Redo-ing Patio - Concrete Question

The numbers represnt the compressive strength of the cured concrete. Normally 28 days is required for full cure.(I think)

For a patio, 3500 psi is more than enough.

Hello, thank you so much for your information it helped a lot.

The 3500 is good enough for your patio. More important is to see that is doesn't dry out, keep it moist to continue the curing, certainly for a couple of days in my opinion, although most contractors get away with just overnight.

Thank you I'm new at t his am want to make sure the right strength is used, be blessed!

My concern would not be compressive strength as this is not critical for a patio.

Reinforcement?

Expansion joints if super large.

Type of finish (steel trowel, float, broom).

Proper curing.

Frost protection?

We don't know were you are.

Hi, Thank you I'm from Philadelphia, Pennsylvania.

OK. Philadelphia is not the desert or a more northern climate where frost can go to 5 or more feet in winter. Still there will be some frost.

Again, the compressive strength does not matter (use 3k psi or more).

The sidewalk finish is used when you don't want to slip (when wet). A steel trowel finish is dangerous when wet. However it is stronger as used inside a garage.

Curing is important. Keep it wet or use burlap or poly for at least one week. The strength at 28 days is close to maximum.

You should use wire mesh for temperature contraction/expansion. Concrete has very little tensile strength.

You need a good subgrade. Compacted granular material, no clay or organics. This will help in the freeze/thaw cycles (winter/summer).

If the patio is large, you need to consider expansion joints (as you see on R.C. sidewalk). Otherwise it will crack.

Good luck.

Yes, also ask for 3 inch slump. Reject the batch if the ready mix truck cannot meet this requirement. Also reject hot loads (been mixed for more than two hours). Look at the batch plant slip.

Don't let the mafia guys onto the jobsite. Well, maybe. You did say Philly.

A 150 lb lady wearing 1/5 inch square stiletto heels and standing on one heel (i.e. walking) is going to produce a pressure of 3750 pounds per square inch. So either ban stilettos or go for the 4000 psi.

Unfortunately some stilettos are just 3/16 of an inch diameter (you do the maths).

Long ago and far away, I did a stent in civil engineerin, specifically soils and materials, and had the opportunity to analyze cylindrical concrete specimens. IN the lab, they are compressivily loaded in a hydraulic compressive loading/tensometer machine. The specimens are tested at 7, 14, 28, and 45 days (typically, but sometimes varies). At each increment, the break strength/load stength reflects the curing process. Depending on the load/psi design of the concrete, the specimens either meet, exceed, or fail the spec.

The psi rating of concrete obviously deals with the compressive load strength. Typically, the higher strength concretes contain less aggregate, and other fillers, and more cement (slightly oversimplified for time reduction). Load designs vary, depending on the application. But to shorten the story, the run of the mill mix design should come in around +/-3000psi and most people park their cars and build their houses on this "standard mix". This should be more than adequate for your purposes, no matter what shoes you wear, so save some money ond buy the consumer grade. Just dont let the finishers add a lot of water. Ask for a 4-6 inch slump when you order it and it should be right by the time it is delivered to you. Finishers like it soupy so that it is easier to place, but this will cause problems later and compromise the integrity of your patio. Good luck with your project.

Z

Best answer so far, and probably needn't be hammered at for a patio, but I would like to add and emphasize that you could pour 9000 psi concrete for this project but without proper substrate prep it will fail. Strength is not an intrinsic factor in concrete.

Rent a jumping-jack to properly compact the soil underneath and a foot or so away from the pad. Also, ensure there are not pockets in the substrate, and that drainage is constructed to get the water away from the slab, no run under it causing a cavity which will cause the slab to sag and crack. Steel rebar #5 at the corners and sides where the most sagging will occur, and proper expansion joints to your climate extremes are also recommended.

Great addendum Oregoon. Good points all. Substrate failure is one of the leading causes of concrete, as well as asphalt, failure. When performing site preparation, it is advisable to perform a proctor test on the soil and determine the density characteristics so that it is possible to quantify maximum compaction. It is possible to tamp with a jumping-jack and still achieve nothing as it relates to compaction. This is probably overkill for a patio. Unless it is going to bear a structural load, as long as water drains away from the slab and doesn't run underneath and freeze, which causes heaving and cracking, removing the old patio and any organic matter will probably be sufficient site preparation. As you pointed out, adequate expansion is a must. Also, fiber reinforced concrete might be a desirable alternative to rebar or mesh.

Z

How large a patio are we talking about?

I myself would use 4000 psi air entrained for a slab on grade. As mentioned use WWM for crack control and make sure that the sub-base has been compacted.

Depending on the size you may need one or more control joints in the slab. A little more info will get you better answers.

Base, base, base.

The strength of the concrete is nearly irrelevant.

Fully reinforced and jointed 8000 psi slabs fail all the time do to poor sub-grade and drainage.

2500 psi. sometimes referred to as '4 sack' is even sufficient for this application and location if the surface it is put on is 'built up' properly.

Size and even thickness dictates if reinforcement and expansion joints are needed.

Same as road building, if you spend the extra money underneath the surface, you will save far more for the maintenance or replacement later.

Having said that, I generally agree with the rest of the comments here.

I should add that most people here will over think and/or over-engineer the hell out of this!

Including me!

1. Check your local codes to see if they apply.

2. Spend as much as you find resonable. It will pay you back in the long run!

The local (Kingston ON) sidewalk standard is 4500 psi after 28 days and I believe most municipal sidewalks in northern climates are similar. If the concrete is wet cured it should reach nearly double that strength after 1 year. I would wait the 28 days before sealing. Drying (not curing) will take 28 days/inch even if it looks dry on the surface. So the seal should be able to breathe.

If you opt for the 3500 psi make sure you wet cure it so that the final psi can increase. I would not imagine that the cost between the two cements is significant enough that you would opt for the lower strength. It may be that a patio does not see the same issues of sidewalk and lower strengths are acceptable. Higher strengths are usually better in most applications. My experience with cement is restricted to sealing wells and abandoning wells where permeability and salt were the largest considerations not strength. We used more bentonite/concrete mixes in these applications.

Never use a pressure washer at anytime, as that will force water into the micro pores and can then create serious deterioration especially if frost is a potential. Scrub out stains with a detergent. Also be careful with deicers or avoid them altogether.

You can check the Portland Cement site for lots of info.

Hello,

This question has been pretty much addressed in another recent blog entry (a few days ago in a post by Don in LA) regarding driveway concrete. You may want to check it out the suggestions of others, including my own posted answer.

A 3,000 or 4,000 psi concrete is adequate, but you need to tell the ready-mix supplier some more vital information so that you end up with a viable, long lasting patio slab.

For the concrete:

Use an initial low slump of 3 to 4" (in truck during transport), but have a mid-range Plasticizer added by the truck driver once the truck reaches your house to bring the slump up around to 7 to 8" for workability----DO NOT ADD WATER to the mix or into the concrete once it is discharged, as it will weaken theconcrete mix. Ask for a low Water-to-Cement ratio (W/C), say around 0.42; try to achieve a 4 to 6% Air Entrainment; and use at least 625 to 650 pounds of Portland Cement per Cubic Yard to produce a creamy dense paste.....you can substitute up to 17% of the cement with Fly Ash Type F or G (Pozzolan) by weight. The Fly Ash will give you a high early compressive strength and very dense (almost waterproof) concrete, but beware it'll also give you a gray-ish finished product, not Alabaster White). If it's a hot day and there's a chance that the concrete temp and ambient air temp goes over 85 Degrees during the concrete placement and finishing ask for dry ice to be added to the mix to keep it cool. Heat kills a concrete faster than Jack Flash. In fact, if the air temp gets over 90 degress it'll flash-up on you faster than you know what and you'll not be aable to finish it properly.

I recommend that you build a 3.5" minimum slab, but 4-inches is more preferable in the NE USA in order for you to get the steel reinforcement in the center of the slab (@ mid-depth) and provide some slab integrity. For minimum Shrinkage and Temperature reinforcement steel use 2 layers of 6 x 6 -W2.9 x W2.9 WWM (ie Welded Wire Mesh), flat sheets only, with 12-inch minimum laps at edges. Install the plastic or Nylon rebar chairs spaced 24-inches o.c e.w.. Do not use rocks, stones or bricks to support the WWM as they'll induce a weakened plain in the concrete.

Once you finish the surface and the concrete begins to finally set, cover the slab with 6-mil poly plastic sheeting and wet cure the slab continually with soaker hoses for no less than 10 days (14 days preferable).

Provide raked joints (Concrete Contraction Joints), 1/4 depth of slab, @10-feet maximum o.c. e.w. so shrinkage cracking occurs at those locations. You can also have poly micro fibers for shrinkage crack control added to the ready-mix concrete plant.

Also, install the patio slab on a gravel (such as NYSDOT Item #4 subbase, placed no shallower than 6-inches). Make sure it is leveled and compacted properly.........water is the best soil compactor if you don't have the mechanical equipment to do it for you..

Signed "Doc" CaptMoosie, BS/BS, MS/MS, PhD, LPE

Civil and Structural Engineer

Upstate NY

Well said CaptMoosie. I find your answer to be of the highest quality; informative; thorough; accurate. Kudos! My hat is off to you. Thanks for the refreshment.

Z

Zenos, I found your posting to be first class, and you had mentioned some very important facts and information that I forgot to mention in my posting. Bravo!!! You get a GA from me.

Goodnight all, and please have a pleasant night and restful nooze.

Either will work well. The PSI relates to the amount of weight the concrete can handle and if it's a patio, I doubt you will drive a car on it. Spend your money on proper expansion joints and reinforcing steel, not the PSI of the concrete. Having said that. I would pour it 5 inches thick with 3000 PSI concrete and 3/8 inch re-bar on 24 inch centers with expansion joints no more than 10 feet apart. For every inch of concrete thickness or depth that you pour over 4 inches, you will almost double it's PSI strength. Gary

Those of you who proclaim that the stength of the concrete is irrelevant are understating its importance. If that were the case we would would not have the first parking garage, concrete bridge ,sewer or drainage pipe. Yes, soil compaction is a good start and all elements of good construction are desired, but there are limits raised by cost. I personally have a 3 car garage over 8 feet of fill which was compacted enough to hold the concrete until it cured. Serious compacting would have blown out my walls before they were back filled. My concrete was 3000 psi with fiber and I poured it between 5/8 rebar on 2 ft centers and I did it 6 inches thick. I have had various pieces of equipment on it including a 10 ton traveling clicker press. I have never had the slightest crack, nor will I. Slump is important as the greater the slump the less strong the concrete is. I have poured and finished enough concrete to equal concrete in I-85 between Spartanburg and Greenville SC. Experience is a great teacher. Cement by itself is not strong at all. It gets it strength from the aggregates used and the percentages of aggregate sizes. It ideal is to have aggregates of various sizes so that when the final mix is complete, there is as little 'void space' as possible. Density is important in strength. The higher the blane of the the cement the greater fill capability it has and hence the greater the strength of the final concrete. The higher the blane the smaller the size of the cement particle.

Michael,

As one of those that proclaimed the strength as being irrelevant (almost);

I, as were most others (I am assuming), was referring to this particular application; A PATIO!

The heaviest thing to load this structure would likely be -insert- (a relative of your own choosing).

I am a certified inspector AND materials tester for both civil infrastructure and commercial vertical construction. I have been on the enforcement side of local codes, and conducted bridge and wall (both retaining and structural) inspections, and been directly involved and responsible for their design, redesign, construction, and maintenance.

AND, before all of that I was a contractor with over 20yrs. experience.

I would never consider the strength irrelevant in any significant load bearing structure of any kind.

State and local codes REQUIRE specific minimum load bearing capacities for even sidewalks and possibly for this application as well. (which I advised to be referred to)

I don't think anyone meant to downplay the roll of good mix design, placement, and/or curing. And I doubt that this person is going to take the advise here and start building parking garages, let alone a driveway. If that was the case, they asked the wrong question!

Nice response unc. The idea that load strength is generically irrelevant, to my knowledge, never entered the discussion. Specifically however, I agree with you that for all intents and purposes, as many of us pointed out, it is not a critical matter with a non-load bearing slab, such as a patio. Maybe Michael misunderstood where we were coming from. We all look at things from different, albeit overlapping, perspectives. It is great to know that such thoughtful and talented people are out there every day attempting to advance and maintain progress. So, to both of you guys, it is great to be associated with persons of your caliber who desire to keep the facts straight. Thanks.

Z