Feature

Long Lasting Concrete and Masonry Protection

Posted on 27 January, 2010 | Tags: Silicone Chemistry

For several years, silanes and silicone resins have been successfully used to protect masonry and concrete constructions. They are not only highly water repellent, but also prevent the penetration of harmful salts, a problem which often affects concrete bridges and buildings along highways or close to the sea. The costs for this kind of protection pay off nicely as expansive maintenance and repair jobs can be avoided.

Concrete bridges age much faster than engineers would like, salt being a special threat. Depending on their location, bridges can be exposed to high levels of salt. Sea water, soil and air may contain a considerable amount of mineral salt which can penetrate the concrete surface and thus affect several parts of the bridge including its piers. Once the penetration begins, the salt attracts further moisture. Thus, water and salts gradually migrate into the pier. At a depth of 40 to 50 millimeters, the corrosive mixture meets the steel rods used to reinforce and stabilize the concrete. The steel starts to rust and in extreme cases may even compromise the structural stability.
Due to the threat of salt to bridges, the German Automobile Association (ADAC) sponsored a study to investigate the state of German bridges. The results gave cause for concern of Germany's roughly 120,000 bridges, about 35,000 are in need of repair. But this would mean spending billions of euros! Besides the actual repair costs, the logistics, too, can be expensive, involving lane closures, redirecting traffic, for months at a time if, for instance, a highway bridge's center pier is involved.

Protecting Bridge Piers
Thus, the expense of renovating a concrete bridge may add up to many times the original construction costs. Preventative measures such as long-term protection against water and salt are becoming increasingly important while bridge piers are still intact. With silicone chemistry, such measures are indeed feasible.
A recent project in southern Germany shows that hydrophobic treatment with a water-repellent product based on iso-octyltriethoxysilan is able to extend a bridge's service life considerably. In order to prove this, a team of engineers, scientists and Wacker chemists applied a formulation containing the special ingredient to the concrete piers of several bridges. The formulation's viscosity was adjusted so that the product adhered to the concrete's surface for several hours. This is important because the active ingredient needs time to penetrate into the pores. Since the formulation remains in contact with the concrete, the highly concentrated iso-octyltriethoxysilan has time to soak in deep, even when a thin layer is applied.

Silanes Provide Long-term Concrete Protection
The project's aim was to achieve an active agent concentration down to a depth of up to six millimeters by hydrophobic impregnation. During penetration, the silane reacts with the silicate matrix inside the capillaries and pores and builds a water repellent barrier against water and salts. This was indeed the outcome,  as an analyses of the drilled cores confirmed. The project as well as similar tests conducted in Sweden several years earlier showed that this kind of treatment has a long-lasting protective effect. The hydrophobic impregnation applied to several concrete bridges Stockholm twelve years ago, is still intact.
Clearly, hydrophobization provides concrete bridges with effective long-term protection. Given the right formulation, a proper application method and a concrete composition in line with general engineering standards, a hydrophobic impregnation of several millimeters is able to protect the concrete for 15 years or more. The treatment can be carried out overnight, which not only makes it very efficient, but also relatively inexpensive compared to the repair costs which are necessary otherwise and certainly to pulling a bridge down and reconstructing it. As in many other fields, prevention is much cheaper than cure.
In the last 20 years, several bridges around the world have been successfully impregnated with SILRES® BS products and with Wacker's expertise. For instance, the same prevention strategy was implemented during the construction of the Stonecutters Bridge in Hong Kong. With a span of more than 1,500 meters, it will be one of the world's largest cable-stayed bridges once it is finished. Its two piers anchored to the seabed between Tsing Yi Island and Stonecutters Island are heavily exposed to salty sea spray. As a precaution, both piers have been treated with Wacker SILRES® BS Creme C which contains iso-octyltriethoxysilan.

Silicone Resins for Residential Buildings
Hydrophobic treatment is also suitable for residential buildings. In this case, silicone resins are applied to a façade. They provide good and long-lasting protection. Ever since Wacker formulated the first Silicone Resin Emulsion Paints (SREP) in 1963, these paints have enjoyed unparalleled success. Though slightly more expensive than conventional materials, they are more durable and therefore extend the intervals between renovations.
But why do silicone resins protect masonry so effectively? What is the secret of hydrophobic treatment? In order to understand the general mechanisms it is necessary to have a closer look at silane chemistry.
Silanes contain a silicon atom bonded to three oxygen atoms each carrying a hydrogen atom, ie an -OH group, also known as a hydroxyl group. The silicon atom is also bonded to an organic group and is responsible for the water-repellent (hydrophobic) effect. The silane's hydroxyl groups react with those of the mineral building material, releasing water. This means that the silane is chemically bound to the building material. The same reaction occurs between hydroxyl groups of different silane molecules to produce a stable silicon-oxygen bond. The resulting hydrophobic silicone resin network lines and strengthens the pores.
The silicone resin acts as a binder in silicone resin emulsion paints. Such a paint also contains a polymer dispersion that 'glues' the fillers and pigments together. The silicone resin coats all the particles with a very thin, continuous film, which gives the resulting coat of paint its key properties, namely durability and water repellency.

Hydrophobic Treatment for Residential Buildings
What's more, whereas water rolls off the surface of a painted facade, such a surface is permeable to gaseous carbon dioxide and water vapor, just like modern sports textiles. The facade can breathe. Plus, there is another aspect that is gaining importance as energy prices continue to rise, silicone resin emulsion paints can improve thermal insulation.
This was confirmed by the results of a 2007 study carried out by a renowned building physics institute in southern Germany. The institute's study shows that damp walls lose heat more quickly because water is a good thermal conductor. For example, a brick with only 4 percent moisture loses about 50 percent of its thermal insulation effect.

Silicone Resins help Improve the Energy Balance
Damp walls thus squander thermal energy. According to experts, a good moisture-regulating coating system such as a SREP can reduce the total heating energy requirement by an average of roughly 5 percent, on its own, without any further thermal insulation.
Besides these savings, silicone resins offer a number of other positive effects the risk of frost damage drops and mildew finds it more difficult to take hold. Not only that, residents benefit from the warmer walls, as these make a house nice and cozy. Such advantages are especially effective where Exterior Insulation and Finish Systems (EIFS) are not desirable or even feasible, for example, in buildings under a preservation order.

Dr Hartmut Ackermann is Technical Marketing Manager Construction Chemicals, Wacker Silicones
E-Mail:hartmut.ackermann@wacker.com

 

Marketing Manager Construction Chemicals, Wacker Silicones
E-Mail:johannes.ihringer@wacker.com