Morphology Characterization of Foam Bitumen and Modeling for Low Temperature Asphalt Concrete

Morphology Characterization of Foam Bitumen and Modeling for

Low Temperature Asphalt Concrete

Biruk Wobeshet Hailesilassie

Doctoral Thesis

Stockholm, Sweden, April 2016

Doctoral dissertation to be defended in Kollegiesalen, Brinellvägen 8, KTH Royal Institute of Technology, Stockholm, Sweden, on 1^st of April 2016, at 13:00.

Main supervisor: Prof. Manfred N. Partl

Opponent: Prof. Kim J. Jenkins

External reviewers: Prof. Michael P. Wistuba Prof. Terhi K. Pellinen Prof. Adam M. Zofka Prof. Andreas Loizos

KTH Royal Institute of Technology

School of Architecture and the Build Environment Department of Civil and Architectural Engineering Division of Building Materials

SE-100 44 Stockholm, Sweden TRITA-BYMA 2016:1

ISSN 0349-5752

ISBN 978-91-7595-865-1

Printed in Sweden by USAB, Stockholm, 2016

Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan i Stockholm framlägges till offentlig granskning för avläggande av teknologie doktorsexamen fredagen den 1 April kl. 13:00 i Kollegiesalen, Brinellvägen 8, Stockholm.

© 2016 Biruk Wobeshet Hailesilassie

Abstract

In the last few years the search for new asphalt technologies to reduce both energy consumption and CO2 production has been intensified. Different techniques are available to reduce production and construction temperatures of asphalt concrete. The use of foam bitumen, as one of them, is attractive due to the low investment and production cost. In the traditional hot mix asphalt process the mineral aggregates are heated to 180 °C and higher. By reducing the temperature to 115 °C, the energy consumption is decreased by about 40 % and CO2 emissions by 31 %. Even higher energy savings are feasible if the production temperature can be further reduced.

Formation and decay of foam bitumen is a highly dynamic temperature dependent process which makes characterization difficult. In this thesis, new experimental tools were developed and applied for characterizing the foam bitumen during the foaming process.

One of the main goals of this study was to improve understanding and characterization of the foam bitumen formation and decay. X-ray radiography was used to study the formation and decay of foam bitumen in 2D representation. Image segmentation analysis was used to determine the foam bubble size distribution. In addition, the main parameters influencing foam bitumen formation, i.e. water content and temperature, were also investigated. The results demonstrate the influence of water content on the morphology and expansion of foam bitumen bubbles. Adding more water in the foaming process leads to quick collapse of bubbles and intensifies coalescence of foam bitumen. Higher temperatures produce larger bubbles at early foaming stage compared to lower temperatures. The morphology of bubble formation depends on the types of bitumen used. An exponential function has been implemented to represent the bubble area distribution. Moreover, theoretical investigation based on the 3D X-ray computed tomography scan dataset of bubble merging showed that the disjoining pressure increased as the gap between the bubbles in the surface layer (foam film) decreased with time and finally was ruptured. The speed of the bubbles also increased with time when the distance between the bubbles decreased.

Ultrasonic sensors were used for accurately monitoring the expansion and decay of foam bitumen as a function of time. Assessment of foam bitumen viscosity was performed using high frequency torsional rheometer (HFTR) and in situ observation by X-ray radiography. A high-speed camera was applied for examining the foam bitumen stream right at the nozzle revealing that foam bitumen at a very early stage contains fragmented pieces of irregular size rather resembling a liquid than foam. Moreover,

infrared thermal images were taken for obtaining information on the in situ surface temperature of foam bitumen during the hot foaming process. The results showed that the average surface temperature of foam bitumen depends on the water content of the bitumen and bubble size distribution, 108 °C for 4 wt% and 126 °C for 1 wt% (by weight) water content respectively. The residual water content in the decaying foam bitumen was determined by thermogravimetric analysis. The result demonstrated that residual water content depends on the initial water content, and was found to be between 38 wt% and 48 wt% of the initial water content of 4 wt% to 6 wt%. Finally, X- ray computed tomography was applied for examining the decay of foam bitumen revealing that the bubbles of foam bitumen remain trapped close to the surface of the foam bitumen.

Furthermore, the influence of water content on the foam bitumen and the asphalt mixture was investigated. The influence of the water content in combination with compaction temperature was studied using gyratory compaction method. AC11N foam asphalt mixture was produced using a lab foamer. Marshall stability and indirect tensile test were used for evaluating the performance of the foam asphalt mixture. The investigation revealed that the Marshall stability is more influenced by compaction temperature compared to water content. Moreover, increasing the water content helps in coating large mineral aggregates when the mixture is produced at low temperature.

Nevertheless, using high water content reduces the Marshall stability to a certain extent.

In addition, the amount of water trapped in the mixture after the mixing process was determined using thermogravimetric analysis. The amount of water remaining in the asphalt mixture was less than 1 wt% of the bitumen mass.

Moreover the influence of viscosity and surface tension on bubble shape and rise velocity of the bubbles using level-set method was implemented in finite element method. The modeling results were compared with well-known bubble shape correlation map from literature. The results indicated that the bubble shapes are more dependent on the surface tension parameters than to the viscosity of the bitumen, whereas the bitumen viscosity is dominant for bubble rising velocity.

KEY WORDS: foam characteristics; evolution of foam bitumen bubbles; image analysis; modeling rising of bubble; foam asphalt mixture

Sammanfattning

Utveckling av ny teknik för att reducera både energikonsumtion och koldioxidutsläpp vid framställning av asfalt har rönt ett stort intresse under de senaste åren. Olika metoder finns tillgängliga för att reducera tillverknings- och läggningstemperaturerna för asfaltsbeläggningar. En intressant metod är skumning av bitumen tack vare dess låga investerings- och driftskostnad. Bildandet och sönderfall av skummad bitumen är starkt temperaturberoende vilket försvårar dess karakterisering.

Syftet med denna avhandling är att, med hjälp av teoretisk och experimentell metodik, öka förståelsen för bildande och sönderfall av skummad bitumen. Nya experimentella verktyg har använts för att karakterisera bitumen under skumningsprocessen. Detta inkluderar även finita elementmodelleringar av skumbubblorna i en vätskematris samt teoretiska och experimentella studier av egenskaper hos skummad asfaltsmassa.

Röntgenradiografi användes för att studera bildandet och sönderfallet av skummad bitumen i två dimensioner (2D). Bildanalys användes för att bestämma skummets porstorlek. Dessutom undersöktes de viktigaste parametrarna som påverkar skummets tillväxt, nämligen vatteninnehåll och temperatur. Resultaten visar vatteninnehållets påverkan på morforlogi och skumbubblornas expansion. Mer vatten i skumningsprocessen leder snabbt till att bubblorna kollapsar samt att skummets koalesens ökar. Högre temperaturer leder till större bubblor tidigt i skumningsförloppet jämfört med lägre temperaturer. Bubblornas form beror på vilken bitumen som används.

En exponentiell funktion har antagits för att representera bubblornas areadistribution.

Teoretisk analys baserad på data från tredimensionell (3D) datortomografi av bubblornas förenande visade att det avskiljande trycket ökade då avståndet mellan ytskiktets bubblor (skumfilm) blev mindre med tiden för att till sist brista. Bubblornas hastighet ökade också med tiden då bubblornas avstånd minskade .

Ultraljudssensorer användes för att övervaka expansionen och sönderfallet av skummad bitumen som en funktion av tid. Utvärdering av viskositeten hos bitumenskummet genomfördes med hjälp av en högfrekvent vridreometer (HFTR) samt med röntgenradiografi. En höghastighetskamera användes för att studera bitumenskummets strömning vid munstycket vilket visade att skummet vid ett tidigt stadium innehöll fragmenterade bitar i olika storlekar, mer likt en vätska än ett skum. En värmekamera användes för att erhålla information om yttemperaturen på det skummade bitumenet under den varma skumningsprocessen. Resultatet visade att den genomsnittliga yttemperaturen för skummad bitumen beror på bitumenets vattenhalt och

storleksfördelningen på bubblorna, 108 °C och 126 °C för 4 respektive 1 % (viktsprocent) vattenhalt. Den resterande vattenhalten i det sönderfallande skummet bestämdes genom termogravimetrisk analys. Resultatet visade att den resterande vattenhalten beror på den ursprungliga vattenhalten och visades vara mellan 38 % och 48 % av den ursprungliga vattenhalten på 4-6 %. Slutligen användes datortomografi för att undersöka sönderfallet av skummad bitumen vilket visade att bubblor kvarhålls nära skummets yta.

Vidare gjordes en undersökning av vattenhaltens inflytande på skummad bitumen och asfaltsmassan. Influens av vattenhalt samt kompakteringstemperatur undersöktes med hjälp av gyratorisk kompaktering. Skummad asfaltsmassa, AC11N, producerades i laboratorium. Marshall-stabilitet samt indirekt dragprov användes för att utvärdera den skummade asfaltsmassans egenskaper. Resultatet visade att Marshall-stabiliteten hos den skummade asfaltsmassan påverkades starkt av kompateringstemperatur jämfört med vattenhaltens påverkan. Dessutom innebär en högre vattenhalt att större aggregat täcks med bitumen när massan produceras vid låga temperaturer. Hög vattenhalt minskar dock Marshall-stabiliteten till en viss gräns. Vidare bestämdes mängden vatten i massan efter blandning med hjälp av termogravimetrisk analys. Mängden kvarvarande vatten i asfaltsmassan var mindre än 1 % jämfört med bitumenmassan.

Viskositetens och ytspänningens inflytande på bubbelstorlek samt tillväxthastighet studerades med hjälp av finit elementmetod. Resultaten från modelleringen jämfördes med kända resultat som återfinns i litteraturen. Resultaten indikerade att bubbelformerna främst beror på ytspänningen medan bitumenets viskositet huvudsakligen påverkar tillväxten av bubblorna.

Preface

The work in this PhD thesis has been carried out as a collaborate project between KTH, Royal Institute of Technology, department of transport science, and EMPA, Swiss Federal Laboratories for Material Science and Technology, Road Engineering/Sealing Components laboratory.

The Commission for Technology and Innovation CTI, Switzerland and Ammann Schweiz AG, Switzerland are greatly appreciated for financing the project.

This research project would not have been possible without the support of many people.

First of all I would like to thank and express the deepest appreciation to my supervisor, Prof. Dr. Manfred N. Partl for his overall guidance and critical review of the research output at various stages. His persistent advise, encouragement and support in various ways from the very beginning were grateful. His follow‐up on the writing phase and careful review of the thesis manuscript are also highly appreciated.

In addition to this I would like to thank Dr. Martin Hugener for providing valuable guidance and support from the beginning of the research until the end, as well as for creating the opportunity to work in a collaboration project between Ammann Schweiz AG and EMPA.

I acknowledge the support of Dr. Andrea Bieder and Dr. Anton Demarmels, Ammann Construction Equipment, CH-4901 Langenthal, Switzerland. I would also like to thank center for X-ray analytics laboratory, and laboratory of multiscale studies in building physics for providing support and opportunities to use the facilities in the lab.

I am very grateful for the support of EMPA colleges Dr. Moises Bueno, Hans Kienast, Christian Meierhofer, Roland Takacs and Simon Küntzel, Road Engineering/Sealing Components, EMPA including other group members of the Lab. Many thanks to all my friends/colleagues for creating great work environment, for their help and moral support during the research project.

Finally, an honorable mention goes to my wife Meheret H. Kassa who has been my constant source of inspiration and encouragement when it was most required. I am grateful for my families for their understandings and endless love.

Dedication

To my parents and siblings, Fitsum and Kidus

Table of Contents

Abstract: ... i

Sammanfattning ... iii

List of symbols ... x

List of Publications ... xiii

Introduction ... 1

1. Motivation ... 1

1.1 Objective and scope of the research ... 2

1.2 Thesis outline ... 3

1.3 Literature review ... 7

2. Foam formation and decay ... 7

2.1 Bubble shape and rising speed ... 10

2.2 Thin film liquid ... 13

2.3 Characterization of foam bitumen... 16

2.4 Foam asphalt mixtures ... 19

2.5 Theoretical back ground ... 21

2.6 Methods and materials ... 31

3. Evolution of 2D-foam bitumen bubble size distribution ... 31

3.1 Evolution of 3D-foam bitumen bubble size distribution ... 32

3.2 Experimental investigation of foam bitumen ... 35

3.3 Influence of water content on foam asphalt mixture ... 39

3.4 3.4.1 Procedures and materials used for foam asphalt mixture testing ... 40

3.4.2 Asphalt mixture compaction and testing methods ... 43

3.4.3 Gyratory compaction and specimen preparation ... 44

3.4.4 Indirect tensile strength (ITS) test ... 44

3.4.5 Air void content determination ... 45

Thermogravimetric analysis ... 45

3.5 Bubbles inside foam decay ... 46

3.6 Dynamic X-ray radiography for foam bitumen observation ... 47

4. Formation and decay of the foam bitumen ... 47

4.1 Evolution of foam bitumen bubbles at different decay times ... 48 4.2

Influence of the water content on foam bitumen bubble area distribution... 49 4.3

Influence of bitumen temperature on bubble area distribution of foam bitumen ... 54 4.4

4.4.1 Stabilized foam bitumen ... 56 Repeatability of the X-ray measurement ... 58 4.5

Evolution of bubble area size distribution, modeling of the distribution ... 59 4.6

3D dynamic observation of the foam bubble merging ... 62 4.7

Summary of findings ... 65 4.8

Foam bitumen formation and decay ... 68 5.

Expansion ratio and half-life time ... 68 5.1

Observation of the foam bitumen stream at the nozzle outlet with high-speed camera ... 72 5.2

Temperature measurement during foam bitumen formation and decay ... 73 5.3

High frequency torsional rheometer ... 76 5.4

Residual water in the foam bitumen decay ... 78 5.5

Summary of findings ... 81 5.6

Modeling of bubble rising velocity and shape for foam bitumen ... 83 6.

Material characterization method ... 83 6.1

Single bubble expansion modeling ... 84 6.2

Predicting bubble shape in motion... 86 6.3

Influence of viscosity and surface tension of bitumen on bubble rising speed ... 97 6.4

Summary of findings ... 98 6.5

Influence of water content in foam bitumen on foam asphalt mixtures ... 101 7.

Influence of water content on foaming bitumen ... 101 7.1

Curing of foam asphalt mixtures ... 101 7.2

Influence of water content on foam asphalt mixtures at different mixing and compaction 7.3

temperatures ... 102 Influence of water content on gyratory compaction and ITS strength ... 104 7.4

Influence of water content on foam asphalt mixture produced using cold aggregates ... 106 7.5

Modeling the heat transfer in the asphalt mixture containing cold mineral aggregates ... 108 7.6

Advantages of increasing the water content in foam bitumen ... 112 7.7

Loss of trapped water in the asphalt mixture ... 115 7.8

Summary of findings ... 117 7.9

7.9.1 Influence of the water content on the foam asphalt mixture performance ... 117 7.9.2 Influence of cold mineral aggregate addition in the foam asphalt mixture ... 118

Conclusions... 120 8.

Foam formation and decay characteristics ... 120 8.1

Characterization of foam bitumen... 121 8.2

Parameters influencing foam asphalt mixtures ... 122 8.3

Recommendation for future studies ... 125 9.

References ... 126 10.

Appended papers

List of symbols

a acceleration

ax lagrangian acceleration

c damping constant

D diameter of a bubble

d_f damping factor

dγ_xy angular strain on the X-Y axis

E_o Eötvös number

ER expansion ratio

F force

F_b body forces

Fg gravitational force

FI Foaming index

Fp pressure forces

Fr Froude number

freq frequency

fres resonant frequency Fs viscous surface forces Fst surface tension force g acceleration due to gravity h thickness of the film

HL half-life

k spring constant

m mass

M₀ Morton number

P pressure

P_c capillary pressure

Pd dynamic pressure

P_L external force for film thinning

Q shear strain

r radius of bubble

Re Reynolds number

T temperature

t time

U velocity

u velocity component along the x-axis v velocity component along the y-axis

VE expanded volume

VfL liquid volume fraction

Vo original volume

VRe Velocity of film thinning

w velocity component along the z-axis

We Webe number

δ fluid element size ε specific heat capacity

η dynamic viscosity

µ viscosity

ξ damping of the system

Π disjoining pressure

Πel electrostatic force Πst steric force

Πvdw Van der Waals interaction

ρ density

σ surface tension

τij tensor notation of stress

Φ energy

ψ data points for expansion height 𝑆⃗ position vector

∆ vector differential operator

List of Publications Journal:

Paper I

Hailesilassie, Biruk W., Philipp Schuetz, Iwan Jerjen, Martin Hugener and Partl, Manfred N. "Dynamic X-Ray Radiography for the Determination of Foamed Bitumen Bubble Area Distribution." Journal of Materials Science 50, no. 1 (2015): 79-92.

Paper II

Hailesilassie, Biruk W., Hugener Martin, Bieder Andrea and Partl, Manfred N. "New Experimental Methods for Characterizing Formation and Decay of Foam Bitumen."

Materials and Structures, (2015): 1-16.

Paper III

Hailesilassie, Biruk W., Martin Hugener and Partl, Manfred N. "Influence of Foaming Water Content on Foam Asphalt Mixtures." Construction and Building Materials 85, no. 0 (2015): 65-77.

Paper IV

Hailesilassie, Biruk W., Iwan Jerjen, Michele Griffa and Partl, Manfred N., "A closer scientific look at foam bitumen", Journal of Asphalt Pavements and Environment of Road Material and Pavement Design (accepted for review).

Paper V

Hailesilassie, Biruk W. and Partl, Manfred N., "Modeling of bubble rising velocity and shape for foam bitumen”, submitted to Journal of Modelling and Simulation in Materials Science and Engineering.

Conference:

Paper VI

Hailesilassie, Biruk W., Philipp Schuetz, Iwan Jerjen, Andrea Bieder, Martin Hugener and Partl, Manfred N. "Evolution of Bubble Size Distribution During Foam Bitumen Formation and Decay." In ISAP 2014 Conference, 2014.

Other relevant publications

The author also published the following publications which are related to this research project:

Journal:

 Hailesilassie, Biruk W., Hean, Sivotha and Partl, Manfred N. "Testing of Blister Propagation and Peeling of Orthotropic Bituminous Waterproofing Membranes."

Materials and Structures, (2013): 1-14.

 Hailesilassie, Biruk W. and Partl, Manfred N. "Adhesive Blister Propagation under an Orthotropic Bituminous Waterproofing Membrane." Construction and Building Materials 48, no. 0 (2013): 1171-1178.

 Hailesilassie, Biruk W., Partl, Manfred N. "Mechanisms of Asphalt Blistering on Concrete Bridges." J. of ASTM International 9, (2012).

Contribution to the list of publication

Paper I - Paper VI, Hailesilassie W. Biruk developed the experimental methods, analyzed the data, carried out FEM modeling, interpreted the results and wrote the manuscripts.

Paper I and Paper VI, Schuetz P. and Jerjen I. carried out X-ray radiographs calibration, images acquisition and advised early stage of radiographic image analysis.

Paper I - Paper III and Paper VI, Hugener M. contributed in advising, planning of the experiments as well as reviewing the manuscript.

Paper II and Paper VI, Bieder A. contributed in discussion, partly planning of the experiments and reviewing the manuscript.

Paper IV, Jerjen I. carried out X-ray CT setup, calibration and radiographic images acquisition. Griffa M. helped and advised in early analysis of CT data set, and reviewed the manuscript.

Paper I - Paper VI, all supervision and final review of the manuscript was done by Partl N.

Manfred

Introduction 1.

Motivation 1.1

Foam bitumen allows the production of so-called foam asphalt, which is a mixture of mineral aggregates, foam bitumen and air. Foam bitumen is highly efficient in wetting and coating the surfaces of fine particles. Foam asphalt is widely used for cold-in-place recycling (full depth recycling), which is gaining recognition and popularity worldwide as cost effective method for rehabilitating distressed asphalt pavements (Stefan, et al., August 2003). However, up to now, the mechanical performance of these foam stabilized pavements is only suitable for sub-base layers but not adequate for base or binder courses, due to insufficient large mineral aggregate coating and high air void contents (Jenkins, 1999, Yin, et al., 2013). This calls for better understanding of the mechanism during the foaming process in terms of morphological changes as well as a sound morphological characterization of foam bitumen.

Compared to hot mixed asphalt (HMA) produced at 150-180°C, temperatures for production, transportation, laying and compaction are significantly lower for foam asphalt. Recent developments consisted in the use of heated mineral aggregates to produce warm mix asphalt (WMA) at 100 – 150°C and half-warm mix asphalt (HWMA) at 70-100°C (Van de Ven, et al., 2012). Thus, coating of the mineral aggregates was improved, with positive effect on the asphalt properties (Chowdhury and Button, 2008, Jenkins, 1999) but still without reaching the performance of HMA. Compared to HMA, foam asphalt has beneficial effects on the environment. Its lower temperatures not only induce a decrease in fuel or energy consumption and CO2 emissions from asphalt mixing plants but also an improvement of the working conditions at the paving site (Chowdhury and Button, 2008, Jenkins, 2000). Moreover, workability improvements have the potential to extend the construction season and the time available for placing the asphalt mixture during any given day (Chowdhury and Button, 2008). Curing of foam asphalt is rapid and sufficient mechanical strength for early trafficking is achieved fast (Chowdhury and Button, 2008, Collings and Jenkins, 2009). These results indicate that WMA can be considered as a promising method to produce asphalt mixtures at lower temperatures.

Several WMA road trials with foam bitumen have been successfully conducted in different European countries (Van de Ven, et al., 2012) and in the USA (Chowdhury and

Button, 2008, Jesse, et al., 2011). However, there are several barriers for wider implementation of foam asphalt such as, improper correlation between laboratory and field results as well as higher void content resulting in inferior performance of road pavements constructed with foam bitumen. So far, no research has been focused on how the foam bitumen properties can be characterized and improve the coating at lower temperatures, except by visual inspection of the mineral aggregate coating with trial and error methods (Rubio, et al., 2012, Van de Ven, et al., 2007). Although there is a lack of fundamental research on foam bitumen, in other foam application fields, foams have been studied in much detail (polymer foams, metal foams, soap, coffee, waste water, etc.).

The main reason for the missing research on this topic can be explained by the difficult characterization of the foam bitumen. The usual characterization of foam bitumen relies on only three empirical parameters called expansion ratio and half-life time, as well as foaming index. However, these parameters give only the volumetric property of the foam bitumen and provide no information on other foam characteristics such as foam morphology, bubble size distribution and viscosity. Dynamic processes during foam formation and decay like drainage, coarsening, coalescence remain unclear due to the difficulty to study the unstable and opaque black foam bitumen.

Objective and scope of the research 1.2

Given the above background, the objective of this research was to develop new experimental methods for characterization and visualization of the unstable and temperature dependent foam bitumen. Focus was laid on improving the understanding of foam morphology during formation and decay of foam bitumen in 2D and 3D representation. Moreover, characterizing the foam bitumen with known methods, by measuring expansion ratio and half-life time was conducted. The study also evaluates the influence of viscosity and surface tension parameters on the rate of foam decay, bubble shape and bubble rising velocity by using finite element method modeling.

Based on the fundamental studies, this research also examined the influences of water content on both foam bitumen as well as foam asphalt mixture. This was also done in combination with mixing and compaction temperature. Moreover, residual foaming water content in foam bitumen and its effects on mechanical performance of asphalt mixtures was studied.

Thesis outline 1.3

As presented in Fig. 1.1, the first topic of the thesis focuses on understanding the evolution of foam bubble size distribution. X-ray radiography was used to study the formation and decay of foam bitumen in 2D representation (paper-I). Image segmentation analysis was used to determine the influence of water content and foaming temperature on foam bubble size distribution over time. In addition, an attempt was made to observe the decay mechanism of foam bitumen with and without surfactant (stabilizers).

The second topic of the thesis focuses on the investigation of formation and decay of foam bitumen. In this part of the thesis, new experimental tools were applied for in situ characterization of the foam bitumen during the foaming process (Paper II). Monitoring accurately the expansion and decay of foam bitumen as a function of time was of interest.

An attempt was made to determine the change of the foam bitumen viscosity over time with the help of X-ray radiography observations and to measure the in situ surface temperature of foam bitumen during the hot foaming process. Moreover, the foam bitumen stream at the foaming nozzle of the foam generator was investigated in order to study the formation of foam in the earliest stage of its origin. Additionally, the residual water content in the foam bitumen decay was determined to quantify the remaining water after the foaming process.

The third topic of the thesis includes a theoretical study on how surface tension and viscosity of liquid bitumen influence the bubble shape, rate of foam decay and bubbles rising velocity (paper-V). A parametric FEM (finite element method) model was prepared using the commercially available software Comsol ^® from Multiphysics. The model considered incompressible Navier-Stokes equations including surface tension. In order to characterize the model parameters, viscosity of bitumen was measured at different temperatures, and the surface tension was adopted from literature. Moreover, theoretical investigations using a 3D X-ray CT (computed tomography) scan dataset was performed to understand bubble merging and disjoining pressure of thin films (Paper-IV).

The final fourth step of the thesis mainly focuses on the investigation of water content influence on the foam bitumen and the asphalt mixture (Paper-III). It includes studying qualitatively the coating of mineral aggregates by foam bitumen at low mixing and compacting temperatures. Moreover, study on the correlation between the foaming water

content and the residual water content in the foam asphalt mix was performed. Influence of the water content in combination with compaction temperature has been investigated using gyratory compaction method. AC11N (Dens asphalt concrete with maximum aggregate size 11mm) foam asphalt mixture was produced in the lab using lab foamer.

Marshall Stability and indirect tensile test were used to evaluate the foam asphalt mixture performance. Further, influence of cold mineral aggregate addition in the foam asphalt mix was also investigated (Paper-III). In addition the amount of water trapped in the mixture after the mixing process was determined using thermogravimetric analysis.

Fig. 1.1 Topics of the thesis.

The study in this thesis focuses on understanding; the foam formation and decay using theoretical and experimental investigation; FEM modeling of foam bubbles in liquid matrix; foam asphalt mixture properties investigation from experimental and theoretical point of view. The dotted boxes in Fig. 1.2 indicate these three main areas.

Chapter 1- states the background in the foam bitumen characterization and its application for low temperature foam asphalt mixture. It includes the motivation of the thesis, steps and structure used to achieve the goals of the research.

Chapter 2- presents a review of the literature on fundamental aspects of foam formation and decay, which includes theoretical background on bubble shape and rising speed as well as known foam characterization parameters. General foam bitumen and asphalt technologies available in present are included. Foam asphalt mixtures properties are discussed.

Chapter 3- reports the materials and methods used to make intensive studies described in chapter 1. New experimental techniques for observation of the hot foaming process in 2D and 3D representation. Procedures and methods used for foam asphalt mixture testing.

Chapter 4-presents new method for observing foam structure evolution using X-ray radiography. It includes study of the foaming water content and bitumen temperature influence on the bubble size distribution and decay of the foam.

Chapter 5-proposes experimental methods for measuring in situ characteristics of foam bitumen expansion and decay.

Chapter 6- states modeling of bubble rising and shape of foam bitumen. The modeling results are compared with well-known bubble shape correlation map from literature.

Chapter 7-presents laboratory investigation on foam asphalt mixture. It includes study of foaming water content influence on coating of mineral aggregates and compaction of foam asphalt mixtures in combination with different compacting and mixing temperature.

Chapter 8- states general conclusions of the thesis from the study of foam bitumen formation and decay characteristics, finite element method modeling, and parameters influencing foam asphalt mixtures.

Chapter 9- proposes recommendations for future studies.

Fig. 1.2 Schema of methods used in the thesis.

Literature review 2.

Foam formation and decay 2.1

Foam bitumen is a mixture of air, water and bitumen. It is produced in an expansion chamber through the injection of small quantities of water into 160 - 180 °C hot bitumen as shown in Fig. 2.1(a). Typically 1 - 6 wt% (% by weight of the bitumen) water is injected. The liquid water is transformed into vapor which expands the bitumen at the nozzle to an expanded volume V_E which is about 5 - 15 times of its original volume V₀ (Koenders, et al., 2000) forming a unstable foam. The ratio of expanded volume in the foamed state to the original volume of bitumen, is defined as expansion ratio (ER=

V_E/V_o). As the foam collapses, most of the water is lost as steam, leaving residual bitumen with properties similar to the original bitumen.

(a) (b)

Fig. 2.1 Schematic of (a) expansion chamber to produce foam bitumen (b) foam bitumen decay curve.

As indicated in Fig. 2.1 (b), the time for the foam bitumen to settle to half of its maximum expanded volume is called the half-life time (HL) (Jenkins, 2000, Muthen, et al., 1999). The half-life time is used to explain the stability of the foam bitumen in general. The desirable design value of the half-life time is difficult to determine since the addition of water varies the half-life. Usually foam bitumen is characterized using the two empirical parameters called maximum expansion ratio ERm and half-life time HL.

However these two parameters describe only the volumetric property of the foam bitumen. Commonly expansion ratio and half-life time are determined using a foam ruler;

nevertheless foam bitumen is very unstable when foamed at higher water content and the measurement of the expansion with the foam ruler can be inaccurate.

The existing technology of the foam bitumen includes foaming either by adding mechanically wet sand \ zeolite or by adding small quantities of water as described above. Wet sand and hot bitumen are used to create in situ foam in low-energy-asphalt process (LEA). Hot aggregates (100 °C - 140 °C) are used in the mix followed by wet sand and bitumen, details of the sequential drying and coating methods are described elsewhere (Olard. F., et al., 2008). Zeolite is a crystalline solid structures having cavities and channels where water or small molecules may reside. It is made of sodium aluminum silicate hydrate, which is hydro-thermally crystalized. It contains considerable quantities of water (21wt% by weight) which is released after mixing with bitumen at a temperature range of 85 °C – 180 °C (Kristjansdottir, 2006). The technologies to make a foam bitumen includes, mechanical mixing, venturi mixing, expansion chamber, shear/colloid mill, air-sprayed water and high pressure sprayed water (Newcomb, et al., 2015).

Depending on the amount of liquid content, foams can be classified as dry or wet, which is represented by the liquid volume fraction VfL as presented in Fig. 2.2. When the liquid volume fraction is less than 1% it is called dry foam, between 1% - 30 % is intermediate foam, and above 30% is wet foam. The amount of liquid may range from less than 1% to about 30 % by volume of the foam (total volume). The gas fraction is expressed as 1 – V_fL and is an index for the foam quality (Paul, 2012). However, this expression tells only about the foamability of liquid foam (two-phase system) in general.

The higher the gas fraction the better the foamability. For liquid foams, as presented in Fig. 2.2, in the dry foam the films that form the interface between bubbles are not spherical in shape, as these bubbles are made of polyhedral cells (Matzke, 1946).

Important geometrical and topological limitations exist in the dry state of the 2D foam.

Experimental investigation on 2D foam soap bubbles showed that the foam films meet an angle 120° forming in between them called Plateau rule, as the surface tension forces at this point should be in equilibrium a (Paul, 2012).

Fig. 2.2 Foam classification (Paul, 2012).

Another classification is based on the porosity of the micro cellular structure. Foams can be classified as open or porous, depending on whether the voids are completely interconnected or closed (Fig. 2.2). In general, both open and closed foam cells are present in a foam. Metal foams commonly have porosity less than 70 % by volume, ceramic foams show a porosity between 45 % and 97 %. In addition, foams are categorized by the way they are formed, by chemical or physical processes. For instance, in polymeric materials as well as metal foams, thermal decomposition of chemical blowing agents can be used to generate the gas for the expansion of the foam. Mechanical whipping of gases into polymer solution, thus entrapping gases in the matrix, is another way of foam formation.

Moreover, the rheology and stability of the foam are mainly depending on the foam's bubble size distribution and gas liquid fraction (Körner, et al., 2002). In order to determine the growth rate of bubbles in dry foam, the bubble size and shape are crucial.

For wet foam the influence of the bubble shape is less significant since the bubble shape is expected to be more spherical. The foam bitumen has more spherical bubbles of different sizes as presented in chapter 4.

Bubble shape and rising speed 2.2

Bubble shape and motion in liquids have been studied since many years due to their importance in different fields including chemical engineering processes, such as fermentation, flotation, and waste water treatment (Yu and Fan, 2008). In the application of dissolved air floatation for drinking water clarification, bubble rising velocity is important to decide the operation time (Lawrence, et al., 2010). Observations in water based foam systems indicate that in pure liquids without surfactants, the bubbles rise velocity increases as the bubble size increases (Kulkarni and Joshi, 2005). When the dynamic viscosity is dominant, for instance for air-glycerol systems, the bubble rising velocity U found was 8.0 ≤ U ≤ 24.0 cm/s for bubble diameters 1.85 ≤ D ≤ 3.9 cm (Talaia, 2007). In foam bitumen technology stable bubbles or low bubble rising velocity can be useful to keep the foam at reduced foam viscosity compared to non-foamed liquid bitumen, as the mixing time in asphalt plants is limited in time. The more stable the foam is the more time is available for coating and mixing mineral aggregates with foam bitumen.

Bubbles in motion are usually classified by shape as spherical, ellipsoidal and spherical cap as shown in Fig. 2.3; the actual shape depends on the relative magnitudes of forces acting on the bubbles, such as surface tension and inertial forces. In general, for predicting the bubble shape, all physical variables affecting the phenomena of bubble rising need to be studied. Eight parameters play a role: the acceleration due to gravity, the characteristic velocity (velocity of the fluid),the diameter of the bubble, the density of liquid matrix, the viscosity of the liquid matrix, the surface tension, the density and viscosity of the gas inside the bubble. The last two parameters are negligible as compared to the corresponding parameters of the liquid. When the bubble size is small, for instance for water D < 1mm at room temperature, the surface tension properties dominate the shape and lead to spherical bubbles. For bubbles of intermediate size both surface tension of the liquid and inertial effects influence the bubble shape. Large bubbles, on the other hands are more influenced by inertia or buoyancy forces as compared to the surface tension forces and the viscosity of the liquid. The bubble shape is influenced by properties of the surrounding media (liquid matrix) and the liquid matrix flow around the bubble. The Reynolds number (R_e) is the most important parameter that characterizes the bubble shape, which mainly depends on the bubble rising speed (Liang-Shih and

Katsumi, 2013). The Eötvös number (E_o) indicates the influence of surface tension forces and gravitational forces on the shape of a bubble rising in liquids. As shown in Fig. 2.3, bubbles with low R_e or low Eötvös number E_o are generally spherical. Details of these two parameters are described in section 2.6. The spherical regime occurs when the interfacial tension forces and/or viscous forces are more dominant than inertia forces (Clift, et al., 2005).

Fig. 2.3 Bubble shape regimes diagram according to reference (Clift, et al., 2005, Grace, 1973).

The rise of bubble in a liquid is a function of different parameters which includes:

bubble characteristics; shape and size, properties of gas-liquid system; density, viscosity, surface tension, concentration of surface active particles/ surfactants, density difference between gas and liquid, liquid motion and direction, as well as temperature, pressure and water content for foam generation (Kulkarni and Joshi, 2005). The bubble rising velocity depends on bubble size and decreases as the bubble shape changes from spherical to ellipsoidal. Increasing the concentration of surfactants and electrolytes minimizes the mobility of the gas-liquid interface in Newtonian fluids. Hence, the rising velocity of bubbles is reduced. This effect is significant up to a certain volume of the bubble.

Viscosity of a liquid influences the magnitude of viscous forces (details of these forces are discussed in section 2.6, that play a big role in shape as well as stability of the

bubbles. Nevertheless, its effect decreases for large bubble diameters (Kulkarni and Joshi, 2005).

Bubble rising velocity of a single bubble has been studied to understand two-phase flows of liquids and air (Bhaga, 1976, Clift, et al., 2005). Reliable bubble rising velocity models have been presented (Clift, et al., 2005) for two extreme cases, for small spherical bubbles and large spherical-cap bubbles. When the bubbles are small enough to create spheres and the Reynolds number is below unity, i.e. in case of slow moving spherical bubbles, its rising velocity can be calculated by considering the buoyancy force and drag force acting on the moving bubble. This means that the surface tension forces and the inertial force terms are insignificant (Tomiyama, et al., 2002). When surfactants are used, the interface becomes immobile and thereby the bubble can be idealized as a rigid sphere.

In this case the well-known Navier-Stokes equation can be used to determine the bubble rising velocity (Tomiyama, et al., 2002). When the bubbles are large enough to produce spherical-cap shapes, the rising velocity is mainly governed by the inertial forces, since these forces are much larger than the viscous forces (Tomiyama, et al., 2002) and addition of surfactants has no major influence on the bubble rising velocity.

Bitumen is a viscoelastic liquid, which changes its rheological properties with temperature. Strength of intermolecular interaction, chemical composition and temperature are influencing rheological properties of bitumen (Bazyleva, et al., 2010).

Asphaltene, which is a main component of bitumen, can contain high-molecular hydrocarbons of mainly aromatic character (Pfeiffer and Saal, 1940). It is insoluble in low-molecular non-aromatic hydrocarbon solution and when heated it does not soften and swell, but decomposes. It can be obtained from precipitation of bitumen with low boiling solvents such as, petroleum ether, pentane, isopentane, and hexane. Detailed investigation on the properties of asphaltene in bitumen showed that, asphaltenes are responsible for high viscosity of bitumen and residues of crude oil (Lin, et al., 1998). Bitumen is a non- Newtonian liquid at temperature below 60 °C, depending on the type of bitumen and chemical composition. The harder the bitumen, the more likely is it showing non- Newtonian behavior below 60°C. It is reported that Newtonian behavior of bitumen can be obtained at elevated temperatures around 137 °C – 160 °C (Bazyleva, et al., 2010).

Moreover, there is a relation between the concentration of asphaltenes and interfacial

tension (Yarranton, et al., 2000). It was shown that the concentration of the asphaltenes increases the surface tension of the bitumen (Yarranton, et al., 2000).

Thin film liquid 2.3

Great progress in understanding the principle of the different foaming processes and the phenomena that take place in the foam films was achieved by investigating free standing films regarding diffusion transfer, molecular and electrostatic interaction, kinetics of thinning and rupture (Birdi, 2008, Sheludko, 1967). Such investigations were used to study the efficiency of antifoaming agents, stabilizing agents, surfactant concentration, thin film stability, disjoining pressure etc.(Birdi, 2008, Ivanov, 1988).

From general foaming technology it is known that the individual foam bubbles approach each other when they grow. Experimental and theoretical investigations (Exerowa, et al., 2006, Karraker and Radke, 2002, Manica, et al., 2008) on different foaming materials, including polymer and aqueous foams, demonstrate that during this process a flat film with thickness h can form between the closest regions of the approaching bubbles, as shown in the Fig. 2.4 (a) and (b). Hydrodynamic interactions as well as buoyancy, electrostatic, van der Waals and steric forces, together with other interactions, can be involved in the formation of the film (2008, Stockle, et al., 2010).

According to the extended Deryagin-Landau-Verwey-Overbeek (DLVO) theory, the disjoining pressure Π(h) responsible for the thinning of a liquid film consists of three types of forces (Cosima and Regine von, 2003, Vance, 1999) following Eq. (2.1): the attractive Van der Waals interaction (Π_vdw), the repulsive electrostatic force (Π_el), depending on the surface charge properties, and the steric force (Πst), which depends on the nature and adsorbed species in the presence of a solvent, but significant when the film thickness is below 20 nm (Birdi, 2008, Exerowa, et al., 2006). Other forces such as oscillatory structural forces, hydration and capillary forces occur due to structural ordering at the interface and are active at a separation distance h in the order of 4 nm (2008). Different researchers studied extensively the disjoining pressure Π (h) in relation to the film thickness in aqueous solution with air bubbles to understand the stability of thin liquid films and the influence of electrolyte concentration (Exerowa, et al., 2006, Vance, 1999, Yaminsky, et al., 2010), surfactant type and concentration of polymeric surfactant(Karakashev and Manev, Karraker and Radke, 2002, Stockle, et al., 2010).

Π(h)=Πvdw+ Πel + Π_st (2.1) Different researchers studied extensively the disjoining pressure Π (h) in relation to film thickness h in order to understand the stability of thin liquid films.

The velocity of the bubbles moving against each other under the action of an external force can be calculated using the Reynolds Eq. (2.2) which gives the velocity of the film thinning VRe (2008). As the film between the bubbles becomes thinner the bubbles approach each other with certain speed. Depending on the approaching speed the bubbles behavior can be described in the following terms; stable for VRe =0.01 -0.04 mm/s, transiently stable fot VRe ~0.04 - 1.2 mm/s and instant coalescence for VRe >1.2 - 140 mm/s (Del Castillo, et al., 2011).

3 2 2

1 2

Re 3 3

1 2

( )

3

P h rL r

V r r

  (2.2)

where PL = Pc – Π(h); Pc is the capillary pressure, Π (h) is the disjoining pressure, r1 and r2 are the radii of the bubbles, η is the dynamic viscosity of the fluid and h is the thickness of the film.

Fundamental results have been obtained over the years and extensive reviews were made for validating Eq. (2.2). For instance, G.E Charles and S.G Mason (Charles and Mason, 1960) investigated oil and water bubble film rupture and analyzed experimental data with Reynolds Eq..(2.2). Moreover, Vassili V. et al. (Yaminsky, et al., 2010, Yaminsky, et al., 2010) studied the film thinning process in aqueous solution between two air phases in a thin film using Reynolds equation for experiments with a pressure balance where they could control the thinning speed while simulating bubbles approaching each other. Further studies on the film thinning process showed that the equation also works when surfactants are used for stabilizing the liquid foam provided that the assumption holds that the amount of surfactant at the surface remains constant during the thinning process and that there are opposing interfacial fluxes (Stockle, et al., 2010). In addition, other researchers (Manica, et al., 2008, Stockle, et al., 2010, Wang and Yoon, 2006) showed experimentally that the film thinning process with and without surfactant and electrolyte is in agreement with Eq..(2.2).

(a) (b)

Fig. 2.4 (a) Schematic of a thin dry foam film between bubbles forming a flat film; (b) schematic description of a bubble group.

Foam instability can be caused by high surface energy associated to the gas-liquid interface, when the liquid viscosity is less, as well as from diffusion of the gas from the foam bubbles to the atmosphere. Thermodynamically unstable liquid foams are critical in different cases, such as oil recovery, fire extinguishing, food and cosmetics as well as to produce light-weight structures (Gonzenbach, et al., 2006, Hench and Polak, 2002).

One of the major reasons for foam collapsing is coarsening. Coarsening of foam mainly occurs in two ways: by the rupture of the film between two adjacent bubbles or cells and or by Ostwald ripening (Lemlich, 1978). The rupture of thin films is related to the drainage (redistribution of liquid) and stability of foam films (Paul, 2012), whereas the Ostwald ripening is caused by inter-bubble gas diffusion by which liquid foam comes into thermodynamic equilibrium, where one large bubble is energetically more favorable than two smaller bubbles (Lambert, et al., 2007, Sabrina, et al., 2009). Because of the above mentioned processes, bubbles smaller than the average size shrink, while large bubbles become larger resulting in an increase of the average size of the bubbles over time (Lambert, et al., 2007). On macroscopic scale previous research studies on polymer/gas solutions have shown that rheological parameters such as viscosity and surface tension have a considerable influence on the foaming properties (Höhler and Cohen-Addad, 2005, Simon, et al., 2004).

In general, foams are stabilized using two methods: using surfactants which are atomically or molecularly dissolved components in the liquid or by addition of small surface active solid particles which can be adsorbed to the liquid surface (Carolin, 2008, stevenson, 2012). Recent studies have shown that liquids with high energy interfaces have been stabilized by colloidal particles in the gas-liquid interface to design long life foams (Eric, et al., 2004, Stocco, et al., 2011). As presented in Fig. 2.5, particles attached

to the gas-liquid interface lower the free energy of the system by replacing parts of the liquid in the gas-liquid interface. The balance between the interfacial surface tensions of solid particle 𝛾SL, liquid 𝛾LG and gas 𝛾SG is responsible for the stabilization effect.

Particle stabilization can be of advantage compared to the use of liquid surfactants, which rather reduce the interfacial tension of the gas-liquid interface (Paul, 2012).

Fig. 2.5 Adsorption of particles at the gas-liquid interface (Gonzenbach, 2006, Studart, et al., 2006) On macroscopic scale research studies on polymer/gas solutions have shown that parameters such as viscosity and surface tension have a considerable influence on the foaming properties (Höhler and Cohen-Addad, 2005, Simon, et al., 2004). Parametric studies of these parameters on foaming are presented in chapter 6.

Characterization of foam bitumen 2.4

Foam bitumen can be characterized by different parameters such as maximum expansion ratio (ERm), minimum viscosity, foaming index (FI) (Jenkins, 2000), coefficient of foamability and half-life time, which is defined as the time that the expanded bitumen takes to settle to half its expanded volume (Muthen, et al., 1999). The half-life time is used to explain the stability of foam bitumen in general. The desirable design value of the half-life time is difficult to determine since it varies with the addition of water. The maximum expansion ratio (ERm= Vmax /Vo) is determined by the ratio of the maximum volume of the expanded foam bitumen (Vmax)and the volume of the original unfoamed bitumen (Vo). Both half-life time and maximum expansion ratio are still taken as main indicators of foam quality, in spite of the fact that these parameters have no direct link to the physical properties of bitumen in the foam. Sufficient expansion of the bitumen is required for adequate coating of mineral aggregates. Inadequate expansion of

foam bitumen lacks dispersion on the mineral aggregate during mixing (Ramanujam and Jones, 2007). The actual expansion ratio (ER_a) considers the expansion of the foam during the spraying for the foam, since the foam decays while spraying the foam in vessel. It is determined by extrapolating the curve for the time of spraying (ts=5 s) as presented in Fig. 2.6 (a). This parameter needs accurate measurement of the foam bitumen decay curve. The FI is the sum of the areas A1 and A2 as indicated in Fig. 2.6 (a) (Jenkins, 2000). Hence FI reads

1 2

4 1

4 4ln * *

ln(2) ^m _m 2 ^{m s}

HL c

FI A A ER ER t

ER c

     

         

 

  

  (2.3)

where HL is the half-life time, ER_m is the maximum expansion ratio, c is the ratio of ER_m/ER_a, ER_a is the actual expansion ratio, t_s is the time of spraying (Jenkins, 2000).

(a) (b)

Fig. 2.6 (a) Determination of foaming index from the foam decay curve (Jenkins, 2000); (b) optimum water content using the Wirtgen approach (Wirtgen, 2005).

The FI parameter was considered as a good indicator of the foam quality since it is calculated from the whole decay curve and includes both the half-life time and expansion ratio. An investigation by (Jenkins, 2000) indicated that the optimum water content for producing a good foam asphalt mixture can be found by characterizing the foam with an empirical parameter called “foaming index” FI. The FI parameter can be used to optimize the amount of water in the foam bitumen. The optimum water content can be found by taking the water content at the maximum of FI as presented in Fig. 2.7 (a). However, it needs accurate measurement of the decay curve (Ozturk and Kutay, 2013). Later on it was found that the FI concept cannot be applied for all types of bitumen. The half-life

time is not always decreasing with increasing maximum expansion ratio (ER_m) (Saleh, 2006, Sunarjono, 2008). Since half-life time may increase or decrease with increasing water content in the foam, it is not a clear indicator for the stability of the foam bitumen.

Therefore, it is not always possible to find the optimum water content from the FI, as demonstrated in Fig. 2.7 (b) with data from literature (Saleh, 2006). The Wirtgen approach (Wirtgen, 2005) states that the minimum expansion ratio should be ER min =8 and the minimum half-life time HLmin = 6 s. Moreover, the optimum foaming water content (FWC) can be found by taking the average of the two water contents required to meet the specified minimum criteria as shown in Fig. 2.6 (b). Rheology and stability of the foam mainly depend on the foam's bubble size distribution and liquid volume fraction, i.e. the ratio of liquid volume to the total volume of the foam (Körner, et al., 2002). Saleh (Saleh, 2006) stated that rotational viscosity measurement allows to determine a water content that produces the lowest average foam viscosity (FV) over a period of 60 s and can be considered as optimum water content as presented in Fig. 2.7(b). This means, if two different bitumen have the same viscosity but different expansion ratio and half-life time, the mixing behavior of the foam bitumen can be similar (Saleh, 2006).

(a) (b)

Fig. 2.7 (a) Optimum water content using FI parameter (Jenkins, 2000) (b) optimum water content based on rotational viscosity measurement (Saleh, 2006).

A minimum expansion ratio ER _min =4 is required for adequate mixing of the foam bitumen asphalt (Jenkins, 2000). The FI parameter can be used to optimize the amount of water in the foam bitumen by taking the water content at the maximum of FI. The minimum expansion ratio and half-life time recommendation from other literatures is presented in Tab. 2.1. The information from Tab. 2.1 is considered to compare with the measured minimum expansion ration and half-life time as shown in Fig. 7.1 in chapter 7.

Tab. 2.1 Minimum expansion ration and half-life time recommendations from different literature (T^* refers to the temperature of the mineral aggregates used in the mixture).

Source Minimum

ERmin

Minimum HLmin (s) The Council for Scientific and Industrial Research, 1999

(Muthen, et al., 1999)

10 12

TRL report 386 by Milton and Earland, 1999 (Milton and Earland, 1999)

10 10

Chiu and Huang, 2002 (Chiu and Huang, 2002) 8 8

Wirtgen, 2005 (Wirtgen, 2005) 8 6

The Austroads guide to pavement technology, 2006 (Leek and Jameson, 2011)

15 30-45

Asphalt academy south Africa, 2009 (Asphalt Academy, 2009)

8 for T^*=10-25°C 10 for T^* >25°C

6

The Queensland department of transport and main road, 2009 (Jothi, et al., 2009)

10 20

Foam asphalt mixtures 2.5

Producing foam asphalt mixtures dates back to 1928 when the first hot bitumen foaming system was patented. Originally, only foam asphalt mixtures with cold mineral aggregates were produced resulting in rather high air void contents from evaporating the water added to produce the foam. Hence, large mineral aggregates were only poorly coated. As a consequence, the performance of asphalt mixtures with cold mineral aggregates was far beyond today's hot mixed asphalt (HMA) and therefore only sufficient for base and foundation layers (D.C. Collings and K.J. Jenkins, 2009). Since then, several improvements have been made such that foam bitumen is currently used in many countries (Van der Walt, et al., 1999). Recent developments dealt with warm or half- warm foam asphalt by using warmed mineral aggregates. By this way, coating of the mineral aggregates was considerably improved with positive effects on the asphalt properties (Jenkins, et al., 1999). Moreover the amount of water was reduced resulting in asphalt pavements with low air void contents and a quality comparable to HMA (Van de Ven, et al., 2007, Voskuilen, et al., 2004). However, these results are mainly based on laboratory experiments and not validated in field. In the traditional hot mix asphalt process the aggregates are heated to 180 °C and higher. By reducing the temperature to 115 °C, the energy consumption is decreased by about 40 % and CO₂ emissions by 33 % (Olard. F., et al., 2008). Even higher energy savings are feasible if the production temperature can be reduced further. Moreover, asphalt produced at lower temperature does not cool rapidly, hence allowing longer hauling distances and/or time periods for