Utilization of Waste Plastic and Recycle Concrete Aggregate in Production of Hot Mix Asphalt

Recycling of waste material is a recent technique aims to change the waste material into new products to reduce the pollution and detrimental effect on the environment and reduce the demand of new fresh natural sources. Plastic bags and Recycled Concrete Aggregate (RCA) are samples of these waste materials can be re-used in road construction. Over one million bags are used every minute worldwide, whereas, aggregate is consist of about 95% of asphalt mixture and can be obtained as RCA from demolished infrastructure. This paper presents laboratory tests results of using waste plastic and RCA in production of asphalt mixture. Since the cement past attached to RCA particles contribute to lower their density and increase the porosity, the waste plastic are used to enhance the engineering properties of asphalt mixture and consume these large amount of waste material. The results showed that Waste Plastic Modified Bitumen (WPMB) mix containing 100% RCA produces higher Marshall Stability, higher retained stability and higher indirect tensile strength compared with conventional mix. The percents of the increase were 10% for Marshall Stability, 7% for Marshall retained stability and 9% for higher indirect tensile


INTRODUCTION AND BACKGROUND
The availability of different waste materials is increasing day by day and the disposal of these materials is a big problem. These wastes are increasing the concern of environmental pollution since many of these materials are non-biodegradable. Plastic one of these materials, which is a very versatile material widely used in packaging of many outputs of industry. A survey has shown that 500 billion plastic bags are used worldwide every year (Plastic bag pollution 2005). Without thinking in a suitable way to utilized these materials in recycling industries, these wastes have In addition to re-use of waste plastic can significantly reduce the disposal problem of these waste and minimize the concern of pollution, they can enhance pavement performance and reduce the cost of construction of roads.
The results of previous studies showed that the using of waste plastic in asphalt mixture improve the engineering properties such as Marshall Stability, resistant to water (measure by retained stability) and resistant to crack propagation (indicated by indirect tensile strength of modified asphalt mixes). This approach of modification also produces better binding property for the bitumen mix and increase the road life ( The other waste material which can be recycled to be used in road construction is the Recycled Concrete Aggregate (RCA). RCA can be obtained from construction debris, where demolished concrete structures are crushed to smaller units for diverse uses. Aggregate is main component of asphalts mixture, it composes about 95% of asphalt mix. The continuous use of this material is threatening the natural resources of it; especially, it is known that the flexible pavement is one of most worldwide pavement type. Therefore, the recycled of this material can protect the environment, provide sustainable construction and reduce the demand on natural resources, moreover, reduce the construction cost of the road (Huang et al. 2007).
However, the engineering properties of RCA differ from natural aggregates due to remaining cement paste on their surfaces after the recycling process. The current article focuses on using both the waste plastic and RCA in asphalt mixture to produce asphalt mixture with higher engineering properties compare to conventional mix and consume the waste materials. This approach also reduces the construction cost of the road and minimising the consumption of natural resources.

METHODOLOGY
The laboratory tests in this research consist of four aspects. These aspects involved production of four different asphalt mixes:  6.0 % and 6.5%) were produced to select optimum bitumen content using Marshall Method. The optimum bitumen content was 4.85%. 2-Mixing of waste plastic with bitumen: generally, there are three different methods to mix the plastic with bitumen as shown below.  Dry Process: In this method, the waste plastic is added to hot aggregate particles to produce a plastic coated aggregate. The plastic coated aggregate is then mixed with optimum quantity of bitumen to get a WPMB Mix.  Wet process: This process involves melting and mixing the waste plastic with hot bitumen a round 150 0 C using a bitumen mixer to get WPMB. The modified bitumen (WPMB) is added to heated aggregate to get a WPMB Mix. This process was adopted in the current research, where waste plastic bags were melted and stirred in bitumen according to above mentioned procedure. The percent of waste plastic was 8 % of the weight of the bitumen, adopted according to previous studies in literature ( where small plastic particles are still insoluble during cooling of WPMB to ambient temperature. In this process, the waste plastic acts as modifier for the bitumen and as a coating for aggregate when it is added to aggregate to get a WPMB Mix. 3-Obtaining of recycled concrete aggregate (RCA): In this work, the RCA was obtained from waste concrete testing cubes. Cubes were crushed and abraded using Los Angles abrasion machine for 20-30 minutes to get different sizes of aggregate particles. These particles were washed and dried using electric oven for 24 hours. The resulting particles where classified according to mid-range of specification of aggregate gradation using sieve analysis for these particles. 4-Evaluating the four mentioned mixes using the following tests: The evaluation process for the four mixes included in this study involved conducting three tests. (1) Where: ITS = indirect tensile strength (MPa) P = applied load (N) h = average height of specimens (mm) d = average diameter of specimens (mm) Fig.1 presents the whole picture of the research frame work.

Standard Marshall Test:
After adopting the optimum content of bitumen, the standard Marshall test was carried out on the four mixes. The Marshall Stability and Flow were record for each type of mix as shown in Fig.2 and Fig.3. Marshall Stability refers to maximum load sustain by asphalt mix, this can be an indication for pavement distortion, rutting and shear stress. Fig.2 shows the Marshall stability for the four mixes. It can be observed that the Marshall Stability increases by about 30% and 15% for the WPMB mix and WPMB mix containing RCA respectively compared with standard mix. While it decreases by about 10% for the RCA mix compared with standard mix. This can be attributed to that, the using of waste plastic in asphalt mixes may lead to better blending and binding of asphalt mix also decreasing of the air voids which consequently lead to higher stability for mixes. For the RCA mix, the cementation material attached with aggregate particles leads to increase the porosity, water absorption and decrease density (Paranavithana and Mohajerani 2006); which, consequently decreases Marshall stability and increases Marshall flow (See Fig.4). Fig.3 shows the comparison of displacement or flow of the mixes during the Marshall test. The addition of waste plastic to asphalt mixture produces less flow due to better binding of mix components. Fig.4 presents the densities for the four mixes, the WPMB mix shows less density compared with conventional mix due to less density of waste plastic. The reduction in density is increasing by adding the RCA due to higher porosity and water absorption for RCA.

Marshall Retained Stability Test:
This test can assesses pavement condition when it subjected to severe conditions such as soaking in water for long time. Fig.5 presents the Marshall Retained Stability results after socking in water for 24 hours at 60 0 C. Fig.5 reveals generally that the Marshall Stability decrease for all mixes, but all values are more than 70% of Marshall Stability values as shown in Fig.6. The Marshall Retained Stability is higher by about 10% and 7% for the WPMB mix and WPMB containing RCA mix respectively compared with Marshall Retained Stability for the standard mix. While it is less by about 15% for the RCA mix compared with standard mix.

Indirect Tensile Strength:
This test is conducted to assess the tensile properties of asphalt mix which can be further linked to the cracking properties of the flexible pavement. Several distresses of pavement such as low temperature cracking, fatigue and rutting are related to tensile properties of asphalt mix. A higher tensile strength leads to higher cracking resistance and higher strain prior to failure of the pavement (Tayfur et al. 2007). Fig.7 presents the comparison of tensile strength for the mixes under consideration. It can be clearly seen that waste plastic improved the tensile properties of the asphalt mix; however, this improvement are slightly decrease with using of RCA in bitumen mixes. The indirect tensile strength increases by 30 % and 10 % respectively for the WPMB mix and WPMB mix containing RCA respectively compared with that of the standard mix. While it decreases by about 20% for the RCA mix compared with standard mix.

SUMMARY AND CONCLUSIONS
This paper has presented an experimental investigation for using waste plastic and RCA in hot mix asphalt (HMA) for road pavement. The results showed that the using of RCA alone (without of waste plastic as modified) causes reduction in Marshall Stability and indirect tensile strength of the asphalt mix; which, consequently produces reduction in the resistance of pavement to deterioration and cracks formulation. On the other hand, the using of waste plastic in asphalt mix which has 100% RCA can produce better asphalt mixture compared with conventional mix.
The use of recycled material (RCA and waste plastics) in the pavement of the road has helped to solve the problem of disposal of waste plastic and demolished concrete structural elements by providing better place for burying them. At the same time, a better pavement can be achieved by improving the Marshall Stability, strength, fatigue life and other desirable properties of asphalt mix, consequently improves the longevity and pavement performance. Finally, the using of these waste materials can significantly reduces the construction cost of the roads.