A statistical Review on the Usage of Reclaimed Asphalt Pavement waste as a recyclable material

Construction is a part of human evolution, in ancient times people used wood and stone as construction materials as they were obtained naturally. As time passed, we learned about new methods and materials of construction. We started using concrete, steel, and asphalt as construction materials but these materials were costly and also had ill effects on the environment. In modern times construction techniques should be eco-friendly and economical as well. So, recycling construction and demolition waste is an option to deal with these problems. Reclaimed asphalt pavement (RAP) is a type of construction and demolition waste, which can be recycled after proper treatment. Reclaimed asphalt pavement materials are obtained from the milling of road pavement during reconstruction or maintenance work. These materials are mainly coarse aggregates and asphalt binders. The use of Reclaimed asphalt pavement with virgin crushed aggregates is widely accepted practice in several construction applications as it reduces the cost of construction, transportation, and handling. Properties of reclaimed asphalt pavement wastes are expected to be in an acceptable range with the conventional material but can be used as a partial or whole replacement for conventional materials. Reclaimed asphalt pavement also reduces the consumption or violation of natural resources. One of the primary advantages of using RAP is cost savings. Incorporating RAP into asphalt mixtures can significantly reduce material and disposal costs since it eliminates the need to extract and transport new aggregates. Additionally, RAP can reduce the demand for virgin asphalt binder In this review, a study was done on the usage of Reclaimed Asphalt Pavement as a recyclable construction material, and its physical and chemical characteristics are compared to conclude and provide its statistical review so that it can be used for sustainable construction works.


Introduction
Asphalt pavements are integral to our transportation infrastructure, providing smooth and durable surfaces for roads and highways.However, the construction and maintenance of asphalt pavements consume vast quantities of natural resources, such as aggregates and bitumen, while also contributing to greenhouse gas emissions.Recognizing these environmental and economic challenges, researchers and industry professionals have turned their attention to Reclaimed asphalt pavement (RAP) , a byproduct of pavement rehabilitation.Reclaimed asphalt pavement (RAP) is essentially old asphalt pavement material that is collected, processed, and reused in new asphalt mixtures.One of the most commonly used construction and demolition waste is Reclaimed Asphalt Pavement (RAP).Reclaimed Asphalt Pavement is a type of construction and demolition waste which is obtained from the removal of top layer of the road during its maintenance or reconstruction, This process is known as milling.During maintenance or reconstruction work, milling of existing pavement can be done partially or in whole.In modern times, RAP can be used as a partial or full replacement for coarse aggregates.This recycling approach not only conserves valuable resources but also reduces landfill waste and lowers the carbon footprint associated with asphalt production.Consequently, RAP represents a sustainable solution that aligns with global efforts to promote circular economies and reduce the environmental impact of infrastructure development.The usage of Reclaimed Asphalt Pavement (RAP) as a construction material varies in its extent and regulation both nationally and internationally.RAP is widely used in the U.S. for road construction and maintenance.Many states have adopted RAP in their asphalt mix designs, with some permitting high percentages of RAP in new asphalt pavements.It is used extensively in Canadian and European road construction, and it is commonly incorporated into asphalt mixes.Some developing countries are also recognizing the benefits of RAP usage, although the extent of adoption may be lower due to limited resources and technology.Efforts are being made to transfer best practices from developed nations.Global Initiatives are being taken by various international organizations and initiatives, such as the United Nations Sustainable Development Goals (SDGs), promote sustainable infrastructure, which often includes RAP usage as a means to reduce the carbon footprint of road construction.In both national and international scenarios, the usage of RAP as a construction material is driven by the need to reduce costs, conserve resources, and minimize environmental impacts.However, the specific regulations, standards, and extent of adoption can vary widely, and these differences are influenced by factors such as regional infrastructure needs, available technology, and local policies.As sustainability and circular economy principles gain momentum, it is likely that RAP usage will continue to expand both nationally and internationally [14].RAP can also be used with geopolymer concrete.Geopolymer concrete is made of Alumino and silicate mixed with thermally activated natural materials like fly ash or rice husk.Silicates and hydroxides of Potassium and sodium acts as activators in geo polymer concrete.Geopolymer concrete releases less carbon compounds during its production and mixing.Due to its economical and ecofriendly properties geopolymer concrete is widely being used these days.The objective of this research is to review the previous research work on the usage of reclaimed asphalt pavement waste as a recyclable construction material.Results of tests conducted on various proportions of reclaimed asphalt pavement waste for different physical and mechanical properties are compared to find out the suitable mix design or proportion of reclaimed asphalt pavement in the construction application [15].

Material and Methods
The following methodology was proposed by different authors to get various results given in table 1 .Table 1 contains the details of the previous research done in this field with the various methods adopted by various authors.It also mentions the type of aggregates used and percentages of recycled aggregates used in the mix .After this under data analysis part with table 2 , comparison of various mechanical and durability properties have been done on the basis of values of results obtained for tests like dry density , moisture content,stress stain effect and grain size distribution as mentioned in the sections given below (Table .

Results & Discussion
From the above data the following results can be inferred as given below: Dry Density is the number of aggregates and the amount of entrained air in the mixture.Results of different mixtures for dry density tests conducted by different authors are given in Figure 1. and the following points can be perceived by it.
x The dry density of RAP can impact the mix design process when incorporating it into new asphalt mixtures.Higher dry densities typically result in increased stiffness and resistance to deformation, which may affect workability during mixing and paving operations.Adjustments to the mix design, such as the binder content or the addition of rejuvenators, may be necessary to achieve the desired workability.
IOP Publishing doi:10.1088/1755-1315/1327/1/0120215 x Dry density test shows that the majority of the mixture with 25% RAP and 75% virgin aggregates developed by Taha.R have the maximum dry density at 2110kg/m 3 as compared to other mixtures.x Constantino J.P. got the lowest value for dry density using 100% RAP aggregates which is 1778kg/m 3 .x Dry density for normal-weight concrete varies from 2300 to 2500kgm/ 3 .
x The average dry density of all the authors for 100% RAP mixture comes as 1896.8kg/m 3 .
Whereas the average dry density for 25% RAP mixture was 1987.6kg/m 3 .

Moisture Content
The amount of moisture a concrete mix can absorb after drying is known as moisture content.Figure 2 shows the percentage of moisture content for different RAP mixtures of various authors and following points can be perceived from it .
x The moisture content of RAP can affect the workability of the asphalt mixture during the mixing and paving processes.x Excessive moisture in RAP can lead to clumping and sticking of the material, making it challenging to achieve a homogeneous mix.On the other hand, too little moisture can result in a dry, dusty mix that is difficult to compact.x The results for moisture content of different authors for different mixtures of RAP are shown in Figure 2. The results clearly show that Bennett T. with 100% RAP mixture got the acceptable amount of moisture that is 5% whereas Kim W. with 100% and 75% RAP mixture got 10% moisture content, which is not the acceptable amount of moisture content in a cement mixture.x Moisture content affects the volumetric properties of the asphalt mixture, including air voids and density.Excess moisture can lead to higher air voids, which may reduce the mixture's durability and resistance to moisture damage.Conversely, insufficient moisture can result in lower air voids, potentially leading to rutting and cracking.

Stress-strain curve:
A stress-strain curve is a graphical way of representing the reaction of a material when the load is applied.The stress-strain behavior of RAP can affect the overall strength and stiffness of the asphalt mixture.RAP typically exhibits lower stiffness compared to virgin aggregates.When RAP is used, the resulting mixture may have reduced stiffness, which can influence its resistance to deformation, rutting, and cracking.The stress-strain characteristics of RAP can also affect the cracking resistance of the asphalt mixture.A more flexible stress-strain response may help mitigate lowtemperature cracking, as it can better accommodate thermal and load-induced stresses without fracturing figure 3 represents the stress value at different percentages of strain for 100% RAP mixture.630 kPa stress at 10% strain was the maximum stress value by Constentino J.P. for 100% RAP aggregates where as Kim W. got 525 kPa stress value at 10% strain as he used a mixture of RAP and crushed aggregates in different ratios thereby getting appretiable results .

Grain Size (Figure.4) Grain Size Distribution
The number of aggregates passes through different sieve sizes during sieve analysis.Percentage of aggregates passing through different sieve sizes for RAP mixtures is given below in figure 4. The result clearly shows that Taha R. got the most favorable results that only 57% passes through 9.5mm sieve which was the lowest among all.Whereas in Bennert T. case 100% of RAP got retained on 0.075mm sieve.The results presented in these papers clearly represented that RAP can be used as a recyclable material when used at 25% in the mixture with virgin aggregates.In the proposed mix, values for dry density is maximum for most of the authors, when only 25% of virgin aggregates were replaced in the mixture whereas moisture content is also acceptable where RAP aggregates were used as 25 to 40% replacement of virgin aggregates.Stress-strain curve and grain size are also favorable for using RAP as a recyclable construction material.

Conclusion
x This study clearly represents that RAP can be used as recyclable material when used at a minimum of 25 -50% in the mixture with virgin aggregates.
x Study shows that, RAP aggregates can only be used as partial replacement for virgin aggregates.
x Replacing virgin aggregates 100% with RAP aggregates gives lower values of dry density and moisture content for most of the authors.Using RAP with a suitable dry density can contribute to sustainability goals by reducing the need for virgin aggregates, thus conserving natural resources and reducing the environmental footprint associated with mining and transportation.x However, Stress-strain relation and grain size distribution shows results in favour of using RAP aggregates more than 50% with mixture of virgin aggregates.x The stress-strain response of RAP can influence the deformation and rutting resistance of the asphalt mixture.RAP with a more compliant stress-strain behavior may be prone to permanent deformation under traffic loading, potentially leading to rutting issues in the pavement.

Future Scope
The scope for future work in Reclaimed Asphalt Pavement (RAP) usage as a construction material is vast, with opportunities to address emerging challenges, optimize current practices, and explore new avenues for sustainable infrastructure development majorly focusing on following research aspects .
x Investigating innovative technologies for rejuvenating aged binder in RAP x Develop strategies to effectively manage the variability of RAP materials, which can differ in composition and characteristics based on the source x Conduct comprehensive life cycle assessments (LCAs) to quantitatively evaluate the environmental benefits of RAP usage compared to conventional construction materials, including its impact on carbon emissions, energy consumption, and resource conservation x Determine the optimal RAP content in asphalt mixtures to balance cost savings, performance, and environmental benefits.x Analyze the effectiveness of existing policies and regulations governing RAP usage, and propose improvements or adaptations to encourage its responsible use in construction projects.
x Conduct studies on public perception of RAP usage in construction projects and design educational campaigns to inform the public about the environmental and economic benefits of recycling asphalt pavement.x Explore how RAP can be effectively used in regions with extreme climates and weather conditions, addressing issues related to freeze-thaw cycles, moisture sensitivity, and temperature differentials