Asphalt Paving - A Durable Finish
Asphalt paving is a popular choice for driveways, parking lots, and roads due to its cost-effectiveness, durability, and flexibility. However, not all asphalt is created equal. Different grades and thicknesses of asphalt are designed to meet the specific needs and requirements of various paving projects. Proper material and ample material, expertly installed with best practices on a suitable base are all essential to a successful paving project.
Asphalt, also known as asphalt cement or bitumen, is a semi-solid, viscous liquid derived from crude oil. It is a key ingredient in the construction of roads, driveways, and parking lots, acting as a binder that holds the aggregate materials (crushed stone, sand, and gravel) together. The combination of asphalt and aggregate forms a flexible and durable paving material that can withstand heavy loads, temperature fluctuations, and various weather conditions. Asphalt is typically heated to high temperatures (over 150 degrees Celsius) to become workable for mixing and paving applications, and it cools down to form a solid, long-lasting surface.
Step by step, how asphalt is paved:
1.) Ensure the gravel base is suitable, fine graded, and compacted as discussed In this article: (Base Preparation).
2.) Manually create a "starting pad" this is a short, paved area, raked and leveled by hand at the start of a paved strip. (some cases where there is enough space beyond where the paved surface is to begin this step can be skipped with some careful use of asphalt spreader controls, or by starting on a previously installed hard surface like asphalt or concrete.
3.) Set the asphalt spreader's screed on the starting pad or "hang" the screed at the starting position (leaving an appropriate gap between it and the gravel base.) An asphalt spreader screed is a heavy flat surface that only lets the correct amount of asphalt pass under it and pushes the rest of the asphalt forward to be used later. As the asphalt passes under the screed it begins to get compressed by the screed plate which acts as a wedge pushing asphalt against the base. The angle of attack or how much higher the front of the screed is than the back, determines the thickness of the asphalt.
4.) The paver is continually filled with asphalt, while the operator or operators drive the machine forward, while making fine tuning adjustments to account for changing slopes, following edges or seams, and even temperature of the material.
5.) Simultaneously the various asphalt workers will get to work on various tasks, which can included filling in areas the paver couldn't, fine tuning quickly changing grades by adding or removing asphalt, initial surface compaction, using both smaller hand rollers, hand tampers, or plate tampers, straightening and tamping the sides/edges of the asphalt, and monitoring the distribution of asphalt throughout the machine.
6.) Towards the end of a strip the last few feet must be finished up by hand, either for another perpendicular strip in the case of a parking lot, or to finish the paved surface to an edge or sidewalk. This requires manually adding or removing asphalt and hand racking to the proper grade.
7.) Compaction is the last major step in asphalt paving. A heavy roller or rollers are driven over the surface to achieve final compaction, often a vibrating drum is used to achieve superior compaction, water is used on the surface to cool and to help stop the asphalt from adhering to the roller drums. Additionally, hand tampers and plate tampers are used to achieve acceptable compaction over the areas a roller isn't able to reach.
8.) The final step is quality control, ensuring that the paved surfaces are as in proper order so they will last for decades. Clean up the site and block off the newly paved surface with barriers.
* Many parking lots and some upgraded driveways will receive a second layer of asphalt, in which case the first layer will act as a base, and not require the same attention to detail.
What are Asphalt Grades?
Asphalt grades refer to the mix proportions of asphalt to aggregate or even the types of aggregate used. The right asphalt grade for a specific project depends on factors like the expected use, local weather conditions, and the desired appearance and smoothness of the finished surface.
The basic nomenclature for hot asphalt mixes is the letters "HL" (Hot Load) followed by a number and sometimes an additional adjective or identifier. For example, "HL3A" or "HL3FINE" represents:
HL: A hot asphalt mix
3: 3/8-inch stone
A or FINE: A modifier signifying additional sand than a regular HL3 (results in a smoother finished surface, but also a slightly softer surface and lower load capacity.
HL4 denotes a hot mix asphalt with 4/8-inch (1/2-inch) stone. This asphalt is very versatile with small enough stone to be produce a relatively smooth surface, but large enough and of high enough quantity of stone to be significantly stronger and more resistant to ware than an HL3. This is a very versatile mix which can be used as either a base layer or finish layer which will be stronger but not quite as smooth or consistent in finish.
HL8 much the same as HL4 but with up to 1 inch stone, though most often actually contains 3/4 inch. This larger stone creates an extremely strong pavement layer, though it must be laid thick enough to maintain the flexibility benefits of asphalt, and to achieve proper compaction, the bare minimum thickness for any asphalt type is 2 x stone size. This asphalt will often have many inconsistences in surface finish, and as such is used as a base layer in the vast majority of cases. Though this is not a rule, in extremely heavy use cases HL8 and even HL8 modified with granite stone and high strength polymers can be used as a finish layer.
Which Asphalt Grade is Right for You?
Choosing the appropriate asphalt grade for your project largely depends on the intended use, traffic load, and desired appearance of the finished surface. Residential and commercial customers often have different requirements and preferences when it comes to asphalt paving. Here's a guide to help you choose the right asphalt grade for your specific needs:
For residential driveways and other light-duty parking lot or sport surface applications, the primary asphalt grades to consider are HL3A and HL3. Both of these options provide a smooth and consistent finish, with some differences in load capacity and surface softness:
HL3A: This asphalt grade is the most popular choice for residential driveways, as it offers a smoother finished surface with the addition of more sand than the regular HL3 mix. The extra sand results in a finer texture, making it more visually appealing and comfortable for walking and driving. However, this also makes the surface slightly softer and reduces its load capacity. HL3A is suitable for most residential driveways that do not expect heavy loads or constant heavy traffic.
HL3: This option is less common for residential driveways but may be chosen if a slightly higher load capacity is needed or if the driveway will experience more frequent heavy vehicle use. HL3 provides a slightly coarser surface compared to HL3A, but it offers increased strength and durability. While still offering a reasonably smooth finish, this grade is a good choice if you expect higher traffic loads or heavier vehicles on your driveway.
Two Layers: In some circumstances the residential customer wants a paved surface to be more robust and long lasting. By paving in two layers, additional thickness and therefore strength can be added to a driveway, Paving in two layers also increases the flexibility of the paved surface before cracking than an equally thick single layer, which can result in a longer life, and less movement over time. a second layer of asphalt can often increase life expectancy by 50% far outweighing the costs in the additional layer.
If you want the absolute best job for your driveway or small parking lot, consider going with a two-layer installation.
Commercial customers usually require the robust and durable asphalt paving solutions due to the higher traffic loads and heavier vehicles they encounter. For commercial parking lots and roads, it's common to use a combination of base and top layers of different asphalt grades. This provides the necessary strength, stability, and durability for these high-demand surfaces:
Base Layers: For the base layer, commercial customers often choose between HL8 or HL4. Both of these grades contain larger stone sizes, providing increased strength and load-bearing capacity. HL8 uses up to 1-inch stone (often 3/4 inch), while HL4 uses 1/2-inch stone. These base layers are crucial for supporting heavy traffic and ensuring the longevity of the pavement.
Top Layers: The top layer of a commercial parking lot or road is typically an HL3 grade. This layer provides a smooth and consistent finish, suitable for driving and walking. The smaller stone size (3/8-inch) offers a balance between surface smoothness and strength, making it an ideal choice for commercial applications. Using HL3 as a top layer ensures that the paved surface is comfortable for users and more esthetically pleasing, while still providing the necessary durability and load-bearing capacity for commercial use.
Asphalt Thickness: Choosing the Right Depth for Your Paving Project
Asphalt thickness is a crucial factor in determining the performance and longevity of your paved surface. The right thickness ensures that the pavement can withstand the expected traffic loads and weather conditions, while also providing a comfortable and smooth driving experience. In this section, we'll discuss the various asphalt thickness options for single-layer and two-layer installations, and how to choose the right one for your residential, medium-duty, or heavy-duty paving project.
For single-layer paving jobs, a base thickness of 2 inches after compaction is often recommended. This thickness provides a good balance between strength, durability, and cost-effectiveness for most residential and light-duty applications. A 2-inch layer can withstand typical traffic loads, temperature fluctuations, and weather conditions, ensuring a long-lasting and functional paved surface.
Residential or Light-Duty Two-Layer Installations:
In residential or light-duty paving projects that require a two-layer installation, it is common to use two layers of 1.5 inches after compaction each. This provides a total thickness of 3 inches, which offers increased strength and durability compared to a single 2-inch layer. The two-layer system also allows for a smoother and more visually appealing finished surface, with the top layer providing a consistent and comfortable driving experience while the bottom layer provides strength.
For medium-duty paving projects, such as commercial parking lots or roads with moderate traffic loads, a specification of 2 inches of HL8 or HL4 as the base layer and 1.5 inches of HL3 as the top layer is commonly used. This combination offers a total thickness of 3.5 inches, providing enhanced strength and load-bearing capacity compared to residential or light-duty installations. The use of different asphalt grades in the base and top layers ensures that the pavement is both strong and smooth, catering to the demands of medium-duty applications.
Heavy-duty paved surfaces, such as those that experience high traffic loads or constant heavy vehicle use, typically require thicker asphalt layers to ensure optimal performance and longevity. Depending on use case both layers may be the same grade, or the surface will still be paved with a finer asphalt for aesthetics, With heavy-duty paving each layer can be 2 inches thick, or a 2.5-inch base layer and a 1.5-inch top layer can be used. This results in a total thickness of 4 inches, providing high strength, durability, and load-bearing capacity for the most demanding paving projects.
Using the appropriate equipment, and selecting the asphalt grade and thickness for your paving project is essential for achieving the desired performance, longevity, and appearance of your paved surface. By considering factors such as the expected traffic loads, application, and asphalt grade, you can choose the right thickness for your residential, medium-duty, or heavy-duty paving needs. As always, consulting with a paving professional can help guide you in making the best decision for your project.
O'Brien Paving experienced team can provide you with their best recommendations for your paving project.