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with hammer testing to detect areas ofdisbonding, delamination, and voids.Hammer testing involves tapping the

surface with a hammer or drawing itgently across the surface, then assessingthe resultant sound(s) to get an indica-

tion of hidden defects.Depths of carbonation are measured

in-situ, using a solution of phenolph-thalein indicator in ethyl alcohol,sprayed onto freshly broken surfaces,

re-stressed tanks, sometimesreferred to as preload or wirewound (concrete) tanks, have

been widely used in the UK in the waterindustry since the early 1950s, primar-ily as reservoirs. The use of high tensilesteel tendons, or cables, to impart com-pressive forces into the structure con-fers the benefits of speed of construc-tion, light weight, and low cost.Instances of pre-stressing wire failure,however, have been fairly common. InDecember 1999, the domed roof of onetank collapsed at Lanner Hill inCornwall, UK, causing a catastrophicstructural failure. Since then, therehave been other sudden failures as pre-load structures continue to deteriorate,presenting significant safety, environ-mental, and financial risks to their own-ers.Against this background a specialist

general contractor has developed spe-cific techniques and site procedures forsafe investigation, repair, and refurbish-ment of these structures. This articlebriefly describes the techniques

InvestigationThe principal investigation techniquesused are the desk study of available “as-built” information and maintenancerecords, with visual inspection andhammer testing, assessments of carbon-ation, screening for chloride, cover sur-veys, half cell potential surveys, specialmetal detection surveys and/or groundprobing radar surveys, and internalinspections using a remotely operatedvehicle (ROV).Influenced by knowledge of the spe-

cific structure, the initial visual inspec-tion for deterioration, including crack-ing of the concrete and overlays, andcorrosion of the wires, is complemented

Repair Techniques for Prestressed Concrete TanksSimon Bladon, CRL Surveys, Mitcham, UK

Continued

P

Top: Inspecting the tankBottom: Final protective coating applied

All photos courtesy of the author

J P C L D e c e m b e r 2 0 0 946

try’s standard guidance documents.More detailed information about the

concrete sub-structure can be obtainedfrom a survey made with a proprietaryportable unit, essentially a metal detec-tor. The unit is a detecting, measuring

and mapping device thatprovides an instant image ofthe arrangement and posi-tion of reinforcement withinthe survey area and thedepth of concrete cover. Formore deeply hidden details,a ground probing radar sur-vey is used. Ground prob-ing radar also gives informa-tion about the location anddetail of the reinforcement,but, in addition, it can locatedeeper pre-stressing ten-dons, major constructionfeatures, and the presenceof voids or cracks.

Through an ROV fitted with a videolink, the concrete surfaces on the insideof the tank can be assessed, without theneed for taking the tank out of service,emptying it, and entering it.Using the methods above, one can

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which appear pink whennot carbonated (pH >9) andcolorless when the concretehas lower pH (carbonated).Screening for chloride is

carried out by chemicalanalysis, generally in a labo-ratory, on samples of theconcrete.Both the depths of car-

bonation and the chlorideion contents give a measureof the presence or ingress ofthe most common agents in,or causes of, the corrosionof encapsulated steel.The measurement of half-

cell potentials can detect corrosion,although care must be taken if galva-nized wires have been used. The equip-ment and methodology, originallydescribed in ASTM C876, has now beenincorporated within many of the indus-

Fig. 1: Hydrodemolition to remove gunite overlay on tank walls. Inset: close-up ofexposed wires.

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identify areas of deterioration and makerecommendations for remedial work.

Repair TechniquesBecause the pre-stressing wires andpost-tensioning tendons are sensitive,particularly where they may be corrod-ed and damaged, hydro-demolition,rather than pneumatic breaking, is usedto remove the gunite overlay on the ten-dons (Fig. 1).“De-stressing” is generally carried out

sequentially. Before deterioratedwires/tendons are de-stressed, theexisting load is reduced by the installa-tion of temporary, post-tensioned ten-

dons (Figs. 2 and 3). The existing wiresare then severed and preferablyremoved before the exposed concretesurfaces are repaired, and remedial ten-dons are installed and jacked to therequired loading (Fig. 4).Mastic joints are then installed along

the junctions between old and new con-crete surfaces where relative move-

ments could initiate cracking and causefuture problems. Conventional concretepatching materials, in accordance withEN 1504, are used to repair spalling andcracked areas of concrete. Patching isfollowed by the surface preparation andapplication of a surface coating toenhance the long-term durability of thestructure.

Fig. 4: Conventional concrete repairs andinstallation of remedial post tensioning tendons

Fig. 3: Temporary tendons installedaround the dome band

Fig. 2: Installation of temporary tendons will befollowed by de-stressing and removal of existing

failed wires.

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