How long can the time between mixing and complete discharge from a truck mixer be?

The recommended time between mixing concrete and complete discharge from a truck mixer is critical to ensuring the quality of the concrete mix. The stipulation of 90 minutes or 300 revolutions serves as a guideline to maintain the workability and performance of the concrete.

The reason this timeframe is significant is that concrete begins to lose its workability as hydration starts shortly after mixing. After about 90 minutes, the mixture may start to stiffen due to setting, which can adversely affect the strength and finish of the concrete once it is placed. Additionally, the limit on the number of revolutions emphasizes the importance of not over-mixing, which can lead to segregation of the aggregate and adversely impact the uniformity of the mix.

By adhering to these limits—90 minutes or 300 revolutions—inspectors and concrete suppliers can help ensure that the concrete retains the necessary qualities to perform effectively in its intended application.