The Ontario Ministry of Transportation (MTO) has experienced success in its trials where precast concrete slabs were used to repair road pavements. “Concrete slabs for highway repairs get an A plus grade from the Ministry of Transportation Ontario.” Though the slabs cost more, savings are found in reduced traffic disruption and extended pavement life. A pilot using concrete slabs was done on a northbound section of Highway 400 in 2016.
As new technology use is introduced with caution by the MTO, with the first test of precast concrete slab repair on concrete pavements in 2004. That trial on Highway 427 in Toronto led to a specification and more precast repair work resulted. At every step, the slabs have been a success leading to a pilot using precast concrete slabs to repair flexible pavement in a fall 2016 trial.
The results are positive: It costs more but there are savings in reduced traffic disruption and extended pavement life which more than offset the initial installation cost.
“We were not surprised by the findings, and we will continue to monitor the long-term performance of this pilot,” said Stephen Lee, Head of Pavements and Foundations Section-MTO Materials Engineering and Research Office.
These slab repairs seem best suited for heavy truck traffic on asphalt pavements with 300 mm or more thickness but with more than 25,000 trucks per day which inevitably lead to rutting and premature deterioration. Shave and pave … the standard treatment, only extends the pavement lifecycle 3 to 5 years. It’s far short of the 8 to 12 years needed before the rutting, cracking and overall deterioration returns. That’s what drove the MTO to look at literally milling out a section of pavement and dropping in precast slabs of concrete. It would be faster and ultimately more durable but would require precision cutting and high-level skills to get them seated and flush with the existing surfaces, said Lee.
He also stated that one of the big advantages is time because this type of rehabilitation work on busy highway sections must be done at night so as not to disrupt traffic flows.
The 2016 pilot project on the northbound section of Highway 400 between the intersections of Highways 88 (to the south) and 89 (to the north) tackled a section with average annual daily traffic in both directions at about 87,300.
They used the Fort Miller Superslab System with fully-cured precast concrete slabs and 3 DIFFERENT SLAB SUPPORT SYSTEMS FOR EVALUATION:
Asphalt
Grade
Grout
—based on factors like load transfer and ease of construction. Instrumentation for long-term monitoring came from a partnership with the University of Waterloo, Centre for Pavement and Transportation Technology (CPATT). Time was also a consideration: could it be done within the overnight 8-hour window? And could horizontal and longitudinal joint details be figured out?
Lee stressed that the slabs are just one option, among many for pavement repair, and will not replace other options. “This will be one of tools in the toolbox for the right pavement candidate. This technique is suitable where ease and speed of construction (product is produced before field work begins) is a premium and where conventional rehabilitation of thick pavement layers cannot be achieved within an 8-hour closure window. The product is manufactured in a controlled environment to ensure enhanced product quality that could equate to longer lifecycle,” he said.
About 22 reinforced precast sections were fabricated in late summer 2016 at the Armtec production facility in Mitchell, Ontario, Canada.
• With a specific micro- and macro-surface texture to ensure friction
• After experimenting, a broom finish was found to be the best option
• Followed by longitudinal “tining” with constant pressure applied throughout
TEST METHOD 1: ASPHALT slab support system
Prior to milling out the sections, crews took core samples to ascertain asphalt depths—on the northbound lanes, these came back at 355, 375 and 375 mm.
This left enough depth to rout out the asphalt leaving 150 mm of base for the 205 mm slabs which are a lane-width wide at 3.7 m and 4.6 m long. They were bonded with two mats of epoxy-coated dowels at 300 mm centres embedded at the transverse joints. Design strength rating was 30 MPa at 28 days with 3-% air void.
During installation over 3 nights, temporary longitudinal slabs were used as sleepers for transition from night to night. Installation started Sept 20, 2016 with 7 to 8 slabs, over 3 nights, for each of the 3 methods, and it was all kept to a tight schedule, said Lee. There were some field adjustments, specifically re-milling an additional 12 mm to achieve the perfect grade. A Wirtgen Model W120 CFI milling machine with a 1.2 m-wide milling head proved to be the right choice for the asphalt supported method which also proved to be the fastest method. It required width with precise tolerances of plus/minus 3 mm.
More interesting was the time factor:
• 8 asphalt-supported slabs were laid in just over an hour—a rate of 9 minutes per slab
• Followed by the temporary end slab
• Fast-setting bedding grout and
• Dowel grout and edge grout for longitudinal joints were applied the following night
TEST METHOD 2:
Grade-Supported Slab support system:
• Milling the asphalt
• Placement
• Grading
• Wetting the cement-treated bedding material (CTBM) prior to placing the precast concrete slab on top. • Graded using a manual leveling screed
• Compacted using the plate tamper to the correct elevation
• Material was wetted to begin hydration
• Slabs were placed directly on this bedding layer … Installation time was about 13 minutes a slab.
TEST METHOD 3:
Grout Supported Slab Support System:
• Embedded with leveling screws
• Quick-set bedding grout then injected
• Installation time was about 12 minutes a slab
The process went well with only one of 22 not meeting the 3mm tolerance and needing diamond grinding.
Falling Weight Deflectometer tests showed the average load transfer efficiency (LTE) for all 23 joints was 80.3%—well above the threshold of 70%. Friction and roughness measurements were equally positive.
The pilot was a learning process with findings to apply to future installations, said Lee.
Lee reported:
• Milling must be precise
• Bobcat-mounted chipping equipment worked faster than manually chipping the edges
• Slabs should be broom-finished at the plant
• Then diamond ground or grooved on site as a post-installation process
• Saw cutting of longitudinal joints prior to milling is not needed
• Transverse end joints should be saw-cut
• For speed, Lee recommends two crews with separate grout mixing equipment for dowel grout and bedding grout—the 2 grouts have different mixing consistency
Overall Lee reported that the slab trial was a success! Moreover, it’s fast: At full production speed, 30 to 40 slabs could be placed in an eight-hour construction window, he said.
While the data suggests the Grout Supported Slab method is slightly better, the Ministry prefers the Asphalt Supported Slab based on a cost-benefit analysis.
It was the fastest of the 3 techniques, precision milling achieves final grading and does not require additional bedding grout. Also, Lee said the Asphalt-Supported method using leveling screws would be a contingency where there are grade issues. There’s still work to be done, in addition to the monitoring to prove out the lifecycle projections.
“Ongoing monitoring results will be used to monitor the lifecycle cost of this option and the disruption cost will need to be analysed on a project-by-project basis,” Lee said. “Based on pilot project experience, a number of changes will be included in the specification to reduce the construction challenges faced by the crew. As with any new process there will still be a learning curve for a new crew.”
For the entire “Daily Commercial News” online Canadian News article, please go to: https://canada.constructconnect.com/dcn/news/projects/2019/05/concrete-slabs-get-nod-quick-highway-repairs
Home Page Photo: MTO pilot using concrete slabs
done on a northbound section of Highway 400 in 2016