Waffle Surface Problem in PS Conrete Beam 1

[WAstruc10]

I have a weird concrete problem that I have never seen before; any advice or thoughts would be appreciated. It is a prestressed, large (2'-6"deep, 6'-0"wide, 20' long) concrete beam, simple span (so PS strands towards bottom), longitudinal mild reinforcing towards top, #5 full circumference stirrups, 7 ksi concrete, 1.5" min. cover (which was special inspected, so not likely to be shortchanged on cover). The contractor used steam curing. When the steaming tarps were removed, it was noted that a "waffle" pattern had developed in the top face of the beam, with low points occurring directly over reinforcing (especially the stirrups). Any ideas why? Or similar problems experienced? My first intuition was that they should not have used steam curing for mass concrete, but I do not have any theory handy that would explain such a phenomenom. Thank you...

 

[ERV]

WAstruc 10,

1.5" of cover over reiforcing bars is somewhat shallow. It could be that due to the shallow cover there is less than the desired amount of aggregate available over the re-bar. In concrete, shrinkage occurs in the cement paste, aggregate will not shrink.

 

[WAstruc10]

Good thought ERV; however, 1.5" of cover is the ACI specified cover, and this was a 3/8" max. aggregate mix. I think there is something else at work here...

 

[ishvaaag]

In any case if the cracks or settlements happen following the rebar it is (plastic?) shrinkage and the phenomena is not only described, but has even an english label I unfortunately do not remember. I once saw an article about this thing I think in ACI's CONCRETE INTERNATIONAL magazine, but I am unable to provide the reference. In the natural state these grooves and afterwards cracks along steel lines use to coincide with hot and windy days, sometimes in spite of high relative humidity and even wet curing. Most likely supply of water by wet curing would have diminished these effects, anyway. Obviously, proximity of the rebar to the surface helps the formation of such cracks.

 

[dik]

Could the depressions be formed as a result of concrete filling in the void caused by air migrating upwards and being trapped by the reinforcing steel... similar phenomena that occurs and is reflected in the increased development lengths for 'top' bars?