concrete spalling

huangyhg

concrete spalling
i have an existing concrete beam (interior) that is subject to constant high temperatures (around 142f, 62c).  at the interface between the roof deck and the beam, concrete is spalling from the face of the beam.  there is no visible stress cracks on the surface of the beam, the beam has not been subject to a fire, yet the spalling does not extend to the reinforcing steel within.   the concrete used was normal density concrete, 5000#/si.  concrete cover to the steel was to be 2".  the depth of the spalls are less than 2" in many areas, and where they extend deeper into the beam 2-2.5", no rebar has been exposed.
any ideas as to the cause of the concrete spalling?   
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per your exposition it looks it should be something thermal, but who knows. maybe radiative cooling from the deck causes stresses enough against the main body of the beam to eject the part at incompatible temperature.
it could be a lot of things; thermal effect as ishvaaag stated; freeze-thaw? carbonation? intermittent spots where the concrete was not properly consolidated.
can you tap the spalled areas with a hammer? if the area has a hollow sound it's possible that the rebar is corroding, which is a cause for spalling.
do you have any photos?
is the 142 degrees the room temperature or the temperature of the concrete at the roof level? is there any constant circulation?
what is the average humidity in the area? the warmer air will hold more moisture that could condense on cooler exterior surfaces.
dick
the beam and underside of the deck are exposed to the same temperature in your description. the top side of the deck is exposed to a different temperature and a gradient several times per day.  
it is likely that your spalling is caused by differential movement.  freeze thaw isn't at play here and carbonation is a possibility, but the depth of spalling is too deep to be primarily influenced by carbonation.
the 142f is not high enough to create an issue with the concrete and also is not high enough to cause sufficient vapor pressure interstitially in the concrete to cause spalling, unless there is some microcracking that occurred during the curing process.
look at the face of the spalled pieces.  if the face shows that the coarse aggregate is fractured, then it is likely that external shearing forces are at work (as the differential movement previously described).  if the face shows that the aggregate did not fracture and that the paste pulled from around it, you could have a placement issue, an initial curing issue, contaminated aggregate, or several other mechanisms.
as bridgebuster suggested, a few photos would help.

the area never gets cold - it is in a co-generator plant.  the heat never gets hotter than about 145f. due to the location of the beam in proximity to the mechanical equipment it is very difficult to get close to the spalls.  i will upload a couple of photos.
another photo
any chance whatever the beam supports is moving due to thermal change and 'pulling' the concrete off the face?
dik
no.  there is no sign of movement of the beam.  the spalling appears only on one side of the beam, and the beam is 24" x 32"high.   
is there a plastic file on the underside of the deck? do you know how long after the concrete placement the spall appeared?
in picture 8, i see a white streak above the spall; that's a somewhat common occurrence in bridge deck construction - a pattern of white lines perpendicular to the stringer/girders. it generally indicates there was too much water in the mix causing the minerals to leach out. back to the picture, i don't see any fractured aggregate or paste; for the time being i think it's a placement problem.
i tend to agree with bridgebuster on this.  the spalling is near the surface and is mostly a paste spall, indicating a lot of laitance at the beam form.  the lack of coarse aggregate involvement shows this to be the case.
check one of the spalled pieces.  you'll probably find that it has little or no coarse aggregate in it.  there's a good chance that the initial fracture took place when the beam forms were pulled...it just took a while for it to fall off.
if my observations are correct, it is a placement issue as bridgebuster said.