Cracks and mix analysis 2

[haynewp]

See attached picture. The footings are all either 5’-6”x5’-6”x1’-6” or 8’-0”x8’-0”x1’-6”. They are all supposed to have #6 at 12” at the top and bottom of the ftgs.

The contractor said they poured these and out of 18 of them, about 5 of them are showing these large cracks less than 24 hours after pouring. He said the rest have some hairline cracking. I don't think he used ANY curing, just poured them in about 60 degree weather and it got down to about 40 degrees that night. Contractor doesn’t know how deep the cracks go yet but said he could insert the edge of a car key down into some of them. The mix they proposed is below. I was thinking heat of hydration made worse by no curing but I didn't see a very high cement ratio when I went back to the mix. They want to inject an epoxy down into the cracks to restore the capacity. I am concerned about the amount of time to wait before they inject the epoxy and if the epoxy can get down far enough into the cracks. I am also concerned the #6 bars got installed in the top of the footings if this much cracking occurred.

Cement C-150, Type 1/2: 400 lbs
Type C Ash: 100 lbs
#57 Stone: 1843 lbs
MFG Sand: 1128 lbs
NAT Sand: 268 lbs
Water: 275 lbs
Air: 6%+/- 1.5%
Slump: 3 to 5 in

Sand/Agg: 0.43%
Sand FM: 3.01%
5.32 bags
water/(c+p): 0.55

 

[Ron]

haynewp...I suspect initial plastic shrinkage cracking. If you have a photo of the whole footing it would help.

What were the wind conditions at and after time of placement? Did they add water at the site?

If plastic shrinkage occurred, rebar placement would have made little or no difference...separation occurred before rebar bonding occurred.

This size of footing and thickness should not have had that much shrinkage strain. Possibility that the aggregates were not saturated at time of mix, contributing to early drying of the mix, which might have been compensated by water addition at the site.

 

[haynewp]

Thanks Ron,

I only have this single photo right now but will be getting more next week. I have been questioned on the w/(c+p) ratio being 0.55. I didn't think it was high enough to cause this even if they didn't cure the footings.

 

[hokie66]

Agree with Ron that these are plastic cracks, maybe shrinkage, or maybe plastic settlement cracks around the bars. How was the compaction done? With 18" deep sections and top bars, it is often necessary to revibrate after some time in order to close these cracks. They would have appeared quickly, but nobody noticed until the next day.

 

[haynewp]

Here is what they said:

Wind was 10 to 20 mph when pouring.
No water was added on site.
Got down into the 30 degree range that night.
They stuck a vibrating rod in the footings for compaction during pouring only.
No curing was done, footings left exposed.

I have attached a better picture. Many cracks are following the top rebar mat that is 2" to 3" below top of ftg. The cracks are about 1/8" wide and some up to 50" long.

 

[hokie66]

Typical plastic settlement cracks. Easily closed by revibration and floating while still plastic, but now they provide a path to the reinforcing steel. Maybe I shouldn't ask, but are the top bars required for uplift or overturning?

 

[haynewp]

hokie

Yes they are definitely required for uplift, I have been worrying about the same thing. They are wanting to epoxy the cracks back closed with an epoxy type that is supposed to flow down into the cracks. I am not sure this will be adequate to replace any compromised bond with the rebar.

 

[hokie66]

I would think that a low viscosity epoxy would work, and would be inclined to allow that solution. For insurance, I would want a coating or some more concrete on top of the footing to protect the bars.

For some reason, plastic settlement cracking seems to be a concrete problem not widely discussed, but it happens all the time in deep sections where consolidation is lacking.

http://www.concrete.net.au/publications/pdf/Settlement.pdf

 

[Ron]

Agree with hokie66...low viscosity epoxy and compatible coating.

 

[TXStructural]

Drying shrinkage would be my guess - or what they call around here "wind cracks". With a 30 degree diurnal swing and 10-20 mph winds, they lost a lot of water through evaporation.

If the footings are under the building, in relatively dry, unexposed soil, I would not be concerned about corrosion in the bars. However, unless you inject the cracks (under pressure) you could get deflection as the cracks try to close with time. Do not let them use gravity to "pour" epoxy into the cracks. The process should be to install injection ports, seal the crack at the surface, and inject under pressure.

 

[hokie66]

TX,
In this instance, I think you are off base. Drying shrinkage cracks wouldn't mirror the bars like plastic settlement cracks do.

Pourable, low viscosity epoxy is fine for this purpose, and injection is overkill.

 

[dik]

They look like plastic shrinkage cracks... or drying shrinkage (the latter because they do not appear to follow any rebar pattern). Top reinforcing often causes plastic shrinkage cracking in proximity to the rebar and has a bit of a pattern sometimes.

What was the actual slump? Looks like 6 or 8 inches and no vibration. For large concrete masses like these footings, the slump could have been 3" max with vibration. It does not appear that there was any curing compound appled.

Dik

 

[hokie66]

Dik,
Did you see his last picture? Checkerboard. Classic plastic settlement cracks. There may or may not have been vibration, but there was no revibration or finishing to close the cracks.

 

[dik]

Hokie... didn't, been so busy of late, other than skimming the OP, I don't usually go through the body of the thread or maybe just skim it...

The pattern shown in the second pix is almost 'classic' plastic shrinkage... thanks.

Dik