A flexible pavement is made up of five distinct layers namely, subgrade, subbase, base course and surfacing or wearing course (Figure 1). Each of these layers requires special treatment to ensure they perform their function satisfactorily.
Subgrade
Subgrade describes the in-situ soil on which a road, an airfield, or a heliport is built. Subgrade includes the soil to the depth that may affect the structural design of the project or the depth at which the climate affects the soil. Depths up to 300 mm may be considered subgrade for pavements carrying heavy loads. The quality and natural density of this material dictate what action(s) to take to prepare the subgrade. For example, a highly organic subgrade material may have to be totally removed and replaced with a higher quality, select material. In most situations this is not the case. Often the in-situ material is suitable, but requires some degree of compactive effort to achieve the required density.
Suitable Compactors for the Subgrade
1. Heavy Pneumatic-tyred Roller: These are preferred for subgrade compaction because of their capability to compact the soil to depths up to 450 mm. It is often necessary to scarify the top 150 mm of the material to adjust the moisture content. Then the material can be re-compacted to a higher density than could be achieved at the soil’s natural moisture content. This process is known as scarify and compact in place (SCIP).
2. The Sheeps-foot Roller: This is good for this operation since it loosens the material, yet compacts it as it walks out of the material. The sheeps-foot roller also helps to break up oversize material or rock in the area. If using a sheeps-foot roller to prepare the subgrade, also use a pneumatic-tyred or smooth-drum roller to compact the top 25 to 50 mm of the final material layer or to seal the layer surface if expecting rain. Shaping and sealing the surface protects the subgrade from the damaging effects of water infiltration. If the layer has been sealed, scarify the top 25 or 50 mm before placement of a succeeding layer. This ensures a good bond between the layers.
3. Vibratory Roller: This is more suitable if the subgrade material is sand as it provides the most effective compactive effort.
Fill Material
Fill material in pavement usually comprises the sub base and base courses. The subbase is usually soil material with higher quality than subgrade materials. Among the fill materials, much attention should be paid to the base course. This because the base course is the primary load-bearing component of a pavement. The material for the base course material depends on the expected wheel load on the pavement while the thickness depends on the subgrade strength. In Nigeria, for instance, the base course for all TRUNK A road are made with stone base which have higher CBR-value than soil. The base course should be made with well-graded granular material with a liquid limit less than 25 percent and a plastic limit less than 6 percent. When placing fill, it is important to spread the material in uniform layers and to maintain a reasonably even surface. The thickness of the layers is dependent on the desired compacted layer thickness.
The thickness of the un-compacted layer is normally 1.5 to 2 times the final compacted layer. For example, place fill in 225- to 300-mm layers to achieve a compacted layer thickness of 150 mm. Place the fill material with a scraper or a dump truck and spread it with a dozer or grader. When spreading material on a prepared subgrade, spread the material from the farthest point from the source to the nearest or vice versa. The advantages of spreading fill from the farthest point to the nearest are as follows:
1. The hauling equipment will further compact the subgrade.
2. Previously undetected weaknesses in the subgrade will become apparent.
3. Hauling will not hinder spreading or compacting operations.
On the other hand, spreading from the nearest point to the farthest point has the advantage of haul equipment traveling over the newly spread material. This compacts the material and greatly reduces the overall compactive effort required.
Before beginning compaction operations, the moisture content of the fill should be determined and compared it to the acceptable moisture range for that material. If the moisture content is below the acceptable range, add water to the fill. If the moisture content is higher than the acceptable range, use one of the previously discussed methods to dry the soil. After achieving the appropriate moisture content, begin compaction operations.
Suitable Compactors for the Fill
Smooth-drum vibratory rollers or dual-drum roller: These are the most ideal type of compactors for base course.
Wearing Course (Binder/Surfacing)
The wearing course is made with asphalt (a composite material made of bitumen and aggregates). The binder course and surfacing have different specifications that are gotten from asphalt mix design. Before laying the asphalt, the surface of the base course should be made moist and sprayed with a bituminous material. The material can be categorized as prime coat, tack coat or seal coat depending on the state of the pavement. Each coat have standard spray rate (This can be found in General Specifications for Roads and Bridges, Nigeria, 1997). The material is sprayed in hot stay using tar boiler. It is common practice to blind the coat with sharp sand to prevent destruction and to facilitate curing. The cured material should be ready to be laid with asphalt after 48 hours. If the surface is too dusty or with sand, it should be swept with a self-propelled machine called Sweeper. The removal of the dust is to ensure proper binding between the new asphalt and the base course or old pavement.
Rolling/Compaction of Asphalt Wearing Course
Rolling of asphalt is one operation in road construction that if not done properly can undermine the performance of the pavement. The three types of rollers suitable for compacting asphalt are:
1. Static-steel wheel roller: This type of roller is suitable for breakdown and finish rolling. They leave smooth finish on the pavement surface, but excessive rolling may result in lateral movement of the mixture causing surface crack and density loss.
2. Vibratory steel wheel roller: This type of roller are suitable for breakdown, intermediate and finish rolling. The breakdown and intermediate rolling are performed in the vibratory mode and finish rolling in the static mode.
3. Pneumatic-tyred roller: This type of roller is used for intermediate rolling. They increase compaction after breakdown rolling and produce water tight surface.
Note
a. When using the roller to aerate soils, travel at the highest practical speed. High speeds tend to kick up the material, which is the objective in this case instead of density.
b. To eliminate non-compacted strips, each pass with the roller should overlap the preceding pass by at least 300 mm.
c. Make gradual turns at the end of each pass. This prevents surface damage to the layer.
Rules when Compacting Asphalts
i. All the rollers should be equipped with a system for watering the drums and should have had scrapers and pads to remove any material that sticks to the drums or tyres.
ii. Quick turns on freshly laid asphalts should be avoided as this can damage pavement surface
iii. Operate the rollers on the asphalt with the drive wheel toward the spreader or asphalt paver. This pulls the mix under the roller and imparts a smooth surface. Placing the rollers on the asphalt with the guide roller toward the spreader will push the hot asphalt forward (Figure 6).This causes the material to mound up in front of the roller. Eventually, the roller passes over the mound, leaving a bump in the finished pavement. In most cases, subsequent rolling cannot correct this bump.
Reference
FM 5-434 (2000): Earth moving operations. Department of Army, Washington, USA.
