The second part of the session focused on the distinction between bound and unbound paving systems, which fundamentally affects how loads are distributed and how water moves through the surface.

Unbound systems consist of:

• Compacted granular sub-base

• Loose laying course such as grit sand

• Pavers laid without mortar

• Joints filled with sand or fine aggregate

These systems offer:

• Flexibility under minor ground movement

• Ease of maintenance and replacement

• Inherent permeability where joints allow infiltration

• Lower embodied carbon due to reduced cement use

However, they are:

• Vulnerable to deformation under heavy or concentrated loads

• Susceptible to rutting if compaction is inadequate

• Less stable on steep gradients

Bound systems, in contrast, use:

• Cementitious or polymer-modified mortars

• Rigid bedding layers

• Fully bonded joints

These provide:

• High resistance to shear forces and point loading

• Increased durability in high-traffic urban conditions

• Greater surface stability and visual consistency

But they introduce:

• Risk of cracking if ground movement occurs

• Higher embodied carbon

• Reduced permeability unless designed alongside SuDS systems

Steintec emphasised that many projects require hybrid solutions, where permeability, structural performance and durability are balanced through careful specification of:

• Sub-base type such as Type 1 or Type 3 aggregate

• Bedding layer gradation and thickness

• Moisture control during installation

• Joint width and tolerance

• Thermal expansion and contraction

• Resistance to freeze–thaw cycles

This reinforces that paving must be understood as a multi-layered system responding to both structural and environmental conditions.