PERFORMANCE OF RECYCLED PLASTIC WASTE AS PARTIAL REPLACEMENT FOR COARSE AGGREGATE IN CONCRETE

CHAPTER ONE

INTRODUCTION

1.1 Background of the Study

Concrete remains the most widely used construction material globally on account of its versatility, durability, compressive strength, and relative affordability. The conventional production of concrete relies heavily on natural coarse aggregate, typically sourced from crushed rock, gravel, or granite quarries, which constitutes between 60% and 75% of the total volume of concrete. The continued extraction of natural aggregate at the current global rate has raised serious concerns regarding the depletion of non-renewable geological resources, environmental degradation from quarrying activities, and the rising cost of aggregate haulage in regions experiencing rapid urbanisation, including Nigeria.

Concurrently, the world is confronted with an escalating plastic waste crisis. Global plastic production has grown exponentially over the past six decades, and it is estimated that a substantial proportion of plastic waste generated annually is neither recycled nor properly disposed of, ending up in landfills, water bodies, and open dumpsites. In Nigeria, poor waste management infrastructure, inadequate recycling culture, and rapid population growth have compounded the challenge of plastic waste accumulation, particularly in urban centres such as Lagos, Port Harcourt, and Uyo, where drainage blockages and environmental pollution linked to plastic waste are recurrent problems.

The convergence of these two challenges, namely the depletion of natural aggregate resources and the growing burden of plastic waste, has motivated researchers within the built environment to explore the incorporation of recycled plastic waste into concrete production as a partial substitute for natural coarse aggregate. This approach aligns with the principles of the circular economy, which seeks to minimise waste generation by re-introducing waste materials into productive use, and it also supports the broader Sustainable Development Goals, particularly Goal 11 (Sustainable Cities and Communities), Goal 12 (Responsible Consumption and Production), and Goal 13 (Climate Action).

Previous studies conducted in various parts of the world have reported varying degrees of success in utilising recycled plastic aggregate in concrete, with most findings indicating a general reduction in compressive strength and workability as the replacement level increases, attributable to the smooth surface texture and hydrophobic nature of plastic, which weakens the bond between the plastic particles and the surrounding cement paste. Nonetheless, several researchers have demonstrated that at moderate replacement levels, typically not exceeding 20% to 25%, plastic aggregate concrete can achieve compressive strengths adequate for non-structural and light structural applications, while simultaneously reducing the density of concrete, an attribute favourable for reducing dead load in building design.

This study is therefore designed to experimentally investigate the performance of recycled low-density polyethylene (LDPE) plastic waste as a partial replacement for natural coarse aggregate in concrete, with emphasis on workability, density, water absorption, and compressive strength characteristics, in order to establish an optimum replacement level that balances structural adequacy with environmental sustainability within the Nigerian construction context.

1.2 Statement of the Problem

The Nigerian construction industry is currently confronted with two interrelated challenges. First, the cost of natural coarse aggregate has risen considerably in recent years due to the depletion of accessible quarry sites, stringent environmental regulations governing quarrying operations, and increasing transportation costs, thereby escalating the overall cost of concrete production. Second, the volume of plastic waste generated in Nigerian cities continues to overwhelm existing waste management systems, resulting in indiscriminate dumping, blocked drainage channels, flooding, and environmental pollution that pose serious risks to public health and urban infrastructure.

While the recycling of plastic waste into construction materials presents a plausible solution to both challenges, there remains limited empirical data, particularly within the Nigerian context, on the actual engineering performance of concrete produced with recycled plastic waste as partial replacement for coarse aggregate. Existing literature is largely dominated by studies conducted in Asian, European, and North American contexts, using plastic waste types, aggregate sources, and mix designs that may not be directly applicable to locally available materials in Nigeria. Consequently, there is a need for a locally grounded, experimentally verified study that examines the performance characteristics of plastic aggregate concrete using materials readily available within the Nigerian construction environment, in order to generate data-driven recommendations for its practical application.

1.3 Aim and Objectives of the Study

The aim of this study is to evaluate the performance of recycled plastic waste as a partial replacement for coarse aggregate in concrete production.

The specific objectives of the study are to:

1.      determine the physical and mechanical properties of the natural coarse aggregate, fine aggregate, and recycled plastic aggregate used in the study;

2.      produce concrete specimens with recycled plastic waste replacing natural coarse aggregate at 0%, 10%, 20%, 30%, 40% and 50% by volume;

3.      assess the workability of fresh concrete at the various replacement levels using the slump test;

4.      evaluate the density and water absorption characteristics of hardened concrete at the various replacement levels;

5.      determine the compressive strength of the concrete specimens at 7, 14 and 28 days of curing;

6.      establish the statistical relationship between plastic replacement percentage and compressive strength using regression and analysis of variance (ANOVA); and

7.      determine the optimum replacement level of plastic waste that satisfies acceptable strength requirements for non-structural and light structural applications.

1.4 Research Questions

This study seeks to answer the following research questions:

1.      What are the physical and mechanical properties of natural coarse aggregate, fine aggregate, and recycled plastic aggregate used in the study?

2.      What is the effect of varying percentages of recycled plastic waste replacement on the workability of fresh concrete?

3.      How does the density and water absorption of hardened concrete vary with increasing plastic replacement levels?

4.      What is the effect of recycled plastic waste replacement on the compressive strength of concrete at 7, 14 and 28 days?

5.      Is there a statistically significant relationship between plastic replacement percentage and compressive strength?

6.      What is the optimum replacement level of recycled plastic waste for coarse aggregate in concrete production?

1.5 Justification/Significance of the Study

This study is significant for several reasons. First, it contributes to the growing body of knowledge on sustainable construction materials by generating empirical, locally relevant data on the performance of recycled plastic waste in concrete, using materials sourced within the Nigerian construction environment. Second, the findings of this study will be of practical benefit to civil engineers, contractors, and construction material manufacturers seeking cost-effective and environmentally responsible alternatives to conventional coarse aggregate, particularly for non-structural applications such as pavement kerbs, walkways, and partition blocks.

Third, the study supports national and global efforts towards plastic waste management and environmental sustainability by demonstrating a viable pathway for diverting plastic waste from landfills and water bodies into productive construction use, thereby contributing to the circular economy agenda. Fourth, the study provides a reference framework for further academic research into alternative and recycled construction materials, and offers policymakers and regulatory bodies empirical evidence upon which guidelines for the use of recycled plastic aggregate in construction may be developed.

1.6 Scope of the Study

This study is limited to the experimental investigation of recycled low-density polyethylene (LDPE) plastic waste as a partial replacement for natural granite coarse aggregate in normal weight concrete, at replacement levels of 0%, 10%, 20%, 30%, 40% and 50% by volume, using a nominal mix ratio of 1:2:4 and a water-cement ratio of 0.55. The study covers the determination of workability (slump), density, water absorption, and compressive strength at 7, 14 and 28 days of curing. Other durability parameters such as tensile splitting strength, flexural strength, chloride penetration, and long-term durability under aggressive environmental exposure are outside the scope of this study and are recommended for further research.

1.7 Limitations of the Study

The study is subject to certain limitations. The compressive strength testing was limited to a curing age of 28 days; longer-term strength development beyond this period was not investigated due to time constraints associated with the academic session. Additionally, the recycled plastic aggregate used was limited to shredded LDPE waste sourced from sachet water and polythene bag waste; the performance of other plastic types such as PET or HDPE was not investigated. The study also did not incorporate surface treatment or chemical modification of the plastic aggregate to improve bonding characteristics, which is an area recommended for further study. Laboratory conditions, though controlled, may also differ from actual field conditions where variations in temperature, humidity, and workmanship could influence performance outcomes.

1.8 Operational Definition of Terms

•        Coarse Aggregate: Granular material, typically greater than 4.75 mm in size, such as crushed granite, used in concrete production.

•        Recycled Plastic Aggregate: Plastic waste that has been collected, cleaned, shredded, and processed into aggregate-sized particles for use as a substitute for natural aggregate in concrete.

•        Compressive Strength: The maximum compressive stress that a concrete specimen can withstand before failure, typically expressed in N/mm² (MPa).

•        Workability: A property of freshly mixed concrete that determines the ease with which it can be mixed, placed, compacted, and finished without segregation.

•        Replacement Level: The percentage by volume of natural coarse aggregate substituted with recycled plastic aggregate in a concrete mix.

•        Water Absorption: The capacity of hardened concrete to absorb water, expressed as a percentage of its dry weight.

•        Circular Economy: An economic model aimed at eliminating waste through the continual reuse, recycling, and regeneration of materials and resources.