Mix design considerations and performance characteristics of foamed bitumen mixtures (FBMs)

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 as time, as temperature has a greater influence on curing rate and also on bitumen 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 the laboratory 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 tests and 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 the addition 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% 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

Structural design of pavements incorporating foamed bitumen mixtures

This paper presents a rational structural design methodology termed the ‘cumulative damage approach’ for road, port and airport pavements incorporating cold bituminous mixtures with foamed bitumen. This has been developed along with a laboratory test, the uniaxial indirect tensile test, to evaluate the fatigue characteristics of these mixtures. The test was developed with a view to addressing the limitations of conventional fatigue tests for foamed bitumen mixtures. The new design approach takes account of the actual stiffness evolution of the mixtures obtained from the fatigue test. It is compared with a traditional approach for conventional flexible pavements, which is based on pavement life as a function of computed tensile strain in the material and interpretation of fatigue data in a conventional way. The results show that the traditional design approach yields conservative outcomes for pavements with foamed bitumen mixtures if the same transfer function or shift factor used for hot mix asphalt is applied. The results also show that if all factors other than induced load influence are the same, the shift factor for foamed bitumen mixtures could be 25–35% higher than for hot mix asphalt

Mix design considerations and performance characteristics of foamed bitumen mixtures (FBMs)

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 as time, as temperature has a greater influence on curing rate and also on bitumen 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 the laboratory 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 tests and 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 the addition 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% 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

Development of a tool to assess in-situ curing of Foamed Bitumen Mixtures

This paper discusses the simplest yet crucial mix design consideration of Foamed Bitumen Mixtures (FBMs); curing. The objective of the present study is to attain better understanding of the curing mechanism of FBMs and lessen the gap between laboratory curing and field evolution of these mixtures. This was achieved by carrying out a laboratory curing study in two phases. The first phase aimed to understand the curing mechanism and to interpret the level of impact of factors such as temperature and time on the curing of FBMs. The effect of these factors on curing was evaluated with reference to stiffness gain and water loss of FBM specimens. The second phase was intended to develop a tool to assess in-situ curing of FBMs. With a view to assessing in-situ strength (or stiffness) the applicability of the maturity method, which is commonly used to estimate in-situ compressive strength of concrete before removal of formwork, to FBMs was evaluated. A strong correlation was found between maturity and the stiffness values obtained from the laboratory 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 illustrate the influence of ambient temperature and the importance of cement addition to FBMs

Mix design considerations of foamed bitumen mixtures with reclaimed asphalt pavement material

In the present work, a mix design parametric study was carried out with the aim of proposing a practical and consistent mix design procedure for foamed bitumen mixtures (FBMs). The mix design parameters that were adopted in the study are mixing and compaction water content (MWC), compaction effort using a gyratory compactor and aggregate temperature. This parametric study was initially carried out on FBMs with virgin limestone aggregate without reclaimed asphalt pavement (RAP) material and a mix design procedure was proposed. This proposed methodology was also found to apply to FBMs with RAP. A detailed consideration was also given to characterising the RAP material so as to understand its contribution to the mechanical properties of FBMs. Optimum MWC was achieved by optimising mechanical properties such as indirect tensile stiffness modulus and indirect tensile strength (ITS-dry and ITS-wet). A rational range of 75–85% of optimum water content obtained by the modified Proctor test was found to be the optimum range of MWC that gives optimum mechanical properties for FBMs. It was also found that the presence of RAP influenced the design foamed bitumen content, which means that treating RAP as black rock in FBM mix design is not appropriate. To study the influence of bitumen and water during compaction, modified Proctor compaction and gyratory compaction were employed on mixes with varying amounts of water and bitumen. By this, the work also evaluated the validity of the total fluid (water + bitumen) concept that is widely used in bitumen–emulsion-treated mixes, and found it not to be applicable

Machine learning techniques to estimate the degree of binder activity of reclaimed asphalt pavement

This paper describes the development of novel/state-of-art computational framework to accurately predict the degree of binder activity of a reclaimed asphalt pavement sample as a percentage of the indirect tensile strength (ITS) using a reduced number of input variables that are relatively easy to obtain, namely compaction temperature, air voids and ITS. Different machine learning (ML) techniques were applied to obtain the most accurate data representation model. Specifically, three ML techniques were applied: 6th-degree multivariate polynomial regression with regularization, artificial neural network and random forest regression. The three techniques produced models with very similar precision, reporting a mean absolute error ranging from 12.2 to 12.8% of maximum ITS on the test data set. The work presented in this paper is an evolution in terms of data analysis of the results obtained within the interlaboratory tests conducted by Task Group 5 of the RILEM Technical Committee 264 on Reclaimed Asphalt Pavement. Hence, despite it has strong bonds with this framework, this work was developed independently and can be considered as a natural follow-up