State of the Practice for Use of RAP in Hot-mix Asphalt

Asphalt Article December 28, 2000
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Since the mid- 1970s, tens of millions of tons of RAP have been used to produce recycled HMA that has the same performance characteristics as HMA made with all virgin materials. Substantial savings have been achieved by using RAP in mixes. The use of RAP has evolved into an everyday occurrence in many areas of the country, generating an estimated 45 million tons of RAP per year. In Florida for example, 75%of the RAP generated is recycled into HMA. The Federal Highway Administration (FHWA) estimates that 333of all asphalt pavement removed is recycled into HMA production.

Performance of the HMA containing RAP has clearly shown that this approach to resource management has proven to be a winner. Significant quantities of both asphalt cement and aggregates have been saved from the landfill. These savings translate into significant economic advantage to both the public and private sectors.

Given this phenomenal growth in the use of RAP, why then do some states limit the use of RAP to certain projects specified by the DOT? Could it be, as the FHWA concluded in its report, Pavement Recycling Executive Summary and Report that specifications and attitudes are major obstacles to the use of RAP? Or have isolated problems with the use of RAP given wrong impressions for use of the product?

Generating RAP

Based on review of existing pavement conditions and grade requirements, the pavement engineer may elect to remove existing HMA in the pavement structure. The removal may be accomplished at partial or full depth of the HMA. Partial depth removal is generally accomplished using a milling machine. The milling machine consists of a series of cutting heads on a rotating drum.

Milling is frequently used in a rehabilitation program where an upper layer of an existing pavement is removed and replaced with new pavement to lengthen the pavement’s service life. In this process, curbs and structural clearances, bridges and overpasses for instance, can be maintained, yet the pavement structure can be improved.

Milled RAP has the additional benefit of being ready to recycle immediately without additional processing. The RAP from a single layer typically has uniform properties (RAP gradation, aggregate gradation, asphalt content and asphalt characteristics). This is because the RAP typically has come from a specific site where the pavement was consistent when placed. For this reason, millings are frequently segregated and identified in separate stockpiles at a storage location.

The pavement also can be removed completely in a total reconstruction. Here, bulldozers or front-end loaders break the entire pavement structure into manageable slabs and load them into trucks for transportation to a reprocessing site. The slabs are then crushed to a usable size for recycling.

RAP that has been removed in full-depth fashion might be segregated at the storage location like milling, but this management practice varies based primarily on the quantity of the RAP. Large quantities of uniformly consistent, full-depth RAP may be segregated for crushing and sizing later because the RAP will prove consistent in stone gradation, asphalt content, and asphalt characteristics after it is crushed and sized.

Frequently, however, pavement that has been removed in full-depth fashion arrives at the plant from various sites and in small quantities. For this reason, full-depth RAP is often stored together in a common pile for blending and processing. The RAP is later crushed down to the largest aggregate size, or the stone in the RAP is actually decreased in size in an effort to create a consistent product from several different sources. Experience has shown that with this type of thorough blending and crushing, a RAP product with consistent stone gradation and asphalt content can be achieved.

Processing RAP

Some engineers place severe restrictions on the use of RAP in the belief that the material is highly variable and, therefore, cannot be properly controlled during processing. However, the FHWA report showed that a proper crushed and sized RAP may have variability that is at least as good as, if not better, than conventional HMA production. As with any other product, the RAP is only as good as the sampling, testing and analysis of the properties.

Millings from a single source are typically consistent in their gradation and their composition (aggregate gradation, asphalt content and asphalt characteristics). Material below 50 mm in size is used at practically all facilities without further processing. Larger particles of millings, however, take a longer time to re-blend with the new hot-mix materials. For this reason, scalping screens are typically installed between the RAP cold feed bin and the transfer belt conveyor in the HMA facility. Many producers use a “RAP breaker” or “lump breaker” which resembles a small roll crusher, positioned between the bin and belt. This can be used with or without a scalping screen. Larger RAP particles passing between the rollers are reduced in size, allowing for better heat transfer and more complete mixing.

Full-depth RAP, or RAP that arrives at the facility in large sizes, must be re-crushed prior to recycling into a new pavement. Several types of crushers and crushing configurations can be used, but most contractors have settled on the following equipment as being most effective and efficient.

Horizontal impact crushers

Horizontal impact crushers have solid breaking bars fixed to a solid rotor. RAP is crushed as a result of impact with the breaking bars and a striker plate. The faster the rotor speed, and the smaller the distance between the striker plate and the breaking bar, the smaller the gradation of the crushed product. The slower the rotor speed, and the greater the distance between the striker plate and the breaking bar, the larger the gradation of the crushed product.

Horizontal impact crushers are typically used as both the primary and as a secondary crusher by recirculating the oversize back through the crusher. However, they can be used as a secondary crusher, with the primary crusher being a jaw-type to handle the very large slabs of full-depth RAP.

Hammermill impact crushers

Hammermill impact crushers are similar to solid bar impactors, but the breaking bars pivot on a rotor creating a swing-hammer type action. During operation, the hammers remain extended based on the centrifugal force of the rotor and function very similar to a solid bar impactor. With foreign material—such as a metal utility access cover—the breaker bars relieve by moving backward to allow the foreign material to pass through the unit.

As with the horizontal impactor, the hammermill crushers are typically used as a primary and secondary unit by recirculating the oversize back through the crusher. However, a hammermill can be installed as a secondary crusher behind a large jaw crusher, just like horizontal crushers.

Jaw/roll combination

The combination jaw/roll crusher is readily available on the used equipment market and has proven effective for downsizing slabs of RAP material. The jaw takes the slab down to a more manageable size, which is then reduced to usable size by a secondary roll crusher. Typically, these combination crushers have recirculating conveyors to recycle the oversized material back to the roll crusher until it is properly sized.

Both jaw and roll crushers can “pancake” the RAP, especially on warm, humid days. “Pancaking” is a commonplace phrase among producers to describe RAP sticking together or agglomerating in the crusher, forming a flat, dense mass of RAP material between the crusher surfaces. This can slow production, as the crusher must be stopped and the “pancaked” RAP material removed to continue crushing. While this proves aggravating for the processor, it does not affect the quality of the processed product. Horizontal impactors and hammermills are not plagued with this problem.

Typically, jaw/roll crushers are used in conjunction with a double-deck screen for producing two products from full-depth RAP: a fine product (typically a 13 mm), and a coarse product (typically a 13 to 19 mm).

Field experience has shown that a remarkably consistent RAP product (stone gradation, asphalt content and asphalt characteristics) can be achieved through careful blending and crushing operations. The key to producing a homogenous RAP product from a “composite” pile is to first blend the RAP thoroughly with a front-end loader or bulldozer and then to down-size the top stone size in the RAP in the crushing operation to one smaller than the top-size in the hot-mix being produced (e.g., 16 mm for a 19-mm top-size mix).

This ensures that the asphalt-aggregate bond is broken as much as possible and no oversize stone appears in the mix. The actual crushing and testing of a given tonnage will generally prove this situation to be true. This crushing efficiency is good news for those interested in conserving landfill space and increasing the percentages of full-depth RAP materials that come from several sources.

When a crusher is available at the processing site at all times, smaller quantities are typically crushed, sampled for consistency and then used by the facility. Crushing smaller quantities provides several advantages to the producer. First, it makes it easier to sample the crushed product for consistency and keep the crushed product identified. Second, smaller piles can be used quickly before they have a chance to gain moisture from being stored outside, which increases drying costs and limits RAP percentages that can be used in the facility.

Milling/grinding reduction units

Another type of crushing equipment that is infrequently found is best described as a milling/grinding reduction unit. These units have milling machine-type heads installed in the discharge area of a bin and large slabs of RAP are deposited in the bin for size reduction into material that can be used directly in the facility.

These units are not crushers and are not designed to reduce the stone size in the RAP. They are designed only to break the asphalt-aggregate bond, much like a cold planing or milling machine does. While not in prevalent use, four or five different manufacturers produce milling/grinding reduction units of various designs.

Storing and handling RAP

When recycling first began in the late 1970s, the literature of the day suggested stockpiling RAP in low, horizontal stockpiles for fear that high, conical stockpiles would cause RAP to re-agglomerate with the weight of the pile. Experience has proven, however, that this is not the case, and indeed, large, conical stockpiles are preferred.
Practical experience has shown that RAP does not have a tendency to re-agglomerate or recompact in large piles. What is peculiar, is that RAP has a tendency to form a crust over the first 200-250 mm of pile depth. This crust tends to help shed water, is easily broken by a front-end loader and may even help keep the rest of the pile from compacting.

Front-end loader operators report that after breaking through this protective covering, each additional load from the inside of the stockpile is easy to manage and that the RAP moves without difficulty through the feeding equipment on the facility.

Speculation is that the “crust” on the RAP pile forms due to a solar/thermal effect from the sun, causing the RAP to gain just enough heat on the surface to partially “melt” back together. Some processors have even experimented with the idea of paving over the stockpile with fresh hot mix by hand in an attempt to seal the stockpile rather than rely on this effect to occur naturally with the sun.

Horizontal stockpiles also have a tendency to form this 200- to 250-mm crust. Yet because the stockpile is smaller and lower to the ground, it can be aggravating for the front-end loader operator to work with the pile. With each bucket load, more crust is encountered. This crust needs to be scalped off, or downsized with a RAP breaker, prior to processing in a hot-mix facility. If a large percentage of each bucket load contains crust, then the material handling equipment in the facility also is taxed and production of RAP mixes can be slowed.

RAP also has a tendency to hold water and not drain over time like an aggregate stockpile. Therefore, low, horizontal, flat stockpiles are subject to larger moisture accumulation than tall, conical stockpiles. It is not unusual to find RAP moisture content in the 7-8-range during the spring at facilities using low, horizontal stockpiling techniques. This drastically affects RAP percentages that can be processed in the facility, raises fuel costs and limits overall production rates.

Material handling machinery, such as front-end loaders and bulldozers, should be kept from driving directly on the stockpile. Compaction will result, making it very difficult for the loader to handle the RAP. With large stockpiles, and restricted space, this sometimes cannot be avoided. In those circumstances, bulldozers must be used later to “shave” the stockpile, removing material a little at a time and the front-end loader can then be used to carry the material to a crusher for processing.

Cold feed bins

While conventional style cold feed bins can feed RAP materials, RAP cold feed bin designs have evolved to generally include steeper side walls, longer feeder belts, longer openings onto the belts, and in some cases, inclined feeder belts or open end walls; all in an effort to promote as free a flow of RAP product as possible.

RAP material requires special handling by the front-end loader operator and cannot be handled in the same manner that aggregate materials are handled when charging cold feed bins.

RAP should be “dribbled” into the RAP cold feed bin by tipping the front-end loader bucket a little at a time, charging the bin slowly. An entire bucket of RAP deposited into the bin at one time, and with force, can cause the RAP material to pack as it drops en masse into the bin—particularly on hot, humid days or with particularly wet RAP.

It also is best not to heap RAP into the bin, filling it to capacity. RAP is not free flowing like an aggregate material. RAP is more likely to “bridge” than virgin aggregate materials. The loader operator, consequently, will need to feed the RAP bin more frequently than a virgin aggregate bin.

Also, RAP should not be left in the bin for extended periods of time—especially on hot, humid days. RAP can recompact in the bin with its own weight under these circumstances. If the HMA facility is going to be shut down for periods longer than an hour or two, the operator should consider clearing the bin to avoid any difficulties on start-up.

Vibrators should be avoided with RAP material to counteract “bridging” and other feed problems. They generally have a tendency to pack the RAP together rather than solving the feed problem. Pneumatic “air cannons,” or “blasters,” which send a charge of compressed air into the material under pressure are the most effective equipment for dislodging RAP material that is not feeding properly.

Covering stockpiles

RAP does not shed water or drain like a typical aggregate, and high percentages of moisture in RAP have dramatic effects on the percentage of RAP that can be processed at a facility. Therefore, covering RAP stockpiles is even more economical than covering virgin aggregate stockpiles.

RAP should never be covered with a tarp or plastic, however. In humid climates, which are the climates where covering RAP stockpiles seems logical, covering stockpiles causes condensation under the tarp and adds moisture to the RAP stockpile. While this moisture may be less than allowing precipitation to fall directly on the RAP stockpile, it still has a detrimental effect on the moisture content in the stockpile and should be avoided.

For this reason, most RAP stockpiles are either left uncovered, or RAP is stored in an open-sided building, but under a roof. In such a structure, free air can pass over the RAP, yet the RAP is protected from direct falling precipitation. Such structures are relatively economical, and with enough RAP production, can be justified on reduced fuel consumption and the possible savings from using higher RAP percentages in the facility.

Many producers pave under RAP stockpiles with the hope that this both contributes to drainage from the RAP pile and reduces possible moisture absorption from the ground. An added benefit to paving under a RAP stockpile is that possible contamination is eliminated as the front-end loader collects material close to the grade on which the stockpile is resting. It is important that the pavement be sloped properly to provide positive drainage.

Segregating stockpiles

The key to producing a quality recycled pavement is in knowing what stone and asphalt properties exist in the reclaimed pavement. When large quantities of RAP from different sources are available, it is advisable to keep stockpiles separated and identified by source.

Throughout the late 1970s and early 1980s, great effort was put forth on keeping each and every stockpile of RAP segregated for processing at a later date. Space constraints and small quantities of RAP from some sources forced processing sites to create a “composite” or “blended” pile. From dealing with these stockpiles, operators discovered that consistent RAP products could be produced using a crushing and screening operation and reprocessing stockpiles.

This discovery has taken some of the emphasis away from separated stockpiles. Whenever significant amounts of an identical or similar RAP product are available, however, it is only logical to attempt to keep these materials separated from other RAP products.

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