General Parameters & Advice
Calibrating and maintaining the equipment used in chip seal, scrub seal, slurry seal or micro surfacing is an important step in achieving good results. Poorly calibrated or mis-calibrated equipment can negate what would otherwise be a quality cape seal coat project.
Calibrating the chip spreader and asphalt distributor ensures that:
- All of the chip spreader gates are applying the same amount of aggregate across its entire width.
- The chip spreader is applying the desired amount of aggregate per unit area.
- The distributor spray bar nozzles are the correct size and all adjusted to the same angle, resulting in triple overlap coverage of the binder across the entire lane.
- None of the spray bar nozzles are plugged with debris.
- The spray bar height is correct.
Slurry seal/micro surfacing pavers require calibration due to their continuous mix operation. Pavers deliver a continuous feed of raw ingredients (aggregate, asphalt emulsion, fines, water and additive) to the front of the pugmill (mixer), while a homogeneous mixture of all ingredients is continuously delivered to the spreader box from the back of the pugmill.
Calibration is a process in which the actual output of the paver is measured by weight for aggregate, emulsion, and fines. These actual outputs are correlated to the revolutions of the aggregate belt since the laboratory specifies all material ratios by weight, to the amount of aggregate.
By performing the calibration process, the slurry seal/micro surfacing paver is set to deliver precise ratios of emulsion and fines with respect to the aggregate, as specified in the mix design from the laboratory. By doing this, the contractor and buyer agency know what they can expect from the mix being applied, since the tests performed in the laboratory determined the ratios. It also allows a precise record of the amounts of materials that were applied on the project for payment purposes.
Calibration Guidelines
Chip Seal Portion
Binder Distributor Calibration Guidelines
1. Check the distributor spray bar’s transverse alignment to ensure it is closely perpendicular to the centerline of the pavement.
2. Select the correct nozzle size to get the correct shot rate and even coverage. If your nozzle is too large the pattern will be streaky, if it is too small the “true” amount of binder will not be able to get out of the nozzle making the shot light.
3. Check and if necessary, adjust the nozzle angles to ensure that they spray at an angle of 30 degrees from the spray bar axis. Often, the outer-most nozzles will be turned in to give a sharp edge with no over spray.
4. Check the distributor spray bar height. The height is usually set so that a triple overlap is obtained. The distributor operator should spray emulsion onto the pavement surface for a short distance and observe where the left open nozzles hit the pavement. If the spray bar height is set too high, it causes ridges in the binder and creates uneven coverage. If the spray bar height is set too low, it causes gaps and streaking in the binder and creates uneven coverage.
5. Determine the "sprayed emulsion weight" by spraying a pre-weighed area of roofing felt and then subtract the initial weight from that of the sprayed felt weight. This weight is the "sprayed weight" of the emulsion. The sprayed weight is then divided by the ara of the roofing felt used. This is the emulsion shot rate for a given area.
Aggregate Spreader Calibration Guidelines
1. Ensure that all gates in the spreader open correctly.
2. Place a 3 foot (1 yard) by 3 foot (1 yard) tarp on the surface.
3. The aggregate spread rate is then determined by spreading aggregate over the tarp and subtracting the weight of the tarp from the total. Since the tarp is 1yd2, the aggregate spread rate will be pounds/yd2 (or metric equivalent).
Here is a short video to describe the aggregate calibration.
Slurry Seal or Micro Surfacing Portion
As there are multiple manufacturers of slurry seal pavers, each manufacturer has their preferred calibration method, however all methods follow the same theory:
Asphalt Emulsion:
- Capture the output of the asphalt emulsion pump accurately while tracking fractions of a revolution of the pump.
- Weigh the output, repeat several times and average the results to ensure there are no errors in the calibration process.
- Determine the weight of asphalt emulsion delivered for each fraction of a revolution (referred to as ‘count’) of the pump.
- Utilize this weight/count in the operating system of the paver.
Mineral Filler:
- Capture the output of the mineral filler hopper accurately while tracking fractions of a revolution of the auger.
- Weigh the output, repeat several times and average the results to ensure there are no errors in the calibration process.
- Determine the weight of mineral filler delivered for each fraction of a revolution (referred to as ‘count’) of the auger.
- Utilize this weight/count in the operating system of the paver.
Aggregate Calibration:
- Capture the output of the aggregate hopper accurately while tracking fractions of a revolution of the conveyor belt. This will include determining the moisture content of the aggregate, as the dry weight of the aggregate must be known.
- Weigh the output, repeat several times and average the results to ensure there are no errors in the calibration process. For pavers with an adjustable aggregate gate, this will be performed at three different gate settings.
- Determine the weight of aggregate delivered for each fraction of a revolution (referred to as ‘count’) of the conveyor belt.
- Utilize this weight/count in the operating system of the paver.
Water and Additive:
A similar process will be followed for the water and additive, however the amount of these materials in the mix will vary (within an acceptable range) as project conditions change, and different brands of pavers will track the amounts in different ways.
Upon completion of the above process, the output of the asphalt emulsion pump and of the mineral filler system is correlated to the output of the aggregate conveyor belt. This is a critical step, as the Job Mix Formula derived from the mix design performed by the laboratory will specify the amount of asphalt emulsion and mineral filler as a percentage of the weight of the aggregate. Follow the manufacturer’s guidelines to complete the configuration of the paver for the specific Job Mix Formula. Once completed, the paver will be configured such that every time mixing is started, the materials will be delivered in the proportions specified in the Job Mix Formula.
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