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History
Otta seal is a type of bituminous surface treatment that was developed by the Norwegian Road Research Laboratory (NRRL). Its name is based off of the location in which it was created in, The Otta Valley. Otta seal was developed to be used as a temporary surfacing on new roads; however, after seeing its strength, it has been used as permanent roads as well. [1]
How It’s Made
Otta seal is formed by adding graded aggregate to a soft bituminous binding agent. The agent is usually emulsified asphalt. Bituminous binder application rates are between 1.9 liter/m2 and 2.4 liter/m2 the value is dependent on aggregate gradation and type. Low quality, local aggregates are often used in Otta seal. The largest aggregate size used is between 13 and 25mm. The aggregate can contain up to 10% fine gravels. Aggregate quantities are usually close to 50 lb/yard2. Otta seal is easily cured by blinding with sand because of its soft binders. The soft binders quickly coat the sand; This is not possible for seals with harder binders. Cutback bitumens in the viscosity range of MC300 to MC800 are the most common binders used.
Otta seal is formed in the following procedure:
1. Remove dust and foreign objects from the base.
2. Add small amounts of water to the base to suppress dust.
3. Apply aggregate at a rate of 1.3 to 2.0 m3 to 100m2 (depending on the grading of the aggregate) of road surface.
4. Apply a hot binder at a rate of 1.6 to 1.9 liters/m2.
5. Flatten the Otta seal using a tired roller until aggregates are pressing up between aggregate particles. This usually has to be done multiple times, typically several days after the first flattening.
6. For the first two to three weeks, allow traffic at low speeds to assist with further kneading. Aggregates will be dislodged by traffic and they should be broomed back onto the seal. [2] [1]
Where Otta Seal is Used
Otta seal is primarily used in places that lack resources and capital. However, even with its cost effectiveness, many countries are resistant to change to Otta seal from traditional seals. As more information about Otta seal is released, there will likely be more widespread use by the construction industry. Currently, Otta seal is most prevalent in Norway, Sweden, Iceland, and Botswana. It is common in Bangladesh, Australia, and parts of Africa. Otta seal is used in places that do not have strict requirements for strength, grading, partical shape, binder adhesion, and dust content. It is used in places that have low capital, and expect relatively low traffic (up to 500 vehicles per day). [1] [3]
Political Impact
Politically Otta seal is a useful alternative versus other types of road surfaces. A city council member of a city that uses Otta seal explained that it is very cost efficient since with Otta seal the can resurface a road ever 5-10 years without having to tear the whole road up every time it needs to be resurfaced. This lengthens the road life and makes it very economical. [4]
Social Impact
Socially Otta seal is preferred by the public at large. It does not cause prolong delays in traffic, it takes few hours to resurface a road and is usable almost immediately afterwards. Since this is mainly used on side roads it is important that it is done quickly to prevent people from not being able to get to their houses. One negative impact is the loose gravel can cause chips or damage to windshields, but odds of this happening is lowered in slower moving traffic areas. [4]
Environment Impact
Environmentally Otta seal helps lowering the dust level. This helps the environment in many ways; helps with driving visibility, decrease health problems caused by dust, and reduce harm done to crops. Another environmental benefit is that the materials used in its production are easily acceptable. There is also a reduction in environmental degradation. [1]
Costs
Otta seal is considered to be a low cost seal. With its low initial cost and less demanding maintenance, it is a very cheap alternative for road surfaces. The cost for a double layer Otta seal is about $2.00 to $2.70 per meter square and will last from 8 to 15 years. For a road that is 1 mile long and 40 feet wide it would cost between $39,242.24 to $52,977.03. [5]
Physical Properties
Otta seal is a type of road surfacing comprised of a bituminous binding substance and aggregate rocks ranging from gravel to fine particles. It is stronger than similar inexpensive surfacing techniques such as chip seal.
The type of bitumen used can greatly affect the strength of the road. Bitumen binders that are more viscous tend to move through the aggregate faster, but are not as hard and cannot handle heavier loads.
A huge advantage of otta seal over other seals is the ability to use almost any type or size of crushed rocks as the aggregate. Depending on where in the world the road is being constructed and therefore what types of rock are available, the strength of an otta seal can vary greatly. Commonly, the gravel used contains sandstone, basalt, and even coral or volcanic stones. Also, size of the particles varies greatly, with a preferred maximum of 16mm and 19mm for single and double otta seals, respectively. Typically, the concentration of fine particles of less than 0.075mm should be less than 10%.
Adhesive agents are generally added to increase the strength between the aggregate and the bituminous binder. It is this interaction that gives otta seal is strength and durability. [6]
Molecular Structure
Since otta seal is a combination of materials, it is beneficial to look at the molecular structures of the materials used.
Rocks are in the material family of ceramics. They have very rigid crystalline structures that stand up well to compressive forces. In contrast, sheer or tensile forces can easily form cracks in ceramics and cause failure. The bitumen acts the opposite way. It is more amorphous, meaning it is weaker under compressive forces, but can hold the rocks together and limit cracking under tensile stress.
Below is a model of the bitumen molecular structure. It is mostly comprised of compressed hydrocarbons, sometimes containing many other elements like nitrogen, oxygen, sulfur, and nickel.
Pros and Cons of Otta Seal
There are many factors when considering what type of road surfacing is necessary. Most advantages to otta seal over other inexpensive surfacing techniques spring from its ability to use a wide range of materials and material sizes during construction. Pros and Cons are listed here:
Pros
- Can be used in remote areas with whatever gravel is available
- Less waste when using crushed material
- Cheaper, lower grade materials can be used
- More tolerant of pavement deflection
- Much less precise proportions, resulting in less mistakes
- More durable and less maintenance
- Workers require very little technical skill
- Creates job opportunities
- Resistance to cracking from solar radiation
- Impermeable, so water can run off
- Can be used in any climate
- Can be broken down and reused
Cons
- Requires more material
- The need for experienced people in charge of operations
- The need for experienced people to do maintenance
- Grade cannot exceed 8% for single otta seal (12% for double)
- Must be applied at above 50°F
Durability and Weathering
Otta seal is more durable than alternative seals. It is impermeable and resistant to cracking from the sun’s radiation. Cracks form in all roads due to changes in temperature, but otta seal performs better than other seals. Overall, otta seal can be expected to last 50 to 60 percent longer than a chip seal. This results in a lifespan between 6 and 12 years if properly maintained, and even longer if a sand seal is used on top of the otta seal.