General Parameters & Advice
Calibrating and maintaining the equipment used in seal coating is an important step in achieving good results. Poorly calibrated or mis-calibrated equipment can negate what would otherwise be a quality 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.
Calibration Guidelines
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.
Follow the nozzle manufacturer's recommendation for proper nozzle selection.
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.
Rule of thumb is that the spray bar height is 12 inches from surface.
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 by 3 foot 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).
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