Construction walling units form the fundamental building blocks for structural and non-structural walls in civil engineering projects across Kenya. Their selection, properties, and appropriate application greatly influence the durability, safety, and aesthetics of buildings ranging from residential houses to commercial complexes and public infrastructure. Understanding the types of walling units, criteria for their selection, and factors affecting their use equips civil engineers to make informed decisions that optimize resource use and meet client specifications under local construction conditions.

2.1 Construction Walling Units

Construction walling units are prefabricated blocks, bricks, or stones designed to be assembled into walls that provide enclosure, support, and partition in buildings. Their material composition, size, strength, and finish vary depending on intended use, environmental conditions, and economic considerations.

2.1.1 Introduction and definition of terms

This subtopic clarifies the terminology related to construction walling units, grounding the learner in precise language essential for professional communication and specification writing.

Definition of Construction Walling Units

Construction walling units are standardized building blocks manufactured from various materials such as clay, concrete, or stone, designed to be laid in mortar to form walls. They serve both load-bearing and non-load-bearing functions depending on their strength and placement.

Common Terminology in Walling Units

• Brick: A small rectangular block typically made from clay or shale, fired to harden, used in masonry.

Stacks of perforated clay bricks (Source: Wikimedia Commons)

• Block: Larger than bricks, blocks are often made from concrete and used for structural walls.

Concrete block on ground surface (Source: Wikimedia Commons)

• Masonry Unit: A general term encompassing bricks, blocks, and stones used in wall construction.

Concrete masonry unit block on ground (Source: Wikimedia Commons)

• Mortar: A mixture of cement, sand, and water used to bond walling units.
• Course: A horizontal layer of units in a wall.

Characteristics Distinguishing Walling Units

Walling units are characterized by size uniformity, compressive strength, density, water absorption, and surface finish, all influencing their performance in construction.

Importance of Standardization

Standardized dimensions and properties ensure compatibility, ease of construction, and structural integrity. For example, in Nairobi’s urban developments, adherence to standard brick sizes facilitates faster building and reduces wastage.

Relation to Structural and Non-Structural Elements

Walling units may be used as load-bearing elements supporting floors and roofs or as partitions providing enclosure without structural support, impacting the choice of material and unit type.

2.1.2 Types of Walling Units

Kenyan civil engineering projects employ diverse walling units selected according to structural requirements, cost, availability, and environmental conditions.

2.1.2.1 Building stones

Stone walling units comprise natural stones cut to size. They are highly durable and provide excellent weather resistance. Stone is commonly used in rural areas for boundary walls and in heritage building restoration projects in Eldoret.

Building stones are natural rocks quarried, cut and dressed into units suitable for walling, and they remain the most widely used walling material in many parts of Kenya because good stone deposits are found in areas such as Nairobi's environs, Machakos, Nakuru and Nyeri. The most common form is machine-cut stone, produced in quarries with circular saw machines to regular sizes such as 200 mm × 200 mm × 400 mm, while dressed stone is shaped by hand to give a fine finished face. The suitability of a stone depends on its strength, durability, water absorption, workability and appearance, all of which depend on how the rock was formed. Example: a secondary school in Nyeri built its new laboratory block from locally quarried machine-cut stone, which cost less than concrete blocks transported from town and needed no external painting.

Classification of Building Stones

Qualities of Good Building Stones

• High compressive strength: The stone must resist the loads it will carry without crushing.
• Low water absorption: Stone that absorbs little water resists dampness, frost damage and weathering.
• Durability: Good stone resists weathering, chemical attack and abrasion over a long period.
• Uniform texture and colour: Even grain and colour give consistent strength and an attractive finish.
• Workability: The stone should be easy to cut and dress to the required shape without excessive waste.
• Freedom from defects: Good stone is free from cracks, cavities, soft patches and clay seams.

Advantages and Disadvantages of Building Stones

• Advantage, durability: Stone walls last for many decades with little maintenance.
• Advantage, local availability: Stone is quarried in many regions, reducing transport costs.
• Advantage, attractive natural finish: Stone can be left exposed without plaster or paint.
• Disadvantage, weight: Stones are heavy, making handling slow and requiring strong foundations.
• Disadvantage, variable quality: Stone from different quarries or beds can vary in strength and size.
• Disadvantage, environmental impact: Quarrying can damage landscapes if not properly managed.

2.1.2.2 Brick units

Brick units are small rectangular walling units, usually made from clay or shale moulded and fired in a kiln, or from other materials such as fly ash, lime or cement. Bricks are small enough to be laid with one hand, which allows flexible bonding patterns, curves and detailed features, and fired clay bricks are durable, fire resistant and attractive. The standard metric clay brick measures about 215 mm × 102.5 mm × 65 mm, although locally made bricks in Kenya often vary in size. Example: a hotel in Kisumu used red facing bricks for the external walls of its new wing, giving an attractive finish that needed no plaster or paint.

Clay Bricks

Clay bricks are produced by molding natural clay and firing it in kilns. They offer good compressive strength, thermal insulation, and aesthetic appeal. For instance, many residential buildings in Kisumu utilize locally made clay bricks for their durability and affordability.

Fly Ash Bricks

Fly ash bricks are made from fly ash, a byproduct of coal combustion, mixed with lime and cement. They are lightweight and environmentally friendly. Some eco-conscious hotel developments in Naivasha incorporate fly ash bricks to reduce carbon footprint.

Classes of Clay Bricks

• Common bricks: General-purpose bricks for walls that will be plastered or hidden, where appearance is not important.
• Facing bricks: Bricks with an attractive, uniform face and colour, used for exposed walls.
• Engineering bricks: Dense, strong bricks with low water absorption, used for foundations, manholes and retaining walls.
• Perforated bricks: Bricks with holes through them, which reduce weight and improve firing and bonding.
• Special shaped bricks: Bricks with rounded, angled or bullnose shapes for corners, sills and copings.

Stages in the Manufacture of Clay Bricks

1. Winning the clay: Clay is dug from suitable deposits, removing topsoil and stones.
2. Preparing the clay: The clay is crushed, mixed with water and tempered to a uniform, workable consistency.
3. Moulding: Bricks are shaped by hand in moulds, by pressing, or by extruding and wire-cutting.
4. Drying: Green bricks are dried slowly under cover to remove moisture and prevent cracking.
5. Firing: Dried bricks are burnt in clamps or kilns at high temperatures to harden them.
6. Cooling and sorting: Fired bricks are cooled, then sorted into grades and defective bricks rejected.

2.1.2.3 Soil stabilized blocks

Soil stabilized blocks are walling units made by mixing suitable soil with a small amount of stabiliser, usually cement or lime, compressing the mix in a manual or hydraulic press, and curing it without firing. The most common type in Kenya is the interlocking stabilised soil block (ISSB), whose interlocking shape reduces the mortar needed and speeds up construction. These blocks are popular for affordable housing because the soil is often available on or near site, the blocks need no firing and they give good thermal comfort. Example: a housing cooperative in Kitui produced interlocking stabilised soil blocks on site for members' houses, reducing walling costs by about a third compared with concrete blocks bought in town.

Steps in Making Soil Stabilized Blocks

1. Select suitable soil: Choose soil with a good mix of sand and clay, free from organic matter, and test it for suitability.
2. Sieve the soil: Pass the soil through a sieve to remove stones, roots and lumps.
3. Measure the stabiliser: Add cement or lime, commonly about 5% to 10% by volume depending on the soil and strength required.
4. Mix dry: Mix the soil and stabiliser thoroughly until the colour is uniform.
5. Add water: Add water gradually and mix to the correct moisture content, so that a squeezed handful holds its shape.
6. Press the blocks: Fill the mould of the block press and compress the mix using a manual or hydraulic press.
7. Stack and cure: Stack the blocks under shade, cover them and keep them damp for at least 14 to 21 days.
8. Test before use: Check samples for strength and durability before using the blocks in walls.

Advantages of Soil Stabilized Blocks

• Low cost: Local soil and little cement make the blocks cheaper than many alternatives.
• Low embodied energy: The blocks are not fired, saving fuel and reducing emissions.
• Reduced mortar: Interlocking blocks need little or no mortar in the joints.
• Good thermal comfort: Earth walls keep buildings cool by day and warm at night.
• Local job creation: Blocks can be made on site by trained local workers.

Limitations of Soil Stabilized Blocks

• Lower strength: They are generally weaker than fired bricks and concrete blocks and suit low-rise buildings.
• Moisture sensitivity: Poorly stabilised blocks can erode when exposed to rain and need protection.
• Quality control needs: Soil type, mix proportions, pressing and curing must be carefully controlled.
• Need for skilled production: Workers need training to produce consistent, good quality blocks.

2.1.2.4 EPS

EPS (expanded polystyrene) walling units are lightweight panels or blocks made from expanded polystyrene, a rigid foam plastic, used as the core of a wall system. The most common EPS system in Kenya consists of EPS sandwich panels, in which a polystyrene core is held between two layers of galvanised steel wire mesh, erected on site and then sprayed or plastered with cement mortar or concrete on both faces to form a strong, insulated wall. Other systems include insulated concrete forms (ICF), which are hollow EPS blocks filled with reinforced concrete. EPS systems are fast to erect, light to transport and give excellent thermal insulation, which makes them attractive for affordable housing and quick-build projects. Example: a contractor building staff houses for a tea factory in Kericho used EPS panels, completing the walls of each house in a few days and reducing the weight on the foundations.

Properties of EPS Walling Units

• Very light weight: EPS panels are much lighter than masonry, reducing transport costs and foundation loads.
• High thermal insulation: The foam core greatly reduces heat transfer, keeping buildings cool in hot weather and warm in cold weather.
• Good sound insulation: Plastered panels reduce noise transmission when properly finished.
• Moisture resistance: EPS absorbs very little water and does not rot.
• Strength from the composite system: Strength comes from the steel mesh and plaster or concrete layers acting together with the core.

Steps in Erecting EPS Panel Walls

1. Prepare the foundation: Cast the foundation and floor slab with starter bars at the wall positions.
2. Set out the walls: Mark wall lines and openings on the slab.
3. Erect the panels: Stand the panels on the wall lines and tie them to the starter bars.
4. Fix reinforcement meshes: Fix corner, joint and opening reinforcement meshes as specified.
5. Brace and align: Brace the panels temporarily and check that they are plumb and in line.
6. Fix services: Cut channels in the EPS core for electrical conduits and plumbing pipes.
7. Apply the first plaster coat: Spray or apply cement mortar to both faces to cover the mesh.
8. Apply finishing coats and cure: Apply the final plaster coats, then cure the walls by keeping them damp.

Advantages and Limitations of EPS Walling

• Advantage, speed of construction: Walls are erected and plastered much faster than masonry.
• Advantage, energy efficiency: Insulation reduces cooling and heating needs.
• Advantage, earthquake resistance: Light weight and the continuous mesh perform well under seismic loads.
• Limitation, fire behaviour: EPS is combustible, so it must be fully protected by plaster and fire-retardant grades used.
• Limitation, specialised skills and equipment: Erection and spraying require trained workers.
• Limitation, fixing heavy items: Special fixings are needed to hang heavy shelves and cabinets.

2.1.2.5 Prefabricate walling units

Prefabricated walling units are wall components manufactured in a factory or yard to set sizes and then transported to site and assembled, instead of being built unit by unit from small masonry pieces. They include precast concrete panels, gypsum board partitions on steel or timber studs, sandwich panels, timber frame panels and light gauge steel frame walls. Prefabrication speeds up construction, improves quality control, reduces site waste and labour, and allows work to continue in poor weather. Example: a private hospital in Nakuru used gypsum board partitions on steel studs to create new consulting rooms over a weekend, with minimal dust and noise for patients.

Types of Prefabricated Walling Units

Advantages of Prefabricated Walling Units

• Fast construction: Units are assembled quickly, shortening the construction period.
• Consistent quality: Factory production gives accurate sizes and finishes.
• Less site waste: Materials are cut and prepared in the factory, reducing waste on site.
• Reduced labour: Fewer workers are needed on site.
• Work in any weather: Factory production continues regardless of site weather.

Limitations of Prefabricated Walling Units

• Transport and lifting needs: Large panels need lorries, cranes and good access roads.
• High initial investment: Moulds, factories and equipment are costly.
• Less design flexibility: Changes after manufacture are difficult.
• Joint detailing: Joints between panels must be carefully sealed to prevent leaks and cracks.
• Specialised skills: Erection requires trained crews.

2.1.2.6 Concrete Blocks

Concrete blocks are manufactured from cement, sand, aggregate, and water. Their larger size reduces laying time, and they can be solid or hollow. County government offices in Mombasa often use concrete blocks for their structural walls due to their strength and ease of reinforcement.

2.1.2.7 Calcium Silicate Blocks

These blocks are produced by mixing sand and lime with cement and curing under steam pressure. They have high strength and good dimensional accuracy, making them suitable for commercial buildings in Nairobi’s central business district.

2.1.3 Criteria for Selecting Walling units

Selecting appropriate walling units requires balancing technical performance, economic factors, and project-specific needs.

Structural Strength Requirements

Walling units must meet minimum compressive strength standards to ensure safety. For example, blocks used in multi-storey buildings in Nakuru require higher strength than those for single-storey residential walls.

Thermal and Acoustic Properties

Units with good insulation properties improve energy efficiency and occupant comfort. Hospitals in Kisii often specify walling units with high thermal mass to regulate indoor temperatures.

Durability and Weather Resistance

Resistance to moisture, chemicals, and weathering affects the lifespan of walling units. Coastal hotels in Malindi select blocks with low water absorption to withstand marine conditions.

Cost and Availability

Budget constraints and local availability influence the choice. Farmers constructing livestock shelters in Machakos prefer locally sourced clay bricks for cost-effectiveness.

Workability and Ease of Construction

Units that are easy to handle and lay reduce labor time and cost. Lightweight fly ash bricks are favored in high-rise apartments in Nairobi for this reason.

2.1.4 Applications of different walling units:

Different walling units suit particular construction functions depending on their properties and cost.

2.1.4.1 Load-bearing walls

Concrete blocks and calcium silicate blocks are commonly used for load-bearing walls in public buildings such as schools in Kisumu, providing the necessary strength to support floors and roofs.

A load-bearing wall is a wall that carries loads from roofs, floors and other walls above it, together with its own weight, and transfers them safely down to the foundation. Because the wall is part of the structure, its walling units must have adequate compressive strength, be laid with full mortar joints in a proper bond, and be thick enough to resist buckling. Removing or weakening a load-bearing wall without providing alternative support can cause floors or roofs to collapse. Example: a SACCO's two-storey office in Murang'a was built with 200 mm machine-cut stone load-bearing walls supporting a reinforced concrete first floor slab through a ring beam.

Characteristics of Load-Bearing Walls

• Adequate compressive strength: Units must be strong enough to carry the loads above without crushing.
• Sufficient thickness: Walls are usually at least 150 mm to 200 mm thick to provide strength and stability.
• Continuous to the foundation: The wall must rest on a foundation sized for the loads it carries.
• Proper bonding: Units are laid in a strong bond so that loads spread evenly.
• Tied together: Ring beams, wall ties and hoop iron tie the walls together and to floors and roofs.

Walling Units Suitable for Load-Bearing Walls

• Machine-cut stone: Strong and durable, widely used for load-bearing walls in low-rise buildings.
• Solid and hollow concrete blocks: Hollow blocks can be filled with reinforced concrete for extra strength.
• Engineering and good quality clay bricks: Strong bricks are used for load-bearing walls and piers.
• Calcium silicate blocks: High strength makes them suitable for load-bearing applications.
• Precast concrete panels: Designed panels can carry loads in panelised buildings.

2.1.4.2 Non-load-bearing walls

Lightweight units like fly ash bricks and hollow concrete blocks are ideal for internal partitions, offering ease of installation and flexibility in room layout.

A non-load-bearing wall is a wall that carries only its own weight and does not support floors or roofs, and its main purposes are to divide spaces, enclose buildings framed in concrete or steel, and provide privacy, sound control and fire separation. Because they do not carry structural loads, these walls can be built from lighter, thinner and faster-to-install units, and they can often be removed or relocated during renovation. However, they must still be stable, fixed securely to the structure and built to resist impacts and wind where they are external. Example: a bank in Eldoret rearranged its banking hall by removing 100 mm hollow block partitions and installing gypsum board partitions, without affecting the building's reinforced concrete frame.

Uses of Non-Load-Bearing Walls

• Internal partitions: Dividing floors into rooms, offices, wards or classrooms.
• Infill walls in framed buildings: Filling the spaces between concrete or steel columns and beams.
• Screen walls: Providing privacy or shading while allowing air movement.
• Fire and acoustic separation: Separating rooms to control the spread of fire and sound.
• Temporary enclosures: Enclosing spaces for limited periods, such as site offices.

Walling Units Suitable for Non-Load-Bearing Walls

• Hollow concrete blocks: Lighter than solid blocks and quick to lay.
• Gypsum board partitions: Light, quick to install and easy to modify.
• EPS panels: Very light with good insulation.
• Fly ash bricks: Lightweight bricks suitable for partitions.
• Sandwich panels: Used for partitions in factories, cold rooms and offices.

2.1.4.3 Boundary and Retaining Walls

Stone units and solid concrete blocks are preferred for boundary walls and retaining structures in farms and estates in Kiambu because of their robustness and resistance to weather.

2.1.4.4 Decorative and Cladding Walls

Clay bricks with attractive finishes are used in hotels and retail outlets in Nairobi for aesthetic wall facades that combine function with visual appeal.

2.1.4.5 Fire-Resistant Walls

Calcium silicate blocks are selected in industrial facilities and hospitals for their fire resistance properties, enhancing safety standards.

2.1.5 Factors influencing walling unit selection:

Several external and internal factors impact the decision on which walling units to use in a Kenyan construction context.

2.1.5.1 Structural requirements

Architectural design and structural load influence unit selection; for example, high-rise buildings in Nairobi require high-strength blocks and reinforced masonry.

Structural requirements are the strength, stability and stiffness that a wall must provide to carry its loads safely throughout its life. They determine the minimum compressive strength of the units, the wall thickness, the mortar strength and any reinforcement or tying required. The designer calculates the loads on each wall from the roof, floors, occupants and wind, and specifies units that can carry them with an adequate factor of safety. Example: the design for a four-storey hostel for a technical training institute in Thika specified high-strength concrete blocks for the ground floor walls, where loads were greatest, and lighter blocks for partitions on upper floors.

Structural Factors Considered

• Compressive strength: Units must carry the vertical loads from above with an adequate factor of safety.
• Wall thickness and slenderness: Thin, tall walls can buckle, so thickness must suit the height and load.
• Lateral loads: Walls must resist wind, earth pressure on retaining walls and earthquake forces.
• Compatibility with reinforcement: Some designs need hollow units that can be reinforced and filled with concrete.
• Weight of the units: Heavy units increase loads on foundations and the structural frame.
• Mortar and bond: The mortar strength and bonding pattern must match the strength of the units.

Worked Example: Stress in a Load-Bearing Wall

A 200 mm thick load-bearing wall carries a vertical load of 150 kN per metre run. Calculate the compressive stress on the wall.

Stress = Load ÷ Area

Load = 150 kN = 150,000 N per metre run

Area = 1,000 mm × 200 mm = 200,000 mm²

Stress = 150,000 ÷ 200,000

Stress = 0.75 N/mm²

The selected units, with appropriate safety factors applied by the designer, must have a compressive strength well above this value.

2.1.5.2 Aesthetic considerations

Aesthetic considerations are the factors related to how a wall looks, including its colour, texture, pattern, size of units and the way joints are finished. The appearance of walls strongly affects how people perceive a building, its value and how well it fits with its surroundings, so clients and architects often choose walling units partly for their look. Some units, such as facing bricks and dressed stone, are attractive enough to be left exposed, while others are plastered and painted. Example: a resort hotel in Diani chose coral stone walling for its cottages so that the buildings blended with the coastal landscape and traditional Swahili architecture.

Aesthetic Factors in Walling Unit Selection

• Colour: Natural colours of stone and fired bricks vary, and the choice affects the building's character.
• Texture and finish: Smooth, rough, rock-faced, dressed or textured finishes create different visual effects.
• Unit size and proportion: Small units such as bricks give fine detail, while large blocks give a bolder appearance.
• Bonding pattern: Patterns such as English bond, Flemish bond or decorative bonds add visual interest.
• Joint finish and colour: Recessed, flush or coloured joints change the appearance of the wall.
• Harmony with surroundings: Materials should suit the local architecture, landscape and planning requirements.
• Fair-faced or plastered finish: Choosing exposed units avoids plastering and painting but requires higher quality units and workmanship.

2.1.5.3 Climate considerations)

Regions with high rainfall or humidity, such as Kisumu, demand walling units with low water absorption and high durability to prevent deterioration.

Climate considerations are the effects of local weather conditions, such as temperature, rainfall, humidity, wind and salt-laden air, on the choice and performance of walling units. Kenya's climate ranges from hot and dry in the north and east, through cool and wet in the highlands, to hot and humid at the coast, and walls must keep the indoor environment comfortable while resisting local weathering. Choosing units suited to the climate improves comfort, reduces energy use for cooling and heating, and extends the life of the building. Example: a boarding school in Lodwar built thick stabilised soil block walls with deep roof overhangs to keep dormitories cool during very hot days.

Climate Factors Affecting Walling Unit Selection

• Temperature: In hot regions, heavy units with high thermal mass, such as stone and earth blocks, delay heat entering buildings.
• Rainfall and humidity: Wet regions need units with low water absorption and good resistance to dampness and mould.
• Coastal salt exposure: Coastal areas need dense, salt-resistant units and good protection to prevent decay.
• Wind: Exposed sites need strong, well-tied walls to resist wind pressure and driving rain.
• Temperature variation: Areas with large day-night temperature changes benefit from insulating or high thermal mass units.
• Sun exposure: Walls facing strong sun may need insulation, light colours or shading.

2.1.5.4 Sustainability in choosing walling materials

Sustainability in choosing walling materials means selecting units that meet the building's needs while minimising environmental harm, conserving resources and supporting local communities over the whole life of the building. It considers how the materials are extracted, how much energy they need to produce and transport, how long they last, how much maintenance they need and whether they can be reused or recycled. Sustainable choices reduce greenhouse gas emissions and resource depletion and often lower costs over the life of the building. Example: a community polytechnic in Busia chose interlocking stabilised soil blocks made from soil excavated on its own site, reducing transport, avoiding kiln firing and training local youth in block making.

Sustainability Factors in Choosing Walling Materials

• Low embodied energy: Prefer units that need little energy to make, such as stabilised soil blocks and local stone.
• Local sourcing: Locally available materials reduce transport emissions and support local economies.
• Renewable and recycled content: Use units made with recycled materials or renewable resources where suitable.
• Durability and low maintenance: Long-lasting units reduce the need for repair and replacement.
• Thermal performance: Units that keep buildings comfortable reduce energy used for cooling and heating.
• Responsible extraction: Use stone, sand and clay from licensed, well-managed sources.
• Reusability at end of life: Choose units and mortars that allow walls to be dismantled and the units reused.
• Health and safety: Avoid materials that release harmful substances during manufacture, use or disposal.

2.1.5.5 Availability of Materials

Remote areas may rely on locally available materials like stone or clay, influencing the choice based on logistics and cost.

2.1.5.6 Project Budget and Timeline

Projects with tight budgets and schedules may prioritize faster-to-lay units like concrete blocks despite higher material costs to reduce labor expenses.

2.1.5.7 Regulatory and Building Code Compliance

Compliance with Kenyan building codes and standards, such as those specified by the National Construction Authority (NCA), governs minimum requirements for walling units to ensure safety and quality.

Chapter Summary

This chapter provided a comprehensive overview of construction walling units, beginning with their introduction and key definitions to establish foundational understanding. Various types of walling units were explored, including building stones, brick units, soil stabilized blocks, expanded polystyrene (EPS), and prefabricated walling units, each with unique characteristics suited to different construction needs. The criteria for selecting walling units were examined, focusing on their appropriate applications in load-bearing and non-load-bearing walls. Factors influencing the choice of walling materials were discussed in detail, covering structural requirements, aesthetic considerations, climate impact, and sustainability aspects. The chapter then delved into the physical, chemical, and mechanical properties of walling units, highlighting how these attributes affect performance and durability. Finally, practical applications were addressed through brick masonry techniques, emphasizing different types of bonds and the importance of selecting suitable mortar to ensure structural integrity. Together, these topics provide a solid foundation for understanding and applying construction walling units effectively in various building projects.