Ask the builder: Stranded cement stucco – why it happened and how to avoid it
My wife and I are the chief ushers of our church. Every Sunday when we get there early, we go up a ramp that gives people attending mass in wheelchairs easy access to the Church of the Sacred Heart in Laconia, New Hampshire. The ramp was built with two poured concrete walls with the ramp between them.
The church is a magnificent red brick edifice built by French Catholics in the late 1800s. At that time, there were competing factions of Catholics in Laconia. It had to be like today’s sporting rivalries. Catholics in each country have built their own churches. From the churches that still exist in Laconia, it is evident that the French had more money and poured a lot into this building that I adore every Sunday.
The contractor who built the ramp maybe 20 or 30 years ago tried to match it to the original red brick. Rather than pigmenting the concrete (yes, you can add dry color pigments to the concrete), he decided to mask the hard gray poured concrete walls by applying a thin cement stucco over any exposed concrete.
Using stucco wasn’t a bad idea, and I guess it could have been a budget issue. The architect or builder of the project could have installed a brick veneer to match the poured concrete for an even better look.
They could have done exactly what the architect and builder did who created a stunning addition to the public library in the nearby city 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 layer of stucco peeled off the poured concrete. Know that Portland cement is an incredible adhesive. When you mix Portland cement with water, you start a chemical reaction where countless microscopic crystals start to grow.
Try imagining those sticky strawberries clinging to your pants as you walk in the woods. Cement crystals are so stubborn. These crystals are so abundant and so 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 started building. There are concrete deposits that can only be removed with a chisel.
The red stucco failed to adhere to the poured concrete on the ramp for several possible reasons. It can be one or all of the following. For starters, the poured concrete walls could be dusty. The stucco batch may not have had enough Portland cement in it, or it may not have been completely mixed with the sand before the water was added.
The stucco material could have started to set in place, and some of the crystals may have already grown and may not have clung to the concrete well. The stucco may not have cured properly, so the crystals could continue to grow for days and weeks after the stucco was applied.
Say you want to apply cement stucco to poured concrete, concrete block, stone wall, or even brick. You want it to last for hundreds of years. Guess what? It is possible to make it last that long.
The first thing to do is to use water and a scrub brush to clean the surface you are applying the cement stucco to. You wouldn’t want a gorgeous summer dress or a coat and tie if you were sweaty and dirty after working all day without taking a shower, right?
It is then important to make the stucco mixture solid. I was trained to do this by a master mason when I entered the construction industry. The stucco I mixed using the following formula never failed: 3 parts medium sand, 1 part Portland cement, and Â½ part hydrated lime.
You mix them in a dry state. Make sure the mixture is uniform in color and texture. You then add clean water and mix until the stucco is about the consistency of applesauce.
Keep in mind that you can increase the formula to three quarters of hydrated lime if you wish. Lime, when hardened, is actually a thin layer of limestone. You know how durable limestone is.
The secret trick is to then slightly dampen the surface that will accept the stucco. Immediately after spraying it, you paint over a coat of cement paint. In the case of my church, some of the dry red pigment should have been added to the cement paint. Cement paint is just a mixture of Portland cement and water mixed with the consistency of regular latex paint.
Apply this cement paint to the surface and apply only the area you can cover with the stucco in a minute or two. Never allow cement paint to dry on the wall.
Do all of these things and you will have stucco that could last 200 years or more. Oh, and don’t forget to heal him. Keep the stucco wet for three days after it has hardened. It is that simple.
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