Construction plastics and rubber products form an essential part of modern civil engineering materials, offering versatile solutions for insulation, sealing, piping, and structural components. In Kenya, the growing infrastructure sector increasingly relies on these materials to improve durability, reduce maintenance costs, and adapt to environmental challenges such as moisture and chemical exposure. Understanding the selection criteria, types, and properties of plastics and rubber is critical for engineers to specify the right materials that meet performance and sustainability requirements in projects ranging from water supply systems to building envelopes.

6.1 Plastics and Rubber Products Selection

The selection of plastics and rubber products in civil engineering demands a thorough understanding of their chemical composition, mechanical properties, and environmental behavior. This knowledge enables professionals to match materials to specific applications such as waterproofing membranes, expansion joints, or protective coatings. Kenyan projects frequently face unique conditions such as high UV exposure, fluctuating temperatures, and variable soil chemistry, which influence material choice. Engineers must balance cost-effectiveness with performance longevity and environmental impact when specifying these materials.

6.1.1 Introduction and definition of terms

The terms “plastics” and “rubber” often appear interchangeably in construction, yet they represent distinct categories of polymer materials with different characteristics and uses. Plastics are synthetic materials made from polymers that can be molded or shaped when heated and retain their form upon cooling. Rubber, by contrast, is an elastic polymer capable of significant deformation and recovery, often used where flexibility and resilience are required.

Definition of Plastics

• Polymers with Thermoplastic or Thermosetting Behavior: Plastics consist of long chains of molecules that respond to heat by softening (thermoplastics) or by curing into a hard, infusible state (thermosets).
• Versatility in Form and Function: Plastics can be manufactured into sheets, pipes, films, and molded components, making them suitable for piping systems, insulation, and finishes in construction.
• Chemical Resistance: Many plastics resist corrosion from water and chemicals, a key advantage in Kenya’s infrastructure where exposure to aggressive soils or water contaminants occurs.
• Lightweight Nature: Compared to metals or concrete, plastics offer reduced weight, easing transport and installation on remote or difficult sites.
• Electrical Insulation: Plastics are excellent insulators, widely used in electrical conduits and cable management within buildings and industrial facilities.

Definition of Rubber

• Elastomeric Polymers: Rubber is characterized by its ability to stretch and return to original shape, providing sealing and cushioning functions.
• Natural and Synthetic Sources: Rubber can be derived from natural latex or synthesized chemically, each with distinct properties relevant to construction uses.
• High Resilience and Impact Absorption: Rubber products absorb shocks and vibrations, ideal for expansion joints, flooring, and vibration isolators in bridges and buildings.
• Waterproofing Capabilities: Its impermeability to water makes rubber essential for membranes and gaskets in wet environments.
• Temperature and Weather Resistance: Rubber maintains flexibility over a range of temperatures, important for Kenya’s variable climate conditions.

Clarification of Key Terms

• Polymer: A large molecule composed of repeating structural units, forming the basis of plastics and rubber.
• Thermoplastic: A type of plastic that softens when heated and hardens when cooled, allowing for reshaping.
• Thermoset: Plastic that undergoes irreversible curing, becoming permanently hard.

• Elastomer: A polymer with elastic properties, capable of significant deformation and recovery.

• Composite Materials: Materials made by combining plastics or rubber with other substances to enhance properties for specific construction purposes.

6.1.2 Types of plastics

Plastics used in civil engineering vary widely in composition and performance. Selecting the correct type depends on factors such as mechanical strength, chemical resistance, temperature stability, and cost. Kenyan construction projects apply different plastics for piping, insulation, and structural components, each type offering unique advantages.

Common Thermoplastics in Construction

• Polyvinyl Chloride (PVC): PVC is widely used for water supply pipes, electrical conduits, and window frames due to its durability, chemical resistance, and affordability. For instance, many county government water projects use PVC piping for potable water distribution.

PVC drainpipe installed in a garage ceiling (Source: Wikimedia Commons)

• Polyethylene (PE): PE, especially high-density polyethylene (HDPE), is favored for underground drainage and sewer piping because of its flexibility and resistance to corrosion and abrasion.

• Polypropylene (PP): PP offers higher temperature resistance than PE and is used in chemical drainage systems where exposure to aggressive substances occurs.

• Acrylic (PMMA): Acrylic sheets serve as glazing materials in roofing and facades, providing transparency with UV resistance.
• Polystyrene (PS): Used mainly in insulation applications, polystyrene foam reduces heat transfer in building envelopes and cold storage facilities.

Thermosetting Plastics in Construction

• Epoxy Resins: Epoxies are used as adhesives and coatings due to their superior mechanical strength and chemical resistance, often employed in floor coatings at hospitals and laboratories.

Bottles of Devcon epoxy resin and hardener (Source: Wikimedia Commons)

• Phenolic Resins: These are used in laminates and molded components that require fire resistance and dimensional stability.
• Polyester Resins: Utilized in composite materials such as fiberglass reinforced plastics (FRP), common in bridge construction and water tanks.
• Melamine Formaldehyde: Used in decorative laminates and surface finishes that require hardness and scratch resistance.
• Urea Formaldehyde: Applied in adhesives and finishes, though less common due to formaldehyde emissions concerns.

6.1.2.1 Principal constituents of plastics

The principal constituents of plastics are the ingredients mixed together to make a plastic material with the required properties, and they consist of a base polymer, called the resin or binder, plus additives such as plasticisers, fillers, stabilisers, pigments and curing agents. The resin gives the plastic its basic character, while the additives adjust flexibility, strength, colour, resistance to sunlight and heat, fire behaviour and cost. The same base polymer can therefore produce very different products, for example rigid PVC for water pipes and flexible PVC for cable insulation and floor coverings, depending on the additives used. Example: a manufacturer supplying water tanks to a county water project in Garissa added extra UV stabilisers to its polyethylene so that the tanks would not become brittle in the intense sunlight.

Principal Constituents and Their Functions

6.1.2.2 Methods of obtaining shapes of plastics

Methods of obtaining shapes of plastics are the manufacturing processes used to form raw plastic, usually in the form of powder, granules or liquid resin, into finished products such as pipes, sheets, tanks, fittings and panels. Thermoplastics are generally heated until soft, shaped and then cooled, while thermosets are shaped and then cured by heat or chemical reaction so that they set permanently. The choice of method depends on the shape, size, quantity and type of plastic, and it affects the cost, quality and properties of the product. Example: a plastics factory in Athi River produces PVC water pipes by extrusion, plumbing fittings by injection moulding and water tanks by rotational moulding, each method suited to the product's shape.

Common Shaping Methods

Steps in the Extrusion of a PVC Pipe

1. Prepare the compound: Mix PVC resin with stabilisers, fillers, lubricants and pigments in the required proportions.
2. Feed the extruder: Load the compound into the hopper of the extruder.
3. Heat and melt: A rotating screw carries the material through heated barrels, where it melts and mixes.
4. Force through the die: The molten plastic is pushed through a ring-shaped die that forms the pipe.
5. Size the pipe: The hot pipe passes through a sizing sleeve that sets the exact outside diameter.
6. Cool the pipe: The pipe passes through water baths or sprays that cool and harden it.
7. Pull and cut: A haul-off unit pulls the pipe at a steady speed, and a saw cuts it to standard lengths.
8. Mark and test: Pipes are printed with size and pressure class and tested for quality before storage.

6.1.2.3 Properties of plastics

The properties of plastics are the physical, mechanical, thermal, chemical and electrical characteristics that determine how they behave in use, and they vary widely between different plastics and grades. In general, plastics are light, corrosion resistant, good insulators and easy to shape, but they are weaker and less stiff than metals, expand considerably with temperature, may soften or burn in fire and can degrade under prolonged sunlight unless stabilised. Understanding these properties helps engineers use plastics where they perform well and avoid failures such as sagging pipes, cracked tanks or leaking joints. Example: the plumber installing exposed PVC drainage pipes on the outside of a hospital in Garissa provided expansion joints and painted the pipes to protect them from heat and sunlight, preventing the distortion seen on an older building.

General Properties of Plastics

• Low density: Plastics are light, typically 0.9 to 1.4 g/cm³ for common types, which eases handling and transport.
• Corrosion and chemical resistance: Most plastics do not rust and resist water, many acids, alkalis and salts.
• Electrical insulation: Plastics do not conduct electricity, making them ideal for conduits, cable insulation and fittings.
• Low thermal conductivity: Plastics, especially foams, are good thermal insulators.
• High thermal expansion: Plastics expand and contract much more than steel or concrete with temperature changes.
• Moderate strength and low stiffness: Plastics are weaker and more flexible than metals unless reinforced.
• Creep: Plastics slowly deform under constant load, especially when warm.
• Sensitivity to UV light: Unstabilised plastics become brittle and discoloured in sunlight.
• Combustibility: Most plastics burn or soften in fire and may release smoke and toxic gases.
• Ease of shaping and joining: Plastics are easily moulded, cut and joined by solvent cement, welding or fittings.

Approximate Properties of Common Construction Plastics

Worked Example: Thermal Expansion of a PVC Pipe

A 6 m length of PVC pipe is installed at 20 °C and later reaches 50 °C in the sun. The coefficient of linear expansion of PVC is 70 × 10⁻⁶ per °C. Calculate the increase in length.

ΔL = α × L × ΔT

ΔL = 70 × 10⁻⁶ × 6,000 × (50 - 20)

ΔL = 70 × 10⁻⁶ × 6,000 × 30

ΔL = 12.6 mm

The pipe must have expansion joints or flexible supports to accommodate this movement.

6.1.2.4 Selection Criteria for Plastics

Selection criteria for plastics are the requirements a plastic product must meet for its particular use, including strength, chemical and UV resistance, temperature range, fire behaviour, compliance with standards, ease of installation and cost. Choosing the wrong plastic leads to early failures, such as pipes bursting under pressure, tanks cracking in the sun or floor coatings wearing away. Engineers therefore compare the service conditions with the properties and certified grades of available products before specifying them. Example: a dairy processing plant in Githunguri selected polypropylene pipes for its hot water lines and HDPE for its cold effluent drains, because ordinary PVC would have softened at the hot water temperatures.

Criteria for Selection of Plastics

• Mechanical Strength Requirements: Load-bearing applications require plastics with high tensile and impact strength.
• Chemical and Environmental Resistance: Materials must withstand exposure to water, UV radiation, and chemicals prevalent in site conditions.
• Thermal Stability: Plastics should tolerate the range of temperatures expected in the Kenyan climate without degradation.
• Cost Considerations: Budget constraints often dictate the choice between high-performance and economical plastics.
• Ease of Fabrication and Installation: Some plastics allow for onsite welding or flexible jointing, reducing installation time and costs.

Additional Selection Criteria

• Pressure rating: Pipes must have a pressure class suitable for the working and surge pressures in the system.
• Fire performance: Plastics in buildings should have suitable fire ratings and low smoke emission where required.
• Health and safety: Plastics in contact with drinking water must be approved as safe for potable water.
• Compliance with standards: Products should meet relevant KEBS and international standards and carry certification marks.
• Durability and service life: The plastic should last for the intended design life in its environment.
• Environmental impact: Recyclable products and responsible disposal options should be preferred.

Steps in Selecting a Plastic Product

1. Define the application: Identify the function, such as water supply, drainage, insulation, glazing or coating.
2. Determine service conditions: Establish pressures, loads, temperatures, chemical exposure and sunlight.
3. Identify suitable plastics: List plastics whose properties match the service conditions.
4. Check standards and certification: Confirm that products meet relevant standards and approvals.
5. Compare suppliers and costs: Compare initial cost, installation cost, service life and availability.
6. Obtain samples and technical data: Review technical data sheets and inspect samples.
7. Specify the product: Write the type, grade, size, class and standard in the specification.
8. Inspect on delivery: Check markings, dimensions and condition of products before installation.

6.1.3 Types of rubber

Rubber products in civil engineering serve critical roles in sealing, vibration damping, and waterproofing. The choice between natural and synthetic rubber depends on the required durability, environmental exposure, and mechanical properties. Kenyan infrastructure projects often utilize rubber for expansion joints in bridges, roofing membranes, and gaskets in water treatment plants.

Natural Rubber

• Source and Composition: Derived from the latex of rubber trees, natural rubber offers excellent elasticity and resilience.
• Mechanical Properties: It has high tensile strength and good abrasion resistance but is vulnerable to ozone and UV degradation.
• Applications: Used in flexible joints, conveyor belts, and shock absorbers in construction machinery.
• Limitations: Natural rubber’s susceptibility to chemical attack and weathering restricts its use in harsh environments.
• Treatment and Vulcanization: Vulcanization improves durability by cross-linking polymer chains, enhancing elasticity and resistance to heat.

Synthetic Rubbers

• Styrene-Butadiene Rubber (SBR): The most common synthetic rubber in construction, SBR offers good abrasion resistance and aging properties, used in roofing membranes and floor coverings.
• Nitrile Rubber (NBR): Resistant to oils and fuels, NBR is ideal for gaskets and seals in water and wastewater treatment plants.
• Neoprene (Polychloroprene): Exhibits excellent weather, ozone, and chemical resistance, widely used in expansion joints and protective coatings.
• Ethylene Propylene Diene Monomer (EPDM): Known for outstanding resistance to UV, ozone, and heat, EPDM is preferred for waterproofing membranes on roofs and basements.
• Chloroprene Rubber: Combines flexibility with resistance to flame and chemicals, suitable for sealing applications exposed to aggressive environments.

6.1.3.1 Properties of rubber

The properties of rubber are the characteristics that make it unique among construction materials, above all its elasticity, the ability to stretch to several times its length and return to its original shape. Rubber is also waterproof, resilient, a good electrical insulator, resistant to abrasion and able to absorb shocks and vibration, but different types vary greatly in their resistance to sunlight, ozone, oils, heat and chemicals. Raw rubber is soft and sticky, so it is vulcanised by heating with sulphur or other agents, which cross-links its molecules and gives it strength, elasticity and durability, and its hardness is commonly measured on the Shore A scale. Example: a contractor replacing the expansion joint seals on a bridge in Kisumu chose neoprene seals because natural rubber seals installed earlier had cracked from exposure to sunlight and ozone.

General Properties of Rubber

• High elasticity: Rubber stretches considerably under load and recovers its shape when the load is removed.
• Resilience and damping: Rubber absorbs energy from shocks and vibrations, reducing their transmission.
• Impermeability to water: Rubber does not allow water to pass through, making it ideal for seals and membranes.
• Electrical insulation: Rubber does not conduct electricity and is used for insulation and protective equipment.
• Abrasion resistance: Many rubbers resist wear from friction, suiting flooring, belts and tyres.
• Flexibility over a temperature range: Rubber remains flexible within its rated temperature range but hardens when cold and softens or degrades when too hot.
• Ageing: Rubber gradually hardens and cracks with exposure to sunlight, ozone, heat and some chemicals, depending on its type.

Properties of Common Rubbers Compared

6.1.3.2 Uses of rubbers

The uses of rubbers in construction take advantage of their elasticity, waterproofness and ability to absorb movement and vibration, allowing structures and services to move, seal and cushion without damage. Rubber components are often small, such as gaskets and seals, but critical, because their failure can cause leaks, noise, vibration damage or structural problems. They are found in bridges, buildings, water supply and drainage systems, roads, machinery and safety equipment. Example: a modern hotel in Nanyuki installed its standby generator and pumps on rubber anti-vibration mounts, preventing noise and vibration from disturbing guests in nearby rooms.

Uses of Rubber in Construction

• Elastomeric bridge bearings: Laminated rubber and steel pads support bridge decks while allowing movement from temperature changes and traffic.
• Expansion joint seals: Rubber seals fill joints in bridges, slabs and buildings, keeping out water and debris while allowing movement.
• Waterstops: Rubber or PVC strips cast into concrete joints in tanks, basements and dams prevent water seeping through.
• Pipe seals and gaskets: Rubber rings and gaskets make watertight joints in water, sewer and drainage pipes and flanges.
• Roofing and waterproofing membranes: EPDM and butyl sheets waterproof flat roofs, basements, ponds and reservoirs.
• Door and window seals: Rubber strips seal gaps around doors, windows and curtain walls against draughts, dust and water.
• Rubber flooring: Durable, slip-resistant and quiet flooring is used in hospitals, gyms, schools and stairways.
• Vibration isolators: Rubber mounts and pads reduce vibration and noise from machines, pumps and generators.
• Hoses and belts: Rubber hoses and conveyor belts are used for water, concrete pumping and material handling on sites.
• Recycled rubber products: Crumb rubber from old tyres is used in rubberised bitumen, playground surfaces, sports tracks and paving.
• Personal protective equipment: Rubber gloves, boots and mats protect workers from water, chemicals and electricity.

6.1.3.3 Selection Criteria for rubber

Selection criteria for rubber are the conditions and requirements used to choose the right type and grade of rubber for a particular application, such as the environment it will be exposed to, the loads and movements it must accommodate, the fluids it will contact and the required service life. Because rubbers differ greatly in resistance to sunlight, oils, chemicals and temperature, a wrong choice can cause seals to swell, crack or harden within a short time. Selection therefore matches the service conditions with the properties of rubber types, and checks hardness, standards and approvals, especially for drinking water. Example: a water treatment works in Kisii specified EPDM gaskets approved for potable water for its treated water pipelines, and nitrile seals for pumps handling oily wastewater.

Selection Factors for Rubber Products

• Environmental Exposure: Resistance to UV radiation, ozone, and chemicals is essential for outdoor and industrial applications.
• Mechanical Requirements: Tensile strength, elongation, and hardness must match the function, whether sealing or cushioning.
• Temperature Range: Rubber must maintain properties across expected temperature fluctuations without cracking or hardening.
• Compatibility with Other Materials: Rubber products often interface with metals, concrete, and plastics, requiring chemical and mechanical compatibility.
• Cost and Availability: Synthetic rubbers may be more expensive but offer longer service life in demanding environments.

Additional Selection Criteria

• Fluid resistance: The rubber must resist the fluids it contacts, such as oils, fuels, sewage or chlorinated water.
• Hardness (Shore A): Softer rubbers seal better under low pressure, while harder rubbers resist extrusion and wear.
• Compression set: Seals must recover their shape after long compression to keep joints tight.
• Approval for drinking water: Rubber in contact with potable water must be certified as safe.
• Fire behaviour: Rubber in buildings may need fire-retardant grades.
• Service life and maintenance: Choose rubbers that last the required time with minimal replacement.

Steps in Selecting a Rubber Product

1. Define the function: Determine whether the rubber is for sealing, waterproofing, cushioning, bearing or vibration isolation.
2. Identify service conditions: Establish exposure to sunlight, ozone, temperature, water, oils and chemicals.
3. Determine mechanical requirements: Establish loads, movements, compression and required hardness.
4. Select suitable rubber types: Compare rubber types whose properties match the conditions.
5. Check standards and approvals: Confirm compliance with relevant standards and approvals, such as for potable water.
6. Review supplier data and samples: Study technical data sheets and inspect samples.
7. Specify the product: State the rubber type, grade, hardness, dimensions and standard in the specification.
8. Inspect and store correctly: Check deliveries and store rubber products away from sunlight, heat and oils until installation.

Chapter Summary

This chapter explored the selection and application of construction plastics and rubber products, beginning with clear definitions and the classification of plastics and rubbers. It detailed the principal constituents of plastics, the various methods used to shape them, and their key properties, followed by the criteria that guide their selection for construction purposes. The types of rubber were examined next, with emphasis on their properties, common uses, and factors influencing their choice in building projects. The chapter then examined the mechanical, thermal, and chemical properties that affect the performance of both plastics and rubber products in construction environments. Finally, it described specific products such as PVC pipes, polythene sheeting, acrylic sheets, rubber roofing materials, and rubber expansion joints, highlighting their roles and suitability in different construction applications. This comprehensive approach provided a solid foundation for understanding how to apply these materials effectively in the built environment.