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-   -   Concrete Patio...what type of mix: 4 sack, 5 sack (http://www.diychatroom.com/f105/concrete-patio-what-type-mix-4-sack-5-sack-140633/)

hammerlane 04-18-2012 05:56 AM

Concrete Patio...what type of mix: 4 sack, 5 sack
 
Planning on a 27' x 21 concrete patio slab for the backyard. Am taking estimates from local contractors.

I know that 6 sack has more "compressive strength" than 4 sack mix.

One of the contractors mentioned 4-sack. I know 4-sack is less expensive than 6 or 6.5 sack mix and I have no problem paying the little extra for the higher sack mix if in the long run the higher sack mix will stand up better to the elements???

BUT is there a reason NOT to use a 6 or even 6.5 sack mix for a concrete patio slab? I live in Ohio where winters get cold with lots of snow.

Thanks.

TarheelTerp 04-18-2012 06:46 AM

27 FEET by 21 FEET by even 2" thick (=3.5 yards) is a LOT of concrete to be doing by hand.
At the far more proper 4" depth and allowance... you're far closer to 8 yards.
Any job using this much 'crete should be getting it delivered.

hammerlane 04-18-2012 06:51 AM

Quote:

Originally Posted by TarheelTerp (Post 901544)
27 FEET by 21 FEET by even 2" thick (=3.5 yards) is a LOT of concrete to be doing by hand.
At the far more proper 4" depth and allowance... you're far closer to 8 yards.
Any job using this much 'crete should be getting it delivered.


Tarheel: Where did you get the impression I was doing this by hand? Are you nuts I'm not mixing 7 yards by hand? Never mentioned that. The second sentence in my first post says: Am taking estimates from local contractors.

Purpose of my post was to get info on whether requesting the higher sack mix would be beneficial and not detrimental to what I want which is a patio slab.

concretemasonry 04-18-2012 07:19 AM

A general rule of thumb is that higher strength mixes have more durability for the same slump and with the same aggregate. Also absorption is lower.

If you have a freezing climate, the best thing you can do is to use air-entrained concrete (5%) to increase the freeze-thaw durability. It is minimal extra cost and any good concrete supplier has the equipment dosing equipment (a few ounces per yard) and had the approved mix designs.

Dick

hammerlane 04-18-2012 07:37 AM

Thanks concretemasonry.

Will the supplier know this for my area and have these additives in the mix?

Guess what Im trying to get at is I do not want the contractor to try save a few bucks on the concrete mix by not having the proper additives or sack mix if this is going to compromise final product.

Im sure our winters are similar to St Paul's

Thanks

jomama45 04-18-2012 07:59 AM

For your climate, I'd suggest a 4000 psi (6 bag/sack equivalent) with air-entrainment as Dick mentioned above. The only difference is that I'd suggest it be a little closer to 6% than 5% (5.8% is the goal here typically).

Here's a good read from your state's ready-mix concrete assoc. Note that they're calling for a minimum 6.5 bag mix, but just remember that they're job is to sell cement/concrete...............

http://www.ohioconcrete.org/LITERATU...work-Final.pdf

Daniel Holzman 04-18-2012 08:34 AM

The "sack" method of specifying concrete is not very precise. Usually concrete is specified in terms of 28 day compressive strength, which is typically at least 3000 psi. How many sacks of cement are required per cubic yard of concrete is usually left to the concrete supplier to figure out, as long as they make the required compressive strength, the number of sacks of cement per yard is not critical, since some suppliers will use admixtures or fly ash to their mix. For a patio, 3000 psi concrete should be adequate.

The comment about air entrained is spot on, although I suspect that by default you are going to get air entrained concrete. Just as important as the 28 day strength, and harder to evaluate, is the quality of the workmanship installing the patio. The concrete has to be properly placed, compacted, cured and finished. The base has to be properly prepared. Poor workmanship will destroy the best concrete.

concretemasonry 04-18-2012 09:00 AM

Daniel & jomama45 -

As far a concrete strength, the better concrete suppliers here will not even supply or dump concrete (at any price) for patios, driveways and sidewalks unless it is 4000 psi and 5-7% air entrainment. Some contractors (not DIYers) can get something less if they sign a waiver provided by the driver. The suppliers are tired of getting dragged into court based on quality claims by people that do not place or finish concrete properly, since it is not worth it.

They prefer to turn their back on the low end (price and quality) concrete business.

Concrete has been referred to as the most expensive way to sell good aggregate.

Dick

jomama45 04-18-2012 02:21 PM

Quote:

Originally Posted by Daniel Holzman (Post 901594)
The "sack" method of specifying concrete is not very precise. Usually concrete is specified in terms of 28 day compressive strength, which is typically at least 3000 psi. How many sacks of cement are required per cubic yard of concrete is usually left to the concrete supplier to figure out, as long as they make the required compressive strength, the number of sacks of cement per yard is not critical, since some suppliers will use admixtures or fly ash to their mix.

It's certainly not uncommon for a contractor to choose between either "Bag/Sack" mixes or compressive mixes. We're often offered a number of options in each, and typically a "bag" mix is a straight cement mix with no admixes. It may not matter as much to a homeowner, but getting the correct concrete is critical for the contractor to succeed. So, simply put a "Bag/sack" mix generally refers to a straight cement mix, as in a "6 bag" mix literally has 564#'s of Portland cement per yard. A 4000 psi mix may have as little as 375#'s of Portland cement, with the remainder of the cementicious material made up from flyash, slag, etc... At the end of the day, they both may have similar break strengths, but have very different finishing qualities.

For a patio, 3000 psi concrete should be adequate.

From a compressive strength stand-point, sure, but from a water-cement ratio, it will likely not be even close to adequate.

The comment about air entrained is spot on, although I suspect that by default you are going to get air entrained concrete. Just as important as the 28 day strength, and harder to evaluate, is the quality of the workmanship installing the patio. The concrete has to be properly placed, compacted, cured and finished. The base has to be properly prepared. Poor workmanship will destroy the best concrete.

:thumbsup:

jomama45 04-18-2012 02:22 PM

Quote:

Originally Posted by concretemasonry (Post 901601)
Daniel & jomama45 -

As far a concrete strength, the better concrete suppliers here will not even supply or dump concrete (at any price) for patios, driveways and sidewalks unless it is 4000 psi and 5-7% air entrainment. Some contractors (not DIYers) can get something less if they sign a waiver provided by the driver. The suppliers are tired of getting dragged into court based on quality claims by people that do not place or finish concrete properly, since it is not worth it.

They prefer to turn their back on the low end (price and quality) concrete business.

Concrete has been referred to as the most expensive way to sell good aggregate.

Dick


I know, you've told me that same thing a number of times here already...... :thumbsup: :laughing:

Daniel Holzman 04-18-2012 03:20 PM

Jomama, are you indicating that as a contractor you actually specify a water/cement ratio for the concrete you use, and actually check if the supplier met your ratio? In 35 years of practice, I have never seen a contractor check water/cement ratio. All they ever check, so far as my experience, is the slump of the concrete, and of course they take concrete samples for lab testing to make sure the mix meets strength requirements. If the mix meets the strength requirement, and passes the slump test, so far as my experience goes, the concrete is fine.

So I have a hard time visualizing 3000 psi concrete failing whatever water/cement test you run.

Ravenworks 04-18-2012 04:29 PM

You guys will think that I'm nuts but I used 7 sack mix with 5% air.
I am going to agree with everyone else and say, at a minimum I would use the 6 sack mix however make sure it's a limestone mix.
If it is someone that you don't know faithfully I would go as far as paying the concrete bill, this way you know what you're getting.
You'll have a nice worry free patio for many years to come.

jomama45 04-18-2012 09:14 PM

Quote:

Originally Posted by Daniel Holzman (Post 901861)
Jomama, are you indicating that as a contractor you actually specify a water/cement ratio for the concrete you use, and actually check if the supplier met your ratio? In 35 years of practice, I have never seen a contractor check water/cement ratio. All they ever check, so far as my experience, is the slump of the concrete, and of course they take concrete samples for lab testing to make sure the mix meets strength requirements. If the mix meets the strength requirement, and passes the slump test, so far as my experience goes, the concrete is fine.

So I have a hard time visualizing 3000 psi concrete failing whatever water/cement test you run.

Where did I say there was a test for w/c ratio??? It's nothing more than a relatively simple equation to figure it out.

Here goes:

- It's known that a gallon of water weighs 8.33#'s.

- It's used as a given value that one cubic yard of concrete needs about 25 gallons of water total (includes moisture available in the fines & aggregate) just to merely hydrate the batch, or in other words get the concrete to a "zero slump."

- It's known that it takes approx. one additional gallon of water for every one inch increase in slump.

- It's known that the compressive strength (28 day) designation given by all ready-mix concrete suppliers is based on a 4" slump.

- It's known that a "bag/sack" of portland cement weighs 94#'s.

- It's known that a 6 "bag/sack" mix will achieve approx. 4000 psi compressive strength at 28 days if cured correctly.

- It's known that a 5 "bag/sack" mix will achieve approx. 3000 psi compressive strength at 28 days if cured properly.

- It's known that a lower water : cement ratio is advantageous. A W/C ratio of .45 or lower is the most often used ratio for the developed world. Although our state isn't under it's jurisdiction, I believe the IRC states that the max. allowable W/C ratio for concrete flatwork to comply is .50 W/C ratio.

- It's known that the W/C ratio is derived by taking the total weight of water divided by the weight of the cementicious material in one yard of concrete.

Examples:

3000 psi mix with straight cement only, no replacement cementicious material:

25 gals. of water to hydrate plus 4 gals. of water to get to a somewhat work-able 4" slump = 29 gals. of water @ 8.33 pounds = ~242 #'s of water

5 bags of cement at 94#'s = 470 #'s

242/470= .515 W/C ratio Well outside of the range for quality concreting, especially for areas prone to excessive freeze/thaw cycles.



4000 psi mix with straight cement only, no replacement cementicious material:

242 #'s of water.

6 bags of cement @ 94#'s = 564 #'s.

242/564= .429 W/C ratio Much better result form the additional cement, and safely within acceptable margins.



These are best-case scenario's for W/C ratio's as well, as replacing the portland cement with cementicious admix's/fillers will only bring the overall cementicious weight down, resulting in even higher W/C ratios..................



Too much info for a DIY forum???? :yawn:

Daniel Holzman 04-19-2012 06:19 AM

This puts things in a bit of perspective. See http://www.ce.memphis.edu/1112/notes...ual/Chap09.pdf

for a complete description of concrete mix design. At a maximum allowable water/cement ratio of 0.5, you are going to end up with 4000 psi concrete. Since IRC apparently specifies a maximum allowable W/C ratio of 0.5 for concrete exposed to freeze/thaw, the minimum practical strength that would be delivered is 4000 psi concrete. Of course, since only the plant actually knows the water/cement ratio used, the only relevant data about the actual water/cement ratio comes by back calculating the water/cement ratio based on the concrete strength test results. This assumes you actually take and measure a concrete test cylinder. Or you could mix the concrete yourself, then you would know the actual water/cement ratio.

Conclusion: for exposed concrete subject to freeze/that, order minimum 4000 psi concrete, take a cylinder, and get it tested to make sure you got what you ordered.

hammerlane 05-28-2012 10:47 AM

Got it done a few days ago...a few pics:


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