May-Day May-Day -- Bluestone on Concrete -- Major Fail
by Frank Rothermel
(Bucks County, Pennsylvania)
Cement Pour prior to Wall & Bluestone Patio
Dear Don -- Learned the stone trade a bit as a kid (not enough as you'll see below), but when our small crew got laid off, decided to make my Mom happy, hung up my trowel, went back and finished college and been a "suit" guy ever since. Fast forward 20 years and my wife and I bought an old farmhouse in Bucks County, PA. Decided to get out my trowel again and build a large stone patio. First, she and I framed the entire area out and poured 55 yards of concrete to create one large, level pad. (Photo #1 attached). Then came back and built the wall (the old Italian way as I'd been taught) with 20 tons of fieldstone. It's about a 100' run, 18" wide, and 30" high with 3' high piers capped with 2" bluestone. After finishing the wall, took a few months to lay 15 tons of irregular Pennsylvania bluestone which was bedded on fresh mortar on top of the fully-cured rough-topped concrete. (Photo #2)
Well, the wall has held up fine but the joints and underlying mortar on the bluestone started to crumble after only 5 years. Now 11 years later it has completely failed. I have no idea what went wrong or how to reasonably fix it. It was a hot summer in 2004 when we laid the bluestone, but we cooled each stone with ice-cold well water, mudded up a moist bedding, and sprayed the joints for a day or two after setting to slow the cure. The mortar recipe was the same one I learned as a kid - 9 shovels of sand, 2 Portland cement, and 1 lime. Didn't
use much mortar on the wall so that was all hand-mixed, but did use a mixer for the bluestone given the large quantities needed to bed even 1-2 large stones.
Anyway, many of the joints started to crumble into sand after a few years - it was insane. You could run a slicker straight through the mortar joint and it looked literally just like sand -- no cement visible at all. And as 1 stone went bad, the surrounding stones started to "pop" with the same crumbling mortar-to-sand mixture found underneath. Now all the bluestones are completely detached from their underlying mortar base.(Photo #3 and Video #4). Have checked with some masons over the past years but there was no consensus on what caused it or how to fix it. Some said I never should have used Portland cement; it should have been a strictly lime mixture. Others said the 1 part lime killed the joints because it allowed too much water to wick through. Still others said never do bluestone over cement because the bluestone is porous and needs drainage underneath no matter what mortar you use.
So what do you and everyone else think? If there's any insights or time-honored secrets you or others can impart, I'd be much obliged. I note that the stone steps leading into my house were likely placed when the large addition was built in 1948. The mortar on these steps is still stone hard, tight and nary a crack -- not even one. Unfortuantely it serves as a constant reminder that maybe I should have stuck to my day-job ;-)
Well, all my best and again, any guidance much appreciated.