Roadway Standards Section 252 - Structural Section

Roadways shall be constructed, reconstructed and repaired with asphaltic concrete over a crushed rock base or Portland Cement Concrete over a crushed rock base.  All construction work and material shall be in accordance with Chapter 1 of these Standards.

In weak soil conditions, where the strength of the standard base rock section is inadequate, Cement Treated Base (CTB) may be used as an alternative as approved by Engineering.

In conditions where the road surface is not structurally sound or is damaged by construction activities or traffic, Full Depth Reclamation (FDR) may be used as an alternative as approved by Engineering.

252.1 Subgrade Evaluation

1. Soil testing may be required by Engineering to determine soil strength and design of the roadway structural section. 
2. Soil tests are needed on undisturbed samples of the subgrade materials that are expected to be within three (3) feet of the planned subgrade elevation. 
3. Samples are needed for each 500 feet of roadway and for each visually observed soil type. 
4. Soil tests are required from a minimum of two locations.
5. The results of the soil testing shall be included in a soils report prepared by an Engineering Geologist or Geotechnical Engineer. 
6. This soils report shall also address subgrade drainage and ground water considerations for year-round conditions, percolation data in areas of proposed drywells or french drains, and recommendations for both summer and winter construction.

252.2 Asphaltic Concrete

1. The standard asphaltic concrete structural section shall be in accordance with Standard Drawing C100. 
2. Asphalt concrete is to be ½ inch or ¾ inch dense HMAC.  Material and installation shall conform to Oregon Standard Specifications for Construction. 
3. No single lift shall be less than 2 inches or greater than 3 inches in thickness.
4. Smoothness of ride characteristics shall meet the Oregon Standard Specifications for Construction Section 00744.70. 
5. When unusually weak soil conditions exist, or in higher elevations with frequent freeze/thaw cycles, or high volumes of truck traffic exist, the pavement and aggregate thickness may be determined by the Asphalt Institute Method.  If it is determined that the street section identified in Standard Drawing C100 is inadequate, the EOR shall provide a pavement design.
6. Design values used in the asphaltic concrete design shall be supplied by an Engineer.  Traffic data shall be obtained and include the following: Design period, traffic volumes, rate of growth, and percent of trucks.
7. Design of asphalt concrete pavement structures shall conform to the guidelines determined by Engineering of the Asphalt Institute publication, Thickness Design - Highways and Streets.
8. The Equivalent Axle Load (EAL) for design of roadways shall be determined by a traffic analysis considering traffic growth, truck distribution determined on the basis of local traffic data and load equivalency factors as set forth in the above-referenced manual.  For collectors and local roadways, the EAL may be determined using simplified procedures which relate the EAL to the average daily number of 18,000 lb. single axle loads estimated for the design lane during the design period.
9. Testing and evaluation of the subgrade soil strength shall be required for all pavement designs.  Testing methods shall include but not be limited to:
   1. The Asphalt Institute Publication, Method of Test for Resilient Modulus of Soil, Manual Series No. 10 or
   2. AASHTO T-193 (CBR Method), or
   3. AASHTO T-190 (R-Value Method)
10. If the CBR value of the subgrade exceeds twenty (20) or the R value of the subgrade exceeds sixty (60), then CBR and R-Value methods shall not be used.
11. Test methods and results shall be incorporated in a soils report in accordance with the requirements of Section 250.10 of these Standards.

252.3 Portland Cement Concrete ("PCC")

The PCC structural section shall be determined using the guidelines and requirements of the Portland Cement Association ("PCA").  The following design parameters shall be used:

1. Design shall be determined by projected estimated axle loading (EAL) of the road.
2. Minimum thickness of PCC shall be seven inches.
3. Jointing plans will be required one week prior to start of construction, delineating intersection and utility structure jointing for final review and approval by Engineering.
4. Design values used in the PCC design shall be prepared by an Engineer.  Traffic data shall be obtained and shall relate to a 20 year projection.

252.4 Cement Stabilized Roadway (CSR) by Full Depth Reclamation (FDR) or Cement Treated Base (CTB)

252.4.1 General

This work consists of in-place construction of cement stabilized roadway utilizing pulverized existing pavement and base materials mixed with Portland cement as per design specifications, and shaping  the roadway to design/plan  grades and cross slopes, including cure sealing or paving and the relocation of excess sub-grade material where required.  The first till pulverizes for shaping to compacted design base grades (minus cement volume) and the second till incorporates (mixes) the cement into the prepared compacted roadway to final design base grades and seals the CSR for curing via fog seal, chip seal or an asphalt base lift per design/plan specifications.

This section details the requirements for full depth reclamation (FDR).  Cement treated base (CTB) will follow the same standards with the exception that there is no existing road surface to pulverize.  The CTB requirements cover the remediation of both base rock and sub-grade.  If the CTB shall be overlain with a full depth of base rock as per these standards, a chip seal or vapor barrier is not required.

252.4.2 Materials, Preparation & Equipment

1. The aggregate shall conform to the requirements of the Oregon Standard Specifications for Construction Section 02630.10.
2. The Portland cement shall conform to the requirements of the Oregon Standard Specifications for Construction Section 02010.
3. The water shall conform to the requirements of Oregon Standard Specifications for Construction Section 00340.
4. Portland cement shall be applied at the rate determined by an engineered design in percent of the dry weight of the material within a depth to be treated to achieve a seven day strength between 300 psi and 400 psi.  The design shall indicate the optimum moisture content. 
5. The Engineer shall obtain samples of the material to be pulverized to determine the design.  Cores will be taken at 7 days and tested as per ASTM C39/C39M-12a and ASTM C42/C42M-12 to confirm that the strength of the CTB is within the correct range.
6. The asphalt used in the curing seal shall be either CRS-1 or CRS-2 emulsified asphalt as designated. The emulsified asphalt shall conform to the requirements of the Oregon Standard Specifications for Construction Section 00710.  Cover aggregate for the cure seal shall conform to the requirements of Section 00710 and shall be ¼ - #10 size.
7. A traveling single or multiple transverse shaft mixer shall be capable of mixing to a depth of 12-inches in one pass.  The cutting and mixing rotor shall be capable of adjustment to conform to the slope of the pavement.  The mixer must have a working water system to bring the CSR to optimum moisture content.
8. Cement shall be spread using a mechanical spreader capable of uniformly distributing the cement across the width of the spread.  The cement spreading equipment shall be in good working condition and shall be equipped with a metering device and travel speed indicator capable of accurately metering and uniformly spreading the required amount of Portland cement on the grade.
9. Equipment used to compact CSR shall be self-propelled 12 ton minimum, vibratory steel wheel, tamping foot, and/or pneumatic tire rollers.  Rollers shall be capable of compacting the material to a firm, even surface.  The tamping foot roller shall be placed immediately behind the tilling operation and before the initial grader operation.

252.4.3 Construction

1. The CSR shall be constructed so that the work will result in a finished sealed or continuously watered, curing CSR section conforming to specifications regardless of the daily or seasonal variations in weather, temperature and humidity under which the work is permitted to proceed.  CSR shall not be constructed during periods of rain.  CSR shall not be constructed out of frozen bases.  Construction shall not occur when descending air temperatures fall below 40° F.  Cement shall not be applied during windy conditions.
2. On the first till the existing road base and pavement materials shall be pulverized to a depth, as specified in the engineered design/plans to a condition such that all material will pass a 2-inch sieve.
3. Roads Without Curbs:  The surface of the pulverized material shall then be brought to the uniform grades and cross sections, as shown in the plans for the final CSR grades (minus the cement volume) and compacted to specifications.
4. Roads With Curbs:  The plans will show the first till depth noted above, as the depth of the existing asphalt and rock sections combined or 12" maximum.  This material is to be stock piled and the subgrade (not tilled) is dug out and removed (to accommodate the depth of the new asphalt section, the cement and redistributed stockpiled material; so that the final finished asphalt grade matches the designed/plan curb exposure).  The stockpiled material is then evenly redistributed, and brought to uniform grades and cross sections, as shown in the plans for the final CSR grades and compacted to specifications.  Any material to be hauled off must go to an approved dump site.
5. The County makes no representation as to the type and size of the material that may be encountered in the existing roadway.  The contractor shall notify the County immediately if the type and size of material (solid objects  3" > dia.) exceeds that which can  be cement treated without excessive damage to the tiller. 
6. In those areas which show excessive deflection or exhibit pumping under the wheel loads of the construction equipment, the pulverized material shall be removed and the sub-grade shall be over excavated to a firm depth as directed by the Engineer.  Backfill the over excavation to the top of the existing sub-grade with 2"-0 size aggregate compacted in 9-inch maximum, loose depth, thickness lifts.  Each lift shall be compacted to at least 95 percent of the maximum dry density determined by inspection.  After backfilling of the over excavation, the remainder of the depth shall be backfilled with stockpiled pulverized material and brought to a uniform grade and cross section.
7. Cement shall be uniformly applied at the designated rate.  The equipment and method used shall ensure the uniformity of cement distribution throughout the material to be treated.  Water shall be added at the tiller during mixing operations to bring the mix to within – 0 to +1 ½ percent of the optimum moisture/density point.  This moisture content shall be maintained until the mixing is completed.      
8. The second tilling/mixing of the cement, water, and aggregate materials shall be started immediately but no later than two hours after the application of the cement.  Mixing shall continue until a homogeneous mixture is obtained.
9. The CSR mixture shall integrate the pulverized material to a depth of 12 inches or design/plan depth.. This CSR mixture shall then be brought to a uniform profile and cross section as noted in the plans.  Shaping and compaction of the CSR mixture shall be completed within two hours after mixing has been completed.
10. Special attention shall be taken around utility structures, survey monument boxes and next to curbs to ensure that the material is thoroughly pulverized, mixed with cement, moistened and compacted to the specified depth.  Material that is inaccessible to the mixer shall be bladed or shoveled into the pulverizing and mixing process after which it shall be returned to its original position.  Vibratory plate compactors shall be used to achieve compaction of the mixture in areas which are inaccessible to the rollers.
11. Special attention shall be given to ensure that the material next to all joints is thoroughly pulverized, mixed with cement, moistened and compacted to the specified depth.
12. Longitudinal and transverse joints adjacent to partially hardened CSR shall be constructed by cutting back with the mixer into the previously constructed work.  The amount of the overlap shall be sufficient to cut back into solid materials.
13. Longitudinal and transverse joints adjacent to existing asphalt, concrete or hardened CSR shall be formed by saw cutting back into the work to form a straight vertical face.  When completed, the face of the joint must be free of loose and shattered material.

252.4.4 Curing

Immediately after the grading, compaction and finishing of the cement treated base has been completed and during the same day while it is still moist, the surface shall be sealed with a fog or chip seal. The fog or chip seal shall be applied in accordance with applicable portions of Section 00710 of the Oregon Standard Specifications for Construction at the rates of 0.26 gallons per square yards of emulsified asphalt and 0.006 cubic yards (truck measure) per square yard for the cover aggregate.  An asphalt surface course may be chosen as a sealing course on high volume roads that must remain in constant service when the County determines that the chip seal does not have sufficient strength to carry the anticipated traffic.  If an asphalt surface course is chosen to seal the CSR, it may be installed no sooner than Day 2. If the fog/chip seal or asphalt concrete surface sealing course is not placed within forty eight hours (Day 2) following start of the mixing operation, then the CSR shall be allowed to cure, while maintaining moisture, for a period of 7 days (Day 7) before placing any asphalt concrete surface course.

252.4.5 Micro-Cracking

1. If specified by the Engineer and approved by Engineering, micro-cracking may be used as part of the design.  Micro-cracking shall occur between Day 2 and Day 3.
2. Micro-cracking consists of 3 full passes, up and back, of a 12 ton vibratory roller with maximum vibrations for the full extent of the CSR.

252.4.6 Performance

1. The cement treated mixture shall be compacted to 98 percent of the maximum dry density as established by AASHTO T 134.  Final finishing shall be accomplished by rolling accompanied by light watering and reshaping to provide a finished surface free of hairline cracking and free of ridges exceeding 0.04 foot in height.
2. If the specified compaction is not obtained, the contractor shall notify the County and Engineer.  The contractor may be required to use a modified compaction procedure or apply additional compactive effort.
3. If approved materials meeting the specifications cannot be compacted to the required density regardless of compactive effort or method, the Engineer may reduce the required density or direct that alternate materials be used.  In no case shall CSR construction proceed until the contractor is able to compact the material to the satisfaction of the Engineer.
4. When directed by the Engineer, the surface of the CSR shall be tested with a 12-foot straight edge provided by the contractor.  No point shall vary by more than 0.04 foot from the testing edge when applied in any direction to the pavement surface.  The completed surface of the CSR shall be within plus or minus .04 foot of the grade required to allow for placement of the specified thickness of asphalt concrete to the designated finished grade height.

252.4.7 Traffic Control Considerations

The CSR construction shall be scheduled so that at the completion of the day the work may be opened to local traffic.  The surface of the CSR shall be protected by placement of the asphalt concrete surface course or by placement of the cure seal.  If a cure seal is placed, it shall be placed a minimum of two hours in advance of opening the road to traffic.  The asphalt surface shall be below 140 degrees before opening the road to traffic.  When approved by Engineering, the road should be closed to through traffic, especially to through truck traffic, for 7 day cure period per Section 290.