Ask the Builder: Failed cement stucco: Why it happened, how to prevent it
My wife and I are the head ushers at our church. Each Sunday, when we arrive early, we walk up a ramp that allows those attending mass in a wheelchair an easy way into Sacred Heart Church in Laconia, N.H.
The ramp was built with two poured concrete walls with the ramp in between them. The church is a stunning red-brick edifice built by French Catholics in the late 1800s. At that time, there were competing factions of Catholics in Laconia. It must have been like sports rivalries of today. Catholics from each country built their own church. Based on the churches that are still standing in Laconia, it’s obvious to me the French had more money and poured lots of it into this magnificent building I worship in each Sunday.
The contractor who built the ramp perhaps 20 or 30 years ago tried to make it match the original red brick. Rather than pigment the concrete (yes, you can add dry color pigments to concrete), he decided to hide the harsh gray cast concrete walls by applying a thin cement stucco to any exposed concrete.
Using the stucco was not a bad idea, and my guess is that it might have been a budget issue. The project architect or builder could have installed a matching brick veneer to the poured concrete for an even better look. They could have done exactly what the architect and builder did who created a stunning addition on the public library in the nearby town of Bristol. The builders there matched the red brick of the original library building perfectly. Only a trained eye can detect that part of the library building is an addition.
Here’s why the thin stucco coating has peeled off the poured concrete. Realize that Portland cement is an amazing adhesive. When you mix Portland cement with water, you begin a chemical reaction where countless microscopic crystals start to grow. Try to imagine those sticky burs that cling to your pants when you walk through the woods. The cement crystals are that tenacious.
These crystals are so plentiful and strong that they can even penetrate the microscopic pores of glass and metal. I have my original magnesium float from 45 years ago when I first started building. There are concrete deposits on it that can only be removed with a chisel!
The red stucco failed to bond to the poured concrete on the ramp for several possible reasons. It could be one or all of the following. For starters, the poured concrete walls could have been dusty. The stucco batch might not have had enough Portland cement in it or it might not have been thoroughly mixed in with the sand before the water was added. The stucco material could have started to set up, and some of the crystals may have already grown and were not able to grab well onto the concrete. The stucco may not have been properly cured so the crystals could continue to grow for days and weeks after the stucco was applied.
Let’s imagine you want to apply cement stucco to poured concrete, concrete block, a stone wall or even brick. You want it to last for hundreds of years. Guess what? It’s possible to get it to last that long.
The first thing to do is use water and a scrub brush to clean the surface to which you’re applying the cement stucco. You wouldn’t get dressed up in a wonderful summer dress or coat and tie if you were sweaty and filthy from working all day without taking a shower, right?
It’s then important to make the stucco mix very strong. I was trained how to do this by a master mason when I first got into the construction business. Stucco I’ve mixed using the following formula has never failed: