How to Identify Concrete PSI

[High5]

We are having a garage floor poured this summer. The spec calls for concrete rated at 4000psi. Most concrete used in home construction is rated at 2500psi. So how do I know what I ordered is in fact in the truck's drum? Do you just take the driver's word for it?

Thanks for your help.

 

[brownbagg]

swiss hammer test

 

[LX-Markham]

Have a testing company sample the concrete coming out of the chute.
3 test cylinders: 7-day, and two 28-day.

Ask to review the concrete mix design. It will likely be a performance based spec.

 

[Shootinok]

Read the batch ticket.
And - Ask your supplier to provide you with the 4K mix design.
The driver will have a ticket that lists everything in the mix that was batched into his drum. The ticket will list the mix ID, some list the PSI and all will list the components by the pound that is in the mix. The concrete companies are regulated to sell you what you paid for, not unlike gas pumps providing the right ooctane gas. Their scales are certified etc.
Check it before they start pouring and you'll be sure they brought the right concrete.

 

[High5]

Read the batch ticket.
And - Ask your supplier to provide you with the 4K mix design.
The driver will have a ticket that lists everything in the mix that was batched into his drum. The ticket will list the mix ID, some list the PSI and all will list the components by the pound that is in the mix. The concrete companies are regulated to sell you what you paid for, not unlike gas pumps providing the right ooctane gas. Their scales are certified etc.
Check it before they start pouring and you'll be sure they brought the right concrete.

This is what I was looking for. Thank you!!!

 

[TheBigMortboski]

I drove concrete mixer for 7 years, what shootinok says is correct. We didn't let a mix design leave quality control until it consistently made 120% of spec psi, so your 4000 mix will likely break at 4800 or higher.

 

[High5]

Thanks fellas. Much appreciated.

 

[Avplayer]

This is great info. Thanx

 

[pop pop]

Experience says LX has ordered spec concrete before. Don't care what the mix plant and ticket say, it's the break test that matter. I've refused concrete on site and the driver got fired. Ticket/spec was perfect however.

 

[LXCam]

If you're truly concerned with it hire a inspections and testing firm. It'll cost you a few hundred bucks but well worth it if you're worried about it or if its required. Out here anything greater than 3,000PSI for commercial construction requires special inspections and testing.

 

[alwaysFlOoReD]

Make sure the driver doesn't add water after the samples are pulled. They can add plasticizers if needed by the finishers.

 

[TheBigMortboski]

Technically nothing can be added after inspection samples are pulled. They have to be tested as placed.

And if this is just a regular garage floor, I think you guys are over thinking this a bit. Usually only large projects or gubment jobs are inspected unless the contractor wants to pay for it, which they never do. And 4000psi is plenty for a garage floor. What I'd be concerned about is the contractor (not the driver! He doesn't care how wet it is as long as it comes out the chute) over watering it. If placed too wet the aggregate and sand sinks to the bottom, leaving just cement powder and water at the top. This lack of consolidation weakens the top, leading to spalting and spider cracks. Compressive strength is arguably the least important thing in a garage slab, unless it's under a lift.

 

[High5]

Technically nothing can be added after inspection samples are pulled. They have to be tested as placed.

And if this is just a regular garage floor, I think you guys are over thinking this a bit. Usually only large projects or gubment jobs are inspected unless the contractor wants to pay for it, which they never do. And 4000psi is plenty for a garage floor. What I'd be concerned about is the contractor (not the driver! He doesn't care how wet it is as long as it comes out the chute) over watering it. If placed too wet the aggregate and sand sinks to the bottom, leaving just cement powder and water at the top. This lack of consolidation weakens the top, leading to spalting and spider cracks. Compressive strength is arguably the least important thing in a garage slab, unless it's under a lift.

We are planning to install two lifts. And I also plan on bringing in commercial machining equipment. So I am concerned about compression strength. If it weren't for the fact that our specifications document has not been followed to the letter, I would not be as concerned. So hopefully reviewing the batch ticket with what the supplier shares with us will be sufficient. We'll take the testing procedure into consideration.

 

[TheBigMortboski]

There will be an allowable water number on the ticket. As long as they don't add any more than that, the supplier will guarantee the material. The rest is up to your contractor.

 

[LXCam]

If it weren't for the fact that our specifications document has not been followed to the letter, I would not be as concerned..

Why, who do you have pouring it - Lydig Construction?



Sorry OP, I don't expect you or anyone else to get that one but ya never know.


BTW, how many yards are you pouring?. If it's a big pour than I would suggest you hire a inspections and testing company to keep everyone honest considering that statement ^^.

 

[TheBigMortboski]

I got it. They come up with some weird s**t.

 

[matt_i]

And I also plan on bringing in commercial machining equipment. So I am concerned about compression strength.

I designed and prepped a slab with the idea of setting heavy machinery all over it, rigged in by forklifts.

Personally I'd be more concerned about your rebar grid, set on chairs, wire-tied intersections, placed with a pump truck to avoid damaging the grid, the compaction and thickness of the crushed, washed stone (not sand) base, vapor barrier, curing sealant.

Getting the compressive strength correct is like the easiest thing in my mind on the list of all the things to get done correctly. All of the above stuff is more of a challenge, imo.

I poured 4ksi, 6" thick. #4 rebar, 16" on center, on 2" chairs. Saw cut 10' to 12' squares.

More info & pics here
http://www.garagejournal.com/forum/showthread.php?t=319602

 

[ilovevocs]

^^ this.

Personally I feel that most failures in residential construction are more a result of poor sub base compaction methods than anything else.

 

[High5]

I designed and prepped a slab with the idea of setting heavy machinery all over it, rigged in by forklifts.

Personally I'd be more concerned about your rebar grid, set on chairs, wire-tied intersections, placed with a pump truck to avoid damaging the grid, the compaction and thickness of the crushed, washed stone (not sand) base, vapor barrier, curing sealant.

Getting the compressive strength correct is like the easiest thing in my mind on the list of all the things to get done correctly. All of the above stuff is more of a challenge, imo.

I poured 4ksi, 6" thick. #4 rebar, 16" on center, on 2" chairs. Saw cut 10' to 12' squares.

More info & pics here
http://www.garagejournal.com/forum/showthread.php?t=319602

Matt, thank you for your insight. We agree with you and others that the sub base is critical. And we have most all the information you guys have shared in our specifications. Thank you for your link. Wow, what a job. I haven't had a chance to go through the whole thing yet. But I will.

When it comes to construction, most all elements of the build are there in front of you to examine and say yes or no to. But when it comes to the concrete, it all looks the same coming out of the drum (doesn't it?). That's why I brought up this question in the first place. I do not envision anyone trying to alter the formula. But we have witnessed communication breakdowns between the general; their sub's and suppliers. I just want to make sure we are getting what we paid for.

Thanks guys.

 

[Chris705]

Same folks who test / break concrete often do soils work and can stop by a couple times to review subgrade compaction (density) if the OP wants to get their input...could probably negotiate a number well in advance of the work. Let the contractor know that the work will be tested and it will probably get him to pay a bit more attention to the work he is performing.

 

[High5]

Same folks who test / break concrete often do soils work and can stop by a couple times to review subgrade compaction (density) if the OP wants to get their input...could probably negotiate a number well in advance of the work. Let the contractor know that the work will be tested and it will probably get him to pay a bit more attention to the work he is performing.

Chris, we had to go through an extensive soil review. The city required a soil log be conducted. So a Geotech was on site during most of the excavation phase. The garage is actually constructed. We have siding, windows, electrical, plumbing, and the slab to go.

Thank you for your suggestion.

 

[LX-Markham]

And if this is just a regular garage floor, I think you guys are over thinking this a bit. Usually only large projects or gubment jobs are inspected unless the contractor wants to pay for it, which they never do. And 4000psi is plenty for a garage floor.

I don't disagree that making some test cylinders for a residential garage slab may be overkill, but this IS the garage journal and the OP did ask how do you tell what strength of concrete you are getting.

I also agree that there are many other more important factors in getting a good slab. Sub-base of course, and proper rebar placement, but often proper curing is overlooked.

 

[lakeroadster]

Going To Pour Concrete, How Will I Know I Got The Strength I Wanted?

I’ve tried to use the saying “Trust, but Verify” whenever I am working with a contractor. The following is what I did when the slab was poured on my barn to ensure I received the concrete I asked for. My contractor and I agreed to this, and everything went great. Additionally I had a local testing lab take 5 samples the day of the pour and perform mechanical tests at 14 day, 28 days and 42 days. More on that starting at Post #163 here: http://www.garagejournal.com/forum/showthread.php?t=289293&page=9

A couple weeks before the pour get a copy of the concrete plants “Trial Mixture Summary Sheet” for the strength that you want. It will look something like this:

Trial Mixture Summary Sheet


Plan ahead.
You’ll want to be at the job site when the concrete truck arrives. Each individual truck that arrives will have a Mix Sheet for the load it is carrying. This Mix Sheet defines the breakdown of the materials and will correlate with the Trial Mixture Summary Sheet. It will look something like this:

Mix Sheet


Be up front with your contractor and tell them you are going to do the following:

• Be onsite for the entire pour
• Review the mix sheet and the amount of water that will be added.
• Either approve or reject the load of concrete if it doesn’t meet the Trial Mixture Summary

Once you get the Mix Sheet you can check it against the Trial Mixture Summary Sheet. The critical thing is the Water Ratio to Cementitious Ratio.

That’s why it is critical that you watch to see how much water they add at the job site.

The ratio on the mix sheet is the maximum.
(Note that as the mix strength increases this number drops).

If they add too much water and the ratio increases above the maximum specified number on the Trial Mixture Summary Sheet, that’s too much water, reject the load.

In the above example:

1,951.6 lbs = 224 Gallons of water was used
4,540 lbs = Amount of cement

1951.6 / 4540 = 0.43 = which is the Water Ratio to Cementitious Ratio

0.43 is less than the specified 0.49, therefore the Water Ratio to Cementitious Ratio is ok.

 

[Dirtydan69]

I wish I had known all this when I had my detached garage built and new driveways/sidewalk/patio poured. It was the shittiest concrete. My neighbor worked building highways. Took one look and told me it was probably rejected concrete from their pours. Oh well the bank and the next owner are stuck with that **** now. At least I got a 25% discount for putting up with the contractors ****** work.

 

[High5]

Re: Going To Pour Concrete, How Will I Know I Got The Strength I Wanted?

I’ve tried to use the saying “Trust, but Verify” whenever I am working with a contractor. The following is what I did when the slab was poured on my barn to ensure I received the concrete I asked for. My contractor and I agreed to this, and everything went great. Additionally I had a local testing lab take 5 samples the day of the pour and perform mechanical tests at 14 day, 28 days and 42 days. More on that starting at Post #163 here: http://www.garagejournal.com/forum/showthread.php?t=289293&page=9

A couple weeks before the pour get a copy of the concrete plants “Trial Mixture Summary Sheet” for the strength that you want. It will look something like this:

Trial Mixture Summary Sheet


Plan ahead.
You’ll want to be at the job site when the concrete truck arrives. Each individual truck that arrives will have a Mix Sheet for the load it is carrying. This Mix Sheet defines the breakdown of the materials and will correlate with the Trial Mixture Summary Sheet. It will look something like this:

Mix Sheet


Be up front with your contractor and tell them you are going to do the following:

• Be onsite for the entire pour
• Review the mix sheet and the amount of water that will be added.
• Either approve or reject the load of concrete if it doesn’t meet the Trial Mixture Summary

Once you get the Mix Sheet you can check it against the Trial Mixture Summary Sheet. The critical thing is the Water Ratio to Cementitious Ratio.

That’s why it is critical that you watch to see how much water they add at the job site.

The ratio on the mix sheet is the maximum.
(Note that as the mix strength increases this number drops).

If they add too much water and the ratio increases above the maximum specified number on the Trial Mixture Summary Sheet, that’s too much water, reject the load.

In the above example:

1,951.6 lbs = 224 Gallons of water was used
4,540 lbs = Amount of cement

1951.6 / 4540 = 0.43 = which is the Water Ratio to Cementitious Ratio

0.43 is less than the specified 0.49, therefore the Water Ratio to Cementitious Ratio is ok.

Thank you! This is the detail I was looking for. We will take your advice. And I think your saying is spot on "Trust but verify".