Strength and permeability of pervious composite prepared by using post-consumer plastic waste bottles

This study focused on developing and evaluating a novel form of pervious polyethylene terephthalate (PET) plastic composite. This novel composite, called as PPC, produced from plastic waste, soil and aggregates, will offer an effective technique for reducing storm water runoff and help divert a large amount of plastic from landfills and incinerators. PPC samples with different PET to soil/aggregate ratios were prepared and then tested for indirect tensile strength and permeability. Permeability was conducted in a unique manner by designing and fabricating a new testing equipment. Both indirect tensile strength and permeability were within the expected values found in the literature for porous pavements. Results showed that indirect tensile strength values increased with PET content. It was also found that using PET alone is not strong enough in binding aggregates and therefore, a soil/PET ratio of one was found optimum for providing maximum strength. Permeability values decreased with the decrease of A/P (aggregate/PET) ratio which in general indicates that lower PET and higher aggregate content is suitable for higher permeability. A soil/PET ratio of one was found to provide higher permeability but strength could compromise. Findings from this study indicated that developed PPC could be used for low-strength construction such as driveways, sidewalks and parking lots.


Introduction
According to Environmental Protection Agency [1], 32.5 million tons of plastic waste is generated annually in the United States (US), which is around 12.8% of entire municipal solid waste. However, only 9.2% of plastic (3.0 million tons) discarded annually is recycled [1]. Among the plastic materials, polyethylene terephthalate (PET) is a form of polyester and the most common consumer plastic. Beverages, food, and other consumer products are delivered in bottles or packages made from PET. In 2015, approximately 5,971 million pounds of PET bottles were sold into the marketplace in the US [2]. PET bottles have taken Both pavement types use the same materials as their conventional counterparts with the exception of including fines in the mix design and keeping the size distribution of coarse aggregates narrow for minimal particle packing (conventional concrete is mixture of Portland cement, coarse aggregates and fine aggregates). A pervious concrete mixture contains little or no fine aggregate, thus creating system of highly permeable and interconnected voids that drain quickly [14][15]. Utilization of post-consumer PET in pervious pavement will provide new market potential and greater likelihood for recycling by consuming the bulk of PET waste bottles and reduce the amount of waste entering landfills. Consequently, the aim of this study was to develop and evaluate novel form of pervious PET composite, called as PPC, by 2 Methodology and Experimental Design

Material Types and Sources
A total of three types of materials namely, virgin aggregates, soil and PET bottles flakes were used in this study. Virgin aggregates were collected from Prairie Material, a local ready-mix concrete plant located in Normal, Illinois. Bulk aggregates were sieved in accordance with AASHTO M 43 test method which indicated maximum aggregate size of 1 inch. Soil was collected from Portico Homes LLC residential construction site located in Normal, Illinois. Air dried soil was tested for sieve analysis and Atterberg limits in accordance with ASTM D 422 and ASTM D 4318, respectively. Test results showed soil with percent passing #200 of 15%, liquid limit of 20% and plasticity index of 15%. The post-consumer PET bottles were collected from Illinois State University (ISU) recycling facility and donated by students/faculty at ISU. These bottles were shredded into flakes by using Nelmor Grinder/Granulator Model No. G1012P1 at Midwest Fiber Recycling, Normal, IL. Fig. 2 shows photographic view of different materials used in this study.

Sample Preparation
A total of five PPC mixtures were prepared in this study. A summary of different composite mixtures is presented in Table 1. All PPC mixtures were prepared in accordance with ACI 211.3R [16] and PCA [14] recommendations; ACI 211.3R [16] provides a procedure for producing pervious concrete mixture proportions. prepared to study influence of soil content on indirect tensile strength and permeability of PPC. Specifically, PPC sample preparation consisted of the following steps: Step 1 -The required amount of ingredients, namely, dry PET waste bottle flakes and soil were weighed according to the mix designs (Fig. 1a). The virgin coarse aggregates were washed a day prior to the sample preparation and oven dried at a temperature of 110±5 o C for approximately 16 hours.
Step 2 -All ingredients were mixed in a stainless steel bowl and kept inside a preheated Ney Vulcan D-1750 oven at approximately 335±10 o C for 15 minutes (Fig. 1b). After 15 minutes mixture was taken out from the oven and stirred using a steel spoon (Fig. 1c). The mixture was then put back into the oven for reheating and stirred again after another 10 minutes. This process was repeated three times until a homogenous mixture was obtained as shown in Fig.  1d.
Step 3 -Once homogenous mixture is achieved, molten mixture was taken out of oven and heated aggregates were added followed by mixing using a stainless steel spoon for 3 minutes (Fig. 1e). Then, mixture was heated again for another 15 minutes in the Ney Vulcan D-1750 oven at 335±10 o C and stirred until homogenous mix is obtained as shown in Figs. 1f and 1g.
Step 4 -Mixture obtained from Step 3 was poured in a pre-heated split cylindrical mold having a diameter of 4 inches and compacted to a height of 4 inches. Then, mixture was compacted in approximately three layers by using a steel rod in accordance with ASTM C 192 guidelines at room temperature.
Step 5 -Split mold containing compacted mixture was allowed to cool down at room temperature for 24 hours. After 24 hours sample was extracted from the mold and labeled using a tag. Figs. 1h and 1i show photographic view of split mold containing compacted mixture and extracted sample, respectively.  Dry cylindrical samples of PPC were tested for indirect tensile strength and permeability. A brief description of each test is discussed in subsequent paragraphs.

Indirect Tensile Strength
Indirect tensile strength was conducted on replicates of PPC samples in accordance with the ASTM C 496M test method. The procedure consists of loading the specimen at a rate (vertical movement) of 0.5 inch per minute. Samples were tested in a universal testing machine using fixture designed for indirect tensile testing. Fig. 4 shows photographic view of sample before testing, test setup, and sample after testing. Tensile strength was calculated using the following equation: where, St,n = tensile strength of the specimen, n, Dn = diameter of the specimen, n, bn = thickness of the specimen, n, and Pf,n = maximum load observed for the specimen, n.
(a) (b) (c) Fig. 4. Photographic view of (a) sample before testing, (b) indirect tensile strength testing setup, and (c) sample after testing.

Permeability Test
Permeability (or hydraulic conductivity) tests were conducted in accordance with ACI 522R [17] with few modifications. Specifically, specimens were tested for permeability using a constant-head permeameter test setup. A permeameter was designed and fabricated for testing PPC specimens (see photographic view in Fig. 5). With the valve closed, water was poured into the cylindrical mold until the water level remains steady at 7.25 inch above the opening valve, i.e., head. Time begins when the valve is opened. The test is complete and time is stopped when the water level in the graduated cylinder reaches 1000 ml. Each specimen was tested for at least three times and using the collected data, permeability was calculated. Additionally, at least two replicates were tested for each PPC mix to ensure repeatability and statistical significance.

Analysis of data
A summary of indirect tensile strength and permeability test results are presented in Table 2.
Aggregate and soil content were added and calculated as a ratio with respect to PET (AS/P). Additionally, aggregate to PET (A/P) ratio was calculated as shown in Table 2. Further, results are analyzed and discussed in sections below.

Indirect tensile strength
The laboratory test results revealed that indirect tensile strength values increased with the increase in PET content. For example, increase in PET content from 5% to 10% (Mix#1-3) increased indirect tensile strength value by approximately 15% (from 52 to 60 psi). Table 2 also indicates that strength values are also dependent on amount of soil in PET-soil glue which binds aggregates together. At a constant PET content of 10% (Mix#3-5), increase in soil content from 0 to 10% resulted in increase in indirect tensile strength values by approximately 150% (from 28 to 70 psi). Table 2 also indicates that same amount of PET was available for soil and aggregates in Mix#3 through 5 (AS/P = 9). However, Mix#5 provided highest strength indicating that using PET alone (Mix#4) is not strong enough in binding aggregates together. A soil/PET ratio of one (Mix#5) is optimum for providing maximum bonding strength between aggregates and PET-soil glue. It is also evident from Table 2 that soil/PET ratio was one for both Mix#1 and Mix#5. However, Mix#5 provided higher strength as it had more PET available for aggregate and soil (lowest AS/P = 9) as compared to Mix#1 (highest AS/P = 19).

Permeability
From Table 2 it is evident that the permeability values decreased with the decrease of A/P ratio. For example, the permeability values of specimens decreased from approximately 2226 ft/day (Mix#1) to 1163 ft/day (Mix#3), as the A/P values decreased from 18 to 8.5. This is due the fact that PET decreases air voids, thus reducing permeability values. Use of equal amount of soil and PET (soil/PET ratio = 1) helped by making specimens porous but decreased indirect tensile strength. For example, both Mix#1 and Mix#4 had 10% PET-soil glue but Mix#1 had more open porous structure (2226 ft/day for Mix#1 and 1242 ft/day for Mix#4) and reduced strength compared to Mix#4. This could be attributed to 5% soil in Mix#1 soil-PET glues compared to 0% soil in Mix#1 soil-PET glue. A/P ratio of 8 results in PPC specimens with very low permeability (42 ft/day). Therefore, specimens with a A/P ratio of less than 8 were not prepared in this study. Additionally, both indirect tensile strength and permeability values of PPC mixtures were within the expected range found in the literature for pervious concrete and porous asphalt.

Summary and Conclusions
This study was undertaken to assess the mechanical and hydraulic properties of an innovative pervious PET composite, called as PPC, material prepared by using post-consumer plastic bottles. Using the mix designs, cylindrical samples of PPC were produced in a steel mold by compacting a mixture containing molten PET and heated aggregates and soil in accordance with proportions determined from mix design. Dry cylindrical samples of PPC were tested for indirect tensile strength and permeability. Indirect tensile strength was conducted in accordance with ASTM standard. However, permeability was conducted in a unique manner by designing and fabricating a new testing equipment.
Results showed that indirect tensile strength values increased with PET content. It was also found that using PET alone is not strong enough in binding aggregates and therefore, a soil/PET ratio of one was found optimum for providing maximum strength. Permeability values decreased with the decrease of A/P ratio which in general indicates that lower PET and higher aggregate content is suitable for higher permeability. A soil/PET ratio of one was found to provide higher permeability but strength could compromise.
Findings from this study show that PPC could be used as a sustainable alternative for pervious pavements for low-strength construction such as driveways, sidewalks and parking lots. PPC is a unique alternative in that it addresses two environmental issues: reducing storm water runoff and diverting plastic waste from landfills and incinerators. As we continue our approach to sustainable and green construction materials, PPC is another step towards ecofriendly development.
Financial support for this study was provided through University Research Grant, College of Applied Science and Technology at Illinois State University. The material collection assistance provided by Prairie Material, Midwest Fiber Inc. and Lafarge North America is gratefully acknowledged. Also, plastic bottles donated by ISU recycling facility and students/faculty for conducting this research is gratefully acknowledged.