Three Keys to Quality Chip Seal Design

If performed correctly, chip seals can be used to stop raveling, seal minor cracks and improve skid resistance, all while extending a pavement’s lifetime.

When it comes to the design of chip seals, Scott Shuler of Shuler Consultants LLC, Fort Collins, Colorado, said during a recent Transportation Research Board webinar that there are three primary factors to consider.

Chip quantity, emulsion quantity, and substrate conditions, he said, are all key to a quality chip seal. Let’s take a closer look at each of those design factors.

“Determining how many chips to apply to the surface depends on the shape, surface texture and size of the chip itself,” Shuler said. “What you’re trying to do is apply the chips one stone thick.”

If the layer is more than one stone thick, Schuler said, not only will the excess chips not stick into the emulsion but they may also pull out chips that were embedded in the emulsion. He added that there should also be some voids so the chips have room to move when the seal is rolled. “If you can’t see some emulsion, you have too many chips on there.”

To determine the spread rate of applying chips one stone thick, Shuler recommends performing a board test in the lab. Set down a 1-square-yard board of known weight, cover it with the chips you will be using on the project one stone thick across the entire board, and weigh the board. Subtract the known weight of the unladen board from the chip-covered board. “That will be the approximate weight to drop on the emulsion per square yard when you’re out on the road,” Shuler said.

The spray rate for emulsion, Shuler said, depends on the geometry of the chips. Each chip should be embedded 30 to 50 percent prior to rolling so that the rollers and traffic will embed the chips 75 to 90 percent eventually.

“To do that, we must figure out what the volume of the voids is between those chips when they’re embedded and calculate that quantity of material,” Shuler said. “The calculation (for an estimated initial shot rate) is based on percent embedment times the average mat depth, or the size of the chip you’re applying.”

Percent embedment is calculated with the following formula (where W stands for loose unit weight of aggregate; 62.4 pounds per cubic foot is constant for the unit weight of water; G stands for bulk specific gravity of aggregate; T stands for traffic correction; and V stands for surface condition correction): {1 - (W / 62.4 G)} x T + V

“The traffic factor is based on how much traffic you have on that roadway,” Shuler said. “Surface condition correction is based on how soft the substrate is.”

3. SUBSTRATE CONDITION To understand the substrate condition, it’s important to measure its rigidity and texture.

Texture is measured with a sand patch test, where a known volume of sand is spread on the surface in a perfect circle and the diameter is measured to define the surface texture.

Rigidity is measured by ball penetration test, where a Marshall hammer is placed on the ball, then the Marshall hammer is raised to its maximum height and dropped on the ball. The depth the ball penetrates the pavement surface indicates the ability of the pavement to resist penetration of chips by traffic.

“If you’re applying a chip seal over a flush surface and it’s too soft, the chips are going to sink into that,” Shuler said. “You’re going to have to correct for that, or you’re going to see some flushing.” He added that extra smooth surfaces will require lower shot rates, while extra rough surfaces will call for an increased shot rate.

With the right amount of chips, appropriate emulsion shot rate, and a solid understanding of substrate conditions when designing chip seals, you’ll be three steps closer to a quality end product.