re: Tying New Concrete to an Existing Foundation - Yes or No?

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What are the #4 bars accomplishing?


Keeping the mesh from sinking.

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And plus if the rebar is at the center on the slab it will be accomplishing nothing.




I guess all those elevated structural slabs designed with one layer of rebar at the center of the slab are hanging on by a wing and a prayer.

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do either of yall even engineer bro?

they are arguing about slabs on grade. This Civil guy is trying to design a beam.


You do know for a slab on grade the reinforcement goes somewhere between midway to the top 3rd of the slab right? the tensile area will be on the top of the slab away from the load or between the 2 loads(vehicle load,wheels)

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See Chapter 5.

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Where does that address the OP's situation??

Let me slide in here and help you out my man. One thing I am an expert on is concrete. Sold it for 15 years from the Georgia Aquarium to 300 ft deep shafts, tunnels, high rises ....and puddly little residential applications like sidewalks, patios and foundations. I will give you a ton of info. About things you should do.

1. What kind of mix?

Most ready mix plants have several option but all have at least two. I will give you the 4 most common and some pros and cons. And yes, it is your choice of what design you want to specify.

A. Straight Portland cement
This mix, as is obvious, has 100% Portland cement. It is the fastest setting and also most likely to incur plastic shrinkage issue like small cracks, etc due to the quick reaction time. It is designed to achieve 70% of design strength in 7 days. A 3000 psi mix should hit 2200 in 7 days. Strength gains will slow down after that, gaining the remaining target strength from day 8-28. It is usually okay for foot traffic within 24 hours. It is primarily used when small vehicle (cars, light trucks)traffic need to be able to utilize it in 7-10 days. Road contractors use what is called Type III (high-early) cement to be able to drive on it it 18-24 hours. That cement is the same as regular Portland but just crushed down to a much smaller size....a very fine powder. You will not need that for sure but just a little piece of info. to have. Straight Portland is the darkest gray of the cementitous materials. If you want a darker slab you should use this. However, you must request it from the supplier. It usually runs about $7-8 more per yard.

B. Fly Ash
Fly ash is a cementitous material that when combined with Portland, reacts like Portland. By itself, it does nothing except get wet and form a weak cake like mass. For commercial applications, the most that structural engineers allow is typically 25% of total cementitous. Many only allow 15%. For residential applications, 25% is perfectly fine. This is most likely the mix any plant will ship you unless you specify straight cement. The reason being is it is about $3-4 per yard cheaper to produce than straight Portland. It is slower to gain strength but this is a good quality to help alleviate plastic shrinkage issues. A target 28 day design strength of 3000 psi will usually get to about 60% design strength in 7 days. However, where a straight Portland mix begins to taper off at day 8 in gains, a fly ash mix is just starting to really kick in. It sees most significant gains between day 12-28. The finished product is a little lighter gray than straight cement.

C. Slag
This is almost the same as fly ash. The main difference being that FA is a coal burning by- product where slag is a by-product of the steel production industry. It reacts, as far as strength gains almost identically to fly ash....maybe even a little slower......which is good in your application. It is the lightest colored finished product. This is very advantageous if you think in terms of lighting in the evening. It reflects light better than any concrete available and looks great in the day as well.

D. Three way mix
Incorporates all 3. Usually 75% Portland , 15% FA and 10% slag. See slag qualities. Just a bit darker than a slag mix.

Every plant will have at least two options. A straight Portland option is a given, as they have to have Portland to make any ready mix. What determines the availability of the others is the plants silo capacity. Two silos means they have Portland and either FA or slag. Three silos and they have all 3.

2. Reinforcement

All you need in this slab, is mild reinforcement. I would not use welded wire mesh. Why? Because there are easier, less labor intensive and less costly options. I would use polypropylene fibrillated fiber at 1.5 LB per yard. It only costs about $7-8 more per yard. That's $28 on a 4 yard slab. But be specific with the person taking the order. If you just ask for fiber at 1.5 LB per yard, they will send you the cheaper mono filament product that is 1/2" and a in single strands. They all keep that by the pallet and it is cheaper for them to ship if the customer doesn't specify. They are really not that good except for minimal plastic shrinkage control. The fibrillated fibers are 1.5" in length and multiple, twisted, strands. They are far superior. Most producers charge the same for both...so get the better, even if they charge a couple bucks more. The newer fibers are not nearly as hairy as the older ones and they all rub out over time anyway.

Placement

If you can pull the truck with chute right to the location, use #57 stone. If it has to be pumped, using a 2.5" line pump from the street, you will need a #7 stone mix. The aggregate is 1/2" as opposed to 1" nominal. A #7 mix will run you about $7 more per yard.


My suggestion:

3000 psi slag mix
1.5" fibrillated fiber at 1.5 lbs per yard.
#57 stone unless line pumped

Good luck

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I don't think he was. We just got off in that discussion regarding the effectiveness of rebar at the center

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Yea we got WAY off topic. I got caught up in the discussion and lost track of the OP. I wasn't trying to design a beam on grade. No need to put you 2 cents in Ramrod. urine already set me straight.

At the end of it all I think we both agreed that doweling into the house foundation is not necessary

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I wasn't trying to design a beam on grade

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if you wanted to put the reinforcement in the lower half... yes you were. slabs on grade had it placed from half to the upper 3rd, like i said. So dont try to belittle me boy.

No need to tie it in unless you plan on closing it in later. If you're no sure if you will, at minimum put a water stop in the joint at the existing house slab. Wire mesh is fine or you could opt for that crap that makes the concrete fuzzy.

On another note I sure hope Lapels didn't issue a CE PE lic # to some of the folks in this thread.

I personally know of two folks who closed in their patio without them. Both times it ended up costing them years down the road and one's insurance refused to cover the damage (moisture wicked up through the joint and did a number on the walls from the inside out).

Got to watch out for termites too traveling through those joints if but I think the only option there is a pest control contract.

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If he makes good on his threat of installing a canopy!!

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And...??

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You mad bro??

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About someone who can't seem to communicate their point? No.

The newer generation fibers are not near as bad as the ones from the 90s and up until about 2006. There would be some visible strands at the surface for a month or two but they would dis-appear with a little time. Just a much cheaper and less labor intensive method. Not that cutting wire mesh to size and having someone fish hook it up to the upper 1/3 during the pour is real difficult. Just a time and money savings.

P.S. - you should see some of the structural fibers that came out around 2002. They are 2 1/2" long and are dosed at up to 7.5 LB per yard. Did some huge 200,000 to 1,000,000 sq ft slabs where they were pouring 20,000 sqft sections with this stuff in place of re-bar, except around columns. Minimal control joints. I mean, areas like 100 x 100. Did one like that in 05 and in 12 there was still not a crack in the whole building slab. But it was like a $40 per yard add on.