Received 22 February 2016; accepted 24 May 2016; published 27 May 2016
Solid waste is the waste arising from human and animal activities that is normally solid and is discarded as useless or unwanted  . According to World Bank Solid Waste Thematic Group, 1.3 billion tons per year of solid waste is being generated in world. This waste is likely to increase to 2.2 billion tons per year by 2025  . World Health Organization (WHO) states that more than 1.5 million people die annually due to poor solid waste management. Waste poses severe hazard to public health through blocking of drainage system, formation of standing ponds, and creating breeding grounds for mosquitoes and flies. This triggers malaria and cholera. 40% of the deaths in Pakistan are due to poor waste management  . In addition, because of lack of proper dumping sites, most of the collected waste ends up in open pits, ponds, rivers, dumping grounds, and agricultural lands. A part of this waste gets decomposed in the environment at dumpsites but non-biodegradable waste stays there for years.
Shredded plastic waste was used in concrete as replacement of sand by Zainab Z. Ismail et al.  . 30 kg of waste plastic was recycled in the study. Tests conducted on concrete included compressive strength, flexural strength, fresh density, dry density, toughness indices, and slump. Curing ages were 3, 7, 14, and 28 days. The results indicated the production of the micro crack in concrete mixtures, but guided the reusing plastic to replace sand in cement concrete provides a decent method to unravel certain of the solid waste evils caused by plastics. Mariaenrica Frigione et al.  then extended the work of Zainab Z. Ismail with different W/C ratio. Samples with changed water/cement ratio and cement content were prepared. Mechanical tests at 28 and 365 days were done on the polyethylene terephthalate (PET) concretes along with on reference concretes comprising of only natural fine aggregate. It was noted that compressive strength, workability, and splitting tensile strength was lowered to some extent than that of reference concrete but ductility was reasonably higher. The importance of curing on durability was examined by V. Silva et al.  . Concrete mixes were prepared in which natural aggregates were substituted by PET aggregate by 0%, 7.5% and 15%. The prepared concrete samples were exposed to laboratory environment, open-air environment, and wet chamber curing systems. Tests for water absorption by immersion, shrinkage, water absorption by capillarity action, chloride penetration were done. Results showed decrease in durability. All samples performed poorly when exposed to drier curing regimes. Nevertheless, sensitivity analyses revealed less deterioration of plastic aggregate concrete, when subjected to progressively drier curing regimes.
Sinan Hınıslıoğlu et al.  used shredded high density polythene (HDPE) as a binder replacement in asphalt. Four percent HDPE, 165˚C of mixing temperature and 30 minutes of mixing time were determined as optimum conditions for Marshall Stability, flow and Marshall Quotient (MQ). Effect of plastic fibers in asphalt concrete was studied by Serkan Tapkın et al.  . Asphalt samples with polypropylenefibers were prepared. For fiber-reinforced samples, the flow values decreased and Marshall Stability values increased. An increased in the fatigue life was also observed. Fiber-reinforced asphalt mixture showed a reduced amount of reflection cracking and good resistance to prolonged fatigue life, and rutting. Wasted plastic bottles were used by Esmaeil Ahmadinia et al. as fine aggregates in asphalt  . Main objective of the research was to see the influence of plastic bottles in asphalt concrete. 2%, 4%, 6%, 8% and 10% shredded plastic bottles by weight of bitumen were added. Suitable amount of PET was established to be 6%. Natural aggregates were coated with plastic and their abrasion, crushing values were calculated by Afroz Sultana et al.  . Various types of plastic for example Low Density Polythene (LDPE), Polypropylene, and HDPE were mixed. Modified and Unmodified asphalt binders were put into rheological test. Marshall Stability tests were also performed. Outcomes exhibited improved values for asphalt concrete. Use of polyethylene terephthalate in asphalt concrete as coarse aggregate replacement resulted in a reduction of bulk density of asphalt mixes  .
3. Materials and Methods
3.1. Plastic Collection and Aggregate Formation
Plastic waste was collected from different hospitals of the city. For sorting and shredding of waste SS Industries and Al-Hafiz Cryptoclastic Industry in Hayatabad Industrial Estate Peshawar, Pakistan were approached. Different categories of plastic were separated from waste. 64% of the waste was LDPE, 32% was HDPE and 4% polypropylene. All three types of waste was shredded (Figure 1(a)), mixed together and then heated (Figure 1(b)). Semi solidform of plastic obtained was cut at regular intervals and further pulverized first manually (Figure 1(c)) then by Jaw crusher.
(a) (b) (c)
Figure 1. (a) Shredded plastic waste (b) Plastic heating pipe (c) Aggregate pulverization.
3.2. Experimental Programme
Following tests were performed on aggregates:
Specific Gravity & Water Absorption (AASHTO T 85 ASTM Designation: C 128-88)
Aggregate Impact Value Test (BS812-112:1990)
Aggregate Crushing Value Test (BS 812-110:1990)
Loss Angeles Abrasion Test (AASHTO designation: T96| ASTM C 535-12)
Soundness of Aggregate Test (AASHTO T 104 and ASTM C 88)
Asphalt Sample Preparation & Testing
Natural aggregates and bitumen used in the asphalt preparations had the properties listed in Table 1. Well graded natural aggregates were used for asphalt sampling.
Replacement of natural aggregates was done on aggregates passing 3/8 in. and retained on sieve #4. Scheme adopted for replacement is described in Figure 2. Well graded natural aggregates were used for asphalt sample making (Figure 3(a)). Natural aggregate were replaced up to 25% (by volume) with 5% incremental increase as shown in Figure 3(b). Trial mixes were made to find the optimum binder content that came out to be 4.5%.
Following tests were conducted on asphalt samples:
Bulk Specific Gravity (AASHTO T-166-05)
Theoretical Maximum Specific Gravity (AASHTO T-209-05)
Marshall Stability and Flow Test
4.1. Specific Gravity and Water Absorption
Specific gravity of plastic aggregates was less than natural aggregates. Low density of LDPE, HDPE and polypropylene was main reason for this decrease in density. Water absorption of plastic aggregates on the other hand was very high. Branched molecular structure of LDPE was the reason of this high water absorption. Figure 4(a) and Figure 4(b) show specific gravity and water absorption of plastic aggregates as compared to natural aggregates.
4.2. Soundness Test
Any type of polythene, whether HDPE or LDPE is generally inert to chemicals. The reason for this is lack of polarity in polythene molecules. HDPE in comparison to LDPE is more resistant to chemicals actions because of its low permeability. Low permeability comes because of high molecular weight of HDPE. Hence soundness test results for HW aggregates were low as compared to natural aggregates as shown in Figure 5.
4.3. Aggregate Impact, Crushing and Loss Angeles Values
As plastic aggregate impact, crushing and Loss Angeles values were less than 10, it came in “exceptionally
Figure 2. Mass of asphalt sample with replaced plastic aggregate.
Figure 3. (a) Well graded aggregates for asphalt sample; (b) Asphalt samples.
Figure 4. Specific gravity of plastic & natural aggregates; (b) Water absorption of plastic & natural aggregates.
Table 1. Natural aggregates and bitumen properties.
Figure 5. Soundness of aggregates.
strong” aggregate category. Major portion of HW aggregates was LDPE. This gave the aggregates more compressibility and hence lesser compression damages. Table 2 presents aggregate impact, crushing and Loss Angeles values of natural and plastic aggregates.
4.4. Density and Percent Voids of Asphalt Samples
Due to lesser density of plastic aggregates, asphalt samples made of replaced plastic aggregates showed lower densities with increasing plastic aggregates percentage. Figure 6(a) shows detail values of densities. As most of the bitumen was used in filling pores of plastic aggregates, less bitumen was available to fill asphalt void. This increased percent voids in asphalt mixes (Figure 6(b)).
4.5. Stability and Flow Results
Stability of the mixes showed increasing trend up to 20% replacement than sudden decrease in stability was observed (Figure 7(a)). This decrease in stability may be because of higher porosity of plastic aggregates. Most of the binder was used in filling the pores of plastic aggregates leaving behind insufficient quantity of bitumen to make stronger bond. As the compressibility of plastic aggregates was greater than natural aggregates, asphalt samples having plastic aggregate had higher flows (Figure 7(b)). Flow values increased with increasing plastic aggregate percentage but decreased at 25% replacement. This decrease was due to decrease in stability of mixes at 25% replacement.
5. Cost Analysis
Preparation cost of plastic aggregates was very high as compared to natural aggregate. The reason for this was that it was a pilot project and there was no prior manufacturing setup for making aggregates from waste plastic. Table 3 shows preparation cost of plastic aggregates. Once proper manufacturing setup has been established, preparation cost of plastic aggregates will be almost equal to that of natural aggregates. Obviously asphalt mixes made of replaced plastic aggregates were expensive than natural aggregates. Cost increased with increase in percent replacement. Table 4 shows details of cost of asphalt samples. Table 5 shows increase in stability of mixes with respective increase in cost.
There is no proper solid waste management system for Peshawar city. Mostly people throw the waste in their streets, where it is either picked up by scavengers or dumped there for years. In relatively developed areas of the city, scavengers collect the waste and dump it in an open area usually at a distance from densely populated area. Existing landfills of the city are not well designed. Open burning of waste on the dumpsites is observed. Major portion of the waste generated by the city is plastic. Plastic waste can be converted to aggregates by proper manufacturing setup. After performing different aggregates tests, it is found that plastic aggregates used in this research are exceptionally strong in impact, crushing and abrasion. Due to higher porosity of aggregates, soundness values are little higher. In asphalt mixes, density and percent voids increased with increasing plastic aggregates content. Stability and flow also increase with increasing plastic aggregate content. But for this increase
Figure 6. (a) Density of asphalt mixes; (b) Voids in asphalt samples.
Figure 7. (a) Marshall stability of asphalt mixes; (b) Flow of asphalt mixes.
Table 2. Aggregate impact-crushing and Loss Angeles values.
Table 3. Plastic aggregate preparation cost.
Table 4. Cost of HW mixes preparation.
*Plastic aggregate; **Natural aggregate; +Bitumen cost; ++Operational charges.
Table 5. Increase in stability of mixes with respective increase in cost.
in stability, cost also increases. After 25% replacement, stability decreases while cost keeps on increasing. Therefore up to 20% of natural aggregates can be replaced by plastic aggregates.