Repair Systems — Concrete & Structural Defects
Backer rods
Technical product reference for closed-cell PE backer rods, open-cell PE backer rods, and bond-breaker tape used before sealant application in concrete joints — covers cell type, sealant compatibility, sizing rules, and depth:width ratio control for Australian joint sealing practice.
Backer rods & bond-breaker tape — why they matter
A correctly performing joint sealant bonds to the two opposing side walls only. If the sealant also bonds to the joint base (three-sided adhesion), it cannot elongate in an hourglass shape under joint movement — instead it tears prematurely. Backer rods and bond-breaker tape prevent base adhesion and control the sealant depth:width ratio to the target of 1:2.
Product Reference
Individual products — one card each — scroll to view all
Soudal / Tremco / Generic PE
Closed-Cell PE Backer Rod — Standard Joint Sealing
Closed-cell polyethylene foam rod — joint depth control
Standard backer rod for concrete joint and crack sealing applications. Insert rod at 25–50% larger than joint width — the compression fit holds it in place without adhesive. Positions the sealant at the correct depth:width ratio of 1:2 and creates a bond-break at the joint base so the sealant bonds to the two side faces only (two-point contact).
Tremco / Soudal / Generic PU foam
Open-Cell PU Backer Rod — Silicone Sealant Applications
Open-cell polyurethane foam rod — silicone sealant joint sealing
Open-cell rods are the correct choice when silicone or acetoxy silicone sealants are specified. Silicone releases acetic acid (or other by-products) during cure — closed-cell foam traps these gases between the sealant and the joint base, creating bubbles and internal voids in the cured sealant that concentrate stress and cause premature failure. Open-cell foam allows the by-products to escape through the back of the joint.
Tremco / Sika / Generic PE tape
Bond-Breaker Tape — Narrow Joint Base Seal Prevention
PE/PTFE bond-breaker tape — joint base treatment
Bond-breaker tape is the solution for joints that are too shallow or too narrow (typically less than 6 mm) to seat a backer rod. A self-adhesive PE or PTFE film strip applied to the joint base prevents the sealant from bonding to the back face, enforcing two-point side-wall adhesion geometry. The tape adds negligible height.
System Comparison
Concrete cracking — confirm all selections against the current manufacturer TDS before specifying.
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Do not confuse backer rods with:
- The sealant itself — backer rods and bond-breaker tape are accessories installed before sealant application; they do not seal the joint — the PU or silicone sealant is the sealing element; both must be present for a correctly performing joint system
- Compriband / impregnated foam tape — pre-compressed impregnated polyurethane foam tapes (Compriband, ISO-BLOCO) are self-expanding joint fillers applied to the joint face before a frame or panel is installed; they are a complete joint seal themselves, not a depth-control accessory for a gun-applied sealant
- Fibre reinforcement in crack injection — polypropylene or steel fibre added to repair mortars and grouts reinforces the cured mortar matrix against shrinkage cracking; backer rods have no structural function and are not fibre reinforcement products
- Masking tape — masking tape applied to sealant joint edges is used to achieve clean sealant lines; it is removed after tooling and must never be used as a bond breaker at the joint base as the adhesive transfers to the concrete and causes sealant bonding at the base
- Compression seals — neoprene compression seals (used in highway bridge and carpark expansion joints) are preformed rubber profiles compressed into routed slots; they are structural joint seals under vehicle loading and are not substitutable by foam backer rods and sealant in trafficked joints
Disclaimer
This page provides general technical information only. Final product selection must be confirmed against the current manufacturer TDS, project specification, substrate condition, and applicable standards. Do not rely on this reference as a substitute for professional engineering advice.