Workshop Concrete Slab PSI

[avayan]

Hi Group!

I have been out for years but finally my workshop dream is transforming into a plausible reality! I think I am weeks away! While all of the paperwork is shuffled in and out, I have a question regarding concrete slabs. For the workshop, I had been quoted for a 4000 PSI concrete slab. This was in October 2019. What I signed was for 4000 PSI. Intriguingly, as time went by, discussions and plans somehow changed to 3000 PSI. I imagine the reason is cost. Of course I will demand 4000 PSI, but want to be informed into what this all means. Some questions:

1. What is the price difference between 3000 PSI and 4000 PSI? Is this easy to determine by any chance, or is it harder than it seems?

2. Will it be safe to erect a 40x60 metal structure in a 3000 PSI slab? Or is 4000 PSI a requirement?

Thanks for your input!

 

[ConCretin]

I would expect to see about a $10-15 per cy cost difference between 3k and 4k concrete. 3000 psi concrete is plenty adequate from a structural standpoint but I'd go with the 4000 due to it's higher cement content, which will give you a more durable surface and buy you some margin for error in the finishing process. Give my Guide to Floor Slabs in the link below a read for some additional thoughts. Congrats on your project!

 

[ard]

how will **you** actually KNOW what PSI the concrete is?



I imagine having them pour a few test cylinders when they pour the slab- and making sure they know in advance- is an excellent way to be sure. Even if you never test them....

 

[ckeggs]

For most automotive needs 3000psi is more than adequate, if you will rolling heavy dense equipment on the floor 4000psi would be a little better. The difference is 94lbs of cement (1 sack) between the two.

A little trick to "encourage" the ready mix company to send a good mix is to request they send concrete test cylinders out with each truck. Low breaks are concrete mans biggest nightmare and knowing the concrete will be tested often leads to an extra touch of cement added to the load. This just verifies ARD's comment that I didn't see to begin with.

 

[ConCretin]

I don't know who you guys are buying concrete from but being shorted cement is the least of my worries. In this age of computerized batching and litigiousness , I doubt many plant operators are tinkering with the mix to make their corporate overlords a few extra bucks.

Low concrete strengths are far more likely to result from the finishing crew wetting up the load than a lack of cement in the truck. Watch your slumps and you'll be fine.

 

[ard]

Low concrete strengths are far more likely to result from the finishing crew wetting up the load than a lack of cement in the truck. Watch your slumps and you'll be fine.

Funny you mention this.... a neighbor has a long 200'+ driveway. 180 of it is perfect..the last 20 feet- poured at the end of the pour as the truck drove along the driveway, has cracked and settled. Probably into 40+ 'pieces'. In my minds eye, I see the crew adding water to get that last bit 'workable, as the day wore on.

 

[mcbane]

Hi Group!





2. Will it be safe to erect a 40x60 metal structure in a 3000 PSI slab? Or is 4000 PSI a requirement?



Thanks for your input!

No idea whether you are in a hurricane zone but...

Whoever has designed your building should provide column reactions. Get those to a local engineer and ask for a footing design complete with anchor bolt specs. Lots of metal building contractors will sub that out for you and get the footings in for you. Either way it needs to be done if you have high wind loads.



Sent from my iPhone using The Garage Journal mobile app

 

[matt_i]

I don't know who you guys are buying concrete from but being shorted cement is the least of my worries. In this age of computerized batching and litigiousness , I doubt many plant operators are tinkering with the mix to make their corporate overlords a few extra bucks.

Low concrete strengths are far more likely to result from the finishing crew wetting up the load than a lack of cement in the truck. Watch your slumps and you'll be fine.

This is my thought exactly. Even the mom & pop concrete plant I like to use sends a computerized printout of the mix with the driver including all ingredients (like CaCl). They supply whatever you order. You could even call them and ask for an emailed copy and they'll send a .pdf.

I think the mix plant has zero incentive to cheat anyone. They'd be out of business very fast if word got around.

 

[tlmartin84]

We test daily on a commercial level. Our 28 day breaks are always well above (1000 psi) whatever the plan mix called for.

I feel pretty confident in getting whatever PSI they tell you. Admixtures are a whole different ballgame....

 

[stillnostrebor]

Getting the mix designs and adding some concrete testing to the project are both advisable if you have concerns. It'll be easier to get your mix designs if you are supplying the readymix direct, but should not be a big issue either way for a good contractor. Just coming to the conversation prepared to discuss this and armed with the willingness to take on cylinder and slump testing might be enough to get them back in line.

I also work in a world where fresh concrete testing is a religion, so this residential stuff seems very cowboy to me.

 

[ConCretin]

We seem to be talking about several issues here.

The first is the ready mix supplier shorting you on cement, which I wouldn't be concerned about.

The second is that the concrete doesn't make strength for some other reason. This is where cylinders would come in but again, not something I'd be concerned about. Low breaks are exceedingly rare if the concrete is placed at the proper slump.

The third issue would be verifying that your contractor actually ordered the 4000 psi concrete you are paying for. This is a real possibility resulting from dishonesty or more likely, incompetence. This can be easily verified with a quick check of the batch ticket, which the driver(s) will have.

Just to put it in perspective, I've placed hundreds of thousands of yards over the last thirty years, the vast majority of it was tested. I can count on one hand how many low breaks we've had and I don't think we've removed a single yard for failing to meet strength. There are many other aspects to placing a slab I would focus my time on.

The Concrete Underground took something over 700 cy to complete. Not a single cylinder was taken and I sleep just fine at night.