Polymer-Modified Cementitious Materials for Concrete Repair Systems
By Hoffmann, Armin
Usually invisible, but always indispensable, dispersible polymer powders have been leaving their mark on the construction industry for 50 years. Whether you’re involved in new building construction or the modernisation and renovation of existing ones, these powders provide high-quality mineral building materials with precisely defined technical properties and help rationalise construction site processing. Particularly useful for successful concrete renovation, polymer-enhanced cementitious materials such as sealing slurries show extremely good results and guarantee high impermeability against water, CO2 and other pollutants. Binders have been around for a very long time. For example, approximately 14,000 years ago, craftsmen in what is now eastern Turkey used quicklime to lay bricks. Three thousand years ago, the Phoenicians were the first to mix lime with volcanic rock, thereby producing a material that hardened even under water. In Roman times, the first impressive structures made with mortar were built, such as the Pont du Gard in southern France. But regardless of mortar’s long history, the innovation potential is by no means exhausted. For example, dry-mix mortar, modified with high-quality dispersible polymer powders, opens up many novel functions and new properties in modern construction.
Polymer powders
The first polymeric binders in powder form for the construction industry were produced back in 1957 in Germany. This revolutionised working methods in this sector, since, for the first time, a one- part, polymer-modified cementitious system was available that only required water to be added onsite. To this day, the system represents a much simpler way of working, with substantial cost advantages.
The binder, marketed under the name VINNAPAS, is a vinyl acetate/ ethylene copolymer. These dispersible polymer powders are known as spray-dried dispersions. The powder particles consist of a water- soluble, protective-colloid matrix in which the water-insoluble, dispersible particles are embedded. An anti-caking agent prevents the powder particles from sticking together. When water is added, the particles disperse. As the mortar sets, flexible polymer bridges are formed between the brittle mineral constituents of the mortar, thus greatly improving its adhesion to a wide range of substrates. The polymer bridges also increase the system’s flexibility. In addition, it is possible to incorporate extra properties such as thixotropy, levelling, superplasticising and hydrophobicity.
Admixtures for concrete renovation
The useful properties of dispersible polymer powders are employed to advantage in many different fields of modern construction. One of the main application areas for polymer-modified concrete admixtures is concrete renovation.
Within a concrete repair system, for instance, polymermodified repair mortar is used to reprofile the concrete. Repair mortars modified with VINNAPAS show remarkable improvements, such as excellent adhesion to old concrete and steel, very good deformability, reduced elastic modulus, higher flexural strength and better workability and wetting properties.
Surface coatings for renovated or repaired concrete buildings are mixed with polymeric binders too. Repaired concrete areas are generally covered with a coating l-3mm in thickness. Extremely good results are obtained in this application with cement-based powders enhanced with VINNAPAS dispersible powders. These powders impart good workability, prevent the thin layers from drying too rapidly, guarantee higher impermeability against CCh and penetration of other pollutants such as salt and improve the freeze/thaw resistance.
The decisive factor in successful concrete renovation is reliable, durable adhesion of the concrete repair mortar to the concrete substrate. Modem sealing slurries provide an ideal method of repairing, for instance, water pipes or sewers. To start with, purely cementitious slurries were used, mainly in the protection of buildings. But they do not provide an adequate barrier against groundwater or driving rain. So today, sealing slurries are enhanced by mixing them with hydrophobic polymeric additives. The most convenient products are today’s dry mortars, which already contain the polymeric component in the form of a dispersible powder. Modern sealing slurries thus comprise cement, a filler, a polymeric binder and another special additive. At the construction site, these dry- mortar systems only need to be stirred with water, which simplifies matters enormously and ensures consistent application properties.
Advantages
About 5% polymer powder usually suffices to modify a water- repellent and rigid mortar (with flexible ones, 15% and more is used). An additional advantage of such mortars is that they adhere firmly even to difficult substrates. For example, they can even be brushed onto a damp substrate with typical coating thicknesses of around 2mm – something that is usually impossible with conventional systems. When added to commercial mortars, polymer powders disperse quickly and completely to form delicate polymer films, which impart the hydrophobic property.
The binder containing the hydrophobic components tends to accumulate in the pores, thus minimising capillary water absorption. This not only reduces the damaging influence of damp but also effectively prevents the uptake and transport of other pollutants. Various dissolved salts, such as chlorides and sulfates, are no longer able to penetrate easily. Sealing slurries also provide reliable protection in applications where hydrostatic pressure, for example of groundwater, would otherwise be problematic. However, these applications require higher proportions of polymer powders, typically 20-40% of a one-part system.
An essential requirement for a healthy indoor climate is that they repel water but are permeable to water vapour. Polymer-based sealing slurries are thus suited to protect cellar walls and foundations, as well as water reservoirs or tanks. Typical applications include all kinds of wet rooms, ranging from swimming pools and fitness areas to water drains.
Time, and therefore efficiency, also plays a major role in construction. Processing speed, material consumption and drying time are just as important as adhesion, surface quality and the load- bearing strength of the finished concrete. It saves expensive downtime if the renovation work can be performed between midday Friday and Monday morning. With polymer-enhanced concrete repair compounds, the materials can be processed rapidly on-site and permit quick and efficient working.
Concluding remarks
VINNAPAS dispersible polymer powders have been used successfully worldwide for decades to improve the technical performance of mortars employed under many different climatic conditions to repair and rehabilitate structural concrete. These modified mortars used for reprofiling are part of a comprehensive system to improve the adhesion of the reprofiling mortar and surfacing mortars, and provide protection against water, CO2 and other pollutants. Subsequently applied finishes and protective coating systems, mostly polymer-based building products, also serve as a protection against further damage and are used for aesthetic reasons.
“Processing speed, material consumption and drying time are just as important as adhesion, surface quality and the load-bearing strength ofthefinished concrete.”
ARMIN HOFFMANN, WACKER POLYMERS
Copyright The Concrete Society Feb 2008
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