1. Introduction
1.1. Background Information
Flash floods are considered to be of major inconvenience to motorists, exacerbated by the limited land we have in Singapore. Singapore, being one of the most densely populated countries in the world, has seen a growing transport demand among individual road users, logistic firms and public transportation agencies in the city. These users “expect reliable and safe road infrastructure for traveling from one location to another and transporting goods and people” (Hartmann & Ling, 2016, p. 29). However, due to the city’s tropical climate, Singapore receives abundant rainfall, which has been steadily rising over the years (Meteorological Service Singapore, 2018 (Fig 1.2)). This increase in frequency and intensity of rain, in conjunction with the increase in the car population in Singapore, is a catalyst for road accidents as wet road conditions reduces the friction car tyres have against the road surface. This in turn leads to higher chances of causing human casualties, property damage, and traffic disruption due to road flooding, landslides, or fallen trees from storms (Chow, Cheong & Ho, 2016).
According to media archives retrieved in a study led by Chow, Cheong and Ho (2016b), there was a total of 212 flood events reported in Singapore from 1965 to 2015, with only seven years that were absent of recorded newsworthy flood events. The data listed in The Straits Times categorized and defined 30.2% of the historical extent of flooding during the fifty-year period as flash floods (Fig 1).
These floods have led to the development of alternative techniques to increase the drainage capability of surfaces, such as the use of porous asphalt. Porous asphalt is a type of asphalt that allows water to permeate through its surface into a collection pond below the pavement. Water-permeable asphalt is made possible by controlling and/or changing the amount of materials used in conventional asphalt mixes, such as the fine and coarse aggregates, along with some variations in construction methods to produce the desired outcome of reducing surface runoff. Porous asphalt has the capacity to conserve water, reduce runoff and promote infiltration which cleanses stormwater, all of which are acquired from the relatively high porosity and permeability of the porous pavement layers (Fwa, Lim, Tan, 2015). According to Hesami, Ameri, Goli and Akbari (2013), several completed or ongoing studies on warm-mix asphalt and steel slag asphalt mixtures have been conducted all over the world.
Exemplifying the advantages porous asphalt would contribute toward sustainability, such as introducing recycled by-products in the production process such as steel slag. Non-potable use for the water collected from the asphalt after rain (Hammes, Thives & Ghisi 2018), together with recycled steel slag, are sustainable aspects of porous asphalt that can serve to urge the Land Transport Authority (LTA) to consider porous asphalt as an alternative to the conventional road mix.
1.2. Problem Statement
Flash floods have always been of major inconvenience to motorists in Singapore. With the aim of being a sustainable city, porous asphalt should be implemented on the roads of Singapore not only to mitigate the occurrence of floods, but also as a sustainable measure by introducing recycled by-products in the production process as well as providing non-potable use for the water collected from the asphalt.
1.3. Purpose Statement
The purpose of this report is to propose the implementation of porous asphalt on busy roads to reduce the occurrence of floods in areas that has lack of drainage system such as the Expressways. The proposed methodologies and application of porous asphalt roads would demonstrate to LTA that porous asphalt not only mitigates the occurrence of floods, it can also be presented as a sustainable approach to mitigate road flooding.
2. Proposed Solution
2.1. Porous Asphalt Roads
The team’s proposed solution to mitigate the occurrence of flood is the implementation of porous asphalt on roads especially on areas where there is lack of drainage system such as the expressways. Incorporating porous asphalt is a strategic stormwater management with its unique functional benefits of having the capacity to conserve water, reduce runoff and promote infiltration with cleanses stormwater, all of which are acquired from the relatively high porosity and permeability of the porous pavement layers (Fwa, Lim, Tan, 2015).
2.1.1. Application
• A proposal to LTA will be made to have a pilot study on a small section of road.
• Instead of using a traditional asphalt mix, our proposed porous asphalt mix can be used during the maintenance of roads
• After studies and research on the small section of road is conducted, if it is sustainable, the porous asphalt can be applied to more roads other than our pilot road
• Substituting crushed stone aggregates with steel furnace slag. According to Ahmedzade & Sengoz (2009b), steel slag used as coarse aggregate improved the mechanical properties of asphalt mixtures as the results of different tests conducted with steel slag indicated that steel slag mixtures have excellent engineering properties. Currently, steel slag is supplied by NatSteel Asia, the only steel mill in Singapore, as a by-product of reinforcement bar production. It is treated by NSL Chemicals Ltd (formerly known as NatSteel Envirotech Pte Ltd).
3. Benefits of Proposed Solution
3.1. Reducing impact of floods and ponding
The higher void content of porous asphalt as compared to the conventional asphalt mix, will allow the seepage of rainwater through the road surface, thus maintaining a relatively dry surface. Reduced ponding on the road surface allows for a safer and pleasant driving experience as the car tyres do not lose friction against a wet road surface.
3.2. Stormwater management
The collection of stormwater from under porous asphalt roads can be collected for non-potable use in buildings, such as “flushing urinals and toilets”, in a building, which led to savings of 53 to 54% of potable water (Hammes, G., Thives, L., & Ghisi, E., 2018). This would make porous asphalt roads dual-purpose and hence sustainable (preventing floods and recycling water simultaneously).
3.3. Recycled by-product: Steel Slag
According to Chen and Wei (2016), the use of steel slag as a coarse aggregate substitute could improve the engineering properties of asphalt mixtures with higher durability, higher skid resistance and has longer road lifespan resulting in lesser carbon emissions. Also, steel slag have angular and rough textured particles that would improve the interlocking mechanism and provide good mechanical properties.
4. Proposal Evaluation
4.1. Case Study
According to Takahashi (2013), the number of traffic accidents on wet surface conditions in Japan since 1998 has decreased drastically because of the spread of porous asphalt used widely across the expressways (refer to Figure 4). Due to the wet surface caused by rain, the loss of vehicle control at high-speed travel was one of the main reasons for these traffic accidents. At that time, with the growing economy and with the unexpected increase in the number of vehicles, many surfaces of expressways in Japan were severely damaged.
4.2. Challenges
Porous asphalt may seem to be highly efficient and sustainable to the environment, but there are certain traits to be forgone in exchange for such benefits.
Strength
The large amount of pore size in the porous asphalt mix, allows water seepage through the surface, but significantly reduces the strength of the road surface as well (Mo, Huurman, Woldekidan, Wu, Molenaar, 2010). This suggests that porous asphalt mix might not be feasible on roads with high occurrences of heavy traffic such as roads junctions. The sheer amount of vehicles at rest at road junctions will generate relatively large amounts of static forces which may not be suitable for porous asphalt to withstand.
To overcome this challenge, the team has suggested for the porous asphalt mix to be implemented on highway roads instead. The reduced congestion in traffic and a swifter road flow, as compared to road junctions, will be a good fit for the use of porous asphalt.
5. Methodology and Procedure
5.1. Primary Research
According to an interview done on March 6, 2019 with Teo Yong Boon, the Laboratory Manager for Samwoh Corporation Pte Ltd, porous asphalt is not currently being implemented on expressways on a large enough scale, and therefore consider proposing the implementation of porous asphalt on expressways.
5.2. Secondary Research
Sources of secondary research include research articles and online journals which are relevant sources that support the contents of this technical report. An example of an article is by Fwa, T. F., Lim, E., & Tan, K. H. (2015) which discussed the characteristics of porous asphalt and pervious concrete pavement materials. Another relevant source is by Hesami, S., Ameri, M., Goli, H., & Akbari, A. (2015), where it talks about the warm-mix asphalt mixtures containing steel slag aggregates. The figures as listed in the appendices were sourced from Meteorological Service Singapore (2018) and several academic articles.
6. Conclusion
The upward trend of increasing annual rainfall that is expected in Singapore due to changes in the global climate will be a major cause of flooding and ponding on roads in the future. There is a compelling need to prevent flooding and ponding and hence accidents from occuring on expressways where vehicles travel at a fast speed, which can also pose health and safety hazards to road users and pedestrians along such roads. While our proposed porous asphalt solution could mitigate floods and reduce ponding on expressways, it is also sustainable approach to solving our problem. Sustainable aspects include the potential of stormwater collection and its various non-potable uses in buildings and the usage of recycled steel-slag aggregates in place of traditional aggregates.
