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MAINTENANCE OF HIGH TRAFFIC ROADS Abstract:The successful use of slurry surfacing on high traffic roads, even in roads subject to extreme conditions of climate and /or studded tires, has been demonstrated in many projects. Such surfacing can provide a hard wearing and skid resistant surface for many years. This paper examines the requirements of slurry surfacing mixtures for high traffic roads in terms of materials (aggregates, polymers and emulsions), design and application. Special emulsion requirements, and the concepts of controlled coalescence of binders to optimize cure times, are discussed. Laboratory results are presented which show the effects of ambient conditions of temperature, humidity and sunlight (i.e. night vs. day work) on mix performance, particularly cohesion build up and abrasion resistance. Some case studies for busy roads are discussed including arterial city roads, Interstate freeways and highways. Roads in areas of very low winter temperature and roads with very high summer temperatures are included. Aspects of application and performance are presented which show the main problems and how to overcome them. 1. IntroductionSlurry surfacing is a cold applied mixture of aggregates and bitumen emulsion. It has basically two forms, slurry seal- a thin application of one stone thickness, and microsurfacing-which may be laid in thick sections of 2-3 stones thickness. The requirements for each type of system are discussed in the International Slurry Surfacing Association (ISSA) Guidelines for Slurry Seal and for Microsurfacing (1). Slurry surfacing is a very versatile, environmentally efficient and friendly (2) system and may be used for many types of maintenance surfacing. This includes surfacing to correct raveling, surface irregularities, rutting and provides a high skid resistance surface (3). In many areas, it may be used in high traffic situations to give long lasting surfaces with life times of 3-10 years. In some cases much longer lifetimes have been reported for State highways. This paper discusses the requirements for long lasting applications in high traffic areas. Currently the following States in USA have used slurry surfacing on state highways and Interstates. For example, in 1996 Georgia DOT laid over 400,000 m 2 on interstates and freeways near Atlanta before the summer Olympics. Interstates that have successfully used slurry surfacing in several states are I-80, I-5,I-99, I-77, I-81, I-44.
Many countries have also used slurry surfacing on high traffic roads, these include, Canada, Germany, Italy, France, UK, Australia, Russia China and others. 2. Mix Design And Materials RequirementsThe mix design of slurry surfacing is carried out according to several methods; mostly these methods are a variation on ISSA methods. In the past, slurry seal has been used for high traffic roads but, in the last decade microsurfacing has mostly been used. The mix design approach was examined by Texas DOT in 1998 and is currently the subject of a larger national study in USA . The main criteria for thedesign of slurry surfacing for high traffic roads are:
These properties are dependent on the materials used and the ratios in which they are used . Also they depend greatly on application conditions and technique. 2.1 Materials RequirementsAggregate must be hard, non-polishing and sound. General requirements of ISSA must be followed for high traffic roads .The following modifications are suggested. The aggregate must be hard and meet all hot mix requirements, and be as cubical as possible, with less than 10% flat stones, especially in the larger fractions. The physical properties are shown in table 1.
The grading is very important and should be type III ISSA.
The emulsion must be compatible with the aggregate used. Generally the emulsion selected is cationic and quick traffic with polymer modification. Polymer modification is about 3% of an SBR or natural latex in the emulsion. Latex should be co-milled to optimize particle sizing. Particle size is an important factor in the performance of slurry surfacing (4). The emulsion must be stable to handle and able to be mixed with aggregate. Generally mean particle size should be no greater than 3-5 microns. The requirements of the residual properties from forced fan recovery are shown in table 3.
The use of Dynamic shear Rheometer, direct tension and bending beam tests to characterize the binders for low and high temperature performance via a PG rating ,may serve as a guide. 2.2 Mixture RequirementsThe mixture requirements are based on two important factors, adhesion and cohesion, and how they are changed by different application and climatic conditions. The basic test requirements are shown in table 4. The suggested values for high traffic roads are indicated. The main requirement is that testing be done under the likely conditions of humidity and temperature. Issues such as film forming rate and cohesion build up need attention too. Aspects of compaction can also be accounted for as well in sample preparation.
2.3 Test ResultsThe effect of conditions on coalescence and film formation in high performance mixtures are shown in figures 1 and 2. These show cohesion build up effects under different conditions and the effect on abrasion resistance.  Figure 1 Effect of Curing Conditions on Mixtures 10C/High Humidity and Lab Humidity 25C/High Humidity and Lab Humidity 40C/High Humidity This figure shows clearly that high humidity and low temperature affect the cohesion build up and so the early mix performance. Indications of performance in both early life and long term performance can be found using this test. The sample treatment conditions are indicated below. A = Curing at 25C for 16 hours and 1 hour soak.  Figure 2 Abrasion loss data 3. Case Studies3.1 Average Life:The following data were collected from DOT and other sources on life times of slurry surfacing on high traffic roads. As the case studies indicate they can be much higher.
Life appears to be more affected by climate than traffic. 3.2 State Highways and Interstates CaliforniaMany state highways have been surfaced with slurry surfacing on high traffic roads, and the list below shows some of these that remain at time of writing, and actual life.
3.3 Interstate I-44The following are two typical microsurfacing jobs in Oklahoma carried out by Vance Brothers,. Tim Harrowwood provided the information. This was project #IMC-172N (232), project length about 6km. Traffic was 84,000 VPD with 10% trucks. The road is 4 lanes wide in two directions and had multiple exit ramps into Tulsa . Road condition was poor. The objective was to provide a suitable surface for 3-5 years. Work was carried out at night 6pm to 6 am . Climate is very hot to very cold with heavy snow in winter. Materials used:
Job Mix:
Equipment:
Traffic times: 1 hour maximum Scope of Work: The microsurfacing was placed in two courses. The first pass was a leveling course, and the second was the wearing course. Total Surface area was about 120,000 m 2 with about equal amounts per layer placed. Grinding was carried out on all pavement markings before the application of microsurfacing. All snow plowable markers were removed. Holes were filled with high performance cold mix. 3.4 US Highway 64/412Project #CMC-172N (238) This was carried out around the same time as I-44. This was 5km of a 4-6 lane 26,000 VPD highway. Traffic times and details are similar to the above. The same materials were used. The expected life is 5-7 years. 4. ConclusionsThe use of slurry surfacing as a maintenance treatment is well established on high traffic roads and the technique can be used in most climatic and traffic conditions. Correct design and applications are essential. 5. References
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