How should concrete batching plants handle returned concrete?

Strengthen the quality control of returned concrete processing to achieve scientific and rational utilization of returned concrete, reduce waste, and increase efficiency. The concrete returned to the station should be considered for downgraded use first. When it is not possible to lower the grade of use, the reasons should be identified according to the situation, the performance of the concrete should be judged, and then reasonable treatment should be carried out.
Collapse too large or too small
If the slump is too large (or segregation), the same proportion of dry material or thick mortar should be added according to the actual situation, and quickly mixed for not less than 90 seconds. After inspection, there are no dry material blocks, the slump meets the design requirements, and the actual water cement ratio is not greater than the theoretical water cement ratio before leaving the station. If the slump is too small, cement should be added appropriately, followed by the addition of admixtures and forced mixing until the slump meets the exit requirements.
Concrete return caused by construction reasons
The returned concrete caused by various reasons such as transportation, pumping, pouring, etc. shall first be weighed by the tank truck driver. The quality inspector shall calculate the remaining volume based on the weighing list provided by the tank truck driver and the tare weight of the tank truck. Then, according to the same or lower grade, the following handling methods shall be adopted:
Same level use
The delay time should be within 2 hours. After processing, the cement dosage should be increased by 10-15 kg/m3 compared to the original mix proportion, and the admixture should be increased by 0.1% to 0.3%. Or directly reduce the strength level by one and adjust the dosage of additives appropriately. The delay time is 2-4 hours. After treatment, the cement dosage should be increased by 20-40kg/m3 compared to the original mix proportion, and the admixture should be increased by 0.4% to 0.6%. Or directly reduce two strength grades and adjust the dosage of additives appropriately.
During the initial setting time of concrete, due to high temperatures, long transportation distances, and long loading times, the slump loss of concrete is significant. It is possible to add the same type of pumping agent or water reducing agent through experiments, but the total amount added cannot exceed 20% of the large amount of admixture added. It should be gradually added, with less added first. After adding, the mixer truck should be quickly rotated for about 2 minutes to determine if the slump meets the requirements. The concrete transported to the site cannot be unloaded immediately. The transport vehicle should be quickly rotated for 2 minutes before unloading to ensure slump and quality. Concrete that has exceeded its initial setting time should be discarded and cannot be reused. The longer the time of stagnation before pouring concrete, the more severe the loss of concrete strength, which should be given sufficient attention.