RCCP Paves Way for Compost Facility

Composting yard waste on a large scale is a growing trend in the management of municipal waste. This trend is the result of increased interest in recycling, as well as, U.S State and Canadian Province bans on disposal of yard waste in municipal landfills to extend the life of these landfills.

Existing composting areas have been paved or unpaved. The benefits of paving the composting area are numerous. Paved surfaces improve the operation of municipal- and commercial-size compost facilities. Paving allows for all-weather access to the facility. Operations such as tipping, sorting, shredding and compost windrow turning are conducted more efficiently on a paved surface. Maintenance costs of material handling equipment can be lower, since the equipment operates on pavement rather than in mud. Paved compost areas produce a more valuable compost by preventing contamination of the product with gravel and other inert debris. Paving allows drainage control and prevents infiltration of compost leachate into groundwater. Finally, paved facilities improve community relations by providing a neater appearance, reduced dust production and tracking of debris onto neighboring roads.

Once the decision is made to pave a facility, it is time to select the paving material. Roller-compacted concrete (RCC) is often the best choice.

Pavements made of RCC had their start in the Canadian forestry industry. Logs are sorted in huge yards. An inexpensive industrial-grade pavement is needed for these yards. A pavement that can withstand high wheel loads and severe abrasive forces even in a rugged freeze/thaw climate.

Roller-compacted concrete is a close relative to the concrete we're all familiar with. The biggest difference is the way it is placed. Rather than being cast into forms, RCC is consolidated and finished using rollers. This method of placement is less expensive since form work is not needed. To accomplish this, RCC is a concrete mixed with less water. A low water cement ratio produces a stronger concrete. It is possible to achieve the same compressive strength as conventional concrete with a lower cement content further reducing cost.

The City of Vancouver's decision to pave its composting facility is a good example of design and construction of an RCC-paved facility. The City had established a recycling goal with a key provision involving increased composting of yard waste. To meet the new recycling goal, the City would need a much larger and more efficient composting facility.

The City had been operating a composting facility for leaves only. About 5,300 metric tons of leaves were taken from city streets or dropped off from residences at the facility. No processing of the leaves was done and the composting leaves were piled on a 600 mm layer of woodchips and bark. While the woodchip and bark base was very inexpensive, (the city received it free), the base did not provided all-weather access to the composting piles for tipping and compost turning nor could it support efficient processing operations (i.e. shredding, grinding) for other kinds of yard waste. Further, woodchips and bark often became mixed in with the composting leaves contaminating the compost product.

The City planned to accept a greater variety of yard waste—branches and grass clippings in addition to leaves would now be accepted. These additional classes of waste would increase the volume of material to be processed and composted to an estimated 22,000 metric tons.

The design of the paved area included areas for processing and composting. Together these areas measured 180 meters by 100 meters (1.8 hectares or 4.4 acres). From the beginning, the planners selected concrete as the paving material for the processing area. They knew that the concrete would withstand abuse from truck, front-end loaders, shredders and other processing equipment.

For the composting area the planners considered both concrete and asphalt. They also considered RCC. Their final decision would be based on consideration of both first cost and long-term maintenance costs.

The City solicited bids for the pavement in both RCC and asphalt. This illustration shows the structurally equivalent pavement sections. Notice the much greater thickness of sub-base necessary to support equivalent loads with an asphalt pavement.

The capital cost for paving the composting area with RCC was $41,000 more than with asphalt. However, when the designers calculated the maintenance costs for the 10-year design life of the facility, they found that the asphalt pavement would require $90,000 worth of maintenance compared to $0 for RCC. In terms of net present value, the RCC pavement was $10,000 less expensive than asphalt. The installed cost of RCC was $31.32 per square meter.

Based on this analysis, the City selected RCC as the material to pave both the processing and composting areas.

The installation of the RCCP involved:

1. Mixing the concrete in a mobile plant erected near the site
2. Transporting the mixed concrete to the paving area by dump truck
3. Placement of the concrete mixture using an asphalt paver
4. Compaction and finishing of the placed concrete using vibrating and non-vibrating rollers
5. Wet curing

Paving with RCC resulted in an accessible, cost effective, productive, and attractive composting facility.

Read more on RCC for Compost Facilities.

 
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