The slump of concrete mainly refers to its plasticizing and pumpability properties. The factors that affect the slump of concrete mainly include changes in gradation, moisture content, weighing deviation of materials, dosage of admixtures, and the easily overlooked temperature of cement.
The slump mainly affects the workability of concrete, specifically ensuring the water retention, flowability, and cohesiveness of concrete during normal construction.
Workability refers to the ability of concrete to be easily constructed and uniformly compacted, which is a comprehensive performance that includes fluidity, cohesiveness, and water retention. The impact and workability mainly include water content, water cement ratio, sand ratio, as well as cement type, aggregate conditions, time and temperature, additives, and other aspects.
The test method for slump: Use a trumpet shaped slump bucket with an upper opening of 100mm, a lower opening of 200mm, and a height of 300mm. After pouring concrete, compact it and then pull out the bucket. The concrete will collapse due to its own weight. Subtract the height of the highest point of the collapsed concrete from the height of the bucket (300mm) to obtain the slump If the difference is 10mm, the slump is 10.
The slump of concrete should be determined based on the structural section, steel content, transportation distance, pouring method, transportation mode, vibration capacity, and climate conditions of the building. When selecting the mix proportion, comprehensive consideration should be given, and a smaller slump should be adopted. The slump is mainly affected by the following aspects:
1. The impact of concrete raw materials
Due to different storage times and batches, the moisture content of Shahe washed sand is unstable, and there are limitations in determining the moisture content through experiments. Generally, the moisture content of coarse aggregates is relatively stable, but sometimes it can also change. The reason is that aggregate factories are mostly open storage, and the moisture content of aggregates changes after rain. Different water absorption rates of aggregates during concrete mixing can cause varying degrees of deviation in concrete slump.
2. Mechanical and mixing time effects
Long mixing time of concrete can increase the water absorption of aggregates, reduce the free moisture in concrete clinker, and cause loss of concrete slump. The measurement system error of concrete mixing machinery can also cause the loss of concrete slump. The concrete mix ratio is obtained through precise calculation and multiple trial adjustments. Any material with inaccurate measurement will cause a change in the specific surface area of the material per unit. The greater the change in the specific surface area of the material, the greater the loss of slump over time.
3. The impact of concrete transportation machinery
The longer the transportation distance and time of the concrete mixer truck, the less free water in the concrete clinker due to various reasons such as chemical reactions, water evaporation, and aggregate water absorption, resulting in the loss of concrete slump over time. The concrete belt conveyor and drum can also cause mortar loss, which is also an important reason for the loss of concrete slump.
4. The influence of concrete pouring speed
During the concrete pouring process, the longer the concrete clinker reaches the warehouse surface, the more free water in the concrete clinker will rapidly decrease due to various reasons such as chemical reactions, water evaporation, and aggregate water absorption, resulting in slump loss. Especially when the concrete is exposed to a conveyor belt, the surface area in contact with the external environment is large, and water evaporation is rapid, which has the greatest impact on the slump loss of the concrete. According to actual measurements, when the temperature is around 25 ℃, the on-site slump of concrete clinker can lose up to 4cm within half an hour.
5. The influence of concrete pouring time
The different pouring times of concrete are also an important reason for the loss of concrete slump. The impact is relatively small in the morning and evening, but greater in the afternoon and afternoon. The temperature is low in the morning and evening, and the evaporation of water is slow. The temperature is high in the afternoon and afternoon, and the evaporation of water is fast. The faster the water loss, the greater the loss of concrete slump, and the worse the fluidity and cohesiveness of concrete, making it difficult to ensure quality
