Construction clay products are fundamental to civil engineering projects across Kenya, from affordable housing developments in rural areas to high-end commercial buildings in Nairobi. Understanding the selection and properties of clay products enables engineers and builders to optimize structural integrity, durability, and cost-effectiveness. This chapter addresses how to apply clay products effectively, focusing on their types, selection criteria, and critical physical and mechanical properties relevant to construction.

4.1 Clay Products Selection

Selecting appropriate clay products is essential for ensuring the performance and longevity of structures. Kenyan civil engineers must consider local availability, environmental conditions, and intended use when choosing clay materials. This section explores the variety of clay products used in construction and the critical criteria guiding their selection.

4.1.1 Types of Clay Products in Construction

Clay products in construction come in various forms, each suited to specific structural and architectural purposes. Understanding these types allows engineers to match product characteristics to project requirements.

4.1.1.1 Bricks

• Bricks: The most widely used clay product, bricks are manufactured by molding clay and firing it at high temperatures. They provide good compressive strength and thermal insulation, making them suitable for walls and partitions in buildings such as schools and hospitals.

Clay bricks are small rectangular walling units made by shaping prepared clay, drying it and firing it in a kiln or clamp at temperatures of roughly 900 °C to 1,100 °C, which turns the clay into a hard, durable ceramic. Their quality depends on the clay, the moulding method and especially the firing, because under-burnt bricks are soft and absorb water while well-burnt bricks are hard, strong and ring clearly when struck. Bricks are selected according to class, strength, water absorption, appearance and size accuracy, and simple field tests help the builder reject poor bricks before use. Example: a contractor building staff houses for a mission hospital in Kapsabet tested samples from three local brick makers for water absorption and soundness and bought only from the yard whose bricks passed, avoiding the damp walls seen in older houses.

Classes and Forms of Clay Bricks

• Common bricks: General-purpose bricks for walls that will be plastered, where appearance is less important.
• Facing bricks: Bricks with a uniform colour and texture for exposed walls.
• Engineering bricks: Dense, strong bricks with very low water absorption for foundations, manholes, retaining walls and damp conditions.
• Solid bricks: Bricks with no holes or only a shallow frog, used where full bearing is needed.
• Perforated bricks: Bricks with holes through them, which are lighter, dry and fire more evenly, and give a good mortar key.
• Hollow bricks: Bricks with large cavities, used in lighter partitions with better thermal insulation.

Typical Properties of Clay Bricks

Qualities of Good Bricks

• Uniform size and shape: Good bricks have straight edges, sharp corners and consistent dimensions for even joints.
• Well burnt: They have a uniform colour and give a clear metallic ring when two bricks are struck together.
• Hard surface: A fingernail should not scratch a good brick.
• Adequate strength: A good brick should not break when dropped flat from about 1 m onto hard ground.
• Low water absorption: Good bricks absorb little water, reducing dampness and decay.
• Free from defects: Good bricks have no cracks, lumps of lime, stones or soluble salts that cause efflorescence.

Steps for Field Testing Bricks

1. Take a random sample: Pick bricks from different parts of the stack, not only from the top.
2. Check dimensions: Lay 20 bricks end to end and compare the total length with the expected value to check size consistency.
3. Inspect appearance: Look for cracks, warping, chipped corners and uneven colour.
4. Carry out the soundness test: Strike two bricks together and listen for a clear ringing sound.
5. Carry out the hardness test: Try to scratch the surface with a fingernail.
6. Carry out the drop test: Drop a brick flat from about 1 m onto hard ground and check that it does not break.
7. Carry out the water absorption test: Weigh dry bricks, soak them in water for 24 hours and weigh them again.
8. Check for efflorescence: Stand a brick in a little water for a few days and look for white salt deposits as it dries.

Worked Example: Water Absorption of a Brick

A dry brick weighs 2.80 kg. After soaking in water for 24 hours it weighs 3.22 kg. Calculate its water absorption.

Water absorption = ((Wet mass - Dry mass) ÷ Dry mass) × 100

Water absorption = ((3.22 - 2.80) ÷ 2.80) × 100

Water absorption = (0.42 ÷ 2.80) × 100

Water absorption = 15%

At 15%, the brick is acceptable as a common brick but not as an engineering brick.

4.1.1.2 Tiles

• Tiles: Clay tiles, including roofing and floor tiles, offer durability and aesthetic appeal. In counties like Mombasa with high rainfall, clay roofing tiles protect structures from water ingress and promote ventilation.

Four square ceramic tiles arranged in a grid (Source: Wikimedia Commons)

Clay tiles are thin, flat or shaped units made from fired clay and used to cover roofs, floors and walls. Roofing tiles are designed to overlap or interlock to shed rainwater, while floor and wall tiles provide hard, durable, easy-to-clean surfaces, either unglazed or with a glazed, glass-like coating. Tiles must be selected for strength, low water absorption, resistance to wear and slipping, and appearance, according to where they will be used. Example: a hotel in Lamu used red clay roofing tiles on its new cottages to match the traditional coastal architecture, and unglazed terracotta floor tiles on the verandahs for their natural, slip-resistant finish.

Types of Clay Tiles

Qualities of Good Clay Tiles

• Uniform shape and size: Tiles should fit together accurately for watertight roofs and level floors.
• Low water absorption: Roofing tiles should not soak up water, which adds weight and causes leaks.
• Adequate strength: Tiles must resist breaking under foot traffic, workers on roofs and handling.
• Wear and slip resistance: Floor tiles should resist abrasion and give a safe grip, especially in wet areas.
• Freedom from defects: Good tiles have no cracks, warping, pinholes or crazing of the glaze.
• Colour consistency: Tiles from the same batch should match in colour and shade.

Steps in Fixing Interlocking Clay Roofing Tiles

1. Check the roof structure: Confirm that rafters are strong enough for the weight of the tiles and that the pitch suits the tile type.
2. Fix the underlay: Lay a waterproof underlay over the rafters with proper overlaps.
3. Set out and fix battens: Mark batten spacing according to the tile gauge and nail battens level and parallel.
4. Load tiles on the roof: Distribute tiles evenly across the roof to avoid overloading one area.
5. Lay the eaves course: Start at the eaves, laying the first course straight and projecting correctly into the gutter.
6. Lay subsequent courses: Lay tiles upwards course by course, interlocking them and hooking their nibs over the battens.
7. Fix tiles securely: Nail or clip tiles as specified, especially at edges and in windy locations.
8. Fix ridge and hip tiles: Bed or clip ridge and hip tiles and seal them against water entry.
9. Inspect the roof: Check for broken, loose or misaligned tiles and replace them.

4.1.1.3 Ceramic and terracotta products

• Earthenware: This includes decorative and functional items such as terracotta elements used in facades and landscaping, providing a natural finish and cultural resonance in public buildings.

Ceramic products are items made from clay and other minerals shaped and fired at high temperatures to become hard and durable, and they range from porous earthenware to dense, glass-like porcelain. Terracotta, meaning "baked earth", is a type of earthenware fired at relatively low temperatures, usually unglazed, with a characteristic reddish-brown colour, and it is used for decorative blocks, facade panels, floor tiles, pots and roof tiles. The firing temperature and type of clay determine how porous, strong and water resistant each product is, and therefore where it can be used. Example: a county museum in Kisumu used terracotta breeze blocks to screen its new gallery windows, providing shade and ventilation while giving the building a warm, natural appearance.

Ceramic Products Compared

Uses of Ceramic and Terracotta Products in Construction

• Sanitary ware: Wash basins, toilets, urinals and sinks are made from vitreous china because it is non-absorbent and easy to clean.
• Wall and floor tiles: Glazed ceramic and porcelain tiles provide hygienic, durable surfaces in kitchens, bathrooms and hospitals.
• Decorative facade elements: Terracotta panels, cornices and grilles add character to building facades.
• Ventilation and screen blocks: Perforated terracotta blocks provide shading, privacy and natural ventilation.
• Drainage pipes and fittings: Vitrified clay pipes resist chemical attack from sewage.
• Landscaping items: Terracotta pots, edging and pavers are used in gardens and courtyards.

Handling and Care of Ceramic and Terracotta Products

• Handle carefully: Ceramics are brittle and chip or crack if dropped or knocked.
• Store properly: Keep products in their packaging, upright or stacked as directed, on firm ground.
• Seal porous terracotta: Apply sealers to terracotta in wet or dirty areas to prevent staining and moisture absorption.
• Use suitable adhesives and grouts: Fix tiles and panels with products recommended for the material and location.
• Clean with mild products: Avoid strong acids and abrasive cleaners that damage glazes and surfaces.

4.1.1.4 Clay Blocks

• Blocks: Larger than bricks, clay blocks are gaining popularity for load-bearing walls due to their size and reduced mortar joints, which speed up construction in projects like affordable housing units.

4.1.1.5 Clay Pipes

• Pipes: Clay pipes are traditionally used in drainage and sewage systems because of their resistance to chemical corrosion, particularly in urban infrastructure managed by Nairobi City Water and Sewerage Company.

Exposed pipes inside a wall cavity (Source: Wikimedia Commons)

4.1.2 Selection Criteria for Clay Products

Choosing the right clay product involves evaluating multiple factors that affect performance, cost, and sustainability. These criteria ensure the material aligns with project demands and environmental conditions.

Key Criteria for Selection

• Strength Requirements: Structural components like load-bearing walls demand clay products with high compressive strength, verified through standardized testing as applied in county government office constructions.
• Durability: The product must withstand local weather conditions, including heavy rains and temperature fluctuations common in Kenya’s diverse climates, to avoid frequent repairs or replacements.
• Porosity and Water Absorption: Low porosity clay products resist water penetration, reducing risks of dampness and efflorescence in buildings such as hotels and hospitals.
• Thermal Insulation: Clay products with good insulating properties help regulate indoor temperatures, reducing energy consumption in office blocks and residential units.
• Cost and Availability: Local sourcing of clay products reduces transport costs and supports the regional economy, a practice encouraged in public works projects funded by the National Treasury.

Additional Selection Criteria

• Appearance: Colour, texture and finish must suit the architectural design, especially for exposed bricks, terracotta and tiles.
• Dimensional accuracy: Products with consistent sizes give neat joints, less mortar and faster laying.
• Fire resistance: Fired clay products are non-combustible and suit walls and roofs requiring fire safety.
• Chemical resistance: Products for drainage and industrial areas must resist acids, alkalis and sewage.
• Compliance with standards: Products should meet the relevant KEBS standards and the project specification.
• Sustainability: Products from responsibly managed clay pits and efficient kilns reduce environmental harm.

Steps for Selecting Clay Products for a Project

1. Define the application: Identify where the product will be used, such as load-bearing walls, roofs, floors or drainage.
2. Review the specification: Note the required strength, absorption, size, finish and standards.
3. Assess site conditions: Consider rainfall, humidity, salt exposure, ground moisture and traffic.
4. Identify suppliers: List suppliers who can provide the product in the required quantity and time.
5. Obtain samples and test certificates: Request samples and laboratory results for strength and absorption.
6. Carry out tests: Conduct field or laboratory tests on samples to confirm quality.
7. Compare costs: Compare total costs, including transport, wastage and laying.
8. Approve and order: Obtain approval from the engineer or client and place the order with agreed quality requirements.
9. Inspect deliveries: Check each delivery for quantity, quality and damage before accepting it. Example: a SACCO building rental units in Kakamega rejected a delivery of roofing tiles after inspection showed many cracked and warped tiles.

Chapter Summary

This chapter focused on the application of construction clay products, beginning with an exploration of the various types commonly used in building projects, including bricks, tiles, and ceramic and terracotta products. It then examined the important criteria for selecting clay products based on their suitability for specific construction needs. The discussion moved on to the physical properties of clay products, such as density, weight, thermal and acoustic insulation capabilities, and water absorption, which influence their performance in different environments. Mechanical properties were analyzed next, highlighting the significance of compressive strength as well as bending and tensile strength for structural integrity. The chapter concluded with a detailed look at the durability and weathering characteristics of clay products, including their resistance to freeze-thaw cycles, chemical attacks, and fire, all of which determine their longevity and safety in construction applications.