Base Stabilization
What is Base Stabilization?
It was known to the ancient Chinese, the Romans and the Indians. In fact, the idea can be traced back more than 5000 years. It is base stabilization, and while the ancients had to be content with using lime or pozzolans to stabilize their roads, today we have something better: liquid calcium chloride. (A good thing, too, when you consider the difference between a horse and a 40-ton truck.) A base stabilization program featuring liquid calcium chloride saves labor, time, money and resources. It strengthens roadbases and other unpaved surfaces, allowing them to handle ever-increasing traffic volumes and axle weights. In short, it enables a roadway to resist change – especially mechanical change – over a long period of time.
We’ll discuss how liquid calcium chloride works – and how to use it – later. We’ll also detail one particularly effective base stabilization technique: Full Depth Reclamation. First, however, it will be helpful to identify some of the factors that you should consider before beginning a base stabilization project.
Cohesion Friction and Density
Cohesion, friction and density are three “mechanical” aspects of a road base which are extremely important in its ability to withstand load. Cohesion refers to the ability of soil particles to resist separation – to “stick together when the going gets tough.” Friction refers to the ability of particles to resist any shifting of their position relative to each other. Density, as we have mentioned, is the weight of a material relative to its bulk.
Now, different road building materials exhibit different “mechanical” properties. Damp clay, for example, has a relatively high density and good cohesiveness. However, it has a low friction coefficient – its particles are prone to repeated shifting and, under load, will yield easily. Gravel, on the other hand, has a high coefficient of friction, somewhat lower density, and little or no cohesiveness. So while gravel can bear large loads, it is extremely susceptible to vibration, hydraulic action and frost heave.
The “Best” Material
An ideal road base would combine a high coefficient of friction and high cohesiveness. But no single material exhibits both these characteristics. A mixture of materials is needed, several aggregate grades – from coarse aggregate to fines passing a #200 sieve. That’s because the largest aggregate components, densely packed, will form voids. These voids will be filled by smaller particles, which are small enough to fit snugly, but large enough to be entrapped. The remaining voids will be filled by yet smaller aggregates.
Important Considerations
Before undertaking a base stabilization or road reclamation project, several issues must be carefully considered. Among the most important are 1) the existing roadway’s overall make-up and configuration, 2) the most desirable final result, and 3) the materials (mechanical equipment, virgin aggregate, and soil stabilizer) to be used. By properly determining the cause of a roadway’s pavement failure, and knowing precisely how the project should turn out, the designer can choose the best materials for the job. Likewise, accurate prediction of future road usage ensures the project’s success. After all, base stabilization creates an entirely new road, from bottom to top.
The Ideal Result
Basically, the ideal result is a road that supports the axle loads and traffic volume it is likely to carry. That is, the road must have adequate bearing capacity; it must be capable of resisting the loads placed upon it. A roadway’s ability to do this is directly related to the density of the material it is comprised of. (Density is the simple ratio of a material’s bulk to its weight.) When discussing a material’s density, however, other factors must be taken into account.
Compaction and Optimum Moisture
Compaction is probably the least expensive and most effective way to strengthen a road. It can increase bearing capacity by as much as 150%, as well as reduce permeability, increase shear strength and improve stability.
The density to which a roadbase can be compacted depends on both the moisture content of the aggregate and the force exerted upon it. Moisture content is critical. Too little water, and particles will not have the “lubrication” necessary to compact properly. Too much moisture, and hydraulic forces develop which may actually force the particles apart under compaction. The limits are strict: a deviation of only 1% from optimum may reduce density by over 2 lbs. per cubic foot and increase voids by as much as 8%.
It is by achieving and sustaining optimum moisture content that liquid calcium chloride makes its major contribution to the overall stability of a road base.
The Benefits of using Liquidow 38% Liquid Calcium Chloride
Calcium Chloride has a number of unique physical properties that make it ideal for base stabilization. Compared to plain water, a calcium chloride solution possesses a stronger moisture film, greater surface tension, reduced vapor pressure, and lower freezing point. These properties, calcium chloride’s “thirst” for water, and its tenacious retention of water provide several benefits in a program of road base stabilization. Here are nine such benefits:
- Greater density
- Less comparative effort
- Optimum moisture control
- Minimizes required binders
- Frost protection
- Surface uniformity
- Effective stage construction
- Improves bond
- Dust-free surface
- Base stabilization via full depth reclamation
Optimum Moisture Control
Calcium chloride’s hygroscopic qualities, together with its ability to lower the vapor pressure of water, work to inhibit evaporation. This is important, because if moisture is not held within optimum limits, adequate densities will not be achieved, resulting in an unstable road base.
Minimizes Required Binders
By increasing the strength of both the moisture film and surface tension of water, calcium chloride provides a binding action of its own. This reduces the amount of fines required for satisfactory cohesion, and lowers the possibility of softening due to capillary action.
Frost Protection
Calcium chloride’s ability to depress the freezing point of water results in a road base with considerable resistance to frost heave. Not much of the chemical is required: in a well-graded aggregate, only one half to one percent calcium chloride by weight is enough to virtually eliminate frost heave.
Surface Uniformity
Calcium chloride’s moisture-retention properties allow surface irregularities to be graded out and re-compacted without affecting the moisture content of the aggregate. This results not only in a surface which is smooth and hard, but a roadbase which is uniformly dense throughout its depth.
Dust-Free Surface
The use of calcium chloride ensures a tough, moist surface, minimal aggregate loss, and a work environment with plenty of fresh air and visibility. This is especially welcome when a portion of the roadbase is open to traffic.
Base Stabilization via Full Depth Reclamation
Full depth reclamation is a particularly effective form of road reclamation. It involves pulverizing both the existing asphalt surface and the underlying base; these are then mixed together to form an upgraded, homogeneous, and highly-stable new base. The process offers several advantages over conventional road reclamation techniques:
- It is performed entirely at the job site. This means that the costs of removing, loading, hauling, crushing and sizing of the old pavement are eliminated.
- Because the old pavement is recycled, the need for disposal is eliminated.
- Fewer workers and less equipment are required, resulting in lower costs, a less cluttered roadway and better traffic flow.
- Faulty roadway cross sections can be adjusted. For example, grades can be lowered to restore curbs that have been lost due to numerous overlays; or sections that drain poorly can be improved. Of course, desirable road features can also be maintained.
- Natural resources are conserved, since all of the existing aggregate and asphalt concrete are reused.
- Reflective cracking, a problem with overlays, is non-existent.
- The structural integrity of the base is increased, resulting in a denser, more compact base that increases road life and improves riding quality.
- Total pavement thickness can be reduced, which makes for lower material costs.
- These benefits provide enormous savings in terms of material and labor. Material costs are reduced because the addition of pulverized asphalts upgrades the previously poor base, thereby reducing or eliminating the need for additional virgin aggregate. Labor costs are minimized because less overall labor is needed.
DUST-FREE SURFACE
The use of calcium chloride ensures a tough, moist surface, minimal aggregate loss, and a work environment with plenty of fresh air and visibility. This is especially welcome when a portion of the roadbase is open to traffic.
Base Stabilization via Full Depth Reclamation
Full depth reclamation is a particularly effective form of road reclamation. It involves pulverizing both the existing asphalt surface and the underlying base; these are then mixed together to form an upgraded, homogeneous, and highly-stable new base. The process offers several advantages over conventional road reclamation techniques:
- It is performed entirely at the job site. This means that the costs of removing, loading, hauling, crushing and sizing of the old pavement are eliminated.
- Because the old pavement is recycled, the need for disposal is eliminated.
- Fewer workers and less equipment are required, resulting in lower costs, a less cluttered roadway and better traffic flow.
- Faulty roadway cross sections can be adjusted. For example, grades can be lowered to restore curbs that have been lost due to numerous overlays; or sections that drain poorly can be improved. Of course, desirable road features can also be maintained.
- Natural resources are conserved, since all of the existing aggregate and asphalt concrete are reused.
- Reflective cracking, a problem with overlays, is non-existent.
- The structural integrity of the base is increased, resulting in a denser, more compact base that increases road life and improves riding quality.
- Total pavement thickness can be reduced, which makes for lower material costs.
- These benefits provide enormous savings in terms of material and labor. Material costs are reduced because the addition of pulverized asphalts upgrades the previously poor base, thereby reducing or eliminating the need for additional virgin aggregate. Labor costs are minimized because less overall labor is needed.
Effective Stage Construction
As construction costs rise and highway funds shrink, there has been a growing tendency (especially in rural areas) to build new roads in stages – with interruptions that can last as long as several years. This can be a maintenance nightmare, because an unpaved wearing surface will likely require frequent bladings and constant replacement of aggregate.
Stabilizing surfaces through the use of calcium chloride in the base aggregate greatly reduces these maintenance costs. Moreover, interim calcium chloride treatments in the spring and fall can further inhibit surface deterioration, which can lead to large savings in aggregate replacement. And, when the time comes for paving, there’s a hard fully-cured and stabilized roadbase ready-and-waiting.
Improves Bond
It is well known that a moist surface more readily absorbs priming materials than a dry surface. In fact, most engineers will wet the roadbase before applying any primers. Such pre-wetting practices are often unnecessary when calcium chloride has been used in the aggregate mix.
Typical Reclamation Projects
There are many potential candidates for full depth reclamation. It is often performed on city streets, as well as “farm to market” county and secondary roads. In addition, the process is commonly used to improve structures such as parking lots, storage areas, and small airport runways.
While every roadway is unique, several types are particularly well-suited to full depth reclamation:
- Base failures
- Insufficient bearing capacity
- Cracked asphalt surfaces
- Narrow two-lane roads
- Parabolic road configurations
- Gravel roads
Base Failures
Secondary roads often suffer from base failure. In all likelihood, these roads began as wagon paths, with their surfaces gradually being updated to dirt, gravel and asphalt.
Their bases were never designed to support modern axle weight and traffic volume. If the base is not improved, the surface will continue to deteriorate, exhibiting potholes, dips, bumps, heavy rutting, etc.
Insufficient Bearing Capacity
Reclamation allows a road to be substantially strengthened because it involves the thorough restructuring of the road itself. A road’s mechanical and chemical properties can be improved with the addition of a stabilizing agent, which can be added in a one-pass or multiple-pass operation.
Cracked Asphalt Surfaces
Water can enter the base and cause structural problems in roads with cracked asphalt surfaces. While overlays will temporarily seal the surface, reflective cracking usually occurs quickly, causing the water penetration problem to persist. It is much more effective to pulverize the asphalt, add a stabilization agent or virgin aggregate if needed, and combine this with the old base. This creates a solid foundation for a new wearing surface.
Narrow, Tow-Lane Roads
These types of roads offer a great deal of potential for improvement. Reclaiming machines can add width to each side of the road. And a uniform base of desired thickness can be created by mixing the shoulder, base and riding surface material.
Parabolic Road Configurations
The rounded shape of many older secondary roads fosters shoulder raveling. It also can inhibit snow removal and make driving more dangerous. Simple overlays will not solve the problem. Reclamation will, since it allows the road to be thoroughly reshaped.
Gravel Roads
Gravel roads also make ideal candidates, since reclamation can provide them with a stable base. Existing aggregates are blended with the proper additives; shaping and compacting prepare them for a wearing surface.
Structures with a pavement layer of less than six inches are ideal for full depth reclamation. In the case of structures with pavement layers exceeding six inches, a cold planer should be used to scrape off part of the layer. (It is economical to collect the RAP produced during this process and later blend it with the reclaimed base.) When such a structure has not been cold planed, it is still possible to use a reclaiming machine on it, but the work will be slow and the tooth replacement expensive.
Important Factors to Consider
Remember: Full Depth Reclamation is, in fact, a base stabilization process. So before a reclamation project is begun, many of the same factors must be taken into account. In particular, the existing roadway’s overall make-up and configuration, as well as the desired end result (a road that can support the axle weights and traffic volumes it is likely to carry) must be considered.
Full Depth Reclamation not only leads to a stronger, more stable base, it also allows roads to be reshaped – parabolic to rooftop crown, for instance. What is more, widths can be changed and shoulders can be enlarged or created from scratch. The possibilities are many; it is up to the designer to choose the ideal result.
Some Final Hints for Effective Reclamation and Base Stabilization
- The importance of drainage cannot be overstated. A road should have the appropriate crown, as well as other drainage features like shoulders and ditching.
- Vibratory rollers can improve road hardness – they offer greater compaction, particularly when calcium chloride has been used, since the lubricating properties of calcium chloride aid in compaction.
- One gallon of LIQUIDOW™ 38% Liquid Calcium Chloride should be used per square yard for full depth reclamation. Also, multiple passes by the distribution truck might be required to prevent run off.
- Curing times are unpredictable – they are often a product of environmental conditions and the weather. However, bases containing calcium chloride should be given several weeks to cure before applying the final wearing surface.
- Anticipated traffic load should determine the choice of wearing surface, be it cold mix, hot asphalt mix, or a single or double seal coat.
In Conclusion
Base stabilization has been practiced for thousands of years – for some very good reasons. It not only results in highly durable roadways, it also helps reduce labor, equipment and material costs. This is particularly true of Full Depth Reclamation: from roads plagued by potholes, dips, cracks and buckling, to deteriorating parking lots and small airport runways, full depth reclamation is the ultimate curative. It corrects the basic structural defects themselves, not just the obvious symptoms. And when practiced with LIQUIDOW™ 38% Liquid Calcium Chloride, the benefits of base stabilization and full depth reclamation are even more pronounced. That’s because LIQUIDOW™’s hygroscopic properties enhance compaction, bonding, curing, and moisture control. The result is a more stable structure.
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