Backfill at reinforced masonry wall

[ACE58]

A project currently under construction is steel framed with 8" bypass girts starting at 5'-4" above finished floor. From the floor to +5'-4" there is an 8" masonry wall with vertical #5 at 1'-4"c/c full height from the foundation. All cells are grouted full, full height. There is horizontal masonry joint reinforcing at 8"c/c full height, & bond beams with continuous(2)#5 with top elevations +2'-8" & +5'-4". On the exterior is an earth berm (assumed 45 psf equivalent fluid weight of soil) which extends to 4" below the top of masonry. The wall was designed with the aid of NCMA tables. There is a foundation drain & 1'-0" wide gravel extends from the foundation to 8" below the top of wall, full length of wall. The contractor will be starting to lay the masonry soon & wants to know how soon he can backfill after erection. If this were a reinforced concrete wall, I would tell him that after the concrete has reached its design strength (even if it's before 28 days)& the wall must be braced during compaction. I have looked through numerous masonry design manuals & have not found guidelines for when the backfilling behind a reinforced masonry wall may start. Can anyone point me to reference that covers this topic? Do I follow a similar route as if concrete & take grout samples to have them tested for design strength? Does grout gain strength on a similar curve to concrete? The contractor is behind schedule due to weather & is looking to cut time out of the schedule wherever he can. Thanks for any help you can provide.

 

[phamENG]

I think you'd be ok using a similar approach to concrete, but remember - you have grout and mortar. As long as they are both coming up, you'll be fine. Another option could be shoring the wall from the beginning. Sometimes, grout will come up a lot faster than regular concrete. Last masonry job I did I was getting full strength from the grout at 7 days. Depends on admixtures, temperature, etc. If you still have a book from your civil engineering materials course it should have plenty of graphs and test data for you, or you can go back and look at past jobs with that masonry sub/grout supplier. You could also call the grout supplier if it's being batched and trucked in - they usually keep good records of their product performance.

 

[masonrygeek]

NCMA TEK 3-11

https://ncma.org/resource/concrete-masonry-basement-wall-construction/

covers this topic a little bit. As mentioned in the TEK, it is often the completion of the floor diaphragm that will allow the masonry wall to resist the backfill. So it may not depend on the masonry strength, but when the bracing/reaction is in place to fully support the wall.

Although its not in the code, most industry publications state that you can start to apply loads at 12 hours after completion of the masonry. If it is a heavy concentrated load then it is recommended to wait 72 hours. In addition, if you look at the "Standard Practice for Bracing Masonry Walls Under Construction", the initial period is considered to be not more than 24 hours so the masonry is gaining strength that quickly.

Finally, I'm not sure that doing grout testing is really going to show you the same strength gain as the concrete masonry, which is what you are really interested in. Most cementitious products gain strength along the same curve so they should all be basically the same, but there may be differences.