How long for concrete to cure?

[dqdave1]

It is not theory. As concrete cures it gets stonger; the day it is fully cured it is at its strongest; from that day on the concrete begins to get weaker. The Hoover dam is still curing; the day it completely cures will be the day it begins to get weaker. Yes it will need to be torn down someday as it wil not be able to withstand the water pressure. Don't worry; it will take a few more years. ( hundreds)
You did not mention the type of concrete you used ( 6 sack, 6.5 sack or 7 sack); this also determines time line. At 4" you are at a miniumum depth; I hope you did not use 2 x 4 lumber; this is only 3 1/2" and will not hold up to weight very well; if you used standard concrete forms; they are 5" and are acceptable to the weights you will be putting on floor.

 

[FL4400]

You should have 75% compressive strength within 7 days of the pour this is usually when ASTM recommends doing the first break test. Most of the time concrete is is within its advertised strength by 7 days. When the 28 day break is done the strength is usually above the advertised strength. Saying that if you poured 4000 psi alot of times the breaks are higher. If it has not achieved the compressive strength within the 28 day period then you have a bad mix of concrete. I worked for a testing for a few years and I am ASTM Certified in concrete. My company now has poured equipment pads and set a 57,000lbs building on them the next with only 4500 psi being poured. You could park your tractor on it in 7 days and not have nay problems

 

[RobA]

You should have 75% compressive strength within 7 days of the pour this is usually when ASTM recommends doing the first break test. ...... You could park your tractor on it in 7 days and not have nay problems

That sounds reasonable. The barnbuilder I hired had the concrete poured. I'm not sure of the mix he used. He did use rebar 24" on center. He suggested that I wait about 10 days to 2 weeks for the tractor. I was going to wait 2-1/2 weeks since it fit my schedule. I am more anxious to get other lightweight stuff in there. Sounds like that won't be a problem.