540-1 DESCRIPTION
Cement treated base course is a mixture of aggregate material, Portland cement, and water that is mixed at a pugmill (Plant-mixed cement treated base course) or mixed on site on the road (Road-mixed cement treated base course).
540-3 LIMITATIONS
Since cement hydration practically ceases when temperatures are near or below freezing, cement treated base should not be placed when the temperature is below
40o
F. In addition, it should be protected to prevent freezing for a period of
seven days after placement. The seasonal limitations outlined in
Article 540-3 of the Standard Specifications shall be strictly adhered to. Any deviations from these limitations will be approved only on a day-by-day basis by the Engineer.
The Engineer should monitor the Contractor's progress to ensure that all cement treated base is covered with a subsequent layer of pavement by
December 1st of the same year. In the event the Contractor does not cover the cement treated base, a sand seal must be placed at the Contractor's expense. This seal does not relieve the Contractor from any damages that may result to the cement treated base.
540-4 PREPARATION OF SUBGRADE
Subgrade areas that are unstable must be corrected prior to placement of aggregate. If the subgrade is relatively dry, it should be moistened prior to placing aggregate.
540-5 CONSTRUCTION METHODS
(A) GENERAL- COMPOSITION OF MIXTURE
The source of aggregate must be documented by a current approved job mix formula. If there is no current approved job mix formula, the proposed aggregate source shall be sampled and the samples shall be submitted to the Department’s
Materials and Tests Unit at least
three weeks prior to beginning production. Refer to
Table 540-1 in the Standard Specifications for job mix formula tolerances.
(B) PLANT MIXED CEMENT TREATED BASE COURSE
(1) MIXING
(a) General
The cement, aggregates, and water must be thoroughly
mixed in an approved central mixing plant. The
approval.
(b) Batch Type Plant
The batch weights of cement, aggregate, and water
must be calculated according to the capacity of the
plant.
(c) Continuous Flow Type Plant
It is very important that the plant be calibrated to
make sure proper quantities of materials are entering
the mixer and/or rotary-vance feeder. Each requires a
surge tank or hopper for proper operation. The speed of
the cement meter must be synchronized with the speed
of the main feeder (aggregate) belt. See below for
examples of the calibration calculations for a pugmill.
(2) HAULING AND PLACING
Protective covers must be used on trucks hauling cement treated base to the roadway in order to avoid moisture loss. Time of loading to beginning of compaction is limited to
one hour. The weigh ticket for each truck shall show the batch time. Place the cement treated base course on a moistened subgrade utilizing approved spreaders so that spreading progresses along the full width of the base in a uniform manner. If one spreader is not wide enough to cover the full width, then additional spreaders are required. Compact the base with approved equipment immediately following spreading to reduce moisture loss. A variety of compaction equipment may be required in order to achieve density. The
Geopavement Section should be consulted for assistance with compaction issues.
(C) ROAD MIXED CEMENT TREATED BASE COURSE
(1) EQUIPMENT
All equipment to be used in the stabilization operation should be inspected and approved to be in good and safe working condition. The cement spreader must have an adjustable rate of flow and be capable of spreading the required amount of cement in one pass. All mixers shall be self-propelled and able to mix to a compacted depth of at least
10 inches. Equipment that leaks oil, grease, water or other materials may be removed from the job site unless repaired or replaced.
(2) SPREADING AND MIXING
The aggregate shall be placed on the prepared subgrade by means of mechanical spreading equipment to ensure uniform depth and to minimize segregation. The amount of aggregate placed prior to the applications of cement and mixing shall be that amount that can be processed within
one week.
After the cement has been uniformly spread over the aggregate, the mixture shall then be thoroughly mixed with the use of a rotary mixer. The appropriate amount of water shall then be added. The mixing should continue until a uniform mixture is obtained. The moisture content at the time of final mixing and compaction shall be as required by
Subarticle 540-5(C)2.
540-6 COMPACTION
Moisture content of the cement treated base must be adequate to achieve compaction. Compaction must be expedited to avoid exceeding the
three-hour time limit. Mixing, spreading, compacting, and cutting grade to specified elevation is included in this three-hour time limit.
540-7 CONSTRUCTION JOINTS
At the end of each day, a straight, transverse, vertical joint should be formed by cutting back into the completed work. In large or very wide areas, a similar longitudinal joint may be required. This procedure is typically performed with a motor grader. Attention should be given to forming a vertical face free of loose or shattered materials.
540-8 TOLERANCES
The thickness of the base shall be within a tolerance of plus or minus
½ inch of the base thickness required by the plans. Base course under concrete pavement shall have a tolerance of plus or minus
¼ inch. The grade and thickness will be determined and recorded in a grade book by the Technician and should not vary more than
½ inch per 100 feet, (e.g., if a location indicates an excess of
½ inch of stone at
one location and is shy
½ inch of stone
100 ft away, then this section
meets the thickness tolerance but does not meet the grade tolerance maximum of
½ inch difference in the established grade within a
100 foot section). When used as a base course under concrete pavement, the grade tolerance shall be a maximum of
¼ inch difference in the established grade within a
100 ft section.
540-9 CURING
Curing of cement treated base utilizes membrane curing, which involves sealing the compacted layer with an asphalt curing seal. Careful attention should be given to the rate of application of curing seal to ensure that the layer is fully covered, but not to the extent that the asphalt runs off excessively. A
7-day curing is required. See
Section 543 of this Manual for more information regarding the curing seal.
540-10 AGGREGATE FOR CEMENT TREATED BASE
Aggregate for cement treated base course may be obtained from previously approved stockpiles or from quarry production (see
Articles 540-10 and
1010-2 of the Standard Specifications).
540-11 TRAFFIC
The stabilized area will be closed to all traffic during the curing period, except for local, lightweight traffic. Areas of roadway, which have to be utilized by local, lightweight traffic should be covered with sand at the rate of
10 pounds
per square yard or other material that has been approved by the Engineer, refer to
Section 818.
Construction equipment shall not use stabilized areas for haul roads, except to discharge material into a spreader during base construction or paving operations.
The heavy loads imposed by hauling and paving equipment may create a major problem in some stabilized layers. The wheel loads from this equipment will produce higher stresses in the stabilized soils than the stresses anticipated from traffic loads after the road is in service.
540-12 MAINTENANCE
The stabilized area shall be protected from freezing and maintained by the Contractor until it has been covered with a layer of pavement. Any maintenance or repair necessary will be performed by the Contractor at no cost to the Department and shall be repeated as often as may be necessary to keep the stabilized base in an acceptable condition until all work is completed and accepted.
540-13 MEASUREMENT AND PAYMENT
The quantity of base course to be paid for will be the number of tons satisfactorily incorporated into the CTBC and will be measured by weighing in trucks on certified platform scales or other certified weighing devices per
Article 106-7. No deduction will be made for any moisture contained in the base mixture at the time of weighing. Measurement will not be made of any base mixture added or replaced for corrective measures during construction or for repairing damaged areas.
The quantity of Portland Cement to be paid for will be the number of tons of cement that has been incorporated into the mix (by weight ticket or theoretical). When bulk cement is used, the quantity will be measured by weighing in trucks on certified platform scales or other certified weighing devices per
Article 106-7. Measurement will not be made of any cement added or replaced for corrective measures.
ADDITIONAL RESOURCES
