Bonded topping joints

[JAE]

With a bonded topping (1 1/2" over 4 1/2" slab), with joints cut about every 12 to 14 feet, would you expect the topping to shrink to a degree that sawcut joints filled with semi-rigid epoxy would tear?

Interior slab - normal weight - bond was tested for tensile-bond limits and passed. Joints about a sawblade wide.

 

[Ron]

JAE...it depends on the mix design. With a 1-1/2 inch topping, it is likely that a "pump" mix was used, thus smaller aggregate, more paste required, and therefore more shrinkage. Do the topping joints correspond to the primary slab joints?

Just did a petrographic examination on a similar condition here locally. Two-inch topping over five inch slab....So much shrinkage that debonding occurred over most of the slab. It had some construction problems as well, though.

 

[JAE]

Topping joints do correspond to slab joint's locations.

Pumped mix. 3/4" aggregate I believe.

I just have a hard time "seeing" a bonded topping (we bond tested the heck out of it) shrink when it is bonded to an older underlying slab.

 

[hokie66]

Any shrinkage in the topping will cause tensile stresses in the topping and perhaps minor cracking rather than end to end change in length. So I don't see the joint opening.

 

[Ron]

JAE...is the bond still intact? Do a sounding on the surface and you might find some unbonded areas.

I don't know if you've measured the joint width now relative to the saw blade width at the time of sawing, but that would give a magnitude of shrinkage.

Any chance that the joint width was very small right after saw-cutting and that there was not sufficient width to get a proper sealant profile? If so, then the sealant might be giving a false indication of the joint shrinkage.

Also, keepin mind that you probably measured tension bond when the failure mechanism for bond failure is going to be shear. Figure the shear bond to be a small fraction of the tension bond.

 

[haynewp]

Could it have gotten very cold after sealing and the epoxy shrank?

 

[JAE]

nope - it is indoors - between 60 and 70 degrees.

 

[haynewp]

Since bonded shrinkage causes random cracking, how could the concrete remain plastic at the interface of the topping to the existing? Was the topping sealed soon after the topside dried?

 

[hokie66]

I imagine Ron has hit on the problem with his comment about sealant profile. Not sure what the filler material actually was, but two things usually make sealant joints fail: joint not clean and primed, or incorrect profile. If the depth to width ratio is too large, even very small opening of the joint can overstress the sealant.

 

[JAE]

The filler was a semi-rigid epoxy with an elongation capability of 24%. The joints were sawn joints which were, at 21 days, re-sawn and then filled with the semi-rigid epoxy.

The joints have cracked - primarily through the epoxy with only minimal bond loss between concrete and epoxy.

With 24% there isn't much room for movement - but again, since it was bonded, I would think the shrinkage ...at the joint....would be minimal. 12 ft. to 14 ft. centers on the sawn joints. So the question is basically, at 21 days of age (the underlying slab was older) would a 1 1/2" bonded topping stretch a sawn joint more than 24%? (i.e. the final joint width is 124% of the original)

 

[kslee1000]

One phenomenon I don't quite understand:

"The joints have cracked - primarily through the epoxy" & "The filler was a semi-rigid epoxy with an elongation capability of 24%."

How exactly the crack looks? Can you post a photo?

 

[Ron]

JAE...the elongation of the sealant was determined at lab temperatures. Its extensibility is severely reduced at lower temperatures, particularly epoxies.

How much separation is there at the sealant crack? The amount of separation should mimick the total joint movement, since cohesive not adhesive failure occurred.

 

[hokie66]

I think that elongation capability would depend on the shape of the filler. If the joints were filled full depth, then the elongation capacity would be much less.

 

[JAE]

Ron - no lower temperatures - again - this was an indoor slab which was always kept above 60 deg F.

I'll see about posting a photo tomorrow.