between various raw materials. After the water is added, the various materials are organically linked together. This is when the hydration of the cement begins. The amount of water plays a big role in the concrete, and it dominates the quality of pumped concrete.
Sometimes from the observation of the appearance of the concrete, it seems that the concrete slump is not too big. After the actual measurement, the concrete slump is very large. Especially above C40 concrete, it is easy to cause the illusion of artificial. The main reason is due to over C40 concrete cementitious materials, viscosity is larger, and the expansion degree is smaller.
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