The effect of porous sound-absorbing concrete slabs on railway noise reduction is examined in this paper. First, the acoustical absorption coefficients of porous concrete materials with various aggregate types, gradations, fibre contents, and compaction indexes are measured in the laboratory. The laboratory results show that porous concrete that uses a composite of expanded perlite and slag as aggregate can not only obtain good acoustical absorption properties but also satisfy mechanical requirements. Also, the gradation of the combined aggregate has a significant effect on the acoustic absorption performance of the porous concrete, with an optimal aggregate gradation of 1~3 mm. Furthermore, the fibre content and compaction index affect both the strength and the acoustic absorption property of the porous concrete, with the optimum value of 0.3% and 1.6, respectively. Then, the findings from the laboratory studies are used to make porous sound-absorbing concrete slabs, which are applied in a test section. The measurements indicate that porous sound-absorbing concrete slabs can significantly reduce railway noise at different train speeds and that the amount of the noise reduction changes roughly linearly with speed when the train is traveling at less than 200 km/h. The maximum noise reduction is 4.05 dB at a speed of 200 km/h.
In recent years, due to accelerated industrialisation, urbanisation, and economic development in China, rail transit, especially high-speed railways, has experienced a rebirth in both development scale and level in China. Compared with ballasted track, ballastless track is advantageous due to its excellent stability, low maintenance, and ability to preserve geometry; therefore, it has been widely used in railway construction in China [
As a result, railway noise is a growing public concern in China, prompting the railway management department to use various techniques and methods to mitigate noise. A noise barrier is one of the most widely used methods in the railway noise treatment field at this time. However, it is very costly to build and maintain a noise barrier, and, in many cases, it generates a strong, uncomfortable visual impact and secondary noise. Furthermore, the central issue of noise barriers is that they do nothing to prevent the noise but merely create local regions of noise reduction [
Porous concrete is a rigid-framed sound absorption material containing open voids and interconnected pores and has been intentionally fabricated for sound absorption [
Many studies have been conducted in the past to investigate noise abatement characteristics of porous concrete and its applications in civil engineering fields [
The research presented herein focuses on an exploratory evaluation of the acoustic absorption of porous concrete using laboratory tests and an evaluation of the noise reduction of sound-absorbing porous concrete slabs using field tests. First, the mechanical and structural requirements of the sound-absorbing slabs are studied in accordance with railway standards. Next, laboratory porous concrete specimens with various aggregates, fibre contents, and compaction indexes are prepared and tested to characterise their acoustic absorption and mechanical properties. Finally, the optimal porous concrete combination is used to manufacture porous sound-absorbing concrete slabs, which are applied in a test section. The railway wayside noise is measured in the test section according to Chinese National Standards (GB 12525-90) and is compared with the noise data obtained from the conventional adjacent section.
Maintaining operational safety is a prerequisite for the application of porous sound-absorbing concrete slabs on railways. Therefore, the mechanical and structural requirements should be adhered to in slab design.
Previous studies have demonstrated that the thickness of the sound-absorbing slab has great influence on its sound absorption performance, especially for low frequencies. A thicker sound-absorbing slab has better sound absorption effects at low frequencies. Therefore, slab thickness should be increased accordingly, within an acceptable range. However, given the distance of approximately 257 mm between the rail head and track slab and that the top surface of the slab cannot be higher than the rail head, the thickness of the slab should be less than 257 mm. Additionally, the maximum vertical compression deformation of fastening systems is approximately 20~30 mm, the largest vertical wear of the rail is 10 mm, and the minimum elevation difference between the slab and the rail head is 10 mm; therefore, the maximum thickness of the slab can be 207 mm. For safety reasons, the thickness of the slab should be 200 mm.
On a railway, the space between the rails should be accessible not only for railway workers performing maintenance but also for emergency work; therefore, the porous concrete sound-absorbing panel must demonstrate good performance as well as sufficient compressive strength. According to Chinese Railway Standards (TB10002.1-2005), vertical load is set at 1.5 kN for calculations. Furthermore, for the cement-based porous sound-absorbing material, the CMT5l05 universal testing machine was used to test its compressive strength, with a 4 cm × 4 cm × 16 cm rectangular column specimen according to Chinese National Standards (GB/T l7671-l999). Using a safety factor of 1.25, the compressive strength should be more than 1.175 Mpa.
When a train travels at high speed, turbulence and strong pressure are produced around the train, while a great negative pressure adhering force is produced in the lower part of the vehicle body. Therefore, the porous sound-absorbing concrete slabs laid on the track slab must have sufficient density. In situ tests and numerical simulations show that the maximum negative pressure of the lower parts of the vehicle body is approximately 423 Pa when the vehicle travels at a speed of 270 km/h. Because the maximum negative pressure is proportional to the square of the train speed, the maximum negative pressure will be 710 Pa when the speed is 350 km/h. Taking the higher speed, crosswinds, installation errors, and other factors into consideration, the maximum negative pressure should be multiplied by a safety factor of 2. Therefore, the density of the sound-absorbing slabs should be greater than 710 kg/m3.
These requirements are the essential prerequisites for the successful development of a porous sound-absorbing concrete slab. The following design of a porous sound-absorbing concrete slab should satisfy these requirements.
To ensure that the porous sound-absorbing concrete slab has enough strength, P.O52.5 cement is used for this study. (P.O means Ordinary Portland Cement. Strength grade 52.5 means that the standard compressive strength value of cement at 28 d is not lower than 52.5 MPa, by the test procedures in standard ISO 679: 1989; 52.5 Mpa is taken as the nominal strength of cement to be used in concrete.) Its main parameters are shown in Table
Parameters of P.O52.5 cement.
80 |
Setting time (min) |
Flexural strength |
|
Stability | |||
Initial setting time | Final setting time | 3 d | 28 d | 3 d | 28 d | ||
|
|||||||
1.5 | 139 | 205 | 4.5 | 7.8 | 25.1 | 55.5 | Qualified |
There are three types of coarse aggregates: expanded perlite, clay ceramsite, and slag, and each type of aggregate is used in three different sizes (number 1 (0–2 mm), number 2 (1–3 mm), and number 3 (1–5 mm)) to prepare all porous concretes. Photos of the three types of coarse aggregates are shown in Figure
Photos of aggregates.
Expanded perlite
Slag
Clay ceramsite
Polypropylene fibres range in length from 8 cm to 18 cm. Polypropylene fibre has many advantages, including chemical corrosion resistance, high wet strength, light weight, small creep and shrinkage, low price, low rates of concrete cracking, a toughening effect, and excellent technical and economic performance. Table
Technical indicators of fibre.
Density (g/cm3) | Compressive strength (MPa) | Elastic modulus (MPa) | Colour | Break elongation ratio | Melting point (°C) | Flash point (°C) |
---|---|---|---|---|---|---|
0.90–0.95 | ≥340 | ≥3500 | White | 10–35% | 165 | 560 |
A commercially available high water reducing agent (density: 1.06 g/cm3), a foaming agent (density: 1.02 g/cm3), and a foam-stabilising agent (density: 0.98 g/cm3) are applied for the porous concretes. A water reducing agent is added to improve the strength and workability of porous concrete. The foaming agent is applied to make cement form open voids and interconnected pores during the hydration process by generating foam via chemical reactions, whereas the foam-stabilising agent helps by increasing the internal friction of cement paste in the flow and then increasing the foam viscosity. As a result, the rupture time of the foam is delayed, and the stability is increased. The amounts of these admixtures are calculated according to cement weight.
The dry mixture is obtained through uniformly mixing cement, aggregates, and other additives in a certain mass ratio. The slurry is adjusted after mixing the water and foaming agent with the dry mixture. After vibrating, moulding, and demoulding, a material with uniform pores is obtained. The samples are first covered with a preservative film for 24 h after demoulding, followed by 28 d to cure under standard conditions. Finally, the sound absorption coefficient and the strength of the samples are examined. The technological process of preparing porous concrete specimens is illustrated in Figure
Technological process of preparation.
In accordance with ASTME 1050-98, the acoustic absorption of porous concrete is evaluated by using a Sheng Wang impedance tube as shown in Figure
Configuration of the impedance tube.
Physical picture of a Sheng Wang impedance tube
Working principle diagram of an impedance tube
The frequency range of interest is limited from 125 Hz to 4000 Hz in this study. A threshold of 125 Hz is established because the acoustic pressures are difficult to stabilise at very low frequencies. The reasons for limiting the high frequency range at 4000 Hz are as follows. (1) The diameter of the impedance tube must be small to sustain a standing wave in the tube for higher frequencies. Due to the aggregate sizes in the material, it is difficult to prepare samples of such small sizes. (2) The range of frequencies in which railway noise is the most objectionable to the human ear is 125~4000 Hz.
Another index noise reduction coefficient (NRC) is introduced when evaluating the sound absorption ability of porous concrete in this study. This value is an average value of sound absorption coefficients at frequencies of 250, 500, 1000, and 2000 Hz. It can be calculated by
In this study, the absorption coefficient and noise reduction coefficient of laboratory-prepared specimens are measured by using standing wave tube techniques, as defined by ISO 10534-2:1998 “Acoustics—Determination of sound absorption coefficient and impedance in impedance tubes-Part 2: Transfer—function method.”
Porous concrete material can reduce noise because there are a large number of voids in the material. When a sound wave enters the material, the air movement is blocked or weakened once it meets the solid walls of these voids. A portion of the sound energy is transformed into heat, which is then dissipated due to the viscosity and heat conduction effects. While a noticeable portion of the sound is absorbed by the porous concrete material, a high proportion of the sound wave is bounced back to the surface and goes into the air, generating noise. This is the process of the transformation and dissipation of sound waves.
To maximise noise reduction, laboratory tests were conducted to investigate the acoustical properties of porous concrete materials using the standing-wave tube method, including tests on various aggregate types, combinations of different aggregates, aggregate grain gradations, fibre contents, and compaction indexes. The frequency of railway noise is 125~4000 Hz. Therefore, the input sound frequency of the standing wave tube was controlled within the range of 125~4000 Hz, and the evaluation was conducted on the acoustic performance of porous concrete material in this frequency range.
To study the effect of aggregate type on sound absorption, the first group of three reference samples with aggregate types of expanded perlite, clay ceramsite, and slag were studied. Cylinders (100 mm diameter and 200 mm height) and rectangular columns (40 mm × 40 mm × 160 mm) were prepared. Three replicates were made for each aggregate type.
The average of the acoustic experiment results from the three replicates is calculated and shown in Figure
Relationship between aggregate type and sound absorption.
Figure There are many pores and apertures inside of the material, and the pores and apertures are small and uniformly distributed. Those pores and apertures are connected with each other. The pores and apertures inside of the material are connected to the outer surface.
Variation of NRC with aggregate type.
Based on the above analysis, the porous concrete with expanded perlite aggregate has the best sound absorption property, followed by the slag; clay ceramsite exhibits the worst sound absorption property.
It can be concluded from the first test group that the expanded perlite is the best choice for porous concrete in terms of sound absorption. However, the density of the product with a single aggregate type of expanded perlite is only approximately 700 kg/m3, less than 710 kg/m3. Thus, to increase the density of the specimen, it would be more practical to apply the composite of expanded perlite and slag as aggregate, for its higher density. In addition, the aggregates can be formed into two layers, with expanded perlite in the upper layer and slag in the lower layer.
A total of twelve combinations were studied: six with 0%, 20%, 40%, 60%, 80%, and 100% slag by volume and the rest with two layers, namely, 0 + 20 cm (the lower is slag, while the upper is expanded perlite), 4 + 16 cm, 8 + 12 cm, 12 + 8 cm, 16 + 4 cm, and 20 + 0 cm. The averages of the acoustic experiment results from the three replicates are illustrated in Figures
Relationship between percent slag by volume and sound absorption.
Relationship between percent slag by thickness and sound absorption.
Variation of NRC with percent slag by volume by mixing method.
Variation of NRC with percent slag by volume with layering method.
Previous studies show that aggregate gradation significantly affects the acoustic absorption property of porous concrete. The third study group was designed with different aggregate gradations of 0~2 mm, 1~3 mm, and 1~5 mm. The average of the acoustic experiment results from the three replicates is calculated and shown in Figures
Relationship between aggregate size and sound absorption.
Variation of NRC with aggregate size.
The effect of fibre was also investigated as a method of increasing the strength of porous concrete. The fourth group was studied with five gradations of fibre content, namely, 0.1%, 0.2%, 0.3%, 0.4%, and 0.5%. Figure
Relationship between fibre content and sound absorption.
Variation of NRC with fibre content.
Figure
Relationship between fibre content and compressive strength.
Compaction index, another important parameter that affects the strength and sound absorption property of porous concrete, was also discussed in this paper. A fifth group was studied with five different compaction indexes: 1.2, 1.4, 1.6, 1.8, and 2.0. The compaction index was calculated using the following equation:
Relationship between compaction index and sound absorption.
Variation of NRC with compaction index.
Figure
Relationship between compaction index and compressive strength.
In summary, the laboratory experiments investigated the effect of aggregate type, aggregate combination, aggregate grain gradation, fibre and compaction index on acoustics, and strength characteristics. The test results showed that the optimum noise absorption could be obtained with the two-layer method, that is, with the combination of 8 cm slag (lower layer) and 12 cm expanded perlite (upper layer) forming a porous sound-absorbing concrete slab, and that the porous concrete material should have an aggregate grain gradation of 1–3 mm, a fibre content of 0.3%, and a compaction index of 1.6. These laboratory findings were then utilised to design a field test section for validation.
To verify the findings from the laboratory research, a test section with porous sound-absorbing concrete slabs was set up at the Suining-Chongqing Railway Line in China, as illustrated in Figure
Mixture design of porous sound absorption concrete slab for the test.
Expanded perlite | Slag | Cement | Water/cement ratio | Polyester fibre | Water reducing agent | Foaming agent | Foam-stabilising agent |
---|---|---|---|---|---|---|---|
35% | 25% | 30% | 0.2 | 0.3% | 0.75% | 1.95% | 1% |
Photo of the test section.
Field core from a porous sound-absorbing concrete slab.
After construction, cylinder specimens were cored from the test cells to test the sound-absorbing property. A typical core is shown in Figure
Average sound-absorbing property of the specimens.
Field measurements at two test sites were conducted with the arrangement shown in Figure
In situ measurement arrangement.
The CRH2-300 vehicle was used throughout the entire experiment at speeds of 80, 100, 120, 140, 160, 180, and 200 km/h to ensure the stability and continuity of the test. The sound-absorbing effect of the concrete slabs at the speed of 140 km/h shown in Figure
Sound-absorbing effect of the sound absorption slab.
The effect of vehicle speed on the noise reduction of the porous sound-absorbing concrete slab was also investigated, and the results are plotted in Figure
Amount of noise reduction at different speeds.
To reduce the noise from the lower part of the train, a porous sound-absorbing concrete slab was developed, and its sound absorption was examined in this study. First, the acoustical absorption coefficients of porous concrete materials were measured in the laboratory, to study the effect of various aggregate types, combinations of different aggregates, and aggregate gradations. Furthermore, the influence of the fibre content and compaction index on sound absorption was examined. Then, the findings from the laboratory study were applied to make porous sound-absorbing concrete slabs that were arranged in a test section. The railway noise in the test section was measured according to Chinese National Standards (GB 12525-90) at different vehicle speeds. The following conclusions can be drawn from the discussion above. The porous concrete with expanded perlite aggregate has the best sound absorption property, followed by the slag; clay ceramsite exhibits the worst sound absorption property. For the two-layer method, the combination of 8 cm slag (lower layer) and 12 cm expanded perlite (upper layer) is better than the other combinations; for the 8 + 12 cm combination, the maximum NRC is 0.623 and the density of the specimen is approximately 820 kg/m3. The aggregate gradation had a significant effect on acoustic absorption properties of porous concrete, with an optimal aggregate gradation of 1~3 mm. Fibre has an effect on not only the strength of porous concrete but also its acoustic absorption property. The optimum fibre content is 0.3%. Similarly, the effect of the compaction index was studied. Compressive strength increases with the compaction index. Therefore, the best compaction index is 1.6. Porous sound-absorbing concrete slabs can significantly reduce railway noise at different vehicle speeds, and the amount of the noise reduction changes roughly linearly with the vehicle speed when the train is traveling at less than 200 km/h. The maximum reduction measured was 4.05 dB at the speed of 200 km/h.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This research is sponsored by the National Natural Science Foundation of China, no. U1234201. This support is gratefully acknowledged. The results and opinions presented are those of the authors and do not necessarily reflect those of the sponsoring agencies.