Slab question 2

[ttuterry]

I am designing a concrete slab on grade. It is going to be 4 inches thick. This slab is 23'x 60' and i decided to put a longitudinal control joint in the center (at 11.5 feet) and control joints every 12 feet transversely. I am thinking about adding #3 reinforcement but i'm not sure I even need it. I don't know if the joints will be enough...I have read about every post on this forum about slab on grade and I still cannot decide what to do...Any advice? Thanks.

 

[bridgebuster]

A Rule of Thumb that I recall: Control joint spacing (in Feet) = 2 to 3 times the slab thickness (in inches) with an Aspect Ratio of 1 to 1.25.

4" is also a standard NY City sidewalk, in which joints are generally placed at 5'.

 

[dik]

The 11.5' should be OK for 4" slab and transverse cuts at 12' should be OK. The reinf should be at 14" min centres, so you might consider using #4 bars. Placing the bars with 2" cover will permit 1-1/2" deep sawcuts. Time of sawcutting is critical. This should occur as soon as possible without the sawblade ravelling the surface; typically about 8 hours after finishing (can use a sofcut saw earlier).

Bridgebuster... most areas I've been involved with in Canada, the sidewalks are 6" thick, with sawcuts at 4' centres and panels separated at 12'-16' centres. Current practice in Winnipeg, or at least a couple of locations with new sidewalks, they have omitted the tentest at 12-16 centres and only relied on sawcutting... sidewalk panels have heaved a foot from thermal movement.

 

[bridgebuster]

dik - frost heave isn't much of a concern in NY city. The 5' joint spacing isn't a hard and fast rule; only when the City installs the sidewalk. Here, property owners are responsible for the cost of installing and maintaing a public sidewalk along their property even though the City owns the land.

 

[SlideRuleEra]

ttuterry - In a 4" slab it will be essentially impossible to get the necessary 3" concrete cover between the rebar and the soil AND get satisfactory top cover. Without enough cover, the rebar will be more likely to corrode, causing more problems than it solves.

I suggest that you consider two options:

1. Forget the rebar and stay with 4" thick unreinforced concrete.

2. Use the money that would have been spent on rebar to purchase more concrete and increase thickness of the unreinforced slab from 4" to 5" (or so).

http://www.SlideRuleEra.net

 

[Taro]

ttuterry,

Slab on grade design is as much art as it is science. There are many acceptable ways to achieve good performance. There are also many ways to run into problems. So when you read various posts, you will find a wide range of opinions. I have found that 4" slabs with rebar work fine if they are done correctly. But besides just the thickness, reinforcing and jointing, you also need to consider many other factors like water content, aggregrate size and grading, vapor barriers, etc. Read up on the subject as much as you can and find an experienced designer to review your work.


slideruleera,

3" cover is not necessarily required for slabs on grade. 3" is the ACI 318 requirement for structural elements that are cast directly against soil and is meant to account for the irregular surface of the soil and the possibility of soil contamination in the bottom layer of concrete.

Slabs that are placed over vapor barriers and/or compacted gravel eliminate the irregular surface and contamination concerns. Also, most slabs on grade are not considered structural elements so ACI 318 does not apply anyway.

 

[SlideRuleEra]

Thanks, Taro, I did not know that this type slab on grade could be exempt - I stand corrected.

http://www.SlideRuleEra.net

 

[dik]

Thanks Taro... saves me responding... Dik

 

[Ron]

ttuterry...your joint spacing is about right for a 4" slab on grade. I wouldn't worry about the reinforcement....use as little as possible. If you can get it in your area, use WWF in sheets (the rolled stuff is a pain for thin slabs)

For such thin slabs, thickness control is extremely important, coupled with early saw-cutting. Cut the joints as soon as you can use the saw without raveling the cut. That can be anywhere from 4 to 8 hours after placement. Don't wait until the next day. As for thickness control, you have a tolerance of -1/4 to +3/8 on a slab of this thickness. Since that is a total variation of 5/8", it is important that you not allow sudden transitions of thickness. This means that your subgrade must be relatively flat and it you use a plastic sheet vapor barrier, it must not have wrinkles in it.