Killing the bounce. Quelle/Publication: European Coatings Journal Ausgabe/Issue: 12/2009 Seite/Page: 1

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1 Seite/Page: 1 Killing the bounce Sprayed concrete is used in large quantities on major construction projects. As rebound during spraying results in loss of material and affects properties, polymer additives were designed specifically to reduce this problem. Large-scale tests confirmed that adhesion to wet substrates, permeability and cracking could all be improved. The added costs can be more than offset by reductions in working time. Polymer additives cut rebound losses and improve quality of sprayed concrete Klaus Bonin* Jürgen Bezler Modern construction cannot exist without sprayed concrete ("shotcrete") whether it is in tunnelling, hydraulic engineering or road construction. The demands made on the concrete are high in terms of the levels of strength, stability and impermeability required. However, the application of sprayed concrete entails rebound from the spraying, which is a major time and cost factor due to the loss of material and changes to concrete properties. For this reason, Wacker has developed a new line of polymeric binders that distinctly reduce spray rebound and offer additional value-added benefits such as adhesion on critical underground structures and waterproofing effects. These binders significantly reduce system costs and at the same time improve the properties of the concrete. Dispersible polymers are established cement additives The combination of inorganic and polymer binders in drymix mortars is essential to modern construction techniques, which demand high quality and safety. For more than 50 years, dispersible polymer powders have been a solid feature of the modern construction sector. As binder components, dispersible polymer powders now enhance plasters and renders, mortars and cements in a wide variety of applications. They impart water-repellent properties to mineral renders and grouts for outdoor use, or to cementitious sealing slurries. In addition, polymer modification significantly improves the properties of tile adhesives. Sprayed concrete, too, is enhanced for a wide variety of uses. Rebound affects quality and adds cost Based upon many years of experience using additives in mortar applications, a variety of additive systems for sprayed concrete have been developed. The technology used determines whether or not the concrete can be successfully applied. Rebound is an important quality factor in the performance of sprayed concrete, as it alters the concrete properties and the excess material must be disposed of as waste. Reducing rebound is therefore important for both technological and economic reasons. Moreover, polymermodified sprayed concrete has some interesting properties and provides yet another means for product differentiation: Outstanding processability without the risk of separation; Good compaction characteristics and high compressive strength; Excellent adhesion to difficult substrates such as tunnel walls; Improved CO2 resistance; Cement-specific interaction with polymers. These new polymer binders can be shipped in either liquid or powder form. Major markets exist for sprayed concrete Underground construction is a vast field and will be of strategic importance in the future, especially in Europe (Figure 1). On the one hand, many existing tunnels are in need of repair, while on the other there is always a demand for the construction of new ones. However, these are expensive and time consuming undertakings. In tunnelling, time and the consumption of material are particularly critical. But modifying sprayed concrete with polymers reduces both. Polymer-modified sprayed concrete can be used for both wet and dry spraying. Effectiveness and consistent quality are of the utmost importance in dry mortar spraying. The advantages of polymer modification in dry spraying are effective hand-ling, since the dry mix already contains the dispersible polymer powder or, in the case of liquid dispersions, it is applied directly. Furthermore, it has a dependably consistent quality that makes quality control easy, shows minimal sensitivity to processing changes and has a long storage life. Wet spray applications can also be improved by using polymer binders. The advantages of this method lie in using the economical, ready-mix concrete technology for standard commercial transport systems. The quality can be checked before addition of the accelerator and can be adjusted to a faster cement setting time for optimal processing. Reduced rebound means minimal system costs, and the addition of polymer powder on site is no longer necessary (i.e., no technical restrictions). Rebound is halved in development tests In the first development tests, the proportion of additive that reduces sprayed concrete rebound yet still provides the desired concrete properties was determined. The investigations were restricted to wet sprayed concrete. Various concrete formulations were applied to standardised test areas to study their effects on properties. Statisticallybased Design of Experiments (DoE) was carried out to determine the parameters needed for planning the next level of tests. Two to three cubic metres of concrete were laid for each trial. To ensure a homogeneous mixture, only midstream concrete was used. By determining the early strength of the concrete, an approximate assessment of its mechanical strength could be inferred. This shortened the time needed to obtain the first results. It soon became apparent that inclusion of the additive significantly reduced rebound, regardless of the formulation. Both modified concrete formulations show that,

2 Seite/Page: 2 with increasing amounts of additive, rebound first declines, reaching a minimum, after which further additive increases rebound again (Figure 2). Concrete loses aggregate through rebound, which causes the cement concentration in the applied sprayed concrete to rise. In general, as the cement concentration rises, the strength of the concrete also increases. As a rule, rising rebound therefore equates to increased ultimate strength. Figure 3 shows the strength profiles of polymer-modified concretes compared to a reference concrete. The reference mixture showed a linear progression of early strength over the entire measuring period. For both mixtures, the materials containing the higher proportion of liquid or powder additives and also the pure superplasticiser displayed the least rebound and, as expected, the lowest strength. The development tests suggested that the mixture with five percent solid additive had the most favourable development of strength. This mixture could be processed longer, and exhibited a similar or even better initial slump than the reference mixture. The rebound values of this mixture also showed that the objective of halving rebound had been met. The concrete formulations modified with liquid additive were the most convenient to process. Large-scale test confirms laboratory predictions In a further series of tests, one of the concrete mixtures was chosen for the purpose of determining whether the experimental findings remain valid on the construction site. An additional goal was to demonstrate the standard compliance of polymer-modified sprayed concrete. For this purpose, a standard Swiss cement, normally used in Switzerland s Hagerbach test gallery, was employed. The results of these tests relate to the control mixture values from the development trials. As expected, rebound could be significantly reduced. The strength profile of the mixture conformed to that seen in the development tests. After two hours, a sharp increase in strength could be observed (Figure 4). Some aspects of these tests, such as the recipe for wet-mix shotcrete, can be determined in the laboratory. However, the complex way that metering, mixing, conveying and nozzle systems interact with the application technology can only be determined by large-scale testing. The access gallery of a salt mine was secured with polymer-modified shotcrete. This imposed special requirements on the mixture s adhesion and impermeability. More extensive tests carried out at a salt mine Wacker Chemie owns a salt mine in Stetten (south-western Germany) which has been producing rock salt for more than 150 years. There are a variety of uses for the salt: most of it ends up as road salt for winter traffic or as industrial salt for the production of sodium hydroxide (caustic soda) and chlorine. The cavities created by mining the salt can be backfilled so as to ensure the site s structural integrity and continued economic viability. The two existing entrances cannot be accessed by typical vehicles, however, due to the roadway arrangement. Because of this, construction of a new entrance that would allow standard trucks to pass through the tunnel was begun in Blasting was the method chosen for tunnelling the 905- metre access route with a ten percent incline. The point of applying polymer-modified sprayed concrete in the salt mine was to investigate its properties under practical conditions including any improvements. To prepare for the on-site use of this technology, basic laboratory tests were carried out. In October 2007, the new polymer-modified sprayed concrete was tested under realistic application conditions. In addition to the decrease in rebound, these trials showed that the mixtures exhibited improved adhesion and impermeability, and, from early 2008, could be used more extensively in the tunnel. Tunnelling ran smoothly for the first few months, but then groundwater began to seep in after about the first 450 metres of tunnel. This necessitated support with injections in addition to the use of sprayed concrete reinforced with steel mesh. The original wet-mix formulation of the sprayed concrete satisfied safety criteria under dry conditions, but showed weaknesses in adhesion when applied to a damp substrate. At this point, it was decided to use the improved polymermodified sprayed concrete, as laboratory tests had shown improved adhesion and impermeability. In the preliminary trials, both powder and liquid additives were tested in the concrete mixtures. The aqueous polymer was used for this large-scale application since it had the advantage of being easy to work with in the mixing plant. Additive was metered into the concrete to give a final concentration of 40 litres per cubic metre of concrete. The sprayed concrete modified in this way could be delivered to the construction site without any difficulty in production or in transport. On inspection of the test surfaces in the side wall, the standard sprayed concrete (to the right in Figure 5) was found to exhibit a large number of wet patches, whereas the polymer-modified sprayed concrete (on left) demonstrates much better adhesion to the wet substrate, exhibits higher impermeability, and is less likely to form cracks. When the performance indicators of the freshly mixed concrete were monitored, the mixtures were found to be homogeneous and suitable for transportation. Stable slump behaviour is desired (Table 1). Practical and economic benefits can be obtained The use of modified sprayed concrete under construction site conditions proved its worth at the access gallery. During the tunnel construction, standard sprayed concrete showed weaknesses in its adhesion to wet substrates or where groundwater had infiltrated. Switching to polymer-modified sprayed concrete prevented a delay in construction and allowed the access gallery to be completed as planned. In another mining study, it was determined that the use of modified steel-fibre sprayed concrete increased the resistance against abrasion. Because of the small size of the samples, comparison of the results from various lab studies on steel-fibre sprayed concrete with different fibre contents revealed no significant differences in performance. During the development work, fundamental calculations based upon experimental results were made on the economic impacts of using sprayed concrete with the aid of the new additive. Reduced rebound has a decisive influence here. It appears that the additional costs of modifying the sprayed concrete are offset by the time saved, and to a lesser extent by the reduction in waste (see Table 2).

3 Seite/Page: 3 These tests and calculations show that, given the right conditions, the use of the new additive can result in significant financial savings due to reductions in time and the consumption of material. Thus, polymer-modified concrete possesses valuable potential, both technical and commercial. Current studies indicate that sprayed concrete is developing in line with the market. It is versatile, and can be applied vertically and on ceilings. It is suitable for tunnelling and mining, as well as for use on slopes and in canals or conduits. í BIBLIOGRAPHY [1] Pittino G., Gharehkhani G., Krenn F., Stelzer G., Golser J., Spritzbeton der etwas andere Beton, Untersuchungen zum Materialverhalten und Prüfung. Beton- und Stahlbetonbau 99 (2004) issue 2, pp [2] Richtlinie Spritzbeton: Österreichische Vereinigung für Beton- und Bautechnik (ÖVBB). Vienna [3] Österreichischer Betonverein: Richtlinie Spritzbeton. January [4] Weigler H., Karl S., Beton. ISBN , Ernst & Sohn Verlag, Berlin, [5] Uetz H., Abrasion und Erosion: Grundlagen, betriebliche Erfahrungen, Verminderung. Carl Hanser Verlag, Results at a glance Sprayed concrete (or shotcrete ) is used in large quantities on major construction projects such as tunnelling. Rebound of the concrete during spraying results in significant material losses and alters its properties. Dispersible polymer additives have been used for decades to improve concrete properties. A range of new additives was designed specifically to reduce rebound during concrete spraying, and it was confirmed that this aim could be met. Largescale testing in a new tunnel constructed within a salt mine showed that the additives also improved adhesion to wet substrates, and reduced both permeability and cracking. Calculations indicate that the cost of the polymer additive is partly offset by the reduction in material used and waste rebound disposal costs, while the reduction in working time produces very large net savings. * Corresponding Author: Klaus Bonin Wacker Chemie AG T klaus.bonin@wacker.com

4 Seite/Page: 4 Figure 1: Overall market for underground construction in Europe ( ) in thousands of cubic metres of material

5 Seite/Page: 5 Figure 2: Relative rebound vs. concentration of additive for four different mixtures of additives, indicated as FLZ, FM, FMVZ, FSZ with the value for the reference material being 100

6 Seite/Page: 6 Figure 3: Comparison of early strength development of different applied mixtures

7 Seite/Page: 7 Figure 4: Early strength development on a further range of concrete mixtures

8 Seite/Page: 8 Figure 5: Area of side wall with local application of polymer-modified sprayed concrete (left) and reference sprayed concrete (right)