Quality of Cold-Mix Asphalt Deserves Attention

By: Michael Pemberton

To manufacturers of high-performance, cold-mix patching products, quality runs deeper than surface appearances. Quality pertains to the appearance and consistency of the patching product, how well it performs, and the unique benefits it offers.

However, quality also pertains to how business is conducted--the commitments producers and suppliers make to their customers and the responsibility taken to ensure that customers are getting what they expect.

What customers expect is cost effectiveness. This value is determined by the cost of the cold patch, its expected life, and the manpower needed to make repairs. A U.S. government study on pothole repair (SHRP-H-353) conducted by the Strategic Highway Research Program recommends that the best materials available should be used to reduce the need for repatching.

According to the study, the cost of repeatedly patching the same potholes, because of poor-quality material, quickly offsets the savings from purchasing a less expensive mix. The study further indicates that in most cases, the poorer performance associated with inexpensive cold mixes will result in greater overall costs for patching because of increased costs for labor, equipment, traffic control and user delay.

To a large extent, the quality of the material begins and ends with the relationship between the cold-patch mix supplier and the asphalt producer. With higher performance mixes, the supplier should work closely with the asphalt producer to ensure that the finished cold patch meets everyone's expectations.

By the same token, a quality producer should take responsibility for the success of the product in the end user's application. Producers should strive to maximize quality by understanding how the material performs, and by fine-tuning their production process in order to achieve the best possible results.

Factors affecting production

Providing consistent, high-quality asphalt patching material is not a routine task. A number of factors affect quality, including raw material selection, how the materials are stored and handled, and the type of production facility being used. In addition, many environmental factors, such as extreme heat or cold, and the presence of moisture or excess dust can dramatically change the complexion of the asphalt mix and cause problems.

For the most part, hot-mix asphalt is produced by coating crushed aggregate with asphalt cement. Standard cold mixes are produced by coating aggregate with an emulsion or with a cut-back asphalt, and some mixes even have some type of modifier in them. In the case of high-performance cold-patch materials, additives specially formulated for each aggregate are incorporated to allow the cold mix to be applied in lower temperatures, and to achieve a "slow cure," which helps to promote long life for the end user. This, along with individual mix designs and quality control procedures, ensures a high quality patching material.

Given the above variables, the only way to achieve consistent quality is for the cold mix blend supplier to monitor the asphalt producer's production on-site to observe and to act as a consultant during every production. To minimize production problems and to maximize the effectiveness of high-performance products, some manufacturers have developed a formalized evaluation program that each batch undergoes in every step of the process.

In the production of its UPM Blend, Unique Paving Materials (UPM) conducts three major tests on asphalt cement: viscosity, penetration, and specific gravity/pound per gallon. The viscosity test ensures that the thickness and flow of the mix is appropriate at a given range of temperatures. The relative hardness of the asphalt is determined by the penetration test. Another test, measuring the specific gravity/pound per gallon determines the weight of the asphalt as compared to the weight of water. Because of the extreme variety of weather conditions around the U.S. these tests help to determine which raw materials are suitable for making cold mix in a given region of the country.

Raw material selection and testing

On the surface, what could be easier than raw material selection? For one thing, there are many different types of aggregates in the U.S.--from the granite and basalt typically available on the West Coast to the limestone and traprock used on the East Coast. Sometimes there are major differences in the rock from the same quarry. These differences can result from diverse factors, such as the depth at which the rock was excavated, the hardness or softness of the rock, and water saturation of the rock over time.

The relative size and surface area of each individual aggregate stone is important because in the production of asphalt, the aggregate is coated with a blend of asphalt and additives. Leading manufacturers use standard testing methods, such as ASTM (American Society for Testing and Materials), to determine the quality of the aggregate that goes into their products.

Before production, a battery of tests should be performed to ensure a quality product. These tests would determine certain characteristics that affect the finished product, including the viscosity of the blend, and the appropriate additive package to make sure the product performs to expected standards.

UPM recommends several tests to conduct on the aggregate to determine how well it will perform.

Surface area: A sieve analysis test would measure the aggregate size and surface area of the rock to be coated. In conjunction with this, a -200 Wash test determines how much dust is present on the rock, which affects the true size of the surface area. The surface area is measured in units of square feet per pound and is used along with the absorption and abrasion of the rock to determine starting Job Mix Formula.

Soundness: A five-cycle sodium test would determine how the rock will stand up to road salt, in addition to simulating the wetting and drying action of five freeze/thaw cycles.

Abrasion: The most rugged of these tests is the L.A. Abrasion test that places 30 lb of aggregate into a steel drum with five steel balls for 500 revolutions. This test determines how much the surface area will increase due to break up during production.

Formulation testing: A series of tests are conducted to help allow for the differences in aggregate and to compensate the amount of water and additives needed to achieve production consistency.

One of these is a "stripping test" that simulates a production of cold patch using various additive packages to determine which is best suited for a particular aggregate and to determine if any special precautions or methods are needed during production.

The major cause of stripping--when the asphalt does not adhere to the aggregate--is the aggregate not being dry enough before the asphalt blend is added.

Another cause of stripping is the presence of excess dust and fines in the aggregate. Measures can be taken during production, such as running the aggregate through a dryer with a dust collector system and raising the temperature to help eliminate the dust.

Production testing

The key to achieving a good blend is to match the right asphalt mixture with the right additives and tweak the process to work well with the type of aggregate available at the plant.

During the production of cold patch, several on-site tests should be conducted throughout the mixing procedure to ensure the consistency of the blend.

High-performance patching materials can be affected by contamination in the stockpile area after production. The remedy is as simple as it seems: storage requirements should call for a clean, paved stockpile area.

To ensure quality control at each production site, representatives should attend each production and to complete a series of forms on each production of cold patch. These records would help laboratories look for trends and to be able to anticipate and solve problems when similar circumstances arise.