For more than a half-century, diesel-fuel screeds have been the market staple, laying millions of tons of asphalt mats in forms ranging from driveways to interstate highways. The fuel is used to fire the burners that preheat the screed bottom so contractors avoid asphalt dragging and lay a higher quality mat.
Although the market principal for decades, considerable drawbacks to diesel-fuel screeds, including operator comfort and maintenance issues, have opened the door for an electrically heated screed alternative. “In the morning, contractors light the diesel burners and run because of the smell,” said Bill Rieken, paver applications specialist for Cedarapids Paving, a part of Terex Roadbuilding.
Today’s electric screeds quickly and uniformly heat the screed bottom, are thermostatically controlled and build better joints. They do so with less fumes and radiating heat, providing for a more operator friendly environment. As an added bonus, the generator capacities on electric screeds deliver the versatility to run auxiliary electrical equipment.
The concept of electrically heated screeds has been around for approximately 10 years, but the idea and benefits took a while to win over contractors. There were drawbacks to the first electric screed designs. They were predominantly designed for commercial paving applications, not highway paving. They also were much heavier and less efficient than their diesel fuel counterparts.
Previous generations of electric screeds also were underpowered. “The first designs employed generators with less than 20-kW capacities,” commented Rieken. This resulted in a longer morning preheat cycle than with diesel screeds, and these generators could not effectively power auxiliary equipment and heat the screed at the same time.
Price also was a drawback. The first electric screeds cost significantly more than diesel-fuel screeds but did not offer many considerable operational advantages. The result was a lack of electric screed sales, with less than a 5% market share.
Throughout the last decade of design and technology advancements, today’s electric screeds have overcome many previous shortcomings. Increases in power and versatility have led to contractor acceptance. According to Mark Hunt, vice president of asphalt pavers for Terex Roadbuilding, “Five years ago, electric screeds accounted for less than 10% of all Cedarapids screed sales. Now they account for at least 50% of our sales.”
Current electric screed models address many of the complaints that contractors have with diesel-fuel screeds, and they do so in a much more operator-friendly design. Three-phase generators with capacities in excess of 30 kW, which are about the same size as the less powerful initial generator designs, now power today’s electric screeds.
A more powerful generator delivers a faster morning heat-up cycle and gives contractors the flexibility of another power source.
Electric screeds are more convenient for contractors who pave a lot of short pulls, work with cold-mix asphalt or work in windy climates. The primary job of either the diesel fuel or electric heat source is to bring the screed bottom close to paving temperature first thing in the morning. Once paving begins, the temperature of the asphalt maintains the heat of the screed bottom.
However, when the screed breaks contact with the hot asphalt, it begins to cool. A strong side wind will have a tendency to cool the screed bottom on the side of the screed facing the wind. Contractors working with cold-mix asphalt will need to keep the heat source on all the time to avoid asphalt from sticking to the screed bottom.
With a diesel-fuel screed, this means reheating a portion of or the entire screed length. This takes time, generates additional heat and creates more smoke and fumes around the screed area.
Electric screeds, by contrast, can handle screed-bottom reheating automatically through thermostatically controlled, multiple heating zones. “Heating for the electric Stretch 20 is broken into four different zones. The operator can set the thermostat for each zone to the desired temperature, and the set minimum temperature is automatically maintained. This allows the operator to focus on laying a smooth mat,” said Rieken.
Electric screeds also require less routine maintenance than their diesel-fuel counterparts. Plugged nozzles and failed glow plugs are common problems with diesel screeds. Although they are easy to fix, this can be costly in downtime. Electric screeds eliminate much of the day-to-day maintenance and operate more reliably throughout the paving season.
Of greater importance to contractors, according to Rieken, electric screeds offer more uniform heat distribution across the entire length of the screed. With diesel-fuel screeds, there are always hot spots on the screed bottom. “Manufacturers do their best to evenly transfer heat throughout the screed bottom, but there will be a hot spot at the burner,” commented Rieken. This intense heat can lead to screed bottom warping and premature replacement.
Heat uniformity contributes to laying a higher quality mat. This prevents the asphalt from sticking to any cool spots on the screed bottom and aids in constructing a better joint. “I have spoken with countless contractors, and they have all told me that the electric screed makes a better joint. This goes right back to uniform heat distribution at the screed,” commented Rieken.
The heating elements in an electric screed attach directly to the screed bottom for more efficient heating. With diesel screeds, the burner not only heats the screed bottom but also the entire screed chamber, resulting in a hotter, less comfortable environment for the screed and paver operators.
Since the electrical heating elements attach to the screed bottom, asphalt that accidentally spills onto the inside of the screed bottom does not affect pre-heating capabilities. The electric screed will still deliver uniform heat across the length of the screed.
A diesel screed will not be able to preheat the screed bottom in the places where asphalt has collected on the inside, because the heat from the burner cannot penetrate the asphalt. This can result in asphalt dragging until the hot-mix asphalt can bring the screed bottom to paving temperature.
Screed of today
Throughout the years, the evolutionary design process has led to significant advancements in electric screed technology. These improvements have not only solved the initial drawbacks of previous electric screed generations, but they also have addressed the limitations of diesel-fuel screeds.
Today’s electric screed has the power, the features and the versatility to rival its diesel-fuel counterpart, but these benefits come at a premium of approximately 20% more than the diesel-fuel screed. However, the electric screed’s reliable day-to-day operation, cleaner heating capabilities and ability to lay a higher quality mat more than offset the additional cost.