What is the maximum time limit after which many project specifications consider concrete as rejectable?

Many project specifications dictate that concrete is considered rejectable if it has not been placed within a specific time frame after mixing. A common standard in the industry is that the concrete must be transported and placed within 90 minutes of being mixed. This time limit is based on the fact that once concrete begins its hydration process, its workability decreases, and the risk of improper mixing and inadequate performance of the concrete increases significantly.

Beyond this time frame, the properties of the concrete may be adversely affected, leading to issues such as reduced strength, potential cracking, and overall compromised integrity of the structure. Therefore, adhering to the 90-minute time limit is critical in ensuring that the concrete maintains its intended performance characteristics for the application it is being used for. This time standard helps to ensure the quality and durability of the concrete, making it a widely accepted guideline in project specifications.