Frequently Asked Questions

Why is zero bleed so critical for PT cable duct grout and what happens if bleed channels form?

Bleed in a cementitious grout refers to the separation of free water from the grout paste before and during hardening — the water migrates upward (in vertical or inclined ducts) or accumulates at high points and direction changes in the duct, leaving a water-filled void above the hardened grout. In PT cable ducts, these bleed water voids are areas where the steel strand is exposed to stagnant water in direct contact with the steel surface. Over time, this water (especially if it contains dissolved chlorides from rain or de-icing salts that have penetrated the concrete cover) initiates electrochemical corrosion of the high-strength steel strand. Because PT strand is under very high tensile stress, even relatively small reductions in the cross-sectional area from corrosion can trigger brittle stress corrosion cracking and sudden strand failure. The consequences of PT strand failure in a bridge are potentially catastrophic and require expensive emergency repair or structure closure. Non-bleed grout such as Conbextra Cable Grout eliminates this risk by ensuring the duct is completely filled with hardened cementitious material in contact with every strand surface.

What equipment is needed for PT cable duct grouting with Conbextra Cable Grout and who should carry out this work?

PT cable duct grouting is specialist work that must be carried out by experienced personnel using dedicated grouting equipment. The key equipment required includes a colloidal mixer (high-shear mixer to fully disperse the cement and additives — a conventional drum mixer is not suitable), a grout pump (capable of pumping low-viscosity grout at the pressures needed to fill long ducts — typically reciprocating piston pumps or progressive cavity pumps are used), pressure gauges on both the injection point and the vent, and a flow-through vent arrangement that allows the duct to be completely flushed of air and water before the grout hardens. Quality control requires measurement of grout viscosity (flow cone test), temperature, and bleed at the batch level. The work should be supervised by a qualified post-tensioning specialist and quality records (batch weights, viscosity, injection pressure, vent outflow confirmation) maintained for each duct. Space Arc Engineering can supply the Conbextra Cable Grout product and provide guidance on specification and quality control requirements.

Can Conbextra Cable Grout be used in HDPE duct systems and what is the required injection sequence for a multi-span bridge?

Yes. Conbextra Cable Grout is compatible with both corrugated metal duct (CMD) and HDPE (high-density polyethylene) duct systems for PT. HDPE ducts are now widely specified for major infrastructure projects in India for their superior corrosion resistance and smoothness (which reduces friction losses during tensioning). The injection sequence for a multi-span bridge should follow the recommendations of the project PT specialist and the bridge contractor PT engineer, but generally the lowest cables are grouted first (to avoid fresh grout from upper cables leaking into lower areas through any imperfect duct connections), and each duct is grouted completely in one continuous injection operation until grout of the same consistency as the batched grout is confirmed flowing from the far-end vent — confirming complete duct filling.