I poured a small slab 4 days ago using Quikcrete 80 lb bags. I`ve had them for quite a while and they were stored in the garage stacked on top of each other. The whole stack was on a sheet of plastic and not on the concrete floor. When I mixed them in my mixer they seemed to be ok and the concistancy looked fine. However, after 4 days the slab is not hard. It has firmed up but not solid and when I walked on it near the edge the concrete just crumbled away.
I`ve never had this happen before. It`s been in the 50s during the day and high 20s at night. Is this a problem be ause of the freezing temps or possibly bad Quikcrete?
Did you blanket it or use anti-freeze mix? (not the right term but I think you know what I mean)
High 20s is too low for concrete, unless you covered it with insulating blanket . Old sacrete does not help.
Did you mix it up good? Concrete has to be agitated a while to start the chemistry.
In my experience those temperatures are fine as long as the ground isn't a bunch colder.
https://www.quikrete.com/media/newsletter/inthemix/2012/1/in_the_mix_cold_weather_masonry8_20_12.pdf
Using too much water can cause the crumbly mix. I use to drive a mixer truck, one time I brought a load home for a slab for a small shop. I was adding a little water, I don't remember what happened but I ended up adding a LOT! I ended up tossing a little sand in to dry it up a touch, but that slab is pretty chalky. People tend to add too much water, maybe not as bad as I did but still😊
Years ago while pouring taxiway slabs on Dover Air Force base not my forman but another forman at the company had a mix truck driver sit for 6 or 7 hours cause the driver added to much water to his mix. When we did help that that foreman out we would pour 22-24 trucks per section about 16-18" thick, I was a new guy to the crew so I got the task of doing all the grunt work pushing or pulling the cement up too or way from the screen board.
Oh and we would blanket sidewalks and curbs when it was in the 40's
Quote from: bluthum on January 26, 2018, 08:46:20 PM
Did you mix it up good? Concrete has to be agitated a while to start the chemistry.
In my experience those temperatures are fine as long as the ground isn't a bunch colder.
I'm going with the temperature issues suggested. The concrete was mixed fine. I have a motor driven mixer and it mixed fine and I made sure it wasn't packed and not mixed. Maybe in a few days it will harden up some. If not I'll scrape it up and do it all over again.
When was your last cold snap and how much water does your ground hold? I've done jobs where we would tarp the ground for several days before a pour with the black side of the blankets up to attract all the heat we could and have even used space heaters before and after the pour.
With the weather we've been having, even though the top skim is thawed I suspect you poured on frozen ground. We were open when it hit the zero-single digits, the frost went deep.
Concrete takes 7 days to set in warm weather then takes 21 day for full set . cold just slows it down . It will be fine .
I agree Gearbox, they add calcium here when it's cold too.
On a side note one of our local drivers was hauling for a state job paving a highway once and one of the engineers had him run a little out then told him to add 1/2 a gallon of water. So to appease him he added 1/2 gallon to a 10yd load ::), he said he laughing all the way back to the plant.
Here is a concrete set chart: Concrete Set Chart (http://www.engr.psu.edu/ce/courses/ce584/concrete/library/materials/Admixture/Link-settime.htm)
pretty sure it froze it will not get harder , i might know what i am talking about ,,,,i have been doing concrete since 1987 in michigan,,,,quickcrete doesnt go bad with age , concrete cures in 28 days at 70 degrees temp, just my 2 cents worth ,,,
Quote from: jmouton on January 28, 2018, 06:59:08 PM
pretty sure it froze it will not get harder , i might know what i am talking about ,,,,i have been doing concrete since 1987 in michigan,,,,quickcrete doesnt go bad with age , concrete cures in 28 days at 70 degrees temp, just my 2 cents worth ,,,
My guess as well. I plan on redoing it when we have a few days where it will be above freezing. I'm also going to add the concrete antifreeze stuff to be sure.
Keep in mind that curing concrete is what's called an exothermic reaction, ie. it gives off heat. The reaction will take place as long as the mix does not get too cold from the environment where it's placed or as long as it does not run out of water. If the external cooling exceeds the internal heat being generated, it will essentially freeze the reaction and depending on what happens with the free moisture, the mix may never cure or will have reduced strength. Probably best to start over.
When you redo, make sure the ground is not frozen, use hot water and keep the bags of mix in where its warm till you mix. After it's placed, cover with something to keep the heat in place and don't let it dry out till cured.
The best laugh I ever got out of a water and concrete deal was in California. We were loading concrete lined water main pipe to go to El Paso. This was big pipe, one joint to the load. When they finished loading the truck in front of me, they broke for lunch, so I helped him tie down and tarp it. The pipe had shrink wrap on the ends, but still had to be smoke tarped, since he didn't have a smoke tarp we used a full tarp. When we finished up he pulled over to a water hose and watered the tarp down. Now it is summer time and that black tarp was hot enough, you couldn't hold your hand on it, so it dried by the time you could move the hose. I walked over and ask him what he was doing, he said beats me, but they told me I couldn't leave till it was tarped and watered. lol
They intended for you to slice the shrink wrap on the back and put a few gallons inside the pipe, the concrete soaked it up and it keeps it from cracking while transporting, his tarp was dry before he got back in the cab.
starmac we were pouring a pad for a grain bin for my neighbor once, nice little circle about 2 loads, easy pour, the first driver left the plant and had forgot to shut the water off. Needless to say that load required no screeding :D. We told them to send the next one extra dry, there wasn't really any way to measure how much more we'd need because it was just kind of running all over, it did set up eventually and I think it's alright but we weren't impressed at the time :D.
In this area they don't add the water until they get on site!
They just mix it dry while in transit!
Don't know why, but that's the way they do it!
Quote from: Chuck White on January 31, 2018, 03:39:38 PM
In this area they don't add the water until they get on site!
They just mix it dry while in transit!
Don't know why, but that's the way they do it!
They do that to set the slump of the mix at the job,
We always had water added.at the plant, to a spec. If the concrete crew wanted more water added when you got on site, that was done with the understanding that they were taking responsibility for the resulting loss of strength of the concrete. That was written right on our load slips. In practice, I never delivered a load that the site crew didn't add water to, you can't really work with a 6" slump. Sandhills, you guys should have rejected that load, that driver blew it big time.