Chapter 7: Slurry Seals

3.0 Mix Design

The performance of a slurry surfacing depends on the quality of the materials and how they interact during and after cure. The mix design procedure looks at the various phases of this process, which include:

• Mixing: Will the components mix together and form true, free flowing slurry?
• Breaking and Curing: Will the emulsion break in a controlled way on the aggregate, coat the aggregate, and form good films on the aggregate? Will the emulsion build up cohesion to a level that will resist abrasion due to traffic?
• Performance: Will the slurry surfacing resist traffic-induced stresses?

The steps in slurry design include:

• Prescreening of Materials
• Job Mix Design
• Final Testing

At each stage, mixing, breaking, curing, and performance issues are addressed.

3.1 Prescreening

Prescreening involves testing the physical properties of the raw materials. The emulsion type is selected based on job requirements and is checked against the requirements laid out in the specifications (Table 1). The aggregate is checked against the agency’s specifications (Tables 2 and 3) and a simple mixing test is performed to assess compatibility with the emulsion. When both of these steps have been completed, the job mix formula can be developed.

3.2 Job Mix Design

3.2.1 Mixing Proportions

The ISSA test method (detailed in Technical Bulletin 102) determines the approximate proportions of the slurry mix components (5). In this test, a matrix of mix recipes is prepared and the manual mixing time is recorded for each mixture. A minimum time is required to ensure that the mixture will be able to mix without breaking in the slurry machine. At this stage, phenomena such as foaming and coating are visually assessed and the water and additive contents required to produce a quality mixture can be determined. Figure 3 illustrates a good slurry mixture consistency.

The mixing time must be at least 180 seconds for a slurry seal at 25°C (77°F). The process may be repeated at elevated or reduced temperatures to simulate expected field conditions at the time of application. Aggregate coating is the criterion used to select the best mix from candidates with mixing times at least as long as the minimum time required through the range of expected application temperatures.

3.2.2 Cohesion Build-up

Once the emulsion content is determined, three mixes are then made, one at the selected emulsion percentage from above, one at -2% of the selected emulsion content and one at +2% of the selected emulsion content. This allows a bracketing of the desired mix proportions. The ISSA test method detailed in TB 139 (5) is used to determine the cohesion build-up in a slurry mixture. This test may be performed at the expected field temperatures to provide the most accurate estimate of the treatment’s characteristics. Table 4 lists [ISSA] mix requirements for slurry surfacing.

3.2.3 Abrasion Resistance (Wet Track Abrasion Test – WTAT)

Mixes are made at three emulsion contents, optimum, optimum +2%, and optimum -2%. These mixes are then cured in circular molds for 16 hours at 60°C (140°F). The samples are then soaked for either 1 hour or 6 days, depending on the abrasion test (TB 100) (5) and the material. Slurry design requires a 1-hour soaking. After soaking, a standard rubber hose is orbitally ground over the surface of the sample (while still submerged) for a set period of time. The wear loss is then calculated. The test equipment is shown in Figure 4, while the abrasion resistance requirements are listed in Table 4.

The results of the abrasion test are plotted along with the specification requirements. This allows selection of the minimum binder content of the mixture.

3.2.4 Upper Binder Limit

The upper binder limit is determined through the use of a deformation measurement. The Loaded Wheel Tester (LWT), the ISSA test procedure detailed in TB 147 (5), is used for the deformation measurement. In this test, a loaded wheel is placed on a cured strip of the mixture and the surface is tested. Once the surface has been tested, hot sand is poured onto the surface and the sample is then retested. When the second round of testing is complete, the amount of sand retained on the sample is measured. This provides a measure of the free asphalt on the surface of the sample. Figure 5 illustrates the test apparatus along with a series of tested samples.

3.2.5 Optimum Binder

The optimum percentage emulsion or binder content is found by plotting the results obtained from the Wet Track Test (TB 100) and the Excess Binder Test (TB 109) (5). Figure 6 illustrates a typical plot of test results. The optimum binder content is close to the intersection of the two plotted lines, but the testing does not account for all the factors influencing the mix. For example, the optimum binder content at the intersection of the plotted results is adjusted for the expected traffic conditions. A rule of thumb is to select the highest binder content that passes both tests for low traffic conditions and the lowest binder content for heavy traffic conditions. Note that this requires an experienced designer to select the optimum and this must be based on field knowledge and experience. This is a weakness in the current design process.

3.3 Final Testing

Once the job mix components have been selected, the mix is tested to determine its properties and ensure compliance with the specifications listed in Table 4. If the mix conforms to the specifications, the emulsion content and aggregate grading is reported as the job mix formula.

Field adjustments may be made to the job mix formula to accommodate climatic variables during application. As a result of the mix design process, adjustments are limited to the amount of additives (cement and retardant) and water content required to ensure a good homogeneous mix at the time of application.