Replies

1. 
   StokestheFire | Oct 12, 2002 06:20pm | #1

   I've always gone with 30 days before doing anything further to concrete (Priming, painting, etc.), but if I can spare the time, I prefer 60 days.  Also, check your directions on whichever sealant you're going with.

   "If left is wrong, then right is the only thing left, right?"

1. 
   MikeOuch | Oct 12, 2002 07:07pm | #2

   It's only 1.5" thick so it will cure a little quicker.  What psi was the mix and did you add any curing agents to it?

   Two weeks and you should be fine with a pour that thin.  Wanna get fancy?  Go buy a moisture meter.

   See...those three summers lugging concrete panels out of holes and pouring slabs finally paid off!

   Mike O.

1. 
   Piffin | Oct 12, 2002 10:53pm | #3

   The thickness has nothing to do with the cure time. Put a piece of plastic over it and if the mix was right, in a week, you'll be able to ring a hammer off of it..

   Excellence is its own reward!

1. 
   MikeOuch | Oct 13, 2002 01:37am | #4

   Not exactly...

   Your point about placing plastic over the concrete has more merit than you might realize.  The "curing" of concrete is actually the time during which a chemical reaction is taking place between portland cement and water.  If you seal the concrete (membrane curing compound) or cover it with plastic you will extend the duration of the chemical reaction and thus maximize the strength of the concrete.  Sealed concrete (left in forms and covered on top, sprayed with a membrane or covered with plastic) will only achieve 40% of its potential after 14 days and 70% after 28 days.  Once the water has been allowed to escape by means of evaporation, the chemical reaction stops.  Reintroducing water after the reaction has stopped does not start the reaction process again. 

   To address your point about thickness having nothing to do with curing time: on a pour that is only 1.5" thick, the voids left by evaporated excess water or air entrapment (during mixing process) are more than sufficient to allow the reacting water molecules to themselves evaporate and the chemical reaction thus ceases.

   The fact is that cured and dry concrete are two totally different things.  After 72 hours, that unsealed 1.5" pour will be dry and little or no chemical reaction is taking place...it has reached it's strength potential.  If the small slab had been sealed and therefore kept moist for a month or two, the little shower would have reached it's full strength potential (fully cured). 

   IMHO his little shower will be plenty dry and plenty strong enough to work with after 10 days.

   Mike O. 

   Edited 10/12/2002 6:40:25 PM ET by MIKEOUCH

   Edited 10/12/2002 6:42:32 PM ET by MIKEOUCH

1. 
   Piffin | Oct 13, 2002 02:29am | #5

   Right. My first responce was from thinking that the reason he said it was thin was he was thinking that by letting it dry faster, he was making it stronger which is in fact, the opposite of reality.

   I was saying basically, that he wasn't making it stronger by making it thin. I hadn't considered elaborating on the opposite.

   BTW, I once experimented with this after studying concrete and cements. I took a litre sized plastic soft drink bottle full of water to maintain shape and wrapped it in a wire mesh with a portland and sand mix pasted on and smoothed over to about 3/4" thick. Then I wrapped it with thin plastic like from the dry cleaners to hold it together. Put it in a warm place and unwrapped it three days later. I was amazed at how glass smooth it was! Then I whaked at it a few times, trying to break it.

   Of course, by now I was getting plenty of jokes from the crew about my new 'thermos bottle' but they werre also astonded when it rang almost like a ping from hitting metal.

   Now this is from memory going back over twenty years ago to an article on curing concreeete block so it could be off a little but what I'm remembering is this; cured at ninety degrees with ninety percent humidity, it will gain ninety percent of its strength in_______(I'm thinking it was one week.).

   Excellence is its own reward!

2. 
   MikeOuch | Oct 13, 2002 05:31am | #6

   The timeframe would be about 10 days in that scenario.  Concrete cures to it's optimal strength when sealed from air at 40 degrees over 60 days.  Anything over 120 degrees causes something to happen to the portland and it actually loses a considerable amount of strength.  The portland cement/water mixture freezes at 20 and is pretty much worthless if the freezing occurs within the first 24 hours (really hard to have happen because curing is an exothermic reaction.  I once poured about 120 yards at the VA Hospital in West Haven, CT in temps around 0 (hot mix with accelerants) and we covered the foundation with crete blankets.  The damn thing was steaming three days later when we stripped. 

   Mike O. 

3. 
   Piffin | Oct 13, 2002 06:07am | #7

   I think a lrage part of the focus of that article was on heat as a cuative and how much is enough. As you probably know, Steam heat is used to cure a lot of concrete block. I think these tests were showing that the strength increase pretty well tapered off at about 98 - 99°F and was a pretty flat curve to 120°.

   Excellence is its own reward!

4. 
   smijal | Oct 13, 2002 04:08pm | #8

   Interesting point Mike.  While taking a tour of the Hoover dam a couple of years back, the guide was saying that all that concrete was still giving off heat, and will be for the next 10 or so years!

5. 
   Don | Oct 13, 2002 05:11pm | #9

   Hardest concrete I ever saw was a WW-II pour of a massive block foundation in a chemical manufacturing building at Pine Bluff Arsenal, Ark.  Nothing elegant about it.  They just hollowed out a large hole in the miserable, wet, floodplain clay and poured.  What was the subterranian temp?  About 58 degrees, probably.  Surrounded by wet clay and covered almost immediately by something that trapped moisture in it, it CURED.  Nearly 40 yrs later, we wanted to do some further construction on the site that required taking the concrete out.  (For some cockamamy reason we didn't know the concrete was even there - as-builts were not very good from wartime construction, and in the intervening yrs, the old facility was cleaned out and a plain dirt fill, covered by a standard 4" floor was poured over it.)  There were two problems - we had no idea how big it was - it was massive, we found out via some core borings, and we could NOT break it up - too hard.  No rebar, just a huge chunk of well cured concrete that had not seen the light of day in nearly 40 yrs.  Wound up modifying our designs to accomodate what was already there, figuring that if it hadn't gone anywhere in 40 yrs, it wasn't going anywhere now.  None of the high-priced suits in the Pentagon would believe what we found and its impact on cost, so I got a small box, grabbed a plastic bag of the wet jello-like clay and stuffed it into the box.  On the way through the Little Rock, AR, airport, I bought a small, plastic alligator in the gift shop in a moment of malicious inspiration.  Threw the alligator into the bag of clay.  Got to the 5 -sided puzzle palace and announced to the appropriate official that I would show him what we found, since he had trouble believing telephonic descriptions.  Plopped the box on his beautiful mahogany desk and let him open it.  He only had one eye that worked, and I can still hear the gasp when he opened the bag and was face to face w/ a plastic alligator staring up at him.  We got the money we needed to fix our problem without further ado.  The rest of the story would do Paul Harvey justice; not enough space to lay out the full Monty here, but it was worth more than one laugh. 

   DonThe GlassMasterworks - If it scratches, I etch it!