Short answer
The economics of on-site recycling do not come from one cost line but four: haulage out, tipping fees, material bought in for backfill, and the haulage of that material to site. Processing rubble on site removes all four at once, which is why the calculation looks far weaker when compared against "crushing cost" alone. The constraint is quality: if where the material produced on site will be used has not been decided in advance, a pile that has been crushed but never used is not a gain.
Two-way haulage: the half of the sum that gets skipped
When demolition rubble leaves site, the truck goes out full and returns empty. Then when backfill material arrives, the truck comes in full and leaves empty. So the same volume is paid for across two separate sets of movements.
Processing on site removes both sets at once. When the sum is done, usually only the first (rubble leaving) is considered; the second is invisible because it is paid from a different budget line.
Add tipping fees and the cost of the material bought in, and four cost lines disappear together. The machine and time cost of crushing should be compared against the sum of all four.
The order for building the sum
- Estimate the volume of rubble leaving site and the backfill requirement separately.
- Work out load counts and cost per load for both.
- Add tipping fees and the cost of material to be purchased.
- On the other side, write down the machine hours, attachment consumption and labour cost of the crushing operation.
- Also put the cost of the space needed for storage and handling on that other side; in a city this is not a small item.
The fifth line in a city
On urban projects the number of truck movements is tied directly to permits and traffic restrictions. Reducing loads saves not just money but programme: needing fewer movements within the permitted hours pulls the project forward in the schedule. That gain is hard to price, but it is the one most felt on site.
The quality constraint: where can it be used?
Where material produced on site can be used depends on its quality and the relevant specification. Use as fill, sub-base and site road material is generally possible; use as structural concrete aggregate requires specification compliance and testing.
So a recycling plan begins before the crushing decision, with the question "where will this material be used". A crushing operation without an answer to that does nothing but break rubble into smaller pieces.
Separation quality also decides. Material still containing timber, plastic, plaster and soil is unsuitable for most uses. Separating before crushing is more effective than screening after it.
Practical requirements for processing on site
- Space: room is needed for the pre-crush pile, the processing area and product stock. On a tight site this is the biggest constraint.
- Separation: rebar, timber and soil have to be removed before crushing. A crusher bucket does not sort them out.
- Dust and noise: in a city, dust suppression and working hour restrictions apply during processing.
- Equipment choice: at low and medium volume a crusher bucket is enough; at high, continuous volume a mobile crushing plant is more economical.
- Permits: in some regions waste processing on site is subject to permit; confirm before starting.