Mix design of foamed bitumen mixtures

8/21/2019 Mix design of foamed bitumen mixtures

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Table of Contents1 Introduction .................................................................................................................................... 1

1.1 Problem statement .................................................................................................................1

1.2 Aims and objectives ......................................................................................................................1

2 Study on mix design parameters ....................................................................................................3

2.1 Introduction ............................................................................................................................ 3

2.2 Mix design considerations ......................................................................................................3

2.2.1 Bitumen type................................................................................................................... 3

2.2.2 Foaming conditions.........................................................................................................4

2.2.3 Foam characteristics .......................................................................................................4

2.2.4 Aggregate properties ......................................................................................................4

2.2.5 Mixing..............................................................................................................................5

2.2.6 Mixing and Compaction water content .......................................................................... 5

2.2.7 Compaction..................................................................................................................... 6

2.2.8 Curing ..............................................................................................................................7

2.2.9 Materials .........................................................................................................................8

2.3 Methodology.........................................................................................................................10

2.3.1 Introduction ..................................................................................................................10

2.4 Results and Discussion..........................................................................................................14

2.4.1 Optimum foam characteristics......................................................................................14

2.4.2 Design gradation ...........................................................................................................15

2.4.3 Optimum mixing water content (pre wet water content)............................................ 18

2.4.4 Optimum compaction effort ......................................................................................... 20

2.4.5 Mixture volumetric composition ..................................................................................23

2.4.6 Selection of Foamed Bitumen content (Mechanical tests)...........................................25

2.5 Conclusions ...........................................................................................................................28

3 Study on accelerated curing..........................................................................................................30

3.1 Scope of the study ................................................................................................................30

3.2 Effect of time and temperature on water loss .....................................................................30

3.3 Effect of time and temperature on stiffness.........................................................................31

3.4 Effect of water content on stiffness...................................................................................... 33

3.5 Effect of RAP and Cement on stiffness .................................................................................33

3.6 Conclusions ...........................................................................................................................34


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3.7 Further curing study..............................................................................................................34

4 Further study................................................................................................................................. 35

4.1 Generating inputs for Pavement analysis and design (June 2013 August 2013)...............35

4.1.1 Specimen Fabrication.................................................................................................... 35

4.1.2 MEPDG inputs frequency sweep test.........................................................................35

4.1.3 Material constants generation for non-linear elastic analysis...................................... 36

4.2 Analysis and design (September 2013 January 2013)........................................................36

4.2.1 Non-linear elastic analysis using KENLAYER.................................................................. 36

4.2.2 Sensitivity analysis of distress types to FBM inputs......................................................36

4.3 Fatigue and durability study (February 2014-June 2014).....................................................36

5 References .................................................................................................................................... 37


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1 Introduction

1.1 Problem statement

Unlike Hot Mix Asphalt (HMA) in which bitumen and aggregate are primary components, in foamed

bitumen (FB) treated mixtures, water and, often, active fillers are the additional ingredients.

Therefore, FB mixtures involve more variables, have a less controllable mixing procedure, and are

expected to exhibit more complex behaviour than HMA. It is a very common practice to add

cementitious additives in FB treated mixtures (FBM) for various reasons such as; to achieve early

strength, accelerated curing. Moreover, this technique of bitumen foaming has commonly been

used for recycling, which means mixtures including Reclaimed Asphalt Pavement (RAP) material.

From the literature review it was understood that the permanent deformation behaviour of FBM

with cementitious and RAP addition is well documented (Halles and Thenoux, 2009,Jenkins, 2000).

However, fatigue behaviour of these mixtures with these additives has not been studied so

extensively. It is a well-known fact that FB mixtures are neither fully bound (like HMA) nor unboundmixtures. The presence of mastic (fines + bitumen) brings a difference in these mixtures in which

aggregates are bonded with this mastic. Therefore, water susceptibility of these mixtures is much

more important than in the case of HMA. Usually, therefore, tests on FBMs are mostly accompanied

by water susceptibility tests. FBMs gain strength with time after compaction. This is due to loss of

mixing water in the mix over the time. This process of gaining strength of FBM with expulsion of

water is called curing. With reduction of water in FBM strength (compressive and tensile) and

stiffness increase (Asphalt academy, 2009). Maturity functions can potentially be used to convert the

actual curing temperature to an equivalent temperature. This concept of a time-temperature factor

can be used to quantify the strength development of FB mixtures.

It has been learned that stress and strain signals occur in the pavement under traffic loading. The

magnitude of the stress varies from one location to the other. HMA exhibits stress independent

behaviour at least to a certain level of stress. So, it has been a common practice to analyse HMA

layers using linear elastic theory. Due to the un-bound nature of the FB mixtures, applicability of

linear (stress dependency) behaviour is questionable. In order to obtain reliable stress and strain

data from analysis, an experimental program with varying stress levels has to be carried out.

After detailed literature review, the following aim and objectives are considered.

1.2 Aims and objectivesThe primary aim of this research is to understand the behaviour of FB mixtures with cementitious

additives and RAP material and to design pavements with FBM layers with some confidence.

The objectives of the research are as follows;

1. Detailed literature review of mix design and structural design procedures that are being

followed by different agencies.

2. Identifying critical mix design parameters and studying their influence on mechanical

properties.

3. Understanding curing (water loss and strength gain) mechanisms in FBM.

4. Developing strength-maturity relationships for FB mixtures.


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5. Studying fatigue behaviour and durability of FB mixtures with cement and RAP material.

6. Understanding the stress dependency behaviour FB mixtures.

7. Conducting mechanistic (non-linear elastic) analysis of pavements with FB layers by using

the results obtained in 4.

8. Designing pavements (structural design) using the Mechanistic Empirical Design Guide

(MEPDG AASHTO design guide).


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2 Study on mix design parameters

2.1 Introduction

Unlike HMA (Hot mix asphalt), there is no universally accepted mix design method for FBM. Most of

the agencies Asphalt Academy, 2009 and 2002, Wirtgen 2004 and 2010 which use FBM have their

own mix design procedures which are the result of numerous efforts over decades (Jenkins, 2000,

Ebels and Jenkins, 2007, Jenkins and vanderVen, 2001, Jenkins et al., 1999, Muthen, 1998, Acott,

1979,Bowering and Martin, 1976,Castedo et al., 1982,Gui-Ping and Wing-Gun, 2008, Kim and Lee,

2006,Ramanujam and Jones, 2007,Saleh, 2003) for improving the mix design guidelines for FBM. In

spite of all these efforts, Foamed Bitumen application in cold recycling suffers from lack of a

standardised mix design procedure and as a result the mix design parameters such as Foam

characteristics, mixing, compaction, curing and testing that are being adopted are far from being

standardised. To overcome this, a research was undertaken at the University of

Nottingham(Sunarjono, 2008) to develop a mix design procedure by identifying critical mix design

parameters. The mix design parameters identified were

Type of bitumen

Foaming conditions- Foaming water content (FWC) and Foaming temperature

Foam characteristics- Maximum expansion ratio (ERm) and Half-life(HL)

Type of mixer

Aggregate gradation

Foamed Bitumen content

Secondary binder (Cement, lime etc)

Mixing and compaction water content

Compaction effort

Conditioning of compacted specimens (Curing)

The research by Sunarjono, 2008was focussed on the influence of the bitumen type, the foaming

conditions, foam characteristics and mixer type on the mechanical properties of FBM. The major

outcomes of the work were recommendations for producing an optimised FA mixture in terms mixer

type and usage, selection of binder type, bitumen temperature, foam characteristics. Therefore the

present study focussed on other mix design parameters such as aggregate gradation, FB content,

mixing and compaction water content, compaction effort and curing conditions. Thus, the primary

objective of the present study is to propose a practical and consistent mix design procedure.

2.2 Mix design considerations

2.2.1 Bitumen type

In HMA mix design, the expected traffic and the regional climate influence the selection of the

bitumen type. However in FBM mix design, fomability (foaming potential) of the bitumen and the

mixture compactability has to be considered during selection of the bitumen type. Harder bitumen

was avoided in past as it produces poorer quality foam leading to poorer dispersion of fines.

However, it was found that FBM with harder bitumen had a positive effect on mixture stiffness due

to high stiffness of the bitumen (Sunarjono, 2008). However, a FBM with hard binders has to be

mixed in a high speed mixture to ensure good dispersion of mastic.


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2.2.2 Foaming conditions

Temperatures of the bitumen during foaming and Foaming water content (FWC) are considered as

important parameters during the bitumen foaming process (Jenkins et al., 1999,Sunarjono, 2008,

Kim and Lee, 2006). In general the higher the bitumen temperature the higher the ERm and lower

the HL. FWC also has similar effect on the foam characteristics (Muthen, 1998, Brennan, 1983,

Maccarrone et al., 1994). Efforts were made to propose a range of foaming temperature and

FWC(Sunarjono, 2008). However, it was understood from the results that the range depends on the

bitumen type. For the type of binder used in this work which is 70/100 (90 pen) a range of 150C -

180C and a FWC of 2.5% - 5% was recommended. However, sensitivity analysis of these

characteristics with FWC and temperature was conducted to verify these values.

2.2.3 Foam characteristics

The first qualitative characterisation of FB was the result of a Mobil Australia study ( Mobil Oil

Austrailia Ltd, 1971) in Australia. In their work quality of bitumen foam was characterised by ERm

and HL. Since then along with ERm and HL, FB has been characterised by other characteristics such

as Foam Index (FI)(Jenkins et al., 1999) and minimum viscosity (Saleh, 2006b) and quantitative

recommendations for these foam characteristics were made by some studies (Bowering and Martin,

1976, Ruckel et al., 1982, Muthen, 1998). Attempts by researchers to apply ERm and HL for

optimising foam characteristics have (Maccarone et al., 1995, Acott and Myburgh, 1983,Fu et al.,

2011) been successful. But, optimising FI for FBM was not successful as no optimum point could be

achieved (Sunarjono, 2008) and measuring foam viscosity was found to be difficult because of foam

distortion and this may lead to unreliable results (Namutebi et al., 2011). Consequently in this study

it was decided to study ERm and HL as foam characteristics to be optimised.

2.2.4 Aggregate properties

Many researchers have showed that a wide range of aggregates can be used with FB ranging from

crushed stone (Ruckel et al., 1982,Saleh, 2006a) to sand (Acott and Myburgh, 1983,Bissada, 1987).

Research has showed that aggregate properties such as aggregate type (Ruckel et al., 1982,Saleh,

2006a, Acott and Myburgh, 1983, Bissada, 1987 ), aggregate gradation (Akeroyd and Hicks, 1988,

Saleh, 2006a,Al-Abdul Wahhab et al., 2012,Namutebi et al., 2011), amount of fines (Bowering and

Martin, 1976,Lee, 1981,Csanyi, 1960,Castedo et al., 1982,Sakr and Manke, 1985,Abel, 1978,Acott,

1979) have significant effect of strength and deformation characteristics of FBM. Alongside these,

angularity of aggregate, Plasticity Index (PI) have also been considered (Sakr and Manke, 1985).

The importance of fines (< 63 (or) 75 micron) has been well documented. It has been argued that the

higher the amount of the fines the more promising the mix (Ruckel et al., 1982,Roberts et al., 1984,

Bissada, 1987, Maccarrone et al., 1994). This argument seems to be true, because foam bonds with

fines to form mastic; more mastic will be formed if more fines are available and the mastic enhances

the FBM properties. However, this was not found to be true in all the findings. Mixtures with higher

amount of fines found to have lower soaked strength (Fu et al., 2011). In addition, the amount of

fines suggested to be in mix ranged from 3% (Sakr and Manke, 1985) to 40% (Lee, 1981) which is a

very wider range. Hence, efforts were made in this study to understand the effect of fines in the

FBM by the use of strength and stiffness characteristics of the mixture. However, work was limited

to a single aggregate type (limestone), four different gradations out of which two are fuller curve


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gradations with 20mm and 32mm nominal maximum size and the other two are gradations with

more fines than fuller gradations.

2.2.5 Mixing

Foamed Bitumen begins to collapse rapidly once it comes into contact with relatively cold

aggregates. Therefore, the mixing process should be a dynamic one. Consequently FB is most often

applied directly from the laboratory foaming plant to the aggregate as it is being agitated in the

mixer. As different mixers can produce up to 25% difference in strength (Academy, 2009) selection

of an appropriate mixer is very important in production of FB mix. It is always recommended to

utilise a mixer that simulates site mixing. From the literature it was found that most of the research

was carried out using a Hobart type mixer (blender type) (Sakr and Manke, 1985,Lee, 1981). Pug mill

drum mixers and milling-drum mixers are the most commonly used mixers on site for the production

of FB mixtures. These mixers provide sufficient volumes in the mixing chamber and energy of

agitation to ensure better mixing (Jenkins, 2000). A pug mill type mixer is therefore recommended

for production of FB mix that is representative of the field (Long et al., 2004). Hence, a twin shaft

pug mill is adopted in this work.

Mixing time should be in accordance with the time required by the bitumen foam to collapse.

Therefore, half-life is an important factor to be considered in the mixing process. The greater the

half -life the better the mixture is predicted to ge. A Half-life of more than 60 seconds can be

achieved with addition of foamant (Maccarone et al., 1995); but this approach is seldom used. In the

laboratory a mixing time of 60 seconds is recommended (Bissada, 1987)which is longer than in situ

mixing but simulates the difference in the energy of the laboratory mixer and field plant and the

same was adopted in this study.

2.2.6 Mixing and Compaction water content

The water content during mixing and compaction is considered as one of the most important mix

design parameter in FBM(Bowering, 1971, Ltd, 1973, Xu et al., 2012). The mixing water content

(MWC) of FBM is defined as the water content in the aggregate when the FB is injected (Fu, 2009).

The MWC helps in dispersion of the mastic in the mix (Brennan, 1983,Jenkins, 2000). However, too

much water causes granular agglomerations which do not yield optimum dispersion of the mastic in

the mix (Ruckel et al., 1982,Fu et al., 2010). It has been considered as best to mix when the water

content of the material is at fluff point; i.e, the water content that gives the material its maximum

loose volume (Sakr and Manke, 1985, Bowering and Martin, 1976, Brennan, 1983). This is

approximately 65% to 85% of OMC as determined by the modified Proctor test (Ruckel, 1978,Abel

and Hines, 1979,Academy, 2009). This range was also validated by Fu et al., 2010 by investigating

the mixng phenomenon by combined micromechanics observations and mechanical testing. In this

study the MWC was always in this range with some exceptions where 65% of OMC has also needed

to consider. It should be noted that, in the present study the mixing and compaction water contents

are the same as the specimens are compacted directly after mixing and therefore it is assumed that

there is no loss of the water content during compaction.

In view of this fact many studies have been focussed on the optimisation of compaction water

content (CWC). (Lee, 1981)) and (Bissada, 1987) optimised CWC with reference to Marshall stability

and found that the optimum CWC was very much dependent on other mix design variable such as

amount of fines and bitumen content. (Sakr and Manke, 1985)), related the CWC to the mix design


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variable and recommended a linear relationship among them to obtain optimum CWC. However, the

work was performed on a FB stabilised sand mixture which did not have any coarser fraction of

aggregate. Moreover, the work was based on optimising the density, without considering any

mechanical properties. The concept of optimum fluid content was later borrowed from emulsion mix

design in which the sum of the water and bitumen content should be close to OWC (Castedo-Franco

and Wood, 1983, Muthen, 1998). This concept considers the lubricating action of the binder in

addition to that of water. Thus the actual water content of the mix for optimum compaction is

reduced in proportion to the amount of binder incorporated. However, the works of ( Kim and Lee,

2006) and (Xu et al., 2012); which optimised CWC based on both density criteria and fundamental

tests (ITS and tri-axial tests) on Marshall specimens, question the lubricating action of bitumen in the

mix. Although the above discussed works are very informative, they have their limitations as

discussed and little attention has been paid to optimising CWC with the gyratory compactor.

Therefore, the present work aims at obtaining a rational range of CWC for mix design and to study

the lubricating action of the bitumen during the gyratory compaction with the help of fundamental

tests such as ITS, ITSM and the volumetric composition of FBM.

2.2.7 Compaction

As density achieved is crucial to the ultimate performance of the mix, special attention needs to be

paid to the compaction phase of mix design. Because of the presence of the water phase in FBM,

this makes the compaction mechanism different from that of HMA. Various laboratory compaction

methods such as Marshall compaction(Brennan, 1983,Muthen, 1998,Kim and Lee, 2006,Xu et al.,

2012), vibratory compactor (Shackel et al., 1974, Bowering and Martin, 1976, Jenkins, 2000),

gyratory compactor (Brennan, 1983,Maccarrone et al., 1994,Jenkins et al., 2004,Saleh, 2006b) in

the past. There are very well established guidelines for Marshall compaction (Wirtgen, 2004) and

vibratory compaction (Wirtgen, 2010,Academy, 2009). However, there are no established guidelinesfor gyratory compaction of FBM in terms of compaction effort (number of gyrations and compaction

conditions (gyratory angle and gyratory pressure). Past studies have evaluated the feasibility of using

the laboratory gyratory compaction on FBM. In these studies efforts were made to obtain the design

compaction effort in terms of compaction pressure, compaction angle and number of gyrations

(Table 2-1). The compaction pressure recommended by Australian guidelines (0.24kPa) and 1.38kPa

from Table 2-1 were given earlier to SHRP work on HMA and seems to require major adjustments.

(Jenkins et al., 2004) tabulated conditions were based on by single water content and single FB

content. From preliminary trials it was found that 30 gyrations (recommended Kim and Lee, 2006)

were too few to achieve modified Proctor densities. The ideal compaction effort has to produce mix

densities that are achieved in the field. However, this is not possible in laboratory compaction as the

field densities are very much mixture specific. Therefore modified Proctor density, which is used

worldwide for monitoring laboratory densities, is used as reference in the present study.
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Table 2-1 Gyratory compaction effort on FBM by different researchers

Summary of gyratory compaction effort on FBM by different researchers

Number of

gyrations

(N)

Compaction

pressure

(kPa)

Compaction

angle

(degrees) reference density

Brennan, 1983 20 1.38 - 2.25kg/m3

Maccarrone et al.1994 85 0.24 2 field density

Jenkins et al., 2004 150 0.6 1.25 Modified proctor density

Kim and Lee, 2006 30 0.6 1.25 Marshall density (75 blows)

Saleh, 2006b 80 0.24 2 Australian guidelines for HMA

The literature review indicates that the past studies although informative still had some limitations

warranting additional study and the mix design compaction effort needs to be established. The

present work aimed at determining the mix design compaction effort with gyratory compactor

required to match practical field densities obtainable.

2.2.8 Curing

Curing is the process in which FBMs lose their water content at elevated temperatures. ( Bowering,

1970) found that FBMs gain their full strength only if they expel a large amount of their mixing water

content. From the literature (Bowering and Martin, 1976,Acott, 1979) it was found that pavements

with FB treated layers exhibited premature distress in days rather than in weeks or months after

construction indicating the need for expulsion of water for performance of pavements with FBM.

Ruckel et al., 1982 , concluded that the sample water content was the most important parameter

affecting mix strength. Therefore, a laboratory mix design procedure needs to simulate the field

curing process in order to correlate the properties of laboratory prepared mixtures with those of

field mixtures. An accelerated laboratory curing procedure which is curing at elevated temperature

is the best available option. From the literature it was found that an elevated temperatures of 40C

or 60C have usually been used to accelerate the laboratory curing mechanism. Most of the previous

researchers (Acott, 1979, Maccarone et al., 1995, Muthen, 1998, Lane and Kazmierowski, 2003)

adopted 60C curing temperature which was proposed by Bowering, 1970. However, Ruckel et al.,

1982 expressed his concern over a curing temperature of 60C which is above the softening point of

many bitumen grades used for foaming. This high temperature may cause a change in mix

properties, which is not desirable. Ruckel et al., 1982 recommended curing at 40C for 3 days which

is for long term curing. Jenkins et al., (2004), (Marquis et al., 2003) adopted this method of

laboratory curing at 40C for 3 days. The present study has also adopted this curing regime of for

mixture design purposes.
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2.2.9 Materials

2.2.9.1 Bitumen

Penetration grade 70/100 supplied by Shell was used in this study. The properties of the bitumen are

tabulated in Table 2-2.

Table 2-2 Properties of 70/100 bitumen used in the study

70/100 grade bitumen properties

Specific gravity 1.03

Penetration Index at 25C 90

Softening Point (C) 45

Viscosity at 135C(mPa-s) 321

2.2.9.2 Virgin aggregates

The virgin mineral aggregate used in this study is limestone from Dene quarry, Derbyshire, UK. It has

been stored separately in stockpiles of size fractions31.5mm, 20mm, 14mm, 10mm, 6mm, dust and

filler (


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2.2.9.3 Reclaimed Asphalt Pavement (RAP)

The RAP material used in this study was supplied by Lafarge Aggregates Limited obtained from

Elstow Asphalt Plant in Bedfordshire. A visual inspection of the material as supplied indicated

that the RAP aggregate material included particles comprising of slate, broken glass, wood and

tiny plastic components. The RAP aggregate material from the quarry was initially air dried at

room temperature in the laboratory at 205C for 24 hours and then placed in a

thermostatically controlled oven at a temperature of 40C for 24 hours and stored in sealed

containers for the further use. A composition analysis of the RAP aggregate material was also

conducted in order to determine the properties of the RAP and its extracted components. The

composition analysis conducted conformed with BS 598-102:2003, BS 598-101:2004 and BS EN

933-1:2012. The results of the composition analysis and the gradation of the RAP after binder

extraction are stated in Table 2-4 and Table 2-4,whileFigure 2-2 Gradation of RAP and recovered

aggregateshows the gradations of both the RAP aggregate material as obtained from the quarry

and the RAP aggregate material after binder extraction.

Table 2-4 Physical properties of RAP used in the study Table 2-5 Properties of recovered bitumen from RAP

Particle Density (kg/m3) and Water

Absorption (%) (BS EN1097-6:2000)

Size (mm) 20mm-4mm


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2.3 Methodology

2.3.1 Introduction

This section discusses the methodology adopted in the study to develop practical FBM mix design

methodology. A detailed and careful experimental design was prepared for the study and is

tabulated in table. The factors were selected considering the findings of previous work done at the

University of Nottingham. Figure shows a flow chart that illustrates the methodology that was

adopted in this laboratory experimental study. For optimising MWC/CWC only 100%VA mixture was

considered. Whereas for optimising compaction effort and bitumen content 50%RAP and 75%RAP

mixtures were also considered.

Table 2-6 Experimental design for mix design study.

Mix design

parameter factorial levels Remarks

Bitumen type 90pen (70/100 grade)

constant throughout the

experiment

Foam Characteristics Erm = 10 Recommended by Sunarjono, 2008

for 70/100 bitumenHL (seconds) = 6

Foaming conditions

Temperature

(C):150,160,170 Recommended range by

Sunarjono, 2008FWC(%): 1,2,3,4,5

Mixer type Pug mill type mixer


experiment

Aggregate type lime stone


experiment

Aggregate gradation

31.5 fuller, 31.5 finer, 20mm

fuller, 20mm finer to study the effect of fines

MWC/CWC % of OMC: 65,75,85,95 variable to be optimised

FB content % of total mix: 2,3,4,5 variable to be optimised

Volumetrics dry density, % air voidsto study bitumen-water interaction

Mechanical tests ITS-dry, ITS-wet, ITCY to obtain design binder content

Step 1

Foamed bitumen was produced using a laboratory mobile foaming plant type Wirtgen WLB 10 in

which the bitumen was foamed at a water pressure of 6 bars and an air pressure of 5 bars. As seen

in Table 2-6 Experimental design for mix design study., the present study is limited to single grade

70/100 (90pen) bitumen. The characteristics of foamed bitumen (ERm and HL) were obtained by

applying different foaming water contents (FWC) (1% to 5% of the amount of bitumen by weight)

and temperatures (150C, 160C and 170C). The magnitude of these foam characteristics were

obtained by collecting foam produced by 500 grams of bitumen in a 275mm diameter steel bucket. A

Wirtgen measuring instrument (dipstick), which is calibrated to measure the ERm if 500 grams of

bitumen is sprayed into the bucket, was used to record ERm and HL. A stopwatch was used to find

out the time the foamed bitumen took to collapse to half of its maximum volume. The Figure 2-4,

Figure 2-5 and Figure 2-6 show the effect of FWC on expansion ratio and half-life respectively. A

minimum half-life of 6 seconds and expansion ratio of 10 were adopted as selection criteria. The


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optimum foam characteristics were obtained by plotting ERm and HL versus FWC. The procedure

recommended by Asphalt Academy, (2009) was adapted to obtain optimum FWC and the findings

are discussed in section 2.4.1.

Step 2

The maximum dry density (MDD) and optimum water content (OWC) of each mixture gradation(Figure 2-8) that was studied were determined using modified Proctor in accordance with BS EN

13286-2: 2004. Obtain Npre, the number of gyrations required to compact the aggregate and water

mixture to attain densities that were obtained, by compacting mixture with gyratory compactor.

Compaction was carried out on the aggregate in the gyratory compactor at different water contents

(95%, 85%, 75%, 65% of OWC). The densities were optimised to obtain optimum trial mixing water

content (MMC-trial). The MMC-trial determination for a mixture is shown in Figure 2-8. The Npre,

MDD, OWC, MWC-trail are tabulated in Table 2-7.

Step 3

In this step each graded aggregate was mixed at a water content of MWC-trial and a FB content of

3% and compacted to the optimised density obtained in Step2. Mixing was carried out in laboratory

scale mixing unit, WLS 30, which is a twin shaft mixer. The mixer was positioned in such a way that

the foamed bitumen can be sprayed directly on the mixture in the mixer and mixing was carried out

for 60 seconds. Before foamed bitumen was sprayed, the aggregate and water were mixed for 60

seconds. After mixing the mixture was riffled and transferred into gyratory compactor moulds. The

gyratory compaction was carried out on the mixture targeting the optimised density that was

obtained in Step 2.

Step 4

The moulds were placed in a forced draft oven at 40C for 24 hours before samples were extracted.

The extracted specimens were cured for 3 days at 40C in oven. The cured specimens were then

tested for indirect tensile stiffness modulus (ITSM) and indirect tensile strength (ITS). The results are

as plotted in Figure 2-9 and Figure 2-10 and are discussed in section 2.4.2. An aggregate gradation

was selected based on ITSM and ITS results.

Step 5

Mixing and compaction was carried out on the aggregate of each selected gradation with varying

water content (95%,85%,75% and 65% of OWC) and varying foamed bitumen content (2%, 3%, 4%,

5% by weight of aggregate) to study the interaction of water and bitumen and to optimise the

mixing water content (MWC). The two compaction methods, modified Proctor compaction andgyratory compaction, were studied. After mixing the mixed material was compacted in using

modified Proctor equipment and densities were obtained. This was done for all possible

combinations and the results are as plotted in Figure 2-11. The same mixtures were also compacted

using the gyratory compactor to Npre gyrations.

Step 6

In this stage, all possible combinations of mixtures were mixed and compacted to the modified

Proctor densities that were obtained in Step 5. Gyratory compacted moulds after compaction were

kept at room temperature for 24 hours and then specimens were extracted. The extracted

specimens were cured at 40C and water content of the mix was monitored. The mechanical tests


14/40

12

were carried out on the cured specimens after 3 to 5 days depending on the amount water in the

specimen. The tests were carried out on all specimens at the same water content (in between 0.6%

and 0.65%) to eliminate the effect of the factor, water content in the mix, on the measured

mechanical properties. The effect of mixing water content on the mechanical properties can be seen

in plots in the figures 2.12-2.15. The findings of optimisation of MWC with respect to mechanical

properties are discussed in section 2.4.3.

Step 7

Plots of wet densities from gyratory compaction from Step 5 versus number of gyrations were

plotted (figures 2.17 2.20) and number of gyrations required to reach the modified Proctor

densities were identified using these plots. Ndesign was identified from the range of gyrations that

were obtained.

Step 8

Mechanical test (ITSM, ITS-dry, ITS-wet) were performed on the specimens compacted to N designgyrations and an OMWC obtained. The tests were performed on all mixture types (100%VA.,

50%RAP and 75% RAP mixtures).


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13

Obtain FB characteristics (ERm and HL)

@T (C): 150, 160, 170

@FWC(%):1,2,3,4,5 (Section 3.2 and figure)

Step1 and Figure 2-4 and Figure 2-5

Determine of Optimum FWC from ERm and

HL data (Step1 and Figure 2-6)

Obtain OMC and MDD for selected four

gradations ( BS 13286-2: 2004) (Step2,

Figure 2-8 and Table 2-7)

Obtain Nprefor all gradations by compacting

the aggregate at OWC to MDD (Step2,

Figure 2-8 and table 2-7)

Optimise densities for all four gradations

and obtain MWC-trial in terms of %OWC

(Step2 and Figure 2-8

Compact aggregates of all four gradations

@MWC-trial and trial FB content of 3% and

cure the specimens at 40C for 3days (Step3)

Test the specimens for ITSM (BS EN 12697-

26:2012) and ITS (BS EN 12697-23:2003)

(Step4, Figure 2-9and Figure 2-10)

Select

deign

gradation

Mix the aggregates

@FB content(%): 2,3,4,5 and

@MWC(%OWC):65,75,85,95,105

(Step5)

Compact the mix using gyratory compactor

to Npre gyrations (Step7)

Obtain densities at all combinations using

Modified Proctor compaction (Step5 and

Figure 2-11)

Obtain Ndesign using gyratory compaction

data and densities obtained by Modified

Proctor compaction (Step7 and Figure 2-16-

2-20)

Compact using gyratory compactor to

modified Proctor densities obtained in Step5

(Step6)

Test the specimens for ITSM, ITS-dry and

ITS-wet (Mechanical tests) (Step6)

Optimise mechanical properties obtain

OMWC (Step6 and Figure 2-12-2-15)

Mix the aggregate at OMWC and compact to

Ndesign at varying FB content of 2%,3%, 4%

and 5%

Test for mechanical properties and

obtain design FB content (Step8 and

Figure 2-24-2-28)

Figure 2-3 Flowchart of methodology implemented in mix design study


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14

2.4 Results and Discussion

2.4.1 Optimum foam characteristics

The results of the foaming experiment that was conducted on 70/100 grade bitumen are shown in

Figure 2-4 and Figure 2-5. The experiment was conducted varying FWC and temperature and

keeping all other factors constant. It can be observed from the plots that as foaming temperature

increased from 150C to 170C, the expansion ratio increased while the half-life decreased. This

trend in expansion ratio and half-life is because of increasing thermal energy available to convert

water to steam at higher temperatures. This leads to formation of foam bubbles of large volume and

hence higher ERm. The trend could also be attributed to the viscosity of bitumen at higher

temperatures. As the viscosity of bitumen has an inverse relation to temperature; its viscosity

decreases and hence the bigger size of the bubbles at higher temperatures. The reason for the trend

of half-life is as the foam temperature increases, the temperature of the foam increases and leads to

less stable foam.

The ERm value increased with increase in FWC while the HL decreased. This trend is because of lack

of enough thermal energy to convert larger volume amount of water available into steam. It has to

be noted that HL values decreased with increase FWC and at higher FWC, HL values tends to be

constant.

Figure 2-4 Sensitivity study on ERm with FWC

0

5

10

15

20

25

30

35

0 1 2 3 4 5 6

ExpansionRatio

FWC(%)

Expansion Ratio vs FWC (70/100 grade)

150C

160C

170C


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15

Figure 2-5 Sensitivity study on HL with FWC

Figure 2-6 Optimising foam characteristics

2.4.2 Design gradation

As discussed in Section 2.2.4, the gradations were selected in such a way that the study should

understand the importance of the amount the fines (63m passing) and their interaction with the

available surface area of the coarser fraction of aggregate. It is a known fact that smaller the

maximum size of aggregate the more the surface area. The results of mechanical tests that were

performed in step 4 are plotted and can be seen in Figure 2-9 and Figure 2-10. For FB operations, it is

recommended that the minimum amount of aggregate passing 63m or 75m sieve size should be

5% and it can be seen from Figure 2-7 that the amount of fines in the aggregate gradation studied

was always greater than 5%. The results of ITSM and ITS tests that were performed on specimens

compacted at OMWC-trial (Table 2-1) and 3% FB and to N pregyrations are presented in Figure 2-9and Figure 2-10. Both ITSM and ITS values on the mixtures with 31.5mm-finer and 20mm-finer

0

5

10

15

20

25

0 1 2 3 4 5 6

Half-life(seconds)

FWC (%)

Half-Life vs FWC (70/100 grade)

150C

160C

170C

0

5

10

15

20

0

5

10

15

20

25

30

35

0 1 2 3 4 5 6

Half-life(seconds)

ER

and

FWC (%)

Foam Characteristics at 170C

ER

HL


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16

gradations were found to be higher than those of mixtures with the fuller gradation. This result

shows the importance of fines in the mixture. However, it has to be noted that though 31.5 mm-

finer and 20mm-fuller have the same amount of fines, their mechanical properties differ

significantly, this can be seen especially ITS values. The reason for this is that the amount of mastic

produced was not enough to weld the coarser fraction of 20mm gradation which has more surface

than 31.5mm gradation coarser fraction. Based on these observations it was understood that though

the amount of fines in the mixtures is a very important mix design parameter, it is not possible to

specify an optimum amount of fines for all gradations. This parameter very much depends on the

maximum size of the gradation.

Figure 2-7 Aggregate gradation considered in the study

Figure 2-8 Modified compaction and gyratory compaction comparison

0

10

20

30

40

50

60

70

80

90

100

0.0 0.1 1.0 10.0 100.0

%Passing

Sieve size(mm)

20mm finer

31.5mm finer

31.5 fuller

20mm fuller

45 55 65 75 85 95 105 115 125

2000

2050

2100

2150

2200

2250

2300

2350

3 4 5 6 7 8

% OMC

Drydensity

(kg/m3)

Moisture content (%)

Modified Proctor

Gyratory compactor-

Npre gyrations

20mm finer

Npre= 200gyrations

OWC = 6.5%

OMWC-trial= 85% of OWC


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17

Table 2-7 Summary of modified Proctor compaction and gyratory compaction study

Gradation (% fines) modified Proctor compaction gyratory compaction

Parameter

OMC

(%) MDD(Kg/m3) Npre

OMMC-trial

(% of OMC) density (kg/m3)

31.5mm fuller (5%) 6.5 2170 190 75 2218

31.5mm finer (7.5%) 7 2210 230 80 224720mm fuller (7.5%) 6 2155 180 80 2178

20mm finer (10%) 6.5 2250 200 85 2298

Figure 2-9 ITSM results on FBM with different gradations

20mm-fuller20mm-finer31.5mm-fuller31.5mm-finer

2700

2600

2500

2400

2300

2200

ITSM(MPa)

Effect of gradation on ITSM


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Figure 2-10 ITS-dry results on FBM with different gradations

Figure 2-11 Modified Proctor test results on FBM

2.4.3 Optimum mixing water content (pre wet water content)

The mechanical properties (ITSM, ITS-dry and ITS-wet) on gyratory compacted and cured specimens

were plotted against MWC in Figure 2-12, Figure 2-13,Figure 2-14 and Figure 2-15. Each ITSM valuein the plot is an average of tests on 8 specimens and ITS-dry and ITS-wet are averages of 4

20mm-fuller20mm-finer31.5mm-fuller31.5mm-finer

550

500

450

400

350

300

ITS(kPa)

Effect of gradation/amount of fines on ITS

4.25 4.75 5.25 5.75 6.25 6.75 7.25

2100

2150

2200

2250

2300

2350

2400

65 75 85 95 105

WetDensity(kg/m3)

Moisture content (% of OWC)

2%FB

3%FB

4%FB

0%FB


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19

specimens. The properties were all measured at the same water content of the specimens (0.6-

0.65%). As can be seen from the figures, the peak ITSM values were 85% of OWC, except for 2%FBM.

When ITS-dry results were considered, the optimum MWC was seen at 85% of OWC for 2%FBM and

3%FBM; and for 4% FBM and 5% FBM the peak was at 75%. For ITS-wet values the optimum was

found at 85% except for 5% FBM. Over all, the optimum MWC for all mixtures was consistently

found between 75% and 85% of OWC of the mixture.

Figure 2-12 Optimisation MWC for 2% FBM


1000

1100

1200

1300

1400

1500

1600

1700

1800

1900

100

150

200

250

300

350

400

60 65 70 75 80 85 90 95 100

ITSM

(MP

a)

ITS(kPa)

% OMC

2 % FBM - Mechanical properties (100% VA)

ITS-dry(kPa)

ITS-wet(kPa)

ITSM (MPa)

1000

1200

1400

1600

1800

2000

2200

2400

100

150

200

250

300

350

400

450

500

60 65 70 75 80 85 90 95 100

ITSM(MPa)

ITS(kPa)

% OMC

3% FBM- Mechanical properties (100% VA)

ITS-dry(kPa)

ITS-wet(kPa)

ITSM (MPa)


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20



2.4.4 Optimum compaction effort

To study the optimum compaction effort and to obtain the design number of gyrations (N design ), the

heights were obtained from the gyratory compactor during compaction. From the height data

density was calculated and plotted against number of gyrations (Figure 2-17, Figure 2-18,Figure 2-19

and Figure 2-20). The marks on the curves are the target densities that were obtained from modified

Proctor data (Figure 2-8). It can be seen from the plots that, though the target densities were

different, the number of gyrations that were required to compact to those target densities are in a

similar range. That means, a design number of gyrations that are required to compact modified

Proctor densities can be established. Ndesignfor all FBMs considered were in the range of 120-160

gyrations. An average of 140 gyrations was adopted as Ndesign.

0

500

1000

1500

2000

2500

3000

0

100

200

300

400

500

600

60 65 70 75 80 85 90 95 100

ITSM(MPa)

ITS(kPa)

%OMC

4% FBM- Mechanical properties (100% VA)

ITS-dry(kPa)

ITS-wet(kPa)

ITSM (MPa)

0

500

1000

1500

2000

2500

3000

0

100

200

300

400

500

600

60 65 70 75 80 85 90 95 100

ITSM

(MPa)

ITS(kPa)

% OMC

ITS-dry(kPa)

ITS-wet(kPa)

ITSM (MPa)


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21

Figure 2-16 Modified proctor test results on mixtures considered in the study

Figure 2-17 Obtaining Ndesignfor mixture with 100%VA and 0% FB at OM WC

2000

2050

2100

2150

2200

2250

2300

3 4 5 6 7 8 9

DryDensity(kg/m3)

Water Content (%)

0%RAP

50%RAP

75%RAP

1800

1900

2000

2100

2200

2300

2400

0 40 80 120 160 200

Wetdensity(kg/m3)

Number of gyrations

0%FB - 100%VA

Target density = 2330kg/m3)

MWC = 80% of OWC = 5.2%


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22

Figure 2-18 Obtaining Ndesignfor mixture with 100%VA and 2% FB at OMWC

Figure 2-19 Obtaining Ndesignfor mixture with 100%VA and 3% FB at OMWC

1800

1900

2000

2100

2200

2300

2400

0 40 80 120 160 200

WetDensity(kg/m3)

Number of gyrations

2%FB - 100%VA

target density

Target density = 2280kg/m3

MMC = 80% OMC = 5.2%

1800

1900

2000

2100

2200

2300

2400

0 40 80 120 160 200

Wetdewnsity(kg/m3)

Number of Gyrations

3%FB - 0%RAP

target density

Target density = 2260kg/m3

MMC = 80% OMC = 5.2%

Ndeg= 120 - 160gyrations


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23

Figure 2-20 Obtaining Ndesignfor mixture with 75%RAP FBM at OMWC

2.4.5 Mixture volumetric composition

The effect of MWC and bitumen on mixture volumetrics was studied by means of two compaction

methods; modified Proctor compaction and gyratory compaction. Bulk densities of specimens that

were compacted to 140 gyrations (Ndesig) for mixtures with different bitumen content were plotted

against MWC. The densities of the specimens increased as the foamed bitumen content increased.

This result indicates that FB increases the compactability of the mixture. The magnitude of this

behaviour can only be studied from water-bitumen interaction curves. This phenomenon was

supported by a shift in peak of the curves towards the left from 100% OWC. However, this shift was

not considerable, and was negligible from 4% FB to 5% FB. Thus replacing water in the mixture with

bitumen doesnt give required level of compactability. Air voids of specimens after curing were

plotted against FB content (Figure 2-22). The theoretical maximum densities were mathematically

calculated from the aggregate and bitumen densities data. Overall, the air voids decreased gradually

as FB content increased.

1800

1900

2000

2100

2200

2300

2400

0 40 80 120 160 200 240 280

WetDensity(kg/m3)

Number of gyrations

75% RAP

0% FB

3% FB

Target density for 0% FBM = 2230kg/m3

Target density for 3% FBM = 2200 /m3

OMMC = 80% OMC = 4.8%

Ndeg= 80 - 120gyrations


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24

Figure 2-21 Influence of FB content on densities

Figure 2-22 Influence of FB content on air voids

2000

2050

2100

2150

2200

2250

60 65 70 75 80 85 90 95 100

drydensity(k

g/m3)

% OWC

2%FB

3%FB

4%FB

5%FB

12.00

13.00

14.0015.00

16.00

17.00

18.00

19.00

20.00

21.00

22.00

60 65 70 75 80 85 90 95 100

Va(%)

% OWC

2%FB

3%FB

4%FB

5%FB


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25

Figure 2-23 Influence of FB content on VMA

2.4.6 Selection of Foamed Bitumen content (Mechanical tests)

The results of mechanical tests on the mixtures that were compacted at optimum MWC which is

80% of OWC and to Ndesign, and varying FB content were plotted in Figure 2-24, Figure 2-25,Figure

2-26, Figure 2-27 and Figure 2-28. As can be seen in the plots there is a clear optimum ITSM value for

all mixtures. For 100%VA mixtures, the optimum was found at 4% FB content. Similarly, the optimum

ITSM values for 50%RAP and 75% RAP mixtures were found at 3.5% and 3% FB content respectively.

It has to be noted that, though the mixing and compaction was carried at room temperature, themixtures with RAP have optimum ITSM values at lower FB content than 100%VA mixture. This

indicates that in FBM design it is not correct to treat RAP just as fresh aggregate, which means that a

separate mix design for mixtures with RAP is imperative.

If ITS-dry values are considered, there was not any optimum for 100%VA mixtures. ITS-dry values for

these mixtures increase with increased in FB content without any optimum value. However, an

optimum could be located for both the mixtures with RAP (50% RAP and 75% RAP mixtures). The

optimum values were found at 3% FB for both mixtures. When ITS-wet results are considered, the

optimum ITS-wet was found only for 75% RAP mixtures, which is at 3% FB content. There was not

any optimum for any mixtures if ITSR was considered. However, it can be noted that, though ITS-dry

values were higher for 100%VA mixture than for mixtures with RAP, the ITS-wet and ITSR values

were found to be superior for mixtures with RAP. This indicates that the mixtures with RAP have

better resistance against water than mixtures without any RAP. This could be attributed to the

presence of fully bitumen coated RAP aggregates in the mixture. Overall, from the results, at 4% and

3% FB contents, optimum mechanical properties were found for 100%VA and 75% RAP mixtures

respectively. However, optimum FB content was not very clear for 50% RAP mixtures. So, a value

between 3.5% and 3%; which is 3.25% was finally adopted as the design FB content for 50% RAP

mixtures. Though UCS tests were performed on all the mixtures, the results (Figure 2-28) were not

considered for the selection of optimum FB content.

18.00

20.00

22.00

24.00

26.00

28.00

30.00

60 65 70 75 80 85 90 95 100

VMA(%)

% OWC

2%FB

3%FB

4%FB

5%FB


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26

Figure 2-24 Effect of Foamed bitumen content on ITSM

Figure 2-25 Effect of Foamed bitumen content on ITS-dry

1500

1700

1900

2100

2300

2500

2700

2900

2 2.5 3 3.5 4 4.5 5

ITSM(MPa)

% Foamed Bitumen

100%VA

50% RAP

75%RAP

300

350

400

450

500

550

2 2.5 3 3.5 4 4.5 5

ITS

-dry(kPa)

% Foamed Bitumen

100%VA50% RAP

75%RAP


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27

Figure 2-26 Effect of Foamed bitumen content on ITS-wet

Figure 2-27 Effect of Foamed bitumen content on ITSR

100

150

200

250

300

350

400

2 2.5 3 3.5 4 4.5 5

ITS-wet(kPa)

% Foamed Bitumen Contnet

100%VA

50% RAP

75% RAP

30

35

40

45

50

55

60

65

70

2 2.5 3 3.5 4 4.5 5

ITSR(%

)

% Foamed Bitumen

100%VA

75% RAP

50% RAP


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28

Figure 2-28 Effect of Foamed bitumen content on UCS

2.5 Conclusions

The conclusions that were drawn in accordance to the mix design parameters that were considered

in the study are:

1. The limited study on the gradation of aggregate with two different amounts of fines showed

the importance of fines in the mixture and dependence of its amount on maximum size of

the aggregate (surface area available). From the detailed literature review in conjunction

with the results of this study, it was understood that for a good FBM, the amount of fines inthe mixture should be more than in an HMA mixture and also even more than in bitumen-

emulsion cold mixtures. However, there is an optimum to this amount as well; where

beyond this optimum the mechanical properties deteriorate. However, finding an optimum

amount of fines is not in the scope of this study.

2. The role of the bitumen during compaction was studied. The study showed that the

bitumen helps in compaction of the mixture but not as effective as water. The bitumen-

water interaction studies also revealed that total fluid (water + bitumen) is not a valid

parameter in mix design of FBM.

3. A rational range of optimum mixing water content (OMWC) was suggested; which is 75-85%

of OWC obtained by modified Proctor test.

4. It was found that a unique design number of gyrations (Ndesign ); the compaction effort

equalling modified Proctor compaction can be established for FBM with a specific aggregate

gradation.

In addition to the above findings, the following conclusions can also be drawn from the present

study on mix design parameters.

5. The study showed that gyratory compaction resulted in higher unit weights and lower

optimum water contents than those obtained by the modified Proctor compaction test.

2.00

2.50

3.00

3.50

4.00

2 2.5 3 3.5 4 4.5 5

UCS(MPa)

% Foamed Bitumen

100%VA

50% RAP

75%RAP


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29

6. The parameter MWC needs to be addressed with reference to OWC obtained from the

modified Proctor test, as this is a parameter obtained from modified Proctor test whose

parameters depends on amount of fines; which an important mix design parameter.

7. It was found that stiffness measure (ITSM) was more sensitive to FB content with clear

optimum than other mechanical properties such as ITS-dry and ITSR.

8. The mixtures with RAP showed more resistance to water than mixtures without RAP.

9. It was observed that the presence of RAP influenced the design FB content; which means

treating RAP as black rock in FBM mix design is not appropriate.


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30

3 Study on accelerated curingFoamed bitumen mixtures gain strength with time after compaction. This is due to loss of mixing

water in the mix over time. This process of gaining strength of FB mixtures with expulsion of water is

called curing. With the reduction of water in the FB mixtures strength (compressive and tensile) and

stiffness increase (Academy, 2009). It is obvious that the laboratory samples used for mix design andto determine engineering properties should be representative samples of field placed mixtures.

Hence, the aim of this study is to develop a standard laboratory curing regime(s) at which it is

sensible to conduct performance tests that represents conditions in the field.

To fulfil this aim the following objectives were considered on FBM.

1. To study influence of curing temperature and time on stiffness and water loss in the mix.

2. To study influence of cement and RAP on curing (stiffness gain and water loss).

3. To develop stiffness maturity (time-temperature factor) relationships.

4. To study performance in terms of permanent deformation of mixtures at the different curing

conditions those are being followed by different agencies.

The need for development of a curing regime protocol and maturity concept was discussed in the

year 1 report and is not repeated here. The specimen fabrication method was discussed was

discussed in Chapter 2. In this method after the specimen was compacted, the specimen was left in

the mould at room temperature for 24hours and then extracted at which time the average water

content was 4.72%. The water content in the specimen was monitored for 30 days. (It is planned to

monitor for a longer period, however, in this report only the first 30 days period is presented) by

observing the change in the specimens weight over time.

3.1 Scope of the study1. In order to avoid ageing of bitumen, it was recommended to cure the specimens below

softening point of the bitumen used in the mixture, which is 45C for the 70/100 grade

bitumen used in this study. Hence, curing temperatures of 40C, 30C, 20C and 5C were

considered. Though curing at 20C and 5C are not to be considered as accelerated curing,

these temperatures were included to study the effect of this temperature on curing.

2. To study the effect of RAP and cement, the following mixtures were considered (a) 100% VA

(b) 100%VA+1%Cement (c) 50% RAP (d) 50%RAP+1% Cement (e) 75% RAP+1% Cement

3. The Non-destructive stiffness (ITSM) test was selected for assessing curing of the specimen.

This is in order to carry out the test on the same set of specimens to nullify variability in the

mixtures and to derive reliable trends for curing evaluation.

3.2 Effect of time and temperature on water loss

The water content in the specimen which was monitored over time is plotted in Figure 3-1. A trend

line which is a power curve is also included. The initial water content in the mixture during

compaction was 5.2% (80% of OWC). The plots showed that water content in the specimen reached

about 25% of the initial amount after 24 hours of curing at both 30C and 40C. Similarly, water

content reached to about 50% of initial amount when specimens were cured at 20C and 5C. The

trend suggests that the rate of water loss is proportional to the amount of water present in the

mixture. In other words, the rate of water loss decreased with time. It is clear from the plot and alsoobvious that the water loss is dependent on curing temperature. The higher the curing temperature


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31

the faster was the water loss. However, all curves, except the 5C curves, seem to reach a constant

amount after which the loss is negligible.

Figure 3-1 Effect of time and temperature on water loss

3.3 Effect of time and temperature on stiffness

The stiffness (ITSM) values that were measured over time on the specimens that were cured at

different temperatures are plotted in Figure 3-2. The plot shows a stiffness monitored for a period of30 days and at curing temperatures of 40C, 30C, 20C and 5C. Trend lines were also included in

the plot. The trend line is a logarithmic with positive tangential slope. As can be seen from the

figure, for all conditions, stiffness increased with time of curing. However, this gain was most rapid

for specimens cured at higher temperatures. This is because rapid curing takes place at higher

temperature; the rapid loss of water yields higher stiffness values with time. This phenomenon can

be seen in Figure 3-3 in which, water loss and stiffness gain of specimens cured at 20C is presented.

0

0.5

1

1.5

2

2.5

3

0 5 10 15 20 25 30 35

Moisturecontent(%)

Number of days

40C

30C

20C

5C

Power (40C)

Power (30C)

Power (20C)

Power (5C)


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32

Figure 3-2 Effect of time and temperature on stiffness

Figure 3-3 Effect of time on curing

0

500

1000

1500

2000

2500

3000

3500

4000

0 5 10 15 20 25 30 35

ITSM

(MPa)

Number of days

40C

30C

20C

5C

Log. (40C)

Log. (30C)

Log. (20C)

Log. (5C)

0

0.5

1

1.5

2

2.5

3

0

500

1000

1500

2000

2500

3000

0 5 10 15 20 25 30 35

WaterContent(%)

ITSM(MPa)

Number of days

ITSM

m/c

Log. (ITSM)

Power (m/c)


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33

3.4 Effect of water content on stiffness

Figure 3-4 shows the plots of stiffness (ITSM) versus water content for the specimens that were

cured at different temperatures. As a general trend, with decrease in water content the stiffness was

increased. However, as can be seen from the figure when individual temperatures were considered

it was found that temperature has a significant additional impact on the stiffness of the mixture. For

example, the specimens which were cured at 40C having water content around 1% showed better

stiffness (ITSM) than the specimens cured at other lower temperatures at same water content in the

specimen.

Figure 3-4 Effect of water content on stiffness

3.5 Effect of RAP and Cement on stiffness

Figure 3-5 shows ITSM results for two curing conditions, 20C for 3 days (early stage of curing) and

40C for 3 days (long term curing). The Figure 3-5shows the importance of cement in FBM especially

during early stages of as construction of FBM layer. The presence of RAP has positive influence on

the stiffness values in early stages of curing. It has to be noted that during early stages of curing,

mixtures with RAP showed better stiffness than mixtures with 100%VA. This could be because of

presence of fully bitumen coated aggregates which provide some deformation resistance. However,

for fully curing regime, 100% VA mixtures showed better stiffness values. This could be because

slower water loss in the mixture with RAP.

500

1000

1500

2000

2500

3000

3500

00.511.522.5

ITSM

(MPa)

moisture content in mix (%)

40C

20C

30C

5C

Log. (40C)

Log. (20C)

Log. (30C)

Log. (5C)


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Figure 3-5 Effect of RAP and Cement on ITSM

3.6 Conclusions

The following conclusions can be drawn based the work so far

1. The effect of curing temperature and curing period on water loss and stiffness gain was

studied.

2. It was found from the experimental results that temperature and time both have a

significant influence on stiffness and water content in the mixture.

3. The presence of cement enhances the mechanical properties (ITSM); its presence is

especially important during early stage of curing.

3.7 Further curing studyThe further study in the present curing study includes (May2013 July 2013)

1. A curing study on mixtures with cement and RAP is in progress.

2. The possibility of Stiffness-maturity relationship development will be studied.

3. Curing study in terms of performance (permanent deformation) and strength (ITS-dry and

ITS-wet) characteristics will be studied. As these are destructive tests only the following

conditions will be considered.

Table 3-1 Curing regimes considered for destructive testing

TemperatureFullyWrapped Unwrapped combination

5C 28 days n/a n/a

20C 28 days 3days

7days (unwrapped) + 21

days(fully wrapped)

40C

28 and 3

days

28 and 3

days n/a

0

500

1000

1500

2000

2500

3000

3500

4000

100%VA 1%Cement 50% RAP 100%VA 1%Cement 50% RAP

20C 40C

ITSM(MPa)after3

dayscuring


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4 Further studyThe further work that has to be done includes

4.1 Generating inputs for Pavement analysis and design (June 2013

August 2013)The structural design of pavement ensures that it serves its purpose structurally and functionally in

an economically viable manner with in estimated design life. Such design can be achieved by

empirical method or mechanistic method or mechanistic-empirical method. The mechanistic-

empirical method of design of pavements will be considered. Huang (2004) reported that the

mechanistic-empirical method is based on the mechanics of materials that relates an input, such as a

wheel load, to an output or pavement response such as strain or stress. The response values are

subsequently used to predict distress from laboratory-test and field performance data. The main

objective of this study is to evaluate the sensitivity of pavement distresses; permanent deformation

on subgrade and fatigue in FBM base layer. For this, two mechanistic analysis tools namely

KENLAYER and MEPDG (Mechanistic-Empirical Pavement Design Guide) will be considered.

The MEPDG is an improved pavement design method adopted by AASHTO (American Association of

State Highway and Transportation). This methodology depends on the pavement material

characterisation of the fundamental engineering parameters. The design method is an iterative

process using the analytical results based on the trial designs postulated by the designer. A trial

design is analysed for adequacy against user input data.

KENLAYER is a flexible pavement response analysis application in KENPAVE computer program

developed by Huang (2004) will be for analysis. Unlike MEPDG, this gives only the response of the

pavement to a given load. Distress models in KENLAYER are fatigue cracking and permanentdeformation. From the preliminary studies and from literature it was understood that FBMs exhibit

non-linear behaviour. Hence, non-linear elastic analysis is considered. For this type of analysis using

KENLAYER, the K- model and its parameters are used. In KENLAYER for non-linear analysis, three

methods were recommended. Out of these three first method which is sub dividing the stress

dependent layers is considered in this study.

4.1.1 Specimen Fabrication

The mixture data consists of dynamic modulus frequency sweep tests on specimens for five

temperatures and four rates of loading. The specimens should have a diameter of 100 mm and a

height-to-diameter ratio of 1.5.

4.1.2 MEPDG inputs frequency sweep test

The MEPDG software uses the material properties to calculate incremental and accumulated

pavement damage based on the expected variation in environmental and traffic loading. This

process, as defined by the user-selected reliability, allows the designer to judge whether or not the

input design thickness and/or materials meet the expected performance during the design period. In

the current version of the MEPDG procedure, three input levels can be used based on the availability

of materials characterization data. The site-specific laboratory-measured values of the material

properties are used as Level 1 input parameters. Predicted values determined from basic volumetric

properties of as constructed mixtures are considered Level 2 input parameters. Level 3 inputparameters are provided as default values in the software based on mixture gradation and the
https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=0CDIQFjAA&url=http%3A%2F%2Fwww.trb.org%2Fmepdg%2F&ei=qjywUd24DYGn0QWYvoGAAg&usg=AFQjCNFd1brpUWBQa_G6er6Qw_grXKVAVQ&sig2=FT1VUbZs-JiXZ30C6EtIcQ&bvm=bv.47534661,d.d2khttp://en.wikipedia.org/wiki/American_Association_of_State_Highway_and_Transportation_Officialshttp://en.wikipedia.org/wiki/American_Association_of_State_Highway_and_Transportation_Officialshttp://en.wikipedia.org/wiki/American_Association_of_State_Highway_and_Transportation_Officialshttp://en.wikipedia.org/wiki/American_Association_of_State_Highway_and_Transportation_Officialshttps://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=0CDIQFjAA&url=http%3A%2F%2Fwww.trb.org%2Fmepdg%2F&ei=qjywUd24DYGn0QWYvoGAAg&usg=AFQjCNFd1brpUWBQa_G6er6Qw_grXKVAVQ&sig2=FT1VUbZs-JiXZ30C6EtIcQ&bvm=bv.47534661,d.d2k


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performance grade (PG) of the binder. In the present study the dynamic modulus (|E*|) which is

Level 1 input will be measured by frequency sweep tests at different temperatures.

|E*| tests will be performed with the NU14 testing machine in accordance with AASHTO TP 62

(AASHTO, 2007a). Tests were performed on 150 mm tall by 100 mm diameter specimens as

previously mentioned. Four testing temperatures ranging from 5C to 50C will be used. Six testing

frequencies ranging from 0.1 Hz to 25 Hz will be used. To measure against damage to the test

samples, the tests will be conducted starting from the coldest temperatures to the warmest

temperatures. In addition, at each test temperature, the tests will be performed starting from the

highest to the lowest frequency. Each sample will be conditioned at the testing temperature for a

minimum period of 3 hr before the test was started. Load levels were selected in such a way that at

each temperature-frequency combination, the applied strain was in the range that ensure testing

will conducted in the linear viscoelastic range of mixture, a necessary requirement for a valid |E*|

test. All tests will be conducted in the uniaxial mode without confinement in line with current

standard AASHTO specifications.

4.1.3 Material constants generation for non-linear elastic analysis

The inputs will be generated in accordance to BS EN 13286-7:2004 which is cyclic load triaxial test for

resilient modulus. The K1 and K2 values obtained from this resilient modulus test.

4.2 Analysis and design (September 2013 January 2013)

4.2.1 Non-linear elastic analysis using KENLAYER

Since, one of the objectives of the present study is to evaluate the performance of FBMs, the

structural analysis will be studied by examining the stress and strain responses distribution inpavement with FBM base layer using non-linear elastic analysis using the parameters obtained in

resilient modulus test. This response data will be used to obtain the life of the pavements with FBM

layer.

4.2.2 Sensitivity analysis of distress types to FBM inputs

For sensitivity analysis on input parameters of MEPDG, the sensitivities of five MEPDG performance

measures to inputs will be studied by varying one input parameter per trial using the MEPDG. The

distresses that will be studied during this analysis are alligator cracking, longitudinal cracking,

thermal cracking, rutting and smoothness for flexible pavements. While for sensitivity analysis using

KENLAYER only fatigue cracking in base layer and permanent deformation on subgrade will only beconsidered.

4.3 Fatigue and durability study (February 2014-June 2014)

For comparing the fatigue resistance of different mixes, and to understand the effect of cement and

RAP addition on fatigue behaviour, a plot of number of cycles (N) against N divided by vertical

deformation (VD) will be used. The N value at which N/VD reaches its maximum value will be

considered as N critical. The N value at which the specimen fails will be considered as N failure. This

method of representing fatigue behaviour was actually used by Read (1996) to define the initiation

and propagation phase of fatigue. It was understood from literature that durability (water

sensitivity) of FB mix is an important factor to be considered for better field performance. The testmethod for durability that will be employed in this research is testing strength parameter (ITS and


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UCS) before and after vacuum saturation conditioning. This permits the calculation of a ratio that

gives an indication of the water sensitivity of the FB mix. The lower the ratio, the more sensitive is

the mix to the effect of water.

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Mix design of foamed bitumen mixtures