29 March 1996

Open day on maintenance

THE application of results of Peter Frosts research on concrete to silo maintenance and repair was featured at a recent Hillsborough open day.

His advice was that power hosing is essential to expose problems like small cracks, and that thorough cleaning is a pre-requisite of effective repairs.

"Remember the corrosive nature of effluent means that small cracks will soon be big ones, and any repair is only as good as the preparation," Dr Frost said. "Cracks should be chased using an angle grinder, or concrete saw, to expose undamaged concrete. The prepared crack should then be primed and filled with a flexible joint sealant. For a successful repair it is important to follow the sealant makers instructions."

Most crack repair sealants have been used at Hillsborough, but no one perfect repair system has emerged. Hot bitumen has been used to successfully repair floor cracks at a reasonable price. When a crack appears where the walls and floor of a silo meet, corroded concrete is cut out and the gap is filled using a sealant. The wall-floor joint is then sealed with a triangular fillet. This is made using a 2:1 mixture of sand and cement.

To ensure good bonding the surfaces are painted with a slurry of cement before the fillet is constructed.

Dr Frost urged farmers to regularly check the sealant in joints installed to allow for expansion and contraction of the concrete.

He said that where concrete was not cracked, there was no risk of structural failure through corrosion of steel reinforcement and surface roughness was not a problem, there was no need for repair. Where repair was necessary cost-effective use could be made of a 3:1 render of sand and cement.

It was important to prevent evaporation of water from the mix after it was placed. Keeping the newly rendered wall wet for three or four days was one way of stopping evaporation. A 20mm (0.75in) deep layer of the same render had been used to successfully repair silo floors, but the work was time consuming, and the repair must cure for 28 days before use.

"Hot rolled asphalt has also been used successfully at the institute, though lack of mechanical strength can be a problem where ring feeders are used," he says. &#42