8.80 USE OF SPECIAL EQUIPMENT
Material transfer vehicles (MTVs) provide mix surge capacity, which allows more constant paver speed and more efficient paving operation. These vehicles operate in front of or beside the paver and receive loads of asphalt mix from delivery trucks. They perform as a mobile 22 – 33 tons asphalt mix surge bin that re-mixes and continually feeds mix to the paver hopper. Use of these vehicles results in smoother pavement by minimizing paver stops and eliminating trucks bumping into the paver. More uniform surface texture and pavement density is also achieved, as mixture and temperature segregation are virtually eliminated by MTV’s remixing capabilities. Two common MTV models used in Iowa are Roadtec SB-2500 “Shuttle Buggy” and Weiler E2850 “Remixing Transfer Vehicle”.
Use of MTVs is restricted to closed construction work zones. Applicable permits must be obtained for moving the vehicles to and from the project on the open highway for compliance with Code of Iowa weight laws. Do not allow the contractor to operate this equipment on the open road.
Due to the possibility of excessive axle loads and tire contact pressures, material transfer vehicles must be approved by the Bridges and Structures Bureau prior to use on a particular project. Approval for use can be requested by the contractor through the project engineer. Approval request is not required if no bridge structures are located within the project limits.
An analysis by the Bridges and Structures Bureau of each bridge to be crossed is required if the vehicle exceeds the allowable weight formula for bridges. This evaluation is based on the MTV in an unloaded condition and must be performed prior to crossing any bridges. The analysis is also requested by the contractor through the project engineer.
Following are the procedures to be followed for obtaining MTV approval to cross bridge structures:
1. Contractor requests permission from project engineer to use MTV on a specific project, preferably no later than at the preconstruction meeting. The contractor must provide the make and model of MTV, and any additional information needed for analysis.
2. Project engineer forwards the request to the Construction and Materials Bureau, who will arrange for review by the Bridges & Structures Bureau.
3. The Bridges and Structures Bureau will perform an analysis of existing bridge structures within the project limits and provide specific requirements regarding MTV operation across the structures. Please note that unless specifically stated otherwise, the following conditions apply to MTV use:
§ MTV hopper level must be properly managed to result in a near empty condition when crossing all bridge structures
§ MTV must cross down the centerline of structures
§ MTV speed may not exceed 5 mph when crossing structures
4. The Construction and Materials Bureau will report results of above analysis to project engineer, who informs the contractor.
Pavements
It is recommended that primary and interstate pavements on which the MTV operates are at least 8 inches in thickness. MTV shall not be operated on shoulders, subbases, or lower base course lifts of asphalt pavement without prior approval from the project engineer.
Per directive by the DOT Highway Administration Chief Engineer, the contractor may operate the MTV on existing pavements at their own discretion, with the contractor assuming all risk for damage caused by MTV use.
MTV use in conjunction with asphalt mix placement must be closely monitored and discontinued if evidence of detrimental distresses in the base or underlying pavement result. Such distresses would typically show up as deformation or rutting of base in full depth paving, or cracking and joint movement in existing pavement during an overlay. Particularly close attention should be paid to placement of the first asphalt pavement lift over cold in-place recycling and over milled/scarified existing pavement. The contractor is responsible for repairing any damage to existing pavement or base caused by MTV operation.
Mat Smoothness Machine
Several contractors have used Terex (Cedarapids) MS‑3 or MS-4 Mat Smoothness Machines on paving and resurfacing projects. This is an asphalt material receiving hopper and elevator that deposits asphalt mix into the paver hopper. Use of this equipment allows for a more consistent paver operation by providing some surge capacity for paver, only on a much smaller scale than MTVs. In some cases, it can also help re-mix material and minimize segregation.
It weighs approximately 18,800 pounds empty and has a hopper capacity of 2.22 cubic yards. Weight restrictions are not a concern with this piece of equipment.
When using Mat Smoothness Machines (or MTVs), the paver hopper should be kept relatively full at all times. If the hopper is allowed to draw down too far, coarse aggregate collected in the sides and corners of the paver hopper might be drawn down and create streaks of segregation in the mat surface.
Windrow Pick-Up Equipment
Many Iowa contractors are equipped to construct asphalt resurfacing and paving projects using windrow pick-up equipment. This process is allowed by specification.
With this process, asphalt mix is deposited in a windrow onto the pavement surface using bottom dump trailers. A windrow pick-up elevator deposits the material into the paver hopper. Again, the primary advantages are contractor efficiency, uniform speed of operation, and elimination of delivery trucks bumping into the paver.
Segregation has occurred on several projects on which this equipment was used. Truckload and longitudinal strip type segregation are potential problems. The contractor should balance their asphalt mix delivery with the mat placement rate to keep the paver hopper at a nearly uniform level, which helps avoid segregation. Balancing delivery and placement also minimize the need to either feed the hopper additional mix or remove excess windrow material with a mini-loader. The windrow should be placed to feed the center of the windrow pick-up machine. A windrow that is improperly located can place an eccentric force on the pick-up machine, which can force the paver to lose proper centerline alignment. It is also important for the contractor to pick up all windrow material from the pavement surface, and not allow the windrow to extend more than two truckloads in front of the paver to avoid excessive cooling of the mix.
Normal asphalt mix laydown temperature limitations apply to this process.
It has been shown that this process can be used successfully for the lower lift of a full depth pavement; however, it is important to make sure the pick-up machine does not disturb (pick up) the subgrade or subbase material.
If streak type segregation is suspected, a trench can be sawed transversely across the lane and the profile viewed for voids and/or a non-uniform aggregate matrix. Cores can also be cut to ascertain if segregation is present. If segregation is determined to exist, costs of the coring or sawing will be at the contractor’s expense.