Wastewater treatment infrastructure - structural asset surveys

The proposed introduction of private-sector finance (PFI) into the Scottish water industry, by means of 25-year concession agreements with commercial operators, prompted a new look at condition and whole life costing of the many and substantial concrete structures in use.

Current condition surveys, no matter how thorough, and even when supported by extensive testing, are alone insufficient for such long-term projections; appraisal must be based on an understanding of pathological processes in concrete, backed by a practical knowledge of available repair and maintenance techniques and their feasibility for a given structure.

Initial circumferential shrinkage of this cylindrical tank is now evident as stained cracks at reinforcement locations.

Andrew Sadleir was the specialist adviser on a team inspecting six wastewater treatment works, and principal author of the comprehensive report for the Client, a PFI consortium, led by a major UK/European contractor.

The detailed condition report also included costed remedial works and maintenance specifications.

Insufficient cover to reinforcement warns of possible widespread future problems in this tank wall, while adjacent poorly-executed column repairs are already failing.

Sadleir's experience in concrete repair contracting and materials consultancy informed the pragmatic approach adopted in the study, for which detailed results were delivered against a tight commercial deadline.

In this case, a reasonably good - and no doubt expensive - repair has been applied to a defective substrate, which exhibits classic signs of frost damage and poor quality mix.

The Operator's preliminary classification of structural condition was revised, taking into account the realistic ‘repairability’ of each structure.

Attention could thus be focussed on specifying remedial solutions for the high-value structures, while proposing low-cost interim measures for those clearly in need of wholesale renewal.