shrinkage cracks in stair slab

[ajcannon]

Recently designed some replacement exterior concrete stairs for a client. We did not use "nose-bars" as they were a reason for the failure of the existing stairs (corrosion/spalling due to inadequate cover and salt applications for snow-melting). I used plenty of temp-shrinkage reinf in a mat of bars that was down in the slab...but now we are getting cracks in the treads themselves (out by where the nose bars would have been). The cracks seem to narrow down at the bottom of each riser (where the concrete gets close to the reinforcement). We had spec'd a low w/c ratio (.42 max)...and the concrete break ended up coming back as 6000psi on most of the stairs...but one stair only made it to 3800 at the 28-day break (specified 5000psi min). In that one there is a large noticable shrinkage crack. The other ones that hit the 6000psi...some of them have very small visible cracks but not nearly as noticeable as in the 3800psi stair.

Two questions: (1)Should we have the 3800psi stair be re-done since it is out of spec? (2) If we did not have them re-done is there anything else we could do to prevent freeze-thaw damage in the small cracks? (3) Is there an inexpensive coating that we shoudl be putting over the stairs that would conceal and protect the small shrinkage cracks that on;y occur at the treads?

Other supporting info...we used corrosion inhibitor add mixture and epoxy coated rebar in the stairs. Thanks in advance.

 

[JAE]

Actually, having a higher strength concrete (6000 psi) implies more Portland Cement in the mix and this can create more shrinkage than 3000 psi concrete. All shrinkage is in the cement paste so the more cement you have the more shrinkage.

What size of aggregate was used? More coarse aggregate helps reduce shrinkage cracks as well.

Intense attention to curing also helps.

 

[Ron]

JAE is correct...more strength isn't always better. Further, deleting the nosing bars because of spalling the last time might not have been prudent. You need to check the cause of the original spalling. Was there too little cover? Was the bar too large for the application? (nosing bars should be very small if you use them at all)

Did you consider using a corrosion inhibitor this time? Was the concrete air entrained?

At this point, you might want to consider a methacrylate sealer to penetrate the cracks and seal the concrete from further moisture/salt intrusion. Be sure to coat the surface and embed sand in it for slip resistance.

 

[ajcannon]

Thanks for the comments. The idea with the high cement content was to get a denser concrete for better durability..

The biggest crack is in the 3800 psi stair...the next biggest crack occurs in another stair where they hit the 5000psi, but they had the plastic blowing around on a hot Saturday afternoon with the hoses off the day after they poured it.

I am not thrilled with how they handled the curing. They used plastic in lieu of wet burlap...they did not add water with hoses - just slowed evaporation rate.

I also can't figure out how they got 3800 instead of 5000. How does that happen? The Ready mix guy did not know either.

The other stairs came out pretty good. just a couple barely perceivable cracks.

I've used a mix similar to this in the past for stairs without nose bars and have had no problems. But those they did a better curing process (wet burlap and soaker hoses) and that mix had fibers in it as well.

I can have them redo the 3800psi stair since it is out of spec...could I epoxy-inject the large crack without making a horrible looking repair? Would one of these penetrating breathable sealers (silane) help with these small cracks for long-term durability?

Thanks for any ideas..

Any thoughts on how these cracks will hold up in harsh freeze-thaw climate if we do nothing?

 

[ajcannon]

I did have 6% air entrainment and a corrosion inhibitor additive. I will look into the methacrylate sealer...something to fill the cracks and maintain a uniform look might be a good option at this point.

 

[ajcannon]

The mix had a lot of cement but it also had very low water content so that should have kept shrinkage (paste) down. For example the mix was intended to have very little if any bleed water and was supposed to be placed using a plasticizer. The plasticizer was supposedly used and that was the reason for the 6.5" measured slump...but I suspect that this load was dosed with water (I wonder how else we could explain the 3800 28-day strength?) hmmm....

We have another problem where the steel handrail toppled over and chipped the noses off all of the treads on the two stairs that had the worst cracks (one was the 3800 stair).

Maybe we have them patch the chips and them apply the penetrating coating to the entire stairs.

 

[JAE]

Sounds like either the poor curing created faster drying/shrinkage behavior or the contractor added water for the 3800 psi mix and increased shrinkage due to higher w/c ratios.

 

[hokie66]

These cracks sound like plastic shrinkage cracks that weren't closed in the finishing of the treads. The hot, windy conditions contributed to rapid drying while the concrete was plastic, and the finisher did not notice the initiation of cracking, which could have been closed at that stage.