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Hot take: the 28-day cure rule for a slab is way off for the mix I actually pour
I was poking around the Portland Cement Association site last week and read that under lab conditions, concrete gains about 70 percent of its strength in 7 days and only creeps to full design strength around 28 days. I always just repeated that number like gospel, but then I looked at the actual cure log from a 1,400 square foot garage slab I poured in Tacoma back in March. It sat at 62 degrees for the first four days and I watched a cylinder test hit 3,100 psi at day 7, and another at 4,050 by day 14. The spec called for 4,000, so by two weeks I was basically done and still went through the motions for another two weeks. I get why engineers default to 28, it's a safe catch-all, but half my crew treats it like a finish line when the mud has already done most of its work far earlier. If you've pulled cylinders at 7 and 14 on your own pours, what kind of gap are you actually seeing between the two?
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