Tendering Asphalt Recycling Work
Cold in-Place Asphalt Recycling is a deep asphalt reconstruction method for existing roads, streets, airfields, and highways. During rehabilitation we have the opportunity to add an asphalt binder through foam or emulsion, plus additives to alter gradations and strength. Well written specifications for Cold in-Place Recycling define some key elements of “process control” that must be in place to ensure competitive bidding to a high technical standard. Those elements are as follows:
1. The existing asphalt is milled away from the surface and elevated up into a processing train of equipment. Milling widths are generally 3.7 to 4.2 meters
2. Particle size is achieved with a combination of screen decks and crushers to ensure a consistent stream of crushed “Recycled Asphalt Product” and additives. This is the first key element.
3. Additives such as underlying granular material, shoulder gravel, course stone aggregate, lime, or portland cement may be blended in a controlled, precise manner to adjust material gradation or increase tensile strengths.
4. Accurate weighing of the variable flow of crushed asphalt and dry additives is the second key element of process control.
5. Job Mix Formula – JMF- The third critical element involves the field testing and retrieval of materials in order to accomplish a laboratory mix design target. This work is done in advance of recycling to provide the owner and contractor with structural targets and additive guides for the recycling process.
6. System Controllers- The fourth critical element is the ability to monitor multiple variables of flow, and constantly adjust accordingly. A simple example is 2% asphalt addition to a flow rate of 400 tonnes per hour. The JMF is downloaded into the computer system and the scales report the actual flow rate of dry additives. 8 tonnes of asphalt per hour is 2.222 kg per second. The control system must be able to adjust the asphalt delivery rate to meet this flow rate and any changes up or down as the machine speeds up, or slows down, or if depth changes occur. This ensures consistency of material.
7. Asphalt additives have evolved into two primary choices. Asphalt Emulsion and Expanded (Foamed) Asphalt. Since the existing roadway is being processed at ambient temperature without heat addition, a standard asphalt cement binder cannot work successfully.
An emulsion allows asphalt cement to be suspended in a solution of soap, water, and additives. (Asphalt Latex Paint). Asphalt in this form will blend, coat, and penetrate into the cold aggregate flow to deliver the required binder to the matrix in the mixing pugmill.
A more recent method is to expand or foam a superheated asphalt cement at the moment of mixing. The foamer spray head blends cold water and compressed air with 170C-335F liquid asphalt cement. The presence and interaction of water and air will expand the volume of the asphalt 15 to 20 times. This foam of asphalt bubbles has a low surface tension, and as the bubbles break down in the pugmill mixing chamber, an even dispersion throughout the matrix occurs.
Jobsite conditions, location, and binder rate are generally evaluated to decide which delivery method has the best advantages.
8. Pugmill mixing is the fifth element. An aggressive paddle mixer capable of high tonnage production will ensure proper blending, coating, and dispersion within the asphalt mix.
9. Controlled placement and compaction is the sixth critical element. Typically large track style asphalt pavers with grade and slope controls are used to place the new mixture into the milled “slot” created in step one. This mix is deep and cold, and requires extra compactive energy in the form of large heavy rollers to lock the material back together. The binders will achieve strength at different rates and timelines as influenced by the in-stream material, ambient temperature, and mix design parameters. The heart of Cold in-Place success is the mix nature of slow to harden, slow to crack. The curing phase is generally strong enough to carry moving traffic with minimal damage until the rest of the roadway structure is placed above the recycled base course material.
10. QC can mean Quality Contracting supplemented by Quality Control. This is also a key element. Generally third party material testing is taking place daily to look for anomalies in the placed material, and to ensure all key items of process control are taking place. In general if the Bulk Density of the in-stream road changes significantly, the recycled matrix Bulk Density will be affected. Testing and acknowledgement of these differences reduce many conflicts in the field regarding compaction densities.
Please take some time to read OPSS 333 and 335 to understand how Seeley and Arnill, and other contractors must deliver a combination of equipment, staff, materials, and testing to a roadway in a competitive bidding environment.