Frequently Asked Questions

What makes Sprayset SL suitable for tunnel lining applications over standard cementitious renders?

Standard renders using sand aggregate have a density of approximately 1,900 to 2,100 kg per cubic metre. In tunnel and underground applications, the render layer is applied to overhead soffits and curved arch surfaces — the dead weight adds load to the structural lining. A standard render at 20 mm thickness weighs approximately 38 to 42 kg per square metre. Sprayset SL with lightweight aggregate has a density of 1,400 to 1,600 kg per cubic metre — the same 20 mm coat weighs approximately 28 to 32 kg per square metre. Across thousands of square metres of tunnel lining, this weight reduction is structurally significant. The lightweight aggregate also provides better thermal insulation and passive fire resistance than dense sand aggregate — in a tunnel fire, the coating slows the rate of temperature rise in the structural concrete beneath.

Does Sprayset SL require a bonding primer before spray application on shotcrete tunnel linings?

Yes. A bonding primer or slurry coat is recommended before applying Sprayset SL on shotcrete tunnel lining surfaces. Shotcrete linings have a rough open texture but may have surface laitance or dust that reduces adhesion. A cement-based bonding slurry (such as a diluted Nitobond SBR slurry) brushed onto the prepared substrate provides an improved bonding interface for the spray mortar. The substrate must be pre-wetted (saturated surface-dry) before application to prevent the dry substrate from absorbing water from the fresh spray mortar too rapidly, which would cause bond failure and shrinkage cracking.

Can Sprayset SL be used over an active waterproofing membrane in a tunnel?

Sprayset SL is designed for direct application to structural concrete or shotcrete substrates and is not designed for application directly over flexible sheet waterproofing membranes. In tunnels with a composite waterproofing system, the inner concrete lining is the element that receives the spray mortar finishing coat. If the tunnel uses a spray-applied waterproofing membrane directly on the shotcrete lining, the compatibility of Sprayset SL over the specific membrane must be confirmed with Fosroc and the membrane manufacturer before application, as bond between a cementitious mortar and a spray-applied membrane varies with membrane type and surface condition.