huangyhg

masonry wall deterioration
i just inspected an 8" cmu wall, built in 1973, that has a strange deterioration going on. the block has had several "pops" take place over the years. in one room i looked at there were at least 1 or 2 per block. something obviously is expanding within the block to cause this. when i checked the problem areas, often there is a dark powder erupting from the block that smells a little like gunpowder. although the owner is concerned only with appearance and the ongoing maintenance, i am also concerned with strength. anyone ever see this before? i think my next step is to get a lab to test one of the blocks.

where are you located? most aggregates in concrete are local, but some unique block plants can use other strange materials. some of these oddities might be(coal, mollasses, newspapers, scrap metal shavings, metalurgical additives for foundries/steel mills, etc.) for molding very unusual products for different purposes and industries.
if i have a location, i may have a knowledge of some possibilities in that area.
what type of block involved? according to astm, are they considered lightweight (less than 105 pcf density), middleweight (102<125 pcf), or normal weight (>125 pcf)?
can you find out what type of aggregate (normal weight - sedimentary, igneous or lightweight - manufactured, or volcanic) was used? does it appear to limited to areas up to the maximum size aggregate used (usually 1/4")?
this deterioration is on the interior, but moisture may be driving some sort of expansion (a guess). what is the wall section and what is the exterior surface?
depending on the source of the expansion, the long term remediation measure would obviously be different.
dick
located in the west georgia area. i think the block are lightweight, but not positive about that. not sure of the aggregate either, but maybe a lab can determine that for me. the exterior walls are block, 2" cavity, and brick. but the problem is on both exterior and interior walls. there is a paint finish on the exposed block. i appreciate your help.
sounds like you have an aggregate reaction type of problem. do you have shales or some type of iron pyrites or some type of material that expands in contact with acid or basic chemicals? you might want to check with a local geotekkie or possibly a universty to get some info.
dik
have a testing company remove several offending particles to determine the chemical composition. i suspect they are not intended to be in the aggregate used.
after knowing the composition, it should be able to determine what is causing the reaction of the particles. most materials need a stimulus for an expansive action that would show on the surface.
i would not be concerned with the strength if there are individual particles. the block strength specifications are so low that most manufactures own standards are 30 to 50% above astm c90 minimums because of the manufacturing process and the green strength needed to handle while not yet cured. while being messy, removing a set of 3 block for compressive strength according astm c140 procedures would document the structural situation.
repair of the cosmetic portion of the problem can be done easily. the challenge is to insure that the process does not occur again. i have seen a situation where some unusual form of western pyrites reacted with lime in the presence of water, but the result was a rust stain and not a black deposit with an odor (sulfur?).
since interior walls are involved and the exterior walls are cavity walls, finding the cause driving the reaction will be a challenge. based on the age and protect situation of the walls, it has been an on-going problem.
dick

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