IMPACT SOUND REDUCTION OF CONCRETE LAYERS CONTAINING CORK GRANULES Anabela Moreira 1, Julieta António 2, António Tadeu 2 1 Department of Civil Engineering, Polytechnic School of Tomar-IPT, Campus Tomar, Estrada da Serra Quinta do Contador, 2300-313Tomar, Portugal {anamoreira@ipt.pt } 2 Department of Civil Engineering, University of Coimbra, Pólo II, Rua Luís Reis Santos, 3030-788 Coimbra, Portugal {julieta@dec.uc.pt; tadeu@dec.uc.pt; } Abstract The reduction of transmitted impact noise achieved by concrete layers containing cork granules is assessed. Several mix designs using varying percentages of cork and cement were studied. Experimental evaluation involved measuring by how much the transmitted impact noise was reduced by the cork-concrete layer on a heavyweight standard floor in acoustic chambers. Different thicknesses of cork-concrete layers were studied. The dynamic stiffness of the cork-concrete layers was also measured, in laboratory conditions. Keywords: impact sound reduction, dynamic stiffness, cork-concrete layer. 1 Introduction Noise inside dwellings causes discomfort to the inhabitants and high levels of noise may physiologically and psychologically damage human health. Many problems are involved in soundproofing residential buildings. Impact noise such as footsteps on rigid floors is easily transmitted through the solid elements of the building and reaches compartments a long way from the source. The use of floating floors with an interposed resilient layer is a good technical solution that reduces impact noise transmission through floors. An alternative to this current solution is the use of lightweight composite layers applied over the structural slab. These lightweight slabs can be obtained with cement mixtures (cement, sand and water) in which sand is partially replaced by cork granules. 1
Cork is obtained from the bark of the cork-oak (Quercus Suber L.). Portugal produces over 50% of the world s cork. It is a natural resource, renewable and recyclable. It is also a low density material which is impermeable to liquids and resilient. Its thermal and acoustic insulation efficiency are well known. Furthermore, the current environmental context demands the use of low impact materials, but also high technical performance [1]. As observed above, cork is a sustainable resource. Its industrial processing consumes energy which is generated by burning industrial cork waste. In Europe, building regulations specify sound insulation requirements for dwellings. These requirements are expressed by parameters defined in several standards. Standard EN ISO 140-8 [2] describes the procedure used to measure reduction of impact noise of floor coverings. It is possible to predict the impact noise insulation of resilient materials by determining dynamic stiffness [3], since resilient materials with low dynamic stiffness improve impact sound insulation of floors. The procedure to determine dynamic stiffness is described in Standard ISO 9052-1 [4]. A lot of work has been done in recent years on the subject of sound insulation of building elements. Yu et al. [1] studied the environmental impact of several acoustic materials in residential buildings. Rushforth et al. [5] compared the impact sound insulation performance of some current acoustic solutions with a range of materials processed from recycled carpet and developed a formulation that yields a sample with a good performance in impact sound insulation. Davern [6] evaluated the impact sound level of timber floors with vinyl coverings over resilient underlays as a typical Australian floor solution. Pereyron et al. [7] carried out measurements at different stages of the construction of a reinforced concrete slab of a residential building. Tadeu et al. [8] developed an analytical model to predict airborne sound and impact sound insulation provided by single and multilayered systems. This paper reports an investigation currently under way that concerns the impact sound insulation of lightweight mortar slabs. A range of cement-cork formulations have been developed. Only two formulations for lightweight mortar slabs are described in this paper. These mortars were obtained by mixing cement, sand, cork granules and water. The cork used is expanded cork granule which is obtained by a hot process at 350ºC and 300 kpa. The dynamic stiffness and the reduction in impact noise transmission index have been obtained experimentally. Laboratory tests have been conducted on samples 1.5, 3 and 4.5 cm thick, in the ITeCons facilities. 2 Experimental program In this study, lightweight mortars were made with Portland cement (CEM II 32.5), sand, expanded cork granules (ECG) and water. Cork particle dimensions used in the mixtures were 3-5 mm (ECG 3/5) and 5-10 mm (ECG 5/10). The specimens were cast and cured in the ITeCons laboratory. The experiments were carried out on 2 different cement proportions. The mortar mix designs are presented in Table 1. The volume proportion of the lightweight granules is 50% for ECG 3/5 and 50% for ECG 5/10. Resilient materials with low dynamic stiffness may reduce the floor impact sound level. In order to establish a connection between impact noise transmission and dynamic stiffness, experimental tests were performed to determine the impact noise transmission reduction index and dynamic stiffness. Laboratory tests were performed after 28 days of curing at 23ºC and 50% relative humidity. 2
Designation Cement Table 1 Mortar composition. Sand ECG 3/5 ECG 5/10 Water (liter) w/c* M 150 151.09 377.67 27.89 32.54 81.36 0.54 M 250 254.63 358.25 25.54 29.79 138.80 0.55 (*) water cement ratio 2.1 Impact sound transmission reduction For each mortar mixture 3 slabs (1.20 mx1.20 m) 1.5, 3 and 4.5 cm thick were cast. Laboratory tests were performed in ITeCons vertical movable chambers on a standard concrete slab 14 cm thick. The experimental procedures were those defined in International Standards NP EN ISO 140-8 [2] and EN ISO 717-2 [9], on impact sound transmission. The test devices used were standard noise sources (tapping machine and omni-directional noise source) and mobile microphones which rotated at a speed of one revolution every 32 seconds. The signal acquisition system was based on a Bruel & Kjaer multi-channel (5 channels) pulse, model 3560-C-T46. The microphones inside the testing chambers were connected to the acquisition system. Experimental tests were monitored in a control room. For the purposes of determining the impact sound transmission reduction the lightweight slabs were manufactured on top of the standard concrete slab. For each test, the tapping machine was placed in 4 positions above the lightweight slab, as recommend in NP EN ISO 140-8 [2]. The sound pressure level was measured in the receiving room of the laboratory facility. Figure 1 Lightweight slabs inside the testing room. The results in Table 2 relate to a 14 cm thick standard concrete slab. 3
Table 2 Impact noise transmission reduction index for the samples tested. Designation Bulk density M 150 430 M 250 740 Thickness (cm) L w (db) 1.5 25 3.0 29 4.5 22 1.5 20 3.0 23 4.5 18 The impact noise transmission reduction index, L w, was calculated in compliance with EN ISO 717-2 [9]. Results show that the 3.0 cm thick M 150 slab had the highest L w and the 4.5 cm thick M 250 had the lowest. The results for mixture M 250 show that the 3 cm thick slab performed best and the 4.5 cm thick slab performed worst. The results for the two mixtures indicate the same tendency, i.e., the best results are for the 3 cm thick slabs, and the worst are for the 4.5 cm thick ones. Figures 2, 3 and 4 show the impact sound reduction for the 2 mixes studied, where the behavior for the same thickness can be compared. All the thicknesses perform better for the M 150 mix. But the greatest difference in terms of impact noise transmission reduction was noted for the 1.5 cm and 3 cm thick slabs, while 4.5 cm slabs exhibited quite similar behavior for the M 150 and M 250 mixes. Figure 2 Impact noise transmission reduction for 1.5 cm thick slabs. 4
Figure 3 Impact noise transmission reduction for 3 cm thick slabs. Figure 4 Impact noise transmission reduction for 4.5 cm thick slabs. 2.2 Dynamic stiffness Experimental tests were performed on samples (200 mmx200 mm) of 1.5, 3 and 4.5 cm thickness. Three samples were tested for each composition and each thickness. The specimens were covered with waterproof plastic foil to which a 0.5 mm thick mixture of plaster and water was applied. The load plate was bedded to the fresh plaster mix, as established by standard ISO 9052-1 [4]. Tests were performed with an impact hammer type 8206, an accelerometer type 4508 B 002, an amplifier type 2646 and a Bruel & Kjaer multi-analyzer PULSE TM type 3560-C. The accelerometer was placed on the center top of the load plate as shown in Figure 5. The specimens were excited by hammer impacts at 4 different positions. The hammer impacts were light and as similar as possible. This procedure allows the determination of the 5
resonant frequency which can be found from the frequency peak. The measurement results (at four points) related to the 4.5 cm thick M 150 specimen are presented in Figure 6. Dynamic stiffness was calculated in accordance with standard ISO 9052-1 [4], which is based on a simplified method. The apparent dynamic stiffness, s t, is given by the following equation: (1) where m t is the total mass per unit area used during the test and f r is the resonance frequency. Figure 5 Test specimen. Figure 6 Dynamic response in 4 excitation points for 4.5 cm thick M 150 specimen. Table 3 presents the dynamic stiffness obtained for the two mixtures studied and the three thicknesses tested. The results presented were calculated from average values of resonant frequencies. The results demonstrate that the 3.0 cm thick M 150 specimen has the lowest dynamic stiffness. The highest values are for the 4.5 cm thick samples for both mixtures. The lowest dynamic stiffness for the M 250 specimens was found for the 3 cm thick one. In accordance 6
with these results, the best performance for impact noise insulation should be exhibited by the 3 cm thick samples and the worst by the 4.5 cm thick samples. Designation M 150 M 250 Table 3 Dynamic stiffness results. Thickness (cm) Resonant frequency (Hz) s' t (MN/m 3 ) 1.5 137.11 159.97 3.0 127.43 148.75 4.5 155.10 228.06 1.5 154.54 206.43 3.0 145.01 191.17 4.5 204.52 405.00 3 Results and discussion The dynamic stiffness results indicate that the 3 cm thick M 150 slab performs best in terms of impact noise insulation. This result is confirmed by the impact sound transmission reduction index, L w, which was established in the laboratory. The two experimental tests show that the M 150 slabs provide better impact sound insulation than the M 250 slabs, for the same thickness. Analysis of results demonstrates that the best impact sound insulation is exhibited by 3 cm thick slabs, while 4.5 cm thick slabs have the highest impact sound transmission. Tables 2 and 3 show that when the cement content is increased the slab becomes more rigid and its performance declines, as concluded by comparing the results of mixture M 150 with mixture M 250. Analysis of the results of mixture M 150 reveals an improvement in impact sound insulation when the thickness is increased from 1.5 to 3 cm, as usually happens for resilient materials (for greater thickness). However, as the mixtures are composed of a resilient material (cork) and a hard one (cement paste), when the thickness changes from 3 to 4.5 cm the slab becomes more rigid and its performance declines such that it is even worse than the 1.5 cm slab. Cement with cork granules mixes and with a higher proportion of binder are still in development. Similar tests will be conducted to analyze and compare impact noise transmission behavior. In light of the present results for the tested materials it may be expected that the higher the concentration of cement the lower the impact sound reduction of the lightweight slabs produced. 4 Conclusions This work has described the impact sound attenuation provided by lightweight concrete layers of different thicknesses. Two mixtures using different percentages of cement were studied. Experimental tests were conducted in ITeCons facilities to quantify the reduction in the impact noise transmission index and dynamic stiffness. Experimental results show that the lowest dynamic stiffness is related to the best impact noise transmission performance. Best results were found for the 3 cm thick slabs, for both cement mixtures. These results suggest that the experimental tests to determine the dynamic 7
stiffness of resilient materials can also be used to predict impact noise transmission in the proposed lightweight slabs. The results indicate good prospects for the use of mortars containing suitable proportions of cork granules to attenuate impact sounds in buildings. Acknowledgments The authors would like to thank Amorim Isolamentos S. A. for providing the cork granulate used to produce the test specimens References [1] Yu, Chia-Jen; Kang, Jian. Environmental impact of acoustic materials in residential buildings, Building and Environment, Vol 44 (10), 2009, pp. 2166-2175. [2] CEN, European Standard EN ISO 140-8: Acoustics Measurement of sound insulation in buildings and of building elements. Part 8: Laboratory measurements of the reduction of transmitted impact noise by floor coverings on a heavyweight standard floor (ISO 140-8), Belgium, 1997. [3] Kim, Kyoung-Woo; Jeong, Gab-Cheol; Yang, Kwa-Seop; Sohn, Jang-Yeul. Correlation between dynamic stiffness of resilient materials and heavyweight impact sound reduction level, Building and Environment, Vol 44 (8), 2009, pp. 1589-1600. [4] ISO, International Standard ISO 9052-1: Acoustics - Determination of dynamic stiffness. Part 1: Materials used under floating floors in dwellings, 1989. [5] Rushforth, I. M.; Horoshenkov, K. V.; Miraftab, M.; Swift, M. J. Impact sound insulation and viscoelastic properties of underlay manufactured from recycled carpet waste, Applied Acoustics, Vol 66 (6), 2005, pp. 731-749. [6] Davern, W. A. Impact noise on two timber floors with vinyl floor coverings on resilient underlays, Applied Acoustics, Vol 24 (2), 1998, pp. 157-163. [7] Pereyon, D.; Santos, J. L. P. Laje nervurada: análise da performance acústica para ruído de impacto, IX Encontro Nacional e V Latino Americano de Conforto no Ambiente Construído, Ouro Preto, Brasil, 8-10 Agosto, 2007. [8] A. Tadeu; A. Pereira; L. Godinho. Prediction of airborne Sound and Impact Sound Insulation Provided by Single Systems Using Analytical Expressions, Applied Acoustics, Vol 68, pp. 17-42, 2007. [9] CEN, European Standard EN ISO 717-2: Acoustics Rating of sound insulation in building elements Part 2: Impact sound insulation (ISO 717-2), Belgium, 1996. 8