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Outdoor Earthen Plasters Outdoor earthen plasters Pavel Svoboda Michal Procházka doc. Ing., CSc, Praha, Czech Republic Ing., Třebíč, Czech Republic e-mail: [email protected] e-mail: [email protected] DOI 10.5592/otmcj.2012.1.7 Construction of ecological buildings has one specific sub-seg- Research paper ment - construction with natural materials. These are buildings made of timber, earth, stone, straw bales, reed, hemp, etc. The majority of these materials need protection against external effects. Architectural or structural elements (roofing overlaps etc.) provide passive protection. Outdoor plasters may be used as an active protection element. The practice to-date has mostly applied lime plasters with high demands for treatment during the first two to three months. An alternative choice were unstabilized earthen plasters with high demands for maintenance. The research intention, therefore, was to develop an outdoor plaster based on natural materials resistant to external weather effects. The desired solution was a plaster of unburnt earth with slightly modi- fied properties, mainly in reaction to water. In the first phase of research, extensive literature search, dozens of potential stabilizing agents were shortlisted. The selection criteria were: water resistance plus low diffu- sion resistance of the resulting material, improved strength characteris- tics, a zero effect on colour and appearance etc. Many alternative agents were rejected. Positive results were reached with special types of polymer binders ap- plied in small doses into earthen plasters. It must be emphasized that it is the plaster quality that plays a significant role here (the analysis would provide a topic for a separate article). The plaster is also suitable for re- pairs and reconstructions of historical earthen structures. These plasters are protected by a utility model. Outdoor earthen plasters were first introduced to the Czech market at the end of 2009. Keywords Introduction Modern earthen structures built in our earthen, plasters, Going back to the starting point of the climatic zone must be fitted with ther- outdoor, clay, polymer, development, the following question mal insulation and treated as composi- stabilize, adobe arises: What are the desirable param- tions open to diffusion. This results in eters of stabilized outdoor earthen the second demand – the plaster must plaster? To be able to reply to this ques- possess a low diffusion resistance. tion, we must first define onto what The former demand logically implies surfaces the plaster will be applied. resistance to the effects of rainwater 420 organization, technology and management in construction · an international journal · 4(1)2012 wind-driven under roofing overlaps. soaked with water which had caused Tested kinds of samples were: the first Among other important demands we clay swelling and thus a loss of the without stabilization, the second with may mention: elasticity, a zero or mini- primary binder (~clay) function. There- lime, the third with cement and the mum effect on colour shading and ap- fore, seeking a solution, I turned to hy- others five ones with different poly- pearance, improved strength charac- drophobization. If water does not pen- mers. teristics – in tension in particular, and etrate into the material, clays do not others. These demands are not met by start to swell. Hydrophobization alone Result existing stabilizers. Lime and cement proved problematic and insufficient. The diffusion resistance factor has in- form a so-called dual system (different The most favourable option, eventu- creased only slightly from the maximum shrinkage of the crystalline grid and ally, were special types of polymers. of unstabilized plaster of ca 10 [-] to ca clays minerals), which causes brit- Polymers act both as secondary bind- 11 [-] for coarse (stabilization with 0.5 tleness. Silicones and bitumen raise ers which substitute primary binder in wt%) and ca 12 [-] for fine plaster (stabi- the diffusion resistance, and bitumen dry state, mainly on the surface, and, lization with 1 wt%). The measurement also changes color. Gypsum is not suf- at the same time, these polymers in was performed in accordance with ČSN ficiently water resistant, etc. Finally, particular are able to ensure protec- EN 1015-19 [3]. Fig. 1 shows the samples we cannot ignore the last parameter, tion against water penetration inside during the measurement process in a i.e. the price, in our considerations. the material. This very combination of constant climate chambers. For the price of construction chemicals secondary binder and hydrophobiza- may substantially exceed the price of tion proved to be of key significance. Both the tensile and compressive e.g. lime and cement, but, on the other The polymers shortlisted for this char- strength have grown (values differ). hand, more environment friendly dos- acteristic were investigated for effi- The growth in tensile strength values ing may be expected. ciency: the selected one with the high- commonly amounts up to 100%, while est efficiency will allow minimizing the the growth in compressive strength is Development path dosage. With regard to the adequate around 50% (stabilization with 1 wt%). Following the first year of literature quality of stabilization, low diffusion The measurement was performed search it has not been found any sat- resistance, environment friendly ap- in accordance with ČSN EN 1015-11 isfactory paper. So I began to scan proach and the price of the resulting [4], shown in Fig. 2 (Tensile Bending Catalogue of ingredients for concrete, product, the current recommended Strength test). mortars and plasters [1] product by dosage selected is 0.5% by weight for product. During research in the area coarse plasters and 1% by weight for The resistance to water is incomparably of alternative stabilization methods fine plasters. higher. It was tested using the modified of unburnt earth, dozens of selected traditional test according to Havlíček stabilizers and binders were removed Tests and Souček – the test of resistance to from the list. Such added binders are Specimens were made from clay and water action [5] which consists in soak- called stabilizing or secondary bind- sand 0/4 (sand for mortars). The ratio ing parts of hanging prisms in water for ers. Dispersion polymer binders were is determined by the shrinkage. The a time period of one hour. The prism di- predominantly shortlisted. The princi- value of shrinkage must be less than mensions were adapted to 40x40x160 pal reason for this choice was mainly 2%, while the dose of water is based mm, and water temperature was speci- the fact that the binder exerts elastic on the real consistence. Consistence is fied at 20°C because inconstancy af- action and, therefore, will be able to defined by value 175 mm by slump flow fects the results. This test we can see act compositely with clay. The second test by ČSN EN 1015-3 [2]. Further tests on Fig. 3, where is tested stabilization reason was the fact that these poly- were carried out: on commercial earthen plasters. mers disperse in water, which is a thin- ning agent for unburnt earth. The final, Test method Specimen dimensions Number of specimens third reason was the hope for a mini- Water Vapour Permeability cylinder Ø 120 mm, h = 15 mm 8 kinds, 3/5 pieces (26 total) mum increase of diffusion resistance Tensile Bending Strength block 40 x 40 x 160 mm 8 kinds, 3 pieces (24 total) of plasters. The result, however, failed to meet all the expectations perfectly. Compressive Strength block 40 x 40 x ca 80 mm 8 kinds, 6 pieces (48 total) After contact with water, the material Resistance to Water Action block 40 x 40 x 160 mm 8 kinds, 6 pieces (26 total) did hold together, but its strength had Test in the Rainfall Simulator plates 300 x 300 x 20 mm 8 kinds, 1 pieces (8 total) dramatically fallen. The reason for this is straightforward – the material was Table 1. Testing program P. Svoboda · M. Procházka · Outdoor earthen plasters · pp 420-423 421 Other specimens were further ex- posed to simulated rain in the rainfall simulator to measure erosion. Rainfall simulator is shown in Fig. 4. The rain was selected to last 60 min. with an intensity of 20 mm/h, followed by a 30 min. break; then there were three torrential rain showers of 15 min. with an intensity of 60 mm/h with 5-minute breaks. The following illustration in Fig. 5 displays the differences between the stabilizer dosing after simulated rain. The specimen stabilized with 1 wt% of polymer does not show any changes, while the specimen stabilized with 0.1 wt% of polymer shows only slight Figure 1. Test pursuant to ČSN EN 1015-19 damage; the specimen without sta- bilization had completely failed. The dose of 0.1 wt% is only for illustra- tion. From practice we may say that it is comparable with the best natural stabilizers (cow manure etc.) dosed at values of 10 to 20 % by weight. Diagram 1. Recommendations The influence The above-mentioned dosing of sta- of addition of bilizing disperse polymer binders is polymer on based on the assumption of observ- the diffusion ing the structural principles for hous- resistance factor es made of natural materials, such as earth, timber, straw bales, reed, hemp and others. The technology of these houses, for example, does not allow non-observance of roofing over- laps, construction of ledges, the ab- sence of a socle, the action of splash- ing water, a combination of water and frost action, thawing snow and related long-term soaking etc. Unfor- tunately, postmodern architecture in- dulges in such approaches, relying on artificial materials resistant to these effects. Outdoor earthen plaster is resistant even to repetitive rainfall. It is, however, suitable if it may dry up after each rain shower. Water in itself does not harm the plaster, but the combination of soaked masonry Figure 2.
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