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Found out why my corners kept cracking - it was the mortar mix ratio

I was having issues with corners cracking on a patio job near downtown Austin last month. Talked to a guy at the supply yard who said my mix was too wet for the 90-degree weather we had. He pointed me to a spec sheet from Quikrete that said Type N mortar needs 5.5 to 6 quarts of water per 80-pound bag, not the 7 I was using. Anyone else had this problem or used a dry mix for hot weather jobs?

4 comments

4 Comments

owens.laura16d ago

Had the same thing happen on a job in San Antonio last August. I was using 6.5 quarts per bag because the sand was damp from morning dew and I didn't account for it. That extra half quart killed me on a long retaining wall. Now I keep a cheap thermometer in the water bucket and aim for 70 degrees or less. Also started using a scoop to measure water instead of just eyeballing from the hose. Saves me headaches every time it gets above 95.

mitchell.avery16d ago

The real problem nobody wants to admit is that nobody checks their water temp. You're mixing in July in Texas, that hose water might be pushing 80 degrees. Hot water makes mortar set faster and makes it more prone to cracking. I've had guys argue with me about this until they actually measured the water temp. Throw some ice cubes in the bucket or let the water sit in the shade for an hour. Also, that spec sheet is assuming your sand is bone dry. If you're using bagged stuff, fine. But if your sand pile has been sitting out in 90% humidity, you're already adding water before you even wet it. That's the corner cracker nobody talks about.

the_john16d ago

Actually saw a study from Portland Cement Association that backed up the water temp thing. They tested mortar at 70 vs 90 degrees and the hot batch lost like 20% of its working time plus had more shrinkage cracks after curing. Not saying it's the only factor but the numbers are pretty clear. You ever try using a thermometer on a hot day just to see what you're actually dealing with?

dianawilson16d ago

Nah man, I gotta push back on this one. I've been mixing mortar in South Carolina summers for 15 years and never once checked water temp. If you're getting cracks, it's way more likely your mix ratio is off or you're working the mortar too much after it starts setting. That whole hot water thing is a theoretical problem that sounds good on paper but doesn't really matter in practice. Also, your sand argument doesn't hold up either because if you're using your eyes and feeling the mix, you adjust for moisture content on the fly. That's just basic masonry, not some hidden secret. Spec sheets are guidelines, not the Bible.