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26 CHAPTER THREE CONSTRUCTION WEATHER CONDITIONS SURFACE PREPARATION AND COMPACTION Equipment used in front of recycling and for compaction after recycling is that typically used in conventional maintenance and HMA overlay placement projects. Surface preparation is not specific to a particular recycling process. Roller selection is typical of that used in standard roadway construction. Vari- ous recommendations for surface preparation are archived in ARRA (2009) presentations and Wirtgen (2004) project sum- maries. The most commonly cited practice is to remove any vegetation in cracks, scrub dirt deposits, and broom the surface before recycling. When lane widening is to be completed, the vegetation along the shoulder needs to be removed. Wirtgen (2004) provides some guidance on selecting the appropriate type of primary roller based on the general gradation of the mix (Figure 23). Sandy (i.e., coarse) mixes, such as FDR mixes, need a sheep's foot roller first, whereas clayey (i.e., fines) mixes, such A number of recommended weather conditions for pav- as CIR mixes, can use either a steel wheel or pneumatic roller. ing were found in several presentations posted to the In- Place Recycling Conference (2009) and ARRA website HIR Construction (2010). Variations found in the presentations depend on the properties of the binders and additives used in the recycling. HIR is an on-site, in-place process that preserves or maintains Typical ranges of weather conditions include deteriorated asphalt pavements while minimizing the use of new material. The HIR method is used to correct surface dis- · Ambient temperature above tresses that are not caused by structural problems (i.e., stable 40°F to 50°F and adequate base). One of three types of HIR processes is used, 45°F to 65°F depending on the distresses present in the existing roadway: · Pavement temperature above 50°F 50°F to 70°F · No anticipated overnight freeze · Dry roadway · Construction dates limited to between May 15 to Oct. 15 May 1 to Sep. 30 · Weather conditions that allow for the proper placement · Curing conditions for CIR or FDR (depend on addi- tives and stabilizers) range from a Minimum of 14 days to maximum of 30 days (CIR) One to 7 days (foamed asphalt stabilized) Once moisture content is below 1.0% (CIR) Better-defined guidance for weather conditions for each in-place recycling process is needed for successful in-place FIGURE 23 Suggested primary roller selection (based on recycling projects. original figure by Wirtgen 2004).
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27 · Surface recycling--heater scarification, · Scarifiers and miller units · Repaving, or Weights range from 7,711 to 39,463 kg (17,000 to · Remixing. 87,000 lb). Heights range from 3.1 to 4.9 m (11 to 16.5 ft). These processes are single-stage and multistage HIR trains. Lengths range from 9.1 to 16.8 m (30 to 55 ft). The single-stage train processes the complete depth in one pass, · Recycling combination units whereas the multistage unit processes half of the depth per pass. Weights range from 15,876 to 83,008 kg (35,000 to 183,000 lb). Surface recycling is a preservation/maintenance process Heights range around 4.3 m (14 ft). that restores cracked, brittle, and irregular pavement in prep- Lengths range from 8.3 to 22.3 m (28 to 73 ft). aration for a final thin wearing course. A single-pass method recycles and places the HIR material in one pass of the Any type of preheater may be in use, according to agency equipment. Repaving combines surface recycling with the and contractor responses (Figure 24). simultaneous overlay of the new HMA overlay. Remixing heats, scarifies, collects material, and places it in a windrow that is picked up and mixed in a pugmill with new aggre- gates, rejuvenators, and new HMA (as needed). HIR processes may not be applicable to recycling pave- ments with multiple seal coats (FHWA 1997), crumb rubber surface treatments, or porous HMA mixes (Stroup-Gardiner, 2008). In these cases, the properties of the upper layers act as an insulator against the heat transfer to the underlying pave- ment. In some cases, the excess binder or additives, such as crumb rubber, may also create smoke and potential fire haz- ards (Caltrans-METS 2005). Excessive crack sealant can pose similar problems. Each category of the HIR process has its own sequence of standard and specialized heavy equipment. FIGURE 24 Types of preheaters currently in use. Percentages are based on the number of agencies and contractors with All of the HIR processes use one or more preheaters experience using HIR processes. to soften the existing pavement so that it can be scarified or milled. The older style of preheater uses a simple set of HIR surfacing equipment consists of a preheater followed burners fueled by propane. The disadvantage of this type of by a combination preheaterscarifier (Figure 25, top). The preheater is that open flames can be seen during use. The heated, scarified HIR material is compacted using conven- open flames may pose a fire hazard when there is dry brush tional compaction equipment. near the roadway or in neighborhoods with landscaping near the project edge, and they can readily burn the pavement HIR repaving uses a standard haul truck to transport new surface. Newer styles of preheaters include infrared, skirted HMA and load it into the front hopper of the recycling unit recirculating heating systems, and most recently, a combina- (Figure 25, middle). The mix is moved through the equip- tion of heating systems. ment to the paving screed at the back. A series of heaters is used to soften the existing pavement, and a scarifier is The size and weight of HIR equipment are highly variable: used to loosen the RAP. This is followed by the addition and mixing of binders or rejuvenators, and the mixture is spread · Preheaters across the lane. Mixing augers blend the materials and place Weights range from 4,990 to 44,906 kg (11,000 to the mix with the recycling screed. For an integral overlay, the 99,000 lb). new HMA is placed on top of the hot recycled mix (paving Heights range from 2.3 to 3.6 m (7.6 to 11.8 ft). screed), and compaction is achieved using standard pneu- Lengths range from 2.1 to 18.9 m (7 to 62 ft). matic and steel wheel rollers.
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28 FIGURE 25 Typical sequence of construction equipment for HIR processes (based on FHWA 1997; ARRA, 2001). HIR remixing mills provide the deepest milling depths of be placed because sufficient time may be needed for water the HIR processes. Typically, two preheating units precede the evaporation after placement. miller-mixer (Figure 25, bottom). The miller-mixer is usually equipped with a front hopper for the new HMA mix, and the Standard haul trucks are used to provide new aggregates bottom front of the miller is fitted with another heater pushed (typical East Coast practice) or new HMA (typical West in front of or pulled behind the recycling. The milled material Coast practice; Figure 26). Unlike the HIR recycling trains, is mixed with fresh materials and additives and placed using one or two nurse trucks are usually in front of the recycling a heated, vibratory, or tamping screed. Compaction is accom- profiler and mixer unit to provide a continuous supply of liq- plished with standard compaction units. uids for the mix (e.g., recycling agents, water). The recycling unit mills, processes, and mixes the recycled materials and CIR Construction then transfers them to a paver. Standard compaction prac- tices are used to place and compact. CIR mills only the existing HMA pavement surface. Screen- ing decks and onboard crushers size the reclaimed asphalt FDR Construction pavement. The sized material is then transferred to a twin- shaft pugmill and mixed with the emulsion or foamed The FDR method pulverizes the existing HMA pavement asphalt. Wirtgen (2004) provides a comparison of the con- along with underlying granular materials. Stabilizers are struction parameters for the typical asphalt binders used in added to the pulverizers or through separate passes of other either CIR or FDR processes (see Table 30). Note that "cold" units. The steps involved in the FDR construction process refers to the ambient temperature of the milling and aggre- are shown in Figure 27. The first construction activity is gate temperature. CIR can still use hot paving-grade binders to deposit fresh materials or additives and spread them with the foamed asphalt process. Breaking time (i.e., rate of evenly over the old roadway surface (Figure 28). Nurse set) for the emulsion will limit when the surface course can trucks provide liquids to the pulverizing and mixing unit.