Articles & Case Studies

Preload Tanks REQUIRES SPECIALIST Investigation and Repair TECHNIQUES

Posted: Wednesday 25th February 2009

Preload tanks, sometimes referred to as pre-stressed or wire-wound tanks, have been widely used in the water industry since the early 1950s, primarily as water reservoirs. The use of high tensile steel tendons, or cables, to impart compressive forces into the structure, conferred the benefits of speed of construction, light weight and low cost.

Instances of pre-stressing wire failure, however, have been fairly common. But it was not until December 1999, when the domed roof of one tank collapsed at Lanner Hill in Cornwall, that a catastrophic structural failure was reported. Since then there have been further sudden and disastrous failures as such structures continue to deteriorate. The failures are presenting significant safety, environmental and financial risks to the water industry and have resulted in an initiative to inspect and repair these tensioned structures as a matter of urgency.

Against this background specialist main contractor, Concrete Repairs Ltd (CRL), has developed specific techniques and site procedures for their safe investigation, repair and refurbishment. CRL Surveys, a division of CRL, has inspected in excess of 30 preload tanks throughout the UK and is involved in a rolling programme of remedial works.

Construction Details

Several design formats were employed for these structures and although current CDM Regulations would require such information to be retained and made available, ‘as-built’ details are scarce and need to be clarified, in order that potentially vulnerable features can be evaluated and evident deterioration/distress put into context. Desk studies of available historical literature have revealed that the Preload System included a menu of design detail options in order to facilitate flexibility.

In summary, this tank system has been likened to a barrel and involves the construction of an in-situ ground slab, with a surrounding ring beam. In-situ cast, segmental walls were then formed onto the ring beam, with a flexible ‘basal joint’ installed so that the walls and floor can move independently. The walls can be vertically pre-stressed (i.e. the stress is applied prior to casting) using either macalloy bars or conventional post-tensioning high-tensile tendons and wedge-anchors. After sequential casting, the walls are wire-wound with the pre-stress applied by forcing a galvanised, 9mm diameter, high-tensile steel wire through a 4.5mm die. The pre-stressed wires are, both during and after installation, encapsulated within layers of sprayed mortar (gunite). Wire-windings can be either a continuous helical from bottom to top, or banded within discrete recesses at various levels up the wall.

Tanks have been built open-topped, or have been fitted with domed roofs. In the case of the latter, the roofs are generally thin ‘shells’, tapering to as little as 75mm thick at the apex and are generally formed from in-situ cast, conventionally-reinforced concrete. The key feature of the roofed tanks, the feature that failed at Lanner Hill, is the critical support provided by a ring beam at the top of the walls of these tanks.

In some cases the ring beams are discrete, cast onto the top of the walls, with an interstitial flexible seal, so that the roof structure and walls can move independently. These ring beams are reinforced, with recesses cast into the external, vertical faces, to accept wire-windings, the critical component for the support of the roof. The roofs are then cast, with connecting reinforcement, into the inside face of the ring beam. In other cases, the ‘ring beam’ is ‘built-in’ into the top of the wall, comprising additional wire-windings, with the roof reinforcement extending down into the top of the wall, inside the windings.

The roof ring beam elements are generally surrounded by upstands to create drainage gullies and in the case of roofed tanks, with ‘built-in’ ring beams to cover the tops of the vertical pre-stressing elements.

Internally, the tanks are commonly, but not always, divided into two halves, with a partition wall enabling maintenance shutdown of one or other of the halves.

Deterioration

Like most structures, Preload tanks exhibit deterioration due to environmental exposure, but they also suffer deterioration as a result of various features in their make-up and also as a consequence of their operation. These include damage as a result of ill-considered retro-fitting of fixtures, fittings and services. Previous remedial works may also suffer deterioration and in the absence of details, careful assessment of a structure’s history will be needed.

The main problems appear to stem from the design and methods of construction and it is useful to note that the website for the American company that now designs the tanks, indicates various design modifications to prevent similar mechanisms of deterioration in the future.

The inner, in-situ cast, segmental walls obviously have construction joints between neighbouring panels. In many cases, perhaps exacerbated by operational cycling of water levels, these joints have leaked from the inside to the outside. Water has consequently seeped behind the gunite and into the wire-windings, resulting in the deterioration of the galvanising and eventual corrosion of the wires. Initial inspections from the outside may not encounter the early stages of deterioration because it occurs on the backs of the wires, or on inner layers of wires and may, therefore, be hidden.

For Preload tanks with discrete ring beams, the gunite thins as it butts up to the soffit of the outer edge of the ring beam. There is also, generally, a movement joint at this location. With time, the gunite delaminates locally and water ingress leads to deterioration of the windings from the top of the wall, downwards. The outer surfaces of the inner walls are generally smooth, ‘as-cast’, which potentially reduces the durability of the bond, especially if the tank water levels rise and fall frequently.

In many cases, the gunite applied to the Preload tanks has also been found to be faulty. Although the designs called for the mortar to be applied during and following installation of the windings so that the wires become completely coated, in many cases there appeared only to be a single coating, applied afterwards.

Methods of assessment

CRL Surveys has inspected more than 30 preload tanks and used a variety of techniques to assess the structures. The suitability of the various methodologies available is dictated by the detail and context of the structure, its future life-requirements and environmental exposure.

The principle investigation techniques used are visual, hammer testing, carbonation and chloride testing, ferroscan surveys, half cell potential surveys, ground probing radar and internal inspections using an ROV. Due to the critical state that some of these tanks are in, it has been important to undertake a thorough risk assessment prior to undertaking any intrusive works on site. The CRL Surveys report identifies the areas of deterioration and provides recommendations for the subsequent remedial works.

Remedial repairs

CRL has so far repaired six preload tanks in the UK and is involved in a rolling programme of inspection and repair.

Due to the sensitive nature of the pre-stressing wires and post-tensioning tendons, particularly where they may be corroded and damaged, hydro-demolition, rather than pneumatic breaking, is used to remove the gunite overlay on the tendons. CRL recommends that existing and deteriorated wires and tendons be de-stressed and preferably removed so that further deterioration and corrosion does not give rise to subsequent failures, or over-stressing of a structure.

De-stressing is generally carried out sequentially. Prior to de-stressing deteriorated wires/tendons, the existing load is reduced by the installation of temporary, post-tensioned tendons. The existing wires are then severed and preferably removed before the exposed concrete surfaces can be prepared, remedial tendons re-installed and jacked to the required loading. Mastic joints are then installed along the junctions between old and new concrete surfaces where relative movements would initiate cracking and future problems.

Conventional concrete patch repairs, in accordance with the BS EN1504, are used to reinstate spalling and cracked areas of concrete. This is followed by the surface preparation and application of a surface coating to enhance the long term durability of the structure.

Further information regarding this interesting remedial work can be obtained from contacting Simon Bladon at sbladon@crlsurveys.co.uk or telephone 020 8288 4848.




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