Equipment World Digital Magazine
Issue link: http://read.dmtmag.com/i/620628
EquipmentWorld.com | January 2016 49 milling pattern, correct grade and slope, cleanliness of cut, consis- tency of millings and cost-effective use. That's in addition to their ob- ligation to maintain uptime for an expensive and critical piece of mobile equipment used in an extremely abrasive application. If a cold mill goes down on a job, it can result in substantial project cost overruns and even nightmar- ish disincentive payments for failure to complete a critical time- sensitive job on schedule. The milling drum, or mandrel, is the heart of the cold milling se- quence. It's the axis around which the entire milling process revolves. A well-designed and well-main- tained cutter drum will help the operator in his mission. "Cutter technology and the way they build the drums is much more advanced than in past years," says Jeff Wiley, senior vice president of Wirtgen America. "A drum is built today to nearly the same preci- sion as a fi ne Swiss watch.The drums are all turned using laser technology. When these drums are assembled new, they are spot-on, with no chance for error." Aside from breakdowns, the proof of success will be in the cut- ter pattern. Variables in control of the cutter pattern involve several elements, including the condi- tion of track pads and the cutter drum, tooth spacing, the cutter tooth and holder condition, the cutter rotation speed, the cutter wrap (tooth pattern on the drum) and tooth spacing. Other variables include the grade control system, the existing pavement condition, the ground speed, the availability of water for dust control and tooth rotation and the overall condition of the machine. Even the housing around the cutter drum is under continuous refi nement. "The industry standard has moved to an angled mold- board wrapping around the cutter Smoothness: A moving target Surveys consistently show that smoothness is the No. 1 criterion by which road users judge pavements. Pavements that are built smooth, stay smoother longer, and provide a longer life, according to engineering reports. That's why state DOTs know that building smooth pavements from the start is a cost-effective proposition. State DOTs routinely employ smoothness specifi cations in their regulations, and are basing contractor payment incen- tives or disincentives on achieved pavement smoothness. The two most commonly used smoothness indexes are the International Roughness Index (IRI) and the Profi lograph Index. A lesser-used method is the Ride Number, an index that estimates user perception of ride comfort, indicated by a number between 0 (poor ride quality) and 5 (excellent ride quality). With the early popularity of the profi lo- graph, the profi le index became a standard index for smoothness measurement in construction specifi cations. Profi lographs are relatively inexpensive, simple to operate and maintain, and display a "trace" of the pave- ment surface that users can easily understand. However, because of its wheeled confi gura- tion, it can't be used for high speed network pavement smoothness data collection. Alternatively, the vehicle-mounted inertial profi ler that measures pavement smoothness while driving at highway speeds, is replacing the profi lograph for gauging smoothness, and is used in compiling IRI-based specs. In a April 2015 report, the Louisiana Trans- portation Research Center (LTRC) studied state smoothness specs, and confi rmed that states are moving away from profi lograph- based smoothness specifi cations to, IRI-based specs. But this change is complicating the contractor's job, since there's a limited history in using IRI-based specs, leading to some confusion on how states can best structure specifi cations. "Additionally," says the report, "contractors accustomed to profi lograph-based specifi cations can struggle to achieve the same level of quality under IRI-based specifi cations." LTRC researched the state-of-practice for IRI-based specifi cations, and summarized how both asphalt and concrete road build- ers could best achieve these specifi cations. It concluded that: • Of the states with IRI-based specifi ca- tions, 85 percent provide incentive and disincentive pay adjustments for asphalt pavements, while 78 percent provide these adjustments for concrete pave- ments. • There is still a fairly wide range of IRI thresholds for incentives, disincentives, full pay, and correction and no general consensus on what thresholds are most appropriate. • There is a wide range of pay adjustments for pavement smoothness, with the ma- jority of states applying pay adjustments on a dollar amount-per lot basis versus a percentage of the contract price. • Although most states have localized roughness provisions, there are a variety of localized roughness methodologies used, and no general consensus as to which is best. "It's important to note that pavement smoothness specifi cations and practices are effectively a moving target," LTRC reported. "Agencies are continually refi ning speci- fi cations based on evaluations of existing programs and improved technology." The methods used to judge pavement smoothness – as part of acceptance testing for new or reconstructed pavements – are growing more refi ned. Also, the equipment used to obtain smooth pavements during construction is improving as digital and laser technology improves. Image: LTRC Summary of incentive/disincentive schemes for asphalt pavements There are real reasons behind the states' demands for smooth pavements.