Realkalisation of a post-tensioned structure
Last year 1500 m² of alumina silo no. 5 at Hydro Aluminium, Karmoy was re-alkalised by Aaronite A/S, Stavanger. The silo was build in 1982, and has 6 vertical columns containing the anchors for 142 tensioned cables. The external diameter of the silo is 37.2 m, with a height of 22.3 m.
An in-service inspection in 1993, revealed large areas of poorly compacted and carbonated concrete together with low cover thicknesses. The consultant recommended a combination of RA and traditional repair to reduce the danger of the reinforcement being bent out of its seating plane due to lack of sideways retention. However,RA causes hydrogen evolution and thereby entails a certain danger of hydrogen embrittlement (HE) with a consequential danger of failure of one or more cables.
The risk of HE depends on steel quality, whether pre- or post tensioned, tensioning degree, geometry, and type of sheath.
Risk evaluation
The steel quality was judged to have a low susceptibility to HE.
The cables are post-tensioned in metal sheaths and grouted with a cement based mixture. Cables and sheaths are considered to be single conductors having large internal voids. Since it is not possible to charge the inner part of a conductor with respect to its surface, the hydrogen evolution will take place on the sheath surface and not on the cable surface.
Due to thicker cover of the steel tendons, the main part of the current will flow from the ordinary rebars to the anode.
All in all, the risk of HE was considered to be low. To reduce the risk of failure further, the cables were unloaded by emptying the silo prior to RA.
Test
Prior to RA, a test program was set up to check and confirm the theoretical assumptions, and to evaluate the efficiency of the treatment. The test included phenolphthalein testing, sodium analyses, continuity measurements, current and voltage measurements and monitoring of the potential at the sheath surface.
The test area was approx. 10 m² and included both wall and column, and covered three cables. There was complete continuity between all rebar connection points.
The anode, of ordinary reinforcement mesh, was covered with cellulose fibre and 0.7 molar sodium carbonate solution. RA was continued for approx. 6 days.
Results
The carbonated zone in the test area was re-alkalised completely. There was a significant increase in the sodium content round the rebars ensuring a durable corrosion protection, while none at the sheath surface.
As assumed prior to RA, there was no hydrogen evolution on the cables, as no voltage difference was found between sheath and cable.
The conclusion of the test was that there is little danger of HE on the tension cables in this structure, since the sheath protects the cable against hydrogen evolution.
A full scale RA was then carried out on 1500 m², documented by the phenolphthalein test alone.
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