127 SECTION 155
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. 128 Use scales that are: − Certified by a scale service company registered with the North Dakota Public Service Commission. If the certification is more than 9 months old when the scale begins project use, have the scale recertified. Submit a copy of the certification before starting weighing operations. − Accurate to within 0.5 percent of the true weight of the applied load throughout the range of use.
b.Dial Hopper Scales. Use dial scales that are marked with the predetermined batch weight of each ingredient by an adjustable pointer on the dial face.
c.Beam Hopper Scales. Equip beam scales with an “over and under” indicator. Include a separate beam for each ingredient.
3.Bulk Cement and SCM . Keep bulk cement and SCM separate from the aggregates. Weigh bulk cement and SCM in a weigh hopper attached to a separate scale. Charge bulk cement and SCM into the weigh hopper from weatherproof bins. Use a bin that is equipped to permit accurate control of the quantity of material withdrawn for each batch within the tolerances specified in Section 802.03 B, “Batching of Concrete Materials”.
B.Automatic Batching Equipment.
1.General. Use automatic batching equipment on projects with over 3,000 CY of concrete. For each batch the only manual operation allowed is a single actuation of a starting device to proportion the designated quantity of each ingredient into the weigh hopper. Interlock the automatic batching equipment so that: − The charging mechanism of any weigh hopper cannot be opened until the scale has returned to zero and the discharge mechanism of the weigh hopper has closed; − The discharge mechanism of the weigh hopper cannot be opened if the charging mechanism is open; − The discharge mechanism cannot be opened until all ingredients have been batched to their designated weights; and − The aggregates are weighed in the specified sequence if separate aggregate components are weighed c umulatively in a single hopper. Preset the designated batch weight of bulk cement and SCM and each separate aggregate component before the batch cycle starts. Equip the automatic scales for weighing bulk cement and SCM with an automatic recording device that graphically records or prints the batched and empty weights. Use a 129 weight recording mechanism that is accurate within one graduation of the chart or tape. Use graphic recording charts graduated for time intervals not exceeding 15 minutes, and with wei ght graduations not exceeding 0.1 percent of the nominal scale capacity. If using printed tapes, use tapes that record the consecutive batch number and ensure the minimum weight graduation printed on the tape does not exceed 0.1 percent of the scale capaci ty. Use automatic batching equipment that is capable of conversion to manual operation. If the automatic equipment breaks down, the Department will allow manual operation for up to 12 hours.
2.Batch Tickets. Generate a batch ticket for each load and send one copy with the load to the work site. Include the following information on the batch ticket: − Ticket Number; − NDDOT project number or NDDOT Project Control Number (PCN); − Delivery date; − Contractor and Subcontractor if applicable; − Time of weighing; − Identification number of truck; and − Quantity of each material in the mix.
155.03 Mixers
A.Stationary Mixers.
1.General. Use mechanically operated revolving drum or revolving blade type mixers. Use a mixer that has a manufacturer certified rating plate attached that shows: − The maximum mixer capacity; − The recommended speed of rotation of the drum or blade (rpm); and − The mixer’s registration number. Provide a NRMCA Certified plant for concrete used in Sections 550, “Concrete Pavement” , 570, “Concrete Pavement Repair” , 602 “Concrete Structures” , and 622 “Pilings”. Use mixers with a rated capacity of at least 10 cubic feet. The Department may approve mixers of smaller capacity if the volume of concrete required is less than 10 cubic yards and is not used for concrete pavement or concrete structures. Use a mixer with a total production capacity that is adequate to furnish concrete at a rate that permits uninterrupted placement and finishing operations.
2.Maintenance and Operations. Keep deposits of hardened concrete from accumulating in the mixer drum. Repair or replace the pick up and throw over blades when blade wear exceeds 10 percent of the original blade depth. Provide a copy of the manufacturer’s design showing the 130 original layout and dimensions of the blades or place permanent marks on each blade showing points of 10 percent wear from new conditions. Interlock the charging, mixing, transfer, and discharge cycles of multiple compartment mixers to prevent intermingling of batches. Use a revolving drum type stationary mixer that has a manufacturer’s guaranteed capacity capable of mixing and holding a volume of concrete equal to its rated capacity plus 10 percent without spillage. Use a revolving blade type stationary mixer that mixes and holds a volume of concrete equal to its rated capacity without spillage. Use a speed of rotation for the mixing drum or blades that is as specified by the manufacturer.
3.Water Measuring. Equip mixers with a capacity of 10 cubic feet or more with a water measuring system. Mount the water measuring system on the mixer or use a separate unit located at the mixing site. Use a water measuring system that: − Delivers the designated quantity of water for each batch within the tolerance specified in Section 802.03 B.4, “Batching Water” ; − Automatically stops the water flow when the designated quantity has been delivered; and − Is adjustable and has a calibrated indicator showing the quantity of water measured for each batch. Calibrate the water measuring system before use and as necessary to maintain the specified tolerances. Use approved manual methods for measuring water when the mixer has a rated capacity less than 10 cubic feet.
4.Admixture Measuring. Use a visual volumetric measuring unit in admixture dispensing systems where the admixture dosage rate does not exceed 32 fluid ounces/100 pounds of cement. Place the measuring unit where the plant operator can read the index points on the measuring unit from the operator’s station. Do not use a measuring unit in a plant when: − Batchi ng office and batch plant are widely separated; or − Batch cycle time is insufficient to allow the fill and discharge of a measuring unit. When a measuring unit is not used, install a flow indicator in the admixture line to indicate to the plant operator the start and completion of each batch. Install a measuring unit in the line to check the dispenser accuracy. Use a separate dispenser for each admixture.
5.Timing. When the stationary mixer has rated capacity of 10 cubic feet or more, equip mixers with controls for automatically timing the mixing cycle. Interlock the automatic timing controls 131 with the discharge mechanism so no part of the batch can be released until the designated mixing time has elapsed. Use an audible or visual signal to indicate the c ompletion of each mixing cycle. Use a lockable timing system. If the automatic timing controls fail, the Engineer may temporarily allow operation of the mixer by an approved manual timer. Use a manual timer that can maintain a mixing time for each batch of 90 seconds or more. Make repairs to automatic timing controls within 72 hours. Use a mechanical batch counter on stationary mixers used to mix concrete for pavement. When the stationary mixer has a rated capacity of less than 10 cubic feet, time the mixing cycle by a method approved by the Engineer.
B.Truck Mixers.
1.General. Use a truck mixer that has the maximum cubic yard capacities for mixing and agitating as certified by the manufacturer on the attached rating plate. Provide a NRMCA Certified plant for concrete used in Sections 550, “Concrete Pavement” , 570, “Concrete Pavement Repair” , 602 “Concrete Structures” , and 622 “Pilings”.
2.Maintenance and Operations. Keep deposits of hardened concrete from accumulating in the mixer drum. Repair or replace the pick up and throw over blades when blade wear exceeds 10 percent of the original blade depth. Provide a copy of the manufacturer’s design showing the original layout and dimensions of the blades or place permanent marks on each blade showing points of 10 percent wear from new conditions.
3.Timing. Equip the truck mixer with a revolution counter that records when the mixer is operated at mixing speed. Use an agitating speed of 2 to 6 revolutions per minute. Keep flush water for cleaning is in a separate compartment from the mixing water. Equip each compartm ent with a calibrated sight gauge. Use a mixing speed for the revolving drum type mixer between 4 revolutions per minute and a speed that results in a peripheral velocity of 225 feet per minute at the largest diameter of the drum. Use a mixing speed for the revolving blade type mixer of 4 to 16 revolutions per minute.
C.Mobile Mixers.
1.General. Use self-propelled mixers. 132 Use a mixer that has the maximum cubic yard capacities for mixing as certified by the manufacturer on the attached rating plate. Keep deposits of hardened concrete from accumulating in the mixer.
2.Measurement. Use a mixer that measures the cement and flow of water introduced into the mix. Use a visible cement recording meter equipped with a ticket printout that shows quantity used. Use a flow meter to indicate water flow that is adjustable to provide for minor variations in aggregate moisture.
3.Calibration. The Engineer will calibrate the mixer and each individual component (coarse aggregate, fine aggregate, water, cement, and admixture). Furnish: − 500 pound capacity platform scale; − 300 pounds of test weights; − Container that can be placed on the scale; and − Deflector to divert the material from the mixer into the container.
155.04 Transporting
A.General. Use equipment that delivers the concrete in a thoroughly mixed and uniform mass. Use equipment that has gates or approved means for controlling the discharge rate and vibrators to aid in the discharge.
B.Agitators. Use revolving-drum or open-top revolving-bl ade type agitating equipment, or a truck mixer operated at agitating speed. Agitate at a speed between 2 and 6 rpm. Use a drum that has a body or drum that is smooth and watertight. Use a unit that has the maximum cubic yard capacities for mixing as certified by the manufacturer on the attached rating plate.
155.05 Side Forms
Use forms: − That are clean and oiled; − Without battered top surfaces; and − That are not bent, twisted, or broken. Do not use repaired forms unless approved by Engineer. Do not use bui lt-up forms. Use metal straight side forms, at least 7/32 inches thick and at least 10 feet long. 133 For curves with a radius of 100 feet or less, use flexible forms or curved forms with the specified radius. Securely set forms to withstand the weight impact and vibration of the consolidating and finishing equipment, without spring or settlement. Use flange braces that extend outward on the base not less than 2/3 the height of the form. Use forms with top faces that do not vary from a true plane more than 1/8 inch in 10 feet and with upstanding legs that do not vary from a true plane more than 1/4 inch in 10 feet. Lock the ends of abutting form sections together.
155.06 Surfacing
A.Spreaders. Use a self-propelled spreader that is capable of spreading the concrete mix to the desired cross section and is adjustable for different thicknesses.
B.Vibrators. Vibrators for full width vibration of concrete paving slabs may be either the surface-pan type or the internal -spud type with either immersed tube or multiple spuds. Attach the vibrators to the spreader or the finishing machine, or mount on a separate carriage. Do not allow the vibrators to contact joints, load transfer devices, subgrade, or side forms. Follow the manufacturer's recommendations for vibrator frequency. Provide equipment for measuring the vibrations per minute.
C.Slip-Form Paving.
1.General. Use self-propelled equipment that places, consolidates, and finishes the concrete in one complete pass without the use of fixed side forms. Do not apply external tractive force to the equipment.
2.Vibrators. Use vibrators capable of consolidating the full width and depth of the concrete in a single pass. Use internal vibrators that operate from 3,500 to 8,000 vpm with an operating position- locking device. Use surface vibrators that operate from 3,000 to 6,000 vpm. Do not exceed the manufacturer’s recommendations for horizontal spacing of vibrators and do not exceed a maximum spacing of 16 inches. Mount the vibrators parall el to the direction of paving with the trailing ends tilted downward 15 degrees. 134 Use vibrators that meet the following specifications at manufacturer’s design frequency of 10,000 vpm: Amplitude 0.070 inches Centrifugal Force 1,000 pounds If two adjacent vibrators fail to operate, stop the paving operation and repair or replace vibrators. Repair or replace damaged or malfunctioning vibrators before beginning paving operations. The Engineer may allow the use of damaged or malfunctioning vibrators to complete that days paving operation.
3.Vibrator Monitoring System. Use a continuously operating electronic monitoring device equipped with a readout display near the operator's controls. Display the vibrator frequencies with manual or automatic sequencing among all individual vibrators. Use a monitoring system that produces a record as specified in Section 550.04 D.3.b, “Consolidation Vibrator Operation” . If the electronic monitoring devic e fails to operate, immediately check the vibrators. Repair or replace damaged or malfunctioning electronic monitoring devices within 72 hours.
155.07 Finishing
A.Manually Operated Screeds. The Engineer will allow manually operated screeds on concrete pavement repair of less than 20 feet in length, driveways, concrete tapers, and sidewalks. The Engineer will allow roller screeds if there is no visible deflection or bounce of the tube. Straightedge the tube prior to using. Use manually operated metal or metal shod screeds that are at least 2 feet longer than the maximum width of the slab. Unless the concrete is satisfactorily consolidated by vibration before screeding, equip the screed with an approved vibratory unit.
B.Transverse Finishing Equipment for Pavement. Use a self-propelled transverse finishing machine that strikes off, screeds, and consolidates the concrete. Use a machine that is adjustable to the specified crown or surface.
C.Bridge Deck Finishing Equipment. Use finishing machine that is: − Self-propelled; − Mounted on wheels which ride on a track; − Equipped with one or more power -driven oscillating or rotating screeds; − Equipped with a mechanical strike off to provide a uniform thickness of concrete; and − Adjustable to the specified cr own and elevation to maintain the required concrete cover over the reinforcing steel. 135 D. Bridge Deck Overlays Finishing Equipment. Use a finishing machine that is: − Equipped with an oscillating screed or screeds with an effective weight of at least 75 pounds for each square foot of bottom face area, and provided with positive control of vertical position, the angle of tilt, and the shape of the crown. At least one oscillating screed shall be capable of consolidating the concrete to the specified density ; − Long enough to uniformly strike off and consolidate the width of lane to be paved − Capable of forward and reverse motion under positive control; − Travelling on rails with fully -adjustable and stable supports; − Supported without the use of shims; and − Not anc hored to the concrete using powder actuated fasteners, unless that concrete will be subsequently overlaid.
155.08 Liquid Curing Compound Sprayers
Use a self-propelled sprayer with a pressure distribution system. Protect the spray nozzles with a hood. Use an approved hand-operated sprayer where necessary.
155.09 Sawing
Use a power saw furnished with suitable blades to cut pavement joints. Maintain each blade of multiple-blade saws in accurate alignment with the other blades. Use a device to guide the saw along the required joint alignment. Provide lighting for night sawing.
155.10 Joint and Crack Cleaning and Sealing
A.Cleaning. Use an air compressor that conforms to Section 156.01, “Air Compressor” .
B.Hot Pour ed Joint Sealant. Use a thermostatically controlled double-boiler heating kettle equipped with a temperature indicator and a mechanical agitator. Use a pressure-type applicator.
C.Silicone Sealant. Use a mechanical device for applying silicone sealant equipped with a nozzle or spout shaped to fit into the joint that completely fills the joint from the bottom up.
D.Preformed Elastomeric Compression Joint. Install the seals as recommended by the manufacturer.
155.11 Grinding
Use a self-propelled grinder equipped with diamond blades that is capable of uniformly grinding or removing the surface to the specified depth without damaging the underlying concrete. 155. 12 CONCRETE BUGGY Use a concrete buggy weighing less than 2,000 pounds empty. Section 156 136 SECTION 156 MISCELLANEOUS EQUIPMENT
156.01 Air Compressor
Furnish a portable air compressor capable of supplying at least 100 pounds per square inch of air pressure at the nozzle. Equip compressors with traps that maintain the compressed air free of oil and water.
156.02 Fogger
Produce a fog spray from nozzles that atomize the droplets and a system capable of keeping a large surface area damp without allowing water to drip, flow, or puddle on the surface. Use high pressure equi pment that generates a minimum of 1200 psi at 2.2 gpm, or low pressure equipment having nozzles capable of supplying a maximum flow rate of 1.6 gpm.
156.03 Milling Machine
Use a self-propelled milling machine with a control system that automatically adjus ts the elevation and transverse slope of the milling head. Provide a 30 foot minimum length skid, rolling traveling straightedge, or other approved device to establish the grade reference for control of the milling head. Use a system that permits the grade reference device to operate on either side of the milling machine and maintains the desired transverse slope regardless of changes in the elevation of the milling head. Provide conveyors capable of side, rear, or front loading and provide the necessary e quipment to transfer the milled material from the roadway to a truck.
Source: North Dakota Standard Specifications for Road and Bridge Construction, 2022 Edition. Pages 162–170 of 550.