Common Problems and Solutions in Concrete Mixing

I. Causes and Solutions for slump Loss and unstable Slump of Pumped Concrete

1. Causes of occurrence

Poor compatibility between concrete admixtures and cement leads to rapid loss of concrete slump.

(2) The dosage of concrete admixtures is insufficient, resulting in unsatisfactory retarding and plastic retention effects.

(3) Due to the hot weather, some admixtures fail at high temperatures. The water evaporates quickly. The overflow of air bubbles causes a rapid loss of slump in fresh concrete.

(4) The initial concrete slump is too small and the unit water consumption is too low.

(5) Poor coordination between the construction site and the mixing station led to prolonged delays and traffic jams in the tank trucks, resulting in excessive loss of concrete slump.

(6) The weighing system for concrete mixing has large measurement errors and is unstable.

(7) Changes in moisture content of coarse and fine aggregates.

(8) Cement should be stored in mixed silos and used together.

2. Solutions

Adjust the formula of the concrete admixture to be compatible with the cement. Before construction, it is essential to conduct compatibility tests between concrete admixtures and cement.

(2) Adjust the concrete mix proportion, increase or decrease the sand ratio and water consumption, and adjust the initial slump of the concrete to over 200mm.

(3) Add an appropriate amount of fly ash to replace part of the cement.

(4) Appropriately increase the dosage of concrete admixtures and adjust the slow-setting components in the admixtures (especially when the temperature is much higher than the normal temperature).

(5) Prevent excessive evaporation of water and rapid overflow of bubbles.

(6) Slag cement or pozzolanic cement should be selected.

(7) Improve the water retention and cooling devices of concrete transport vehicles.

(8) The accuracy of measuring equipment should comply with relevant regulations and have valid certificates of conformity issued by the legal metrology department. Self-inspection should be strengthened to ensure the accuracy of measurement.

(9) Strengthen the detection of the moisture content of aggregates. When it changes, adjust the mix ratio in a timely manner.

(10) Cement entering the warehouse should be stored and used separately according to the manufacturer, type and grade.

3. Summarize experiences

Regarding the issue of slump loss in pumped concrete, especially in pumped concrete and underwater cast-in-place pile foundation concrete. Through learning, exploration and experimentation, a set of solutions to problems that are in line with the actual situation has been summarized. For instance, communicate with the admixture manufacturer to improve and adjust the retarding components in the admixture; Adjust the construction time of concrete and try to avoid construction not under high-temperature conditions. The secondary feeding of admixtures in the case of poor road conditions and long distances on construction access roads; Construction using concrete mix ratios with high fly ash content, etc. The thorny problem of concrete slump loss has been greatly alleviated. No matter what causes the slump to decrease, making it impossible to pump or failing to meet the construction requirements, we can adjust it with admixtures to achieve the required slump. This eliminates the bad habit of adding water at will to increase the slump and fundamentally determines the quality of the concrete!