Several types of stabilized bases are used in air- port concrete pavement construction. The most commonly used materials include cement- treated base (CTB), lean concrete base (LCB and also known as econocrete), and asphalt- treated base (ATB). These materials provide an excellent base over a properly prepared sub- base or subgrade.
The stiffness of stabilized base layers has an im- pact on the performance of the concrete pave- ments. They affect the curling/warping behavior of the slab and they increase the restraint on the slab during the initial curing period. In the case of LCB, the base stiffness can be extremely high. The result is an increased potential for random cracking, reflective cracking, or cracking due to unsupported edges of the pavement slabs. But, a well designed and constructed sta- bilized base will increase the fatigue life and im- prove the constructability of a concrete
pavement.
Cement-Treated Base (CTB)
Soil cement and CTB are two different materials that can be used for base course material. Soil cement is usually of a slightly lower quality be- cause it uses on-site subgrade or fill material, while CTB is typically made from processed ma- terial.
PAVING ALERT
The Case for Maximum Strength:
Many specifications attempt to address the potential for a cracking problem by limiting the strength of the stabilized layers. This works to a degree, but there are times the strength is needed and methods are used to reduce the cracking potential.
PAVING ALERT
Recycled Concrete Aggregate (RCA):
Whenever possible, designers should allow the use of RCA as a granular subbase. Research sponsored by the IPRF came to the following conclusions regarding the use of RCA:
• RCA can be used as unbound base material if produced from uncontaminated concrete. RCA should not be used where there is a potential for sulfate exposure from subgrade soils, ground water, or other external sources.
• RCA is not a hazard to the environment. Localized environmental effects from raised pH in
leachate are insignificant. Discharge of heavy metals or organics from common sources of RCA, if any, is insignificant.
• RCA typically is a better economic option, especially when transportation costs of virgin aggregate and disposal costs of concrete are considered.
Designers and contractors should refer to IPRF Report 01-G-002-03-5, “Evaluation, Design and Con- struction Techniques for the Use of Airfield Concrete Pavement as Recycled Material for Subbase” for de- tailed guidance on the use of RCA.
A laboratory mixture design is necessary to es- tablish the optimum cement content. In freezing environments, a freeze thaw durability test is also performed. This process requires 20 to 25 days, assuming that the aggregate is durable. If there is little experience with local materials, more than one mixture design should be pro- grammed into the test schedule.
CTB materials are nominally designed for a 7- day compressive strength of 750 psi (FAA speci- fications call for 7 day strengths between 500 and 1,000 psi). Material at this strength level usually passes the freeze thaw test and is not susceptible to shrinkage cracking. If a higher strength is required to pass the freeze thaw test, positive steps must be taken to reduce the po- tential for random cracking in the base.
Important mixing and placement procedures in- clude:
1. CTB is mixed in a central mixing plant.
• Water and cement are introduced into a pug mill mixer at the appropriate amounts to achieve the mixture design proportions.
• Cement introduction needs to be performed so that balling of the mate- rial does not occur.
2. The optimum moisture and density prop- erties of the mixture are determined using ASTM D558 procedures.
3. Delivery and placement equipment need to be matched to allow compaction of the mixture to occur within 60 minutes of ini- tial mixing.
4. Final grading and compaction needs to be completed within 2 hours of mixing.
5. It is essential to monitor density using a nuclear density gauge. A density of 98 percent of the maximum is required for full payment under FAA specifications.
The moisture content should be within 2 percent of the optimum. For summertime construction conditions, a moisture con- tent of +2 percent is recommended. Me- chanical spreaders, asphalt pavers or other specialized paving equipment are typically used to place the material (Fig- ure 5.1).
PAVING ALERT Material Alert:
Lean porous concretes such as CTB are more susceptible to sulfate attack than pave- ment concretes.
Consideration should be given during the de- sign phase of the project to investigate possi- ble detrimental effects on the CTB caused by sulfates present in the soil, groundwater, or aggregates.
Figure 5.1. Typical placement of cement treated base (CTB).
Chapter 5 – Subbase and Base Construction
6. The placement plan needs to be devel- oped to minimize the number of longitudi- nal and transverse joints. Longitudinal joints in the CTB should be placed di- rectly underneath planned longitudinal joint locations in the concrete pavement.
7. A transverse header is constructed at the end of the day or when continuing place- ment is interrupted for more than 60 min- utes. Full-depth saw cutting is the easi- est method to form the transverse joint.
8. Longitudinal joints are typically cold con- struction joints that are placed nearly ver- tical (a 1 to 1 ½ in. offset between the top surface and bottom surface of the CTB is tolerable). Trimming the edges with a saw may be necessary before placing the abutting lane of CTB when the edges have sloughed, raveled or were placed with too much offset (>1 ½ in.)
9. The temperature at the time of placement needs to be greater than 40°F. If tem- peratures are expected to fall below 35°F within 24 hours of placement, placement should be suspended.
10. Common design thicknesses for CTB are 4 to 6 in. Although some compaction equipment can effectively compact layers up to 12 in., the need for a CTB greater than 6 in. thick should be justified in the pavement design process. Placing and compaction equipment must be matched to the lift thickness.
11. A combination of rubber tired and vibra- tory compaction provide the best results.
12. Avoid designing CTB that must be placed in multiple lifts. If multiple lifts are needed to achieve design objectives, the surface of the underlying layer must be kept moist until the next lift is placed.
Using water trucks with side spray bars is an acceptable method to accomplish the sprinkling of the surface.
13. Finished grade tolerance is typically ⅜ in.
when tested with a 16 ft straightedge.
• Using a trimmer is the best way to achieve the required grade tolerance.
• Because CTB is a rigid material, it is not practical to re-grade after com- paction. Thus, care needs to be taken to achieve the specified tolerance the first time.
14. CTB must be cured. The curing seal is usually an asphalt emulsion applied im- mediately after final compaction. The surface of the CTB is to be kept moist until the layer is sealed. The seal should be protected for 7 days or until the open to traffic strength is attained.
PAVING ALERT
Cement Treated Base (CTB) Construction:
• Apply a single coat of white pigmented curing compound.
• Utilize a choke stone bond breaker to reduce restraint.
• Reduce the joint spacing of the pavement.
• Saw cut joints as soon as possible.
PAVING ALERT Winter Exposure:
It is best that a CTB layer be covered with the pavement layer prior to winter and/or a freezing environment. If a CTB layer must be left ex- posed, it must have attained design strength.
Before construction resumes in the spring, check the grade.
Lean Concrete Base (LCB or Econocrete) LCB, also commonly known as econocrete, con- sists of aggregate and cementitious materials uniformly blended together and mixed with water. The term econocrete is used because the materials used are of marginal quality. The mixtures typically use 2 to 3 bags of cement per cubic yard (per cubic meter) of material. Fly ash replacement of approximately 15% of the port- land cement is acceptable. The resulting mixture should pass the freeze thaw durability test and the procedures used for placing LCB are the same as conventional concrete. Common de- sign thicknesses for LCB are 4 to 6 in.
The primary issue with LCB is the strength of the mixture. Strength requirements are as follows:
1. 7-day compressive strength between 500 psi and 800 psi.
2. If 3 day strengths are greater than 500 psi, the contractor must place transverse joints in the LCB layer.
When necessary, joints in the LCB may be sawed or tooled. The design of the jointing pat- tern in the LCB must match the joint pattern of the pavement or reflective cracking may occur.
Care must be taken to align the joints when the concrete pavement is placed. Working joints in the LCB that have opened enough to allow intru- sion of mortar during the concrete paving
process should have a strip of geotextile fabric placed over them prior to paving.
The LCB must be cured. This is typically done by applying a single coat of white pigmented curing compound. A choke stone bond breaker should be applied prior to placing the concrete pavement.
Traffic is typically not permitted on the LCB until it attains a compressive strength of 350 psi.
Asphalt-Treated Base (ATB)
ATB consists of aggregate and bituminous mate- rials mixed at a central mixing plant. The follow- ing are considerations for constructing ATB:
1. The asphalt and the aggregates used in the asphalt mixture need to meet the re- quirements that are listed in the project specifications.
2. The asphalt mixture needs to satisfy the job mixture formula specified for the proj- ect.
• The asphalt mixture needs to be sam- pled and checked to see if it meets the job mixture formula (e.g., Marshall density, theoretical maximum density, air voids, and asphalt content).
3. The layer thickness of ATB is normally limited to 4 to 5 in.
• Some compaction equipment may be able to compact these materials to a depth of 6 in. A test strip may be con- structed to verify the contractor’s abil- ity to compact a thicker layer. The compaction is verified by coring and checking the top and bottom core den- sity.
PAVING ALERT
Lean Concrete Base (LCB) Construction:
• Apply a single coat of white pigmented curing compound.
• Utilize a choke stone bond breaker to reduce restraint.
• If 3 day strength exceeds 500 psi, provide for joints in the LCB and match them with the pavement joint pattern.
• Reduce the joint spacing of the pavement.
• Saw cut joints as soon as possible.
Chapter 5 – Subbase and Base Construction
4. The density after compaction can be checked using a nuclear density gauge.
5. The layer thickness and air voids are de- termined by obtaining cores.
6. ATB placement must be stopped if the temperature is below 40°F. If asphalt is being placed in cold weather, it is impor- tant that compaction begin immediately after placement by keeping the rollers close to the paving machine.
ATB layers affect the early age performance of the pavement. In summer conditions, the sur- face of these layers can reach 140°F. The ex- cessive heat impacts both the strength gain and the shrinkage rate of the fresh concrete. There- fore, these layers should be whitewashed using a lime-water solution or water sprinkled to re- duce their surface temperature immediately be- fore concrete placement. A wax based curing compound should not be used because of the high surface temperature. Also, trimming of these layers with a milling machine is not usually employed. This process leaves a coarse sur- face texture, thus increasing concrete pavement frictional restraint forces that in turn increase the potential for random cracking.