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Here’s a quick quiz for you: What’s similar between the workings of the human brain and the acceptance criteria for a freshly batched load of ready-mixed concrete? Both brain activity and the concrete mix have a similar period of optimal use – 90 minutes.
Have you ever wondered about the ideal length of events like concerts, movies, and football games? These events reflect our biological rhythms. But researchers have also identified a pattern that is shorter in duration and that can play an important role in our workplace productivity. These shorter cycles are called ultradian rhythms. In fact, researchers have shown that our actions are controlled in cycles that have alternating periods of high-frequency brain activity (about 90 minutes) followed by lower-frequency brain activity (about 20 minutes). Now you know why after about an hour and a half of work you need to take a break from a difficult task to refocus your efforts.
So intrinsic is our biological rhythm that productivity experts are using them to create systems to optimize work activities. For example, Hive is a start-up that markets a project management and collaboration platform incorporating this 90-minute brian activity rule. Hive’s users employ their platform to coordinate social media, meetings, and project managements to maximize productivity. The company’s growth has been steady as many organizations try to incorporate work-at-home practices into corporate success.
So now perhaps we understand why Section 12.9 of ASTM C94 was added in the 1930s. This section currently reads “Discharge of the concrete shall be completed within 1 1⁄2 h after the introduction of the mixing water to the cement and aggregates or the introduction of the cement to the aggregates.”
I believe that the genesis of the 90-minute rule was the optimal attention span of the concrete truck driver. It’s as good a reason as any for the 90-minute rule.
Today, there's a debate on why the 90-minute rule was adopted. Some believe the rule was based on the amount of fuel in a pony motor that operated the drum’s rotation on mixer trucks of the 1930s. Others believe that the duration had a mathematical relationship to the 300-rotation rule for the drum that was also incorporated in C94 in those days. And there are some who believe that there must be a rheological relationship to the ingredients used at the time and strength gain.
The 90-minute rule has been the bane of the concrete industry ever since it was adopted. Many good loads of concrete have been rejected when delivery occurred 91 minutes after batching. And just as important, many professionals believe that the rule has allowed suspect yards of concrete to be used only because the load was within the time limit. Currently, there is a great debate on whether the 90-minute rule should be modified to match current operating conditions and practices. This section of ASTM C94 is under ballot, with hopes of a resolution in December.
Regardless of the merits of the current language, I believe it’s time to set the matter straight on why the 90-rule was originally established. Knowledge of the past can provide a path for the future
As chair of the ACI History Committee (ACI 120), I’m asking my committee to create a document that will provide a historical perspective on how the 90-minute standard was first developed. I’ve scheduled a virtual meeting on May 27 to create a task group to search the historical records. Let me know if you’d like a meeting invite.
I also encourage you to send me any information you might have on the historical background of this aspect of ASTM C94. What seems a simple consideration is actually very complex. In addition to the historical reasons for the language adoption, I hope to provide some perspectives on the challenges this important consideration has created. Resolving the status of the 90-minute rule in ASTM C94 could determine the future of the ready-mixed concrete industry.