I am remodeling my kitchen and will be soon installing cabinets. The home has hollow CMU exterior walls with 1.5" furring strips to which the 1/2" drywall is secured. I had read mixed opinions on attaching cabinets to furring strips alone, so I notched out the furring strips and attached 2x6s that span the length of the upper cabinets using Loctite PL Max adhesive and 2-3/4" Tapcons, every 24" , directly to the CMU wall. Will this be sufficient to support the weight of the upper cabinets?
The adhesive is useless in the long run. I suppose if you used enough of it, then it might provide a moisture barrier between the block and the wood. It would have made more sense to put a layer of plastic moisture barrier between the block and the 2x6.
The 2-3/4" Tapcon length was a good choice to avoid threads in the wood layer. Any thread-engaged wood must tear before the Tapcons can pull the wood layer tight to the block, where this unnecessarily leverages the withdrawal strength of the fasteners. As number of threads in the wood layer goes up, the probability of bad installs increases.
@isherwood is correct that Tapcons with sufficient frequency will be okay. There's specific technical information on the strength of Tapcons embedded in hollow CMUs at https://www.itwredhead.com/portals/0/fmproductcatalog/documents/products/37/tapcontechnicaldata.pdf. Pessimistically assuming 3/16" diameter Tapcons and lightweight hollow CMU blocks, each anchor is good for 220# tension and 400# shear.
Given 2 ft from your ledger attachment points down to the bottom of the cabinet and given load P centered 1 ft from the wall, that's P = 220#(2ft)/(1ft) = 440# for a tension failure and P = 400# for a shear failure. At your current 24" spacing and assuming a 50% bad install rate, your 24" fastener spacing implies 4 ft of shelf weight per fastener, so (400#)/(4ft) = 100#/ft is your current load carrying capacity. That seems inadequate to me. With @isherwood's 2 on 12" layout, you've got (400#)/(1ft) = 400#/ft. I like that better. It allows for some idiot to hang on the cabinets. The cabinets should fail before their anchorages fail.
Recall that the analysis was predicated on the 2 ft distance from ledger attachment down to bottom of cabinet. For a different distance (and different fasteners and different block), a different pounds per foot falls out of the analysis.
The big virtue of the Tapcon fasteners in this application is that you can drill and install them without needing to put oversized stuff like lag shields behind the ledger. Flat head sleeve anchors would do well in this application too.
Adhesive applied in the field should never be considered structural. It can augment mechanical fasteners to reduce noise and movement, but it should not be a primary connector.
In this case, I'd consider the original furring strips more robust than the new lumber without more anchors. The furring strips are at least trapped behind a continuous sheet of drywall and locked in place at the top.
I would add expanding or toggling anchors at 16-24" intervals along the 2x6, staggering position vertically to prevent rocking. Counterbore for heads so they're below flush. If you want to use up your Tapcons instead, pairs every 12" should do. Some portion of them won't grab well due to the nature of masonry, but if you use enough they'll average out to be adequate.
I would not count on the Loctite PL Max adhesive bond between the sheetrock and 2x6 to hold up the cabinets. I installed cabinets for a few years, and usually used large head screws specific for cabinets to secure them directly to your studs. That should be enough.
