Yes, the tarps keep moisture in. Concrete is fully cured in 30-40 days. Giving the caps some pitch at the top is also important. I'm betting that big hole is from water standing on top, popping out a bit in the freeze and growing from there. You don't need much, maybe 1/4".
I know Dad knew about wet cement curing. He had a job back when I was a kid helping to replace a millionaire's fairly long dock. The iron pipes supporting the dock were cemented in place to ground rocks on the lake bottom possibly 50 years earlier and under water ever since. Damn that was hard cement. We had to break it up and remove it. I remember asking him why the cement was so hard, he told me something about being under water for long periods made stronger chemical bonds. Still, I don't recall him misting the pillar caps he formed nor covering them with tarp to keep them wet. Thanks for those great tips!
I'm betting that big hole is from water standing on top, popping out a bit in the freeze and growing from there.
Quite possible. Dad was a clever soul but he may have left the tops flat not realizing long term implications. I've been scratching my head about why that hole opened up. All I could think of were some kids must have been pounding a rock on it or something. Your bet sounds more plausible.
Is there an easy way to verify the center is ~¼" higher? Poke a thin rigid wire into the uncured cement to check heights while trowelling?
Build your forms 3.5 inches tall. If you're using molding for detail set that at the 3" mark. If not just notch a half inch out of one end of a piece of 2x4. Fill with cement to the top of molding and vibrate. Add a couple shovels to the middle. Put one end of a piece of wood on the molding and the other end on top of the form. Screed. Repeat on each side several times, adding or removing cement as needed.
Maybe I'll post about this with pictures when I finally get around to it this spring/summer.
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