29 March 1996

CONCRETE SILOS TO REALLY LAST

Concrete clamp silos that resist the corrosive effect of silage effluent could become reality if Ulster research

lives up to expectations. Robert Davies reports

CONCRETE silos that resist the corrosive effect of silage effluent are still a pipe dream, but Ulster research could make them a reality.

Peter Frost, head of farm mechanisation research at the Agricultural Research Institute, Hillsborough, Northern Ireland, has high hopes of success with still confidential work on changing the chemistry of cement. This continuing investigation of admixtures that improve the durability of concrete, and completed trials of alternative aggregates, should eventually provide farmers with long life, low maintenance concrete for silo walls and floors.

The Hillsborough work, which began in 1991, is being done in association with Queens Universitys departments of civil engineering and chemistry. It was prompted by the £6m a year silo repair bill faced by Ulsters farmers, and the risk of environmentally catastrophic effluent leaks.

"Effluent is 200 times as polluting as raw domestic sewage, and effluent from silage produced on Northern Ireland farms has a polluting potential equivalent to a population of 5m," Dr Frost says. "It also very acidic, and has a severe corrosive effect on concrete, which is alkaline. The two problems are inseparable."

When the investigation started, X-ray analysis of cores taken from the walls and floors if in-service silos showed that corrosion starts on the surface of the concrete.

Normal concrete in normal silos corrodes at the rate of 0.5 to 1mm a year. This exposes aggregate to give a rough surface, which can damage the feet of cattle. As corrosion continues, aggregate become dislodged from the surface. Small cracks allow effluent to pass deeper into the concrete, and can allow the escape of effluent.

By immersing slabs, cylindrical cores and cubes of concrete in tanks of effluent, Dr Frost can accelerate corrosion. In the test facility, constant exposure to the effluent for 60 days is the equivalent to the corrosion that would occur over 20 years.

Work has shown that the rate of corrosion is linked to the free lime content of the concrete. The aggregate used is very important. When concrete mixes containing four commonly used aggregates were compared, there was nearly 20 times the corrosion in the one containing limestone as in the one made with the most resistant aggregate. Corrosion was measured by weight loss (see table).

"For maximum resistance aggregates should have a calcium content of less than 3%. However, because using a corrosion-resistant aggregate results in a rough surface through corrosion of the cement binder, it may be useful in some circumstances to use an aggregate that corrodes at the same rate as the binder.

"While total corrosion in these circumstances is likely to be increased, the surface concrete will remain smooth. Matching the corrosion rates is very difficult. The effect of aggregate size is small."

The trials revealed that using the correct water to cement ratio is crucial. Water is needed to hydrate the cement. This is a chemical reaction that results in the formation of the solid mass of set concrete. Adding more water makes mixing simpler, and the concrete is easier to work. Adding excess water reduces the strength of concrete. In practice the quantity of water used is a compromise between workability and strength.

During the trial it was found that concrete with a ratio of 0.5 water to 1.0 cement was most resistant, but the mix was difficult to work. Increasing the cement content of the mix, or adding a plasticiser improved workability. Ratios of 0.6:1 and 0.7:1 provided reasonable corrosion resistance.

Increasing cement in the concrete improved impermeability and durability. It also allowed a lower water/cement ration for a given level of workability. Alternatives to ordinary Portland cement were assessed, but none proved cost-effective.

Adding materials to change the properties of cured concrete did improve durability. Greatest benefit came from replacing up to 30% of cement with pozzolanic materials, which react with lime to form a cement. The most effective was fly ash, which cut corrosion by 33%.

Use of protective coatings on concrete to prevent or delay corrosion produced mixed results. Only 11 out of 29 systems showed some potential to protect against attack by effluent. Bitumen coatings and sand and cement renders showed the best cost-effective potential. The 2.5:1 mixture of sand and Portland cement with a water to cement ration of 0.5 to 1, formed a sacrifice layer that corroded away over 10 years.

The trials showed the extending the time between mixing the concrete and first exposure to effluent significantly improves resistance. At the very least the concrete must be given 28 days to cure. Dr Frost also advises the laying of a polythene sheet on silo floors during first season use.


Corrosion after nine weeks immersion in silage effluent

Aggregate type BasaltGravel 1Gravel 2GritstoneLimestone

Weight loss (as % of initial dry weight)

20.74.55.46.388.2

Calcium content 9.52.70.30.748.7


&#8226 Concrete in normal silos corrodes at the rate of 0.5 to 1mm/year.

&#8226 For maximum resistance aggregates should have a calcium content of less than 3%.

&#8226 Correct water to cement ratio (0.6:1 and 0.7:1) is crucial.

Corrosion varies with different concrete mixes. The blend on the left contained more limestone.

By immersing concrete cubes in an effluent bath Peter Frost can accelerate the effects of corrosion.