HMA Overlay Construction With One Pass/Lane Width PCCP Rubblization
|
By:
Marshall R. Thompson
Professor Emeritus of Civil Engineering
University of Illinois at Urbana-Champaign
Frank R. Van Matre
Project Implementation Engineer
Illinois DOT- District 7
David Lippert
Pavement Technology Engineer and
Paul Jenkins
Project Engineer
Bureau of Materials and Physical Research
Illinois DOT
|
Various Portland Cement Concrete Pavement (PCCP) types were originally constructed on the Illinois Interstate System. The PCCP included Jointed Reinforced Concrete Pavements (JRCP), Jointed Concrete Pavements (JCP), and Continuously Reinforced Concrete Pavements (CRCP). Major PCCP distresses are slab cracking, joint deterioration and faulting, CRCP "punchouts," PCC deterioration (extensive D-cracking in IL), and Hot Mix Asphalt Overlay (HMA OL) reflective cracking and subsequent crack deterioration. PCC deterioration extent and severity increase considerably as the pavements age.
Many of the original PCC pavements on the Illinois Interstate System have been rehabilitated. Illinois DOT (IDOT) used repair/rehabilitation techniques such as slab jacking, slab patching, joint repair/replacement, diamond grinding, milling, and HMA overlay in initial Interstate PCCP rehabilitation projects.
Many complicating factors arise in selecting PCCP rehabilitation options, particularly when the section has been previously rehabilitated:
|
-
-
-
-
-
|
It is difficult to accurately estimate PCCP patching quantities (especially when prior section rehabilitation included a HMA OL);
PCCP patching is very expensive (IL unit prices of $100 to 120/square yard are common);
PCCP patching is very time consuming (HMA OL construction time is small compared to PCCP slab repair/ patching), exposing traffic and construction crews to increased safety hazards for extended time periods;
PCCP patching procedures greatly interfere with traffic flow through the construction zone (open patches, etc.) and raise more safety concerns than other operations; and
PCCP patch performance reliability is quite variable.
|
In some cases, total pavement reconstruction has been used. Total reconstruction is very expensive and time consuming, and can only be justified in special circumstances, e.g., various sections of I-80 in northern Illinois and the Chicago Expressway System.
|
| |
Pavement Fracturing Techniques
|
Thompson's 1989 NCHRP Synthesis of Highway Practice No. 144 summarized Breaking/Cracking/Seating (B/C/S) practice and technology. The primary goal of B/C/S is to reduce (hopefully eliminate) HMA OL reflective cracking.
IDOT constructed and monitored several crack & seat with HMA OL projects in the 1980s. Follow-up monitoring studies indicated that the crack & seat retarded, but did not eliminate HMA OL reflective cracking. The delay period typically varied from three to five years, and longer delays were achieved with the thicker HMA OLs. Similar trends were noted in the NCHRP Synthesis.
At the time of the NCHRP Synthesis, rubblization applications were not as widespread as B/C/S, but several states had used the procedure. A comprehensive nation-wide National Asphalt Pavement Association (NAPA) study published in 1991 indicated that rubblization was the most effective procedure for addressing reflective cracking.
Rubblization destroys PCCP slab continuity and eliminates transverse joints and the associated joint opening/closing, which cause reflective cracking. PCCP rubblizing breaks the concrete into pieces that are substantially debonded from any reinforcement. Typical specifications require most of the concrete piece sizes to be less than about 9 inches at the surface and 9 inches in the lower part of the slab.
|
| |
Equipment Developments
|
During the late 1980s and early '90s, PCCP rubblization gained favor, and rubblization equipment and construction procedures were considerably improved. The PB-4 Resonant Pavement Breaker (RPB) was a particularly significant development. The RPB vibrating head breaks the roadway concrete in 6-inch wide longitudinal strips. Thus, numerous passes are needed to cover a lane width of pavement. If the pass spacing is too large, unbroken strips of concrete (longitudinal ribs) may remain.
The maximum RPB wheel load is around 20 kips. During multiple pass rubblization operations, this wheel frequently traverses the rubblized slab. If the combined thickness of the rubblized PCCP and subbase and subgrade conditions at the time of construction are not adequate to support multiple RPB passes, severe rubblized PCCP and subbase/subgrade shoving/distortion may occur. In some instances, the RPB may "punch through" the degraded pavement section during construction.
Shoving/distortion/"punch through" considerations limit RPB use. Careful pre-construction subgrade evaluation is required to ensure that the existing subgrade stability is adequate to support RPB operations.
Multiple pass operation and subgrade stability limitations are eliminated with the use of new equipment, including the Multiple Head Breaker (MHB), developed by Badger State Highway Equipment, Inc., Antigo, WI. The prototype MHB was first used in the 1995 construction season.
The manufacturer's specification literature describes the MHB as a rubber-tired, self-propelled unit which carries 1,000-pound hammers mounted laterally in pairs. Each pair of hammers is attached to a hydraulic lift cylinder which operates as an independent unit, develops between 2,000 and 7,000 foot pounds of energy depending upon lift height selected, and cycles at a rate of up to 35 impacts per minute.
The 8-foot wide machine carries 12 hammers 8 inches in width. A 2.25-foot wing carrying two l,500-pound hammers, can be added to each side for a total breaking width of 12.5 feet Due to individual control of each lifting cylinder, breaking can be as narrow as 2 feet or increased in two-foot increments to as wide as 12.5 feet. Working speed ranges from 10 to 52 feet/minute which permits complete pulverization or selective cracking of PCC pavement.
A 12.5-foot wide MHB was used on the rubblization project described in this paper. The MHB's hammers are fitted with a transverse "breaker foot" in the center. A typical production rate is about 500-600 feet per hour (approximately one lane-mile for a l0-hourday).
The maximum MHB wheel loads (12.5 kips) are on the back of the power unit and operate on the intact slab. Thus, the MHB can operate on any PCCP of reasonable thickness. The two trailing 3-km wheels (behind the drop hammers) travel over the rubblized slab and do not cause rubblized PCCP and subbase/subgrade shoving/distortion. The RPB heavy wheel loading and multiple pass problems are thus completely overcome by the MHB.
|
| |
IDOT Rubblization Experience
|
IDOT's first rubblization project was constructed in 1990 on I-57 near Champaign, IL, as part of the SHRP Long-Term Pavement Performance (LTPP) SPS-6 PCCP rehabilitation study. Six- and 8-inch HMA OLs were constructed over a rubblized 10-inch JRCP with a 6-inch granular subbase. The RPB was used to rubblize the sections. The rubblized sections were 500 feet long. Periodic condition surveys and Falling Weight Deflectometer (FWD) tests have been frequently conducted.
Excellent performance (no HMA fatigue cracking, limited thermal cracking in the 6-inch HMA OL) has been achieved on the I-57 rubblized project which has accommodated approximately 6 million ESALs through 1996. Periodically collected FWD data indicate the sections have retained their structural capacity/integrity.
IDOT also constructed two rubblized JRCP projects on low traffic volume Illinois State routes in the early 1990s. These projects have also performed very well. IDOT's favorable rubblization experience (particularly the 1990 I-57 project) prompted planning of the construction of a larger scale rubblization project on an interstate route carrying a moderate traffic volume. This new project will provide additional construction experience, pavement response, and performance information/ data to further evaluate the rubblization construction method.
Many miles of IDOT's PCCP Interstate System are, and/or soon will be, candidates for extensive rehabilitation. Rubblization is a particularly appealing option for projects where extensive slab patching is required and/or PCC deterioration is a major distress. It is estimated (for IL unit prices) that when 12-15 percent slab patching is required, rubblization becomes more economically favorable.
|
| |
Rehabilitating I-57
|
In the Fall of 1995, IDOT District 7 was considering rehabilitation options for the northbound lane of I-57 near Edgewood in Effingham County. The pavement was an 8-inch CRCP over a 4-inch bituminous aggregate mixture (BAM) subbase. The project was constructed in 1970, and was resurfaced with a 3 1/4-inch HMA OL in 1985.
The section has experienced significant concrete D-cracking distress. An emergency 1.5-inch HMA inlay was placed in the driving lane in the summer of 1994. The initial inlay did not provide satisfactory performance. Subsequently, the HMA inlay was milled and replaced with a 1.75-inch emergency HMA inlay to carry the project through the winter and into the 1996 construction season.
Prior to placing the 1995 HMA inlay, several investigative activities were conducted: 1) Falling Weight Deflectometer (FWD) deflection tests; 2) PCCP coring (the cores were subsequently subjected to free-thaw cycles for assessing the remaining D-cracking life); 3) Dynamic Cone Penetrometer (pop) testing of the sub grade support; 4) Auger samples of the subgrade; and 5) Ground Penetrating Radar evaluation.
The investigative activities indicated that:
|
-
-
-
|
The PCC was very vulnerable to further rapid D-cracking deterioration (the cores disintegrated following 80 freeze/thaw cycles);
Extensive PCCP patching (approximately 12 percent) would be required; and
The fine-grained subgrade strengths/moduli were adequate (DCP data indicated the in site CBRs were greater than 4) to successfully support rubblization and HMA OL construction operations.
|
Based on current and projected traffic levels, the estimated l0-year design 18-km ESALs are about 7.1 million. The rehabilitation options considered were: 1) extensive PCCP patching and a 4.75-inch HMA OL, and 2) PCCP rubblization and a thicker HMA OL.
Based on studies conducted by IDOT District 7, IDOT's central Bureau of Materials and Physical Research-Pavement Review Team, and the University of Illinois, the project was divided into two parts with different rehabilitation procedures: rubblization and standard (slab patching and HMA OL). Rubblization eliminates the PCCP patching, but additional HMA OL thickness is required. The comparative performance information will be helpful in evaluating the procedures.
The standard section at the south end of the project is approximately 1.1 mile long. The existing HMA OL was milled to bare CRCP and pavement patching completed. A HMA OL was then placed. The HMA OL included two binder course lifts (1 3/4 and 1 1/2-inches thick) and a 1 1/2-inch surface course for a total of 4 3/4-inches.
The rubblized section at the north end of the project is 3.2 miles long. The existing overlay was milled off and the CRCP pavement rubblized. The 8-inch rubblized CRCP base plus 4-inch BAM subbase provided a 12-inch pavement section adequate to support the HMA OL construction. Rubblized PCCP behaves like an "unbound material."
The NAPA rehabilitation study indicated:
The product of the rubblization process leaves an in situ layer of material similar to the appearance of unbound base layers. However, it has been concluded that the inherent strength of the rubblized layer is between 1.5 to 3 times as effective in load distributing characteristics compared with a high quality dense graded crushed stone base.
Fatigue considerations control the HMA OL thickness requirement for rubblized PCCP. HMA OLs for rubblized PCCPs are thicker than those used in the traditional PCCP patching and HMA OL procedure. An 8-inch HMA OL was selected based on the excellent structural behavior (characterized by the FWD data) and performance of the 6-inch and 8-inch HMA OLs on the SHRP LTPP SPS-6 rubblized test sections constructed in 1990 on I-57 near Champaign, IL.
The HMA OL binder lift thicknesses were (bottom up) 3-inches, 1 3/4-inches, and 1 3/4-inches for a total 6 1/2-inch OL binder thickness. The specifications required construction of all of the binder lifts before traffic could use a respective lane. The binder thickness is adequate to minimize construction-related cumulative fatigue damage and minimize rutting in the rubblized PCCP layer. Finally, a 1 1/2-inch surface course lift was placed for a total 8-inch HMA OL.
Two 500-foot sections were constructed for comparison to the 8-inch HMA OL section over rubblized CRCP. HMA OL thickness effect is considered in a reduced thickness (6 1/4-inches) HMA OL section. The control section is an 8-inch HMA OL constructed over the "non-patched/non-rubblized" existing CRCP.
|
| |
Considering Equipment
|
The RPB was used to rubblize previous IDOT projects. The RPB multiple pass operations and the occurrence of some rubblized PCCP shoving/distortion/ "punch through" problems prompted the careful consideration of available rubblization equipment for use on the Edgewood project.
IDOT, a potential paving contractor, and University of Illinois personnel initially reviewed some video tape and slide presentations showing MHB operations. Favorably impressed, the group visited an MHB operation on I-65 in Indianapolis, IN. Subsequently, it was decided that the MHB should be specified for the I-57 project.
The one-pass MHB operation minimizes traffic interference and delays, enhances traffic safety, and considerably expedites construction. The reduced MHB wheel load configuration also eliminates any potential subgrade stability related to construction problems.
|
| |
Producing the HMA
|
Two dense-graded mixtures (HMA binder and HMA surface) were produced for the project. An AC-20 asphalt was used for both mixtures. The coarse and fine aggregates were crushed dolomite. Superpave mix design principles were used in establishing the HMA binder mixture.
The HMA binder course has a 3/4-inch nominal maximum size aggregate structure, contains 4.6 percent asphalt by weight of the mixture, and was constructed to approximately 7 percent air voids. Georgia Loaded Wheel testing data indicated that the binder mixture had excellent rutting resistance. The surface course has a 3/8-inch nominal maximum size aggregate structure, contains 5.4 percent asphalt, and was constructed to approximately 8 percent air voids. Similar HMA surface mixtures have excellent interstate overlay performance records in IDOT District 7.
The mixtures were produced at Howell Asphalt Company's Effingham, IL, facility using a 5-ton batch plant. Semitrailers hauled the HMA to the paving site.
|
| |
Constructing the Roadway
|
The rubblization project was a continuous operation beginning with the milling of the existing asphalt concrete surface, continuing with PCCP rubblization and compaction, and immediately followed by HMA paving. The 3.2-mile long rubblization and HMA OL paving operations were constructed in six weeks, start to finish. The accelerated pace was achieved by using the MHB, capable of one-pass/lane-width rubblization of a lane-mile of PCCP per day. The HMA binder was initially placed (driving lane, then passing lane) followed by HMA surface course paving.
The construction sequence started with milling a 12.5-foot width of the existing HMA OL. The milling operation provided a one-day head start. Rubblization began immediately. The MHB rubblized the CRCP lane (12.5 feet wide) in one pass. The centerline joint was rubblized during this pass.
The MHB was followed by two passes of a vibratory roller (53-km centrifugal force) fitted with an Elliot Z-pattern grid on the roller face. The Elliot grid face is a segmented bolt-on shell for the roller and projects approximately 1 inch from the roller face. The MHB left particle sizes ranging from 3 to 4 inches on the surface and many particles were flake-shaped. The vibratory roller reduced the "flake-shaped" surface particles to 1 to 2 inches in size. The compacted, highly keyed surface facilitated HMA OL placement. The vibratory roller with the Elliot Z-pattern grid is essential to the rubblization process.
Several test pit areas indicated the MHB and vibratory rolling provided a rubblized PCCP slab that met the project specification:
The equipment shall break the majority of the existing concrete into particles 3 inches or less above the reinforcing steel. Below the reinforcing steel, the majority of the particles shall be 9 inches or less. Concrete to steel bond shall be broken and there should be enough overlap in the rubblized pavement between lanes to provide continuous coverage.
Before the HMA paving operation, the rubblized PCCP was compacted with one pass of a rubber-tired roller and (immediately before paving) one pass of a double drum vibratory roller. After rubblizing approximately one mile, Howell Asphalt Company started conventional paving operations.
A Blaw-Knox PF-200 rubber-tired paver was initially used to lay the first lift of HMA binder. It became quickly apparent that some limited rutting of the rubblized PCCP base was occurring under the rubber tires. Contractor and IDOT personnel concurred that a track paver was needed. A Barber-Greene SA-150 track paver continued the paving operation with a 40 foot ski running off the existing pavement After placing the first HMA binder lift, the contractor switched back to the rubber tired paver in order to use an over the top/mat reference ski. HMA compaction was accomplished with two double drum vibratory rollers, one rubber-tired roller, and an 8-12 ton static tandem roller.
At times, within less than one mile, milling, rubblizing, compaction, and HMA paving were in progress in a very tight and continuous operation. HMA production/placement rates of 2,000-2,500 tons per day were typical on full-paving days. Since the rubblizing, compaction, and paving operations were so tight, weather was not a significant factor because only a very short stretch of rubblized concrete was left exposed at any one time. The work zone traffic pattern closed only one lane while the adjacent lane remained open to traffic. This traffic pattern eliminated the need for costly crossovers or road closures.
|
| |
Post-construction Evaluations
|
FWD testing (9-kip loading/12-inch diameter plate) was conducted on various sections in September 1996. Pertinent FWD data and back calculated subgrade moduli are summarized in Table 1. The AREA term (in inches) typically varies from about 15 to 18 for an unsurfaced granular base pavement to the high 20s and low 30s for an thick intact PCCP slab.
The HMA OL thickness effect is apparent Increased HMA OL thickness decreases the maximum deflections and increases the AREA term. AREA is a "measure" of the combined effect of HMA OL modulus and thickness, and increases when HMA modulus and thickness increase.
The rubblized sections are very uniform. The deflection COVs (Coefficient of Variation) are quite low compared to the 20 percent COV value used in IDOT's Full-depth HMA thickness design procedure, indicating excellent support uniformity.
As expected for the control section (8-inch HMA OL over the 8-inch CRCP), the maximum deflection is less than for the rubblized section, and the AREA is larger indicating a larger degree of "slab action."
IDOT's Video Inspection Vehicle ran the section on November 22, 1996. The profile data indicated the average IRI for the driving lane was 58 inches/mile (very good for typical IL construction). The average rut depth was 0.05 inches. Estimated accumulated ESALs for the section (from mid-August through November 22) are about 200,000. The Video Inspection Vehicle data indicate that the paving is very smooth and the HMA has good rutting resistance. FWD testing, Video Inspection Vehicle runs, and on-site pavement distress inspections will be frequently conducted in the future.
|
| |
Indications for the Future
|
IDOT's PCCP rubblization construction experience and pavement evaluation/monitoring data for three I-57 projects indicate:
|
-
-
-
-
-
-
-
-
|
PCCP rubblization and HMA OL pavements provide excellent performance. Reflective cracking and associated crack deterioration distress are eliminated.
The MHB can effectively rubblize a lane-width of PCCP in a one-pass operation.
Typical MHB rubblization production rates (approximately 500 - 600 feet per hour) significantly contribute to reducing the project construction schedules.
MHB PCCP rubblization and HMA OL construction operations can be sequenced to minimize work zone traffic interference and delays and enhance traffic safety.
The MHB wheel configuration and wheel loads eliminate the rubblized PCCP shoving/distortion/"punch through" problems associated with multiple pass/high wheel load rubblization equipment.
Rubblized PCCP behaves like a very high-quality granular base. Fatigue considerations control the HMA OL thickness. HMA OLs for rubblized PCCPs are thicker than those used in the traditional PCCP patching and HMA OL procedure.
The major factor limiting PCCP rubblization is the structural adequacy of the rubblized PCCP section (PCCP thickness plus subbase thickness) to support (for in situ subgrade conditions) subsequent construction operations. Considerations of overhead clearance requirements, shoulder pavement section buildup, and similar cross-section geometry items may also be limiting factors.
The success of the Edgewood rubblization project has prompted IDOT to consider PCCP rubblization and HMA OL a viable and cost effective rehabilitation option that is particularly appropriate when PCCP patching quantities are high and/or concrete deterioration is in an advanced stage.
|
| |
| |
Max. Deflection |
AREA |
Subgrade Modulus |
| Pavement Section |
(mils) |
(inches) |
(ksi) |
| |
Avg. |
COV(%) |
Avg. |
COV(%) |
Avg. |
COV(%) |
| 6.25-inch HMA OL |
9.3 |
9.9 |
25.3 |
2.2 |
9.4 |
12.1 |
| |
| 8-inch HMA OL |
8.5 |
10.5 |
26.2 |
2.2 |
9.2 |
12.5 |
| |
| CONTROL |
3.5 |
--- |
29.3 |
--- |
--- |
--- |
| (8-inch HMA OL/ |
| non-rubblized PCCP) |
| |
| NOTE: All deflections are normalized to 9-kip FWD loading (12-inch load plate) |
| |
Source: National Asphalt Pavement Association, Focus on HMAT, Vol.2, No.1
|
|
