Post-tensioning (PT) duct grouting is a critical process that fills the voids around prestressing tendons with cementitious grout to protect them from corrosion and ensure a monolithic bond between the tendon and the concrete structure. Proper execution directly impacts the long-term durability of bridges, parking garages, and high-rise buildings.
The procedure begins with pre-grouting inspection. Ducts must be free of debris, oil, or standing water. Air vents and outlet ports should be checked for blockage. Grouting equipment such as the colloidal mixer and piston pump must be calibrated to maintain continuous flow. The grout mix typically uses Portland cement (Type I/II), water, and a high-range water reducer to achieve low bleed (under 0.3%) and high fluidity.
Mixing follows a strict sequence: first, measure water into the mixer (using potable water at 15-25°C). Add cement slowly, then chemical admixtures. Mix for 3-5 minutes until a homogeneous, viscous fluid is achieved. The water-cement ratio normally ranges from 0.35 to 0.40 by weight. Test the grout immediately for flow cone efflux time (target 15-25 seconds), temperature (below 35°C), and fluidity retention.
Injection is performed from the lowest grout inlet ports to prevent air entrapment. The grouting pump applies pressure between 0.5 to 2.0 MPa. Maintain unbroken flow until pure grout exits from the highest vent ports. Then, cap the vents while keeping pressure on. For long tendons, intermediate vents may be used. A critical step is the overflow test: collect grout from the final outlet and ensure it matches the fresh grout in consistency and color.
Post-injection quality control involves monitoring bleed water and volume change. Allow the grout to set for 12-24 hours, then check for surface cracking. Vacuum-assisted grouting may be specified for extra assurance. All records of mixing, pumping pressure, and vent sealing times must be documented per project specifications. Following these procedures ensures the PT system delivers design strength and durability for decades.