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:
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.
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:
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.