Mix Design Considerations and Performance Characteristics of Foamed Bitumen Mixtures (FBMs)

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Mix Design Considerations and Performance

Characteristics of Foamed Bitumen Mixtures (FBMs)

By

Kranthi K Kuna

Thesis submitted to the University of Nottingham for the degree of

Doctor of Philosophy

DECEMBER 2014

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ABSTRACT

The sustainability issues in pavement materials and design form a strong incentive for the

present work. Using recycled materials in pavements is a sustainable practice that is gaining

adoption, particularly for flexible (bituminous) pavements. One approach is to incorporate

large quantities of Reclaimed Asphalt Pavement (RAP) into base and sub-base applications

for pavement construction. Numerous studies have reported that RAP can be reused as an

aggregate in Hot Mix Asphalt (HMA) as well as in cold mix asphalt, granular base, sub-base,

and subgrade courses. Cold recycling technology, like hot mix technology, has also become

popular in various countries for rehabilitation of damaged bituminous pavements. RAP

stabilized with bitumen emulsion and foamed bitumen has been used as a base layer. The

present study focuses on Foamed Bitumen treated Mixes (FBMs). Most of the agencies

which use FBMs have their own mix design procedures which are the result of numerous

efforts over decades. In spite of all these efforts, Foamed Bitumen application in cold

recycling in the United Kingdom suffers from the lack of a standardised mix design

procedure. To overcome this, the present research objective was to develop a mix design

procedure by identifying critical mix design parameters. The mix design parameters that

were optimised were Foamed Bitumen content, mixing water content (MWC), and

compaction effort. Special attention was given to the simplest yet crucial mix design

consideration of FBMs; curing. The thesis also attempted to simulate what should be

expected in terms of the performance of flexible pavements containing FBMs as road base.

The mix design parametric study was initially carried out on FBMs with virgin limestone

aggregate (VA) without RAP material and a mix design procedure was proposed. Optimum

MWC was achieved by optimising mechanical properties such as Indirect Tensile Stiffness

Modulus (ITSM) and Indirect Tensile Strength (ITS-dry and ITS-wet). A rational range of 75-

85% of Optimum Water Content (OWC) obtained by the modified Proctor test was found to

be the optimum range of MWC that gives optimum mechanical properties for FBMs. The

proposed methodology was also found to apply to FBMs with 50% RAP and 75% RAP. It was

also found that the presence of RAP influenced the design FB content, which means that

treating RAP as black rock in FBM mix design is not appropriate. This work also evaluated

the validity of the total fluid (water + bitumen) concept which is widely used in bitumen-

emulsion treated mixes.

The present work was also intended to better understand the curing mechanism of FBMs

and to lessen the gap between laboratory curing and field evolution of these mixtures. This

was achieved by evaluating different curing regimes that are being followed by different

agencies and researchers, as well as identifying important parameters that affect curing. In

achieving this, a link was established between laboratory mix design and field performance

by evaluating applicability of the maturity method. The curing regime study provided a

valid investigation into the behaviour of FBM taking into account the effect of temperature,

curing conditioning (Sealed or Unsealed), curing duration and the influence of cement with

different curing regimes. It was found that the temperature is as important a parameter astime, as temperature has a greater influence on curing rate and also on bitumen

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properties. Moreover, higher curing temperatures resulted in higher rate of stiffness gain.

This trend is not only because of rapid water loss but also implies an increase in binder

stiffness at higher curing temperatures. Though the presence of RAP improved the early

stage stiffness of FBMs, it slowed down the rate of water loss from the specimens which

resulted in smaller stiffness values at a later stage. The experimental results also indicated

that cement addition has no influence on water loss trends, but improved the stiffness

significantly during all stages of curing.

The study also evaluated the applicability of the maturity method as a tool to assess the in-

situ characteristic of FBM layers in the pavement. It was found that replacing the time term

with an equivalent age term in the maturity function aided in estimating stiffness rather

than relative stiffness. This was possible because of the characteristic curing of FBM in

which the limiting stiffness these mixtures reach strongly depends on the curing

temperature at least for the length of the curing stages considered in the present study. A

strong correlation was found between maturity and the stiffness values obtained from thelaboratory tests which resulted in development of maturity-stiffness relationships. The

application of the method to assess the in-situ stiffness was presented using three

hypothetical pavement sections. The results showed the influence of ambient temperature

and the importance of cement addition to FBMs.

The permanent deformation resistance was assessed by performing RLAT tests on

cylindrical specimens compacted by gyratory compactor. The RLAT test results indicate that

both test temperature and stress level have significant influence on permanent

deformation characteristics as expected. The effect of stress on permanent deformation

was increased with increase in test temperature. It was also found that from limited testsand mixture combinations, RAP content has only a slight influence on permanent

deformation of FBMs. However, the presence of cement led to significant improvement.

FBMs were also found to be less temperature susceptible than HMA in terms of permanent

deformation and, within FBMs, mixtures with cement were found to be more sensitive than

FBMs without any cement.

For assessing the fatigue performance of FBMs, the ITFT was initially used to investigate the

effect of cement on the fatigue life. The ITFT tests results showed that the FBMs without

cement (50%RAP-FBM) have lower fatigue life than HMA (DBM90) at any initial strain level.

Nevertheless, similar to permanent deformation, the fatigue life was improved with theaddition of 1% cement to FBMs. However, the above discussion was not found to be

completely valid when uniaxial tests were carried out. In stress controlled uniaxial tests, a

sinusoidal load of 1Hz frequency was applied axially to induce tensile strain in the radial

direction. The failure criterion considered in the study was the number of cycles to reach

50% stiffness and this was plotted against the measured initial strain values. Results

indicated that there was not much difference in fatigue life among different mixtures and

also between FBM and HMA. However, stiffness evolution curves showed that FBMs fail in

a different pattern compared to HMA. Unlike HMA, which showed a three stage evolution

process, for FBMs the stiffness actually increased initially to reach a maximum and

decreased at a slower rate until failure. It was also found that by plotting curves according

to Hopman et al.,(1989) which identifies the fatigue failure transition point, use of the 50%

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stiffness criterion for fatigue life evaluation is not a conservative approach. Uniaxial tests

also revealed that, although in fatigue the FBMs were found to behave differently from

HMA, in terms of permanent deformation, FBMs behave similarly to HMA in that a steady

state strain rate was achieved.

Keywords: Foamed bitumen, Reclaimed Asphalt Pavement (RAP), Recycling, Mechanical

Properties, Performance Characteristics

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would like to dedicate my thesis to my

beloved parents and all my teachers

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ACKNOWLEDGEMENTS

I wish to express the deepest gratitude to my supervisors Prof. Gordon Airey and Dr. Nick

Thom for their belief in me and for the opportunity to pursue this research work. I cannot

thank them both enough for their time, advice, guidance, concern, and assistance which

kept me on track for the entire duration of my study and above all, for painstakingly

correcting the write-ups. My sincere thanks are due to Mr. Andrew Dawson whose

constructive criticism contributed in no small measure to the success of this study. Also, for

selecting me for the joint University of Nottingham and Virginia Tech schola