Thinking of a day out on the job site as game day in professional football, there are some parallels to draw that will achieve winning results in both worlds. Site preparation can be thought of as the pre-game set up for achieving your goals and a successful outcome, which happens long before the game clock starts. Adequate site preparation is essential to long-term pavement performance. For example, pavements constructed without adequate site preparation may not meet smoothness specifications or may fail due to inadequate subgrade support. In this edition of the RoadReady newsletter, we will explore some critical aspects of site preparation to gear up your team for pulling out a “big win” on paving day.
The first step is site inspection. Start by visually inspecting the construction site conditions to evaluate for any potential deficiencies, such as improper grade and slope, inadequate soil quality, cracking, rutting, and localized failures like potholes. Remember, it’s better to burn a timeout and identify the holes in your defense than to blow it on the big play.
Grade is the change in elevation in the direction of travel, for example, driving down a hill would be a change in grade. Slope is change in elevation 90 degrees to the direction of travel, so driving on the side of a hill would be driving on an uneven slope. Grade and slope affect roadway drainage, and allow for sheet flow, where water drains to the edge of the pavement instead of sitting on the roadway surface. 2% cross slope is a generally accepted standard value. These are based on a surveyor’s profile gradeline (PGL), which is indicated by marks and a stringline. In order to correct any deficiencies in grade and slope, remove or add material to match PGL with grading equipment, preferably that has electronic controls.
Correcting Soil Deficiencies
Soil quality is an important indicator of a pavement’s support. With poor soil underneath the pavement, no matter how well it’s constructed, the pavement could fail. Repair depends on the nature and extent of the problem. If a few weak spots have been identified, these can usually be excavated and replaced with a better-quality fill. When the soil is of poor quality generally, it requires more extensive work.
For soils that lack stiffness or have a tendency to swell with moisture, it may be possible to stabilize the subgrade with a binding material. A prime coat of asphalt can be placed to help with sandy soils, as long as the soil material is not too fine. Most problem soils tend to be predominantly silt or clay. To stabilize these soil types, only lime or portland cement will work, and even this kind of stabilizing material is not likely to be effective for organic soils.
For marginal cases of poor soil, additional layers may be added to the design of the pavement structure to compensate. Over-excavation, where the poor-quality soil is completely removed and replaced, can be quite expensive, and avoiding poor soil is generally preferred if possible.
Cracking is a common form of pavement distress, which can reflect through the different layers of pavement through to the top layer, or wearing course. Cracking allows water infiltration and negatively affects the pavement’s surface. Depending on the number and/or severity of the cracks, it could be enough to fill the cracks individually. If the cracking is severe, it is best to lay a seal coat. Crack seal 6-12 months before the planned overlay, so that the crack seal asphalt can cure, and be sure that the cracks are not filled overfull – any bumps from poorly applied material can reflect through and create bumps on any asphalt treatment, which is not desirable. No one wants those preaseason injuries showing up at the super bowl.
For more information on various forms of cracking, read our December newsletter, “Say No to Cracks!”
Establishing a defense for rutting is a key component to setting up a paving victory. Rutting is when there are depressions following the wheelpath, and could be caused by one of two things. First, if the rut is free of cracks, with bumpers on either side, then the mix was too plastic. Rutting with alligator cracking in the wheelpath indicates subgrade deformation. Both of these could cause hydroplaning if filled with water, and are always hazardous because the rut can pull vehicles into the rut path as the car is steered across the rut. For the crack-free rut, mill to remove, and use a less-plastic mix this time around. For alligator cracked ruts, completely redesign the pavement section to strengthen the pavement structure.
Localized Pavement Failure
Localized pavement failure describes a wide variety of problems, but the most well-known is probably the pothole. Like a nagging injury, if not properly treated, it can lead to devastating effects down the road. Potholes damage the pavement surface, causing roughness which can damage a vehicle, and allows moisture to infiltrate the pavement structure, which will eventually destroy the underlying structure. This kind of failure must, of course, be corrected before paving, such as by patching. In short, figure out what’s going wrong, and then fix it. Especially important is if moisture is in the hole, it must be made to drain correctly. Moisture problems will come back to haunt you, and can be expensive to repair.
Moving on to the ballgame – constructing your pavement
Paving hasn’t started yet, but by following these tips and correcting deficiencies prior to paving, you can set your team up for success. Be sure to continue that winning strategy in to the ballgame, with proper construction practices and a successful final inspection.