What is the driving surface difference among asphalt, concrete and polyester concrete?
- We apply asphalt with a smooth steel roller. The oil and small pieces of rock are tightly compacted and it leaves a very smooth ride.
- We apply concrete with a paving machine that finishes the surface with a steel edge. Because the concrete material is liquid-like when applied, the larger, coarser pieces of rock (aggregate) sink as the concrete sets. The fine sands rise to the top. Once paving is complete, we comb the top of the concrete to create shallow grooves to increase the skid resistance. The end result is a smooth driving surface capable of providing enough resistance to stop some skidding.
- We apply polyester concrete in the same way we apply concrete. But polyester is coarse and sticky and the aggregate does not sink. When we comb the top to increase skid resistance, the larger pieces of aggregate stick up giving the road a slightly noisier sound and rougher feel.
It sounds like asphalt, concrete and polyester are very different. How do you decide which to use where?
- Asphalt is the most common material. It is low-cost, versatile and has the broadest application for roads that have a road bed under them.
- We use concrete for high traffic areas where there is a lot of stopping and starting. We use concrete for heavy traffic like buses and trucks. Concrete is durable and strong and is used for structures like bridges. The concrete encases rebar which hold the bridge structure in place. There is no road bed to sit on.
- We use polyester as a top layer when we repair bridge deck surfaces. We need a rougher surface because bridges are the first to freeze in cold weather and we want to provide as much skid resistance as possible. Additionally, polyester is a very light material and it dries quickly, in as little as two hours. It is the most expensive product we use so we use it judiciously.
Why did WSDOT use polyester concrete for this job?
We had certain criteria for contractors bidding on this job.
- WSDOT wanted to keep at a minimum the hours I-5 was closed. To keep the closures at a minimum for this busy stretch of interstate, we needed a product that would cure quickly. Two products – latex modified concrete and polyester concrete – met that criteria.
- WSDOT wanted a product that would extend the life of the roadway by 25 years.
- WSDOT wanted a product that was safe for drivers.
Why does polyester work for this job in particular?
The contractor, Concrete Barrier Inc., bid the contract using polyester concrete based on his previous experience and his expertise in applying the product. It met our three requirements: short cure time, long-lasting material, and safe for drivers.
I expected the new road to be smooth and quiet like the new road surface on SR 520 in Bellevue or I-5 in Everett.
Those two roads have brand new asphalt pavement made with pieces of rubber. Those new roads are not bridges and can therefore use asphalt as the road surface.
What is cure time?
When engineers design new roadways or rehabilitate existing roadways, they can choose among a variety of substances to use as the surface of the road. The most popular surface products are concrete and asphalt. Certain products harden and cure faster than others. Faster curing means vehicles can be on that surface sooner. Cement concrete can take up to 24 hours to cure; during the cure period, the road must remain closed to traffic. Getting drivers back on the I-5 lanes was an important piece of this project. Polyester concrete has one of the fastest curing times available. The product hardens within two hours.
If it hardens so quickly, why don’t you use it everywhere?
Polyester concrete is more expensive than cement concrete so it is important to use this product when time is a critical piece of the work.
How is the polyester concrete intentionally roughened?
The small grooves in the pavement are installed though a process called tining (rhymes with lining). After the material is placed on the road, a rake-like attachment on the finishing machine makes grooves in the pavement. It improves skid resistance and can reduce the incidence of skids by up to 40 percent. In the case of polyester concrete, the grooves are about 3/16 of an inch deep.
Is that typical? How deep are the grooves in cement concrete and asphalt?
Cement concrete is also finished with 3/16 of an inch tining. Asphalt pavement produces a naturally rough finish; no tining is used with asphalt.
Where else have you used tining on polyester concrete?
We used this process on SR 18 and bridges on the Olympic Peninsula.
Is polyester as safe as asphalt and other concrete?
Yes. Polyester concrete, cement concrete, and asphalt all provide solid driving surfaces with adequate roughness for skid resistance.
Is this safe for motorcycles?
Tined finishes in cement or polyester concrete pavement are safe for motorcycles.