Concrete PSI?

[karoc]

Doing little research for concrete psi, just the normal concrete slab that be for my small wood/metal shop and for living quarters. I believe that for this kind of usage that 3000psi is normal. Then I read that 4000psi concrete rating would be better and less subject to cracking. But what’s the average cost increase to go say from 3000 vs 4000psi or in between?

 

[1MtnGoat]

Definitely go with 4000psi. If will give a nicer/smoother finish due to higher cement content. Cost difference is only $10-$15 per cu yard. Cracking in concrete is not a matter of if it will crack but where. You should plan on sawing a control cut every 10-12' in both directions to control cracking. This cut can be under an interior wall to hide it. There are good self leveling urethane concrete crack fillers that can also be used to fill the cuts. Use 6"x6" #10 WWM (welded wire mesh/aka highway mesh) in the slab to help control cracking. Do not pour on a hot day and keep slab wet for several days to slow the shrinking process. My shop floor is 4000psi and 40'x76' with filled cuts and no visible cracking. Make sure you tell your finisher how smooth you want it finished.

 

[Renegade1LI]

One big factor of cracking is using too much water, keep it to a 5-6" slump the less water the better. I see guys pour it like soup & it will have a greater probability at cracking. Expansion joints at 20' max intervals & control joints around columns. Make sure they use a vibratory screed & the wire should be pulled up to the center of the slab, we use 2.9 x 6 wire on continuos chairs helps keep the wire off the ground.

 

[ConCretin]

Up here the increase in cost to go from 3000 to 4000 psi concrete is less than $10. From a structural standpoint, you don't need any more than 3000 but upping the compressive strength means more cement, which will provide a more durable surface among other benefits and is worth doing in my opinion.

As long as your slab is uniformly supported by a well constructed base, cracking is almost entirely the result of shrinkage. 4000 psi concrete will shrink a bit more than 3000 psi concrete but as long as you keep the water content i.e. slump under control and cut properly spaced control joints, it shouldn't be a problem. Unless you are using a water reducer, slump should be kept in the 4" range. A mid range water reducers will get you a more workable 6" without the negative side effects additional water would bring.

Take a look at my Guide to Floor Slabs in the link below for some additional thoughts and good luck with your project.

 

[nadogail]

My buddy who has years of experience working with concrete tells me that every concrete slab comes with Two Guarantees; #1 your slab will crack and #2 Nobody will steal your slab.

 

[rburke65]

Ya….and #3 is that it will be gray. Call you local batch plant and ask. Use some rebar and maybe some mesh to be safe, watch the slump, cut it when you can walk on it and youll be ok.

 

[Hobby_Man22]

One big factor of cracking is using too much water, keep it to a 5-6" slump the less water the better. I see guys pour it like soup & it will have a greater probability at cracking. Expansion joints at 20' max intervals & control joints around columns. Make sure they use a vibratory screed & the wire should be pulled up to the center of the slab, we use 2.9 x 6 wire on continuos chairs helps keep the wire off the ground.

When is it that they add water? Does the cement truck driver add the water to make it easier to work with then they tell them to add more and more just because they're lazy?

 

[Hobby_Man22]

What's the deal with the control joints? It will crack on its own where it wants anyways.

 

[ConCretin]

When is it that they add water? Does the cement truck driver add the water to make it easier to work with then they tell them to add more and more just because they're lazy?

The batch plant will generally hold back some water from the amount specified in the mix design so it's not unusual or harmful to add some water onsite. But it's also not unusual for the finishing crew to want to make their job easier and add too much.

Slump is generally thought of as a measure of workability but it's primary purpose is as a measure of mix water. The advertised compressive strength of a particular mix is generally based on the water required to achieve a 4" slump. Most finishers these days aren't going to want to drag around concrete that stiff and will add too much water.

I recommend requesting a mid range water reducer and an allowable slump of 6". This mix will often require less water than an unmodified 4" slump. Less water means better concrete.

 

[ConCretin]

What's the deal with the control joints? It will crack on its own where it wants anyways.

Kinda reminds me of a Seinfeld bit. Whats the deal with......?

It's simply a matter of aesthetics. Most people would rather see nice straight lines as opposed to random ragged cracks.

 

[Hobby_Man22]

I have a 30x50 slab and it has no control joints. Only one minor crack.

 

[Hobby_Man22]

Haven't really noticed control joints in too many warehouse syle slabs, but I may not have been looking.

 

[Skooterj]

Haven't really noticed control joints in too many warehouse syle slabs, but I may not have been looking.

My factory has a joint every 20 feet, East West, North South....

 

[marak]

Ordering a 6 bag mix will get you the 4,000 psi strength. Seems like you get a 5 bag mix (3,000 psi) if you leave it unspecified. Very small difference in price here in Alaska. I always go for the 6 bag variety.

 

[Renegade1LI]

Don't forget a good subase with 95% compaction is very important. We do a lot of concrete for NYCSCA, and they spec item 2 stone subase, not rca, plus a vapor barrier for indoor slabs. There's a lot to doing good concrete slabs, correct mix, proper handling & placement, good consolidation with the use of a vibratory screed, reinforcement in the correct location plus guys that want to work & know how to finish.

 

[stillnostrebor]

Haven't really noticed control joints in too many warehouse syle slabs, but I may not have been looking.

They more than likely have them. I have participated in construction of hundreds of commercial spaces and they all have received control joints. Usually warehouse floors also get Polyurea sealant in the joints or similar, so that makes them less noticeable.

 

[yugami]

Haven't really noticed control joints in too many warehouse syle slabs, but I may not have been looking.

Now days they cut control joints then refill if you want a perfectly smooth floor.

The really old places I worked had monolithic slabs but the really new buildings have had 12' squares with grout.

 

[Yankee2bbq]

What's the deal with the control joints? It will crack on its own where it wants anyways.

It gives concrete a place to crack. Concrete will crack every 10 foot.