Initial pulverization
For the best results, a pavement reclaimer is recommended. Other types of equipment could be used, such as tiller-type mixers or motor graders, but they are less efficient in getting the necessary gradation for a successful project. Typically, the required gradation can be achieved in one pass. More passes with a reclaimer may be required if the material is more difficult to break up. Reclaimers can typically pulverize materials to a depth of one foot. If deeper lifts are required for your project, a top layer may need to be pushed aside in windrows to pulverize and treat the lower layer first.
Additional material can be added to the roadway to develop a thicker and stronger pavement layer, and this added material should be blended thoroughly with the original roadway material.
Preliminary Grading
Prior to adding cement needed to strengthen the pavement, a motor grader is used to shape the road surface and restore the pavement grade and cross-slope.
Many times, a minor edge or shoulder widening is part of the project. If it is determined that adequate support is found in the existing subgrade, then a motor grader or road widener can be used to get the additional width required for the project. To build a much wider road, it will typically be necessary to grade and build a supportive subgrade and re-establish impacted roadway ditches before adding new or reclaimed materials for this part of the project.
Spreading Cement
After establishing the grades, cross-slope, and width for the roadway, it’s time to spread the cement (typically dry) on the re-established road template. The cement should be spread evenly across the area designated for the material trucked to the project site. The rate of cement is determined by the pavement engineer for the project, which can be calculated, or one can use an industry rate table to determine the amount of cement needed for spreading.
Mixing
The reclaimer used for initial pulverization mixes the cement with the reclaimed road base. Sometimes, a water truck is used to add water to the road base before mixing, but the best results are found when the reclaimer has a water line directly feeding into a mixing drum where the rate of water can be more accurately controlled. Cement slurry can also be fed directly into the mixing drum, but this method is uncommon and requires the use of the equipment. It is important to achieve near optimum moisture for the base-cement mixture. In the field, optimum moisture—determined through laboratory testing—can be assessed using nuclear density gauges or, more commonly, a simple ball test: if the material forms a cohesive ball without being overly saturated, it is near optimum moisture.
Compaction and Final Grading
Proper compaction is important for FDR to achieve its full strength and durability. In accordance with ASTM D558 or AASHTO T 134, the FDR base should be uniformly compacted to 98 percent of the maximum dry density. The type of equipment for compaction is dependent on the material properties, but the types that can be used are tamping (sheep’s foot), steel drum vibratory, and pneumatic rollers. Avoid over-compactions as it could lead to aggregate crushing and loosening and drying of the surface.
The typical time for mixing and final compaction is 2 hours. On cooler days, more time may be needed. Final Grading is important to ensure the proper roadway template is achieved. The goal is to achieve a cohesive, uniform layer-free of cracks and loose material. Final grading should be completed within 4 hours after the start of mixing.
Curing and Opening to Traffic
Curing is important for the FDR to achieve full strength and avoid damaging the newly completed surface. Curing is typically done with a bituminous or other approved membrane. Moist curing is allowed but requires continual attention for seven days or until a surface layer is placed on the FDR. The new FDR must be cured sufficiently to prevent visible deflection under heavy loads. Curing membranes should be dried before opening to full traffic to avoid vehicles picking up material. It is common to open traffic to a newly completed segment of FDR within 2 to 3 hours after placing the curing membrane if it is determined that the unfinished pavement can withstand the loading. Microcracking is also recommended, and I will discuss it in detail in a subsequent blog post.
Surfacing Operations
The completed base is suitable for a chip seal, asphalt pavement, or concrete overlay surface. It is common to place and use a chip seal as a temporary pavement surface to prevent damage to the FDR. A rapid-curing liquid asphalt and a small grade of aggregate can provide a good temporary surface that will allow good cohesion of subsequent pavement layers.
Weather Considerations
For a successful project, placing FDR under the proper weather conditions is important. Ambient temperatures between 50ºF and 90ºF are the most favorable. Freezing weather causes problems with pulverization and will retard the set of the FDR and excessive heat will accelerate the set. More than light rain and thus excessive moisture will cause the FDR not to be cohesive and thus susceptible to damage. Windy conditions should be avoided due to the dust of dry cement.
No-Cost Assistance Available
I have many years of experience with FDR projects, and I am happy to say the ones constructed with my colleagues have long lives and are still operational. The Cement Council of Texas will be happy to discuss the details of FDR with you or your group and is pleased to make no-cost presentations to large groups. Email me at rbowers@cementx.org or call me at 817-919-5858.