Farm Waste Structures: Guidance on Construction, Repair and Maintenance

The use of hot rolled asphalt (hra) surfacing for agricultural forage silos

Note No: CGN 012, December 2002
Prepared by ADAS National Building Design Team. Funded by The Department for Environment, Food & Rural Affairs.



Figure 1. Hot rolled asphalt paver laying surface.

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Silo bases need to be impermeable and hard wearing to resist degradation by acidic effluent (<pH 4.5) and farm machinery. Self-feed systems also need to resist abrasion from cattle hooves and the effects of dung and urine. Traditionally, high strength reinforced concrete (C35A) has been used as the main material for constructing floors but HRA can provide a flexible, durable surface. Hot Rolled Asphalt (HRA) is a dense bituminous mix, formed by the combination of high viscosity bitumen, filler and fine aggregate, to which coarse aggregate is added as bulk. HRA should not be confused with other bitumen-bound surfaces like coated macadams, which are generally more permeable. Appropriately designed mixes of HRA can provide the impermeability required by the Control of Pollution Regulations 1991. The use of HRA in forage silos is to be included in BS 5502.

  • Requires no formwork and is usable within 24 hours, allowing rapid use.
  • Can be laid in a continuous process with little shrinkage, minimising the need for joint formation.
  • Resistant to acid attack.
  • Not susceptible to frost.
  • Reduced cost per m2 in comparison to most other construction materials.

New Forage Silo Construction

A new floor constructed with HRA surfacing will comprise a series of layers (Figure 2) formed over the sub-grade (natural ground). The thickness and type of these layers depends on the stability of the sub-grade. If site conditions are satisfactory and traffic is limited (self-feed systems) then the construction could comprise a sub-base (compacted crushed stone), a binder course of dense bitumen macadam or HRA and a top surface course of HRA. Where vehicles regularly use the floor, a base of dense bitumen macadam will also be required.

Figure 2 Suggested structural layers in a new HRA silo floor

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Forage Silo Resurfacing and Repair

Defective concrete or macadam silos can be resurfaced using HRA. A detailed survey of the existing floor slab should be carried out to confirm that a new surface could be laid without causing any structural damage. Once the base is approved as structurally sound the old floor should be prepared in the following manner:

  1. Power wash floor to create a clean surface.
  2. Thoroughly clean all existing cracks or open joints and fill with a sealant.
  3. Repair potholes and failed edges.
  4. Form chases in floor at the front of clamp to key the new surface (figure 3).
  5. Apply tack coat.
  6. Lay and compact HRA surface course and caulk joints with sealants or waterbars.

(NOTE. Any joints or cracks in an existing concrete floor will eventually give rise to sympathetic reflective cracking in asphalt overlays. Bridging the cracks with geotextiles may reduce the rate of development of this cracking).

Figure 3 Detail of chase (with effluent channel).

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HRA mixes consist of coarse aggregate, fine aggregate, filler dust and bitumen (50 pen) and their composition is determined either from traditional recipes or by design, according to the tables in BS594: Part 1. It is likely that mixes with a coarse aggregate content in the range of 30% to 55%, but with an enriched bitumen content 0.7% to 1.3% higher than given in BS594: Part 1 will prove the most durable. (Note: those mixes with a coarse aggregate contents of 30% or 35% should provide the best impermeability characteristics). In self-feed silos a 30% or 35% mix will provide a less abrasive surface to animal feet. A 55% mix may be more permeable but better able to resist heavy point loads e.g. from vehicles. Aggregates should be sourced from hard stone (e.g. basalt, dolerite and gritstone) quarries. Limestone aggregates should be avoided because they are prone to degradation by the acidic silage effluent.Workmanship

Good workmanship is essential. The Quarry Products Association (QPA) can provide an information sheet giving guidance on choosing a surfacing contractor. On large paved areas, HRA is usually fed into a combined hopper and spreader, known as a paver to control the application depth. The material is then rolled to the necessary finished thickness. In some cases limitations in silo dimensions will not permit the use of a paver and the only option is to hand lay. In this method of application quality control must be much more rigorous, to ensure that the correct thickness of HRA is spread, prior to rolling, and reference should be made to BS594: Part 2 for guidance. All joints must be fully compacted and sealed to prevent effluent seepage. Pay attention to any joints between the asphalt and retaining walls to ensure an adequate seal. It is normal practice to offset joints in the different layers (surface course/binder course/base) of a fully flexible construction by at least 300mm.Operational Maintenance and Repairs

Newly laid HRA should not be trafficked until it has cooled to ambient temperatures - normally about 24 hours. Certain machinery oils soften HRA and sustained point loadings can damage the surface. HRA surfaces should not be used as permanent parking or for pallet/feed trough storage areas. Regular maintenance should be undertaken. For example dung and mud should be regularly removed as these can disrupt the HRA surface.

Where small areas of the surface are damaged propriety bituminous products are available. These should be checked for their impermeability characteristics. They must be used strictly in accordance with the manufacturers instructions and be inert after curing. For major repairs consult an experienced contractor.References

  1. The Control of Pollution (Silage, Slurry and Agricultural Fuel Oil) Regulations 1991 (as amended).
  2. British Standard 5502 Part 21: Code of practice for selection and use of construction materials.
  3. British Standard 594: Parts 1 & 2: Hot rolled asphalt for roads and other paved areas.
  4. Quarry Product Association Asphalt Application notes.
  5. Code of Good Agricultural Practice for the Protection of Water (DEFRA).
  6. CIRIA Report 126 Farm waste storage: guidelines for construction.

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HRA floors need proper watertight joints, especially between the walls and perimeter channels. With the correct specification, good site preparation and thorough detailing of joints HRA floors can perform effectively for the 20 year design life required - given the appropriate maintenance and care.
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