6.10 Commonly used components and materials for corrosion protection barriers
6.10.1 Plastic sheaths and ducts
Plastic sheaths and ducts shall conform with relevant European product standards and in particular are required to be continuous, impermeable to water, resistant to age brittleness and resistant to ultra-violet radiation damage during storage, transportation and installation. Joints between plastic components shall be fully sealed against ingress of water by direct contact or by sealants. Where used, PVC shall be resistant to ageing and shall not produce free chlorides.
The minimum wall thickness of an external corrugated duct common to one or several tendons shall be:
- 1,0 mm for internal diameter < 80 mm;
- 1,5 mm for internal diameter > 80 mm but < 120 mm;
- 2,0 mm for internal diameter > 120 mm.
The minimum wall thickness of an external smooth common sheath or duct shall be 1 mm greater than that required for the corrugated ducts or it shall be reinforced.
The minimum wall thickness of an internal sheath shall be 1,0 mm and an internal corrugated duct shall be 0,8 mm.
NOTE Where two plastic barriers are provided some protection to the inner barrier is provided during installation by the outer barrier.
Where used for load transfer, plastic ducts shall be deformed or corrugated. The amplitude and pitch of the deformations or corrugations shall be related to the wall thickness and shall be able to transfer load in a manner not susceptible to creep losses.
| Verification of protection offered a) All corrosion protection systems shall have been subjected to test(s) to verify the competence of the system. The results of all tests shall be well documented; b) the Client's Technical Representative will carry out a technical assessment of the results of the corrosion protection system tests in order to verify that the protection offered by each barrier in the system is achieved. It should be noted that in certain systems the integrity of the inner protective barrier itself depends on the maintenance of the integrity of the outer barrier; c) where only a single protective barrier is provided in the tendon bond length the integrity of this barrier may be checked by an insitu test such as an electrical resistivity test. |
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| 1. Tendon bond length The encapsulation may consist of one of the following: a) a single corrugated plastic duct containing the tendon(s) and cement grout;b) two concentric corrugated plastic ducts containing the tendon(s), fully pregrouted {with cement or resin) within the core and the annulus between the ducts prior to installation; c) a single corrugated plastic duct containing a bar tendon or tendons and pregrouted with cement grout. A minimum cover of 5 mm is provided between the duct and bar. The bar tendon(s) have a continuous ribbed outer surface. The crack width of the cement grout between the duct and the bar does not exceed 0,1 mm under service loading; d) a single steel or corrugated plastic tube-a-manchette duct not less than 3 mm thick, surrounded by a minimum of 20 mm grout cover injected under a pressure of not less than 500 kPa at intervals along the tube-a-manchette no greater than 1 metre. A minimum cover of 5 mm is provided between the duct and the tendons. The crack width of this cement grout does not exceed 0,2 mm under service loading; e) a single corrugated steel duct (compression tube) closely surrounding a greased steel tendon. The duct and plastic cap at the restraining nut are protected by the surrounding cement grout having a thickness of not less than 10mm, and where the crack widths do not exceed 0,1 mm under service loading. |
Protective barriers Offered insitu a) one plastic duct b) two plastic ducts c) internal cement grout and surrounding plastic duct; d) internal cement grout and surrounding steel or plastic duct; e) steel duct and surrounding cement grout |
| 2. Tendon free length The protection system allows free movement of the tendon within the borehole. This may be achieved by one of the following: a) a plastic sheath to individual tendon(s) filled completely with flexible corrosion protection compound plus the inclusion of A, B, C or D below; b) a plastic sheath to individual tendon(s) filled completely with cement grout plus A or B below; c) a common plastic sheath for multiple tendon(s) filled completely with cement grout plus B. a) Common plastic sheath or duct filled with flexible corrosion protection compound; b) common plastic sheath or duct sealed at the ends against ingress of water; c) common plastic sheath or duct filled with cement grout; d) common steel duct filled with dense cement grout. A lubricant or bond free contact is present within either the individual sheaths or the common sheath to ensure free movement of the tendon(s) during stressing. |
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| 3. Transition between anchor head and free length A coated, grouted or cast-in metal sleeve or fixed plastic duct is sealed or welded to the anchor head. It is sealed to the free length sheath or duct and filled with corrosion protection compound, cement or resin. |
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| 4. Anchor head A coated and/or galvanised metal cap with a minimum 3 mm wall thickness or a rigid plastic cap with a minimum 5 mm wall thickness is connected to the bearing plate and if removable it is filled with a flexible corrosion protection compound and sealed with a gasket. If nonremovable it may be filled with cement or resin. |
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Where a corrugated duct, used for grout injection under pressure, is considered as a protective barrier then it should be demonstrated that the grout ports do not allow the ingress of water after grout injection.
The duct shall be not less than 3 mm thick and the pitch and amplitude of the corrugations shall be suitable for load transfer, proven by a system test (see 6.12).
The integrity of the protective barrier should also be demonstrated in the stressed state (see 6.12).
Where a single plastic duct forms the sole protective barrier of a permanent anchor an in situ test shall be carried out to verify the integrity of the plastic duct throughout the length of the anchor. This may be by electrical resistivity test and carried out after borehole grouting and after stressing to establish the complete isolation of the steel tendon from the ground. Details of an acceptable testing method are given in annex A to this standard.
