Rust stained Masonry?

[SteelPE]

I was onsite yesterday with a client who wanted to put an opening in his existing masonry building. The existing building is approx 15-20 years old and is constructed out of red split fact CMU. While we were onsite he asked me what I thought about some rust stains that were on the outside of his building. The red split face CMU has little rust spots on the split face section of the masonry. Little dots every once in a while. It almost looked like some pieces of the aggregate (that makes up the CMU) has high concentrations of iron which rusted over time and caused the spots. I would say that one out of every three or four blocks had a small rust spot which is only noticeable if you are close to the building (because the block is red). What do others think may have caused this problem?

Unfortunately I didn't think to take a picture as he was in a hurry and we had other things that needed to be figured out.

 

[BAretired]

Don't know, but your hypothesis sounds reasonable. Is there a concern about the rust spots?


BA

 

[concretemasonry]

Not a structural problem.

In any event, the aggregate contained some iron bearing particles either from the source, trucking or contamination from before the the products were made.

If he really concerned, contact the possible supplier of the units because they need to be aware of the problem and future changes can be made.

It could be worse if it was a white split CMU unit made with white cement.

The aggregates (ASTM C33 and others) are the prime cause and the block are in the wall. There is little to be done, except if the are larger, they can be drilled out and hopefully patched to make them less noticeable. The ASTM standards are woefully weak for architectural units because of the lack of interest by the professional community and the problem and and solution lies with the supplier(s).

Since it is a red CMU, the most likely pigment was a red synthetic (refined and processed) and used in such small quantities because of the purity and high tinting strength it would almost an impossibility for that to be the source.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[Ron]

Agree with CM...aggregate is contaminated with iron-bearing particles. Common. No repair for it other than clean the spots and seal the split-face block with a clear sealer....that's temporary of course.