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22 TABLE 14 ENVIRONMENTAL IMPACT COMPARISON OF MICROSURFACING VERSUS HOT MIX ASPHALT OVERLAY Greenhouse Gas Annualized Percent Energy Use Life Treatment Composition Emissions Savings vs. 2-in. Extension BTU/CY MJ/CM Lbs/SY Kg/SM Hot-Mix Overlay 1.5 in. Hot-Mix 46,300 59 9.0 4.9 510 yr Energy (3.8 cm) Greenhouse Asphalt Use 2.0 in. Gas Savings Overlay 61,500 77 12.3 6.7 510 yr Savings (5.0 cm) Micro- Type III 5,130 6.5 0.6 0.3 35 yr 83%86% 90%92% surfacing Type II 3,870 4.9 0.4 0.2 24 yr 83%84% 91%92% Adapted from Chehovits and Galehouse (2010). This conclusion is especially valid given the current wide- residual asphalt) (Caltrans 2009). Cationic emulsions are typ- spread focus on sustainable design and construction practices ically used in microsurfacing. The literature cites CSS-1h (Takamura et al. 2001; Uhlman 2010). The crux of sustainable (cationicslow settinglow viscosityhard; see the Glossary engineering practices revolves around judicious selection of for abbreviations of emulsion types) as the most common materials, which is the subject of the next section. (Moulthrop et al. 1999). Some agencies prefer quick setting emulsions such as CQS-1h to reduce the amount of traffic dis- ruption and delay (ISSA 2010a). SELECTION OF MATERIALS Emulsions can also be specially formulated to ensure com- Once a road has been identified as a microsurfacing project, patibility with local aggregates and to meet appropriate mix the next step is to select the appropriate materials. The compo- design parameters. The survey asked respondents to indicate nents of a microsurfacing job mix consist of emulsion, aggre- which type of emulsions were commonly used in their micro- gate, mineral filler, additives, and water. It is important that surfacing program and the content analysis of the microsur- each of these ingredients be compatible with each other for the facing specifications also sampled for that information. The microsurfacing to work as designed. Therefore, the mix design results are shown in Table 15 (note the table only lists binders process is necessarily based on laboratory results, which are in that were cited by survey respondents). A trend in the data turn used to optimize the job mix formula. was found that showed that only 3 of 28 U.S. agencies used more than a single emulsion type. All of the specifications Emulsion contained only a single emulsion, which was also true for the Canadian agencies. The use of a single microsurfacing binder "Asphalt emulsions are dispersions of asphalt in water sta- coupled with the reported microsurfacing performance results bilized by a chemical system" (National Highway Institute in Table 8 (i.e., 100% of the agencies rated their microsur- 2007). They are manufactured by blending emulsifying agents facing performance as "Fair," "Good," or "Excellent") indi- with the base asphalt to permit it to disperse uniformly, creat- cates that an agency can select a single binder that works best ing a temporary mixture. This mixture "breaks" upon place- for its specific climatic and traffic environment and use it ment and releases the water leaving behind the asphalt (called exclusively in their microsurfacing program. Agency satis- 70 60 50 40 30 20 10 0 Energy CO2 NO2 Ozone Raw Material Microsurfacing Hot-mix Overlay Polymer-modified Hot-mix overlay FIGURE 10 Comparative environmental impacts of three pavement preservation and maintenance treatments (adapted using data taken from Takamura et al. 2001).
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23 TABLE 15 MICROSURFACING BINDER USAGE SUMMARY FROM SURVEY AND CONTENT ANALYSIS Quick Set CSS- CSS- CSS- CQS- CQS- Mixing CRS- CRS- Nation 1P 1h* 1hP 1h* 1hP RalumacTM Grade 1P 2P U.S. 1 6 7 5 5 2 1 1 1 Canada 3 2 1 0 2 0 0 0 0 Total 4 8 8 5 7 2 1 1 1 Content 2 9 4 1 4 0 0 0 0 Analysis Totals *Note all the specifications that called for CSS-1h and QS-1h also specified that it be polymer or latex modified. See Glossary and Abbreviations for emulsion grade abbreviations. faction with the performance of a single binder can be iden- density. To correct this, the latex is remixed by circulating it tified as the following effective practice: and the emulsion in the tanker before it is transferred to the microsurfacing machine for installation (ISSA 2010a). The Microsurfacing programs implemented with a single binder survey had 23 responses citing polymer use, 7 citing natural type can yield satisfactory performance in a given agency's latex, and 2 styrene butadiene styrene. climate and traffic conditions. Typical emulsion specifications are shown in Table 16. Microsurfacing emulsion specifications are similar stan- These include binder content and residual asphalt properties. dard emulsion specifications and contain requirements for Both viscosity and storage stability are vital to ensure effective physical characteristics such as stability, binder content, and emulsion performance on the jobsite. viscosity. Polymers are added to microsurfacing emulsions to reduce thermal susceptibility and promote aggregate retention after curing and opening to traffic (ISSA 2010a). Addition- Aggregates ally, polymers improve the binder's softening point and its According to the National Highway Institute's Pavement Pres- flexibility, which translates to better thermal crack resistance. ervation Treatment Construction Guide (2007), the key char- However, there is a low effective limit to the amount of reflec- acteristics of aggregates used in microsurfacing are as follows: tive cracking microsurfacing will resist. Finally, the polymers permit microsurfacing to be placed in thicker sections of two · Geology: This determines the aggregate's compatibility with to three stones thick, which enables its use in rut filling. the emulsion along with its adhesive and cohesive properties. · Shape: The aggregates must have fractured faces in order to Emulsions are usually modified with latex in an emulsion form the necessary interlocking matrix. Rounded aggregates of polymer particles. The asphalt and latex do not combine. will result in poor mix strength. · Texture: Rough surfaces (crushed aggregate) form bonds more The latex and the asphalt particles intermingle to form an easily with emulsions. integrated structure as shown in Figure 11 (note the aggre- · Age and Reactivity: Freshly crushed aggregates have a higher gate particles contained in a microsurfacing mixture are not surface charge than aged (weathered) aggregates. Surface charge shown in this figure. Figure 11 depicts only the interaction of plays a primary role in reaction rates. · Cleanliness: Deleterious materials such as clay, dust, or silt the latex with the asphalt in the microsurfacing mixture). can cause poor cohesion and adversely affect reaction rates. Microsurfacing emulsion is modified with either natural latex · Soundness and Abrasion Resistance: These features play a or styrene butadiene styrene latex. It is possible for the latex particularly important role in areas that experience freeze-thaw to separate from the emulsion owing to the differences in cycles or are very wet. The physical properties of the aggregate are important to it achieving its design service life (Fugro-BRE/Fugro South 2004). The quality of microsurfacing aggregate is typically measured by the four properties shown in Table 17. The con- tent analysis found that some agencies also specified the fun- damental material from which the aggregate was made. A typical example from the Missouri DOT is as follows: The mineral aggregate shall be flint chat from the Joplin area, an approved crushed porphyry or an approved crushed steel slag. Blast furnace slag may be used from sources with a documented history of satisfactory use and that have been previously approved by MoDOT for use in micro-surfacing. For non-traffic areas such FIGURE 11 Emulsion with latex in dispersion and after as shoulders, the mineral aggregate may be crushed limestone or breaking and curing (Watson 2005). crushed gravel (Missouri DOT 2004).
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24 TABLE 16 TYPICAL PROPERTIES OF MICROSURFACING EMULSIONS Test Typical Specification Method Residue 62% min. AASHTO T59 Sieve Content 0.3% max. AASHTO T59 Viscosity at 77°F (25°C) 1590 AASHTO T59 Stability (1 day) 1% max. ASTM D244 Storage Stability (5 days) 5% max. ASTM D244 Residue Penetration at 77°F (25°C) 4090 ASTM D244 Elastic Recovery 5%60% AASHTO T301 Softening Point 135°F (57°C) min. ASTM D5 Distillation at 350°F (177°C) 62% min. ASTM D6997 Polymer Content 3.0% min. ASTM D6372 Sources: National Highway Institute (2007) and ISSA (2010b). Gradations tionally, many of those that cited specifying a nonstandard gra- dation added an explanatory note that theirs was a slight mod- Table 18 shows the gradation for standard microsurfacing ification of the standard gradations. One can see the split aggregates. ISSA (2010b) is the proponent of the A143 micro- between Type II and Type III aggregates. When asked to iden- surfacing design method and is the source for the various type tify the gradation that was most often specified, Type II was classifications of aggregates for both microsurfacing and slurry indicated more frequently than Type III and other gradations. seals. The primary difference between the two gradations is top The seeming preference for the smaller aggregate may have size, with Type III furnishing a coarser aggregate than Type II. been caused by the concerns about road noise, which are dis- The gradation determines the appropriate amount of residual cussed in detail in chapters seven and eight. asphalt in the mix design as well as the particular applications, such as rut filling, for which a given mix design is effective. Type III aggregate will also produce a surface with deeper Type II is recommended for "raveling and oxidation on road- macrotexture, which will result in a surface that drains faster ways with moderate to heavy traffic volumes. Type III . . . is than Type II. Conversely, it will also produce more road noise. appropriate for filling minor surface irregularities, correcting Road noise was the complaint most often reported in the raveling and oxidation, and restoring surface friction . . . on survey. A study completed at the National Center for Asphalt arterial streets and highways" (National Highway Institute Technology (NCAT) found that a "surface with a smooth 2007). The fines (i.e., aggregate particles 75 µm and finer) in texture using small maximum size aggregate" (de Fortier the mix create "a mortar with the residual asphalt to cement Smit 2008) reduced macrotexture and minimized road noise. the larger stones in place" (National Highway Institute 2007). Because Type II aggregate has a lower top size, it appears Fines are essential for creating a cohesive hard-wearing mix. to conform to the NCAT finding. Therefore, the preference The National Highway Institute Pavement Preservation Man- for Type II may also be to mitigate road noise complaints. ual (2007) recommends that the fines content be at the mid- point of the grading envelope. Additionally, a 2002 research study suggests that the distribution of the fraction that passes Mineral Filler the #200 sieve (75 µm and finer) is critical in effectively con- trolling reaction rates in microsurfacing emulsions (Shilling Portland cement or other fine materials are used as a "mixing 2002). This finding makes the amount of material that passes aid allowing the mixing time to be extended and creating a the #200 sieve an important gradation factor with regard to creamy consistency that is easy to spread . . . hydroxyl ions microsurfacing performance. counteract the emulsifier ions, resulting in a mix that breaks faster with a shorter curing time" (National Highway Insti- tute 2007). Portland cement also has a fine consistency, Surface Texture which absorbs water from the emulsion and causes it to break faster. As previously discussed, fine materials in the min- Eight of the responses indicated that the agency used more eral filler also promote cohesion by forming a mortar with than one gradation in their microsurfacing programs. Addi- the residual asphalt. The Minnesota DOT specification for TABLE 17 GENERAL AGGREGATE PROPERTIES AND AGGREGATE REQUIREMENTS Test Microsurfacing Test Number and Purpose Sand Equivalent (min.) 65 ASTM D2419 Clay Content Soundness (max.) 15% ASTM C88 (using NaSO4) Abrasion Resistance 30% max. AASHTO T96 Resistance to traffic Crushed Particles 100% ASTM D5821 Source: ISSA (2010b).