Bituminous Surface Treatments: Types, Material, and Application

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A bituminous surface treatment (BST) is a chip seal or seal coat. It's a liquid asphalt surface treatment with a thin layer of aggregate placed at a thickness of around 0.5 inches or less. BSTs are normally used on pavements with lower traffic volumes.

Types of Bituminous Surface Treatments

1. Single Surface Treatment

The bituminous surface is produced by applying bitumen to a prepared surface for one time, and the surface is immediately rolled, preferably with a pneumatic-tired roller.

2. Multiple Surface Treatment

In this type of treatment, the application of bitumen is repeated for 2 or 3 layers with aggregate size usually becoming smaller with each application. Each layer is immediately rolled, preferably with a pneumatic-tired roller.

Material Requirement of Bituminous Surface Treatments

1. Aggregate 

A. Size

The aggregate should be uniform in size, preferably in the range of 1⁄2 to 1⁄4 in (13 to 6 mm) for single surface treatments. For multiple surface treatment, the aggregate size shall be in the range of 1 to 1⁄2 in (25 to 13 mm) for the bottom layer, with each successive layer using the aggregate of approximately 1⁄2 in size. 

The larger size aggregates may be used in multiple treatments. Aggregates larger than 1⁄2 in (13 mm) can cause objectionable tire noise. 

B. Shape

The ideal shape of the aggregate shall be cubical. Flat or elongated aggregates are undesirable. Flat aggregates tend to become aligned on their flat sides and may be completely covered with bituminous material. Rounded aggregates tend to roll and have poor retention and, therefore, are undesirable.

C. Cleanliness

The cleanliness of the aggregate is extremely important. If the coarse aggregates are dusty or coated with fine material, the bitumen will not adhere to the aggregate, resulting in poor performance and loss of cover aggregate. It is recommended that the aggregate passing the No. 200 mesh screen not exceed 1% by weight.

2. Bitumen

The bitumen used in the surface treatment is classified into three types.

A. Asphalt Cement

The asphalt cement shall be homogeneous, free from water, and shall not foam when heated to 347°F (175°C). The asphalt cement shall conform to the requirements given in the table below:

B. Cutback Asphalt 

The cutback asphalt shall not foam when heated to application temperature and shall conform to the requirements in the table.

C. Emulsified Asphalt

The emulsified asphalt shall be homogeneous after thorough mixing, provided that separation has not been caused by freezing. Emulsified asphalts separated by freezing shall not be tested. Emulsified asphalt shall conform to the requirements prescribed in the table below:

Application Process of Bituminous Surface Treatments

1. The potholes in the road surface shall be patched, and the cracks shall be filled.  
2. The road surface to be covered with the surface treatment should be cleaned with a rotary broom or other approved means.
3. The bituminous material is sprayed at a specified rate and temperature based on the type and grade of material.
4. The recommended viscosity range for spraying is 20-120 cSt.
5. The cover aggregate is spread at a specified rate immediately behind the bituminous material spray application to achieve maximum possible chip wetting depth.
6. This is required because of the quick rise in viscosity with cooling in asphalt cement and road tar, but it is very desired in asphalt emulsions and cutback asphalts to optimize the meniscus effect.
7. The cover aggregate should be placed on the bituminous material within 2 min of the bituminous application.
8. A self-propelled mechanical spreader or other aggregate spreading device capable of uniformly distributing the aggregate at the specified rate and width should be used to apply the aggregate.
9. The spreader must be calibrated to apply the amount of cover aggregate specified by the project's design specifications. 
10. The surface is rolled immediately after applying the cover aggregate to seat chips in the bituminous membrane, ideally with a pneumatic-tired roller.
11. The speed of the roller should not exceed 5 mph. Depending on environmental conditions, type, and amount of bituminous material, three passes of a 12 to 15-ton pneumatic-tired roller are normally used. 
12. The aggregate should be rolled out within 5 minutes of the bituminous material being applied, and the remaining three coverages should be completed within 30 minutes.
13. Control traffic speed until bituminous material has been set. It is recommended that traffic speed not exceed 20 mph for 4 h after placement of the surface treatment. 
14. For multiple surface treatments, repeat steps 3 to 6.
15. Application of subsequent coats shall be carried out only after the preceding bituminous surface has set and hardened, which normally takes one day. 

Precautions during Construction of Bituminous Surface Treatments

1. Surface treatment operations should not be carried out during cold and wet weather periods or both. The surface shall be clean and dry while performing the work.
2. The air temperature should be at least 50°F (10°C) in the shade and rising before starting the operation.
3. The operation should not be permitted when the temperature is 60°F or less and falling.
4. The operation should not be carried out in the rain or when rain is expected.

FAQs

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