Collar-jointed blockwork refers to a robust masonry construction method where walls are built from two separate leaves of masonry, typically concrete blocks, held together with stout wall ties, with the gap between the leaves – known as the collar-joint – being fully filled with mortar. This construction is also often termed double-leaf wall construction, emphasizing its dual-layer nature.
This technique creates a solid, monolithic wall structure by bonding the two leaves together with a continuous mortar infill, differing significantly from traditional cavity wall construction that maintains an air gap.
Understanding the Core Concept
In collar-jointed blockwork, the strength and integrity come from the intimate bond created by the mortar-filled gap between the inner and outer block leaves. This contrasts with a cavity wall, which features an air gap for thermal and moisture protection. The collar joint, being completely filled with mortar, ensures that both leaves act as a single structural unit, distributing loads more effectively and offering enhanced performance characteristics.
Key Components of Collar-Jointed Blockwork
Understanding the individual elements helps clarify this construction method:
- Two Leaves of Masonry: These are the inner and outer layers of the wall, typically constructed from concrete blocks. The blocks are laid in a conventional manner for each leaf.
- Collar-Joint: This is the space between the two masonry leaves. Crucially, it is completely filled with mortar, ensuring a solid connection between the leaves. The width of this joint can vary but is generally consistent to facilitate proper filling.
- Stout Wall Ties: These are robust metal connectors embedded in the mortar joints of both leaves at regular intervals. They mechanically link the inner and outer leaves, preventing them from separating and ensuring they act cohesively under load. Learn more about wall ties.
- Mortar: Beyond just holding the blocks together, the mortar plays a critical role in filling the collar-joint. The type of mortar used (e.g., cement-lime-sand mix) is essential for achieving the desired strength, durability, and bond.
Why Choose Collar-Jointed Blockwork?
This construction method offers several advantages, making it suitable for specific applications:
- Enhanced Structural Strength: By creating a solid cross-section, collar-jointed walls provide superior load-bearing capacity and rigidity compared to single-leaf walls. This is particularly beneficial for tall structures or those subject to significant lateral forces.
- Improved Thermal Mass: The increased density and mass of a collar-jointed wall contribute to higher thermal mass, which can help regulate internal temperatures by absorbing and slowly releasing heat. This can lead to more stable indoor environments and potentially reduced energy consumption for heating and cooling.
- Superior Acoustic Insulation: The solid, dense nature of these walls makes them highly effective at blocking sound transmission, providing excellent acoustic separation between spaces. This is ideal for buildings where noise reduction is critical, such as apartments, hotels, or educational facilities.
- Fire Resistance: The solid masonry construction inherently offers good fire resistance, providing a robust barrier against the spread of fire.
- Durability and Longevity: Collar-jointed blockwork creates a very durable and long-lasting wall system, capable of withstanding harsh weather conditions and everyday wear and tear.
Collar-Jointed vs. Cavity Walls: A Key Distinction
It's important to differentiate collar-jointed blockwork from the more common cavity wall construction. While both involve two leaves of masonry, their fundamental difference lies in the gap between them.
| Feature | Collar-Jointed Blockwork | Cavity Wall Construction |
|---|---|---|
| Gap Content | Fully filled with mortar | Air gap (often with insulation) |
| Structural Unity | Acts as a single, solid unit | Two separate leaves with limited interaction |
| Primary Benefit | High structural strength, thermal mass, acoustic insulation | Moisture protection, thermal insulation (with air gap/insulation) |
| Load Transfer | Distributed across entire wall thickness | Primarily by inner leaf, outer leaf is cladding |
Construction Process at a Glance
The construction of collar-jointed blockwork typically involves:
- Laying the Inner Leaf: The inner block leaf is constructed to the required height, following standard masonry practices.
- Laying the Outer Leaf: The outer block leaf is then constructed parallel to the inner leaf, maintaining the specified width for the collar-joint.
- Inserting Wall Ties: As the leaves rise, wall ties are embedded into the mortar joints of both the inner and outer leaves at the appropriate spacing and staggering.
- Filling the Collar-Joint: Once a section of the leaves and ties is in place, the gap between the leaves is completely filled with mortar. This can be done by pouring or pumping, ensuring thorough compaction to eliminate voids. This step is repeated as the wall progresses upwards.
Typical Applications
Collar-jointed blockwork is frequently specified for:
- Load-Bearing Walls: Where significant structural support is required, such as in multi-story buildings.
- Acoustic Walls: In environments demanding high sound insulation, like between residential units or along noisy corridors.
- Basement Walls: Providing a dense, strong, and durable barrier against earth pressure and moisture, often combined with appropriate waterproofing.
- Retaining Walls: Its robust nature makes it suitable for retaining earth.
The use of collar-jointed blockwork provides a sturdy, monolithic wall system that offers distinct advantages in structural performance, thermal mass, and acoustic properties.
