DEVELOPMENT AND STUDY OF ANTI-NOISE BARRIERS BASED ON ALTERNATIVE RAW MATERIAL SOURCES

Transcription

1 DEVELOPMENT AND STUDY OF ANTI-NOISE BARRIERS BASED ON ALTERNATIVE RAW MATERIAL SOURCES Jiri ZACH (1), Jiri BROZOVSKY (1), and Jitka HROUDOVA (1) (1) Brno University of technology, Faculty of Civil Engineering, Institute of Building Materials and Components, Veveri 95, Brno, CZ Abstract Since the second half of the twentieth century, the noise level within the outside worldwide environment began to grow rapidly. At the present time and at many places, the highest permitted values stipulated by the national hygienic prescriptions and other legislative requirements are being significantly exceeded in plenty of cases. Transportation corridors (roads as well as railways) belong to the main noise origins within this external environment. During the recent years and especially in the settled areas, there were built up so called antinoise barriers around these noise sources which should absorb the respective noise and avoid its spreading into the surrounding space. The paper is devoted to development and manufacture possibilities as regards the antinoise screens which involve the usage of easily renewable raw material sources from the agriculture, forestry and also secondary raw materials proceeding from the building industry. Keywords: renewable raw material from agriculture, secondary raw materials proceeding from the building industry, anti-noise barriers 1. INTRODUCTION Nowadays the anti-noise walls are being built in order to reduce spread of noise from line noise sources which are mainly transport corridors and railway lines. Parameters of the antinoise walls are defined in accordance with EN-1793-(1-3). In accordance with this standard and on the basis of measuring tests in an acoustic laboratory, there is a calculation of onefigure numbers in the field of coefficient of sound absorption and soundproof level. These one-figure values are used to facilitate the evaluation of acoustic properties of the anti-noise walls which are frequently dependent. In both cases, determination of the one-figure value represents a weighted average value expressed in levels of db (decibel) where weight values for individual partial frequencies are given by standardized spectrum of road noise according to EN : 305

2 A) Sound absorption. Determination of the sound absorption of these construction products is declared in accordance with EN For evaluation of sound absorption of the antinoise walls, there was established a one-figure value of DL α [db] in which the individual sound absorption coefficients are weighted by a standardized spectrum of road noise. According to the DL α values, the anti-noise walls are divided into five categories from A0 (the worst values) to A4 (the best values). B) Airborne sound insulation. This is about ability of anti-noise walls to reduce the acoustic pressure of noise waves passing through these walls. Testing of these products is described in EN where a one-figure value DL R [db] for evaluation of soundproof level is declared. This value is determined by individual values of airborne sound insulation values weighted by a standardized spectrum of road traffic noise. According to the DL R values, the anti-noise walls are divided into four categories: B0 (the worst) - B3 (the best). 2. PROPOSAL OF ANTI-NOISE WALL WITH HIGH SOUND ABSORPTION The anti-noise wall with high sound absorption should meet requirements of A4 Class, it means that its one-figure value of sound absorption DL α should be higher than 11 db. As it is evident in the standardized noise spectrum of traffic (see figure No. 1), the greatest importance is given to middle frequencies in the range from 630 to 2,000 Hz when calculating the one-figure value of sound absorption. The resulting anti-noise wall should therefore show the highest values of sound absorption in this range. -5 Level of acoustic intensity [db Frequency [Hz] Figure 1: Graphic chart of standardized noise spectrum related to road traffic according to EN Frequency progression of the sound absorption value of a particular material is dependent on thickness of this material d [m]. The value of the particular material's sound absorption coefficient is dependent on relation of its thickness, frequency f of incoming sound and also pore size in the material. When passing through the pore material, the intensity of the incoming sound decreases down at every bounce back from the wall of its pores. Generally 306

3 and in the field of low frequencies, the value of sound absorption coefficient increases together with raising thickness of the material. It reaches the maximum with thickness of d just if: d = c/4f (1) Where: c - speed of sound in the particular enviroment [m.s -1 ] Optimum thickness [mm Frequency [Hz] Figure 2: Dependence of optimum thickness of absorption layer on the frequency When calculating the one-figure value of sound absorption, the greatest importance is given to middle frequencies at the range from 630 to 2,000 Hz. Proposed thickness should therefore be higher than theoretical effective thickness for the frequency of 630 Hz which equals about 134 mm. 3. PROPOSAL OF TESTING PRESCRIPTION Research was aimed at development of acoustic-insulation material with the use of alternative raw material sources from agriculture industry, which could be used for manufacture of absorbent part of the acoustic absorbers for anti-noise walls. When developing the material, the main requirement was aimed at its high value of sound absorption coefficient at wide frequency range, low density and good mechanical characteristics which guarantee consistency of materials and enable its application for building industry structures. Table 1: Showing the average shape representation for individual screens Mesh size [mm] ,5 0,25 0,125 0,063 0 Weight of partial rest [g] Average shape of d grains on individual t screens [mm] s Note: d = length of a grain [mm], t = thickness of the grain [mm], s = grain width [mm] In this case, there was technical hemp used as the main raw material, respectively it was hemp chaff which is generated as waste material when processing the technical hemp into 307

4 hemp fiber. There were macroscopic analysis and screen analysis performed on this hemp chaff. There were percentage portions of each grain sizes represented in the delivered waste materials. Then there were also average grain sizes determined in each fraction (average value taken from min. 20 measurements). Rests on individual screens [%] 25,00 20,00 15,00 10,00 5,00 0, ,5 0,25 0,125 0,063 0 Mesh size [mm] Figure 3: Rests on individual screens during implementation of screen analysis It is obvious in the measured values that the largest representation is reached in fractions of 4 and 2 mm screens. Powder density of loosely strewn waste material was 80 kg.m -3, specific density equalled the value of 1500 kg.m -3, equilibrium moisture is relatively high, it reaches the levels around 9 %. Water absorption is considerably high too due to natural origin of the material, it reaches %. In total, there were 9 testing prescriptions proposed. When proposing the test samples, there was an effort to develop a new material similar to wood-cement which is commonly used for production of the anti-noise walls. Therefore, the basic prescription lies in processing of common wood-cement on the basis of wood chips and Portland cement. Other five prescriptions were designed as a combination of hemp bullen and Portland cement. The last three prescriptions were designed on the basis of hemp bullen and alkali activated blast furnace slag. The prescriptions were designed in such way so that the effects of each individual components could be determined, e.g. the effects on both thermal-insulation and acousticinsulation characteristics as well as mechanical properties. There was surface-active water dispersion of Sokrat 2084 continuously added to the mixture at the amount of 5 % taken from the weight of binder material. The dispersion was increasing the wettability of hempy elements and decreasing surface tension of water. This process helped in foaming formation during mixing of the test mixture and it consequently got the foam stabilised. All individual prescriptions are detailed in the following table in which there are ratio representations of individual components recorded. All the components were dosed by using weight proportions: 308

5 Table 2: List of weight ratios for formulas of testing samples Mixture n. Cement Slag Water Water glass Wood Hemp RESULTS AND MEASURED VALUES Based on the proposed testing mixtures (see Table No. 2), there were test samples of dimensions 100x100x100 mm for determining the physical and mechanical characteristics. Further, there were also some testing roller-shaped samples with a diameter of 100 and 30 mm and thickness of 40 mm for determination of acoustical characteristics. Density of testing samples was determined on the samples of 100x100x100 mm according to EN The results stated here below, in Table No. 3, are arithmetical average values for every individual testing set (5 samples). Table 3: List of density of tested samples Mixture n Density [kg.m -3 ] Density [kg.m-3] Mixture n. Figure 4: Overview of measured density values on every individual testing mixtures As it is evident from the measured values, the density of every individual mixtures ranged from 276 to 630 kg.m -3. As for the mixtures of technical hemp, both lower and higher density values were achieved depending on the amount of binder material. E.g. lower and higher 309

6 values than those in comparative mixture No. 1 (on the basis of wooden chips) in which the density value equalled 564 kg.m -3. The most important endpoint for each of the samples sound absorption. Determination of the sound absorption coefficient was performed on tested samples by means of the system of impedance tubes (Kunt tube). The measurement was taken according to ISO in third octave ranges from 100 Hz to 5000 Hz. Table 4: Dependence of the sound absorption coefficient αs [-] of testing samples on frequency f [Hz] Mix.n Sound absorption [-] 1,00 0,90 0,80 0,70 0,60 0,50 0,40 0,30 0,20 0,10 0, Frequency [Hz] Figure 5: Dependence of the sound absorption coefficient of testing samples on frequency As it is obvious in Table No. 4, the reached values of sound absorption coefficients considerably varied in individual testing mixtures. In order to balance the individual sets of values of sound absorption which were measured in every individual testing samples for 310

7 individual frequencies, there was a calculation of one-figure values of DL α [db] in accordance with EN The results are contained in the following table and graph: Table 5: List of single number values DL α [db] of testing samples Mixture n. Density [kg.m -3 ] DL α [db] ,00 6,00 5,00 DLa [db] 4,00 3,00 2,00 1,00 0, Mixture n. Figure 6: Review of single number values DL α of testing samples When evaluating the measured values of density related to the testing samples and their density, some very strong correlation between two variables was found (as for the sample with technical hemp: samples 2-9; correlation coefficient r = 0.9). DL α = -3x10-5 x ρ v x ρ v (2) 311

8 7,00 6,00 DLa [db] 5,00 4,00 3,00 2,00 y = -3E-05x 2 + 0,016x + 3,9065 1, Density [kg.m -3 ] Figure 7: Dependence single number values DL α on density (calculation for samples 2 9) Compressive strength was determined on testing cubes of 100 x 100 x 100 mm. When determining the mechanical characteristics, the samples were continuously burdened. If the samples did not reach the deformation up to 10%, tension of 10% was used as the result value. Table 6: List of compressive strengths R c of testing samples Mixture n. Density [kg.m -3 ] Compressive strength [N.mm -2 ] With the measurements, it is obvious that samples with technical hemp show, at the same level of density, some lower mechanical characteristics in comparison with comparative sample No. 1 based on wood chips. Positive effect of alkali-activated binders was found only in the case of higher density. 5 CONCLUSION Anti-noise walls are an important means of protecting human health against adverse effects of noise, mainly from traffic transport, which is a direct consequence of the present human society development. The anti-noise walls form a protective barrier in areas of main transport corridors in many developed countries. Currently, when working on designs of the anti-noise 312

9 walls, the high emphasis is given not only to own acoustic characteristics but also to the nature of raw materials of which these structures are made. Production of acoustic absorbers made of natural easily recoverable raw material resources, such as technical hemp or other plants of similar kind, is very interesting not only for the point of environmental view but also as for the view of constantly sustainable development. On the basis of performed laboratory measurements, it was proven that the technical hemp bonded with inorganic binder has good mechanical and acoustic characteristics. In comparison with sample No. 1 based on wooden chips and bonded with cement, it was found the samples with technical hemp show better acoustic characteristics, with comparable mechanical characteristics and comparable density - assuming that alkali-activated binder is used for their production. Main benefit of the use of technical hemp lies in the field of constantly sustainable development because 1 ha area which is sown with hemp provides times more cellulose containing less lignin in comparison with 1 ha area of forest. Then also - the speed of recovery is 100x higher in the areas with technical hemp in comparison with the most of common tree species. ACKNOWLEDGEMENTS This outcome has been achieved with the financial support of the Ministry of Education, Youth and Sports of the Czech Republic, project No. 1M , within activities of the CIDEAS and GACR 103/08/P265. REFERENCES [1] Zach, J. The utilization of waste raw materials for development of thermal insulating building materials. Structures and Techniques, 8th International Conference. Vilnius, Lithuania, Vilnius, Lithuania p ISBN [2] Zach, J. Possibilities of insulation materials production from organics rows and waste. In ICAMS Rumunsko, ICAMS p ISBN [3] EN Road traffic noise reducing devices. Test method for determining the acoustic performance. Intrinsic characteristics of sound absorption [4] EN Road traffic noise reducing devices. Test method for determining the acoustic performance. Intrinsic characteristics of airborne sound insulation [5] EN Road traffic noise reducing devices. Test method for determining the acoustic performance. Normalized traffic noise spectrum 313