Problem
A major concern with existing bridges and some post-tensioned floors has been quality of grout and protection of tendons. A properly grouted duct will protect tendons from corrosion by providing an alkaline environment around and between the tendons. However, such factors with the grout as poor flow, segregation, air entrapment combined with procedural problems such as maintaining pumping pressure, protection of end plates and correct choice of ducting have led to many challeges facing the industry.
1. Choosing the right grout
The selection of a suitable duct grout has been simplified by the introduction of the CARES Appendix PT10 that now recommends solely the use of a prebagged premixed grout for use in posttensioning ducts. This in a way supercedes BS EN 447: Specification for common grout, which allowed the use of site-mixed grouts. weber.cem duct grout was developed several years ago in conjunction with the work done in the preparation of Concrete Society Technical Report TR47: Durable Posttensioned Concrete Bridges. It has been used successfully on many major civil engineering and building projects without any problems.
2. Voids in ducts
The best way to ensure complete filling of the ducts and hence protection of the tendons is to use a top quality grout. The favoured option is to use a combined colloidal mixer and pump with a constant pressure monitoring system. This ensures a steady flow of grout with a measured quantity of water without segregation and without air entrainment. BS EN 446 provides directions on the correct grouting procedures to be adopted by the specialist grouting contractor.
3. Chemical reaction within the ducts and tendons
Incorrect selection of ducting material can lead to chemical attack as well as swelling of the grout. Recent tests carried out on galvanised steel ducts have shown that gas evolution as a result of a chemical reaction between zinc or aluminium and the alkaline grout can result in swelling of the ducting and possible disruption of the concrete surrounding the ducts. Plastic ducting has been in favour in many countries worldwide.
4. Corrosion of the end anchorages
End plates can be susceptible to corrosion as they are exposed to the elements. They can be coated with a resin coating such as weber.tec EP bonding aid to resist formation of anodic sites on the steel.
Solution
Correct grout selection and application
The stress in the mortar shall not exceed that defined in Appendix 21/1 for bridge bearings and Appendix 26/2 for other applications. Where the mortar is required to resist stress before attaining its 28-day strength the compressive strength shall be confirmed by tests on mortar cubes stored under conditions that simulate the field conditions.
