Effective vehicle noise reduction with two layer porous asphalt

Transcription

1 Effective vehicle noise reduction with two layer porous asphalt Gijsjan van Blokland M+P, The Netherlands Bert Peeters M+P, The Netherlands Summary Two layer porous asphalt was developed in the Netherlands about 30 years ago by contractor Heijmans and M+P Consulting engineers as a high-end noise-reducing pavement. Since then, it has been thoroughly tested and improved in several Dutch as well as international studies. The road pavement exhibits a significant noise reducing performance, not only in new condition but also over its entire lifetime. It is now used as the preferent measure to reduce traffic noise on Dutch motorways, covering about 1000 km (17%) of the highway system. Its area of application is extending to countries outside Netherlands.. This paper gives an overview of the construction principles, it explains the mechanisms that account for the acoustic performance and it distinguishes between its effect on cars and on trucks, for both rolling noise and powertrain noise. The paper presents data on the aging effect and relates it to traffic conditions. The paper formulates developments that will result in further improvements in both new conditions and during its life time. 1. Introduction It is generally acknowledged that the road pavement exhibits a large potential to suppress road traffic noise. Developments and applications of noise reducing road surfaces are found over all Europe [1]. One type that is found particularly effective is the two layer construction of porous asphalt concrete, the Double Layer Porous Asphalt (DLPA) construction. This paper presents an overview of the underlying noise reducing mechanisms, presents data on its effect on different categories of road vehicles and addresses a general concern with noise reducing pavements, namely that they age faster than a reference pavement. 2. Constructive concept of DLPA The DLPA concept consists of a fine graded top layer that acts as a sieve and a course graded sublayer through which drainage water can flow to the side of the road. Dirt on the surface is either washed away through the surface or remains on top and is then washed away without clogging the surface. The combination of the fine top layer and course sub layer prevent dirt to stuck in the system (see Figure 1). Figure 1 Principle of the anti-clogging mechanism of DLPA. Essential is that the top layer is of a finer gradient than the sub layer. An additional advantage is that it prevents the buildup of water in the porous system. In case of single layer porous asphalt (SLPA) system this might take place after some clogging. The existence of water in SLPA not only affects the absorption properties but even more important, may cause major winter failure of the surface when due to a series of frost/thaw cycles large parts of the surface may break out, causing pot holes. Experiences over several decades in the Netherlands have demonstrated that the DLPA Copyright 2018 EAA HELINA ISSN: All rights reserved

2 concept works. Winter failure of DLPA is significantly less than that of SLPA. Its effectivity in preventing dirt build up is indirectly proven by its better preserving of its acoustic performance over time. The application of open surfaces is mainly on highways and there dirt build up is already less due to the high speed passing of car tyres causing a vacuum cleaning effect. feasible when the absorption effect is located at a slightly higher frequency. 3. Effect of pavement on traffic noise The importance of the pavement for the amount of traffic noise is depicted in Figure 2. It presents passby levels of a large population of vehicles on two pavement types. It can be seen that the most silent passage on the concrete pavement is more noisy than the noisiest passage on the low noise pavement. This effect is also found at low speeds, indicating that it holds for tyre noise and for powertrain noise. S.P.B. level [in db(a)] 89,0 84,0 79,0 74,0 69,0 Brushed concrete drain asphalt 0/8, 80 mm thick 64,0 50 v [in km/h, log scale] 150 Figure 2 Pass-by levels at fine brushed cement concrete (CC) and DLPA (PAC) over a speed. Data composed for measurements at several locations. The green arrows indicate the variation found between the pavements, the red and blue arrows within the pavement type. 4. Understanding pavement effects on vehicle noise The noise reducing effect of a pavement can be explained by its surface characteristics, texture, flow resistance and acoustic absorption. The graphs in Figure 3 depict the effect of variations in each of the three characteristics relative to an SMA 0/8. Figure 3c presents the optimal choice of a rather smooth texture, a low flow resistivity and a wellplaced acoustical absorption. Compared to the most noisy version, 5/8 mm surface dressing, a reduction of 10 db is achieved. An even better result is Figure 3 Coast-by levels (LAmax) of cars at 80 km/h on varying surface types. All types exhibit a maximum gradient of 8 mm. Fig 3a (top) compares SMA 0/8 with a surface dressing of 5/8 mm. The effect of texture is clearly visible below 1 khz. Fig 3b (mid) compares SMA 0/8 with a semi-open layer TSL0/8. It shows the effect of lowering flow resistance. In fig 3c (bottom) the added effect of acoustic absorption is shown for 40 and 70 mm thickness. For the rolling noise of heavy duty vehicles (HDV s) a similar effect is found although here the effect of the tyre type becomes more relevant. The graphs in Figure 4 show the texture and the absorption effect for a vehicle consisting of two steer tyres, four traction tyres and four trailer tyres

3 This effect can be explained by the fact that most of noise of the power train propagates to the side of the road by multiple reflections between the car under side and the road surface (see Figure 5) Figure 5 Scheme of propagation of power train noise to the road side by multiple reflection at the road surface 6. Acoustic absorption of grain like materials Figure 4 Coast-by levels (SEL) at 70 km/h of a composite truck with 2 steer, 4 traction and 4 trailer tyres. Average over four different manufacturers. Fig 4a (top) presents the effect of texture. Figure 4b (bottom) the effect of flow resistance and acoustic absorption. SLPA indicates a single layer porous asphalt of 40 mm thickness. Both SLPA and DLPA have a top layer with 8 mm max grading For a porous pavement there exists an inverse relation between the total thickness and the frequency of the maxima in the spectral distribution of the absorption. Doubling of the thickness cause halving of the frequency of the lower (and other) absorption peaks. In Figure 6 the spectrum is presented for two common thicknesses, 40 mm for a single layer surface and 70 mm for a double layer surface. The results in Figure 4 show a similar behaviour for truck tyres as for car tyres. The different appearances are mainly caused by the averaging over rib and block patterns and the SEL metric versus the LAmax metric. The main effect is that truck tyres, especially the traction types are very insensible to texture changes, Also here the largest reduction effect is found for DLPA. Since for truck tyres the relevant spectral bands are found at lower frequencies, the DLPA absorption effect is well-placed. Compared to the SMA 11 surface the DLPA reduces rolling noise with nearly 7 db(a). 5. Effect for power train noise The graphs in Figure 3 and Figure 4 present coastby events with the engine switched off or idle. It can be concluded from Figure 2 that a significant effect of DLPA can also be expected for power train noise. Figure 6 Spectral distribution of acoustic absorption for a grain like material. Flow resistivity: 20Ns/m4, porosity 20%. Top: 40 mm thickness, bottom: 70 mm thickness Decreasing porosity has as main effect the narrowing of the absorption peaks. For a grain like

4 material the maximum achievable porosity is in the order of 25%. For a given porosity the flow resistance can be varied by varying stone size. Smaller sizes lead to increased resistance. The effect of resistance is mainly a variation in the height of the peak. noise reducing performance was studied for a wide range of road surface types and for several countries in Europe. The study included time related data from several DLPA sections in the Netherlands, Germany and Spain over a period of up to 10 years. 7. Noise reducing effect of DLPA DLPA is constructed with two gradings of the top layer, 4 to 8 mm and 2 to 6 mm. The latter is referred to as DLPA-fine. For both types the total thickness is 70 mm. The graphs below show the overall road traffic noise reduction relative to a dense asphalt concrete pavement (ACsurf 11). 4 effect relative to reefernce pavement [db] speed [km/h] Double Layer Porous asphalt 4/8-11/16 70 mm effect relative to reefernce pavement [db] speed [km/h] SMA-NL5 Thin Surface Layer 8 mm Fine Double Layer Porous asphalt 4/6-11/16 70 mm Porous asphalt concrete 6/16 40 mm transversely brushed cement concrete Surface dressing 6 mm Figure 8 Repeated SPB measuremenst at a series of DLPA sections in the Netherlands. Top: data for cars, bottom: data for trucks. The avarge increase/year of SPB levels is 0,38 db/yr for cars and 0,36 db/yr for trucks -6 Figure 7 Overall traffic noise reduction of several types of road surfaces relative to an ACsurf 11. The light blue and green curve indicate the DLPA and DLPA-fine pavements. Top: cars, bottom: trucks. Thin surface layer perform well for cars, but fail for trucks. SLPA 6/16 fails for cars, especially at lower speeds The graphs in Figure 7 show the superior reduction of the DLPA and DLPA-fine type for both cars and trucks. This can be explained by the combination of a smooth texture, important for cars and the rib tread profile found at steer and trailer tyres of trucks, and the high acoustic absorption that suppresses both the horn amplification of the tyre and the emitted power train noise. 8. Aging of the acoustic performance Porous road surfaces are susceptible to losing their noise reducing performance over time quicker than dense types. In the QUESTIM project [2] aging of In Figure 8 the results of repeated measurements at about 25 test sections on Dutch highways are shown. An average yearly increase of pass-by levels of about 0,35 db/yr was found. Aging of individual sections varied significantly. At some locations (A28 near Staphorst) slopes around 0,2 db/yr. were observed. At another location (A30 near Ede) slopes were around 0,5 db/yr. The spectral changes in the SPB events found at both locations give insight in the aging mechanisms (see Figure 9). For the sections at the A28 location both absorption and openness is preserved over a period of 8 years while at a comparable site (the A30 at Ede) the porous structure of the test sections are nearly lost. One observes a difference in build-in geometry that can explain the differences in acoustic performance over time. The A28 geometry allows an easy

5 outflow of water from the construction, while the A30 with its grass shoulder, is more locked-in (see Figure 10). 9. Further developments DLPA belongs to the 2 nd generation of noise reducing surfaces that at the moment is already 25 years old. The 3 rd generation will use the know-how acquired since then to generate a second break through. One can define two challenges. The first challenge is how to extend the effective frequency range of acoustic absorption. When the narrow bands found with grain like materials (see Figure 6) van be broadened, not only better overall reduction can be achieved, but also the tuning to either cars or truck tyre noise can be omitted. The second challenge is the implementation of elasticity in the construction. The Sperenberg project [3] already demonstrated its potential and the PERSUADE project corroborates the findings at some road tests. The durability issue has not yet been solved. In the Dutch USW (ultra silent road surfaces) project scientists, road authorities and road builders are working to develop solutions that incorporates broader absorption bands and elastic top layers with an extended life time. 10. Conclusions Figure 9 Spectral changes of SPB for cars at the A28 and the A30 location. The spectral change at the A28 location indicates a slight decrease of acoustic absorption an preservation of the openness of the surface. For the A30 a nearly complete loss of acoustic absorption and openness can be concluded. Figure 10 Top: laying geometry of the DLPA construction at the A28 site, bottom: at the A30 site. The geometry at the A28 site allows a quicker drainage of water compared to the A30 site, possibly explaining the differences in clogging. The aging data refer to the condition of the slow lane of a 2x2 or 2x3 lane highway. Conditions of the medium and fast lane remain better due to the lower intensity of heavy vehicle transport. Double Layer Porous Asphalt (DLPA) has demonstrated its effectiveness in reducing the sound production of road traffic on high ways and regional roads over a series of years. It exhibits superior acoustic performances compared to other noise reducing surface types like TSL and SLPA for both car and truck noise. The noise reducing capabilities remain over a period of at least 10 years. DLPA is in the Netherlands the standard solution for noise hotspots along highways and, even after taking into account the more frequent maintenance compared to a standard ACsurf, it shows a lower LCC than noise barriers. The standard DLPA type has a top layer of 4/8 mm. An extra 1 db effect can be obtained by the application of a 2/6 mm top layer. The basic of DLPA enables further optimization with a potential of an 8 to 10 db noise reduction for a mixed traffic stream compared to a standard ACsurf

6 References [1] CEDR, State of the art in managing road traffic noise: noise-reducing pavements,, Technical Report , downloadable at [2] G.J. van Blokland et al., Modelling of Acoustic Aging of Road Surfaces, QUESTIM project, Deliverable D2.2, downloadable at project website [3] Sperenberg project, Einfluss der Fahrbahntextur auf das Reifen-Fahrbahn-Geräusch, Report nr. FE /1995/MRB, Müller-BBM/M+P