Acoustic Properties of Cenosphere Reinforced
Cement and Asphalt Concrete

Abstract A detailed experimental study has been conducted to determine the effect of adding hollow ceramic-micro balloons, also known as cenospheres, on the acoustic properties of cement matrix and asphalt concrete. The motivation for this study was to explore the feasibility of using cenospheres in developing lightweight sound-absorbing structural materials. Cement and asphalt concrete specimens with different volume fractions of cenospheres and varying diameters and thickness were tested to determine their acoustic characteristics over the range of frequencies (0–4000 Hz). Experimental results showed that a 40% volume fraction addition of cenospheres to the cement matrix increased the Noise Reduction Coefficient by 100%. In contrast to cenosphere-rich cement, the sound absorption coefficient of asphalt concrete decreased with an increase of cenosphere, volume fraction. #2003 Elsevier Ltd. All rights reserved. Keywords: Cenospheres; Cement; Asphalt; Sound absorption coefficient; Lightweight materials; Noise reduction coefficient

1. Introduction Noise is becoming an increasinglysignificant concern because of its adverse effects on the lives of manypeople in urban societies and in rural areas. Noise arising from different sources such as vehicles, aircrafts, power plants and machineryis not only uncomfortable but also hazardous to health. These concerns have led to major developments in the field of sound absorbing materials. For homogenous and isotropic materials, acoustic performance is defined bya set of experimentallydetermined constants, namely: absorption coefficient, reflection coefficient, acoustic impedance, propagation constant and noise reduction coefficient (NRC). In recent years extensive research studied the usages of various types of materials in making sound barriers. Wolfe et al. [1] investigated the usage of cement-wood composite as highwaysound barriers and established the effectiveness of such sound absorbing materials in reducing traffic noise. Watts et al. [2] studied the effects on roadside noise levels byapplying sound absorptive materials to the traffic face of noise barriers. Watts et al. [3] also studied the combined effects of porous asphalt surfacing and noise barriers on traffic noise using the boundaryelement method. Significant research efforts have also examined noise reducing mechanisms in order to develop new design recommendations for pavements. Iwase et al. [4] performed an estimation of road traffic noise reduction bymeasuring the basic acoustic properties of porous pavement. Yamaguchi et al. [5] studied the sound absorption mechanisms of porous asphalt surface bycomparing it with other porous sound absorbing materials, such as mineral wool and synthetic foam. They also investigated the relationship between void ratio and sound absorption properties of porous asphalt pavement. Meiarashi et al. [6] and Oshino et al. [7] studied noise reduction mechanisms and characteristics of asphalt pavement, and proposed a model for predicting vehicle noise propagation on asphalt pavement. However, studies involving the use of cenospheres for sound absorption applications have not yet been reported. In this work the effect of the addition of cenospheres on the acoustic properties of cement matrix and asphalt concrete was studied to determine the feasibilityof using cenosphere enrichment as a counter measure against noise. Cenospheres [8] are hollow micro balloons made of aluminum silicate. Being a waste product of thermal power plants, theyare relativelyinexpensive and their use has the added benefit of decreasing the strain on the environment. The focus of this studyis to characterize the sound absorption parameters of cement matrix and asphalt concrete as a function of cenosphere content and material thickness over the frequencyrange of 0– 4000 Hz, and to identifythe optimum amount of cenospheres required to maximize sound absorption.

2. Theoretical considerations The absorption of sound results from the dissipation of the sound energyas heat. The dissipation mechanisms are mainlydue to one of two phenomena. The first is the energyloss due to flexural vibrations in the specimen. The second is porosity effects, where energyis dissipated due to multiple reflections of sound waves within the voids in the structure. For most porous materials like synthetic foam and mineral wool with interconnected pores, incoming sound is reflected within the pores, causing them to vibrate and convert sound energyinto heat.