ULTRA-THIN WHITETOPPING FOR OTTAWA TRANSITWAY BUS STATION REHABILITATION

Transcription

1 ULTRA-THIN WHITETOPPING FOR OTTAWA TRANSITWAY BUS STATION REHABILITATION Rico Fung, P. Eng., Cement Association of Canada, Toronto, Canada Michael Corbett, P. Eng., City of Ottawa, Ottawa, Canada Kathy Keegan, P. Eng., LAW PCS, Beltsville, Maryland, USA Mike Richards, P. Eng., City of Ottawa, Ottawa, Canada Paper prepared for presentation at the Pavement Technology Advancements Session of the 2001 Annual Conference of the Transportation Association of Canada Halifax, Nova Scotia

2 Abstract Ottawa is the Capital City of Canada and has established an efficient transit system over the past fifteen years. The Transitway is an exclusive bus corridor, with stations dotting the routes providing rapid transit service to the population across the Ottawa-Carleton region. Initial construction on the Transitway was completed using asphalt concrete as the paving material. Buses, including articulated models, travel in excess of 70 km/hr and brake from this high speed when stopping at the stations. This tremendous speed and braking force have caused severe rutting of the asphalt pavement under the wheel paths and in some cases have shoved the asphalt over the curb creating an unsafe condition. The bus station examined in this paper, Campus Station, had been recently rehabilitated with a mill and overlay using Stone Mastic Asphalt (SMA). Within two years of the SMA rehabilitation, the rutting in the wheel paths was severe enough to require additional rehabilitation. Ultra-Thin Whitetopping (UTW) was chosen as a possible solution for rehabilitating severely rutted asphalt at Transitway Stations to restore safety and ride requirements. This paper will examine the design, construction and performance monitoring of the UTW pavement at Campus Station. Details include skid resistance, resistance to deterioration and scaling in cold climate and de-icing salt conditions. The goal is to assess the suitability of UTW technology for use in bus station rehabilitation in Ottawa. 2

3 Introduction Ultra-Thin Whitetopping (UTW) is the newest and most innovative application of portland cement concrete for rutted asphalt pavements. This technology entails bonding a relatively thin layer of concrete overlay, usually fifty (50) to one hundred (100) mm thick, onto an existing asphalt pavement. This produces a bonded composite pavement with closely spaced transverse and longitudinal joints to reduce the tensile stresses in the concrete layer from traffic loads and environment conditions such as temperature and shrinkage. Fibres are usually specified in the concrete mixes to improve the cracking resistance, toughness and ductility of the thin bonded concrete overlay. The first North American UTW project was in Kentucky in 1988; a second followed in 1990 in Colorado. The first controlled experiment was in 1991 in Kentucky [1]. The performance of this UTW installation has been outstanding from the outset and is still in service today. This excellent performance generated much interest worldwide and has resulted in more than three hundred (300) UTW projects today [1]. This technology was first utilized in Canada by the City of Mississauga in Since 1997, the City of Brampton has completed three UTW contracts involving four locations. In 1999, the Ministry of Transportation of Ontario completed a highway project in the Windsor area. These applications generated significant interest on the part of government agencies. The Region of Ottawa-Carleton, now the City of Ottawa, decided to evaluate this technology for severely rutted bus stopping areas by rehabilitating one of its Transitway stations, Campus Station, in The former Region of Ottawa-Carleton has built an efficient dedicated transit system over the past fifteen years. This Transitway is an exclusive bus corridor, with stations dotting the routes providing rapid transit service to the population throughout the Ottawa-Carleton region. Traditionally, asphalt has been the paving material used in the construction of the Transitway pavement including at bus stops. Buses, including the articulated models, travel in excess of 70 km/hr and brake at high speed when stopping at the stations. This tremendous speed and braking force have rutted the asphalt pavement under the wheel paths badly and in some cases shoved the asphalt over the curb creating an unsafe condition. In the Campus Station, premium stone mastic asphalt (SMA) was used twelve years after initial construction to address the rut problem. Unfortunately, it was placed with a thickness of only 40 mm. The SMA and underlying asphalt rutted severely two years into service. The bus volume is 200 buses/hour in each direction and the scheduled daily trips are approximately1200 in each direction. This pavement structure experiences severe freeze-thaw cycles in the winter and high asphalt temperatures in the summer. To rehabilitate the Campus Station, the former Region of Ottawa-Carleton chose to evaluate UTW as a rehabilitation strategy to restore the safety and ride requirements, and to resist the accelerated rutting that has consistently been a problem. The heaviest bus used on the Ottawa Transitway has a gross vehicle weight (unloaded) of 87 KNS (19,490 lbs) and a rear axle weight of approximately 41 KNS (9,080 lbs). The maximum allowable number of passengers on an articulated bus is 90. This will add 6300 kg to the unloaded gross vehicle weight, assuming an average weight of 70 kg per passenger. 3

4 Campus Station Pavement History Campus Station was opened in It forms a part of the City of Ottawa s (formerly Region of Ottawa-Carleton) Transitway bus only roadway system. The location of Campus Station on the current Transitway system is shown in Figure 1. Campus Station is situated near downtown Ottawa, next to the University of Ottawa campus. The station endures about 1200 bus trips per day in each direction, the majority of which stop at the station. Buses on the Transitway include traditional two axle units, but more often consist of articulated buses (a two-axle unit basically towing a single axle trailer ) as shown in Figure 2. The buses typically travel at speeds in excess of 70 km/hr and brake very rapidly at the stations when they stop to service the public. The original pavement at Campus Station consisted of traditional Ontario HL3 surface course hot mix asphalt (HMA) over HL8 base course HMA. The subgrade is consistent and competent. Over the years, since the bus only roadway was open for traffic, the pavement has deteriorated under the tremendous bus stopping. In particular, rutting has been extreme at stations where the buses stop for passengers. In 1997, the Campus Station pavement was milled to a depth of 45 mm and repaved with 45 mm of Stone Mastic Asphalt (SMA), Performance Grade Unfortunately, the pavement rutted again within two years. A single layer of SMA was insufficient to mitigate the rutting problem. Subsequent testing of the pavement revealed that rutting was occurring to a depth of up to 100 mm, that is, deeper than the SMA layer. Figure 1 Campus Station Location Map 4

5 Figure 2 Articulated Buses on Ottawa Transitway Pavement Site Condition Investigation The transportation engineers of the former Region of Ottawa-Carleton, now City of Ottawa, started the preliminary site condition survey in June 1999 for Campus Station. The north and southbound bus/curb lanes were visually inspected and rutting measurements were recorded. Four drilled core samples were also taken from each asphalt pavement and were found to have minor to severe rutting in the wheel paths. Based on these preliminary results, this station was identified as a candidate for continuous monitoring and possible future rehabilitation due to rutting in the areas where buses stop. A rehabilitation strategy, that would outlast the previous repair, was required and UTW was considered as a viable option. In March 2000 the transportation engineers met with Cement Association of Canada technical staff to further evaluate the distress condition of the Campus Station asphalt pavement and assess whether the station was a good candidate for Ultra-Thin Whitetopping technology. It was identified that the rutting had continued and some of the asphalt had been shoved up onto the face of the curb. The bus/curb lanes were in need of immediate repair before the next winter to restore safety and ride requirements in the stopping areas. In April 2000 two open cross-sectional vertical cuts were done in the severely rutted area, one in each direction of travel. The extraction of these samples enabled the evaluation of the existing asphalt pavement structure, layer thickness and extent of distress. The rut depths were in the range of 145 mm to 170 mm, measured from top to bottom of rut. The northbound pavement consisted of three construction lifts, 45 to 50 mm SMA surface course, on two lifts of HL8 base course, for a total asphalt thickness of approximately 140 mm. The SMA surface layer and approximately 50% of the underlying HL8 layer were found to be rutted. The southbound pavement consisted of four lifts of asphalt, about 40 mm SMA surface course, on 40 mm HL1/HL3 with two lifts of HL8 base course below for a total asphalt pavement thickness of about 180 mm. The SMA surface layer and HL1/HL3 underlying layer were found to be 5

6 completely rutted. The rutting also extended into the top 10 mm of the top HL8 layer, as shown in Figure 3. There was no evidence of rutting in the granular base under either the northbound or southbound pavements. This layer thickness and rut depth data were eventually used in the engineering determination of milling depths and UTW thickness. Figure 3 Cross-section of Campus Station Southbound Asphalt Pavement Prior to UTW Reconstruction Design and Construction The design of UTW differs from traditional concrete pavement design, because it is constrained by the following existing pavement factors: Elevation of adjacent pavement lane or curb and gutter, Depth of the existing asphalt, Depth of milling. The thickness design process is essentially an evaluation rather than design and involves the determination of two important factors: Load-carrying capacity, Expected service life. 6

7 Based on these design principles and tables from the American Concrete Pavement Association (ACPA) publication [1], appropriate UTW thicknesses were determined. The design incorporated the site investigation, daily bus traffic volume (1200 trips per direction) and axle loads. In the northbound curb lane, due to the thickness constraint of existing asphalt pavement (140 mm ±) and the rut depth, it was decided to mill a maximum of 75 mm to remove the failing asphalt. This would leave 75 mm of structurally sound asphalt below, meeting the minimum asphalt thickness requirement as recommended in the ACPA Ultra-Thin Whitetopping publication [1]. To bring the roadway back to its original elevation/profile, 75 mm of Class C- 2 concrete would be placed. It was understood that using the minimum recommended thickness of remaining asphalt pavement did not provide a conservative design, however, it was decided to implement this design to evaluate and better understand the behavior and performance of UTW. The station is 54.8 m long and the bus/curb lane rehabilitated is 3.0 m wide. The southbound lane had an asphalt pavement structure of about 180 mm. The UTW design mill depth was 100 mm based on the extent of rut depth in the asphalt. The new UTW concrete would be of the same thickness to maintain the existing elevation and curb height. The station is 55.0 m long and the bus/curb lane rehabilitated is again 3.0 m wide. The saw cut joint spacing was 10 times the UTW thickness, or 0.75 m and 1.00 m respectively for the northbound and southbound curb lanes. This guideline is based on the experience gained from previous projects in Mississauga and Brampton, Canada [2]. Soff-Cut technology was specified for saw cutting to eliminate any early shrinkage cracking potential. There was no thickened edge provision in either direction at the entrance and exit ends of the lanes, due to the limited thickness of residual asphalt in both pavements. Table 1 Material Specification Material Specification Compressive Strength at 24 hrs 20 MPa 28-Day Compressive Strength (Class C-2) 32MPa Cement Type Type 10 Cement Dosage 390 kg/m 3 W/C Ratio 0.35 Air Content (in-situ) 7 ± 1.5 % Fibre Content 1.6 kg/m 3 Fibre Length 20 mm The material specification (Table 1) was for high early-strength concrete with a compressive strength of 20 MPa at 24 hours, to facilitate early opening to transit traffic. A minimum in-situ air content of 7 ± 1.5% was also specified for resistance to freeze-thaw cycles and de-icing chemicals. Synthetic fibres were added to the ready-mixed concrete mixture to provide additional resistance to crack and fatigue loading. Longer fibres were not considered to avoid any potential of balling up in mixing and during placing. 7

8 The stipulated work schedule was from 10:00 p.m. Friday, June 9 to 4:00 a.m. Monday, June 12, 2000, at which time the station could be completely shut down and the buses rerouted. The Region of Ottawa-Carleton issued a Call for Quotation for Campus Station in May The call was followed by a mandatory pre-quotation meeting to familiarize the potential contractors with UTW construction techniques and practices. The Cement/Concrete industry assisted the regional transportation engineers in hosting this important contractors meeting. The construction began with asphalt milling on the evening of June 9 th as soon as the station was secured from traffic for the construction period. Milling was completed at around midnight, at which time rain poured down heavily. It took substantial effort to clean up the milled asphalt surface the following morning after the heavy rain. It is a prudent practice to properly clean the milled surface as bonding is crucial in the UTW technology to ensure that new concrete will bond to the old asphalt to form a composite pavement. The contractor was not prepared for the required level of surface cleaning of the milled asphalt. This resulted in a long delay before placement of concrete could begin. A high-pressure power-washer truck was brought in to complete the cleaning effort, as shown in the following Figure 4. Figure 4 Power-Washing Milled Asphalt Surface Prior to Concrete Placement at Campus Station The first concrete placement started in the northbound lane at around 11:00 a.m. on June 10 th. A horizontal truss-type bridge vibratory screed was employed to consolidate the concrete mixture as past Canadian experience has shown that this is the best method to ensure a homogenous mixture and enhance strong bond between the two materials [2]. Figure 5 shows the vibratory screed used at Campus Station bus/curb lane reconstruction. Following consolidation and strike- 8

9 off by the screed, the concrete was floated with an aluminum highway straight edge 3.0 m long and sealed with two coats of white-pigmented curing compound. The concreting of the southbound lane finished around 7:00 p.m. prior to an additional period of rain. Approximately 63 m 3 of ready-mixed concrete were placed that day. Soff-Cut [3] joint sawing started at 6:00 p.m. in the northbound lane approximately three hours after the concreting finished in this lane with no raveling in the green concrete. Saw cutting began in the southbound lane at around 8:00 p.m. This was too soon as the concrete had not yet reached its set. The southbound saw cutting was halted because of raveling and the rain that had started to fall. This lane was not sawcut until the following morning. Figure 6 shows the saw cutting operation at the Campus Station. Figure 5 Vibratory Screed used at Campus Station UTW Construction Figure 6 UTW Sawcutting at Campus Station 9

10 The longitudinal and transverse contraction joints were sawcut to a maximum depth of one-third of the UTW thickness and 3 mm wide. These joints did not need to be sealed. The texture of the UTW surface was specified to have a stiff broom finish followed by transverse tining at 3 mm wide and 18 mm spacing with a groove depth of 3-5 mm. The texture is required to provide adequate skid resistance. The transverse tining was completed before the sawcutting. A total of twelve (12) standard cured cylinders (100 mm x 200 mm) were cast for the concrete strength test program. These tests were carried out to ensure that the contract stipulation of 20 MPa compressive strength was achieved prior to reopening the station to bus traffic: Four laboratory cured cylinders tested had an average compressive strength of 27 MPa at 24-hour, Two field cured cylinders showed an average of 20.5 MPa at 36-hour, at which time the station was opened to buses, Two autogenous cylinders were tested at 48-hour with an average of 26.3 MPa, The average 28-day compressive strength of eight cylinders was 41.5 MPa. Thermocouples were installed in the concrete pavement in two locations for maturity testing. It was monitored throughout the construction weekend with the following results at 4:00 a.m. on June 12 th, the morning before opening to bus traffic: Location 1: the average time temperature factor (TTF) reading is 1075 degree-hours, equivalent to 19.6 MPa, Location 2: the average TTF reading is 915 degree-hours, equivalent to 18 MPa. The testing results indicated that the concrete met the 20 MPa compressive strength requirement and the station was opened on schedule for the travelling public. Post Construction Monitoring Regional engineers conducted British Pendulum testing soon after construction to monitor the skid resistance of the UTW surface. Safety at the bus stop is an important issue, and buses require frictional resistance when traveling at high speed into the station and braking. A higher British Pendulum Number (BPN) indicates higher skid resistance. Normal values for Ottawa asphalt surfaces range from 35 to 75. The skid tests were carried out on six (6) occasions with results shown in Table 2. The former Region of Ottawa-Carleton, now City of Ottawa, uses a minimum friction limit of BPN = 45 as an indication that skid resistance is insufficient. 10

11 Table 2 Surface Skid Resistance Monitoring, Campus Station UTW, 2000 Surface Condition British Pendulum (BPN) Mean Locations June 19 June 26 July 11 July 24 Sept. 27 Oct. 27 Good Good Good Good Good Good Good Good Good Good Polished Polished Good Good Good Good Good Good Table Notes: Northbound Locations 1 and 2 are at the exit, 3 is in the center, and 4 and 5 are at the entrance. Southbound Locations 6 and 7 are at the exit, 8 is in the center, and 9 is at the entrance. Northbound Lane Southbound Lane The low skid numbers occurred mainly in the southbound lane at locations 6 and 8, where there was difficulty in the early sawcutting during construction. The raveling affected the tining workmanship. In all other cases, the skid numbers appear to be stabilized and the concrete pavement surface meets the safety requirement. At the end of August 2000 cracks were observed, mainly in the northbound lane: Two cracks radiate from the existing manholes, one at the center (Figure 7) and the other at the exit end of the station, A half moon crack in two adjacent panels butted against the asphalt at the entrance of the station (Figure 8). 11

12 Figure 7 -- Crack at Manhole Located at Midpoint of the Station, Northbound Lane Figure 8 -- Half Moon Crack in Two Adjacent Panels at the Entrance of the Station, Northbound Lane There were no cracks found in the southbound lane at that time. There were, however, some local polished spots with low British Pendulum Number (BPN) for skid resistance. Note that the UTW thickness (100 mm) in the southbound lane is 25 mm thicker than the UTW in the northbound lane. 12

13 The most recent visual inspection on January 24, 2001 showed some additional cracks have developed in the northbound lane. These cracks are mainly concentrated at the entrance and exit ends of the UTW where it butts against the asphalt. The three earlier cracks have also expanded. Of a total of 292 panels, there are only 20 panels exhibiting cracks. These cracks appear to be very tight and there is no sign of scaling despite heavy use of de-icing salts. In the southbound lane, there are 165 panels in total. One panel shows a corner crack adjacent to the manhole located at the middle of the station. In addition, there is some minor spalling at various joints and some of the tining grooves appear to be very shallow. These are probably due to the saw cutting problem that occurred during the construction. Figures 9 through 12 show the condition of Campus Station UTW pavement, taken in January Figure 9 Campus Station UTW Northbound Lane January

14 Figure 10 Campus Station UTW Southbound Lane January 2001 Figure 11 Campus Station UTW Northbound Lane Cracking at Manhole/Ironwork January

15 Figure 12 Campus Station UTW Southbound Lane at Curb Showing Excellent UTW Surface Condition Observation and Conclusion The Campus Station bus lanes rehabilitation has provided further insight into the design and construction of UTW. These insights can be summarized as follows: Fast track concrete pavement construction is achievable with rapid concrete compressive strength development, Proper preparation of the milled asphalt surface is critical for bonding, especially after rain, Where UTW butts against catch basins and manholes, special details should be considered. Isolation joints must be installed at all times, Thickening the concrete edges at the transition from UTW to adjoining asphalt pavement may help to reduce the edge stress and cracking potential, Soff-Cut technology is useful in reducing early cracking, but the timing of sawcutting is crucial. As evident in this project, even the Soff-Cut technology was too soon for the green concrete in the southbound lane that resulted in a poor, rough finish, and may have caused low skid resistance in some local spots. The concrete for the northbound lane was placed during noon hours with approximately 16º C air temperature, which helped the concrete mixture to hydrate and set at a faster rate, and resulted in a successful Soff- Cut sawing three hours after the final finishing. The concrete for the southbound lane was placed at around 3:00 p.m. and did not finish until approximately 6:30 p.m. The air temperature had started to drop and it might have slowed the concrete mixture s 15

16 hydration and setting. This effect would have caused the Soff-Cut saw to ravel green concrete at the joints when saw cutting was started at approximately 8:00 p.m. The proposed solution is to raise the concrete mixture s temperature to compensate for the drop in air temperature. Plastic sheets or thermal blankets are also effective in protecting in-place concrete from cold air temperature. These measures would help to maintain the concrete temperature required for hydration and thus proper setting time. UTW has proven to be a viable rehabilitation strategy for the Campus Transitway Station, hopefully addressing the longtime asphalt rutting and shoving problem. 16

17 References 1. [ACPA 98] American Concrete Pavement Association, Concrete Information -- Ultra- Thin Whitetopping, American Concrete Pavement Association, Skokie, IL, [Fung 00] Fung, Rico; Morris, Dave & Sizer, Colin, Ultra-Thin Whitetopping The Canadian Experience, Paper at the Transportation Association of Canada 2000 Annual Conference, Edmonton, AB., Soff-Cut Technology Information from Web Site: 17