Resteel cover/cracking 1

[markg]

We placed the concrete floor slab for a pedestrian bridge and are plagued with cracks. The slab is 2" composite metal deck with 2-1/2" concrete cover for total thickness of 4-1/2". The design called for #4 rebar at 12" on center in both directions. There was very little room to install the resteel since we only had 2-1/2" concrete thickness above the metal deck. The bars running perpendicular to the deck flutes had to be installed in the 2-1/2" space. Thus, even if we had perfect placement of the resteel there is only 1" concrete cover above the bars and 1" from bottom of bars to top of metal deck. The slab cracked at 12" on center (i.e. direcly above each bar). Tooled control joints were indicated and installed every 12'.

I believe the 2-1/2" concrete thickness above the metal deck was too thin to accomodate the #4 bars and thus the resteel caused the cracks. In essence, the resteel actually created the weak plane at which the concrete relieved itself while shrinking. The owner (Federal Goverment) is directing us to remove and replace the slab at our own expense. I have concrete test reports showing mild air temperatures the morning we placed (65), acceptable concrete temps (75) and excellent 28 day breaks (5000 psi +). The design only required 4000 psi.

Has only experienced similar problems due to inadequate resteel cover? I'd appreciate reference to any code requirements or other published literature that would help support my position.

 

[Taro]

If I understand the situation correctly, the bars parallel to the deck span direction were placed in the low flutes and the transverse bars were placed in the topping above the deck. A couple comments...

1. The cracking at 12" o.c. probably occurred because the perpendicular bars were placed in the low flutes where they are ineffective in controlling cracks at the top surface.

2. Control joints don't do much good in composite slabs because the metal deck is bonded to the concrete and causes restraint.

My preferred detailing for composite slabs includes welded wire fabric near the top surface of the slab. This provides shrinkage reinforcing in both directions. The WWF can be supplemented with rebar where required for flexural resistance.

 

[concretedoc]

I essentially agree with Taro's points 1. and 2. The further you place the bars from the surface, the less effective they are in controlling crack width. And control joints are worthless in a composite slab. The #4 bars run through the joints and prevent them from opening and the Nelson studs also restrain concrete movement. So cracks are just as likely to form between the joints as at the joints.

Reinforcing steel doesn't prevent cracking in many cases. It may even provide enough restraint to cause cracking. It simply holds the cracks tightly closed when they do appear. For an excellent discussion of this topic as related to composite-deck slabs, see "Let It Crack" by J. Thomas Ryan in the August 1999 issue of Concrete Construction, pages 37-39.

I don't agree with using welded-wire fabric near the top surface of the slab because, unless it's heavy gage and chaired at close intervals, you can't keep it near the top surface where it's most effective in holding crack faces together. Ryan makes this same point. He recommends using no contraction joints, allowing random cracks to form, then patching any that are wide enough to affect serviceability.

I assume this walkway is indoors. Will it be covered with a floor covering? How wide and long was it?

 

[JAE]

You should tell us if the bridge is exterior (exposed concrete) or interior. If interior, then the cover required over #4 bars is typically only 3/4" so your 1" cover should have been adequate. Cracking at 12" o.c. seems pretty excessive, even with 3/4" cover.

If your strength and air temp were OK...what about water/cement ratio? You can get samples of the concrete taken and have a lab perform petrographic analysis to estimate the water content -

What was your aggregate size? Small aggregate mixes tend to shrink more.

The interior slab situation - the cracks may be no big deal if the slab is to be covered. If exterior - I'm surprised that you chose metal decking as even galvanized deck will rust eventually.

 

[markg]

Thanks for your thoughts so far. Here's some additional info in response to questions asked.

The slab is an interior slab. Aggregate was #57 stone. The concrete was pumped at a 4.5" slump.

 

[Ron]

markg...JAE is on the right track. What you have experienced is something call rebar slump. This is where you have rebar close to the surface and the combination of aggregate size and cover distance do not allow the same distribution of aggregate over the rebar as you have in the rest of the concrete matrix. In your case, #57 stone top size is about the same as your cover, so no room to get good coarse aggegrate distribution over the rebar.

This creates a differential shrinkage condition (micro level) in the mix, that is exacerbated by the overall mix shrinkage (macro level). Because of your different rebar elevations, you lost some of the effect of the rebar at critical times during the curing/shrinkage process. You had more shrinkage at the top which was not countered by the lower rebar, thus giving an opportunity to concentrate the shrinkage at the upper rebar locations.

In this case, a smaller coarse aggregate would have been better for the noted conditions, being aware of the issues that JAE noted about more shrinkage with smaller aggregate (that's because you need more cement to cover the surface area of the aggregate). Based on your compressive strength results, you likely had a high cement mix anyway. Next time, sacrifice some compressive strength (which you probably didn't need all of anyway!), match the coarse aggregate size to the cover, and keep the w/c ratio low. You can always use admixtures to gain workability.

I would bet that your specification mentioned a minimum cement content in your mix. If you see that, start asking questions! That type of specification does not serve to provide better concrete for the application, just stronger concrete!

 

[jheidt2543]

JAE and Ron's comments address the heart of the problem, the concrete mix design was not matched to the proposed use. While we don't have all the facts, it seem clear from the above responses that this is a DESIGN problem, not a CONSTRUCTION problem.

So, the one thing that hasn't been addressed is who should pay for the slab replacement? If the contactor follows the specifications and the results turn out to be less than expected, shouldn't the designer take the hit?