Taking the stress out of preload
Failing wire-wound tanks across the UK present safety, environmental and financial risks to the industry. John Drewett, director of Concrete Repairs, outlines his company's investigation and remediation service
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 benefits of speed of construction, light weight and low cost were due to the use of high tensile steel tendons or cables to impart compressive forces into the structure.
Instances of pre-stressing wire failures 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 catastrophic failures in the water industry as these structures continue to deteriorate.
These failures are presenting significant safety, environmental and financial risks to the industry, which has resulted in an initiative to inspect and repair these structures as a matter of urgency. Against this background, Concrete Repairs Limited (CRL) has developed techniques and site procedures for the safe investigation, repair and refurbishment of these tensioned structures. The surveys division of CRL has inspected 32 preload tanks in the UK and the company is involved in a rolling programme of remedial works to the tanks.
Several design formats have been employed and, although Construction, Design & Management (CDM) Regulations would require design details to be retained and made available, as-built details are scarce and need to be clarified. This is so potentially vulnerable features can be evaluated and evident deterioration and 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 are then formed on to 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; 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 – which taper to as thin as 75mm 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 on to 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 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.
Preload tanks exhibit deterioration, like most structures, due to environmental exposure. However they also suffer deterioration as a result of various features in their make-up and also as a consequence of their operation, including damage as a result of ill-considered retro-fitting of fixtures, fittings and services.
Previous remedial works may also be suffering deterioration and in the absence of details careful assessment of a structure’s history will be needed. With preload tanks, the main problems seem to stem from the design and methods of construction (it is useful to note that the website for a company that now designs the tanks in the USA 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 inside to 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, delamination of the gunite, 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, potentially reducing 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 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.
In CRL Surveys’ inspections of 32 preload tanks, it used a variety of techniques to assess the structures. The suitability of the various methodologies available will be dictated by the detail and context of the structure, its future life-requirements and environmental exposure.
The principle techniques are visual, hammer testing, carbonation and chloride testing, ferroscan surveys, half cell potential surveys, ground-probing radar and internal inspections using a remotely operated vehicle.
Due to the critical state that some of these tanks are in, it is 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.
CRL has 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 done sequentially. Prior to de-stressing deteriorated wires or tendons, the load on the existing is reduced by the installation of temporary, post-tensioned tendons. The existing wires are severed and preferably removed, with remedial tendons then installed and jacked to the required loading.
Once the original tendons have been removed, the exposed concrete surfaces can be prepared and remedial tendons installed and jacked to the required loading. Mastic joints are installed along the junctions between old and new concrete surfaces where relative movements would initiate cracking and problems in the future.
Conventional concrete patch repairs in accordance with the European Standard 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.
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