Concrete Crack Acceptability Guidance 5

[ChipperB]

I was asked to look at a concrete slab today as part of a one year warranty inspection. The slab is outdoors and its purpose is to support aluminum bleachers. The slab is basically like a big sidewalk. It is 25'x60', 4" thick, reinforced with welded wire mesh. It has control joints every 12 feet perpendicular to the longtitudinal axis of the slab. The only crack I saw was one in the center panel and it spanned the width of the slab between the control joints. The crack was 1/64 in. up to 3/64 in. wide. My gut instinct was that this crack was probably acceptable, however in my research I could find nothing that specifies when cracks are acceptable or when they are not. Does anybody have any web sites that give guidance on when cracks are acceptable and when they are not?

 

[bridgebuster]

Here's something from ACI224R - Control of Cracking in Concrete Structures

(of course 224R isn't for sidewalks but perhaps you want to seal the 3/64" crack, depending on your climate.In bridge work, we ignore cracks in substructures, sidewalks, and parapets under 1/32" wide.)

Table 4.1—Guide to reasonable* crack widths,
reinforced concrete under service loads

Exposure condition Crack width
Dry air or protective membrane 0.016
Humidity, moist air, soil 0.012
Deicing chemicals 0.007
Seawater and seawater spray, wetting and drying 0.006

* It should be expected that a portion of the cracks in the structure will exceed these values. With time, a significant portion can exceed these values. These are general guidelines for design to be used in conjunction with sound engineering judgement.

 

[Ron]

For slabs on grade, cracks have essentially no structural implication. They are shrinkage cracks and are unsightly, but functionally innocuous. You may ignore the crack or fill it with a semi-rigid epoxy filler.

 

[bimr]

1. Gergely, Peter, and Lutz, Leroy A., "Maximum Crack Width in Reinforced Concrete Flexural Members," Causes, Mechanism, and Control of Cracking in Concrete, Special Publication SP-20, American Concrete Institute, 1968, pp. 87-117.

2. "Debate: Crack Width, Cover, and Corrosion," Concrete International, May 1985, pp. 20-35.

3. Hognestad, E., "Design of Concrete for Service Life," Concrete International, June 1986, pp. 63-67.

4. Beeby, A.W., "Cracking, Cover and Corrosion of Reinforcement," Concrete International, February 1983, pp. 35-40

5. Halvorsen, G.T., "Code Requirements for Crack Control," Concrete and Concrete Construction, Special Publication SP-104, American Concrete Institute, 1987, pp. 275-322.

 

[southard2]

Chipper,

I agree with Ron. The cracks don't change your load path much which was basically from slab to ground. I would only be concerned about the crack if the crack is really close to a bleacher column or something like that. But most likely this crack was probably cause by shrinkage, or when the control joints were not cut quick enough. The welded wire fabric if properly placed should limit the crack width.

If the crack is a load related problem, then the crack would be more likely to be caused by a bending failure due to weak soil subgrade conditions. Four analyzing the resulting soil bearing pressues just take away the portion of the slab.

That said, this is all speculation because I have very limited information. For your run of the mill 3 -5 bleacher row set a 4" slab is OK. If the bleachers are large be careful cause the foundation itself may be inadequate.

John Southard, M.S., P.E.

http://www.pdhlibrary.com

 

[BigH]

bimr - thanks for the memory - Peter Gergely was one of my structural professors at uni.