That mortar mix ratio everyone swears by failed me on a retaining wall

Two weeks ago I followed the standard 3:1 sand to cement mix that gets recommended around here all the time. Laid about 40 feet of retaining wall in my own backyard with it. Three days later I noticed cracks forming at the base joints where the wall meets the footing. Turns out that ratio is fine for above ground work but not when there's any hydrostatic pressure behind it. I ended up tearing out 12 feet of that section and switching to a 2.5:1 mix with some lime added. Has anyone else run into this or am I the only one who trusts the old timers too much?

3 comments

3 Comments

parker_webb8d ago

Been mixing mortar for fifteen years and I gotta push back on this one. That 3:1 mix has held up just fine on dozens of retaining walls I've done in this area, even some that sit in wet clay soil half the year. Sounds more like your footing might not have drained right or maybe the base wasn't compacted enough before you poured. I've seen guys blame the mix when really the problem was they didn't put in proper weep holes or gravel behind the wall. Adding lime is fine but 2.5:1 is gonna be a pain to work with and might even shrink more as it cures. Next time check your drainage before messing with the recipe.

margaretc428d agoMost Upvoted

My folks had a similar issue with a retaining wall they did back in 2018. They used the 3:1 mix like everyone said and got hairline cracks within a week. Turned out the ground behind it stayed wet way longer than they expected because their soil drains slow. I ended up helping them redo a section with a 2.5:1 mix plus a handful of lime like you did, but what really fixed it was digging out the dirt behind the wall and putting in a 6 inch layer of clean gravel with a perforated pipe at the bottom. The stronger mix helped with the pressure but the drainage was the real fix. That wall is still standing fine now like 6 years later.

shane_wilson7d ago

hairline cracks within a week" - man that's rough. I feel your pain though, I once mixed a batch that was so bad it looked like a dried out riverbed after three days. Drainage is always the thing I forget about until it's too late, guess that's why I stick to pouring drinks not concrete.