Chemical production facility - surveys of concrete process structures

In June 1999, Andrew Sadleir was commissioned to carry out condition surveys on two reinforced concrete structures at a major UK production facility of an international chemicals manufacturing company.

On this project, SADLEIR technical consultancy acted as specialist consultant to the Client's consulting engineers, Cardiff-based C.D.Gray & Associates.

Each of the structures supports a function critical to a manufacturing process chain, affecting a large part of the plant's operation. In such situations, advance warning of structural repair or replacement is essential to the avoidance of unplanned, and expensive, production downtime.

Both structures evidenced extensive areas where steel reinforcement had been given an insufficient depth of concrete cover, at the time of construction. While this would explain much of the visible damage to the concrete, further quantitative in situ NDT and laboratory testing were carried out, with a view to identifying all actual or incipient pathological processes which might be posing a threat to the structural concrete.

Concrete spalling in this beam may well be attributable to corrosion of shallow reinforcement, but other processes are causing the problems in the column beneath. Note also the fractures formed near the column base.

In the case of one structure, approaching 60 years old, previous appraisals had been mainly confined to consideration of the mechanical aspects of the visible degradation. The damage had been largely attributed to obviously deficient reinforcement cover; although certain aspects observed during Sadleir's subsequent inspections were not wholly consistent with simple reinforcement corrosion.

Expansive disruption of the concrete, causing sections outside the reinforcing cage to burst off. Fracture planes all align with the predominantly axial service stresses.

Enquiry revealed a former use entailing prolonged and continuous exposure to sea water, the mode of exposure being itself particularly onerous. Suspicions thus aroused, and strongly reinforced by initial visual inspection, were eventually confirmed by chemical and petrographic analyses. Crucial structural elements were found to be heavily contaminated with marine salts, with resultant degradation of the concrete by several processes, principally sulphate attack.

While basic conclusions became evident at an early stage in the investigation, on the basis of visual observation and known history, any such conclusions - given their economic implications - must be supported by testing and analysis. Sadleir's report provided a definitive statement of the structure's condition, concluding with a recommendation to plan for its early replacement.

Given the overriding limitations on service life, recommended ‘repairs’ were therefore confined to measures addressing short-term structural utility; and periodic re-inspections are focussed on those areas identified as critical.

Several aspects of poor construction workmanship are exemplified at this construction joint, which serves as a starting point for structural deterioration.

The second structure appeared, at first sight, to be suffering from no more than localised damage resulting from poor construction workmanship. The potentially aggressive local atmospheric environment, however, underlined the need for checking chloride levels in the concrete. This was backed up with half-cell electropotential survey to selected areas. In the event, moderately elevated chloride levels were discovered, sufficient to cause concern as to long-term serviceability, but still susceptible of treatment.

Subsequent re-testing has been undertaken to assess development of electro-potential around the reinforcement.

In cases where Cl^- ion concentrations are excessive, identification of the problem before its effects become visible (by which time the damage may be considered severe), can make the difference between a viable treatment and repair programme and abandoning the structure to a curtailed service life.

Lack of care in positioning the reinforcement cage has obvious consequences. Note also the earlier attempt at repair.

In this instance, the adoption of relatively-inexpensive standard test procedures, their interpreted results complementing a thorough inspection of the structure and its environment, were essential to the identification of the most important factor affecting the structure's continued serviceability; and at a sufficiently early stage for preventive measures to be still viable. By comparison, the visible and localised damage was of relatively low significance to the structure's overall durability.

It is always necessary to look beyond the obvious.

SADLEIR technical consultancy has since been appointed to specify and supervise further testing, remedial, and preventive maintenance works to this structure.