*Article* **The Influence of Environmentally Friendly Flame Retardants on the Thermal Stability of Phase Change Polyurethane Foams**

## **Dong Liu and Anjie Hu \***

School of Civil Engineering and Architecture, Southwest University of Science and Technology, Mianyang 621010, China; dtld123@126.com

**\*** Correspondence: anjie@swust.edu.cn; Tel.: +86-15281112512

Received: 9 December 2019; Accepted: 17 January 2020; Published: 22 January 2020

**Abstract:** To improve thermal insulation, microencapsulated phase change materials (micro-PCMs), expandable graphite (EG), and ammonium polyphosphate (APP) were introduced into polyurethane foam (PUF) to enhance the thermal stability and improve the thermal insulation behavior. The morphology of the PUF and micro-PCM was studied using a scanning electronic microscope (SEM), while the thermophysical properties of the PUF were investigated using a hot disk thermal constants analyzer and differential scanning calorimetry (DSC). The thermal stability of the PUF was investigated by thermogravimetric analysis (TGA), and the gas products volatilized from the PUF were analyzed by thermogravimetric analysis coupled with Fourier transform infrared spectrometry (TGA-FTIR). The results revealed that the thermal conductivities of the PUF were reduced because micro-PCM is effective in absorbing energy, showing that the PUF functions not only as a thermal insulation material but also as a heat sink for energy absorption. Moreover, the EG and APP were found to be effective in improving the thermal stabilities of the PUF, and the optimized formulation among EG, APP, and micro-PCMs in the PUF showed a significant synergistic effect.

**Keywords:** polyurethane foam; thermal property; phase change material; flame retardant

## **1. Introduction**

Widely used for sound absorption [1–3], furniture [3,4], and insulating materials [5], polyurethane foam (PUF) plays an important role in our daily life. Compared with other insulating materials, PUF has the advantage of low thermal conductivity, high mechanical and chemical stability at both high and low temperatures, and the ability to form sandwich structures with various facer materials [6,7].

The properties of PUF depend on the density, structure, and geometry of the foam, and a large number of fillers are required to get the desired properties [8–10]. To improve the thermal characteristics of PUF, adding phase change material (PCM) into PUF has been studied since the 1990s [11–13], since PCM can absorb or release significant latent heat while the temperature of the material stays almost constant in the process of phase change [11,14]. Among the investigated PCMs, the paraffin shows desirable properties, such as high heat of fusion, little or no supercooling, self-nucleating behavior, and thermal and chemical stability. However, liquid paraffin has a relatively low viscosity, and it has to be kept in a closed tank or container to prevent leaching. Microencapsulation of paraffin provides a perfect way to solve the leaching problem. Microcapsules containing the polar phase change material (PCM) n-dodecanol were synthesized by in situ polymerization, using melamine-formaldehyde resin as a shell and styrene-maleic anhydride copolymer (SMA) as an emulsifier. Results show that anionic SMA emulsifier is suitable for the encapsulation of n-dodecanol [15]: when the mass ratio of emulsifier to n-dodecanol is 4.8%, the phase change latent heat and encapsulation efficiency reaches the maximum

values of 187.5 J/g and 93.1%, respectively. A series of PU microcapsules containing the PCM of n-octadecane was successfully synthesized, using an interfacial polymerization in SMA dispersion with diethylene triamine (DETA) as a chain extender reacting with toluene-2,4-diisocyanate (TDI), and the average diameter of the microencapsulated phase change materials (micro-PCMs) was in the range of 5–10 µm. Earlier research showed that PU-shells regularly fabricated with the influence of SMA varied with a weight ratio from 1.0% to 4.0% in diameter, encapsulating ratio, release properties, and thermal stability [16].

Another drawback of PUF and paraffin is that, like other organic materials, their flammability is high and the flame-spreading rate is fast, which limits their use due to the high possibility of a fire hazard. Hence, a flame retardant should be added to improve the thermal stability of PUF and paraffin. There are generally two kinds of flame retardants. The first are halogen-containing flame retardants, which are most effective at improving thermal stability. However, they always release a large amount of noxious material during their decomposition, which has brought ecological and physical problems. The second kind are halogen-free flame retardants, such as EG and decabrominated dipheny ethane (DBDPE). The effect of the retardants is influenced by many factors. In the literature [17], the EG/PU composites prepared with their method showed a V-0 flame retardance level, whereas EG/PU composite prepared by conventional blending only showed a V-2 flame retardance level. Results in the literature [9] showed that the flame-retardant efficiency got better with the increase in the density of PU foam at the fixed EG weight percent, or with an increase in the EG weight percent at the fixed foam density. According to the literature [8], when the flame-retardant loadings were 20 wt%, the limiting oxygen index (LOI) value of DBDPE-filled rigid polyurethane foam (RPUF) increased to 33 vol%, while the EG-filled RPUF reached 41%. However, when they were simultaneously added into RPUF, there was not any flame-retardant synergistic effect. In the literature [18], they tested polystyrene composites with 21.4 wt% ammonium polyphosphate, and found that the peak of the heat release rate was reduced by 77.5% as compared to those of virgin polystyrene. In the literature [19], it was found that the peak heat release rate of PEG could be decreased by 19.2% by introducing graphene aerogels (GA) with 1.60 wt% and phosphorylated polyvinyl alcohol (PPVA) with 15.0 wt%. In the literature [20], it was found that, with 0.25 phr of functionalized graphene oxide (fGO) and 10 phr of EG/dimethyl methyl phosphonate (DMMP), the LOI value of RPUF/fGO composites reached 28.1%, and its UL-94 test reached V-0 rating, showing that the fGO could significantly enhance the flame retardant properties of RPUF composites. Carbon nanotubes and zinc aluminum-layered double hydroxide (CNT/ZnAl-LDH) could improve the thermal degradation stability as well as the fire safety of PU foams [21]. These researches indicate that halogen-free flame retardants can effectively improve the stability of the organic materials, and their effects are mostly influenced by the content of the retardants and the synergistic effect of different retardants. Among these retardants, the EG and APP are widely applied due to their good flame-retardant efficiency and economic efficiency. Thus, we applied them in the PUF, and optimized the content of the retardants for the best effect.

In this work, we focused on the effects of EG, APP, and micro-PCMs on the thermal characteristics of PUF. The EG and the APP were added to increase the thermal stability of PUF synergistically, and micro-PCMs were added to improve the temperature-controlling capability of PUF. Besides thermal stability, the EG can also improve the effectiveness of the energy exchange of PCM because of its high thermal conductivity.

#### **2. Experimental Section**

#### *2.1. Materials*

Paraffin (melting temperature T<sup>m</sup> = 58.15 ◦C, latent heat 163.31 J/g) was used as the phase change material. The melamine (MA), formaldehyde, and dodecyl sodium sulfate were supplied by Sinopharm Chemical Ltd. (Shanghai, China). The polymeric MDI (methane diphenyl diisocyanate, NCO% = 30.2~32.5, viscosity at 25 ◦C equals to 150~250 mPa·s) was purchased from Yantai Wanhua

Polyurethanes Co., Ltd. (Yantai, China). The polyether polyol (YD 4450, n◦OH = 350 ± 10 mgKOH/g, viscosity at 25 ◦C equals to 220 ± 50 MPa·s) was supplied by Hebei Yadong Chemical Group Co., Ltd. (Shijiazhuang, China). The ammonium polyphosphate (APP, average particle size 92% < 10 µm) and expandable graphite (EG, average particle size: 320 mesh) were provided by Hefei Keyan Chemical Material Technology Development Co., Ltd (Hefei, China). Other additives are given as follows: dibutyltin dilaurate (DD), triethylenediamine, triethanolamine, silicone oil, and water.

#### *2.2. Preparation of Micro-PCMs*

Micro-PCMs were fabricated by in situ polymerization, using a melamine–formaldehyde copolymer as the shell and paraffin as the core material. The detailed fabrication process included the steps of prepolymer solution synthesis, PCM emulsion preparation, and mixing of the prepolymer solution and the PCM emulsion for an in situ polymerization. The resultant microcapsules were filtered and washed with distilled water at 70 ◦C until a pH of 7 was reached. The wet powders were then dried in a vacuum oven at 80 ◦C for 12 h to remove the water. The final mass fraction of the paraffin was approximately 70%, and the total latent heat of the micro-PCM was 118.9 J/g.

#### *2.3. Fabrication of PUF*

The PUF formulations are given in Table 1. Making the PUF, materials of catalysts, surfactants, blowing agents, EG, APP, and micro-PCMs, etc. were added to a YD 4450 blend in a proper mold and stirred intensively for 15 s. After that, the calculated quantity of MDI was added and stirred at 2500 rpm for 10 s at a room temperature of 20 ◦C. Then the foam was placed in an oven at 60 ◦C for 24 h.


**Table 1.** Formulations and thermophysical properties of PUFs.

#### *2.4. Characterization of Flame Retardant PUF*

The morphology and microstructure of the PUF were observed using an electronic scanning microscope (Hitachi X650, Hitachi Limited, Tokyo, Japan). The latent heat and the phase change temperature of the PUF were studied using differential scanning calorimetry (DSC), which was carried out in an argon atmosphere with the DT-50 (Shimadzu, Quioto, Japan) thermal analyzer, with a heating rate of 10 ◦C/min and a flow rate of 30 mL/min. The thermal conductivities of the PUF were tested by the hot disk thermal constants analyzer. A constant voltage was applied, and a thermocouple recorded the temperature rise by circulating a current along the platinum wire. The precision of the thermal conductivity and the thermal diffusivity were ± 2.0% and ± 5.0%, respectively. The thermal stability properties of the PUFs were tested by a TGA Q5000 IR thermogravimetric analyzer (TA Instruments, New Castle, PA, USA). In the test, the samples were heated from room temperature to 600 ◦C with a linear heating rate of 20 ◦C/min in a nitrogen atmosphere. The thermogravimetric analysis-Fourier transform infrared spectrometry (TGA-FTIR) of the sample was performed using the DT-50 instrument interfaced to the Varian 2000 FTIR spectrometer. About 3–5 mg of the product was put in an alumina crucible and heated from 30 to 600 ◦C. The heating rate was set as 20 ◦C/min in a nitrogen atmosphere.

## **3. Results and Discussion**

## *3.1. SEM Photos of Micro-PCMs and Pure PUF Structure*

Figure 1a shows the particle profiles of the micro-PCMs. The micro-PCMs have a spherical shape, and the microcapsules range in size from 0.3 to 0.8 µm. The entire honeycomb structure of the pure PUF is shown in Figure 1b, and the honeycomb structure can contain the additives, such as EG, APP, and micro-PCMs. Observing the PUF (Figure 1c), it can be found that the admixture inserted into the pure PUF, showing that the PUF could be used as a supporting material for the admixture.

**Figure 1.** SEM photographs of the micro-PCM (**a**) and PUF (**b**,**c**).

## *3.2. The Thermal Conductivities of the PUFs*

As a widely used thermal insulation material, the heat transfer efficiency of the PUF is the most important property in the application of the material [22,23]. The heat transfer efficiency of PUF is characterized by thermal conductivity [24]. The tested thermal conductivities of PUFs with different formulation are listed in Figure 2, which show that both the additions of EG and APP increase the thermal conductivities of PUFs. However, the increases in the conductivities are relatively small compared to the conductivity of the pure PUF. According to the heat transfer theory, the thermal conductivities of PUFs are determined by the thermal conductivity of the foams and gases within the cell, the radiative heat transfer among these foams, and the convection heat transfer within the cells. Filler particles, including EG and APP, not only fit into the structure of the foam but also lie within the cells, which increases the thermal conductivity of foams and cell gases. However, the filler particles also decrease the radiative and convection heat transfer within the cells, which reduces the effects of the heat transfer enhancement. This conclusion also can explain the result that the thermal conductivities change indistinctively when increasing the content of EG in the PUF. Double side effects also exist in adding the micro-PCMs. When the micro-PCMs are added to the PUF, the thermal conductivities increase, due to their capability for absorbing energy. However, this effect does not reduce the thermal insulation of the PUF, because the energy is restricted into the phase change material.

#### *3.3. The Latent Heats of the PUFs*

The latent heats of the PUFs are tested by the DSC. The DSC curves of the micro-PCM and PU6 to PU9 are exhibited in Figures 3 and 4, respectively. It can be seen that there are two peaks on the DSC curves. The sharp peak represents the solid–liquid phase change of the paraffin, and the minor peak is associated with the liquid–solid phase transition of paraffin. The values of the latent heat obtained with DSC curves are presented in Table 1. As shown in the table, the total latent heats of the PUFs are a little lower than the theoretical values, which were calculated by multiplying the latent heat of the dispersed micro-PCMs with their weight percentage. DSC is also used to investigate the changes of latent heat values under conditions of thermal cycling with the temperature from 80 to 25 ◦C. From Figure 4 and Table 1, we can find that the energy was released when the temperature decreased, which is useful in order to keep a constant temperature for environment and energy saving. These results demonstrate that the PUFs are not only thermal insulation materials but also storage materials for thermal energy.

**Figure 3.** The DSC curves during thermal cycling of micro-PCM.

**Figure 4.** The DSC curves during thermal cycling of PUFs.

#### *3.4. The Thermal Stabilities of PUFs*

The thermal stability of the PUF as a thermal insulation material is important for practical applications. Since PUF and micro-PCM both have low thermal stability, they are ignited easily. Improving the thermal stability of PUF is imperative. Figures 5 and 6 show the thermograms of micro-PCMs and PUFs, respectively. The thermal degradation of pure PUF occurs in a two-step process. The first step of degradation is due to the formation of polyol and isocyanate groups. The isocyanates from this deployed condensation reaction are very reactive to dimerize, and they form carbodiimide with the evolution of carbon dioxide. The second step of degradation was due to the further reaction between carbodiimide and polyol to impart the substituted urea [25,26]. From Figure 6a, it can be found that there are no significant changes for the thermal degradation of the PU2 and PU3 when the EG is introduced into the PU1. This is probably due to no reaction/interaction between EG and PUF during the thermal degradation process. However, the rates of thermal degradation for the PU2 and PU3 are accelerated at about 300 to 400 ◦C in comparison with that of PU1, which may be due to the degradation of intercalated compounds in EG, such as H2SO<sup>4</sup> decomposing to H2O and SO<sup>2</sup> gases and the oxidation of EG, and the distance between the basal planes of EG increasing [27,28]. The structure of EG could prevent the heat and oxygen transfer and improve the thermal stability of the PUF at high temperatures because the EG could cover the surface of the matrix. At higher loadings of EG in PUF, the content of the char layer increases in the temperature range from about 650 to 750 ◦C, and this char layer could act as a thermal barrier for the PUF and prevent further decomposition. When the mass fraction of APP replaces the EG in equal amounts of PU2 and PU3, the thermal stabilities of the PU4 and PU5 are both increased. This is because the APP can form polyphosphoric acid and carbonaceous residue with PUF at high temperatures, and help in inhibiting the thermal degradation of the PUF. When the micro-PCMs are introduced into the PU2 and PU3, the thermal stabilities of the PU6 and PU7 are both lower than that of PU2 and PU3 (Figure 6b). This is because the thermal stability of the micro-PCMs (listed in Figure 5) is lower than that of PU2 and PU3. However, if part of the EG of PU6 and PU7 is replaced with equal amounts of the mass fraction of APP, the thermal stability of PU8 becomes higher than that of PU6 (Figure 6c), while the PU9 degrades faster than PU7 for temperatures lower than 550 ◦C and slower for higher temperatures, showing that the ingredients influence the thermal stability improvement due to the synergistic effect of APP and EG. These results show that the APP, melamine and polyol can be effectively taken as an integrated intumescent flame retardant, and the intumescent flame retardant can be thermally decomposed at a higher temperature to form

three-dimensional network structures. Besides, the EG acted in great synergistic effect with the APP with a certain formulation in the PUF/micro-PCM system [29].

**Figure 6.** The thermal stabilities of PUFs (**a**) the effects of EG/APP on the thermal stability of PUF, (**b**) the effects of micro-PCMs on the thermal stability of PUF/EG composite, (**c**) the effects of EG/APP on the thermal stability of PUF/micro-PCMs composite.

#### *3.5. The Volatilized Products of the PUFs*

The TGA-FTIR study is to identify the volatilized products, and the results can provide a better understanding of the thermal degradation mechanisms [30]. The whole range of spectra of the samples is shown in Figure 7. It can be seen that the main decomposition product is carbon dioxide, which is in the range of 2410–2220 cm−<sup>1</sup> . In order to investigate the influence of EG, APP, and micro-PCMs on the thermal degradation of the PUF, the evolution of the intensity of the characteristic peak carbon dioxide is plotted in Figure 8, in which the characteristic peak is at 2356 cm−<sup>1</sup> . It can also be found that the curves of the carbon dioxide show two peaks at about 16 and 32 min, respectively. These peaks are attributed to the depolymerization of PUF to form monomer precursors such as polyol and isocyanates, and the dimerization from isocyanate to carbodiimide and carbon dioxide. The ratio between the second peak value and the first peak value are calculated, i.e., the ratio of PU1 is 0.71, while the ratios of the PU2, PU4, PU6, and PU8 are 1.09, 2.10, 2.84, and 2.23, respectively, showing that EG can highly reduce the carbon dioxide. Comparing the results of PU2 and PU6 with PU4 and PU8, we can also find the reduction of carbon dioxide when replacing EG with APP in PUF. From these results, it can be concluded that EG increased the thermal stability of the PUF, and by replacing part of EG with APP, the efficiency in improving the thermal stability of PU can be further improved.

**Figure 7.** The 3D surface graphs of the FTIR spectra of the evolved gases produced by PUFs.

**Figure 8.** Intensity of characteristic peak versus time for the volatilized carbon dioxide of PUFs.

#### **4. Conclusions**

The preparation and characterization of PUF containing EG, APP, and micro-PCM are reported. EG and APP were used to improve the thermal stability of PUF, and micro-PCMs were used as the phase

change material for thermal energy storage. The SEM images showed that PUF could be an excellent supporting material for EG, APP, and micro-PCM. The thermal conductivity results indicated that the thermal conductivities of the PUF changed owing to the added EG and APP, but the values do not increase continually with the increase of EG and APP contents in the PUF. A possible reason is that the EG and APP could increase the thermal conductivities of the foams and cell gases while decreasing the radiative heat transfer within the foams. The thermal conductivities increased when micro-PCMs were introduced into the PUF because micro-PCMs could absorb the thermal energy; hence, the temperature could be maintained more consistently by the PUF. The DSC results showed that the total latent heats of the PUFs were a little lower than the theoretically calculated values. The TGA and TGA-FTIR results showed that APP and EG have synergistic effects for improving the thermal stabilities of the PUF. When the micro-PCMs are added into PUF, an integrated intumescent flame-retardant system is formed at high temperatures, and optimal formulation among EG, APP, and micro-PCM could provide the optimal synergistic effect in improving the thermal stability of the PUF. All in all, the temperature regulating and controlling effect could be ameliorated owing to micro-PCM, the thermal conductivity of PUF improved with the appropriate amount of EG, and EG and APP showed a great synergistic effect to improve the thermal stability of the PUF/micro-PCM composite.

**Author Contributions:** Conceptualization, D.L. and A.H.; methodology, A.H.; software, D.L.; validation, A.H.; formal analysis, D.L.; investigation, D.L.; resources, D.L. and A.H.; data curation, D.L.; writing—original draft preparation, D.L.; writing—review and editing, A.H.; visualization, D.L.; supervision, A.H.; project administration, A.H.; funding acquisition, A.H. and D.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by National Natural Science Foundation of China, grant number 51503173 and 51306156; Sichuan Provincial Department of Education, grant number 17ZA0405; Doctoral Program of Southwest University of Science and Technology, grant number 11zx7156; The Open Fund of Jinhe, grant number 11zh9002. The APC was funded by Sichuan Provincial Department of Education, grant number 17ZA0405.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Review* **Acoustic Properties of Innovative Concretes: A Review**

**Roman Fediuk <sup>1</sup> , Mugahed Amran 2,3,\*, Nikolai Vatin <sup>4</sup> , Yuriy Vasilev <sup>5</sup> , Valery Lesovik 6,7 and Togay Ozbakkaloglu <sup>8</sup>**


**Abstract:** Concrete is the most common building material; therefore, when designing structures, it is obligatory to consider all structural parameters and design characteristics such as acoustic properties. In particular, this is to ensure comfortable living conditions for people in residential premises, including acoustic comfort. Different types of concrete behave differently as a sound conductor; especially dense mixtures are superior sound reflectors, and light ones are sound absorbers. It is found that the level of sound reflection in modified concrete is highly dependent on the type of aggregates, size and distribution of pores, and changes in concrete mix design constituents. The sound absorption of acoustic insulation concrete (AIC) can be improved by forming open pores in concrete matrices by either using a porous aggregate or foam agent. To this end, this article reviews the noise and sound transmission in buildings, types of acoustic insulating materials, and the AIC properties. This literature study also provides a critical review on the type of concretes, the acoustic insulation of buildings and their components, the assessment of sound insulation of structures, as well as synopsizes the research development trends to generate comprehensive insights into the potential applications of AIC as applicable material to mitigate noise pollution for increase productivity, health, and well-being.

**Keywords:** concrete; noise; acoustic properties; sound-absorbing; sound-reflecting

#### **1. Introduction**

Building regulations and planning authorities have not properly focused on resolving the ongoing issue of noise pollution faced by the residents of urban areas [1]. Typically, concrete is used as an external cladding to inhibit the propagation of sound transmission, which reflects the sound waves away from the structure [2]. Although the sound waves get reflected away, its magnitude does not reduce and becomes an issue in enclosed spaces such as apartment complexes, factories, and narrow thoroughfares, respectively (see Figure 1) [3,4]. As a result, this leads to several problems such as masked warning signals, higher chances of impaired hearing, and increased work-related stresses [5].

**Citation:** Fediuk, R.; Amran, M.; Vatin, N.; Vasilev, Y.; Lesovik, V.; Ozbakkaloglu, T. Acoustic Properties of Innovative Concretes: A Review. *Materials* **2021**, *14*, 398. https:// doi.org/10.3390/ma14020398

Received: 30 November 2020 Accepted: 13 January 2021 Published: 14 January 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

**Figure 1.** Sound waves reflections in a surrounded narrow thoroughfare [3]. Reprinted with permission from Elsevier [3]. **Figure 1.** Sound waves reflections in a surrounded narrow thoroughfare [3]. Reprinted with permission from Elsevier [3].

Concrete is a widespread and cheap structural material for the construction of residential buildings of any volume and number of storeys [6]. During the construction and operation of these buildings, it is necessary to ensure comfortable living conditions for people [7]. The most important step along this path is ensuring the acoustic well-being of residents through the optimal design of building envelopes [8–10]. Usually, when a sound wave strikes or hits the surface of a building, the sound energy gets absorbed, transmitted, or reflected away [11]. Hence, there is a relationship between the incident energy and the absorbed, transmitted, or reflected energy [12]. The absorption of sound in a porous material is related to the loss in energy wherein the incident noise energy converts to heat energy and some other energies due to vibration, friction, and air viscosity [13,14]. Sound insulation is the sound transmission loss of the building partition [15,16]. Lightweight porous materials absorb sound well, while heavyweight and hard materials reflect a significant portion of sound energy [3]. Despite the fact that there are some studies on the acoustic behavior of different concrete material, it is necessary to systematize the current state of sound-insulation concrete composites and structures made of them [17,18]. Concrete is a widespread and cheap structural material for the construction of residential buildings of any volume and number of storeys [6]. During the construction and operation of these buildings, it is necessary to ensure comfortable living conditions for people [7]. The most important step along this path is ensuring the acoustic well-being of residents through the optimal design of building envelopes [8–10]. Usually, when a sound wave strikes or hits the surface of a building, the sound energy gets absorbed, transmitted, or reflected away [11]. Hence, there is a relationship between the incident energy and the absorbed, transmitted, or reflected energy [12]. The absorption of sound in a porous material is related to the loss in energy wherein the incident noise energy converts to heat energy and some other energies due to vibration, friction, and air viscosity [13,14]. Sound insulation is the sound transmission loss of the building partition [15,16]. Lightweight porous materials absorb sound well, while heavyweight and hard materials reflect a significant portion of sound energy [3]. Despite the fact that there are some studies on the acoustic behavior of different concrete material, it is necessary to systematize the current state of sound-insulation concrete composites and structures made of them [17,18].

To reduce build time, labor savings, and thermal efficiency, most architects and designers prefer the use of insulated concrete forms. However, the acoustic properties of these insulated concrete forms are often overlooked or under looked. To provide better acoustic properties, the use of acoustic insulation concrete (AIC) is a recent advancement in the field of concrete technology. The acoustic component installed in an AIC reduces the sound in the building structure and keeps the pollution created by noise at bay while improving the acoustical effects of the sound generated within. To achieve a desired acoustic protection, the installation of an AIC wall can provide excellent exterior insulation as well as a soundproofing envelope. Due to a better soundproofing envelope, AIC becomes a suitable choice for hotels, airports, and schools where the requirement of thick traditional walls can be eliminated. Meanwhile, the use of lightweight concretes such as foam or fabric are usually porous and do not allow sound to be reflected. As a result, the sound passes through the porous medium, and the energy gets converted to heat while reducing its magnitude [3]. Hence, the use of lightweight concrete is effective when used in movie halls or in recording studios. This reduces the resonance time in the room [19,20]. Although the use of lightweight concrete is effective internally, it is not suitable in external conditions, and hence, concrete is still the preferred material for oth-To reduce build time, labor savings, and thermal efficiency, most architects and designers prefer the use of insulated concrete forms. However, the acoustic properties of these insulated concrete forms are often overlooked or under looked. To provide better acoustic properties, the use of acoustic insulation concrete (AIC) is a recent advancement in the field of concrete technology. The acoustic component installed in an AIC reduces the sound in the building structure and keeps the pollution created by noise at bay while improving the acoustical effects of the sound generated within. To achieve a desired acoustic protection, the installation of an AIC wall can provide excellent exterior insulation as well as a soundproofing envelope. Due to a better soundproofing envelope, AIC becomes a suitable choice for hotels, airports, and schools where the requirement of thick traditional walls can be eliminated. Meanwhile, the use of lightweight concretes such as foam or fabric are usually porous and do not allow sound to be reflected. As a result, the sound passes through the porous medium, and the energy gets converted to heat while reducing its magnitude [3]. Hence, the use of lightweight concrete is effective when used in movie halls or in recording studios. This reduces the resonance time in the room [19,20]. Although the use of lightweight concrete is effective internally, it is not suitable in external conditions, and hence, concrete is still the preferred material for other commercial and residential building structures. Based on the past researchers, it was reported that various types of concrete behave differently as a sound conductor; especially dense mixtures are superior

sound reflectors, and light ones are sound absorbers. It is found that the level of sound reflection in modified concrete depends on the type of aggregates, size, and distribution of pores and changes in concrete mix design constituents. The sound absorption of AIC is enhanced by forming open pores in concrete matrices by either using a porous aggregate or foam agent. To this end, this article reviews the noise and sound transmission in buildings, types of acoustic insulating materials, and the acoustic properties. This literature study also delivers a comprehensive review on the type of concretes, the acoustic insulation of buildings and their components, the assessment of sound insulation of structures, as well as to synopsize the research development trends to generate comprehensive insights into the potential applications of AIC as applicable material to mitigate noise pollution for increase productivity, health, and well-being. **2. Transmission of Noise and Sound in Buildings**  To produce a noise and sound-proof AIC, the walls and roofs of a building could be engineered with modifications on the level at which the sound energy (Figure 2a) [21]: Reflects back from the building wall, Absorbs within the wall, and Transmits through the building wall. Structural elements such as glazing and roof lights, which can reduce the performance, should also be considered. The transmission of noise between different zones of a building structure is shown in Figure 2b. Two types of noise transmission occur: direct and flanking transmission, and noise is transmitted when any two areas are separated from each other. The noise travels in two separate routes [22,23]: (i) through the separa-

er commercial and residential building structures. Based on the past researchers, it was reported that various types of concrete behave differently as a sound conductor; especially dense mixtures are superior sound reflectors, and light ones are sound absorbers. It is found that the level of sound reflection in modified concrete depends on the type of aggregates, size, and distribution of pores and changes in concrete mix design constituents. The sound absorption of AIC is enhanced by forming open pores in concrete matrices by either using a porous aggregate or foam agent. To this end, this article reviews the noise and sound transmission in buildings, types of acoustic insulating materials, and the acoustic properties. This literature study also delivers a comprehensive review on the type of concretes, the acoustic insulation of buildings and their components, the assessment of sound insulation of structures, as well as to synopsize the research development trends to generate comprehensive insights into the potential applications of AIC as applicable material to mitigate noise pollution for increase productivity, health, and

#### **2. Transmission of Noise and Sound in Buildings** tion structure, which is termed direct transmission, and (ii) around the separation struc-

*Materials* **2021**, *14*, x FOR PEER REVIEW 3 of 28

To produce a noise and sound-proof AIC, the walls and roofs of a building could be engineered with modifications on the level at which the sound energy (Figure 2a) [21]: ture through adjoining structural elements, which is termed flanking transmission [21,24]. Meanwhile, the insulation of sound for both types of transmission is controlled by its mass, isolation, and sealing characteristics [22,25]. The direct transmission of noise


well-being.

• Transmits through the building wall. elements make it challenging to predict the flanking transmission of noise [23,27,28].

**Figure 2.** Noise and sound transmission in a building. (**a**) Factors of acoustic insulation performance; (**b**) Flanking and direct transmissions of sound between different zones [21]. Reprinted with permission from DATA STEEL [21]. **Figure 2.** Noise and sound transmission in a building. (**a**) Factors of acoustic insulation performance; (**b**) Flanking and direct transmissions of sound between different zones [21]. Reprinted with permission from DATA STEEL [21].

It should be noted that at various conditions, the separation wall could be of high standard AIC, while the side wall could be of low standard, which is continuous be-Structural elements such as glazing and roof lights, which can reduce the performance, should also be considered. The transmission of noise between different zones of a building structure is shown in Figure 2b. Two types of noise transmission occur: direct and flanking transmission, and noise is transmitted when any two areas are separated from each other. The noise travels in two separate routes [22,23]: (i) through the separation structure, which is termed direct transmission, and (ii) around the separation structure through adjoining structural elements, which is termed flanking transmission [21,24]. Meanwhile, the insulation of sound for both types of transmission is controlled by its mass, isolation, and sealing characteristics [22,25]. The direct transmission of noise can be determined by laboratory tests, since it directly depends on the properties of the separation wall and floor [26]. Meanwhile, the details of junctions between the structural elements make it challenging to predict the flanking transmission of noise [23,27,28].

It should be noted that at various conditions, the separation wall could be of high standard AIC, while the side wall could be of low standard, which is continuous between the rooms. In such cases, flanking transmission can propagate more noise than direct transmission [21]. Hence, the joints between the separation elements should be properly detailed to minimize noise due to flanking transmission.

#### **3. AIC Properties 3. AIC Properties**

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properly detailed to minimize noise due to flanking transmission.

The ability to reduce sound transmission through a structural element is what defines the properties of AIC. Usually, concrete is a good insulator of sound, which can reflect up to 99% of its total energy. However, it is also a relatively poor absorber of sound and can propagate within enclosed spaces, causing echoes [29–32]. Sound, a form of energy, can travel through mediums of solid, liquid, and gas in longitudinal wave by the oscillation of vibrating particles [1,33]. As the sound waves dissipate, it gets expanded outward while distributing the intensity over a larger area. The larger the vibrating particles in the medium, the more the energy that passes through the medium [34]. Meanwhile, audible sounds are classified into two types: airborne and impact sounds. Airborne sounds can be recognized as speech, loudspeakers, and musical instruments, respectively. These sounds waves travels through the air only and cannot travel through solids [3]. The airborne sound produces vibration in the concrete, which causes the vibration of suspended particles on the other side of the wall, thus, causing it to be heard. On the other hand, footsteps, closing doors, and falling objects are categorized as impact sounds, which can vibrate between walls and floors, leading to airborne noise in adjoining rooms [2]. Typically, noise and sound differ from each other, with the former being subjective and dependent on the receptor. Due to it being subjective, architects and designers should consider reducing noise in the building, in urban areas in particular [35]. However, since it is challenging to lower the volume or the propagation of sound, noise mitigation measures should be implemented to bring down the level of annoyance [36,37]. In this regard, remedial measures such as insulation, reflection, and isolation provide the best ways to reflect sound energy [3,26]. The ability to reduce sound transmission through a structural element is what defines the properties of AIC. Usually, concrete is a good insulator of sound, which can reflect up to 99% of its total energy. However, it is also a relatively poor absorber of sound and can propagate within enclosed spaces, causing echoes [29–32]. Sound, a form of energy, can travel through mediums of solid, liquid, and gas in longitudinal wave by the oscillation of vibrating particles [1,33]. As the sound waves dissipate, it gets expanded outward while distributing the intensity over a larger area. The larger the vibrating particles in the medium, the more the energy that passes through the medium [34]. Meanwhile, audible sounds are classified into two types: airborne and impact sounds. Airborne sounds can be recognized as speech, loudspeakers, and musical instruments, respectively. These sounds waves travels through the air only and cannot travel through solids [3]. The airborne sound produces vibration in the concrete, which causes the vibration of suspended particles on the other side of the wall, thus, causing it to be heard. On the other hand, footsteps, closing doors, and falling objects are categorized as impact sounds, which can vibrate between walls and floors, leading to airborne noise in adjoining rooms [2]. Typically, noise and sound differ from each other, with the former being subjective and dependent on the receptor. Due to it being subjective, architects and designers should consider reducing noise in the building, in urban areas in particular [35]. However, since it is challenging to lower the volume or the propagation of sound, noise mitigation measures should be implemented to bring down the level of annoyance [36,37]. In this regard, remedial measures such as insulation, reflection, and isolation provide the best ways to reflect sound energy [3,26].

tween the rooms. In such cases, flanking transmission can propagate more noise than direct transmission [21]. Hence, the joints between the separation elements should be

#### **4. Types of Acoustic Insulating Materials 4. Types of Acoustic Insulating Materials**

The common aspects concerning the conservation of acoustic energy according to the classic ratio (1) are shown in Figure 3. The common aspects concerning the conservation of acoustic energy according to the classic ratio (1) are shown in Figure 3.

$$E\_i = E\_r + E\_{\mathcal{e}} \tag{1}$$

where *Ei*—energy of sound falling on the building envelope; *Er*—reflected sound energy; and *Ee*—comprises the absorption and transmission of sound energy. where *Ei*—energy of sound falling on the building envelope*; Er*—reflected sound energy; and *Ee*—comprises the absorption and transmission of sound energy.

**Figure 3. Figure 3.** Sound energy conservation [14]. Reprinted with permission from Elsevier [14]. Sound energy conservation [14]. Reprinted with permission from Elsevier [14].

For the design of enclosing structures, it is important to use extremely sound-absorbing or extremely sound-reflecting materials.

#### *4.1. Sound-Absorbing*

The acoustic absorption coefficient α is computed using Equation (1) is expressed as (2).

$$\alpha = 1 - \frac{E\_r}{E\_i} \tag{2}$$

$$a = 1 - \frac{E\_r}{E\_l} \tag{}$$

The absorption coefficient varies between 0 and 1, where 0 denotes the materials that reflect sound, and 1 is an ideal sound absorption material [38–40]. The presentation of the acoustic absorption coefficient as one value is an intricate problem. The absorption coefficient varies between 0 and 1, where 0 denotes the materials that reflect sound, and 1 is an ideal sound absorption material [38–40]. The presentation of the acoustic absorption coefficient as one value is an intricate problem.

For the design of enclosing structures, it is important to use extremely

The acoustic absorption coefficient is computed using Equation (1) is expressed as

This is attributable to the fact that the coefficient of acoustic absorption, α, is varied for each frequency of sound. As a simplification, the division of the audible frequency spectrum into octaves (with a further 1/3 octave division) is used, as well as, for example, such characteristics as noise reduction coefficient (NRC) and sound absorption average (SAA) [8,16,41]. NRC is a part of the easiest approaches that could be estimated as the arithmetic mean of sound absorption coefficient at frequency ranging between 250 and 2000 Hz. Meanwhile, SAA is the arithmetic mean of the coefficients of sound absorption at 12 one-third octave frequency ranging 200–2500 Hz in conformity with ASTM C423-17 [42]. Construction materials are commonly ranked by their NRC [42,43] (Figure 4). This is attributable to the fact that the coefficient of acoustic absorption, α, is varied for each frequency of sound. As a simplification, the division of the audible frequency spectrum into octaves (with a further 1/3 octave division) is used, as well as, for example, such characteristics as noise reduction coefficient (NRC) and sound absorption average (SAA) [8,16,41]. NRC is a part of the easiest approaches that could be estimated as the arithmetic mean of sound absorption coefficient at frequency ranging between 250 and 2000 Hz. Meanwhile, SAA is the arithmetic mean of the coefficients of sound absorption at 12 one-third octave frequency ranging 200–2500 Hz in conformity with ASTM C423-17 [42]. Construction materials are commonly ranked by their NRC [42,43] (Figure 4).

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sound-absorbing or extremely sound-reflecting materials.

*4.1. Sound-Absorbing* 

(2).

**Figure 4.** Building materials versus noise reduction coefficient (NRC) [43]. Modified with improvements from [43]. **Figure 4.** Building materials versus noise reduction coefficient (NRC) [43]. Modified with improvements from [43].

> Summary of the noise reduction coefficients for different concrete composites are presented in Table 1. It can be concluded that an acoustic insulation concrete (AIC) building structure should possess a relatively good absorption coefficient with the NRC exceeding 0.45.


#### **Table 1.** A summary of NRC for different concrete composites.

#### *Materials***2021**, *14*, 398

#### **Table 1.** *Cont.*


#### *4.2. Sound-Insulating*

Sound reflectors are materials that reflect noise, preventing the further propagation of a sound wave. Concrete, glass, wood, plastic, and metal are used as sound-reflecting materials.

In the concept of "Sound reduction index", two quantities should be distinguished [47]:


The last parameter is more adequate; therefore, it is the one that is important for assessing sound insulation. It varies widely for different types of concrete. For example, rubber concrete with a smooth surface has this value at the level of 30–35 dB [54], and for concrete with crumb rubber, it is 30–50 dB [3]. Moreover, for hemp concrete, it fluctuates in the range of 3–11 dB [56], and for homogeneous concrete floors, this value reaches 60–70 dB, and theoretically, it can reach 90 dB [63].

The same indicator is "Transmission Loss (TL)", which differs depending on the type of material. However, the sound reduction index R is defined by ISO standards, so, in general, it is usually preferred in the American literature. In addition, ISO 717-1 distinguishes between frequency-dependent R (or R') and weighted values (R<sup>w</sup> or R'w). The latter values are accepted for frequencies around 500 Hz; therefore, they are considered the most presentable, and the fact is that a significant part of the sound (noise) spectrum is concentrated in the region of 500 Hz. Accordingly, ISO 717-1 requires the weighted Rw (or R'w) value precisely in the frequency range of about 500 Hz. Thus, this is a somewhat simplified, but sufficient for practical application, interpretation for sound reduction index R (or R'). For the sake of clarity, soundproof building materials are commonly designed in such a way as to have a maximum sound absorption coefficient, specifically for noise with frequencies in the region of 500 Hz.

The efficiency of sound reflectors is evaluated by the following features of materials and structures:


#### **5. Type of Concretes**

Table 2 shows acoustic property of different type of concretes. It can be seen that normal concrete detects the highest level of sound reflection followed by glass-based concrete.


**Table 2.** Acoustic property of different type of concretes.

#### *5.1. Normal Concrete*

The acoustic absorption coefficient for normal concrete is almost about 0.02, signifying that around 98% of the sound dynamism is a surface reflection. The more denser/heavy concrete, the higher the sound insulation rate that can be detected [3,18].

#### *5.2. Aerated Concrete*

In aerated concrete, alumina reacts with calcium hydroxide and leads to the appearance of gaseous hydrogen, which later forms microscopic bubbles in the mix. Lightweight aerated concrete has a density typically ranging 250 to 500 kg/m<sup>3</sup> and has a higher porosity of about 82.1% to 91.5% [72]. The pore wall of this concrete is very thin due to which sound waves can be transmitted very easily. According to Laukaitis and Fiks [74], the porosity decreases as the density increases, but on the other hand, the volume of open pores also increases. The open pore volume of aerated concrete is more than that of foam concrete. The absorption coefficient of autoclaved aerated concrete is best categorized by the ratio of open pores to total porosity.

#### *5.3. Foam and Porous Concretes*

Zhang et al. [51] developed a synthesis geopolymeric foamed concrete (FC) with absorption coefficients of 0.7 to 1.0 at 40–150 Hz and 0.1 to 0.3 at 800–1600 Hz, and 12 MPa of compressive strength was almost attained. Luna-Galliano et al. [75] studied the coefficient of absorption curves of geopolymers with silica fume used as a porous former. For different proportions of the starting components, the coefficient of sound absorption curves was the same, including with two peaks at 400 and 2500 Hz. It was noted that the mix contained more silica fume, which leads to the curve becoming wider (which indicates the stability of the coefficient of sound absorption), which can correlate with the highest open porosity. It was also found that an FC with low density exhibits a higher sound absorption coefficient (α) than a high-density FC when determined using a standing wave apparatus [76–78]. In Figure 5 [79], it can be observed that at the target void ratio (TVR) of 20%, the absorption coefficient revealed the highest value in the frequency range of 315–400 Hz; for the TVR of 25%, it ranked the second highest in the frequency range of 400–500 Hz; and for the TVR of 30%, it became the highest in the frequency range of 500–630 Hz [79]. This indicates that the frequency of the highest absorption coefficient was found for sound as the TVR augmented.

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sorption coefficient was found for sound as the TVR augmented.

standing wave apparatus [76–78]. In Figure 5 [79], it can be observed that at the target void ratio (TVR) of 20%, the absorption coefficient revealed the highest value in the frequency range of 315–400 Hz; for the TVR of 25%, it ranked the second highest in the frequency range of 400–500 Hz; and for the TVR of 30%, it became the highest in the frequency range of 500–630 Hz [79]. This indicates that the frequency of the highest ab-

**Figure 5.** Sound absorption coefficient for the porous concrete with different densities and target void ratios (TVRs) [79]. Reprinted with permission from Elsevier [79]. **Figure 5.** Sound absorption coefficient for the porous concrete with different densities and target void ratios (TVRs) [79]. Reprinted with permission from Elsevier [79].

In another study, it was found that the inclusions of 30% slag and fly ash into geopolymer FC (GFC) show no significant effect on the absorption coefficient at lower frequency but increases at higher frequency, 800–1600 Hz in particular [51,80]. This can be explained by the variation in the pore size, porosity, and tortuosity owing to inclusions of the slag particles [81]. Even increasing the dosage of foam (5 to 10%) shows little or no effect on the absorption coefficient at lower frequency but is efficient at medium frequency only, i.e., about 600–1000 Hz. Nevertheless, in comparison to a conventional plain concrete, which has an absorption coefficient typically less than 0.1 at 125–2000 Hz, GFC shows excellent acoustic absorption properties [82]. However, GFC has low sound absorption at medium to high frequencies relative to expanded clay porous concrete, which usually has an absorption coefficient of about 0.5 in this region [83]. However, a GFC shows the same absorption coefficient as the thickness of the GFC increases (Figure 6). In another study, it was found that the inclusions of 30% slag and fly ash into geopolymer FC (GFC) show no significant effect on the absorption coefficient at lower frequency but increases at higher frequency, 800–1600 Hz in particular [51,80]. This can be explained by the variation in the pore size, porosity, and tortuosity owing to inclusions of the slag particles [81]. Even increasing the dosage of foam (5 to 10%) shows little or no effect on the absorption coefficient at lower frequency but is efficient at medium frequency only, i.e., about 600–1000 Hz. Nevertheless, in comparison to a conventional plain concrete, which has an absorption coefficient typically less than 0.1 at 125–2000 Hz, GFC shows excellent acoustic absorption properties [82]. However, GFC has low sound absorption at medium to high frequencies relative to expanded clay porous concrete, which usually has an absorption coefficient of about 0.5 in this region [83]. However, a GFC shows the same absorption coefficient as the thickness of the GFC increases (Figure 6). *Materials* **2021**, *14*, x FOR PEER REVIEW 11 of 28

**Figure 6.** Acoustic absorption spectra of geopolymer foamed concrete (GFCs) at varying thickness [51]. Reprinted with permission from Elsevier [51]. **Figure 6.** Acoustic absorption spectra of geopolymer foamed concrete (GFCs) at varying thickness [51]. Reprinted with permission from Elsevier [51].

Crumb rubber concrete (CRC), which is produced using different sizes of crumb rubber typically ranging 6 to 19 mm for coarse and 1 to 6 mm for fine, has been a subject of recent research [3,31,32,62,84]. However, the use of CRC in structural applications is not practical owing to considerable reduction in the strength [85,86]. Although CRC has lower mechanical strength, it can be used as a durable composite material capable of reflecting and absorbing sound [87]. In fact, the use of CRC as an exterior surface of a building can reduce the propagation of street noises into the buildings. High noise levels caused by busy streets and passing into dwelling spaces are often uncomfortable for inhabitants in high-rise apartments. An extremely dense material has an absorption coefficient of about 0 and can reflect away the sound completely. Hence, the use of CRC can provide a solution to both sound absorption and recycling used tires. Table 3 presents a list of classic absorption coefficients for some common materials used in structural con-

**Table 3.** Sound absorption coefficients for some common materials.

**Type of Material SACs**  Concrete 0.02–0.06 Hardwood 0.3 Unpainted block-work 0.02–0.05

Materials with good sound absorption prepared by a concrete matrix incorporated with crumb rubber were investigated in [56]. The sound absorption coefficient α was above 0.5 for nine out of 12 specimens and attained a maximum value of 0.82 and 0.93 under favorable conditions. The concrete panels were prepared with crumb rubber as a replacement to natural fibers and investigated its acoustic properties in references [3,88]. The results showed that rubber concrete has proven itself in terms of sound absorption, especially with a high percentage (15%) of rubber crumb. Sukontasukkul [45] reported an improvement of 46% in the NRC upon the inclusion of 20% crumb rubber in precast

*5.4. Crumb Rubber Concrete* 

cretes.

concrete panels.

#### *5.4. Crumb Rubber Concrete*

Crumb rubber concrete (CRC), which is produced using different sizes of crumb rubber typically ranging 6 to 19 mm for coarse and 1 to 6 mm for fine, has been a subject of recent research [3,31,32,62,84]. However, the use of CRC in structural applications is not practical owing to considerable reduction in the strength [85,86]. Although CRC has lower mechanical strength, it can be used as a durable composite material capable of reflecting and absorbing sound [87]. In fact, the use of CRC as an exterior surface of a building can reduce the propagation of street noises into the buildings. High noise levels caused by busy streets and passing into dwelling spaces are often uncomfortable for inhabitants in high-rise apartments. An extremely dense material has an absorption coefficient of about 0 and can reflect away the sound completely. Hence, the use of CRC can provide a solution to both sound absorption and recycling used tires. Table 3 presents a list of classic absorption coefficients for some common materials used in structural concretes.

**Table 3.** Sound absorption coefficients for some common materials.


Materials with good sound absorption prepared by a concrete matrix incorporated with crumb rubber were investigated in [56]. The sound absorption coefficient α was above 0.5 for nine out of 12 specimens and attained a maximum value of 0.82 and 0.93 under favorable conditions. The concrete panels were prepared with crumb rubber as a replacement to natural fibers and investigated its acoustic properties in references [3,88]. The results showed that rubber concrete has proven itself in terms of sound absorption, especially with a high percentage (15%) of rubber crumb. Sukontasukkul [45] reported an improvement of 46% in the NRC upon the inclusion of 20% crumb rubber in precast concrete panels.

Materials containing crumb rubber have been known to be a good absorber of sound, with absorption coefficients ranging between 0.3 and 0.7 [32]. In fact, the combination of crumb rubber and concrete further can increase the absorption coefficient as well as reduce the level of reflected sound [89]. Apart from structural concrete, the use of crumb rubber has also been known to be a good sound absorber when utilized in highways. The incorporation of crumb rubber in asphalt mixes could lower the noise produced by vehicular traffic. The sound absorption coefficients of asphalt pavements containing crumb rubber are considerably improved with time due to its higher absorption energy [46]. The damping vibration was about 230% higher when crumb rubber was used to replace 15% of natural fines in CRC [90]. Incorporating 20% crumb rubber in concrete blocks was also reported to produce a lighter, flexible, and durable sound-absorbing material [91]. The durability of CRC against freeze–thaw could also be improved using air entraining admixtures [92,93]. It is found that CRC when exposed to excessive heat shows a substantial enhancement in the energy absorption, but only when lower grades of rubber and natural fines substitution were considered [85]. In comparison to conventional aggregates, crumb rubber is highly elastic, which deforms easily under an applied load, and its ductility can increase up to 90%, subsequently improving energy dissipation [3]. As the crumb rubber content increases, the density of CRC reduces. Since a material's acoustic properties depend mostly on its density, lightweight concrete such as CRC containing a higher proportion of crumb rubber could absorb more sound. Hence, the ability to reflect more sound energy increases [32,94]. Due to this, CRC panels have been used mostly in office buildings as exterior cladding or on balconies of buildings. The compaction degree of a CRC also greatly influences its sound absorption, since the larger grades of CRC absorb more sound when fully compacted [62,90]. Meanwhile, CRC cladding panes are often used as an alternative to the conventional ones to protect the structure with an added benefit of reducing the overall

weight. A proposal provided by the City of Vancouver Noise Control Manual [95] shows that the installation of exterior CRC panels around the balconies of high-rise buildings can improve its acoustic performance with a reduction in the level of reflected noise as well (as see in Figure 7). to protect the structure with an added benefit of reducing the overall weight. A proposal provided by the City of Vancouver Noise Control Manual [95] shows that the installation of exterior CRC panels around the balconies of high-rise buildings can improve its acoustic performance with a reduction in the level of reflected noise as well (as see in Figure 7).

Materials containing crumb rubber have been known to be a good absorber of sound, with absorption coefficients ranging between 0.3 and 0.7 [32]. In fact, the combination of crumb rubber and concrete further can increase the absorption coefficient as well as reduce the level of reflected sound [89]. Apart from structural concrete, the use of crumb rubber has also been known to be a good sound absorber when utilized in highways. The incorporation of crumb rubber in asphalt mixes could lower the noise produced by vehicular traffic. The sound absorption coefficients of asphalt pavements containing crumb rubber are considerably improved with time due to its higher absorption energy [46]. The damping vibration was about 230% higher when crumb rubber was used to replace 15% of natural fines in CRC [90]. Incorporating 20% crumb rubber in concrete blocks was also reported to produce a lighter, flexible, and durable sound-absorbing material [91]. The durability of CRC against freeze–thaw could also be improved using air entraining admixtures [92,93]. It is found that CRC when exposed to excessive heat shows a substantial enhancement in the energy absorption, but only when lower grades of rubber and natural fines substitution were considered [85]. In comparison to conventional aggregates, crumb rubber is highly elastic, which deforms easily under an applied load, and its ductility can increase up to 90%, subsequently improving energy dissipation [3]. As the crumb rubber content increases, the density of CRC reduces. Since a material's acoustic properties depend mostly on its density, lightweight concrete such as CRC containing a higher proportion of crumb rubber could absorb more sound. Hence, the ability to reflect more sound energy increases [32,94]. Due to this, CRC panels have been used mostly in office buildings as exterior cladding or on balconies of buildings. The compaction degree of a CRC also greatly influences its sound absorption, since the larger grades of CRC absorb more sound when fully compacted [62,90]. Meanwhile, CRC cladding panes are often used as an alternative to the conventional ones

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**Figure 7.** A crumb rubber concrete (CRC) cladding installed on a multi-storey structure to avoid the diffusion of sound [95]. Modified with improvement from *Wakefield* [95]. **Figure 7.** A crumb rubber concrete (CRC) cladding installed on a multi-storey structure to avoid the diffusion of sound [95]. Modified with improvement from *Wakefield* [95].

In reference [96], the importance of preliminary processing of the surface of secondary rubber particles to increase the sound-insulating properties of CRC was proved. The elastic modulus of concrete combinations was determined, and the surface adhesion prepared from pretreated/untreated rubber aggregate was studied [3]. Conclusions indicated that the method of pretreatment caused weaker adhesion between the cement paste and crumb rubber; therefore, it improved the ability to absorb the vibration and sound-insulating properties of concrete. It was shown that the effects of freezing and thawing do not significantly affect the insulating properties [3]. It was been found that the insulation characteristics for all concrete improved at high frequencies. The findings show that rubber concrete can be used on the outside of building envelopes to absorb sound throughout multi-storey urbanized buildings, but it requires full-scale on-site testing.

#### *5.5. Expanded Polystyrene Concrete*

The use of foamed polystyrene aggregates has some effect on noise reduction owing to its lower stiffness, which has a high ability to dissipate energy. Expanded polystyrene granules (EPS) can easily be added in concrete to produce a lightweight concrete of varying density [60]. EPS are ultra-lightweight aggregates with a density <33 kg/m<sup>3</sup> , non-absorbent, hydrophobic, and closed cell nature [97]. The NRC of a 50 mm-thick layer of EPS is around 0.32 [98]. This means that about 32% of the incident sound energy gets absorbed and does not reflect back, hence improving the absorption characteristics of the material. Oancea et al. [60] replaced the aggregate with 50% polystyrene foam while obtaining lightweight concrete with a density of 1810 kg / m<sup>3</sup> and NRC at the level of 0.18. Due to the closed porosity of polystyrene beads, effective damping of the sound wave is not possible.

#### *5.6. Fibered Concrete*

At present, synthetic fibers such as glass and mineral fibers are used in buildings as a sound-proofing material [99]; however, these pose risks to human health [100]. It is also testing.

not possible.

*5.6. Fibered Concrete* 

*5.5. Expanded Polystyrene Concrete* 

found that the airflow resistivity is an important parameter that needs to be considered in the acoustic behavior of fibrous concrete [12]. When choosing raw materials for concrete production, it is necessary to select materials with a reduced density that is capable of damping the sound wave. In particular, when choosing fiber, polypropylene has one of the lowest densities (Figure 8). found that the airflow resistivity is an important parameter that needs to be considered in the acoustic behavior of fibrous concrete [12]. When choosing raw materials for concrete production, it is necessary to select materials with a reduced density that is capable of damping the sound wave. In particular, when choosing fiber, polypropylene has one of the lowest densities (Figure 8).

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taining lightweight concrete with a density of 1810 kg / m<sup>3</sup>

In reference [96], the importance of preliminary processing of the surface of secondary rubber particles to increase the sound-insulating properties of CRC was proved. The elastic modulus of concrete combinations was determined, and the surface adhesion prepared from pretreated/untreated rubber aggregate was studied [3]. Conclusions indicated that the method of pretreatment caused weaker adhesion between the cement paste and crumb rubber; therefore, it improved the ability to absorb the vibration and sound-insulating properties of concrete. It was shown that the effects of freezing and thawing do not significantly affect the insulating properties [3]. It was been found that the insulation characteristics for all concrete improved at high frequencies. The findings show that rubber concrete can be used on the outside of building envelopes to absorb sound throughout multi-storey urbanized buildings, but it requires full-scale on-site

The use of foamed polystyrene aggregates has some effect on noise reduction owing to its lower stiffness, which has a high ability to dissipate energy. Expanded polystyrene granules (EPS) can easily be added in concrete to produce a lightweight concrete of varying density [60]. EPS are ultra-lightweight aggregates with a density <33 kg/m<sup>3</sup>

non-absorbent, hydrophobic, and closed cell nature [97]. The NRC of a 50 mm-thick layer of EPS is around 0.32 [98]. This means that about 32% of the incident sound energy gets absorbed and does not reflect back, hence improving the absorption characteristics of the material. Oancea et al. [60] replaced the aggregate with 50% polystyrene foam while ob-

Due to the closed porosity of polystyrene beads, effective damping of the sound wave is

At present, synthetic fibers such as glass and mineral fibers are used in buildings as a sound-proofing material [99]; however, these pose risks to human health [100]. It is also

,

and NRC at the level of 0.18.

**Figure 8.** Densities of materials of various fibers [101,102]. Data adopted with permission from Fe-**Figure 8.** Densities of materials of various fibers [101,102]. Data adopted with permission from Fediuk and Klyuev [101,102].

diuk and Klyuev [101,102]. Stainless steel, nickel fiber, alloy fiber, and iron metallic fiber are some of the noise reduction materials available [12]. A review of existing literature on the acoustic absorption of metallic and inorganic fibrous composites is presented in Table 4. The use of fibers is widely known to provide a wide and varied solution to noise reduction in buildings, some of which are listed in Table 4.

#### *5.7. Recycled Aggregate Concrete (RCA)*

The sound absorption of a cellular material is mostly influenced by the energy loss occurred due to the friction induced within the pores of the walls [79]. Hence, the higher the open void ratio, the greater the SAC of concrete at all frequencies [103]. In a porous concrete, the pores are interconnected to each other, and the pores increase as the amount of coarse aggregate increases [104]. Consequently, the sound absorption of a porous materials is significantly influenced, which is directly proportional to its airflow resistivity [105]. Hence, to produce a sound-absorbing concrete, it is imperative to generate a tortuous path wherein the sound energies could dissipate, mostly in frequencies of noise traffic, ranging between 300 and 1000 Hz [106]. Some researchers used recycled aggregate with particle sizes greater than 5 mm as a substitute for aggregates in concretes [13,107].


**Table 4.** Acoustic insulation of different fibrous concrete composites.

At the same time, the SAC was found to be enhanced with the use of recycled concrete aggregates [11]. An SAC of about 0.91 was recorded when 100% of RCA was used, which is 0.24 higher than the control mix [122]. This was as a result of the greater void content in the RCA concrete. Figure 9 [13] clearly portrays that the SAC increases as the demolition wastes aggregates (CDW)/fly ash (FA) increases. In fact, the porous concrete corresponding to the CDW80-FA20 mix showed the best SAC. Meanwhile, a huge variation can be observed when the amount of coarse fraction increases to 80%.

#### *5.8. Mollusk Shell Waste Aggregate Concrete*

It is also known that the particle size plays a crucial role in the acoustic properties of porous materials made with shell waste [49]. To examine this, the grain size distribution of the mollusk shell waste (7 and <−2 2 mm) was studied [41]. It was found that the porous concrete containing shell waste aggregates of about 5–10 mm showed the best acoustic absorption properties [49,79]. Meanwhile, it is reported that the internal pores have no contribution to the SAC of a porous concrete [55]. Hence, it was concluded that concretes made with mollusk shell waste (2 to 7 mm particle size) increase the SAC by about 40% with respect to the porous concrete containing natural aggregates only. [123]. For particles of the same size, the sound transmission lost showed absorption of more than 4 dB at a thickness of 12 cm. An improvement in the acoustic characteristics of the composites is possible due to the pyrolysis of mollusk shells, resulting in a convoluted surface capable of

effectively damping the sound wave [73]. This finding coincides with the study by Paridah et al. [124] wherein the sound waves hitting the pores of a wall produce an acoustic wave due to the dissipation of energy. Due to the porosity and tortuosity in the specimen, an incident sound wave is produced, dissipated into sound energy due to internal friction, and results in high SAC [125]. Logically speaking, large particles cause a reduction in the energy dissipation due to its higher porosity, and hence, the absorption coefficient also decreases. Hence, sizes up to 7 mm are crushed and allowed to be utilized [41,49]. Owing to this, the SAC of a porous concrete is usually lower than a recycled concrete, mostly in frequencies below 2000 Hz [123]. *Materials* **2021**, *14*, x FOR PEER REVIEW 15 of 28 corresponding to the CDW80-FA20 mix showed the best SAC. Meanwhile, a huge variation can be observed when the amount of coarse fraction increases to 80%.

**Figure 9.** Sound absorption coefficient (SAC) for demolition wastes as aggregates for geopolymer concrete [13]. Reprinted with permission from Elsevier [13]. **Figure 9.** Sound absorption coefficient (SAC) for demolition wastes as aggregates for geopolymer concrete [13]. Reprinted with permission from Elsevier [13].

#### *5.8. Mollusk Shell Waste Aggregate Concrete 5.9. Polymer Concrete*

It is also known that the particle size plays a crucial role in the acoustic properties of porous materials made with shell waste [49]. To examine this, the grain size distribution of the mollusk shell waste (7 and <−2 2 mm) was studied [41]. It was found that the porous concrete containing shell waste aggregates of about 5–10 mm showed the best acoustic absorption properties [49,79]. Meanwhile, it is reported that the internal pores have no contribution to the SAC of a porous concrete [55]. Hence, it was concluded that concretes made with mollusk shell waste (2 to 7 mm particle size) increase the SAC by about 40% with respect to the porous concrete containing natural aggregates only. [123]. For particles of the same size, the sound transmission lost showed absorption of more than 4 dB at a thickness of 12 cm. An improvement in the acoustic characteristics of the composites is possible due to the pyrolysis of mollusk shells, resulting in a convoluted surface capable of effectively damping the sound wave [73]. This finding coincides with the study by Paridah et al. [124] wherein the sound waves hitting the pores of a wall produce an acoustic wave due to the dissipation of energy. Due to the porosity and tor-Arenas et al. [13] report that the activation solution in the geopolymerization of fly ash has a considerable effect mainly on the strength parameters but not on acoustic properties. However, according to Luna et al. [75], incorporating a higher amount of silica fume in the mix and increasing the setting temperature leads to an increase in the open voids. In turn, this reduces the compressive strength but increases the SAC. Similarly, Mastali et al. [59] developed cinderblock alkali-activated concrete with significant acoustic characteristics for the interior walls of buildings in residential areas. Stolz et al. [58] produced geopolymers with a density of about 1000 kg/m<sup>3</sup> as well as heat and sound insulation comparable to the properties of the commercial existing options. In comparison to ordinary Portland cement (OPC)-based composites, alkali-activated FC shows excellent acoustic absorption characteristics [44]. Porous concrete was also found to exhibit average SACs higher than 0.5, at mid to high frequencies [83]. The alkali-activated FC was also observed to exhibit good SAC (>0.5) at the same frequency levels as that of the porous concrete [59].

#### tuosity in the specimen, an incident sound wave is produced, dissipated into sound energy due to internal friction, and results in high SAC [125]. Logically speaking, large *5.10. Foam-Glass Based Concrete*

particles cause a reduction in the energy dissipation due to its higher porosity, and hence, the absorption coefficient also decreases. Hence, sizes up to 7 mm are crushed and allowed to be utilized [41,49]. Owing to this, the SAC of a porous concrete is usually lower than a recycled concrete, mostly in frequencies below 2000 Hz [123]. *5.9. Polymer Concrete*  Arenas et al. [13] report that the activation solution in the geopolymerization of fly ash has a considerable effect mainly on the strength parameters but not on acoustic properties. However, according to Luna et al. [75], incorporating a higher amount of sil-The use of foam-glass concrete (FGC) can also provide a good acoustic insulation wherein the annealing step is not required, and the density can also be greater than 150 kg/m<sup>3</sup> [126]. In fact, foamed-glass blocks of high costs can exhibit better aesthetics for indoor acoustic insulation properties [127]. However, an FGC composite is a unique material with good acoustic properties, with an almost unlimited service life [126,128–131]. The main reason for the insufficiently widespread use of foam glass in structures is not its operational features, but the cost related with the technological features of its manufacturing. Glass fiber is widely used in cement materials, which do not show a considerable improvement in sound absorption, but it improves sound reflection well due to the creation

larly, Mastali et al. [59] developed cinderblock alkali-activated concrete with significant acoustic characteristics for the interior walls of buildings in residential areas. Stolz et al.

of a dense and low-porous structure [128,132–135]. Although significant studies have investigated FGC, very few characterized its open and closed porosity in detail [136–138]. The majority of the available research focused only on the partially closed, open porosity, closed porous with high density, or open porosity with lower-density FCs [139–141]. FGC with open porosity is usually due to the use of a decomposition foaming agent that crystallizes [139–144]. This type of concrete is produced using a replicate synthesis that was obtained from organic foams, the sol–gel process, or inorganic gel casting [127,145,146]. Meanwhile, only a few studies are available on FGCs with closed porosity and a density lower than 150 kg m−<sup>3</sup> [139,147–152]. The SAC as a function of frequency containing 20% glass and 80% GDL (sample 6 and 10) has also been studied (Figure 10) [127]. From this study, it has been found that findings were insignificant at frequencies lower than 100 Hz. Meanwhile, a few researchers observed that the SAC of glass foam is higher than rock wool at medium to high frequencies [153–155]. This is mainly due to the porous nature of the material. Since foam is composed of an open cell structure, it shows excellent performance at a frequency range of 1250–3150 Hz [156]. Similarly, tortuosity also causes the SAC to vary while in transit from an open cell structure to a fibrous one [157]. In addition, due to the dimensional stability of FGC with open porosity, it is suitable as immovable acoustic insulation elements or as a provision for catalysts [145]. *Materials* **2021**, *14*, x FOR PEER REVIEW 17 of 28

**Figure 10.** Average SACs of samples compared with rock wool [127]. Reprinted with permission from Elsevier [127]. **Figure 10.** Average SACs of samples compared with rock wool [127]. Reprinted with permission from Elsevier [127].

#### **6. Building Components 6. Building Components**

Evaluation of the acoustic characteristics of the building elements allows you to deal with the noise penetrating the premises and determine the degree of its impact [158,159]. Building acoustics study the transmission of sound (for example, the steps of people and the noise of passing cars) through the walls and entrances of a building [158,160]. First, sound is measured inside and outside the building, and then work is carried out in the room to change the difference in the levels of reverberation and background noise [161,162]. Information about noise (for example, about its frequency spectrum) allows you to effectively deal with it, for example, using isolation and shielding [163,164]. Evaluation of the acoustic characteristics of the building elements allows you to deal with the noise penetrating the premises and determine the degree of its impact [158,159]. Building acoustics study the transmission of sound (for example, the steps of people and the noise of passing cars) through the walls and entrances of a building [158,160]. First, sound is measured inside and outside the building, and then work is carried out in the room to change the difference in the levels of reverberation and background noise [161,162]. Information about noise (for example, about its frequency spectrum) allows you to effectively deal with it, for example, using isolation and shielding [163,164].

#### *6.1. Reinforced Concrete Wall System 6.1. Reinforced Concrete Wall System*

*6.2. Steel Plate Wall System* 

A reinforced concrete wall has better sound-insulating properties than a brick of the same thickness, because it is denser [96]. A thick concrete wall is better than a thin concrete wall in terms of sound insulation [19]. However, in order for a single-layer reinforced concrete wall to provide standard sound insulation for urban conditions, it is necessary to ensure that its thickness is more than 1 m, which is very expensive and sig-A reinforced concrete wall has better sound-insulating properties than a brick of the same thickness, because it is denser [96]. A thick concrete wall is better than a thin concrete wall in terms of sound insulation [19]. However, in order for a single-layer reinforced concrete wall to provide standard sound insulation for urban conditions, it is necessary

for building facades is very relevant [166]. Figure 11 shows a clear model for the construction of RC walls with extremely high acoustic insulation. Carefully selected layer thicknesses and their sequence ensure a sound reduction index of up to 60 dB [167].

Good sound insulation properties are provided by wall systems using steel plates and an air inner layer, which are called ventilated facades [168]. The system of ventilated facades is not only an architectural solution, protection from adverse weather conditions, and reliable thermal insulation, but it is also an optimal sound-insulating property (Sound reduction index, Rw) that can be improved up to 15 dB [169]. One of the widely used approaches in reducing airborne noise transmission is by the installation of a denser acoustic mineral wool wall, as shown in Figure 12 [21]. This dense mineral wool slab shows an excellent SAC and also acts as an additional layer to the structural concrete.

to ensure that its thickness is more than 1 m, which is very expensive and significantly reduces the internal space [165]. Therefore, the use of multilayer wall systems for building facades is very relevant [166]. Figure 11 shows a clear model for the construction of RC walls with extremely high acoustic insulation. Carefully selected layer thicknesses and their sequence ensure a sound reduction index of up to 60 dB [167]. *Materials* **2021**, *14*, x FOR PEER REVIEW 18 of 28

**Figure 11.** Model for the construction of reinforced concrete walls with extremely high acoustic insulation [167]. Reprinted with permission from ECOSISM [167]. **Figure 11.** Model for the construction of reinforced concrete walls with extremely high acoustic insulation [167]. Reprinted with permission from ECOSISM [167].

#### The insulation layer is usually installed in addition to the insulation quick, which maintains the low-level coupling between the two skins. It also maintains the thermal *6.2. Steel Plate Wall System*

properties of the structural concrete. Meanwhile, the usage of a mineral wool acoustic wall can compromise with the acoustic performance due to its higher degree of coupling [98]. However, the additional mass provided by the acoustic mineral wool wall can reduce the impact noise propagated through the structure during rains [21]. Good sound insulation properties are provided by wall systems using steel plates and an air inner layer, which are called ventilated facades [168]. The system of ventilated facades is not only an architectural solution, protection from adverse weather conditions, and reliable thermal insulation, but it is also an optimal sound-insulating property (Sound reduction index, Rw) that can be improved up to 15 dB [169]. One of the widely used approaches in reducing airborne noise transmission is by the installation of a denser acoustic mineral wool wall, as shown in Figure 12 [21]. This dense mineral wool slab shows an excellent SAC and also acts as an additional layer to the structural concrete.

The insulation layer is usually installed in addition to the insulation quick, which maintains the low-level coupling between the two skins. It also maintains the thermal properties of the structural concrete. Meanwhile, the usage of a mineral wool acoustic wall can compromise with the acoustic performance due to its higher degree of coupling [98]. However, the additional mass provided by the acoustic mineral wool wall can reduce the impact noise propagated through the structure during rains [21].

permission from DATA STEEL [21].

printed with permission from ECOSISM [167].

Finishing materials

Reinforcement bars

Core layer

**Figure 11.** Model for the construction of reinforced concrete walls with extremely high acoustic insulation [167]. Re-

duce the impact noise propagated through the structure during rains [21].

*Materials* **2021**, *14*, x FOR PEER REVIEW 18 of 28

**Figure 12.** Effect of the of wall's thickness built-up on acoustic insulation [21]. Reprinted with permission from DATA STEEL [21]. **Figure 12.** Effect of the of wall's thickness built-up on acoustic insulation [21]. Reprinted with permission from DATA STEEL [21]. *Materials* **2021**, *14*, x FOR PEER REVIEW 19 of 28

The insulation layer is usually installed in addition to the insulation quick, which maintains the low-level coupling between the two skins. It also maintains the thermal properties of the structural concrete. Meanwhile, the usage of a mineral wool acoustic wall can compromise with the acoustic performance due to its higher degree of coupling [98]. However, the additional mass provided by the acoustic mineral wool wall can re-

Insulation layer

Horizontal studs

Vertical studs

#### *6.3. Masonry Wall System 6.3. Masonry Wall System*

The compositional nature of the construction of the three-layer facade, combined with a brick wall, allows combining materials with excellent sound-absorbing and soundreflecting properties (Figure 13) [170,171]. The mineral wool layer located inside has good sound absorption, which reduces the noise load by more than two times (TL and Rw), especially from low-frequency sounds [172,173]. The compositional nature of the construction of the three-layer facade, combined with a brick wall, allows combining materials with excellent sound-absorbing and sound-reflecting properties (Figure 13) [170,171]. The mineral wool layer located inside has good sound absorption, which reduces the noise load by more than two times (TL and Rw), especially from low-frequency sounds [172,173].

**Figure 13.** Masonry wall system [174]. Reprinted with permission from ORION-SD [174]. **Figure 13.** Masonry wall system [174]. Reprinted with permission from ORION-SD [174].

#### *6.4. Concrete Sandwich Panels 6.4. Concrete Sandwich Panels*

[177].

A study by Frazao et al. [175] developed innovative structural panels, a combination of sisal fiber–cement composite as the thin outer layer, and a polypropylene-reinforced lightweight concrete as the base layer. Such multilayer composition makes it possible to efficiently perform sound-insulating functions by distributing sound-absorbing and sound-reflecting functions between the layers. Experimentally determined layer thicknesses and their sequence provide a sound reduction index Rw up to 50 dB [175]. Cuypers and Wastiels [176] made sandwich panels with outer layers of textile-reinforced concrete. Due to the fact that textile concrete is a light type of concrete, this allows for the creation of the effective sound absorption of a wave's incident on the wall, both outside and inside. In this case, the sound reduction index Rw of the developed design is 45 dB. Good acoustic properties (Rw = 40 dB) are predicted for multilayer panels with corrugated steel surfaces and a core of plain or reinforced foam, as developed by Flores-Johnson and Li A study by Frazao et al. [175] developed innovative structural panels, a combination of sisal fiber–cement composite as the thin outer layer, and a polypropylene-reinforced lightweight concrete as the base layer. Such multilayer composition makes it possible to efficiently perform sound-insulating functions by distributing sound-absorbing and soundreflecting functions between the layers. Experimentally determined layer thicknesses and their sequence provide a sound reduction index R<sup>w</sup> up to 50 dB [175]. Cuypers and Wastiels [176] made sandwich panels with outer layers of textile-reinforced concrete. Due to the fact that textile concrete is a light type of concrete, this allows for the creation of the effective sound absorption of a wave's incident on the wall, both outside and inside. In this case, the sound reduction index R<sup>w</sup> of the developed design is 45 dB. Good acoustic properties (R<sup>w</sup> = 40 dB) are predicted for multilayer panels with corrugated steel surfaces and a core of plain or reinforced foam, as developed by Flores-Johnson and Li [177].

In urban apartment buildings, ensuring the sound insulation of floors between floors is of great importance [63,178]. To do this, it is necessary to lower the level of impact noise through the usage of various damping additives in the ceiling, for example, rubber [179]. The transmission of impact sound through hard-walled ceilings is greatly

Traditionally, hollow concrete slabs are utilized to lower the volume of concrete in the slab and to minimize its own weight; however, in this case, the resistance to impact noise is reduced (Rw = 36 dB) [182]. This is due to the fact that, unlike wall structures, the sounds of steps, moved furniture, etc., which are effectively extinguished by massive structures, are transmitted through the ceilings [183]. Al-Rubaye et al. [184] developed a hollow composite reinforcing system with four flanges with an aim to enhance the adhesion of concrete, stabilize holes in concrete elements, and improve sound insulation performance (Rw = 43 dB). It is also reported that the built-up system of Cobute precast

increased (Rw = 45 dB); therefore, rigid joints must be avoided [180,181].

#### *6.5. Reinforced Concrete Slab System*

In urban apartment buildings, ensuring the sound insulation of floors between floors is of great importance [63,178]. To do this, it is necessary to lower the level of impact noise through the usage of various damping additives in the ceiling, for example, rubber [179]. The transmission of impact sound through hard-walled ceilings is greatly increased (R<sup>w</sup> = 45 dB); therefore, rigid joints must be avoided [180,181].

Traditionally, hollow concrete slabs are utilized to lower the volume of concrete in the slab and to minimize its own weight; however, in this case, the resistance to impact noise is reduced (R<sup>w</sup> = 36 dB) [182]. This is due to the fact that, unlike wall structures, the sounds of steps, moved furniture, etc., which are effectively extinguished by massive structures, are transmitted through the ceilings [183]. Al-Rubaye et al. [184] developed a hollow composite reinforcing system with four flanges with an aim to enhance the adhesion of concrete, stabilize holes in concrete elements, and improve sound insulation performance (R<sup>w</sup> = 43 dB). It is also reported that the built-up system of Cobute precast slab (Figure 14) had a 50% reduction on self-weight, dramatically assisted with services distribution, and effectively contributed acoustic insulation (R<sup>w</sup> = 40 dB) [185]. *Materials* **2021**, *14*, x FOR PEER REVIEW 20 of 28 slab (Figure 14) had a 50% reduction on self-weight, dramatically assisted with services distribution, and effectively contributed acoustic insulation (Rw = 40 dB) [185].

**Figure 14.** Details the Cobute precast slab system [185]. Reprinted with permission from Cobute [185]. **Figure 14.** Details the Cobute precast slab system [185]. Reprinted with permission from Cobute [185].

#### *6.6. Steel–Concrete Composite Floorings 6.6. Steel–Concrete Composite Floorings*

In comparison to reinforced concrete slabs, composite floor systems offer a more economical solution (Figure 15) [186]. The constituent structural elements of the reinforced concrete slabs can optimize the strength and performances of the steel and the concrete as well. However, due to the connection of the rigid system "reinforced concrete slab–steel beam", impact and structural noise quite easily find their way to the lower floor. To improve the acoustic characteristics of the floors, various modifications of these composites have been developed, such as slim floor beams [187], Thor and Delta beams [188], composite slim floor beam [189,190], iTECH composite beams [191], and Ultra-Shallow Floor beams [192–194] are the other few composite flooring systems used. Some of these composite flooring systems use a profile steel sheet or small mesh beam [187,188,195]. This provides a better sound insulation to the floor (Rw = 30–50 dB). In comparison to reinforced concrete slabs, composite floor systems offer a more economical solution (Figure 15) [186]. The constituent structural elements of the reinforced concrete slabs can optimize the strength and performances of the steel and the concrete as well. However, due to the connection of the rigid system "reinforced concrete slab– steel beam", impact and structural noise quite easily find their way to the lower floor. To improve the acoustic characteristics of the floors, various modifications of these composites have been developed, such as slim floor beams [187], Thor and Delta beams [188], composite slim floor beam [189,190], iTECH composite beams [191], and Ultra-Shallow Floor beams [192–194] are the other few composite flooring systems used. Some of these composite flooring systems use a profile steel sheet or small mesh beam [187,188,195]. This provides a better sound insulation to the floor (R<sup>w</sup> = 30–50 dB).

Under the conditions of the current world development, noise pollution associated with population growth and industrial development is the main problem faced by people living mainly in municipal areas. This condition highlights the importance for long-term research of new energy materials that can reduce the acoustic power of a sound wave as a result of absorption and reflection. From the literature reviewed, it is found that each

**Figure 15.** Steel–concrete composite floorings [186]. Reprinted with permission from Elsevier [186].

**7. Conclusions** 

Concrete slab

**Figure 14.** Details the Cobute precast slab system [185]. Reprinted with permission from Cobute [185].

*6.6. Steel–Concrete Composite Floorings* 

*Materials* **2021**, *14*, x FOR PEER REVIEW 20 of 28

slab (Figure 14) had a 50% reduction on self-weight, dramatically assisted with services

In comparison to reinforced concrete slabs, composite floor systems offer a more economical solution (Figure 15) [186]. The constituent structural elements of the reinforced concrete slabs can optimize the strength and performances of the steel and the concrete as well. However, due to the connection of the rigid system "reinforced concrete slab–steel beam", impact and structural noise quite easily find their way to the lower floor. To improve the acoustic characteristics of the floors, various modifications of these composites have been developed, such as slim floor beams [187], Thor and Delta beams [188], composite slim floor beam [189,190], iTECH composite beams [191], and Ultra-Shallow Floor beams [192–194] are the other few composite flooring systems used. Some of these composite flooring systems use a profile steel sheet or small mesh beam [187,188,195]. This provides a better sound insulation to the floor (Rw = 30–50 dB).

distribution, and effectively contributed acoustic insulation (Rw = 40 dB) [185].

**Figure 15.** Steel–concrete composite floorings [186]. Reprinted with permission from Elsevier [186]. **Figure 15.** Steel–concrete composite floorings [186]. Reprinted with permission from Elsevier [186].

#### **7. Conclusions 7. Conclusions**

Under the conditions of the current world development, noise pollution associated with population growth and industrial development is the main problem faced by people living mainly in municipal areas. This condition highlights the importance for long-term research of new energy materials that can reduce the acoustic power of a sound wave as a result of absorption and reflection. From the literature reviewed, it is found that each Under the conditions of the current world development, noise pollution associated with population growth and industrial development is the main problem faced by people living mainly in municipal areas. This condition highlights the importance for long-term research of new energy materials that can reduce the acoustic power of a sound wave as a result of absorption and reflection. From the literature reviewed, it is found that each construction material has a different NRC and fundamentally relies on its density. However, to avoid sound waves, in any building materials, as far as possible, mass is required—that is, high-density materials, such as bricks, are best suited for partitions, while cellular concrete is lightweight and inferior to bricks in sound insulation of wave noise. However, it is imperative to note that regarding the structural noise of lower frequency, a cellular concrete isolates better than brick. The construction industries worldwide have started to mainly use the sound-absorbing concrete to reduce the level of sounds in opened and closed areas and increase the amount of sound insulation.

To increase sound insulation, wall structures should be designed as multilayer, including with the use of air gaps. Slab floorings that work as absorbers of structural noise should be designed using damping components, such as chopped rubber, and it is also necessary to prevent rigid "sound bridges". It is proposed to expand the range of measured acoustic characteristics by expanding the lower boundary of the frequency range from 100 to 20 Hz.

However, it is obligatory to consider all structural parameters and design characteristics, when designing concrete structures, such as acoustic insulation property. This is, in particular, to ensure comfortable living conditions for people in residential premises, including acoustic comfort. Different types of concrete behave differently as sound conductors; especially dense mixtures are superior sound reflectors, and light ones are sound absorbers. It is found that the level of sound reflection in modified concrete highly relies on the type of aggregates, distribution and size of pores, and changes in concrete mix design constituents. The sound absorption of AIC can be improved by forming open pores in concrete matrices by either using a porous aggregate or foam agent. To this end, this article reviews the noise and sound transmission in buildings, types of acoustic insulating materials, and the acoustic properties. This literature study also provides a critical review on the type of concretes, the acoustic insulation of buildings and their components, and the assessment of sound insulation of structures, as well as synopsizes the research development trends to generate comprehensive insights into the potential applications of AIC as an applicable material to mitigate noise pollution for increase productivity, health, and well-being. Nevertheless, AIC is deemed as one of the superior materials that should be used in the building constructions, in particular, concert halls, cinemas, theatres, music venues, etc. Several further research investigations are recommended for the production of renewable and green concrete composite;

• New applications of AIC are worth exploring and can be found; for example, EPSbased concrete can be produced as a class of innovative lightweight soundproof concrete.


**Author Contributions:** Conceptualization, R.F., and M.A.; methodology, M.A., R.F., and N.V.; validation, R.F., M.A., T.O., N.V., Y.V. and V.L.; resources, M.A., R.F., and N.V.; data curation, M.A., R.F., Y.V., T.O., and V.L.; writing—original draft preparation, M.A., and R.F.; writing—review and editing, M.A., R.F., N.V., Y.V., V.L. and T.O.; supervision, M.A., and R.F.; Visualization, N.V., Y.V., T.O., and V.L.; Revision, T.O.; project administration, R.F., M.A., and N.V.; funding acquisition, M.A., N.V., and Y.V. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research is partially funded by the Ministry of Science and Higher Education of the Russian Federation as part of World-class Research Center program: Advanced Digital Technologies (contract No. 075-15-2020-934 dated 17.11.2020).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data sharing not applicable.

**Acknowledgments:** The authors gratefully acknowledge the financial support by Deanship of Scientific Research at Prince Sattam bin Abdulaziz University, Alkharj, Saudi Arabia, the Peter the Great Polytechnic University, Saint Petersburg, Russia, and the cooperation of the Department of Civil Engineering, Faculty of Engineering and IT, Amran University, Yemen, for this research.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Abbreviations**


#### **References**

