Stucco has a reputation for cracking, one solution is to add fiber to the mix. The fiber creates an interconnected three dimensional web suspended within the stucco or plaster moving in every direction and linked, to reduce cracking and improve the overall strength of the entire installation. Premixed bagged stucco and custom mix designs with fibers are readily available online and positive reviews as well as heated discussions abound for each type of fiber including metal, PVA, Fiberglass, Polypropylene, Nylon and Asbestos.
The fibered mixes come from the historical practice of using animal hair often horse hair in plaster mixes to enhance the strength and reduce cracking of plaster and stucco mixes. Below is a short description and a few pros and cons of each type of fiber.
Asbestos has a long history going back to the time of the Romans. It was found to enhance the flexural strength of concrete but was also used for textiles to make table cloths and other utilitarian fabrics. It was thin, lightweight and when proportioned properly it was a great benefit to any concrete mix, plus it seemed to last forever. More recently, asbestos was used into the 1950s for siding, roofing, and pipes. Once it was found to cause cancer it fell out of popularity but an alternative that is as effective and economical has yet to be developed.
Fiberglass was developed shortly before World War II, it was marketed as a replacement to steel mesh for stucco as it could be mixed directly into the concrete. With time, the makers of fiberglass fiber for concrete switched to advertising it as a way to enhance a concrete mix but not as a replacement to the structural elements of a project. Fiberglass is still a popular choice for mixing into concrete, there are two main grades, E-glass which is less expensive and AR-glass which stands for alkali-resistant.
Polypropylene fiber came onto the market after fiberglass and was promoted as a way to increase compressive strength and tensile strength. It was pointed out though that the polypropylene fibers would melt at fairly low temperatures and would often become quite brittle once added to a wet concrete mix, therefore losing any of its benefits over fiberglass.
Nylon has been around since before fiberglass or polypropylene but it was not used in concrete. The nylon industry jumped on board as the polypropylene fibers began to fizzle out. Fiberglass salesmen pointed out the pitfalls of nylon such that nylon was very stiff and would stick out of any stucco or plaster wall. Also, it would absorb moisture when the wall became wet and then shrink once it dried out losing its bond to the stucco.
When one hears about steel fiber for the first time, they may imagine something like a chopped up piece of steel wool. The steel fiber sold for the stucco industry is often 1 ¼” long by ¼” wide and 1/16” thick. This is most used and effective in the shotcrete industry, not recommended in stucco.
Other fibers are available on the market and there is a long explanation of adding fibers to mixes in Herb Nordmeyer’s book The Stucco Book, The Basics.