acoustical and thermal properties of porous concrete - sustainable

MOURRE Guillaume

Material Science Engineering, 4th year

Acoustical and Thermal Properties of Porous Concrete

Acoustics and Noise Research Group, University of British Columbia, Vancouver

Internship mentor: Pr. HODGSON Murray Internship supervisor: Mr. BEAUGNON Eric

Year 2012/2013 From May 13th to August 2nd

Acknowledgements

First and foremost, I would like to acknowledge my internship mentor, Pr Murray Hodgson for

everything during the internship. He was available and helped me a lot to do progress on my project. I

also thank Mr Beaugnon for having accepted to be my internship supervisor and for his follow-up during

the internship.

I also would thank my acoustic lab co-workers for their good moods which has created conviviality in the

lab: René, Mina, Kyle, Li Yi-Zhong and Vivek. I especially would like to thank Banda, who dedicated a lot

of his time to my project. Then, many thanks to Mrs Margaret Gardiner, the laboratory administrator for

her help before, during, and after my internship.

I thank people from Civil Engineering Department: Pr Nemkumar Banthia, Dr Cristina Zanotti, Negar

Roghanian, Sudip Talukdar, Jane Wu and Ricky N Ratu for the help they provide me and the time they

dedicated to my project. I especially thank Jean-Lou for his help and for some nice travels we did.

Even if tests were not conclusive, I thank Marcus Schmieder from Metro Testing for thermal tests.

Finally, last but not least, I sincerely thank my family and my friends for their support during all the

internship.

Table of contents

Introduction .......................................................................................................................... 1

I. Context of the project ..................................................................................................... 2

1) Location of the internship ................................................................................................................. 2

2) Internship steps ................................................................................................................................. 3

3) Previous work .................................................................................................................................... 3

a) Properties of porous materials ...................................................................................................... 3

b) Improvements of existing thermal slabs ....................................................................................... 4

c) Concrete material .......................................................................................................................... 5

i. General properties and composition ........................................................................................ 5

ii. Mechanical properties and setting time ................................................................................... 6

iii. Acoustical properties ................................................................................................................. 7

iv. Thermal properties .................................................................................................................... 7

d) Propagation of sound and prediction models ............................................................................... 7

i. Propagation of a sound wave .................................................................................................... 7

ii. Parameters ................................................................................................................................ 8

iii. Acoustic and non-acoustic parameters ..................................................................................... 8

iv. Common models ....................................................................................................................... 9

3) Creation of a new type of concrete; requirements and samples .................................................... 10

a) Properties targeted ..................................................................................................................... 10

b) Fibres characteristics ................................................................................................................... 10

c) Samples created .......................................................................................................................... 11

II. Techniques used ............................................................................................................ 14

1) Impedance tube .............................................................................................................................. 14

a) Principle of the measurement ..................................................................................................... 14

b) Measurement protocol ............................................................................................................... 15

c) Determination of acoustic and non-acoustic parameters .......................................................... 16

2) Air flow resistivity apparatus ........................................................................................................... 17

3) Compression test ............................................................................................................................. 17

4) Study of thermal properties of porous concrete ............................................................................ 18

III. Results and discussion .................................................................................................... 20

1) Acoustical properties: absorption and transmission ....................................................................... 20

a) Absorption properties, general behaviour .................................................................................. 20

b) Time effect on acoustical properties ........................................................................................... 23

c) Fibre effect on acoustical properties........................................................................................... 23

d) Effect of side ................................................................................................................................ 24

e) Effect of segregation ................................................................................................................... 28

f) Concrete parameters influence on acoustical properties ........................................................... 29

g) Comparison with another sound insulating concrete material................................................... 29

2) Models and parameter determination ............................................................................................ 30

a) Air flow resistivity determination ................................................................................................ 30

b) Porosity ........................................................................................................................................ 32

c) Unknown parameter determination – Attenborough model ..................................................... 33

3) Mechanical properties: compression tests ..................................................................................... 35

Final conclusion .................................................................................................................... 36

Future prospects ................................................................................................................... 37

Personal impressions ............................................................................................................ 38

References ............................................................................................................................ 39

Appendix .............................................................................................................................. 40

Table of illustrations and tables .............................................................................................. I

1

Introduction

My second-year internship took place at UBC, the University of British Columbia, in Vancouver,

Canada, and more especially in the ANRG (Acoustics and Noise Research Group). The project was carried

out in cooperation between my laboratory and the Civil Engineering Department of UBC. In this way, my

project was supervised by Dr. Murray Hodgson, my supervisor, and Dr. Nemkumar Banthia, from the Civil

Engineering Department.

Today, concrete radiant slabs inside buildings must meet several requirements. To ensure the well-being

of inhabitants and to decrease energy consumption, acoustical and thermal properties are important. In

this way, the goal of my internship was to measure and characterise the acoustical and thermal

properties of a porous concrete, which is supposed to be sound absorptive. One of the major constraints

of the project was to achieve sound-absorptive properties without deteriorating the thermal properties.

Indeed, concrete must conduct heat and also provide good acoustic insulation. The main idea of this

project was to put steel fibres inside the porous concrete to enhance the heat conduction.

This project being multidisciplinary, two departments were linked, to share knowledge. On the first hand

the ANRG for characterisation with the impedance tube and on the other hand the Civil Department for

the concrete side, the mix design and the study of concrete.

This report will be made up of three parts. The first part will summarize the previous works on the

subject and will describe all the parameters and models used for sound propagation description that we

will need further. In the second part we will focus on the different apparatuses used in this study. Lastly,

in the third part, all the obtained results will be discussed and future prospects will be detailed.

2

I. Context of the project

1) Location of the internship

The internship took place in the CIRS (Centre for Interactive Research on Sustainability) and brings

together the ANRG and the Civil Department, two laboratories at the University of British Columbia.

The ANRG, headed by Pr Murray Hodgson, is a research laboratory in the acoustic field. The studies are

all linked to the study of, its propagation, its interaction with obstacles and its effects on people. The

goal of these studies is to optimise the acoustical environment of buildings to ensure the comfort of

inhabitants. In this way, the ANRG develop predictions models, control and measurements tools.

The ANRG is directly linked with the NSERC CREATE Sustainable Building Science Program (SBSP), a

research and training program also headed by Pr. Murray Hodgson, which focuses its activity on building

sustainability. The aim of the SBSP is to develop ecologically friendly buildings with green materials.

Research subjects are various, but all are on building properties, interaction between inhabitants and

building, building design and development of sustainable technologies.

These two laboratories are located in the CIRS building which is a remarkable building. Opened in 2011,

the CIRS is known to be the most sustainable building is the North America and one of the only buildings

in the world with positive effects on the environment. Indeed, all of the building was though in

agreement with sustainability; hence the solar panels on the roof, the wood structure and the green wall

in front of the building, which allow to regulate the inside temperance all year long.

Lastly, this study was carried out in cooperation with the Civil Engineering Department which provided

its knowledge about concrete. This laboratory focuses its research activity on all of the field of concrete,

fibres in concrete, green concrete, corrosion of reinforcement bars, etc.

Figure 1. CIRS building, outside and inside, UBC Campus, Vancouver.

3

2) Internship steps

The internship is only the foundation of a study which will be continued in the future. Consequently, this

project will be the initial study and will be a reflection of the properties we want to achieve with this kind

of concrete. The first prototypes will be cast and characterised with the aim of improving the properties

in the future. The steps of the internship will be the followings:

Reflection and bibliography study on the different properties we want to obtain

Casting of the samples

Acoustical and Thermal characterisation

Sound absorption modelling

Analysis of the results, conclusions and future work.

3) Previous work

a) Properties of porous materials

Porous materials are often used for their sound absorptive properties. For instance, fibre glass, a

fibrous material, is used in anechoic chambers (special rooms used to block the outside noise and to

decrease significantly the reverberation time).

This sound absorptive property rests on the intern structure. Indeed, pores permit a part of the

incident wave to be absorbed and, consequently, they permit the sound to be reduced [1]. Nevertheless,

a difference must be made between open and closed pores.

Figure 2. Open and closed pores, two different behaviours.

[J.P. Arenas, M.J. Crocker, “Recent Trends in Porous Sound-Absorbing Materials”, Sound & Vibration, Materials Reference Issue, 2010]

4

Indeed, closed pores are not as efficient as open pores concerning sound absorption. In this way, they

will not be considered in this report. Contrary to closed pores, open pores interact with the fluid phase

(air in our case) [1] and will trigger energy dissipation. These energy losses can have three origins [2]:

- Losses caused by viscous effects: energy dissipation by friction between air and pores (more

precisely between air and porous skeleton)

- Losses caused by thermal effects: energy dissipation by heat exchange between air and skeleton

- Losses caused by viscoelastic effects: energy dissipation caused by internal friction of the matrix

in distortion.

Losses caused by viscous effects are the most efficient. Sound waves lose energy and, in this way, the

sound attenuation is better. Pore size cannot be overlooked; when the pore size decreases, the

absorption coefficient increases. Indeed, the more the pores are narrow, the more collisions will occur

between the incident wave and the pore walls. [3]

With the same reasoning, sound absorption will be better with a higher surface area. Sound absorption

in porous concrete depends on the friction between incident sound waves and the pore walls.

b) Improvements of existing thermal slabs

One of the main ways to heat a room is to put inside the slab pipes. Made of copper in the past, they are

now made of PEX (cross-linked polyethylene) to avoid corrosion. These pipes are linked to a water heater

and a heat transfer medium (essentially water) which circulates in the pipes. This fluid exchanges calories

with the slab. After that, heat propagates in the slab and heats the room.

Thermal slabs must be able to meet two special requirements: they must be heat conductors in order to

optimise the energy efficiency, and be sound absorptive. Concrete is a commonly used material due to

its high thermal conductivity and capacitance.

As explained in the previous part, porous concrete could be used, but the porosity must be high enough

(usually the porosity of sound absorbing materials is higher than 0.9). Nevertheless air, being a low heat

conductor, porosity will deteriorated the thermal properties of the slab. The main challenge is to create

enough porosity to make the slab sound absorptive but not deteriorate too much the thermal

properties.

Figure 3. Simplified drawing of a thermal slab.

Floor

Ceiling

Floor 2

Porous

concrete

Pipes

Reinforced concrete

Floor 1

5

Table 1. Different thermal conductivities at 20°C.

Material Thermal conduction at 20°C (W.m-1.K-1)

Air 0,0262

Concrete 0.74 [4]

Stainless steel 26

The solution tested in this report is to put steel fibres inside the concrete porous matrix to achieve at the

same time optimal sound absorption and thermal conduction properties. These fibres, by creating a kind

of fibrous network inside the structure, should increase the thermal conductivity and allow better energy

efficiency. Even if the mechanical properties are not directly investigated in this report, the fibre’s role

on the compression properties will be studied.

Figure 4. Simplified drawing (section) of concrete with fibres.

Moreover, even if thermal conductivity is the main parameter concerning thermal slabs, thermal storage

(capacitance) is also a key parameter.

c) Concrete material

Concrete is, with steel, the most used material in the field of building. It is particularly liked because of its

ease of use and its good mechanical properties in compression. Nevertheless, the concrete created in

this project will be very weak, because of its porosity, and the concrete will not be adapted for use as a

foundation. Whereas usual concrete cast in the Civil Department is very strong and non-porous, the

concrete considered in this report will be very porous and not strong

i. General properties and composition

The usual concrete is made of the following elements [4]:

- Cement (made of lime, silica, aluminium oxide and iron oxide)

- Water, which is necessary to the hydration reaction

- Aggregates

- Sand

- Admixtures (optional) to increase or slow down the setting time

- Fibres (optional) to strengthen the structure or in specific applications

Hot pipe

Steel fibres

Porous matrix

6

Table 2. Main components of the Portland cement.

From A.M. Neville, “Properties of concrete” [4]

Name Formula Abbreviation

Tricalcium silicate 3CaOSiO2 C3S

Dicalcium silicate 2CaOSiO2 C2S

Tricalcium aluminate 3CaOAl2O3 C3A

TetraCalcium aluminoferrite 4CaOAl2O3Fe2O3 C4AF

Table 3. Usual composition of Portland cement.


Oxide Composition (%)

CaO 60-67

SiO2 17-25

Al2O3 3-8

Fe2O3 0.5-0.6

MgO 0.5-4.0

SO3 2.0-3.5

Alkalis 0.3-1.2

Besides the above, silica fumes (SiO2) can be used to “catalyse” the setting speed and improve the

compactness of the final structure because of its small diameter (between 0.03 μm and 0.3 μm).

Portland cement (widely used) is made of the oxides in Tables 2 and 3 [4]:

To create a cohesive structure, a chemical reaction called hydration occurs. This reaction permits to

concrete increase its strength.

ii. Mechanical properties and setting time

Generally, the strength of a concrete structure is supposed to be almost full after 28 days (99% of the

final compression resistance). In this way, this 28 day length of time is always taken as a reference

concerning the hardening of concrete. Nonetheless, concrete is supposed to reach its final strength one

year after ifs fabrication (Table 4.).

Table 4. Mechanical properties evolution with time.


Days after casting Percent of the finale strength

1 16%

3 40%

7 65%

14 90%

28 99%

The compression strength depends on the density of concrete. A usual concrete (2240-2400 kg/m3) have

a compression strength between 20 and 40 MPa (after 28 days) whereas for a “no-fines concretes”, with

7

no sand and no admixtures, this value is only around 10 MPa (after 28 days), depending on the density

and the mix design. With decreasing density, the compression strength decreases too [4].

The mechanical properties of concrete can be improved by putting metal fibres in the structure. By

acting like a Band-Aid, fibres can stop crack propagation. However, fibres must be inserted in a sufficient

quantity to improve these properties, and this requires enough paste. The workability of concrete, which

is the maneuverability, is often tested and consists on the study of the deformation of a cone of concrete

with time. Nevertheless, this test is not feasible with highly porous concrete.

iii. Acoustical properties

Concrete can have two different uses, depending on its porosity. The maximum reflection occurs for a

non-porous concrete whereas concrete, is a good absorptive material when porous. However,

transmission properties must be considered too. Given that the concrete usually used is not porous,

concrete is not considered as being a good sound insulating material. In general, sound absorptive

properties of floors are often ensured by a porous layer on the surface: a carpet or carpeting provide

these properties. Moreover, if concrete is used as a sound absorptive material, no coating can be put on

the surface which could trigger some design issues.

iv. Thermal properties

The two thermal properties considered in this report are thermal conductivity and thermal capacity.

Thermal conductivity is ability of a material to transfer heat, while thermal capacity gives information

about the ability to store energy. Concrete has a high specific heat (cp) which gives it, because of a high

mass (m), a high thermal capacity.

The thermal conductivity of concrete also depends on its density and porosity, air being a very low

thermal conductivity material. For instance, an aerated concrete has a thermal conductivity around

0.109 W/m.°C, while the thermal conductivity of a usual concrete is more than 2 W/m.°C. Thermal

conductivity also depends on moisture content, which enhances thermal transfer.

d) Propagation of sound and prediction models

i. Propagation of a sound wave

When a sound wave strikes a material, part of this wave is reflected, part is absorbed by the material and

part is transmitted (Figure 5).

8

Figure 5. Simplified drawing of the sound wave propagation.

ii. Parameters

The widely used parameters in acoustics to describe sound propagation in a medium are the following:

is the complex surface impedance of the material. It can be determined with an impedance tube and

depends on frequency.

is the characteristic impedance of a medium. This parameter gives us the complex resistance of a

medium to the transmission of a wave. Zc depends on frequency.

is the complex reflection coefficient at normal incidence. It is a complex number because it comes

directly from transfer function obtained with the impedance tube.

is the energy absorption coefficient at normal incidence. It characterises the sound attenuation of a

material depending on the frequency of the incident wave. This is the most important property to

characterise sound absorptive materials.

iii. Acoustic and non-acoustic parameters

Parameters obtained with the impedance tube are of two types: acoustic and non-acoustic parameters.

Acoustic parameters allow us to describe the absorption and transmission properties of a material. Non-

acoustic parameters are not directly linked to acoustics, but they can be obtained with the impedance

tube [5].

φ is the porosity, which corresponds to proportion of the volume of the material occupied by pores.

is the tortuosity. In a porous structure, open pores create paths ways where fluids can go. These

pathways can be very complex, with curves. Tortuosity gives information about the linearity of these

paths. If the way is straight, tortuosity equals 1, and consequently, 1 is the lowest value for tortuosity.

Λ, the characteristic length (viscous length) is a weighted ratio of the volume to surface area of the

pores. It is weighted according to the squared modulus of the microscopic velocity evaluated, including

the effect of the viscosity [6].

Wall

Transmitted wave

Absorbed wave

9

For materials with non-cylindrical pores and complicated internal structures [6], the thermal

characteristic length Λ’, is often used. This length characterises thermal exchange between the incident

wave and the porous skeleton.

Table 5. Examples of acoustic and non-acoustic parameters.

Acoustic parameter Non-acoustic parameters

Zc Zs Characteristic and surface impedances φ Porosity

γ, τ Propagation and transmission coefficients αγ Tortuosity

TL Transmission loss Λ, Λ’ Viscous and thermal characteristic lengths

α Absorption coefficient σ Airflow resistivity

iv. Common models

Theoretical models used to describe sound waves in porous materials depend on the microstructure of

the material. Indeed, some models are usable for fibrous materials, whereas others are used for granular

materials.

The Delany and Bazley model is the most widely used model because of its simplicity. Indeed, this model

needs only one parameter, the air flow resistivity. However, this model is only practical with materials

with porosity close to 1, which is not the case with porous concrete.

In previous works [1], the Attenborough model was tested for granular and fibrous materials and gave

good agreement. Indeed, materials with non-homogenous pores (random dispersion of size) like

concrete or WhisperWall (made by Armtec, a Canadien company) have a sinusoidal answer for

absorption coefficient with frequency and the Attenborough model, after parameter optimisation,

predicts these fluctuations well. The Attenborough model is developed in Appendix 3.

Once the airflow resistivity value is known, we can find the complex characteristic impedance and :

Lastly, the normal incidence absorption coefficient is equal to:

| |

Many others models are used and they depend on other parameter. In this project, only the

Attenborough model, which gives us the better prediction (see Table 6) with materials like concrete, will

be investigated.

Table 6. Summarize of two models used for prediction in porous middle.

Model Used for Parameters

Delany-Bazley All the materials with high porosity σ

Attenborough Non-uniform pores σ, α, ε, Sp

10

3) Creation of a new type of concrete; requirements and samples

a) Properties targeted

New concrete must respond to two special requirements: having the best acoustic properties without

deteriorating the thermal properties too much, porosity being a favourable parameter for sound

absorption but not for thermal conductivity.

Firstly, concerning acoustic properties, concrete must have the highest porosity. The best solution would

be to be use mono-disperse aggregates with a diameter of 10 mm (± 1 mm), the most rounded possible

and without small particles. However, this kind of aggregate is very hard to find. In this way, concrete

would be created with rounded aggregates but with larger size dispersion. In order to get pores, there

will be no sand in the concrete.

Secondly, thermal tests should be done to know if the fibres add something or not concerning the

thermal properties. Because there will be a very small amount of paste, the quantity of fibres has to be

very low, otherwise the structure will not be cohesive and the concrete not homogeneous at all.

Lastly, as for the mechanical properties of concrete, compression tests after 7, 14 and 28 days will be

carried out to follow the evolution of the compression results for a very weak and porous concrete. To

make this test doable, we will need to do capping of samples, which means the adding of a paste at the

top and at the bottom of the cylinder to make them even. In any case, the contribution of fibres to the

mechanical properties will remain very limited.

b) Fibres characteristics

Fibres available in the Civil Lab will be used. Though copper or aluminium, with high and moderate

thermal conductivities would certainly be better concerning the heat transfer, they are excluded because

of the corrosion risk they would bring to the structure. Steel wool, which would be more suitable to a

network creation, had been envisaged but had been excluded for the same reasons as copper and

aluminum. In the end, the fibres were made of steel, and are 30 mm long with a thickness of about 0.5

mm.

Figure 6. Fibres used.

11

c) Samples created

The influence of water/cement ratio, fibre and the quantity of fibres on the acoustical and mechanical

properties will be studied. An increase of the water-cement ratio is supposed to improve porosity but

decrease the mechanical properties and workability when fibres are added in the mixture (see Table 7).

Table 7. Summarize of studied samples (mix design).

Unit Density (kg/m3) % vol

Mix Cement Water Aggregates w/c Fibres

Mix 1 200 100 1550 0.5

Mix 2 200 100 1550 0.5 0.5

Mix 3 200 110 1550 0.55 1

Mix 4 140 90 1634 0.64 1

To determine the last mix design, three different mixes were tested in very low quantity (See Table 8).

The mix differs only in the aggregates proportion; the goal of these tests was to decrease the paste

proportion which fills the holes, decreases porosity, and make the material non homogeneous.

Table 8. Tests for determining the best final sample (%weight).

Test 1 Test 2 Test 3

Aggregates 84.1% 88.2% 92.2%

Water 4.7% 3.7% 2.7%

Cement 7.2% 4.1% 1.2%

Fibres 4.0% 4.0% 4.0%

During the mixing, the workability of these three mixes was good, except for Test 3, because of the very

low quantity of cement. After mixing, no conclusions can be directly drawn, except that Test 3 is non-

usable. 24h after, cylinders were removed from the molds and allowed us to make a difference between

three mixes:

- It was not possible to demold Test 3, because of the low amount of cement; there were no

bonds between aggregates.

- It was possible to demold Test 2 and Test 1 cylinders, but the structure of cylinder 2 was not

cohesive at all, whereas cylinder 1 was compact and did not lose any aggregate.

According to these observations, Test 1 was chosen to create the last slab. With this mix design, the

quantity of paste seems to be enough to create bonds between aggregates, but not too so great as to fill

the holes and make the material heterogeneous.

Proportions of all the mixes are summarized below (see Table 9 and 10).

12

Table 9. Weight composition of the four samples.

Weight %

Mix 1 Mix 2 Mix 3 Mix 4

Aggregates 87.1% 82.1% 80.0% 84.1%

Water 1.9% 5.3% 5.7% 4.7%

Cement 11.0% 10.6% 10.3% 7.2%

Fibres No fibres 2.1% 4.0% 4.0%

Table 10. Volumetric composition of the four samples.

Volumetric %

Mix 1 Mix 2 Mix 3 Mix 4

Aggregates 77.3% 75.8% 80.7%

Water 13.5% 14.5% 12.1%

Cement 8.6% 8.4% 6.0%

Fibres 0.7% (0.5%) 1.3% (1%) 1.3% (1%)

Concerning the 1st mix, numbers given are not really exact. Indeed, the moisture content was high (rain)

and aggregates were wet, so water quantity was reduced and aggregate quantity increased. For the

three others mixes moisture contents were not measured because aggregates were outside and dried by

the sun.

The fabrication protocol was the followed:

Weighting of aggregates, cement, water

Place cement and aggregates in the mixer

Mixing

Addition of water and mixing

Addition of fibres (progressive addition, depending on workability)

Homogenization of the mixture (see Figure 7).

Stopping of the mixer and filling of the molds

Small vibrations to fill the whole mold but to not decrease porosity too much

Removal from the molds after 24 hours (see Figure 8 and 9)

Cut with the water-jet cutter the next week

13

Figure 7. Final mixture (concrete with fibres) before the filling of the mold.

Figure 8. Water jet cutter.

Figure 9. Cut with the water-jet cutter (big cylinder on the left, small cylinder on the right).

After casting, cylinders and slabs are put in a bath of water and lime (curing step) to enhance the

hydration reaction and increase the strength.

14

II. Techniques used

1) Impedance tube

a) Principle of the measurement

To get acoustic and non-acoustic properties of the samples, an impedance tube (or Kundt’s tube) has

been used. This test is normalized by the standard ISO 10534-2:1998 Acoustics -- Determination of sound

absorption coefficient and impedance in impedance tubes -- Part 2: Transfer-function method.

This test is essentially based on an article [5], which allows us to transform a two-microphone tube into

three-microphone tube in order to study transmission properties. The apparatus is actually made up of

two tubes; one large one for low frequency and one small one for high frequency (see Table 11).

Table 11. Impedance tubes characteristics.

Length (cm) Diameter (cm) Frequency range (Hz)

Tube 1 (big) 100 9.898 255-1750

Tube 2 (small) ≈ 18 2.895 900-6000

Figure 10. Big impedance tube (low frequency). Figure 11. Small impedance tube (high frequency).

The apparatus is sketched in Figure 12. The distance between microphone 2 and the sample and distance

between the two microphones are important and directly linked to the final results; they determine the

frequency range. Microphones with a diameter of 6 mm are placed in supports adapted to the cylindrical

shape of the tube and after preamplifiers.

15

Figure 12. Impedance tube.

The sample is located at one end of the tube and the loudspeaker at the other one. A white noise, with a

constant spectral density is generated at the other end by the loudspeaker. The signal of the two

microphones is analysed with the Soundbook. A standing wave is created in the tube and the system

measures its characteristics [6].

When the absorption is studied, only positions M1 and M2 are used, whereas when transmission

properties are required, M1 and M3 positions are used and position M2 is plugged. The impedance tube

can be used is two orientations, horizontal and vertical, to analyse liquid or non-solid samples like sand

or soil.

The frequency range is determined by the system geometry. The upper one is set to avoid the

occurrence of non-planes waves (1710 Hz for the large tube and around 6000 Hz for the small one). The

lower limit is also given by the system geometry (tube length and microphone spacing) and is around 200

Hz for the large one and 900 Hz for the small one.

b) Measurement protocol

Firstly, two microphones and the pre-amplifiers are connected to the Soundbook. Then, the microphones

are calibrated (1000 Hz and 94 dB). These ones are not in phase, which is why we need to correct this

phase difference (we do not use phase matched microphones). In this way, the microphone position are

swapped: firstly the reference channel is microphone 1 and the response channel is microphone 2 and

secondly these two positions are interchanged. Afterwards, Excel spreadsheets are generated by the

Soundbook and analyzed with a MATlab code, to study the evolution of the parameters with frequency.

s L d

x

M1 M2

M3

Loudspeaker

Porous

mat. Amplifier

Sound generator Frequency

analysis

system

16

Figure 13. Microphones positions (at the top, normal position, at the bottom, swapped position). Absorption study Transmission study

c) Determination of acoustic and non-acoustic parameters

Acoustic parameters are determined with by function transfer method. Indeed, transfer functions

between microphones 1 and 2 first, and between microphones 2 and 3, are studied to deduce the

pressure ratio between two sides of the sample (x=0 and x=d, with d the sample thickness). All the

formulas come from the article [5].

where is the complex reflection coefficient:

Wavenumber, and the two impedances are obtained using the following expressions:

( )

(

)

( )

Lastly, absorption coefficient is found.

| |

All of these formulas are not directly accessible. They are in the Matlab code previously done and

improved during the internship to combine low frequency and high frequency results.

The four main non-acoustical parameters are: the airflow resistivity, the tortuosity and the two

characteristic lengths (viscous and thermal) are determined by an indirect method. They will be found

with formulas deduced from the theoretical model.

M1 M2

M2 M1

M1

M2

M2

M1

17

2) Air flow resistivity apparatus

The study of the air flow resistivity is a relatively simple test. The sample is set in a tube, which has the

same diameter as the impedance tube and a pump to suck out the air. Two manometers are located

before and after the sample; they will allow us to determine the pressure drop caused by the porous

sample. Airtightness is provided by a rubber band which rolls up around the sample.

Figure 14. Airflow resistivity apparatus.

Then, the slope of the variation of pressure drop with the air flow gives us the air flow resistance (Pa.s.m-

3). By multiplying this value by the area of the sample, we get the specific resistivity (Pa.s.m-1) which,

when divided by the sample thickness, allows us to obtain the air flow resistivity (Pa.s.m-2). This value is

used in several theoretical models as a main parameter (σ).

Figure 15. Simplified draw of the airflow resistivity apparatus.

3) Compression test

Even if we are not directly interested in mechanical properties, we carried out some compression tests

after 7, 14 and 28 days. Because this concrete is very weak, we needed to do capping of samples. The

capping permits us to get two flat surfaces, which is important because they are directly in contact with

Δp (Pressure drop)

Air flow rate Compressor

Sample

Porous sample

Compressor

P1

Inflow

Patm

Pressure drop ΔP

Pump

18

the apparatus. The white paste, which sets with time (after 20-25 minutes), is obtained by mixing

powder with water to get a viscous mixture.

Then, cylinders are put between two plates and a strain is applied with a certain speed (0.24 MPa/s).

Generally, because this kind a concrete has a very low strength, cylinders usually broke quickly.

Figure 16. Capping (on the left) and compression test (on the right) for a porous concrete.

4) Study of thermal properties of porous concrete

Initially planned, the Heat Flow Meter analysis was not possible. A new quantitative solution has

been found to know if fibres, in a very low quantity, provide something concerning the thermal

properties of concrete.

Fibre glass was used to provide thermal insulation around the sample and to prevent leaks.

Thermocouples were set at the beginning and at the end. The goal was to study the temperature

Thermocouples

T (°C)

Time (hours) Hot plate

Fiber glass

Concrete

specimen

Figure 17. Study of the thermal properties

19

increase at the top of the sample. This test does not allow obtaining Thermal conductivity but Thermal

diffusivity, for which formula is given as follows:

Thermal diffusivity :

where is the thermal conductivity, the density and the specific heat capacity.

This test was carried out for one cylinder but, because of the relatively bad insulation and the cylinder’s

length, the test was not reliable and took too long. Moreover, boundary conditions were not stable with

time. To be sure that the temperature remains the same, steam (100°C) would provide a better

boundary condition. At the other side, ice would be a solution (0°C). As for the insulation, instead of

using fibre glass, insulating foam would be better. Lastly, the cylinder should have a smaller height to

make the test faster.

20

III. Results and discussion

1) Acoustical properties: absorption and transmission

a) Absorption properties, general behaviour

Acoustical properties and especially absorption coefficient were studied with both impedance tubes. For

every graph, the most important frequency range concerning noise absorption is represented by a blue

band. This particular frequency range corresponds approximately to the frequency range of sound

emitted in a building, even if some lower and frequencies cannot be studied with this impedance tube

because of its length.

Mix 1 : no fibres and w/c=0.5

Figure 18. Absorption properties against frequency after 7 days (Mix 1).

This sample gives us a particular response: a narrow peak at around 610 Hz which reveals a high

absorption, a low absorption between 1000 and 1500 Hz, and a medium absorption for high frequency

(more than 0.7). Even if this materials seems to be sound absorptive, the usual sound absorptive

materials present a high absorption coefficient in the middle frequency range, which is not the case here.

The usual frequency range sound absorptive materials want to absorb is between 200 and 2000 Hz.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 1000 2000 3000 4000 5000 6000 7000

Co

eff

icie

nt

d'a

bso

rpti

on

α

Fréquence (Hz)

Evolution of absorption properties of a porous concrete without fibres

High frequency

Low frequency

21

Mix 2 : 0.5% fibres and w/c=0.5


The second sample has very low acoustic properties at low frequency (less than 70%) and good for very

high frequency, but only for one side. The effect of the segregation layer will be detailed below

Mix 3 : 1% fibres and w/c=0.55


For the 3rd mix, only low frequency behaviour was studied, but twice. Like the previous sample,

acoustical properties are poor, even if a peak around 1000 Hz is obtained.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 1000 2000 3000 4000 5000 6000 7000

α A

bso

spti

on

co

eff

icie

nt

Frequency (Hz)

Evolution of absorption coefficient with frequency (Mix 2)

Side 1

Segregation side

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Ab

sorp

tio

n c

oe

ffic

ien

t

Frequency (Hz)

Sound absorption properties (Mix 3)

Test 1 Side 1

Test 1 Side 2

Test 2 Side 1

Test 2 Side 2

22


The last mix is supposed to be more porous than the previous ones regarding the test cylinder cast first,

the very low amount of cement between aggregates and the mass of the sample.


Mix 4 provides the best acoustical properties, with several peaks with high amplitude (more than 90% of

absorption).

In every case, the behaviour of granular materials, like this concrete, is made up of several peaks, with

highly absorption (especially at low frequency). However, these peaks, which correspond to a certain

Helmholtz resonance frequency, are quite narrow. Mixes 1 and 4, which provide the most satisfactory

sound absorption results, have almost the same peaks, at almost the same frequency (Table 12)

Table 12. Peaks for Mixes 1 and 4.

Mix 1st peak 2nd peak 3rd Peak 4th peak

Mix 1 605 Hz 2363 Hz 3975 Hz 5694 Hz

Mix 4 546 Hz 2506 Hz 4000 Hz 5838 Hz

Then, the zone between the low frequency results and the high frequency results seems to be not

accurate. Indeed, results are shifted.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 1000 2000 3000 4000 5000 6000 7000

α A

bso

rpti

on

co

eff

icie

nt

Frequency (Hz)

Sound absorption properties (Mix 4)

Side 2

Side 1

23

b) Time effect on acoustical properties

Concrete’s porosity is, like the majority of parameters, a function of time and depends more exactly on

the hydration reaction. However, only microporosity or nanoporosity will increase with time.

Table 13. Time effect on the 1st

Mix.

Sound absorption response is quite different between 7 days and 21 days, especially at high frequency.

Indeed, whereas after 7 days we got an almost constant sound absorption coefficient from 2000 to 6000

Hz, we got three different peaks, relatively sharp and narrow, after 21 days. It is particularly remarkable

that all four peaks are separated by around 1700 Hz. These three peaks reveal high absorption properties

of porous concrete without fibres, but only at particular frequencies (2369, 3994 and 5694 Hz).

While porosity is supposed to increase slightly with time (micro and nano porosity) and results are

supposed to be better than after 7 days, results after 21 days do not show a more absorptive material in

all the frequency range, but only at certain frequencies.

The 34 day results seem to have the same behaviour as the 7 day results; peaks are almost at the same

frequency but absorption is lower than at 7 days.

Results only differs at high frequency; at low frequency, the main peak do not change with time. The

background noise at high frequency after 7 days can be explained by a correction we made, tape was

wrapped around the sample to prevent leaks.

c) Fibre effect on acoustical properties

It is not possible to study the direct effect of fibres on acoustic properties because of the segregation

layer. Indeed, to draw conclusions about fibre effects, sample 1 and sample 2 should be compared.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 1000 2000 3000 4000 5000 6000 7000

α A

bso

rpti

on

co

eff

icie

nt

Frequency (Hz)

Time effect on the sound absorption properties of a porous concrete

After 7 days

After 21 days

After 34 days

24

However, because the mix design did not change between Mix 1 and Mix 2, it could be possible that

fibres enhance segregation, which triggers poor sound absorption properties.

d) Effect of side

The effect of the sample side should not be important concerning acoustic properties. Actually, when

concrete is cast, light vibrations are applied to ensure the correct filling of the mold, and a tool is used to

make the surface smooth. As a consequence of these two steps, the surface at the top side (Side 1) is not

as flat as the one at the back of the mold (Side 2). Moreover, because of gravity and the excess of paste,

Side 2 can be denser than Side 1, which can result in different properties.

Mix 1 – No fibres

Side 1 – High frequency Side 2 – High frequency

Figure 23. Influence of sample side on the absorption properties (High frequency) – Mix 1.

Side 1 – Low frequency Side 2 – Low frequency

Figure 22. Influence of sample side on the absorption properties (Low frequency) – Mix 1.

25

The side of the concrete sample has no influence on sound absorptive properties for the 1st mix; we get

almost the same behaviour for both sides for low and high frequency.

Mix 2 (0.5% fibres)

Whereas results were the same for the first sample, the effect of the sample side for the second sample

is different. These results are especially surprising given that a peak was detected at low frequency for

the “non-porous” side.

Mix 3 – 1% fibres

For the 3rd mix, two samples were tested, to study the influence of the sample location.

Side 1 – Low frequency Side 2(Segregation side) – Low frequency

Figure 24. Influence of sample side on the absorption properties (low frequency) – Mix 2.

Side 1 – High frequency Side 2(Segregation side) – High frequency

Figure 25. Influence of sample side on the absorption properties (high frequency) – Mix 2.

26

Side 1 - Low frequency Side 2 - Low frequency

Figure 26. Influence of sample side on the absorption properties (low frequency) – Mix 3 – Test 1.

Side 1 - High frequency Side 2 - High frequency

Figure 27. Influence of sample side on the absorption properties (high frequency) – Mix 3 – Test 1.

27


Figure 28. Influence of sample side on the absorption properties (low frequency only) – Mix 3 – Test 2.

The 3rd mix confirms the previous conclusions. Absorption properties are more important for Side 2,

which is the side at the back of the mold. Moreover, even if the behaviour of both tests is similar, these

properties are not isotropic and depend on the sample location.

Mix 4 – 1% fibres and water/cement ratio equals 0.64


Figure 29. Influence of sample side on the absorption properties (low frequency) – Mix 4.

28

Side 1 – High frequency Side 2 – High frequency

Figure 30. Influence of sample side on the absorption properties (high frequency) – Mix 4.

For the last mix, properties do not change at low frequency, but are slightly different are high frequency.

Side of analysis seems to be important; indeed, absorption is different depending on the side for the

majority of the samples. This observation is direct proof of the non-homogeneity and the isotropy of the

concrete samples.

e) Effect of segregation

When the samples were removed from the molds, a thin layer of paste was discovered at the back of the

mold. This phenomenon, called segregation, generally occurs due to bad mix proportioning or excessive

vibration during casting. This thin layer makes the sample sound reflective (see Figure 31).

The segregation layer, by reducing the porosity, decreases the sound absorption properties of the

concrete.

Figure 31. Both sides of the Mix 2, effect of segregation. (Left: porous top side, right: segregation side)

29

f) Concrete parameters influence on acoustical properties

Four mixes were tested and they all provide different results. In any case, increasing the water-cement

ratio by increasing the aggregate proportion was apparently the right solution to decrease paste which

made the material non-homogeneous. A high water-cement ratio triggers a higher porosity.

g) Comparison with another sound insulating concrete material

A sound absorptive material, called WhisperWall, is used to absorb sound along highways. WhisperWall

is a kind of concrete made of very small aggregates and used tires, which provides sound adsorption due

to frame vibration. Its porosity is supposed to be around 0.4%. The paste proportion is very low, but the

structure is cohesive even so (See Figure 32).

Figure 32. Whisper Wall (left) and porous concrete (right).

Figure 33. Acoustical behaviour of two different materials.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 1000 2000 3000 4000 5000 6000 7000

Ab

sorp

tio

n c

oe

ffic

ien

t α

Frequency (Hz)

Comparison between the new concrete and the Whisper Wall

Concrete Mix 1

Whisper Wall

30

The behaviours of these two materials are relatively similar. WhisperWall is more adapted for medium

frequencies, which makes it more suitable for the sound absorption. They both have a special peak (1st

peak), which is not at the same frequency. This peak seems to relate a Helmholtz resonance because of

its narrowness. Indeed, the frequency position of this peak is directly linked to the internal structure of

the material. Helmholtz resonance comes from the resonance of air in a cavity.

In any case, this kind of concrete, without fibres and with a water-cement ratio equal to 0.5, is suitable

for sound absorption applications, but it depends on the frequency range targeted.

2) Models and parameter determination

a) Air flow resistivity determination

Air flow resistivity is a parameter used to predict the characteristics of sound absorbing materials. This

parameter is obtained with the air flow resistivity apparatus (cf Part II.2).

Mix 1 : Without fibres

Figure 34. Airflow resistivity (Mix 1).

Table 14. Determination of the airflow resistivity (Mix 1).

Airflow resistance (Pa.s.m-3) 44641

Specific airflow resistance (Pa.s.m-1) 1347

Airflow resistivity (Pa.s.m-2) (σ) 16836

y = 44641x + 0,2023 R² = 0,9648

y = 47196x - 3,2419 R² = 0,9626

0

5

10

15

20

25

30

35

40

45

50

0 0,0002 0,0004 0,0006 0,0008 0,001 0,0012

Pre

ssu

re d

rop

(P

a)

Air flow rate (m3/s)

Air flow resistivity determination (Mix 1)

31

Mix 2 : 0.5% fibres and w/c=0.5

As explained before, a segregation problem was encountered with Mix 2. In this way, the airflow

resistivity test was not suitable for this sample, because of a non-porous side. Because of the thickness of

the layer, it was not possible to carry out this test, even after grinding.







The airflow resistivity obtained is not relevant. Indeed, the slope of this graph is five times as high as the

1st mix case, whereas this mix is supposed to be more porous. A thin non-porous layer may stop the flow

transfer and trigger this very high airflow resistivity.

y = 202152x - 9,709 R² = 0,9984

0

10

20

30

40

50

60

0 0,00005 0,0001 0,00015 0,0002 0,00025 0,0003 0,00035

Pre

ssu

re d

rop

(P

a)


Airflow resistivity determination (Mix 3)

32

Mix 4: 1% fibres and w/c=0.64






Table 17. Summarize of the airflow resistivities.

Airflow resistivity (Pa.s.m-2)

Mix 1 16836

Mix 2 N/A

Mix 3 76241

Mix 4 7003

The air flow resistivity is very different depending on the sample. Airflow resistivity is not directly linked

to porosity, because it depends on viscous parameters, but we can assume that Mix 1 and particularly

Mix 4 are more porous that the other ones.

b) Porosity

Even if no direct methods were available to determine porosity, we can compare samples qualitatively.

Because samples have the sample shapes, a cylinder with a diameter of 98 mm and 8 mm height; we can

consider that the samples have the sample volume. The density is supposed to be almost the same,

because fibres are used in a very low quantity, so if we weigh samples, it should give us an indication of

the porosity (see Table 18).

y = 18569x - 3,1771 R² = 0,9926

0

5

10

15

20

25

0 0,0005 0,001 0,0015

Δp

Pre

ssu

re d

rop


Aiflow resistivity detetermation (Mix 4)

33

Table 18. Samples’ weight.

Weight (g)

Mix 1 1105.0

Mix 2 1137.0

Mix 3 1143.5

Mix 4 935.5

With these values, we can qualitatively compare the samples densities:

φMix 4 > φMix 1 > φMix 2 > φMix 3

Even if it is only a rough estimation based on hypothesis, we can considerer that the most porous sample

is Mix 4.

c) Unknown parameter determination – Attenborough model

Attenborough model [7] have been used in previous work to describe the acoustical behaviour of sound

absorptive materials, especially for granular and fibrous materials [1]. The four main parameters of the

Attenborough model are airflow resistivity, porosity, and tortuosity and shape factor. Only the airflow

resistivity is directly determined, the three others parameters are estimated.

Tests were only carried out on Mix 1 and Mix 4 which present clear peaks.

Mix 1: No fibres, w/c=0.5

The Attenborough model predicts well this kind of behaviour with peaks. The model gives the best

approximation for porosity equal to 0.3, a shape factor equal to 0.2 and a tortuosity of 1.7. Airflow

resistivity was determined previously with the airflow resistivity apparatus. A special comment could be

made on the match at low frequency. Indeed, whereas the model predicts very well the real material

behaviour at high frequency, it is worse at low frequency. The first peak seems to be shifted for no

apparent reason.

34

Figure 37. Attenborough model prediction for the 1st

Mix (no fibres, 21 days).

Φ=0.3, Sp=0.2, α=1.7, γ=1.4, σ=16836 Pa.s.m-2.


Attenborough’s model was applied to the 4th Mix (Side 1). The link between low and high frequency

being really poor, the model predicts well the peak positions, but with a frequency shift.

Figure 38. Attenborough model prediction for the 4th

Mix (fibres and w/c=0.64).

Φ=0.35, Sp=0.5, α=1.7, γ=1.4, σ=7003 Pa.s.m-2.

35

The acoustical behaviour, at least the amplitude, can be predicted with the Attenborough model. This

model is in keeping with the fluctuating acoustical response of porous concrete.

3) Mechanical properties: compression tests

Compression strength is the most used mechanical quantity for comparing the strengths of two different

concretes. Because concrete is not homogeneous, three measurements by sample were done, which will

give us a better indication than one. Three tests cannot be sufficient to determine reliably the

compression strength of a concrete; however, because mechanical properties are not the principal

aspect of this project, three cylinders will be enough (see Figure 39).

Figure 39. Mechanical properties of a porous concrete.

Mechanical properties do not really change with time for Mix 1 contrary to the behaviour which was

expected. The graph is based on an average of three cylinders tests values. On the contrary, concerning

Mix 2, an increase can be noted, but the standard deviation is quite large for this sample (cf Appendix III),

so that we can really draw conclusions on this sample. This large standard deviation (around 2 MPa after

21 days) comes from the segregation. Indeed, the excess of paste, which is not the same depending on

the sample, reinforces the mechanical properties. Concerning Mix 3, results do not make sense. Indeed,

strength decreased with the time, which is not possible. The accuracy of the test, and the capping layer

could be the reasons for this strange behaviour.

In any case, because of their low quantity, fibres do not contribute anything concerning compressive

strength. On the contrary, as expected, compressive strength is very low, between 4 and 7 MPa, whereas

a non-porous usual concrete can reach 25-30 MPa.

Acoustical properties can be reached for no-fines concrete because of cavities in the concrete structure.

However because of its random structure, these properties are limited. Mechanical properties were also

affected by the heterogeneity of the structure; thus no evolution was noticed and the compression

strength, around 5MPa, is very low.

0 2 4 6 8

7

14

28

Compression strength (MPa)

Days

Mix 3

Mix 2

Mix 1

36

Final conclusion

No fines concrete, which is a particular concrete without small particles, is only made of three

different components; aggregates (with the highest proportion), cement and water. The goal of this kind

of concrete is to create a porous structure, which is necessary to absorb sound waves. Usually used as

underpinning and walls, concrete can also be used as an architectural component, to ensure a special

function; sound absorption for example. In this report, acoustical properties of three different types of

no-fines concrete, which differ in composition were investigated

Sound absorption properties were tested with an impedance tube. An impedance tube allows

the acoustical properties of a porous sample to be obtained. For all of the samples, absorption peaks

were detected. These peaks, fairly narrow and sharp, refer to Helmholtz resonance of concrete. Indeed,

because of porosity, some cavities with random size and length are located in the structure and create a

resonance phenomenon. However, because of the random organisation, cavities can have different

sizes; that is why the peaks are not really sharp and narrow. At high frequency, the absorption coefficient

is often higher than 0.5 which is the case for the majority of acoustic materials. Nevertheless, acoustical

properties are often targeted in a frequency range from a few hundred Hertz to some thousand Hertz, so

the most important behaviour to consider for concrete is at low frequency. The Attenborough model

was tested with different parameters to predict the acoustical behaviour. This model is particularly

adapted to porous concrete.

Thermal properties were not directly investigated in this project because of the lack of

equipment to characterise concrete. However, a solution must be found to increase the proportion of

fibres; which seems to be not sufficient to create a steel network which could improve thermal

conductivity. Indeed, after a quick observation of the internal structure the fibres distribution is not as

good as expected; piles of fibres were found even if special attention was paid to fibres addition, and

contacts between fibres are very rare. Because of the porous structure and the very low amount of

paste, fibres quantity cannot be increased a lot; in this way, some solutions are proposed in the last part,

Future prospects.

Mechanical properties were tested as well, even if this concrete will not be used as a load-

bearing wall. As expected, and because of porosity, compression strength is very low and not more than

7 MPa, even after 28 days. This result has to be compared with a usual concrete, which is between 25

and 30 MPa, and up to 150 MPa for a very high strength concrete.

Heterogeneity of concrete was the main limit to obtaining high sound absorption properties.

Indeed, acoustical behaviour depended on the sample’s location. A special attention was focused on the

paste reduction because paste created some non-porous areas which are harmful for the acoustical

properties. In this way, the last mix (Mix 4) offers good sound absorption behaviour.

Finally, porous concrete tends to be suitable for sound absorption, but depends on the targeted

frequency range.

37

Future prospects

To further increase the porous concrete’s properties, some adjustments must be made to

achieve high sound absorptive properties and, at the same time, thermal conductivity. It would be

interesting to test the following ideas in the future.

To begin, porosity can be and must be improved by material selection. Aggregates must have small size

dispersion (around 10 mm diameter) and with no small aggregates which decrease porosity. However,

with aggregates with the same size, we will get a highly porous structure, but with large pores, which will

reduce the tortuosity. Consequently, the dispersion size of aggregate should be around 2-3 mm to get a

porous concrete with a high tortuosity. In addition, aggregates must be as round as possible. Porous

aggregates could be investigated to study the effect of “double porosity”. Cement composition could

also be studied: its thermal conductivity and specific heat can be improved.

Fibre effects could also be increased. Even if fibre effects on thermal conductivity have not been directly

investigated in this report, the volume fraction of 1% seems to be not enough. A solution must be found

to create a real network in the concrete. By creating a fibre network before casting and adding

aggregates after, would ensure contact between fibres, and would surely have an effect on thermal

properties. In addition, fibre nature plays an important role in thermal properties. Aluminum or even

better, copper fibres would be better than steel fibres. Wool of these materials could also be interesting,

after resolving corrosion problems involved and process protocol.

Nano fibres would also be useful for this project. Moreover, copper powder could be directly inserted in

the cement or in the mixture. While these two solutions would be very expensive, they would increase

thermal conductivity on a small scale and also, if used in high quantity, of the all structure. Finally, in

perfect agreement with sustainability, these fibres could come from recycling, to minimize the impact on

the environment of building materials and to make the product as green as possible.

38

Personal impressions

To begin, I was very happy to do my internship in the ANRG lab at the UBC. I was interested in

acoustics, and I had a short experience with porous materials in a previous work. When I arrived I was

pleasantly surprised by the good atmosphere in the lab. This atmosphere contributes to make a good job

on our respective projects.

Concerning my own project, I began with a bibliographical study which helped me a lot to acquire

knowledge on concrete, a subject I did not know well, and on the impedance tube. I was impatient to

start casting my samples, and worried after one month, because of the 28 day period which cannot be

reduced. Then, I decided to start casting, even if the aggregates I found were not exactly the ones I

looked for and without knowing if the thermal analysis would be realised. After casting my samples, I

was disappointed about the “Heat Flow Meter”, because it would have been very interesting to carry out

at the same time the acoustical analysis and the thermal analysis. This project was challenging because

the goal was to link two antagonistic properties. Unfortunately, I tried without success to find a solution

to analyse my sample even so, but it seems to be difficult to get an accurate value, even a comparative

test, by building an apparatus.

I tried to do some good acoustic tests and interpretations, which was not very easy because official

previous work on the subject did not exist. Nevertheless, I have learnt a lot on different subjects, and the

project has required some organisation. Indeed, I had to coordinate my work in two different

laboratories, which was sometimes tricky. It was the first real whole project I had to carry out by myself

and I am proud to have created the last mix design by myself, especially because the test was conclusive.

More generally, I was very interested by my project and the work I did. Porous materials and their

acoustical behaviour is an interesting subject. I hope that my work will be the beginning of another

project and I hope that it will be continued fruitfully in the future.

39

References

[1] “Modelling of normal absorption coefficient”, M. Samimi. UBC Acoustics and Noise Research Group.

29p, 2012.

[2] “Mesure des propriétés acoustiques des matériaux poreux”. B. Brouard, B Castagnède, M. Henry, D.

Lafarge and al, Techniques de l’ingénieur, 2003. Available online. [05/22/2013]

[3] “Sound absorption property of open-pore aluminum foams”, F. Wang, L. Wang, J. Wu, X. You. 2007.

3p. Available online.

[4] “Properties of concrete”, A.M.Neville, 844p.

[5] “Evaluation of the acoustic and non-acoustic properties of sound absorbing materials using a three-

microphone impedance tube”, O. Doutres, Y. Salissou, N. Atalla, R. Panneton, 4p, 2010

[6] “Acoustic Absorbers and Diffusers, Theory, Design and Application”, T.J. Cox, P. D’Antonio, Second

Edition, edition Taylor and Francis. 476, 2009.

[7] "Acoustical characteristics of rigid fibrous absorbents and granular materials", K. Attenborough,

J.Acoust. Soc. Am. 73: 785-799, 1983.

40

Appendix

Appendix 1 Protocol of making concrete (Mix 3) ........................................................................................... I

Appendix 2 Attenborough Model equations .................................................................................................. I

Appendix 3 Results of the compression test ................................................................................................ III

I

Appendix 1. Protocol of making concrete (Mix 3).

Time Event Fibres quantity

0 min Adding of cement and aggregates 0%

3 min Adding of the half of the quantity of water 0%

5 min Adding of 0.5% of fibres 0.5%





15.5 min Adding of 0.5% of fibres 1.0%

18.5 min End of the mixing 1.0%

Appendix 2. Attenborough Model equations.

Deduced from the MATLab code developed by Mina Samimi

The Attenborough model

ρ0=1.213kg/m3 air density Pr= 0.72 Prandtl number (Pr=ν/α Kinematic viscosity over thermal diffusivity) γ=1.4 ratio of specific heat for air P0=101320 Pa, air pressure η=18.6x10-6 Pa.s, viscosity of air c=342.2 m/s speed of sound in air f=0:8000 frequency range Ω=2πf, angular frequency

√

where:

ρ0=1.213 air density = angular frequency = tortuosity

= air flow resistivity porosity

Sp = pore shape factor

T1 is the ratio between Bessel functions of 1st and 0 orders:

( ) ( )

( )

(√ )

(√ )

Complex density function :

II

( ( √ )

( √ ))

in the report

Complex density of the medium:

( )

( √ )

( √ )

Complex compressibility function :

( ( )

( √ )

( √ ))

in the report

Complex compressibility of the medium:

Bulk modulus :

Characteristic impedance:

√

Propagation wave number:

√

The surface impedance for a finite porous medium:

( )

Complex reflection coefficient:

( )

( )

Absorption coefficient: | |

III

Appendix 3. Results of the compression tests.

Mix 1 Mix 2 Mix 3

Day 7 - 4.45 5.65

Day 7 2 6.47 6.03

Day 7 - 6.12 5.81

Day 7 average Not possible 5.68 5.83

Standard deviation 0 0.88 0.16

Day 14 6.24 5.02 6.32

Day 14 5.92 10.51 4.69

Day 14 5.05 5.34 5.41

Day 14 average 5.74 6.96 5.47

Standard deviation 0.62 3.08 0.82

Day 28 6.85 9.59 4.67

Day 28 5.60 4.75 4.03

Day 28 5.03 6.64 6.42

Day 28 average 5.83 6.99 5.04

Standard deviation 0.93 2.44 1.24

Table of illustrations and tables

Figure 1. CIRS building, outside and inside, UBC Campus, Vancouver. ......................................................... 2

Figure 2. Open and closed pores, two different behaviours. ........................................................................ 3

Figure 3. Simplified drawing of a thermal slab. ............................................................................................. 4

Figure 4. Simplified drawing (section) of concrete with fibres. .................................................................... 5

Figure 5. Simplified drawing of the sound wave propagation. ..................................................................... 8

Figure 6. Fibres used.................................................................................................................................... 10

Figure 7. Final mixture (concrete with fibres) before the filling of the mold. ............................................. 13

Figure 8. Water jet cutter. ........................................................................................................................... 13

Figure 9. Cut with the water-jet cutter (big cylinder on the left, small cylinder on the right). ................... 13

Figure 10. Big impedance tube (low frequency). ........................................................................................ 14

Figure 11. Small impedance tube (high frequency). ................................................................................... 14

Figure 12. Impedance tube. ........................................................................................................................ 15

Figure 13. Microphones positions (at the top, normal position, at the bottom, swapped position). ........ 16

Figure 14. Airflow resistivity apparatus. ...................................................................................................... 17

Figure 15. Simplified draw of the airflow resistivity apparatus. ................................................................. 17

Figure 16. Capping (on the left) and compression test (on the right) for a porous concrete. .................... 18

Figure 17. Study of the thermal properties ................................................................................................. 18

Figure 18. Absorption properties against frequency after 7 days (Mix 1). ................................................. 20

Figure 19. Absorption properties against frequency after 7 days (Mix 2). ................................................ 21



Figure 22. Influence of sample side on the absorption properties (Low frequency) – Mix 1. .................... 24

Figure 23. Influence of sample side on the absorption properties (High frequency) – Mix 1. ................... 24

Figure 24. Influence of sample side on the absorption properties (low frequency) – Mix 2. ..................... 25

Figure 25. Influence of sample side on the absorption properties (high frequency) – Mix 2. .................... 25

Figure 26. Influence of sample side on the absorption properties (low frequency) – Mix 3 – Test 1. ....... 26

Figure 27. Influence of sample side on the absorption properties (high frequency) – Mix 3 – Test 1. ...... 26

Figure 28. Influence of sample side on the absorption properties (low frequency only) – Mix 3 – Test 2. 27

Figure 29. Influence of sample side on the absorption properties (low frequency) – Mix 4. ..................... 27

Figure 30. Influence of sample side on the absorption properties (high frequency) – Mix 4. .................... 28

Figure 31. Both sides of the Mix 2, effect of segregation. .......................................................................... 28

Figure 32. Whisper Wall (left) and porous concrete (right). ....................................................................... 29

Figure 33. Acoustical behaviour of two different materials. ....................................................................... 29

Figure 34. Airflow resistivity (Mix 1). ........................................................................................................... 30



Figure 37. Attenborough model prediction for the 1st Mix (no fibres, 21 days). ........................................ 34

Figure 38. Attenborough model prediction for the 4th Mix (fibres and w/c=0.64). .................................... 34

Figure 39. Mechanical properties of a porous concrete. ............................................................................ 35

Table 1. Different thermal conductivities at 20°C. ........................................................................................ 5

Table 2. Main components of the Portland cement. .................................................................................... 6

Table 3. Usual composition of Portland cement. .......................................................................................... 6

Table 4. Mechanical properties evolution with time. ................................................................................... 6

Table 5. Examples of acoustic and non-acoustic parameters. ...................................................................... 9

Table 6. Summarize of two models used for prediction in porous middle. .................................................. 9

Table 7. Summarize of studied samples (mix design). ................................................................................ 11

Table 8. Tests for determining the best final sample (%weight). ................................................................ 11

Table 9. Weight composition of the four samples. ..................................................................................... 12

Table 10. Volumetric composition of the four samples. ............................................................................. 12

Table 11. Impedance tubes characteristics. ................................................................................................ 14

Table 12. Peaks for Mixes 1 and 4. .............................................................................................................. 22

Table 13. Time effect on the 1st Mix. ........................................................................................................... 23

Table 14. Determination of the airflow resistivity (Mix 1). ......................................................................... 30



Table 17. Summarize of the airflow resistivities. ........................................................................................ 32

Table 18. Samples’ weight. .......................................................................................................................... 33

Etudiant (nom et prénom) : MOURRE Guillaume MATERIAUX 4ème année

Entreprise : Acoustic Noise Research Group, The University of British Columbia

Adresse complete : Centre for Interactive Research on Sustainability (CIRS), Room 2160 3rd floor, 2260

West Mall, Vancouver, BC, Canada V6T 1Z3

Téléphone (standard) : +1-604-822-3073 Fax : +1-604-822-9588

Responsable administratif (nom et fonction) : M. HODGSON Murray, Professeur, Acoustique & Noise

Research Group

Téléphone : +1-604-822-3073 Fax : +1-604-822-9588 Mél : [email protected]

Maître de stage (nom et fonction) : M. HODGSON Murray, Professeur, Acoustique & Noise Research

Group

Téléphone : +1-604-822-3073 Fax : +1-604-822-9588 E-mail : [email protected]

Tuteur enseignant (nom et fonction) : M. BEAUGNON Eric, Professeur

Téléphone : +33 4 76 88 90 45 Fax : +33 4 76 88 12 80 Mél : [email protected]

Titre : Propriétés acoustiques et thermiques du béton poreux

Résumé :

Le béton, très utilise dans le domaine de la construction n’est pas connu pour être un bon isolant

acoustique en raison de sa très faible porosité. En effet, la porosité est néfaste pour les propriétés

acoustiques du béton, c’est pourquoi une porosité la plus faible possible est visée lors de sa fabrication.

Cependant, le béton sans petites particules, donc sans sable, a été étudié durant ce projet. Même si sa

porosité est moins importante qu’un isolant acoustique, ce type de béton peut absorber les ondes

sonores jusqu’à plus de 90% mais dans une gamme de fréquence relativement restreinte. L’effet de

l’addition de fibres métalliques dans la structure du béton a aussi été étudié, bien qu’aucun résultat n’ait

pu être obtenu. L’hétérogénéité et l’anisotropie du béton, deux propriétés connues du béton, ont

clairement été constatées durant ce travail et constituent une limite aux propriétés d’absorption sonore

du béton.

Student (Name and First Name) : MOURRE Guillaume MATERIAUX 4th year

Laboratory: Acoustic Noise Research Group, The University of British Columbia

Adress: Centre for Interactive Research on Sustainability (CIRS), Room 2160 3rd floor, 2260 West Mall,

Vancouver, BC, Canada V6T 1Z3

Téléphone (standard) : +1-604-822-3073 Fax : +1-604-822-9588

Administrative responsible (Name): Mr. HODGSON Murray, Professor, Acoustics & Noise Research

Group

Téléphone : +1-604-822-3073 Fax : +1-604-822-9588 E-mail: [email protected]

Advisor: Mr. HODGSON Murray, Professor, Acoustics & Noise Research Group

Telephone: +1-604-822-3073 Fax: +1-604-822-9588 E-mail: [email protected]

Tutor-scientis : Mr. BEAUGNON Eric, Professor

Téléphone : +33 4 76 88 90 45 Fax: +33 4 76 88 12 80 E-mail: [email protected]

Title: Acoustical and Thermal Properties of Concrete

Abstract:

Concrete, one of the most used materials to build houses and buildings, is not known to be a

good sound absorptive material because of its low porosity. Indeed, porosity is harmful for mechanical

properties; that is why zero porosity is often reached with usual concrete. No-fines concrete, which

refers to concrete with no small particles (sand), has been investigated in this project. Even if its porosity

is smaller than for usual sound absorptive materials, no-fines concretes can absorb sound waves more

than 90%, but in a relatively narrow frequency range. Fibre effects on thermal properties of concrete

have also been studied without results. Heterogeneity and anisotropy of the concrete material, two

well-known properties of concrete, have clearly been noticed as a limit to its sound absorption

properties.

acoustical and thermal properties of porous concrete - sustainable

Documents