Replies

1. 
   Dave_Richeson | Jul 07, 2001 11:18pm | #1

   *
   28 days to reach full compressive strengt. Someone else may know the known the various crush numbers or percentages for 7, 14,& 21 days.

   What mix are you pouring, and are you taking test cylinders?

   Dave

1. 
   Johnnie_Browne | Jul 08, 2001 03:08am | #2

   *It depends on the slump and the mix. If it was poured dry with a strong mix, 100 % in 3 to 7 days is common. If it was wet and a weak mix maybe 28 to 56 days. It really hard to tell not knowing the mix, producer, method, crew.

1. 
   Mad_Dog | Jul 09, 2001 12:55pm | #3

   *28 days, full strength. Can carefully backfill at one week, but longer time is recommended if possible-usually good to get the floor framed in and sheathed to keep everything where it should be, but obviously you need to plan for a chute to reach the basement floor here and there.

1. 
   Andy_Engel_ | Jul 09, 2001 05:21pm | #4

   *Also, to reach the mix's design strength, you have to keep the concrete damp for those 28 days, something I've never seen done in residential work. The numbers are pretty interesting; in the worst drying conditions, the concrete might reach only half of the design strength. It's likely that you won't be able to leave the forms on to hold in the moisture for that time, but hosing down the wall and covering it with poly for even a week can dramatically improve the concrete.Visit the Portland Cement Association's website, http://www.pca.com, for more info on this. Andy

1. 
   d_j_k | Jul 11, 2001 05:12am | #5

   *Thanks for the info.I'll have ask about the slump and the mix. Neverseen anyone spec that or any foundation peoplestate that here in Iowa ( for residential work).Looks like 4 weeks is good to go with.Good point about keeping it damp, I'm not hooked up to the water main yet, or I'd be out there spraying it now. Obviously you can tell I'm current building my own personal home.dj

2. 
   Mad_Dog | Jul 11, 2001 01:10pm | #6

   *If you pour on a Friday, you can feel pretty good about plating it on Monday. With your tall walls, I'd give it as much time up to the 28 days as you can before serious backfilling though. Just don't fall into the gap...

3. 
   Wayne_Law | Jul 12, 2001 11:20pm | #7

   *Because concrete gets stronger over time the "strength" of a concrete mix is defined to be the compressive strength of the mix after curing for 28 days under proper conditions. That is, a "3000 psi" concrete has a comperessive strength of 3000 psi at 28 days. Before 28 days it's not as strong, after 28 days it's stronger. After 5 years it can be one-third stronger than its 28 day strength, so a 3000 psi mix can be nearly 4000 psi eventually.Quoting fromi Architectural Graphic Standards,"the majority of the strength is acheived in the first days of curing. Approximately 50% of the total compressive strength is reached in 3 days; 70% is reached in 7 days. The remaining 30% occurs in the last 21 days at a much slower rate."It takes three days to even reach half strength, so a few extra days of paying attention to curing conditions can make a real difference in ultimate strength. This would be especially true of something like a slab. If the surface dries out the morning after a pour, the surface will be much less durable than if kept moist for a number of days.

4. 
   piffin_ | Jul 13, 2001 12:35am | #8

   *Theoretically, it never stops curing.90 degrees at ninety percent humidity does ninety percent of design strenghth in about three days. But that's ideal situation. Don't happen in field. Less of any of the three variables equals different answer.eg cold weather can mean leave forms on for a week.Other times you can be building and backfilling by then. Most conditions will give 90% by 28 days

5. 
   Johnnie_Browne | Jul 16, 2001 04:28am | #9

   *I test concrete everyday, that my job. It depends on the mix,slump,temp,method. But an average job that was not poured wet will meet 100 % of strength in seven days. If it was poured from a four to six slump it will come in around 21 days. Now this is for a good producer not one of these cheapy house builder producer. We got some that only produce mixes for houseslab that might make 2500 psi in 56 days. Secret, you can get a weak,high flyash mix, pour dry and not get any cracks. The mix is slow to cure so it does not dry out as quick. But yes you get 2/3 of psi in three days. also I tested some concrete that was 83 years old. It went over 8000psi

2. 
   d_j_k | Jul 16, 2001 04:28am | #10

   *
   Anyone have any knowledge on how long 8"thick concrete walls,some
   10' tall some 12' tall (with plenty of steel), takes to reach various stages of strength?

   Thanks
   dj