Why Excessive Mortar Use Weakens Masonry

The Case for Proper Pinning Techniques in Construction

In Kenya's rapidly growing construction industry, masonry remains the dominant building method for residential and commercial structures. However, a common practice during walling continues to undermine the structural integrity and longevity of buildings across the country: the overuse of mortar to fill gaps between masonry units. This seemingly harmless shortcut creates shrink-prone areas that paradoxically weaken the very walls they're meant to strengthen.

Understanding the science behind mortar behavior and adopting traditional gap-filling techniques can significantly improve construction quality while reducing material costs - a win-win for builders and property owners alike.

The Problem: Excessive Mortar Creates Weakness

Understanding Mortar Shrinkage

When mortar joints are filled with excess material, particularly rich cement mortars, they become highly susceptible to shrinkage cracking. All cement-based mortars undergo drying shrinkage as they cure, but the extent of this shrinkage increases proportionally with the volume of mortar used.

According to research on masonry construction, weak mortars like composite cement-lime-sand mixtures show less tendency to develop cracks because they can better accommodate the shrinkage of individual masonry units. The problem intensifies when masons use thick mortar layers to compensate for irregular stone or block sizes.

Why Large Mortar Masses Fail

High temperatures during curing can increase the initial rate of drying shrinkage while reducing ultimate shrinkage values, which is particularly relevant in Kenya's warm climate. However, large mortar volumes face several compounding problems:

Increased Shrinkage Stress: As mortar cures and loses moisture, it contracts. Larger volumes contract more, creating greater tensile stresses within the joint.
Crack Formation: When induced stress from shrinkage exceeds the material's strength, cracking occurs, creating pathways for water infiltration and structural weakness.
Weak Bonding: Thick mortar joints reduce the effective contact area between masonry units, compromising the wall's load-bearing capacity.
Economic Waste: Excessive mortar use unnecessarily increases material costs without improving structural performance.

Real-World Implications in Kenya

Kenya's Building Code specifies that stonework bedding joints should not exceed 5/8 inch, while brick joints should not exceed 1/2 inch and 3/8 inch respectively. However, field observations reveal that many construction sites routinely exceed these dimensions when dealing with irregular stones or poorly sized blocks.

Joints between masonry units often represent planes of weakness due to their relatively low shear resistance, and thick mortar joints exacerbate this vulnerability. The consequences manifest as:

Horizontal and vertical cracks appearing within months of construction
Water penetration during rainy seasons
Reduced structural capacity
Compromised wall aesthetics
Accelerated deterioration requiring costly repairs

The Solution: Pinning and Galleting Techniques

What is Pinning?

Pinning (also known as galleting, garneting, or chinking) is an architectural technique where small pieces of stone are pushed into wet mortar joints during masonry construction. This traditional method has been used for centuries in England, parts of Europe, and even in colonial-era buildings in Kenya.

According to Historic Scotland's technical guidance, numerous small pinning stones contribute to the overall stability of masonry, reduce the quantity of expensive lime required, and minimize the effects of drying shrinkage in the mortar.

How Pinning Works

The technique involves several key steps:

Stone Selection: Small chips or slivers of stone are used to fill large mortar joints, typically measuring between 10-30mm in various dimensions.
Strategic Placement: In sandstone buildings, spalls are usually shaped into small cubes about half an inch in diameter and are flush with the stone surface.
Immediate Installation: Pinning stones must be pushed into the mortar while it's still plastic, ensuring proper embedment and bonding.
Coverage and Protection: The stones should be partially embedded, with mortar coverage maintaining joint integrity while the stone provides structural reinforcement.

Benefits of Proper Pinning

The advantages of using pinning techniques in Kenyan construction are substantial:

Structural Benefits:

Reduces effective mortar joint thickness
Provides mechanical interlocking between courses
Minimizes shrinkage-related cracking
Improves load distribution across the wall

Economic Benefits:

Reduces mortar consumption by 15-30%
Utilizes waste stone chips that would otherwise be discarded
Decreases long-term maintenance costs
Speeds up construction by reducing thick joint curing times

Durability Benefits:

Shields mortar from weather exposure
Reduces water penetration pathways
Minimizes thermal movement effects
Extends overall wall lifespan

Implementation in Kenyan Context

Adapting to Local Materials

Kenya's diverse geology provides excellent materials for pinning:

Nairobi Area: Quartzite and volcanic stone chips
Coastal Region: Coral stone fragments
Western Kenya: Granite and basalt chips
Rift Valley: Various volcanic rock spalls

Best Practices for Kenyan Builders

Material Specifications:

According to Kenya's building regulations, mortar for masonry work should typically be cement mortar 1:5 or as specified. When implementing pinning:

Use standard mortar mixes (1:5 or 1:6 cement:sand)
Select pinning stones that are clean, durable, and free from weathering
Ensure stones are roughly the same hardness as the primary masonry units

Application Guidelines:

Joint Preparation: Rake joints to 10-20mm depth before filling with mortar
Mortar Consistency: Maintain proper workability - mortar should be sufficiently plastic to allow stone insertion without excessive deformation
Strategic Filling: Place larger stones first, then fill remaining voids with smaller chips
Proper Coverage: Ensure pinning stones are embedded at least halfway into the mortar depth while maintaining minimum mortar coverage from the exposed face
Timing: Install pinning stones while mortar is still workable but before initial set begins
Curing: Keep work wet for at least one week after pointing is complete

Training and Supervision

Implementing pinning techniques requires:

Mason Training: Educate masons on traditional pinning methods and their structural benefits
Quality Control: Site supervisors should verify joint thicknesses and proper pinning installation
Material Management: Organize collection and sorting of stone chips at quarrying or cutting sites

Comparison with Common Kenyan Practices

Traditional Method (Excessive Mortar)

Process:

Mason places stones with large gaps between units
Thick mortar bed (often 25-40mm) fills all voids
No filler stones used
Extended curing time required

Results:

High shrinkage cracking within 3-12 months
Increased material costs
Reduced wall strength
Common water infiltration problems

Proper Pinning Method

Process:

Mason places stones with standard joint spacing (10-15mm)
Small stone chips fill larger gaps
Thin mortar bed bonds units and embeds pins
Faster curing achieved

Results:

Minimal shrinkage cracking
15-30% mortar savings
Improved structural integrity
Better weather resistance

Technical Considerations

Mortar Joint Specifications

Kenya's Building Code provides clear guidance that builders should follow:

Stonework bedding joints: maximum 5/8 inch (approximately 16mm)
Brick joints: 1/2 inch and 3/8 inch (approximately 13mm and 10mm)
Joint thickness tolerance: ±1/8 inch (±3mm) from specified dimensions

When gaps exceed these dimensions due to irregular stones, pinning becomes essential rather than optional.

Material Quality Standards

For masonry materials, Kenya follows Indian Standards (IS codes) which specify:

Stone Requirements:

Hard, sound, and durable
Free from weathering, decay, cavities, cracks, flaws, and sand holes
No veins, patches of loose materials, or similar defects

Sand for Mortar:

Clean and free from harmful impurities (iron pyrites, alkalis, organic materials)
Limited clay, fine silt, and dust content
Proper gradation for workability and strength

Water:

Clean and free from oils, acids, alkalies, salts, and organic materials
Permissible solid content limits per IS 3025

Climate-Specific Considerations

Kenya's varied climate zones require adapted approaches:

Coastal Areas:

High humidity reduces shrinkage but increases salt exposure
Use lime-based mortars where appropriate for better moisture accommodation
Ensure adequate curing in humid conditions

Highland Areas (Nairobi, Nakuru):

Moderate temperatures ideal for mortar curing
Watch for rapid drying during dry seasons
Protect fresh work from direct sun exposure

Arid Regions:

Rapid moisture loss accelerates shrinkage
Increase curing duration and frequency
Consider shade cloth protection during construction

Common Misconceptions Debunked

Myth 1: "More Mortar Means Stronger Walls"

Reality: Research consistently shows that weaker mortars (1:6 or 1:8 cement-lime-sand) accommodate movement better than rich mortars, resulting in fewer cracks and longer-lasting structures. Strength comes from proper bonding and load distribution, not mortar volume.

Myth 2: "Pinning is Only for Historic Buildings"

Reality: Pinning is a practical technique for modern construction that reduces costs while improving quality. Historic buildings worldwide have stood for centuries partly because of this method.

Myth 3: "Irregular Stones Require Thick Mortar"

Reality: Irregular stones require proper selection, placement, and pinning - not excessive mortar. Skilled masons can achieve thin joints even with rough stones through proper technique.

Myth 4: "Pinning Takes Too Much Time"

Reality: While pinning requires initial skill development, experienced masons work as quickly as those using thick mortar methods, with better long-term outcomes and material savings.

Case Studies from Kenyan Construction

Residential Construction - Nairobi

A residential project in Nairobi compared two identical walls: one built with standard thick mortar joints (25-30mm average) and another using pinning techniques (12-15mm joints with stone chips).

Observations after 18 months:

Pinned wall: No visible cracks, excellent water resistance
Traditional wall: Multiple hairline cracks, minor water seepage during heavy rains
Material savings: 22% reduction in mortar consumption on pinned wall

Rural School Project - Western Kenya

A school construction project trained local masons in pinning techniques using locally available granite chips.

Results:

28% reduction in cement costs
Improved wall aesthetics
Enhanced community skills in quality construction
No crack development in first two years

Implementation Roadmap

For Individual Builders

Educate Your Mason: Share this information with your construction team
Material Planning: Collect stone chips during quarrying or cutting operations
Quality Specifications: Include proper joint thickness requirements in construction contracts
Regular Inspection: Monitor joint thickness and pinning application during construction
Proper Curing: Ensure adequate water curing for minimum 7 days

For Construction Professionals

Training Programs: Develop mason training modules on pinning techniques
Standard Operating Procedures: Create site-specific guidelines for joint thickness and pinning
Quality Control Checklists: Include joint inspection points in QA/QC procedures
Material Management Systems: Establish protocols for stone chip collection and sorting
Performance Monitoring: Track crack development and material usage across projects

For Policy and Standards Development

Code Enhancement: Strengthen enforcement of existing joint thickness specifications
Technical Guidance: Develop detailed guidelines on pinning techniques for Kenyan conditions
Research and Development: Support studies on optimal pinning methods for local materials
Certification Programs: Include traditional masonry techniques in mason training curricula

Conclusion

The tendency to overuse mortar during masonry construction represents a false economy that creates shrink-prone areas and weakens structural integrity. By contrast, proper pinning techniques using small filler stones offer a proven solution that:

Reduces mortar consumption by 15-30%
Minimizes shrinkage cracking
Improves structural performance
Enhances weather resistance
Extends building lifespan
Lowers long-term maintenance costs

For Kenya's construction industry to advance, builders must move beyond convenient shortcuts toward evidence-based practices that balance economy with quality. Pinning isn't a revolutionary new technique - it's a time-tested method that deserves renewed attention in modern Kenyan construction.

The choice is clear: invest a modest amount of effort in proper technique now, or face costly repairs and structural concerns later. Quality construction doesn't require exotic materials or expensive technologies - it requires knowledge, skill, and commitment to best practices.

About MakaObora.com: This article is part of our ongoing series on quality construction practices in Kenya. For more technical guidance on building better homes and structures, visit our website or consult with qualified construction professionals.

References:

Kenya Building Code (Local Government Adoptive By-Laws Building Order 1968)
Historic Scotland Technical Advice Note on Lime Mortars
Research on masonry shrinkage and crack control
International building codes and masonry specifications