- Title
- Environmental salinity and bitumen-sealed unbound granular road pavements
- Creator
- de Carteret, Ryan Steven
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2015
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Areas affected by dryland salinity in Australia are predicted to more than double over the next 40 years as a result of land use and climatic changes. This presents a significant risk to road asset managers as concentration of salts within road pavements has been shown to cause damage to bituminous surfacings. This study examined how salts move and concentrate within road pavements, and the effects of natural occurring salts (environmental salinity) on pavement performance. The primary aims of the study were: to evaluate the potential for accumulation of salts in bitumen-sealed unbound granular road pavements; to assess the effects of naturally occurring salts (environmental salinity) on bitumen-sealed unbound granular road pavements; and to identify and assess improvements to construction and maintenance practices to mitigate the effects of salts on roads. To evaluate the potential for accumulation of salts in bitumen-sealed unbound granular road pavements a multifaceted approach was adopted incorporating testing (laboratory soil columns), field investigations (field study of a purpose-built pavement) and numerical modelling (water movement and salt transport in roads). This approach was deemed necessary due to the lack of quantitative data available, either from previous laboratory studies or field investigations. The different approaches also allowed investigations to be conducted under controlled, ‘real’ and simulated conditions, and at different timescales. To assess the effects of naturally occurring salts (environmental salinity) on bitumen-sealed unbound granular road pavements, a series of tensile and shear strength tests were conducted on compacted specimens of road pavement material with different combinations of salt concentration and moisture content. To achieve a broad understanding, tensile and shear strength tests were conducted on pavement samples with different moisture contents and salt concentrations ranging from very low values to very high values exceeding those likely to occur naturally. Microstructure analysis techniques, including chemical analysis, optical and scanning electron microscopy, and mercury intrusion porosimetry, were then employed to analyse tested specimens in order to explain the physical mechanisms which occur as a result of salt concentration within bitumen-sealed unbound pavement materials. Through an in-depth understanding of how salts move and accumulate within road pavements and the effects salts have on road pavements, a number of practical measures to mitigate or minimise the impacts of salts on roads were identified and qualitatively assessed. One of the potential mitigation measures was then quantitatively assessed using a numerical model to determine its effectiveness in minimising salt accumulation and, therefore, potential for salt damage. This extension of the project provides a methodology that can be applied to design potential mitigation measures for use in specific areas of high salinity risk or to assess other pavement and drainage measures for their effectiveness in mitigating the risk of salt damage to road pavements. From the laboratory column testing, field investigations and numerical modelling undertaken it was observed that significant water movement and evaporative moisture loss at the surface are likely to occur in unbound granular road pavements. Water movement is likely to occur vertically, towards the surface, within a road pavement and from the subgrade to the pavement, primarily driven by surface evaporation and capillary flow advection. The moisture content of a road subgrade was found to change in response to rainfall infiltration, regardless of whether or not the pavement has a surfacing. Additionally, horizontal water flow, from the surrounding environment into and out of a bitumen-sealed pavement and subgrade, was found to be likely to occur in response to rainfall, or lack of, and is likely to have a more significant influence on subgrade moisture content than direct infiltration through the overlying pavement. Salinisation of a road pavement was found to be possible under certain circumstances (e.g. saline groundwater of a particular concentration at a particular depth) in arid or semi-arid climates. While salinisation is likely to occur very slowly under typical climatic conditions, it may occur relatively rapidly (years rather than decades) under drought conditions. Salt transport within a bitumen-sealed granular pavement was found to occur due to both advective and diffusive solute transport, though not always in the same direction. Solute transport associated with advective water flow is likely to occur upwards within a road, according to the suction gradient, driven by evaporation at the pavement surface. However, under very highly salinised conditions, such as may occur near the pavement surface, solute diffusion is likely to occur downwards (counterflux) driven by chemical gradient. Salt transport from a saline groundwater into a road subgrade is likely to occur due to advective flow (upwards) and diffusive flow (upwards or downwards depending on the concentration of the groundwater), though is likely to take a long time under typical in situ boundary conditions. Peak salt concentration was found to occur in the upper zone of the pavement close to, but not at, the pavement surface (approximately 18-30 mm below the surfacing) in column experiment two and the numerical modelling, corresponding to the location of the evaporation front. However, the salt concentration at the surface was only marginally less than the peak solute concentration. Under certain boundary conditions (e.g. shallow saline groundwater and drought conditions), peak solute concentration was found to have the potential to exceed the salt saturation threshold, the point where salt precipitation may occur. The tensile and shear strength testing confirmed that the presence of natural salts (environmental salinity) can have a positive effect on granular road pavements, which contrasts with the negative impact salt crystallisation can have on bituminous surfacings. While in solution, salts were found to have a small positive effect on the tensile strength of the pavement material studied due to clay chemistry interactions. Upon crystallisation, calcite crystals were observed to form bonds within the pores of the pavement material, which led to a marked increase in both tensile and shear strength. At the interface between the surfacing and the pavement material, large whisker-shaped halite crystals were observed to grow from the bitumen, which is consistent with microstructural observation samples from in situ salt-damaged road pavements. The two different crystal habits observed within the bitumen-sealed granular pavement specimens explain how salt crystallisation can both stabilise pavement material and also cause debonding of the surfacing from the pavement. Five practical measures to mitigate or minimise the impacts of salts on roads were identified and qualitatively assessed. These measured included increasing the granular pavement thickness, addition of a low-permeability earthworks capping layer, addition of an earthworks drainage layer, addition of a low-permeability pavement interlayer and substitution of a sprayed-seal surfacing with a surfacing of lower permeability. Each of these measures was assessed as having some potential to mitigate or minimise salt accumulation within road pavements, though each had limitations, such as installation difficulties or impacts being temporary rather than permanent. The effectiveness of increasing the granular pavement thickness as a salt mitigation treatment was then quantitatively assessed using a numerical model. Increasing the pavement thickness was found to significantly reduce the rate of salt accumulation and increase the time to reach salt saturation threshold, while also increasing the design traffic capacity. Overall this option was determined to be a practical and cost-effective mitigation measure for arid and semi-arid areas where there are shallow saline groundwaters. The findings from this study have a number of implications for road pavement design and maintenance in areas at risk of environmental salinity. Specifically, environmental salinity is likely to present a risk to the performance (serviceability) of some bitumen-sealed unbound granular roads in arid and semi-arid areas where there are shallow groundwaters due to the potential for salt damage to bituminous surfacings. However, the risk, and cost, of salinity-related damage to road pavements is unlikely to be a significant issue in most areas because the preconditions required for salt damage to occur (presence of very shallow, ≤ 1 m deep, saline groundwater and significant, ≥ 5 year, drought climatic conditions) are unlikely to be widespread. Environmental salinity, while presenting a risk to the performance of road surfacings, is unlikely to be detrimental to the strength of many bitumen-sealed unbound granular pavements. Additionally, environmental salinity should not have a detrimental effect on the surfacing of many bitumen-sealed unbound granular roads provided measures, such as those assessed in this study, are implemented to ensure salts do not concentrate and precipitate in the upper zone of a road pavement, where salt crystallisation could cause damage to the surfacing. Therefore appropriate geochemical investigations should be conducted in arid and semi-arid areas where salinity is identified as a potential issue, followed by detailed assessment of the potential for salt accumulation, applying the methodology and numerical model utilised in this study. In areas where environmental salinity is assessed as having the potential to cause damage to the road surfacing suitable mitigation measures, such as those discussed in this study, should be implemented.
- Subject
- pavements; salinity; tensile; shear; microstructure; laboratory columns; field site
- Identifier
- http://hdl.handle.net/1959.13/1309566
- Identifier
- uon:21907
- Rights
- Copyright 2015 Ryan Steven de Carteret
- Language
- eng
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