anchor brick failure - could cause be water

huangyhg

anchor brick failure - could cause be water
we have an indurating furnace operating at 2300 deg f as part of making iron ore pellets. the roof is a castabel refractroy 9vesuvius) and is held in suspension by anchor bricks. we experienced a roof failure where all of the achnors broke at hte interface with the refractory. 14 inch achnor bolts and 9 inches of cast refractory. all that was left was the 5 inches of the bolt that was suspended from free moving steel achors on the beams.
can anyone shed some light into what would have caused such a failure (machine had just been cooled down)and does water impregnation affect the tensile strength of the anchor brick to the point of breaking? there were signs of excesive water being directed to the area of the roo colapse. it was a hole in the building roof and all of the water was aimed at the location of failure.
any help is much appreciated
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you'd probably get more responses if you posted this in the iron and steel forum.
yes, water intrusion can affect the strength of brick, both compressive and tensile.  many older brick were not consistently fired through their cross-sections,resulting in a hard "shell", but a softer core.  when the hard shell is compromised by repeated wetting/drying, the core ends up being the primary strength of the brick and has much lower strength than the full brick.
have a petrographic examination done on the brick to check for this.
thoughts:
thermal change from the water
steam created
corrosion   
as ron noted... over a period of time, with frequent wetting and drying the hard outer skin (fireskin) can exfoliate leaving the softer core.  this softer core can deteriorate very quickly.  are there signs of brick 'facing' around the area of deterioration?
dik
in a furnace the change in temperature can produce high stresses within the refractory itself to the point that the refractory fails either in compression or in tension. generally refractory is in compression as it expands and is restrained. more so at the hot face, where the higher compressive stresses can cause the refractory to fail. when it is cooled then the differential thermal expansion can cause tensile stresses. in tension refractory has little or no strength and either fails or joints between the bricks open up.   
corus
i would like to thank everyone for the great feedback. this information if shedding a lot of light on this failure. the brick is only 2 1/2 years old.
i am not sure what is meant by "signs of brick facing" so i attached some pictures of the brick. also, as corus mentionned, thre were vertical cracks in the section of roof that seemd to fall first and those crack were darkened indicating tht the crack was not new. what is a petrographic examination?
do people use specific procedures to enter a cold furnace that has a refractory roof? are any special inspections performed?
here is a picture of the repairs showing the placement of the anchor bricks that hold up the castable refractory roof. this may help.
again,  i appreciate eall of the help i have been getting on this topic
brick facing... shards of the brick surface.  if only 2-1/2 years old, you might look elsewhere for the problem.  saturated brick exposed to heat can cause the surface to fail.  possible 'bad' batch of refractory brick, or something else.
dik
dik, thanks for the info. did you look at the picture or the brick? does it help?
bad bricks, is thee anything we as a consumer can do to ensure the quality of the brick?
are there quality parameters, tests or sops that can be obtained from the manufacturer or done independently. could we doa  field verification on site (not sure what to look for) to guard agains this or idenitfy bad bricks before they get installed.
ron

the anchor bricks in question, pr-85 from snow shoe refractories are an 855 alumina brick with a specified tensile strength of 2500 psi @ 68degf (20 deg c). what happens to the tensile strength at 2200 degf. does it go down?
is this the right brick for the application?