CMU Block Construction 1

[QCtech]

I have two questions: 1) I have read somewhere either ACI or a spec book that a CMU wall should not be grouted on the same day as the block was placed. Does anyone know if there is a spec for this? 2) What would turn CMU block or brick veneer the color black (new construction)?

 

[jike]

Mildew?

 

[QCtech]

Mold - Mildew ? You are probably correct. If they didn't construct their weep holes properly and moisture got trapped within the cavities. I just thought maybe there was some known impurity that can contaminate the water or cement during the block casting or mortar or grout mixing processes.

 

[concretemasonry]

You can grout the same or next day if you follow proper grouting procedures. Just make sure the wall is properly braced. That is sort of an old wives from back when people just dumped the large cavity of a pilaster as fast as they could. Some people advocate grouting soon if it is possible, but many times the schedules and logistics do not permit it.

Make sure your grout is approximately the same strength as the block. An other, more practical terms, avoid rich grouts. The ideal grout will have the same cured properties as the masonry units.

How soon after construction did the black color appear? Did you spray a sealer(?) on the block wall before veneering? some could be water soluable. Black is an unusual colore and suggests either organics or magnesium (very rare). Unlikely it is from the units since they are essential cured (most chemical reactions completed) before delivery to the site. On rare cases, I have seen block "sun burn" (orange stain) due to UV and strange mortar mixing water reactions.

 

[QCtech]

I actually had a chance to go see what's up with the block. It is not mildew/mold. However, the mortar joint itself, along the roof line, in conctact with wood blocking, is actually turning colors (gray/ black). The block wall has been up for a few months. I am still stumped though. I am not a masonry expert so I don't know what impurities can be doing this. Any ideas?

 

[concretemasonry]

QCtech -

Could it be coming from the pressure treated lumber? Also it is a possiblity that the nails were not compatitble with the P.T. wood. The P.T. wood is very hard on certain metals (nails, aluminum flashing, etc.). Both of these would require moisture to carry a stain, but P.T. lumber is usually very wet and can take a long time to dry out.

The mortar may have been relatively fresh and could be receptive to any staining moisture. - Just a thought.

Dick

 

[QCtech]

PT wood? possibly. Again I am not an expert at this, so, how about a petrographic exam.

 

[boffintech]

http://www.pci-il.com/data/pci/tech...scence(NCMA TEK)(Cleaning)(Efflorescence).pdf

NCMA TEK 8-3A Control and Removal of Efflorescence

 

[concretemasonry]

The NCMA Tek notes certainly are a good reference for all subjects and do a very good job on efflorescence. I have never run across a black efflorescence, but there could be such a thing.

QCtech - Are the stains in the mortar? The block, brick or both?

My gut reaction is deleterious materials in the mortar if it is not pre-proportioned or possibly from a water soluable black damp proofer that was leached out. The treated wood is a long shot. Since you have a veneer, the block backup and the brick veneer should be separated by an air space.

Dick

 

[QCtech]

The black discoloration is in the mortar only. The mortar was mixed by hand and not preproportioned (we had numerous problems with getting them to have the proper mix just to achieve compressive strength). I am awaiting the damp proofing submittal info from the contractor. I will post as soon as I get it. Thank you so much for helping me out guys.

 

[concretemasonry]

One thing about the mortar strength and proportions. - Do not do both since you are totally responsible. ASTM C270 states that you should specify mortar by proportions OR by properties, but never by both.

Also, you may have been trying to acheive the wrong strength. Most people specify a higher strength because they think it is better. The appendix of ASTM C270 clearly states that you should use the weakest mortar possible that is consistant with the loads. Normally, Type N or S is adequate, since mortar does not have a major effect on the strength of the masonry. I have seen 4500 psi hollow prisms made with Type S mortar. What strength and testing procesure did you specify or enforce?

Good luck in the results of the damp proofing and it is just a cosmetic problem that can be easily corrected.

Dick

 

[QCtech]

I agree, the point with the strength was that we could not achieve 1800 psi at 28 days, very sandy, could crumble in my hand. They were not utilizing a bucket with a predetermined volume to weigh out the materials (just shoveling in).

As for the discolored mortar, I just received the submittal they used - Karnak 100AF non-fiber emulsion dampproofing and it is the color black and was brushed on. I couldn't see where it was noted to be water soluble, however it does note that it should not be exposed to water before curing / drying. So - maybe you have it the mark there Dick. Well, I will keep you posted to the out come of our findings. Thank you for your help again.