Special silanes protect bridge piers against salt

Media

More media…

On any winter’s day, thousands of vehicles pass under and over bridges, subjecting bridge piers to a constant barrage of salt-laden spray. The salt collects on the structure and attracts further water – a permanent layer of moisture enabling the salt to infiltrate the concrete in a slow process. Over the years, the porous nature of concrete allows these chlorides to infiltrate ever deeper into the piers until they finally reach the reinforcement. The ensuing corrosion has consequences: as the products of corrosion need more space than the iron itself, the concrete cover spalls off. Gradually, the bridge pier loses its load-carrying capacity and has to be refurbished.

Expensive refurbishment

Rectifying such damage is extremely costly. For instance, if a central pier of a motorway bridge is affected, traffic lanes have to be closed off for months, resulting in congestion and delays. Scaffolding has to be erected, concrete removed, reinforcement exposed, the entire bridge might even need temporary support during the work. Added to all this is the costly and time-consuming repair work itself: only careful priming of the steel can provide permanent protection against rust. The new concrete has to match the existing material exactly because otherwise the result will be further damage after just a few years. Furthermore, concrete takes between 15 and 28 days to cure properly so only after this additional waiting time can the temporary works be removed and all lanes opened again. Refurbishment work therefore takes weeks and costs much more than the pier cost to build in the first place!

Deep impregnation treatment protects concrete reliably

All this can be avoided by impregnating the intact bridge piers with water-repellent silanes, for example. Such a deep impregnation treatment can be carried out very quickly and protects the concrete reliably against water and salts because it lines the pores of the concrete with an active water-repellent substance. Vital, however, is that the silanes can penetrate several millimetres deep into the concrete; only then can reliable protection be assured.

Special silanes from WACKER are frequently used for deep impregnation treatments on concrete structures. And for good reasons: they are stable in alkaline environments, i.e. cannot be decomposed by the concrete itself over time, and they are either colourless or can be matched to the colour of the concrete so that the appearance of the piers is not impaired by the silicone resin network within the concrete. The active substance is applied directly to the surface of the concrete, which results in a thin, approx. 0.2–0.5 mm covering of silane that gradually seeps into the concrete. The application work is typically completed within three or four hours.

Scientific studies prove that such treatment prolongs the intervals between bridge pier refurbishment work considerably; indeed, refurbishment may become totally unnecessary in some cases. Pilot projects in southern Germany, Sweden and Switzerland have demonstrated that impregnation with a cream or gel made from silanes provides the best possible protection. The bridge piers of 30 bridges in Stockholm, which were given a hydrophobic treatment within the scope of a scientific study, still showed no visible signs of damage even 12 years after the treatment. And the chlorides had penetrated only a few millimetres into the concrete.

Pilot project in Bavaria

On unprotected bridges, however, salts can penetrate 40–50 mm into the concrete within just a few years, a fact that was confirmed by a pilot project carried out in Bavaria in 2005. The core-drilled samples taken 16 motorway bridges confirmed that in some circumstances the salts were penetrating much faster into the fabric of the structure than had been generally assumed. For example, bridges built in 1990 had higher chloride infiltration depths than bridges dating from 1975. The assumption that the piers investigated would last for about 90 years turned out to be inappropriate in many cases. The concrete often needed repairing after just 20 years. How fast and how deep the salts penetrate into the piers depend on many factors: type of concrete, volume of traffic, quantity of de-icing salt used.

During the pilot project, the bridges in the survey were given a deep impregnation treatment with special silanes. The final scientific analysis showed that the active substance had penetrated approx. 50–60 mm deep into the concrete and had formed a water-repellent silicone resin network. That is enough to protect both concrete and reinforcement reliably against chlorides for many years.

Professional application guarantees success

The following sequence should be adhered to on site in order to guarantee a successful deep impregnation treatment of concrete structures: Firstly, an analysis of the condition of the concrete should be carried out to assess the chloride load. At the same time, a survey to find out the cause of the damage, and to rectify this, is recommended. Existing concrete components, e.g. bridge piers, in which the salt has not yet reached the reinforcement can be protected against further damage at any time through a deep impregnation treatment.

When using products containing silane, proper application by skilled professionals is just as important as the use of a good-quality product with the right formulation. Technical details regarding the proper application of hydrophobic treatments for concrete can be found in several publications (in Germany, for example, the SIVV manual [1], the DAfStB guidelines for protecting and repairing concrete components [2] and ZTV-ING document from BASt [3]). Apart from that, the quality and durability of the hydrophobic treatment should be checked afterwards in order to guarantee long-lasting protection. The compilation of active substance profiles is also recommended in addition to the measures stipulated in the directives and guidelines.[4] All in all, deep impregnation with special silanes offers clear advantages over full refurbishment. The treatment lasts 15–20 years, can be renewed at any time, costs much less than refurbishment and can be carried out very quickly. Compared with the typical repercussions of refurbishment work, i.e. lane closures and tailbacks for many months, these are advantages every road user will appreciate.

References

1. SIVV-Handbuch: Schützen, Instandsetzen, Verbinden und Verstärken von Betonbauteilen, 2008 ed., Berlin, 2008.
2. German Reinforced Concrete Committee (DAfStB): Richtlinie für Schutz und Instandsetzung von Betonbauteilen, Oct 2001 ed., Berlin, 2001.
3. Federal Highway Research Institute (BASt): Zusätzliche Technische Vertragsbedingungen und Richtlinien für Ingenieurbauten (ZTV-ING), part 3, section 4, Apr 2010 ed.
4. Gerdes, Andreas; Wittmann, Folker H.: Hydrophobieren von Beton – Welche Einflussfaktoren bestimmen die Wirksamkeit? In: Moderne silanbasierte Schutzsysteme für mineralische Oberflächen, Zurich, 2001.