request recommendations for rebar spacing for concrete project

[Tractor Seabee]

Science Lesson first:
The technology has changed a lot over the last 20 years a lot; due to all the studies and experimentation of the American Concrete Institute (ACI). They are the industry experts and their process and procedures have ANSI recognition and are specified standards in the Building Codes. For slabs on grade the current trend is fibered concrete placed over a compacted structural base (crushed gravel) and no rebar or mesh (glad nobody mentioned that). Compaction of the base rock is the key as it is supporting the slab structurally. A proper base you could have good results with 300 PSI concrete.

Rebar is a must though if your base is not compactable as then you are building a Structural Slab that has to support itself and any imposed loads. A structural slab is an engineered solution where all the variables are considered and a solution designed.

Now to your DIY project:

Do not tie the new slab to the old as that is a "construction joint". It gets a piece of joint material placed between so the new slab is not restrained. As the new slab tries to support the old movement will occur and the old will probably fracture.

The most important part of this job has not been covered: CONTROL JOINTS These are fracture plane place deliberately to tell the new concrete where to crack not where it wants/will. They can be troweled in or saw cut after finishing. They need to be cut 1/3 minimum depth of the slab thickness. If you are using an old timer finisher he will poh poh this and want to use his edging tool to make a fashion statement to keep you happy; but, believe me it is necessary one way or another. Saw cutting has to be done as done right behind the finisher, the next morning is too late, the concrete has already cracked where it wants to.
I would do a control joint every 10-12 feet across the slab.

15 years ago; previous house I lived at we did an 80 foot x 14 foot drive way done in this manner; 4,000#, 5% air, fiber, 6" slump (it was pumped), no water added at site, well dampened crushed gravel base, here in the NW water cured (just beat the rain that day) heavy broom finish as it was a slope. The finisher used the control joint tool I specified. Saw cutting is easier if there are a lot of joints.

Today, No cracks anywhere yet. A year after placing it had a gravel truck and a concrete truck drive over it for a project behind the house; never phased it. I parked my 40 MH on it for 8 years. SUCCESS ? Your call.

 

[/pine]

...Compaction of the base rock is the key as it is supporting the slab structurally....Rebar is a must though if your base is not compactable as then you are building a Structural Slab that has to support itself and any imposed loads....

I must be missing something...I realize things have changed and it's been a long, long time since I was educated on this topic but...In this scenario if the substrate/base will not support the slab ("not compactable")...how is it supposed to support the rebar that is structurally supporting the slab?

 

[paulharvey]

Thank you all for the responses so far. The supplier told me it is 5$/yd for 4000psi, and 6$ for fiber per YD, so at a approx 1.5 yds the 'extras' would not even add $20 to the bill, even if the 4000psi mix and fibers are not 'worth it'/needed, I think I will find value in mere peace of mind.... I may try to dig some small footers, but as the area is already lower than the existing driveway, and likely low enough, I worry that digging my loosen/disturb the soil, and could do more harm than good.

Couple points; 1.5 yards costs the same as 3 cy with many companies now. I believe both Argos(Vulcan/Florida Rock) and CEMEX both have 3 yard minimum order; meaning you can order as little as you want but you'll pay for 3 cy, plus full fuel and environmental charges. Those charges are same for 1 cy as for 10 cy, so 1.5 cy could be around $500 even though 10 cy might be $1200.

About the sand; sand is ideal for a base. To compact keep in mind that you can't compact dust or mud. Once you have it graded and formed, water the dirt. Then while still damp, compact. When you actually pour, quickly wet dirt and steel before placing concrete.

Rebar, I would say 6" from out side edge, run a bar, many one down center. Driveways don't generally spec rebar, only wire or fiber (Fdot doesn't allow fiber to replace wire like Florida Building Code does). The county I worked for (some times inspecting driveway for ups, but 95% road inspecting) didnt recognize fiber as replacing wire.

 

[disney]

Couple points; 1.5 yards costs the same as 3 cy with many companies now. I believe both Argos(Vulcan/Florida Rock) and CEMEX both have 3 yard minimum order; meaning you can order as little as you want but you'll pay for 3 cy, plus full fuel and environmental charges. Those charges are same for 1 cy as for 10 cy, so 1.5 cy could be around $500 even though 10 cy might be $1200..

I will be using additional concrete for other areas, and when I had contacted cemex, I had asked for the price of 2&2.5yds and the 2.5yd quote was about 60$ higher, so maybe two is their minimum...

 

[Streetcar]

I must be missing something...I realize things have changed and it's been a long, long time since I was educated on this topic but...In this scenario if the substrate/base will not support the slab ("not compactable")...how is it supposed to support the rebar that is structurally supporting the slab?

The slab is supported by the subgrade but for design purposes is assumed to be supported at the ends.
Control joints should match the joints in the existing pavement.

The saw to cut joints in wet concrete is expensive.

 

[disney]

Do not tie the new slab to the old as that is a "construction joint". It gets a piece of joint material placed between so the new slab is not restrained. As the new slab tries to support the old movement will occur and the old will probably fracture.

The most important part of this job has not been covered: CONTROL JOINTS These are fracture plane place deliberately to tell the new concrete where to crack not where it wants/will. They can be troweled in or saw cut after finishing. They need to be cut 1/3 minimum depth of the slab thickness. If you are using an old timer finisher he will poh poh this and want to use his edging tool to make a fashion statement to keep you happy; but, believe me it is necessary one way or another. Saw cutting has to be done as done right behind the finisher, the next morning is too late, the concrete has already cracked where it wants to.
I would do a control joint every 10-12 feet across the slab.

OK; So if I don't try to tie the new and old slabs together, what is joint material that you would recommend? Lowes carries a strip of what I will call a rubberized wood, but it is sized more for a sidewalk at about 4ft long, 3-4 inches wide, and about 3/4 thick. How would I attach it to the old slab so it stays in place while I pour, a few tapcons on each strip? If the old slab is thicker than the strip is wide/high, do I use two pieces to more closely match the height/thickness of the new slab?

I was also planning on using plastic on the ground, any reason I shouldn't do that?

About the control cuts, I understand (at least I think I do) but if the new slab is a max of 8ft from front to back, and i'll guess 14ft wide, would you/ should I still put a joint it, and if so where? front to back (approx. 6ft joint), or side to side (14ft joint, but would leave the smaller end with only 2 to 2.5 feet of concrete on either side of the joint)?

Again thanks to all who have replied.

 

[Streetcar]

The preformed expansion joint material, rubberized wood , is the material to use, if you want expansion.
Since your slab has concrete on one side and soil on the other, there is ability to expand without causing damage to concrete.
Building supply stores sell various widths of expansion material. It can be cut to piece solid coverage for exposed concrete.if you don't have solid coverage with expansion material, it can't do its job. It can be held in place with tapcon screws.
Match joints from existing concrete. Fifteen feet is the maximum joint spacing typically for unreinforced concrete

Concrete is porous, the plastic will stop water from flowing down through concrete. This is a problem in freezing areas

 

[Tractor Seabee]

The slab is supported by the subgrade but for design purposes is assumed to be supported at the ends.
Control joints should match the joints in the existing pavement.

The saw to cut joints in wet concrete is expensive.

That is why I discussed the control joint trowel. You can rent the saws but they are not DIY recommended, takes an experienced operator.

Ron

 

[Tractor Seabee]

OK; So if I don't try to tie the new and old slabs together, what is joint material that you would recommend? Lowes carries a strip of what I will call a rubberized wood, but it is sized more for a sidewalk at about 4ft long, 3-4 inches wide, and about 3/4 thick. How would I attach it to the old slab so it stays in place while I pour, a few tapcons on each strip? If the old slab is thicker than the strip is wide/high, do I use two pieces to more closely match the height/thickness of the new slab?

I was also planning on using plastic on the ground, any reason I shouldn't do that?

About the control cuts, I understand (at least I think I do) but if the new slab is a max of 8ft from front to back, and i'll guess 14ft wide, would you/ should I still put a joint it, and if so where? front to back (approx. 6ft joint), or side to side (14ft joint, but would leave the smaller end with only 2 to 2.5 feet of concrete on either side of the joint)?

Again thanks to all who have replied.

The Lowes stuff is OK. If the old slab is thicker than the joint material is wide stack one on the other. I put some roof mastic to stick it to the old slab. Start your concrete there, it won't move once the new concrete is against it. You can cut that stuff with a saw, just butt together and saw off the last piece. Plastic is a waste of time and $ unless you are doing it on a very sunny day. Plastic under will slow down water loss which can cause stress cracks if the concrete cures too fast and lowers strength due to incomplete curing of all the materials comprising the concrete. Interior work plastic is used to prevent ground water from migrating through the concrete upwards. Concrete can be water tight but it is not moisture tight.

Ron

 

[dodge man]

One joint should be enough. I also agree, don't tie the new into the old. If the old was thicker, it would be OK to tie to two together.

I also agree, no plastic under it, that is more of a indoor application, like under a garage floor.

We often break the test cylinders at 7 days and or 14 days. 7 days is if we want to open a road right away. Usually we consider most concrete cured enough after 14 days to put it in service.

You can also spray the concrete with a curing compound so you don't have to try and keep it wet.