VAA Virginia Asphalt Fall/Winter 2020

VAASPHALT.ORG 21 would receive a 5% bonus for that lot. After meeting with VDOT, it was decided to concentrate on AC standard deviation and production tolerances for the 2016 pilot projects. In addition to the bonus, the penalty for variable mixes was also increased. After the 2016 pilot projects, data for the 2017 to mid-2018 mixes were evaluated by VDOT to assess the AC consistency bonus’ impact. Overall, the effect was positive but not as dramatic as the field density bonus. Addition- ally, it was noted that some mixes were receiving bonuses for consistency, even when the final AC content was below the design value. The 2019 specifications addressed this concern with language that stated that the average AC content could not be more than 0.10% below the design value to be bonus eligible, along with the existing standard deviation language. This was done to ensure that sufficient asphalt was going into the mixes to improve durability. Recently, VDOT shared an analysis of AC content standard deviation data from 2016 to 2019 (see Figure 3). In general, the trend lines fromnon-pilot 2016 mixes to bonus eligible mixes for 2017 to 2019 are similar. All four years had between 60 and 70% of the mixes meeting a standard deviation of 0.15. This analysis raised the question, “What value exists in this bonus?” It also raised the question, “Is the threshold for bonus correct?” As mentioned earlier, the 0.15 was based on the average standard deviation per approved jobmix formula for the 2014 production data of VDOT’s surface mixes. A separate analysis of this data showed that approximately 60% of mixes (not tonnage) met the 0.15 (see Figure 4). Overall, more analysis is required to answer both questions. Is there value in producing consistent mixes—most definitely! Less variability in good designs improves material life and overall performance. When a more uniformmaterial is delivered to the job site, the paver and roller operators spend less time adjusting to variability (if they manage to adjust to it at all). An available bonus is an implicit recognition that superior consistency requires more effort for the asphalt plant operation from stockpile management to THE POWER OF THE CARROT In late 2011, then VDOT Chief Engi- neer Mal Kerley established the Asphalt Quality Task Force. This task force was charged with developing what the Chief Engineer described as “…A simple, workable contracting mech- anism that would help encourage quality.” continues on page 22 △ Table 1 - Pay Tables % of Theoretical Maximum Density Old Pay Percentage (%) 1 Current Pay Percentage (%) 2 Greater than or equal to 92.5 100 100 90.0 to 92.4 95 90 88.0 to 89.9 90 80 Less than 88.0 75 75 Note 1 – 2016 VDOT Road and Bridge Specifications Note 2 – 2020 VDOT Road and Bridge Specifications 2017 2018 2019 Average Density, % 98% 97% 96% 95% 94% 93% 92% 91% 90% SM IM BM 93.8% 93.9% 93.8% 93.4% 93.8% 93.9% 94.9% 94.6% 95.1% FIGURE 2: Change in In-Place Density Since 2007