Rennick Yard Roller-Compacted Concrete Pavement
BNSF Rennick Yard
In 1986, the Burlington Northern Railroad (BNSF Railway) converted an existing yard in Denver, Colorado to an intermodal facility, paving it with roller-compacted concrete (RCC). This facility has maintained continuous service over the last 29 years. In 2015, it underwent its first significant rehabilitation, consisting of a 2 inch cold mill and replace with stone matrix asphalt (SMA), along with localized full-depth repair (patching).
Originally, this intermodal facility was used for loading and unloading piggy-back truck trailers onto rail cars. Over time, the use has shifted with the major current activity being the exchange of standard 40 foot shipping containers between rail cars and truck trailers.
The designer of the facility, Centennial Engineering, developed both conventional jointed plain concrete pavement (JPCP) and RCC pavement design alternative at a 10 percent savings. The RCC option was selected by BNSF as the best value alternative.
Centennial Engineering completed the design of the pavement, assuming 10,000 repetitions of an 110,000 pound wheel load over a 20 year period. The design vehicle was a Piggypacker PC-90, a specialized vehicle similar to fork-lift used for lifting loaded semi-trailer onto and off of railroad cars.
Due to the way the trailer is supported in front of the Piggypacker and the need to counterbalance the trailer, extremely high loads are generated through the two wheels on the front axle.
The design methodology used was the Portland Cement Association’s “Thickness Design of Concrete Pavements Carrying Heavy Industrial Vehicles” (1973). The flexural strength of the RCC was set at 700psi, which was determined from laboratory testing of beams sawn from a test section. The modulus of subgrade reaction used in design was 170pci. These inputs yielded a required RCC thickness of 15 inches. In some areas adjacent to the rail lines where the traffic loading would be most concentrated, the RCC was placed additional 5 inches of RCC. This bottom 5 inches of RCC was not purposefully bonded to the upper 15 inches of RCC, and thus the bottom 5 inches of RCC acted as a stabilized base.
The RCC mix design was performed by Ground Engineering. The aggregate gradation was the standard Colorado DOT hot-mix asphalt gradation of the time for a 3/4 inch nominal maximum aggregate size.
The subgrade consists of sandy clay, with pockets of unconsolidated clay. These soft clay pockets were excavated and filled with granular material and then the entire site was covered with two (2) feet of granular borrow.
The design incorporated a four-foot wide trench drain running the length of the facility, with the outfall being to the south.
RCC pavement construction ran from April to May 1986. The RCC mix was produced and placed by Peltz Companies as a subcontractor to Judd Brothers. Pelts set up an ARAN ASR 2000 continuous mix pugmill on site, which kept the typical haul time from the plant to the pavers under 5 minutes. The plant produced an average 1,000 cubic yards of RCC for every day of paving. Two ABG Titan 410 pavers with tamping screeds along with Bomag 10-ton vibratory rollers were used for paving and compaction. The pavers alone were able to place the RCC mix at 95% density, with remaining compaction provided by the rollers. The pavers incorporated a 15-degree shoe for placing and densifying the longitudinal edges of the mat.
In the high trafficked areas, paving started with placement of the 5inch thick RCC base layer. This base layer was allowed to cure for seven days before the 15 inches of RCC was placed. Placement of the 15-inch thick RCC layer was originally performed in two 7.5 inch lifts; however this practice was changed during construction to first placing one 9 inch lift followed by one 6 inch lift. This change was made to reduce the amount of rolling necessary to densify the surface lift and to produce a t and to produce a smoother surface. Placement of the two lifts were performed concurrently, with no more than one hour passing between the placement of the first and second lifts, and typically far less.
No controlled or sawn joints, either longitudinal or transverse were created. Load transfer across adjacent paving lanes was enhanced by staggering the edge of the two lifts by approximately 1.5 inches.
Curing was accomplished by continually keeping the surface wet by periodically sprinkling the RCC with water. Water trucks were employed for this purpose for a period of seven days, with two or three trucks operating during the day, and one operating through the night. No curing compound membrane was used.
Since 196, the Rennick Yard has supported an average of 600 trailer or container movements per day. Container picks are currently performed using two Taylor-TETCP-1100I loaded trailer/container handlers, and one MI-Jack-800 rubber tire gantry crane. The maximum loaded axle weight of the TETCP-100I is 302,200 pounds, distributed over four, 140psi tires. the maximum wheel load of the MJ-800 is 83,890 pounds at 91psi.
The condition of the Rennick Yard RCC pavement as of July 2015 ranges from excellent to poor. In areas solely used for trailer parking, the pavement is mostly in excellent condition. The joints remain tight and the surface texture is remarkably similar to just-constructed RCC pavement.
The RCC pavement adjacent to the gantry crane runway (eastern portion of the yard) ranges from fair condition in the north, to good condition in the south. Distress are predominantly related to raveling of joints, with some load-related cracking. Notably, raveling was more severe on the joints between paving lanes than cracks within the paving lane, indicating that the raveling is likely exacerbated by lower density at the joint. This damage was mostly removed in areas that received a two (2) inch mill.
In areas that required full depth patching, delamination of the top two lifts was evident and moisture was often visable at the lift interface. No coring or test pits were performed in intact areas as part of this study, making it impossible to determine whether delamination is widespread, or if it is only associated with the more severely distressed areas.
The RCC pavement at the western portion of the yard, which is primarily trafficked by trailer/container handlers, ranges from poor condition in the north, to fair condition in the south. Load-related cracking, having an appearance similar to alligator or block cracking, becomes widespread at the northwestern portion of the yard. Cracks and joints have substantial raveling. Site operations personnel note that this raveling has predominantly occurred after treatment with magnesium chloride brine anti-icing began in 2011.
After 29 years of minima maintenance costs, BNSF has budgeted $2.5 million for rehabilitation of the Rennick Yard pavement in 2015, with HDR serving as the design engineer. The planned treatment is 2 inches of cold-milling, partial and full depth patching as necessary, and overlay with stone matrix asphalt. HDR estimated that approximately 20% of the area will require partial depth patching and a further 20% will require full depth patching. Partial depth patching includes removing the in-place RCC to the bottom of the surface lift, and replacing with SMA. Full-depth patching includes removing all of the existing RCC and filling to the bottom of the surface lift with conventional concrete, and then overlaying SMA to match the existing grade.
Life Cycle Cost
The initial cost of paving in 1986 was $3.3 million. Including $2.5 million for the current rehabilitation and an assumed $50,000 for minor maintenance every tree years, the total cost of the pavement to date is $6.2 million in non-discounted dollars, with 29 years of documented good service and an expected 10 years or more of remaining service life. In 1986 dollars, the total life cycle cost comes to $4.0, using the OMB nominal rate of 5.7% for the period of 1986 to 2016. It should be noted that this nominal rate is significantly higher than the discount rate typically used for public agency life cycle cost analyses, as those analyses typically do not use inflated future costs.
The Rennick Yard RCC pavement, although designed for 20 years of service, is now receiving its first rehabilitation after 29 years of service. For most of that life, the pavement provided good to excellent serviceability, with a notable decline in condition at about year 25. Based on conversations with site personnel, this decline coincided with the facility implementing magnesium chloride brine anti-icing practices.
The most predominant distress type evident in July 2015 is raveling of the joints. It is most likely that this deterioration is related to the relatively low density or poor curing at joints, which would increase localized permeability and reduce strength. But the role of the magnesium chloride brine anti-icing/deicers should be ignored. It is well documented that chemical deicers physically amplify freeze-thaw distress in concrete, increasing the saturation and the development of damaging pressures within the concrete as pore solution freezes (Taylor 2011). Further, recent evidence suggest that magnesium chloride deicers can contribute to chemical degradation of concrete as well (Weiss and Farnam 2015). In combination, the switch to these magnesium chloride brine anti-icing/deicing solution may have contributed to more rapid development of joint raveling than would be expected with a standard solid sodium chloride deicer or no deicer at all. While accounts of the construction process do not reveal any deficiencies in the process of densifying the joints relative to current RCC construction practice, the use of curing compound in lieu of wet-curing is now recommended. The quality of partial-depth patching would likely have been substantially reduced had maintenance been performed a few years earlier, before the joints raveled to their current depth.