Concrete solutions

Ammonium nitrate fertiliser, produced by Terra Nitrogen at its Severnside Works, is highly aggressive to reinforced concrete. Consequently the concrete prill tower that forms the core of the production plant has deteriorated rapidly, despite regular maintenance and repair.

The Severnside plant of Terra Nitrogen (UK) Limited produces up to 500 000 tonnes of ammonium nitrate per year, which the company markets as NITRAM fertiliser. It is a highly competitive market.

The final stage of the production process is known as prilling. This involves converting liquid ammonium nitrate into granules by feeding it through spray heads at the top of a tall tower. As the substance falls, it solidifies to form small pellets, or 'prills'.

The prill tower at Severnside was constructed in 1964 and consists of a 15.6m diameter reinforced concrete shell, 178mm thick and 86m high, on concrete piled foundations. A reinforced concrete plant room, supported on the dropshaft, liftshaft and single column, is attached to the tower between the 75m and 100m levels.

Ammonium nitrate may be a valuable commodity as a fertiliser, but as a solution, dust or vapour it presents a long-term aggressive environment for reinforced concrete. It attacks both the concrete and steel, reducing the ability of the reinforced concrete sections to redistribute loads.

The primary deterioration mechanism is the reaction of ammonium nitrate with calcium hydroxide in the cement paste. This leads to a reduction in the calcium hydroxide content and an increase in porosity and decrease in alkalinity. The calcium nitrate resulting from this then reacts with hydrated calcium aluminate, present in cement, to form calcium nitro-aluminate, which has a higher volume.

This reaction leads to an expansion of the weak matrix and subsequent bursting of contaminated layers. Ammonium nitrate also promotes stress corrosion of steel reinforcement. Coupled with a tensile stress, this causes brittleness and results in direct tensile failure of the reinforcement.

Ammonium nitrate attack is generally first seen as a removal of surface dust (known as laitance) followed by a loss of aggregate in the weak cement matrix that eventually exposes the reinforcing bars. Where wetting and drying exist, the laitance may not be removed and the apparently sound concrete surface may simply burst. The degradation increases with ammonium nitrate concentration. Just 0.5 per cent of ammonium nitrate by weight of cement appears sufficient to cause considerable damage.

Protective measures were taken during the original design of the tower. The main dropshaft walls were lined with aluminium foil, and other vulnerable surfaces were coated with epoxy or tiling. However, these have not proved sufficient, and with a 15-year design life needed to meet nitrate demand, Terra and its technical advisors at ABB Eutech outlined a strategy to prolong the tower's life.

ABB Eutech formulated an outline repair strategy based on previous inspection and repair works. The project management role was entrusted to design and engineering consultancy Taywood Engineering, which has a 25-year history of combining practical experience with innovation, as Matt Gascoigne, project manager, explains.

'We have a long history of working on nuclear sites,' he says, 'so we're used to planning around continuous plant operation and restricted access.'

Investigations at Terra Severnside indicated ammonium nitrate concentrations up to 3.8 per cent within the tower dropshaft wall.

Results from the prill tower investigations showed localised evidence of brittleness consistent with stress corrosion. Taywood's initial investigations confirmed that nitrate attack was affecting the internal and external surfaces of the dropshaft, and found evidence of stress corrosion cracking in local areas of the plant room. This was a potential cause for concern, as the structure supports on-line equipment weighing approximately 60 tonnes.

As a precaution, a temporary steelwork support system for the section of plant room adjacent to the cracking was installed and areas where loose concrete may present a hazard were removed and repaired as necessary.

Finite element analysis revealed that the lift shaft and support column on the west side of the plant room were more flexible than the dropshaft. This had encouraged the plant room to deflect down and away from the dropshaft under certain loading conditions, concentrating stress in the plant room's floor support beams and in the walls of two access walkways.

The high stresses had caused the initial cracking of the reinforced concrete section observed in an earlier inspection. Further investigation at the crack itself revealed that stress corrosion cracking had resulted in failure of the steel reinforcement. The structural survey also revealed significant areas of 'soft' concrete. Some of these were behind patch repairs, indicating that nitrate attack was continuing.

On the basis of the FE results, Taywood recommended some initial repairs. These included the replacement of the deteriorating access walkways between the prill tower and the plant room, and the removal and replacement of a small section of damaged concrete on the dropshaft wall.

With the essential repairs completed, Taywood formulated a full repair scheme and budget estimate. Spanning three years, the maintenance programme was developed to allow Terra to make more accurate financial forecasts for the future production of the plant. The client accepted the maintenance programme in 1998 and Taywood was appointed to carry out the design and construction management, including the role of planning supervisor.

Earlier monitoring had identified significant movement where the plant room connects to the tower roof level, so Taywood installed a steelwork restraint to prevent the roof collapsing completely while the structure was strengthened.

As a further safety precaution the condition of the structure is monitored constantly. The instrumentation measures parameters such as wind speed and direction, tilt and crack displacement and provides real-time information on any movement during the repair works.

Main contractors Bierrum have completed most of the construction phase of the main repair works. The work consists of replacing damaged areas of beams, slabs and walls within the plant room. Props are used where necessary to support beams, plant and pipework during breakout.

Logisitics are critical here, and have had to accommodate the access and time restrictions imposed by the requirement for continuous production. Safety planning has also been a major factor: for example, some repairs have involved personnel working adjacent to pipework operating at temperatures in excess of 100°C.

Taywood continues to use the finite element model to ensure that the stability of the structure will be maintained throughout all phases of repair. Ammonium nitrate tests are performed on concrete dust samples at each repair site to determine the level of chemical infiltration and ascertain the extent of the repairs required. The concrete is broken out and replaced until an acceptable concentration of ammonium nitrate is recorded.

The main repair works at Terra are scheduled for completion at the end of 2001. By then it will have taken two years to install 15 tonnes of steelwork and replace 120m3 of concrete. It has been as slow and meticulous in execution as in the planning, but there have been no interruptions to production during the rehabilitation project.