Cast in place concrete code 2

[jjk4985]

I was trying to find out the code concerning spacing for expansion joints in cast in place concrete sidewalks meeting at a nintey degree angle.

Any info would be very helpful.

Thanks,

Jim

 

[TDAA]

If they meet, the spacing is 0. ;P

 

[jjk4985]

what my problem is, is that I have a concrete sidewalk coming off a foundation wall about 10'. From there the sidewalk takes a nintey and goes another 100'. 3 days after the pour, a crack at the exact point where the two walks meet formed at a 45. I didnt know if there was a code specifying distance for expansion joints from that point.

 

[Maury]

The crack is more likely to due contraction than expansion. Were contraction joints placed at the joint? Was the concrete covered with polyethylene or sprayed with curing compound? Was there any loading that could have broken it? Was the water/cement ratio kept to spec? Was the thickness adequate?

What makes you think that expansion joint spaceing was the problem?

 

[TDAA]

Agree with Maury.

You are likely looking at contraction. If it was expansion, you would most likely have just gotten displacement. Over 100 feet, this would have been fairly minimal (assuming "normal" temp variations).

Allthough, some questions: From the point of intersection, does the crack travers the portion of the walk that is shared, or does it go at 45 degrees across one of the legs? If the latter, which one? Is the crack width uniform?

 

[Ron]

jjk4985...your cracks were caused by drying shrinkage, something for which control joints should be placed. There are numerous references for this but typical codes do not address this as the code is concerned about structural sufficiency, not cracks which have little or no significance to structural integrity.

There should have been control joints placed in the sidewalk. Lets assume the sidewalk was 4" thick. The maximum spacing for a control joint for this thickness would be about 36 times the thickness or 144 inches (12 feet). That's assuming a relatively square placement. Since sidewalks are not square, but are rectangular, shrinkage naturally tries to make them into squares as the drying shrinkage process is relatively uniform and the concrete is relative uniform at that stage of curing. So the controlling factor then becomes the geometry. Control joints should be placed so that the length-to-width ratio is no more than about 1.2:1. So if your sidewalk was 4 feet wide, your joints should have been no more than about 5 feet apart.

The joints should have been specified to have been tooled into the plastic concrete or saw-cut within about 8 to 12 hours of initial placement.

As noted, there are numerous references for this, particularly from American Concrete International. Check their publication for slabs on grade. The Portland Cement Association also has good publications on this subject.

As a rule of thumb, control joints should be spaced at 24 to 36 times the thickness of the concrete for plain concrete.

It is a common misconception that concrete needs "expansion" joints. Those are rarely needed. Concrete rarely gets bigger in volume from the time it is placed. There are a few exceptions such as long, thin strips of concrete (like sidewalks, runways, and roadways), but even then expansion joints are spaced relatively far apart (100 feet is common); whereas control joints are needed at close spacing in concrete to control drying shrinkage, a much more prominent condition than expansion.

In order of use, the types of joints used in concrete are:
1. Control
2. Construction
3. Isolation
4. Expansion

 

[irasamm]

Check out the American Concrete Institute (ACI) 224.3R concerning contraction joints. As Ron stated previously, you won't find absolute guidance in the ACI but will find a few standard sections and reccomended advice. Also, many state/municipal websites offer standard drawings online for contractor use which show generally accepted methods for their respective parts of the coutry.