What is Reclaimed Asphalt Pavement (RAP)?
RAP (Reclaimed Asphalt Pavement) is the material obtained after asphalt pavement is removed and reprocessed.The pavement may be removed for various reasons including resurfacing or reconstruction.RAP is therefore made up of quality aggregates coated with asphalt cement.
How is RAP obtained?
There are a few steps involved in acquiring RAP:
Step 1. The pavement can be removed by one of two processes; milling, using a milling machine, or by full-depth removal.As the name suggests, milling uses a milling machine, which can remove up to 2 inches in one pass.On the other hand, full-depth removal requires pneumatic pavement breakers or a bulldozer fitted with a “rhino horn”.
Step 2. This next step involves the processing of the resultant crushed material.This can also go either of two ways.The crushed material can be collected and loaded into trucks for transportation to a central processing facility, where the RAP is crushed, screened, conveyed and stacked among other things;Or, using an automatic pulverizing machine, the pavement is crushed in place and added to stabilized or granular base aggregate.
Common highway uses of RAP and their processing requirements.
There are multiple highway construction applications for pulverized reclaimed asphalt pavement.These include, granular or stabilized base aggregates, recycled asphalt paving using cold mix or hot mix, or as an aggregate substitute or supplement.
AggregateTo produce granular base aggregate the RAP is crushed and screened, then blended with conventional granular aggregate, to give it higher load-bearing strength.
AggregateThe process is similar to that of granular base aggregate, but in place of granular aggregate, various stabilization reagents are blended into the mix giving it greater strength when compacted.
Hot Mix Asphalt
Hot mix asphalt can be produced in either of two ways; at a central processing facility, or through in-place recycling.At the central processing facility, the reclaimed asphalt pavement undergoes crushing, screening, and stacking, which results in a granular RAP product. The product is then added to hot mix asphalt as an aggregate substitute.
Hot in-place recycling uses a single or multi-pass procedure, where the material is heated, scarified, rejuvenated, laid down, then finally compacted.No processing is required prior to the recycling operation.
Cold Mix Asphalt
Similar to the hot mix asphalt, the cold mix asphalt can also be produced in either of two ways; at a central processing facility, or through in-place recycling.In the central processing facility, the reclaimed asphalt pavement processing needs for the cold mix recycling are also similar to those for the hot mix recycling.The difference is that, in the cold mix processing, the RAP product is added to the cold mix asphalt mixtures as an aggregate substitute.
Cold in-place recycling makes use of special “processing trains”.Using a milling machine, the pavement is milled to a maximum depth of 6 inches. After processing it, asphalt emulsion is added to the crushed pavement which is then laid down and compacted in a single pass. Just like in hot in-place recycling, no processing is required prior to the recycling operation.
Asphalt Cement Supplement and Asphalt Concrete AggregateAs seen above, RAP can also be used as an aggregate substitute material. This way, it reduces the need for asphalt cement in asphalt mixes containing RAP, by providing asphalt cement binder.