Asphalt Paving Charleston SC is a popular choice for paving projects because it’s safer and more economical than other materials. It’s also easy to recycle, promoting environmental sustainability.
The process of laying asphalt pavement involves several steps. First, contractors slope the site to create a solid base. Next, they put down a thick layer of load-bearing aggregates. Then they add a binder course of bitumen and additives to hold the aggregates together.
The subgrade acts both as a platform for construction equipment and as the foundation of the asphalt pavement structure. Consequently, proper subgrade preparation is essential to the success of the project. Subgrade soils should be capable of supporting traffic loads without excessive deformation and graded to a specified elevation and slope.
A poor or unstable subgrade will fail under dynamic wheel loading, causing rutting and ultimately pavement failure. The most common causes of poor subgrade are expansive soils, moisture and lack of compaction. Expansive soils contain clay minerals that absorb water, resulting in an increase in volume, or “heaving”. This swelling creates pore pressures that can exceed surcharge pressures (such as the weight of the pavement).
To prevent this phenomenon from occurring, expansive soils should be stabilized by adding a cementitious material such as lime, portland cement or emulsified asphalt. Alternatively, a marginally poor subgrade soil can be made acceptable by adding additional base layers to spread pavement loads. However, this approach has several disadvantages, including increased cost, deterioration of the base layer and subgrade, and reduced pavement life due to overloading of the base.
In some cases, HMA surface courses can be placed over poor subgrades if the design thicknesses are sufficiently large and the base layer is designed with a high enough strength to support the traffic load. However, this is a risky approach and the pavement life will likely be short.
In most cases, the primary application for asphalt pavements is for walkways such as sidewalks, bicycle and golf cart paths, and fire lanes. These are usually designed to withstand light traffic and occasional heavy-load applications such as maintenance or emergency vehicles.
Mix Design
Mix design is the process of determining what aggregates to use and what the optimum asphalt binder (asphalt cement) should be. The result of a good mix design is a recommended mixture, commonly referred to as a job material formula (JMF) or recipe, for the paving contractor to produce. Typically, the JMF will include aggregate gradation and asphalt binder type along with performance testing for rutting, fatigue cracking and moisture damage.
The first step in the mix design process is to select a gradation for the aggregates. This is usually done using a laboratory test called the PG (performance grading) system. Several different grades of aggregates are tested under various conditions to determine the one that best meets project requirements for rutting, fatigue and cold weather resistance. Depending on the expected climate conditions and pavement temperatures, this grading is then used to establish a blending target for the HMA.
Once the gradation of the aggregates is determined, the laboratory creates a number of trial mixes with different asphalt binder content levels. These mixes are compacted into molded specimens and then tested for their desired characteristics. Mixes that pass all of the required tests are then selected for production.
The mix design process is becoming increasingly based on performance testing. The objective is to produce quality HMA that will provide long-lasting service in the field. Many highway agencies, engineers and contractors have realized that traditional prescriptive methods of combining ingredients in prescribed volumes is not producing HMA that will resist damage from traffic loads and other forces. A new approach to mixing HMA is balanced mix design (BMD). This method uses performance tests that evaluate the suitability of the mixture for a specific application.
Laying
The base layer of asphalt is a vital part of the pavement that enhances its load-bearing capacity. It consists of a mix that has been crafted from different aggregate sizes. This mix is usually prepared at an asphalt plant to ensure consistency and quality. Depending on the intended use of the pavement, the mix design may vary to accommodate the expected amount of traffic and environmental conditions.
Ideally, the sub-base must be able to stand up to the weight of cars and trucks while also being resilient against erosion, weather, and other factors that can affect its structural integrity. It is important to check the drainage of the area and install proper slopes to prevent water accumulation and rutting.
Once the sub-base is ready, the paving crew can start working on the actual laying of the asphalt. This process requires specialized paving equipment known as an asphalt paver. This machine is designed to deliver the asphalt mix evenly over the surface and compact it for a firm, durable, and smooth finish.
When constructing an asphalt pavement, the crew must follow strict guidelines. Failure to do so can result in numerous problems such as poor drainage, rutting, and cracking over time. The best way to avoid these issues is by prioritizing meticulous compaction, prudent mix selection, thorough base preparation, and careful construction.
Once the paving is done, it’s crucial to allow the surface to fully harden before using it. This can take 30 days or more, so it’s important to be patient and exercise caution while the asphalt is hardening. Proper maintenance will also help extend the life of your asphalt driveway, parking lot, or road.
Compaction
When it comes to asphalt, compaction is a critical factor for achieving the best possible results. That’s because the quality of a finished asphalt pavement directly correlates to the density, or air-void content, of the mix.
The goal of compaction is to get all the air out of the mixture and essentially make it dense or tight, so that the particles knit together better, improving stability and resistance to rutting. Compaction can also help improve fatigue life and durability of the finished pavement.
It’s important to understand that compaction is a dynamic process that can be influenced by multiple factors on the job site. For example, the air temperature plays an important role in how fast a lift of HMA cools and therefore limits the time available to compact it.
Another key variable is the base temperature of the road on which you’re paving. Increasing the temperature of the base helps to give the mix more time to be compacted. And finally, wind velocity has a significant impact on the speed at which the lift of asphalt cools and the time available for compaction.
In addition, the type of roller used to compact the material has a big impact on the outcome of the compaction. The breakdown roller, which works immediately behind the paver, is responsible for developing the bulk of the HMA’s final target density and needs to match the production and speed of the paver. This roller is usually a vibratory steel wheel or pneumatic tire roller. The intermediate roller, which follows the breakdown roller, is responsible for finishing and typically uses a vibration frequency of 60 to 90 hertz. These frequencies are often used to achieve higher levels of compaction and provide the best finish for the asphalt pavement.
Finishing
Once the sub base layer is in place and any soft areas have been repaired, it is time for the binder course to be added. The binder layer is large aggregate mixed with oil to create a strong, sturdy asphalt surface. It is installed directly over the runner-crush stone base and is considered to be the “weight-bearing” course of asphalt. This layer is much thicker than the top course and will provide an additional structural benefit to the paved surface.
After the binder course has been rolled and compacted, the final asphalt layer is placed. It is composed of small aggregate, sand and oil to produce the smooth, jet-black surface we’re all used to seeing on roadways, parking lots, and driveways. The surface asphalt is then rolled and compacted once again to achieve proper compaction.
It is important to note that a well-made asphalt pavement requires constant monitoring. Load and usage, as well as weather conditions, all impact how long an asphalt surface will last. Regular inspections can help extend the life of a paved surface and prevent costly repairs.
It is also important to choose a contractor that has the experience and equipment to do a quality job. Ask for referrals from friends and neighbors and do your homework. Study the contract and ensure that everything you want to be done is spelled out in detail. Be wary of contractors that have extra asphalt they’d like to put down at a discounted price. This is an indicator of poor workmanship and materials. Finding a contractor who is experienced and has the right equipment will result in a long-lasting, high quality paved surface. Using the right tools for each step of the process is critical to asphalt longevity.