123 SECTION 154
154.01 Mixing Plants
A.General. Equip the mixing plant with individual bins, or a bin with individual compartments, for each aggregate component. Construct bins to prevent intermingling of aggregate. Provide uniform production that is uninterrupted under normal operating conditions. Use mixing plants capable of calibrating and checking the proportions of each material.
B.Storage and Heating Equipment. Use storage tanks that uniformly heat the bitumen to the required temperature without damage or changes to its characteristics. Do not use direct flames against the storage tanks. Equip the tanks with a ther mometer that registers the temperature of the bitumen. Provide measuring sticks and outage tables for calibration and determining quantity.
C.Feeders. Mechanically feed aggregates into the drier.
D.Driers. Use driers that continuously agitate, heat, and dry the aggregate.
E.Thermometric Equipment. Use thermometric equipment that automatically records mix temperature as the mix is discharged from the plant. Record and submit each day’s production on a chart. Use a graduated with time increments of 15 minutes and temperature increments 10°F. Record temperature variations of 20°F within one minute of the time the variation occurs. Place the equipment where it is unaffected by plant vibration. Place a transparent dust-proof cover over the equipment.
F.Flow Rate Meter . Measure and record the quantity of bitumen being discharged into the mixer with a flow rate meter with totalizer. Use a totalizer capable of recording up to 1,000,000 gallons with a certified accuracy within 0.20 percent. Use a flow rate meter and totalizer that automatically temperature-corrects to 60°F. Calibrate the flow meter before the start of the Project and as necessary. Provide the flow rate meter’s operations manual. Submit a chart, generated by the flow meter, showing the as phalt percentage of the total mix on a time-coordinated basis. Use a chart that has maximum increments of 0.10 percent of the total mix. Keep the display and recorder accessible to the Engineer. Correct the flow if it varies by more than 0.25 percentage p oints from the set percentage of asphalt. Adjust the plant to meet tolerance within 30 minutes after the first start-up of the day and 5 minutes after any additional start-up. 124 G. Cold-Feed Control. Remove plant screens when aggregate is handled by cold -feed control, with the exception of a scalping screen. Use a scalping screen with an opening size that does not exceed the mix’s maximum size aggregate plus 1/2 inch. Provide a mechanical feeder with a separate bin for each aggregate. Equip bins with dividers and separate positive feed controls. Calibrate the positive feed controls. Use quick adjusting feed controls that provide constant and uniform flow of aggregate throughout the range of its calibration.
H.Sampling Devices. The Engineer may obtain representative samples of the aggregate before it enters the mixing unit at any time. Either stop the belt entering the mixing plant or provide an easy to operate sampling device that can be used without interrupting the mixing plant operation. Do not use the mixing plant on the project until the Engineer approves the sampling device.
I.Batch-Type and Continuous-Type Mixing Plants. Use batch-type and continuous -type mixing plants that conform to AASHTO M 156. The Department will not require a flow rate m eter as specified in Meter” for batch-type and continuous -type mixing plants.
J.Drum -Dryer Mixing Plants.
1.General. Use a drum -dryer mixing process that provides positive weight control of the cold aggregate feed with a belt scale or other device to automatically regulate the feed gate and permit instant correction of variations in load. Use a cold-feed flow and bitumen flow that is automatically coupled. Equip the system with automatic burner controls. Use systems that provide for temperature sensing of the bituminous mixture at discharge. Provide a drum with a pitch no greater than 0.75 inch per foot or as recommended by the manufacturer. When plant production is changed, maintain mix temperature within 20°F of the average temperature of mix produced during the preceding 30 minutes. Provide the manufacturer’s drum -dryer mixing plant manual.
2.Surge Bins. Use a plant that has adequate bin storage for the hot mix to ensure continuous production operations. Equip surge bins for hot storage with batchers, baffle plates, or other approved devices to minimize segregation during loading and unloading. Do not allow uninterrupted free fall of material from the loading conveyor into the surge bins.
154.02 Surfacing Equipment
A.Pavers. Use self-propelled paver with: − full-width vibrating screed, heated if necessary, capable of spreading and finishing the mix to specified lane widths and lift thicknesses; 125 − an auger that extends to within 1 foot from either edge of the vibrating screed; − receiving hopper with sufficient capacity for a uniform spreading operation and a distribution system that places the mixture uniformly in front of the screed; − screed or strike-off assembly that produces an undamaged surface; − control system that automatically controls the elevation and transverse slope of the screed; and − automatic grade reference device that operates on either side of the paver and maintains the specified transverse slope, with manual override control capabilities. The Engineer will allow hydraulic extendable screeds for variable width pavements. Establish the grade reference for control of the screed height using one of the following methods: − 30 foot minimum length travelling straightedge; − erected stringline with adequate tension to prevent deflection from the weight of the grade sensor; or − other approved device. The Engineer may allow the use of manual controls during breakdown or malfunction of the automatic screed control for the remainder of that working day.
B.Liquid Bitumen Distributors. Use a truck that maintains a uniform speed equipped with a visible speedometer that displays the truck speed in feet per minute. Use truck mounted distributors with: − circulating heati ng system that evenly controls the heating of the bitumen to the specified temperature; − thermometer that displays the temperature of the tank contents; − pressure pump capable of maintaining a uniform and sufficient pressure throughout the full length of the spray bar during operation; − pressure pump gauge that shows the pump output in gallons per minute; − adjustable full -circulation spray bars that uniformly distribute bitumen within a tolerance of 0.02 gallon per square yard; − cutoff valves to start and stop the flow quickly and uniformly over the length of spray bar without dripping; and − volume gauge that displays the quantity of material in the tank. Provide data showing the manufacturer’s instructions and recommendations for spray bar height above the road surface, nozzle size, and angle of spray fan with the spray -bar axis. The Engineer may require that the distributor be calibrated at any time. Provide for each distributor, charts, tables, or other means for accurate, rapid determination of application rates in gallons per square yard for: − spray bar widths; − truck speeds; and − pump pressures or outputs. 126 C. Aggregate Spreader. Use a spreader capable of uniformly placing aggregate over a width of one lane plus one foot, not to exceed 13 feet. Pr ovide cutoff plates to reduce the width of spread when necessary. Do not allow the equipment wheels to contact bitumen not covered by aggregate.
D.Micro Surfacing and Slurry Seal Equipment.
1.Proportioning Devices. Use a proportioning device that continuously proportions and delivers aggregate, emulsified asphalt, mineral and field control additives, and water to the mixer. Provide and label individual volume or weight controls for proportioning each material to be added to the mix.
2.Emulsion Pumps. Use heated positive displacement emulsion pumps.
3.Mixing Equipment. Use mixing equipment that: − is self -propelled; − has sufficient storage for aggregate, emulsified asphalt, mineral and field control additives, and water; − has a revolving multi -blade twin shafted mixer; and − discharge the mixed product on a continuous flow basis. Use a machine equipped with self-loading devices that loads materials while continuing to lay the micro surfacing or slurry seal coat.
4.Spreading Equipment. Equip the mixer with a mechanical type spreader box that uniformly spreads the surfacing mixture. Equip the spreader box with: − paddles that agitate and uniformly spread the materials throughout the box; − a hinge near the center to compensate for a quarter cr own; − a front seal to prevent material loss at the road contact point; − an adjustable rear seal for strike off; and − the capability to side shift to compensate for variations in pavement geometry. For micro surfacing, attach an additional strike off to the s preader box after the adjustable rear seal that provides a smooth finished surface. Do not use a burlap drag.
5.Rut Box for Micro Surfacing. Use a rut box to pre-fill wheel ruts. Use a 5 foot wide rut box with hydraulically adjustable strike off screeds. Use a rut box that is capable of an inverted or regular crown. Section 155 127 SECTION 155 CONCRETE EQUIPMENT
155.01 General
Do not use aluminum in any operation where concrete could come in contact with the aluminum.
155.02 Batch Plant
A.General. Fully load bins that have suspended weighing equipment for a minimum of 2 hours before testing. Construct the weighing mechanism so that it is unaffected by wind and other weather conditions. Provide a NRMCA Certified plant for concrete used in Sections 550, “Concrete Pavement ”, 570 “Concrete Pavement Repair”, 602 “Concrete Structures”, and 622 “Pilings”.
1.Bins. Equip the batch plant with individual bins, or a bin with individual compartments, for each aggregate component. Construct bins to prevent intermingling of aggregate. Use a scalping screen or other device on each bin that is capable of removing deleterious and oversized material from the aggregate.
2.Weighing Hoppers.
a.General. Use beam or springless dial -type weigh hopper scales. Adjust the balance to read zero before weighing each load. Do not allow the freely suspended hopper to contact anything that could affect proper functioning of the scale. Provide one standard 50 pound test weight for every 500 pounds of the scales capacity. Provide a minimum of 10 test weights up to a maximum of 20. Replace or repair scales that cannot be maintained accurately within the specified tolerances. Relieve air pressure build up by venting hoppers that are loaded by compressed air. Charge the weigh hopper to ensure the batched weights are within the tolerances as specified in Section 802.03 B, “Batching of Concrete Materials”. Weigh the separate aggregate components for each batch cumulatively in a single hopper or separately in individual hoppers. When weighed cumulatively, weigh the aggregate i n the specified sequence. Use weigh hoppers that discharge material without loss and without manual assistance. If necessary, equip the hoppers with a vibrator to obtain complete discharge. Use hoppers that hold the materials without spillage. Use dischar ge gates that do not leak when closed.
Source: North Dakota Standard Specifications for Road and Bridge Construction, 2022 Edition. Pages 158–161 of 550.