There are many causes for slump loss in concrete, which are mainly as follows.
The influence of raw materials on slump loss in concrete
Whether the cement and pumping agent match and adapt should be derived from the adaptability test. The amount of pumping agent should be determined through the adaptation test with cement cementitious materials to determine the best amount. The amount of air-entraining and retarding components in the pumping agent has a greater impact on the slump loss in concrete. If the air-entraining, retarding components are high, the slump loss in concrete is slow, otherwise, the loss is fast. The slump loss of concrete formulated with naphthalene high-efficiency water-reducing agent is fast. At a low positive temperature of +5℃ or below, the loss is slower.
If the setting agent in cement is hard gypsum, it will cause the concrete slump loss faster. C3A content in cement, the use of “R” type cement, fine cement, fast setting time of cement, etc. will cause faster slump loss in concrete. The slump loss in concrete is related to the quality of the mixed materials in the cement and the amount of admixture. The C3A content in cement should be within 4% to 6%. If the content is lower than 4%, the air-entraining and retarding agent should be reduced, otherwise, the concrete will not be set for a long time. If the C3A content is higher than 7%, the air-entraining and retarding agent should be increased, otherwise, the concrete slump will be lost soon or the false condensation phenomenon will occur.
The excessive content of cement and mud lumps of coarse and fine aggregates used in concrete, and the excessive content of needle-shaped particles of crushed stone will cause slump loss in concrete faster. If the coarse aggregate absorption rate is large, especially the crushed stone used, after high-temperature exposure in summer, the coarse aggregate put into the mixer will absorb a lot of water in a short time, resulting in concrete slump loss in a short period of time (30min) faster.
The influence of the mixing process on slump loss in concrete
The concrete mixing process has an effect on slump loss in concrete. The type of mixer and mixing efficiency are related. Therefore, the mixer should be regularly serviced and the mixing blades should be replaced regularly. Concrete mixing time should not be less than 30s. if it is less than 30s, the concrete slump is unstable, resulting in relatively faster slump loss.
The influence of temperature on slump loss in concrete
The effect of temperature on slump loss in concrete should be of special concern. When the temperature is greater than 25°C or above 30°C in the hot summer, the concrete slump loss is accelerated by more than 50% relative to that at 20°C. When the temperature is below +5℃, the concrete slump loss is minimal or no loss. Therefore, when pumping concrete production and construction, pay close attention to the impact of temperature on the concrete slump.
The high working temperature of raw materials will cause the concrete temperature to increase and slump loss to accelerate. In general, the temperature of concrete out of the machine should be within 5 to 35 ℃. Beyond this temperature range, we must take the appropriate technical measures, such as adding cold water, ice water, and groundwater to cool the water and raw material’s working temperature, etc.
Generally speaking, the working temperature of cement and admixtures should not be higher than 50℃ at the highest. The working temperature of heating water for pumping concrete in winter should not be higher than 40℃. Otherwise, it will cause the concrete slump loss to accelerate. It will even cause the concrete to set quickly, and in the mixer in a false condensation state, which can not get out of the machine or transport to the site unloading difficulties.
The higher the working temperature of the cementitious material, the less effective the water-reducing component of the pumping agent is in plasticizing the concrete, which will accelerate the loss of concrete slump. Concrete temperature is proportional to slump loss. For every 5-10°C increase in concrete, the slump loss can be about 20-30mm.
The influence of concrete strength grade on slump loss in concrete
Concrete slump loss is related to the strength grade of concrete. High-grade concrete has fast slump loss relative to low-grade concrete. Crushed concrete has faster slump loss than pebble concrete. The main reason for this is related to the amount of cement used per unit.
The influence of the concrete state on slump loss in concrete
Static concrete has faster slump loss than dynamic concrete. When dynamic, the concrete is constantly stirred, so that the water-reducing ingredients in the pumping agent cannot fully react with the cement, which hinders the progress of cement hydration, thus making the slump loss small. When static, the water-reducing ingredients are in full contact with the cement, which accelerates the cement hydration process, so the concrete slump loss is accelerated.
The influence of transportation machinery on slump loss in concrete
The longer the transport distance and time of the concrete mixing and transportation truck, the concrete clinker will have less free water due to various reasons such as chemical reaction, water evaporation, and water absorption of aggregates, causing concrete slump loss. Concrete belt transporters and stringers can cause mortar loss, which is also an important cause of concrete slump loss.
The influence of pouring speed and time on slump loss in concrete
The different time of concrete pouring is an important reason for the loss of concrete slump. The influence is smaller in the morning and evening, and bigger in the noon and afternoon. The temperature in the morning and evening is low, the water evaporates slowly. The temperature at noon and afternoon is high, the water evaporates quickly. The faster the loss of water, the greater the loss of concrete slump, the worse the fluidity of concrete, cohesiveness, etc., and the more difficult to ensure the quality.
Also to read:
How to Improve Cohesiveness of Concrete?
Methods of Winter Construction for Concrete
Slow-setting Problems of Ready-mixed Concrete & the Preventive Measures