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CHAPTER EIGHT
CASE STUDIES
INTRODUCTION interpretation of the information contained in the literature.
Each case will be briefly described and then the analysis' focus
As with most pavement preservation tools, microsurfacing has will shift to specific lessons learned. Finally, the results of the
many uses and those uses differ from agency to agency based case study will be compared with the information derived from
on individual experiences, climatic conditions, and traffic vol- the other study instruments to generate conclusions and effec-
umes. The previous chapters have chronicled the very aspects tive practices.
of microsurfacing from a general practice level. This chapter
will review six case studies. Each was selected because it
detailed a specific aspect of microsurfacing's state of the prac- Microsurfacing as Pavement Preservation--
tice. Kohn (1997) posits that case study research can be used Maine Department of Transportation
for the following reasons:
This case study consisted of a 5-year field test of microsurfac-
· To explore new areas and issues where little theory is ing placed on asphalt pavements on two highways in northern
available or measurement is unclear; Maine. The test compared microsurfacing with thin (1.7 in. or
· To describe a process or the effects of an event or an 43 mm) hot-mix asphalt (HMA) overlays. The objective of the
intervention, especially when such events affect many trials was to evaluate microsurfacing's ability to "extend the
different parties; and service life of two projects" (Marquis 2009). Table 42 con-
· To explain a complex phenomenon. tains the salient facts about the project.
Microsurfacing is the domain of specialty contractors who
Results of the Maine Case Study
not only install the product but, as shown in chapter three, also
furnish the technical design for their product. This puts the This 5-year field evaluation found that microsurfaced sections
agency at a disadvantage in terms of theoretical knowledge showed considerably more wear than the HMA overlay sec-
and experience. The survey found that fully one-fourth of the tions. The details are as follows:
agency respondents did not even use microsurfacing, making
it a new area for those entities. Pavement maintenance and
preservation is by definition an intervention whose process · Most high spots of the microsurfaced roadway have been
needs to be clearly described to ensure that it is properly uti- abraded by winter snow removal equipment. In some
lized and its inherent benefits are accrued by the owner agency. areas the microsurfaced treatment has been worn away
Finally, chapter three shows it to be a complex phenomenon completely.
that needs to be explained so that its desirable qualities can be · Microsurfacing has higher IRI [International Roughness
properly exploited for pavement maintenance and preserva- Index] values and Frictional Resistance is slightly higher
tion programs. Hence, all three of these reasons apply to this than the HMA sections.
synthesis report, making the case studies particularly valuable · Microsurfacing appeared to slow the progression of
to this type of study. reflective cracking up [for 2 years] . . . [then] cracks
reflected through the microsurfaced sections at a higher
rate than the HMA sections.
CASE STUDY DESCRIPTIONS · Microsurface treatments claim to add between five and
seven years life to existing pavements. This appears to be
Table 41 summarizes the case study programs that will follow the case on the Limestone project where the material is
this section. Scanning the table will show that the cases were performing as expected. The only apparent issue is snow
drawn from agencies across the United States and in Canada. plow abrasion (Marquis 2009).
They also encompass both warm southern climates and cold
northern climates where snowplowing impacts microsurfac-
ing performance. Each was selected to demonstrate a spe- Lessons Learned
cific aspect of microsurfacing practice. The case studies were
drawn from the literature and fleshed out with telephonic or A full analysis of the three reports available in this case study
face-to-face interviews where necessary to ensure an accurate (Marquis 2002, 2004, 2009) provides two lessons learned. The
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TABLE 41
CASE STUDY PROGRAM SUMMARY
Case Study Agency/Location Reason for Inclusion Remarks
Microsurfacing as a Maine DOT Specific use for Demonstrates
pavement preservation Caribou, Maine pavement preservation; microsurfacing
treatment long-term performance performance in cold, snowy
in an area with heavy climate; answers concerns
snowplowing that it is not appropriate on
roads with heavy
snowplowing
Use of microsurfacing York Region Focus on safety; Demonstrates a use for
as a preventive Ontario, Canada specific use for microsurfacing that does
maintenance treatment preventive maintenance not focus on pavement
to improve safety distress
Long-term comparative Oklahoma DOT Used for filling deep Very comprehensive look
performance of Tulsa and Oklahoma ruts and treating at the treatment in a variety
microsurfacing on City, Oklahoma alligator cracking on of situations
asphalt and concrete high-volume interstate;
pavements 9-year record
Microsurfacing on a Georgia DOT Heavy urban traffic Agency survey response
high traffic interstate Atlanta, Georgia volume; road noise indicated they do not use
highway evaluation microsurfacing
Microsurfacing on Kansas DOT Evaluation of ride Ride quality is of prime
Jointed plain concrete Cowley County, quality improvement; importance on concrete
pavement Kansas use of microsurfacing pavements; comparison is
on a concrete surface with a hot-mix overlay
Microsurfacing using a Minnesota DOT Evaluation of cracking Provides an alternative for
softer binder Albertville, Minnesota and rut filling situations where cracking is
performance the primary issue
first lesson regards the impact of snowplowing on microsur- (AADT)] and only rut depth was unacceptable on the Presque
facing. Both microsurfacing sections were visibly abraded by Isle (8,600 AADT). Thus, the following lesson learned can
snow removal equipment. The report states: "Most high spots be noted:
of the microsurfaced roadway have been abraded by winter
snow removal equipment." Given this premise, the three per- Snowplowing will abrade microsurfacing and eventually
formance measures studied (International Roughness Index, wear it away. Although this is an issue if the treatment is
rut depth, and friction number) were all within acceptable lim- used to act as a seal to water intrusion it does not signifi-
its for the Limestone road [1,100 average annual daily traffic cantly impact the use of microsurfacing to enhance ride
TABLE 42
MAINE DOT CASE STUDY FACTS
Item Data
Binder CSS-1H
Aggregate Type III
Mineral Filler Non-air entrained portland cement
Job Mix Design
Aggregate 100%
Portland cement 1.0%
Water 10.0%
Binder 12.0% + 1%
Test Specification
Residual asphalt 8.3
Wet track abrasion--1 h 470.0
Wet track abrasion--6 day 680.0
Excess asphalt loaded wheel 453.2
Wet stripping 96.0
Compatibility 11 pts
Location Route 1 between Presque Isle and Caribou, Maine
Route 1A between Limestone and Caswell, Maine
ADT 8,600 and 1,100, respectively
Distress Level Before Microsurfacing
IRI (m/km) 0.97 to 2.14 (62 to 136 in./mile)
Rut depth (mm) 9.95 to 12.75 (0.39 to 0.50 in.)
Friction number Average 53.2
Length of Test Period 5 years
Snowplowing? Yes
ADT = average daily traffic.
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quality and skid resistance or as a rut filling technique on Microsurfacing as Preventive Maintenance--
a structurally sound pavement. York Region
The second lesson in this case deals with skid resistance. This case study consisted of the analysis of microsurfacing's
Both the Presque Isle and Limestone microsurfaced test sec- impact on safety when used as a preventive maintenance tool
tions were found to furnish higher skid numbers throughout to restore skid resistance. The study evaluated accident rates
the 5-year test period than the HMA test sections on the same over a 4-year period of microsurfacing placed on pavements
roads. The higher volume road lost 3.4% and the lower volume on two highways in the Region of York in Ontario, Canada.
road lost 1.1% over the period. Therefore, the lesson learned is: The test compared 28 microsurfacing sites with 12 HMA over-
lays. The objective of the study was to evaluate the inclusion
Using microsurfacing to correct the loss of frictional resis- of safety issues in an agency's preventive maintenance pro-
tance on a structurally sound pavement works well in a gram (Erwin and Tighe 2008). It used the two types of pave-
Northern climate. ment surface treatments and an accident rate before and after
to test the study's hypothesis. Table 43 contains the salient
facts about the project.
Effective Practices
One effective practice can be derived from this case study. The
Results of the York Case Study
analysis of the literature review shown in Table 7 (chapter
three) found that among those authors that specifically men- Figure 26 is a map that shows the location of the York Region
tioned snowplowing that microsurfacing was found to be suit- in the Canadian Province of Ontario. The results of the study
able for use in those areas more often than it was cited as a are summarized as follows:
concern (four positive versus one negative citations). Next, the
survey received responses from 23 states where winter snow · Microsurfacing has a positive safety effect when applied at loca-
removal is an issue and only 6 did not include microsurfacing tions with an AADT greater than 3,000 vehicles per lane.
in their pavement maintenance programs. Additionally, all of · Microsurfacing has been demonstrated to have a positive safety
the Canadian provinces except one use microsurfacing. There- effect on locations with higher traffic volumes susceptible to
any one or combination of these conditions:
fore, the intersection of those two lines of information with the
occurrence of wet or slick (not dry) road surface conditions;
one contained in this case study yields the following effective trend in severe crashes;
practice: frequent intersection-related crashes; and
high occurrence of rear-end crashes.
· Another point to consider is that contractors [furnish lower]
Microsurfacing can be effectively employed on roads
treatment prices for larger jobs. To capitalize on that opportunity
where routine winter snow removal is a factor if the under- and keeping in mind that microsurfacing was demonstrated to
lying pavement is structurally sound. be very effective at reducing intersection-related crashes, when
TABLE 43
YORK REGION CASE STUDY FACTS
Item Data
Binder CSS-1h
Aggregate Type III
Mineral Filler Non-air entrained portland cement
Job Mix Design--Typical
Aggregate 100%
Portland cement 2.0%
Water 10.0%
Binder 11.5% + 1%
Test Specification--Typical
Residual asphalt 6.0% to 11.5%
Wet track abrasion--1 h 538.0
Wet track abrasion--6 day 807.0
Excess asphalt loaded wheel 538.0
Wet stripping 90.0
Compatibility 11 pts
Location 28 sites through the Reg ion of York
AADT 1,000 to 7,000+
Distress Level Before Microsurfacing
IRI (m/km) Not available
Rut depth (mm) Not available
Friction number Not available
Length of Test Period 4 years
Snowplowing? Yes
AADT = average annual daily traffic.
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survey responses reflected that using it to improve friction is
not currently a primary reason for selecting microsurfacing for
a given project.
Long-Term Evaluation of Microsurfacing
Performance--Oklahoma Department
of Transportation
This case study reports on an early large-scale field test of
microsurfacing used to fill deep ruts and alligator cracking
on high-volume four-lane divided highways in Oklahoma.
The three sites studies were located in Oklahoma City and
Tulsa. The study lasted 9 years and yielded valuable informa-
tion that the Oklahoma DOT used to modify its microsurfacing
program. That the Oklahoma DOT still uses microsurfacing as
a major tool in its pavement maintenance and preservation pro-
FIGURE 26 Map of the York region case study.
gram amply demonstrates the value of including the case study
in this synthesis. Table 44 contains the important data about
prioritizing treatment sites; one could group intersection to and this case. Table 45 shows the aggregate gradations used by the
tender them out as single job. Such foresight in the planning DOT specifically for deep rut filling and alligator cracking
process can help agencies stretch their budgets farther while compared with the standard microsurfacing gradation.
making the roads safer.
Results of the Oklahoma DOT Case Study
Lessons Learned
The major findings of this robust study are as follows:
Two lessons can be drawn from this case study. First, the idea
of including safety in a public highway agency's pavement · Microsurfacing reduces the level of rutting and retards the rate
maintenance/preservation program is appropriate. A struc- of rutting for up to four years of service.
· Microsurfacing provides good friction characteristics for up to
turally sound pavement could be rendered unsafe merely to loss nine years of service.
of skid resistance resulting from the polishing of the pave- · Microsurfacing can be used effectively to fill ruts up to 38 mm
ment's aggregate (Gransberg 2009). The literature shows that (1.5 inch) deep.
countries with a tradition of performance-based pavement · Microsurfacing works well for filling depression cracks and
alligator cracks.
maintenance contracting such as Australia and New Zealand · Microsurfacing worked successfully with mine chat (cherty
include accident rates as a key performance indicator (Grans- limestone) and dolomite/granite aggregate mixture (Hixon and
berg et al. 2010). Therefore, adding an analysis of accident rates Ooten 1993).
to the pavement maintenance/preservation project selection
process makes sense. The lesson learned here is as follows: Lessons Learned
Because microsurfacing has shown itself to be particu- Two lessons learned can be derived from this case. The first
larly effective in reducing intersection accidents, adding regards the sustainability of microsurfacing and the ability to
safety issues to the project-specific treatment selection increase its "greenness" by utilizing recycled waste materials
process may furnish added value to an agency's pavement such as mine chat. Not only does microsurfacing require
maintenance/preservation program. less bituminous material, because it is cold-laid it con-
sumes less energy than other treatment alternatives. There-
The second lesson learned is that it would be beneficial to fore, the lesson learned in this case is as follows:
use microsurfacing in localized areas that are expected to
experience frequent stopping. For instance, freeway ramps Microsurfacing is a "green" alternative and can be used
would benefit from a higher friction surface to enhance emer- to promote sustainable maintenance practices by using
gency stopping during unexpected situations. Thus, the lesson recycled waste products such as mine chat for aggregate
learned is: and products such as fly ash and cement kiln dust as min-
eral filler.
Microsurfacing can be effectively used to enhance skid
resistance in areas where a reduction in stopping distance The second lesson learned comes from the special aggre-
is critical to safe operation of a given highway feature. gate gradations used by the Oklahoma DOT. The literature
contradicts the Oklahoma experience with regard to alligator
This approach has not been developed enough to yield an cracking as can be seen in Table 9. However, because the
effective practice. It does intersect with the literature, but the Oklahoma DOT used a special gradation designed specifically
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TABLE 44
OKLAHOMA DOT CASE STUDY FACTS
Item Data
Binder CSS-1h
Aggregate See Table 45
Mineral filler Non-air entrained portland cement
Job Mix Design--Typical
Aggregate 90%
Portland cement 2.0%
Water 9.0%
Binder 9% + 1%
Test Specification--Typical
Residual asphalt 8.0% to 13.0%
Wet track abrasion--1 h Not available
Wet track abrasion--6 day Not available
Excess asphalt loaded wheel Not available
Wet stripping Not available
Compatibility Not available
Location I-40 in Oklahoma City: 2 sites
US-64 in Tulsa: 1 site
AADT 11,000 to 40,000
Distress Level Before Microsurfacing
IRI (m/km) Not available
Rut depth (mm) 72 to 81 mm (2.8 to 3.2 in.)
Friction number 3244
Length of Test Period 9 years
Snowplowing? No
AADT = average annual daily traffic.
to treat alligator cracking, the two lines of information are not case study and the literature, the following effective practice
directly comparable. Thus, it appears that a one-size-fits-all is proposed:
approach to microsurfacing design may optimize microsurfac-
ing's rut filling ability at the expense of its crack filling ability. Microsurfacing is the proper alternative to enhance skid
This idea is validated because the Oklahoma DOT used a spe- resistance in areas where the frictional characteristics of
cial gradation for filling deep ruts rather than the "normal" gra- the road's surface are to be restored to safe operating limits.
dation. Therefore, the lesson learned is:
The aggregate gradation in the job mix design is to be cus- Microsurfacing on High-volume Roads--
tomized to match the primary purpose of utilizing micro- Georgia Department of Transportation
surfacing on a given road with specific gradations being
This case study (Tables 46 and 47) consisted of an experimen-
developed for cracking versus rut filling.
tal trial of microsurfacing to address wheel path raveling and
cracking on 92 lane-kilometers of Interstate 285 in Atlanta.
Effective Practices Because the motivation for the project was part of the prepara-
tion for the 1996 Summer Olympics, aesthetics was also a con-
Taking the findings in this case study regarding microsurfac- sideration. The project used both a tack coat and a scratch
ing's ability to furnish long-term surface friction characteris- course. The Georgia DOT evaluated friction, crack propaga-
tics and intersecting it with the information found in the York tion, and road noise.
TABLE 45
OKLAHOMA DOT CASE STUDY AGGREGATE GRADATIONS
Percentage Passing
ODOT-Type I ODOT-Type II ODOT-Type III
Sieve Size (alligator cracking) (normal) (deep ruts)
3/8 (9.5 mm) 100 99100 98100
#4 (4.75 mm) 98100 8094 7585
#10 (2.36 mm) 6886 4460 4555
#40 (420 µm) 2241 1230 1525
#80 (177 µm) 1025 820 825
#200 (75 µm) 515 515 28
Source: Hixon and Ooten (1993).
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TABLE 46
GEORGIA DOT CASE STUDY FACTS
Item Data
Binder CSS-1HLM (Ralumac with 3% natural latex)
Aggregate See Table 47
Mineral filler Type I portland cement
Job Mix Design--Typical
Aggregate 100%
Portland cement 1.0%
Water 10.0%
Binder 7.4%
Test Specification--Typical
Residual asphalt 6.8%
Wet track abrasion--1 h 807
Wet track abrasion6 day 538
Excess asphalt loaded wheel 538
Wet stripping 90%
Compatibility Pass
Location I-285 in Atlanta, Georgia
AADT 55,650
Distress Level Before Microsurfacing
IRI (m/km) 0.573 (36.1 in./mile)
Rut depth (mm) 19 mm (0.75 in.)
Friction number 4650
Length of Test Period 2 years
Snowplowing? no
AADT = average annual daily traffic.
TABLE 47 The unique feature of this study was the comparison of
GEORGIA DOT CASE STUDY noise levels with other surface courses. Table 48 provides
AGGREGATE GRADATION
the comparison with several locations in the Atlanta metro
Percentage Passing area. It shows that the change is virtually negligible. When
Sieve Size GDOT
3/8 (9.5 mm) 100
this is compared with the survey results where the respon-
#4 (4.75 mm) 80 dents cited road noise as the most frequent public complaint
#8 (2.36 mm) 6886 about microsurfacing a dichotomy exists. One possible expla-
#50 (300 µm) 2241 nation is that public road noise complaints are the result of
#200 (75 µm) 515
the differential change from the original surface, which may
have seemed quieter owing to low friction characteristics,
Results of the Georgia DOT Case Study and the microsurfacing that increased the texture of the wheel
paths.
The Georgia DOT case study is summarized as follows:
· The microsurfacing used on I-285 has performed quite well. Lessons Learned
· No additional problems with raveling or load cracking have
been encountered. The major lesson from this case study deals with the qualita-
· The mix has provided excellent smoothness and good friction,
with a minimal increase in pavement noise levels.
tive aspects of microsurfacing and its use as a "quick fix" to
· It is aesthetically superior to slurry seal because of its resem- enhance the appearance of a road at a low cost while extend-
blance to hot-mix asphalt (Watson and Jared 1998). ing its life and enhancing the safety of the traveling public by
TABLE 48
COMPARISON OF MICROSURFACING ROAD NOISE TO
OTHER COMMON SURFACES
Surface Course Average Decibels Microsurfacing Difference
Microsurfacing 74.9 --
Conventional OGFC 73.9 +1.0
Modified OGFC 72.8 +2.1
Porous European Mix 72.7 +2.2
Dense Graded Surface Mix 73.1 +1.8
Portland Cement Concrete 73.1 +1.8
Source: Watson and Jared (1998).
OGFC = open-graded friction course.
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increasing friction numbers. This speaks to the public relations Results of the Kansas DOT Case Study
aspects that impact public highway agencies. It also demon-
strated that road noise complaints are largely perceptional and The results of the Kansas DOT case study project in Cowley
that the public can be educated by showing them the numbers County, Kansas, can be summarized as follows:
such as the Georgia DOT did. Thus, this lesson can be stated
as follows: · A relatively thin application of microsurfacing
(20.6 kg/m2) placed in two lifts improved the ride
Microsurfacing can be used as a cost-effective means to quality of JPCP [jointed plain concrete pavement].
enhance the visual quality of a high-volume road while · The contractor was able to complete the 1.6 km (1 mile)
simultaneously enhancing skid resistance, smoothness, and test section including the sealing of joints and cracks in
addressing raveling and cracking issues on a high-volume 10 working days.
highway. · The ride quality improvement when a short-span, 2.4-m
ski was attached to the paving box indicated a minor
This case study did not yield any effective practices. increase (16.7% on average) in smoothness from the
original pavement.
· The use of a 4.9-m ski produced a marked improvement
Microsurfacing Performance on Concrete (49% on average) in smoothness from the original
Pavement--Kansas Department of Transportation pavement.
· The average final profile index on the project for the
The objective of this case study was to test microsurfacing's 14.1-km (8.8-mi) section where the 4.9-m ski was
ability to improve ride quality on a jointed, plain concrete used was 436 mm (27.48 in.), well within the limits
pavement on US-77 in Cowley County, Kansas. Kansas DOT established by Kansas DOT of 254 to 762 mm for
engineers investigated a number of alternatives (diamond a 100-mm-thick bituminous pavement (Moulthrop
grinding, HMA overlay, and cracking and sealing) and selected et al. 1996).
microsurfacing based on cost and time required for installa-
tion. The concrete pavement was structurally sound, although
the ride was rough owing to joints that were faulted approx- Lessons Learned
imately 6 mm (0.25 in.). Before installation, the joints and
cracks in the substrate were sealed and a tack coat consisting This case study project documents the successful enhancement
of SS-1h emulsion was applied. Tables 49 and 50 contain the of ride quality on jointed plain concrete pavement using micro-
details of this case. surfacing. It was included because much of the nation's Inter-
TABLE 49
KANSAS DOT CASE STUDY FACTS
Item Data
Binder CSS-1HLM (Ralumac)
Aggregate See Table 50
Mineral filler Type I portland cement
Job Mix Design--Typical
Aggregate 100%
Portland cement 1.75% + 0.25%
Water As required
Binder 7.6% + 0.4%
Test Specification--Typical
Residual asphalt 6.8%
Wet track abrasion--1 h Not available
Wet track abrasion--6 day Not available
Excess asphalt loaded wheel Not available
Wet stripping Not available
Compatibility Pass
Location US-77 in Cowley County, Kansas
AADT 4,000
Distress Level Before Microsurfacing
IRI (m/km) 0.848 to 0.929 (52.6 to 57.6 in./mile)
Rut depth (mm) Not applicable
Friction number Not applicable
Length of Test Period 2 years
Snowplowing? Yes
AADT = average annual daily traffic.
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TABLE 50 rigid strikeoff is used, it does not flex. This rigidity allows for bet-
KANSAS DOT CASE STUDY ter reprofiling of the pavement section (Moulthrop et al. 1996).
AGGREGATE GRADATION
Percentage Passing Therefore the following effective practice is found:
Sieve Size KDOT
3/8 (9.5 mm) 99100 When using microsurfacing to improve ride quality on
#4 (4.75 mm) 8694
#8 (2.36 mm) 4565 jointed plain concrete pavements, the spreader box can be
#16 (1.19 mm) 2546 modified to furnish better support across the joints and the
#30 (300 µm) 1535 flexible rubber strike-off can be replaced with a rigid strike-
#50 (297 µm) 1025
#200 (75 µm) 515 off to improve smoothness.
Microsurfacing Performance with a Softer
Binder--Minnesota Department of Transportation
state Highway System was constructed using this pavement
type, and ride roughness is a major issue on roads with this The objective of this case study was to test the impact of a softer
type of pavement. Often concrete pavements are found in binder on microsurfacing's ability to resist reflective cracking
urban areas where the high traffic volume initially warranted and act as a surface preparation measure for subsequent level-
the higher construction cost and lower life-cycle costs that ing or rut-filling courses. Minnesota DOT engineers investi-
concrete furnishes. The cardinal outcome of the case was the gated this treatment on four test sections originally paved in
finding that microsurfacing delivered a smoothness that was 1993. Before installation, cracks in the substrate were sealed
comparable to hot mix (Moulthrop et al. 1996). Thus, this case in only one test section and a tack coat consisting of diluted
provides a valuable tool for pavement managers dealing with CSS-1h emulsion was applied. Table 51 contains the details
this issue. Two lessons learned can be derived from this case of this case.
study project.
· Microsurfacing furnishes a cost-effective means to Results of the Minnesota DOT Case Study
improve ride quality on jointed concrete pavements; and
The results of the Minnesota DOT case study project at the
· Microsurfacing provides an expeditious means to
Minnesota Test Road Facility can be summarized as follows:
improve ride quality while minimizing disruption to
traffic.
· The construction phase demonstrated the viability of
producing and placing microsurfacing slurry mixtures at
Effective Practices 12.5% and 16.5% emulsion levels. Mixture consolida-
tion did not appear problematic when very-low-volume
In this case study, the standard microsurfacing equipment traffic was involved.
needed to be modified for use on jointed concrete pavement. · Following a 6-month service period that included a
A detailed explanation of the modifications is as follows: northern climate winter, the project was evaluated for
reflective cracking, smoothness, and rutting. Approxi-
The standard load-bearing support for the laydown box consists of
mately 71% of transverse cracks and 5% of longitudinal
three steel skis, on which the box rides as it is pulled along the pave- cracks had reflected through the microsurface.
ment. These skis are normally 1.8 m (6 ft) long at the outside sup- Transverse cracks in lanes constructed with scratch
port location and shorter at the middle of the box. For this project, and wear course mixtures had reflected through the
the skis were initially changed to 2.4 m in length to help support the
laydown box when it passed over the faulted joints. This did not
microsurface to 88% of preconstruction numbers.
produce the desired smoothness and, after consultation with the Transverse cracks in lanes constructed with rut-fill and
project engineers, it was decided that the box needed to have better wear course mixtures had reflected through the micro-
support to stop it from tipping when the skis passed over the joints. surface to 60% of preconstruction numbers.
The contractor fabricated a 4.9-m-long supported-beam leveling
arm that attached solidly to the laydown box at the outside edge.
Patched locations were not reflecting through the
This beam had attached at each end of it small metal skis that piv- microsurface.
oted when the ski passed over the joints. The beam was attached to · Pavement IRI measurements showed little change from
the laydown box so that the standard 1.8 m skis were left attached the post-construction condition. The 6-month IRI was
to the bottom of the box. The beam supported the box, which elim-
inated the tipping. The other equipment adjustment made to the
found to have decreased by 22% for lanes constructed
standard laydown box configuration was the use of a steel (rigid) with scratch and wear course mixtures, and by 58% for
strikeoff plate instead of the rubber (flexible) strikeoff that is nor- lanes constructed with rut-fill and wear course mixtures.
mally used. Usually a microsurfacing laydown box uses a rubber · Rut conditions as measured after construction showed
strikeoff to finish the surface. When a flexible strikeoff is used,
downward pressure is applied to the fresh mix, which causes a
the following results:
small amount of deformation in the surface. Rubber tends to follow A 4% to 6% decrease for lanes constructed with
the natural surface contours, thus restricting the leveling. When a scratch and wear course mixtures,
