Frequently Asked Questions

How is Proofex Swellmastic applied to seal the tie rod holes remaining in a basement wall after formwork striking and what preparation is needed before application?

Sealing tie rod holes in a basement wall after formwork stripping is one of the most common applications for Fosroc Proofex Swellmastic on below-grade construction sites in India, and the quality of this sealing is important because tie rod holes are consistent, known leak paths in basement walls — the steel tie rod passes through the full thickness of the wall, leaving a conical or cylindrical hole and a steel rod (or rod cut-off stub) that creates a direct hydraulic connection between the external groundwater and the basement interior. The application procedure is: after formwork is stripped, the tie rod is cut flush with or slightly below the concrete surface using a disc cutter (cut at least 20 to 25 mm below the wall face to provide adequate concrete cover over the Swellmastic plug); the tie rod hole is cleaned of all laitance, loose concrete, and steel swarf from the cutting operation using compressed air; the hole is lightly dampened with water but must not have free-standing water in the hole during Swellmastic application; load the Proofex Swellmastic cartridge into a standard cartridge gun; insert the gun nozzle into the tie rod hole and extrude mastic to fill the hole from the back toward the surface — fill the hole completely from bottom to top to avoid air pockets; press the extruded mastic firmly into the hole with a gloved finger or a wooden dowel to ensure full contact with the sides of the hole with no voids; trowel the surface of the Swellmastic plug smooth and set slightly below the wall face; on the external face of the wall (positive pressure face), the Swellmastic plug should be protected from UV exposure and mechanical damage by the subsequent applied waterproofing membrane (Proofex 30 or Nitoproof 400) — the membrane applied over the external wall face covers all the tie rod Swellmastic plugs, providing an additional layer of protection.

Can Proofex Swellmastic be used to seal around a PVC pipe penetrating a basement wall and is the PVC surface compatible with the hydrophilic mastic for a watertight bond?

Yes — Fosroc Proofex Swellmastic is specifically recommended for sealing around pipe and conduit penetrations through basement and below-grade concrete walls, including PVC pipes, HDPE conduit, and steel pipes. The sealing mechanism for pipe penetrations uses the same hydrophilic swelling principle as for construction joint sealing: Swellmastic is applied into the annular gap between the pipe outer diameter and the concrete wall penetration hole; on contact with groundwater penetrating through the annular gap, the Swellmastic swells and fills the gap, creating a compression seal around the pipe. For PVC pipes specifically: the Swellmastic does not rely on chemical adhesion to the PVC surface for its primary sealing mechanism — the sealing is achieved by mechanical compression of the swollen mastic against both the concrete wall and the PVC pipe outer surface; PVC surfaces do not require priming or special preparation for Swellmastic application beyond cleaning; the Swellmastic fills the annular gap around the pipe uniformly under the gun application pressure; and the hydrophilic swelling on water contact provides the final compression seal. The installation procedure for a pipe penetration is: the annular gap between the pipe and the wall should ideally be minimum 10 to 20 mm on all sides for adequate Swellmastic volume — larger gaps may need to be packed with mortar first, leaving a 10 to 20 mm deep channel around the pipe for Swellmastic; clean the gap and pipe surface of any oil, release agent, or contamination; extrude Swellmastic into the annular gap, filling completely from the back of the penetration to the wall face; press the mastic into full contact with the concrete wall and pipe surface; for water pipes carrying water at higher pressure (domestic water supply, fire sprinkler systems), the sealing requirement is more stringent — consult Fosroc technical for the appropriate pipe penetration sealing system for the specific water pressure and pipe material combination.

Is there a risk that Proofex Swellmastic will over-expand and damage the surrounding concrete if the groundwater pressure is particularly high and the mastic swells excessively?

The concern about over-expansion of hydrophilic swelling materials damaging surrounding concrete is a valid engineering consideration that has been studied and addressed in the design of Fosroc Proofex Swellmastic and similar hydrophilic swelling products. The expansion pressure that swelling mastic develops is a function of its swelling ratio, the confinement provided by the surrounding concrete and joint geometry, and the water pressure driving the swelling. In a fully confined joint (a tie rod hole or construction joint where the swelling mastic is surrounded by rigid concrete on all sides), the swelling of the hydrophilic mastic is restrained by the concrete walls of the joint — instead of expanding volumetrically, the swelling generates internal pressure against the concrete faces. The design of Proofex Swellmastic is calibrated so that the maximum swelling pressure developed in full confinement is well below the tensile strength of properly compacted concrete (concrete tensile strength approximately 2 to 4 MPa for M25 to M35 grade) — the swelling pressure at maximum water saturation is sufficient to close micro-gaps and provide a compression seal, but does not reach levels that would crack or spall the surrounding concrete. However, there are situations where swelling pressure could become a concern: very weak concrete (below M20 grade, heavily carbonated, or already cracked) around a swelling mastic plug may not provide sufficient confinement resistance; large-volume swelling in a wide joint or large hole filled to a significant depth with Swellmastic generates greater total swelling force than a small volume in a narrow joint — for large holes, a two-stage approach (pack partially with mortar then seal the remaining gap with Swellmastic) reduces the total Swellmastic volume and the resulting swelling force; and Swellmastic applied in an unconfined or partially open joint (where one face of the joint is not solid concrete) cannot develop swelling pressure and the seal may be expelled from the joint rather than sealing it. In standard below-grade construction joint and tie rod hole sealing applications with normal-strength concrete, the risk of concrete damage from Swellmastic over-expansion is negligible — the product is designed for these applications and the swelling pressure is within safe limits.