Subject: Split-Face Block 101
I don’t know if it was the recent eclipse, the hurricanes, or the wild fires in the Northwest or all of them together, but something has knocked the masonry stars out of alignment. I have received a rash of calls regarding architectural concrete masonry units. The calls were all specific to split-faced (SF) concrete masonry units (CMU).
This old “bricky” is just naïve enough to believe that participants in the masonry industry know what they are doing. But, boy, am I wrong! First, let me explain what the TMS 402/602 and ASTM C90 says about the split-faced unit. TMS 402/602 is a structural code specification that deals with the structural use of a CMU. It does not address color or face texture.
ASTM C90 on the other hand is the manufacturing specification. It is used to guarantee consistency of the CMU. The discussion here is not whether C90 should or should not cover both smooth units and architectural units. It does cover both of them, so you will just have to deal with it. According to C90 6.2.2, the size tolerance is +/- 1/8 inch for all non-split faces. Since there could be dimensional variations in the split-faces, Note 9 suggests consulting the supplier for the acceptable tolerances in the block you are using.
Paragraph 7.1.2 disallows chips of 1 inch or greater in any direction on the smooth finish face block. That is basically the size of a quarter. Although, Note 10 says that this does not apply to the split face. Many times 6”, 8”, 10”, and 12” units are installed with both sides finished. When this happens, greater care than usual should be used to insure deviations that are acceptable on one face of the wall are not exaggerated on the other face. Typically, one should work from the split-face side of the wall to be able to get a better look at the side that is not laid to the line.
Paragraph 7.3 and Note 11 discuss color and texture. Suffice to say, communication between the specifier and supplier is of great importance. The best consistency is found when all units are manufactured at the same time.
Variation in the thickness is another issue that can raise its ugly head. There are numerous reasons for occasional thick and thin differences in split faces. There is no tolerance for the thickness except a minimum of ¾ inch is allowed. The specifier should state if a wider variation is expected or allowed. If the “bad” split-face units are highly random, then at a minimum the masons should cull those and set them aside.
The bottom line is that the mason/mason contractor has a greater responsibility when installing split-face CMU. I have heard of architects wanting a line pulled on both sides to insure uniformity. (Ha Ha!) They missed their calling. I’m not sure what they should be doing, but designing masonry is not it. In order to help your project architect, engineer or specifier design the best possible split-faced masonry, contractors should have an understanding and knowledge of ASTM C90 and NCMA TEK 2-3A at the very least. REMEMBER ASTM C90 is a manufacturing specification. There is NO standard for workmanship.
Contractors, this is another example of the critical importance of the mockup. Selection of the CMU should be based on a minimum submittal of four units. Final approval of workmanship lies with the accepted mockup. If I get a call to come to one of your projects, I will ask to see your accepted submittals and your accepted mockup. Without those items, judgment becomes totally subjective. The best way to insure success is to adequately communicate, plan for success, and be diligent in doing your work correctly. Masonry last forever, so get it right the first time. Your MCAA stands ready to help.
Always remember to, “Raise the line and come on around the corner.”