In the early 1930s, a coating consisting of a mixture of very fine aggregates, emulsified asphalt, and water was placed on a road in Germany.

This new and promising technique in preserving road surfaces marked the beginning of slurry surfacing development. Later in that decade, extensive worldwide experimentation began in earnest. But it was not until the 1960s, with the introduction of improved emulsifiers, continuous flow machines, and set control additives that real interest was shown in the usage of slurry surfacing for a wide variety of applications.

In the mid-1970s, Screg Route, a French company, designed Seal-Gum, a micro-asphalt concrete that was subsequently improved by the German firm Raschig and marketed in the United States under the trade name "Ralumac®". Since the introduction of Ralumac in 1980, micro surfacing has been used by many state DOTs and local agencies in the United States on moderate to high volume roads. In fact, micro surfacing applications have conclusively demonstrated that the treatment can be a highly successful pavement preservation technique. However, despite this success, it is crucial that managers and engineers fully understand micro surfacing’s limitations, performance variables, and materials/mixture requirements to ensure that the treatment is used only when appropriate.
Continuing advancements in mixing methods, emulsified asphalts and machinery have made slurry surfacing systems today’s choice in providing highly durable, low-cost pavement preservation surface treatments. As a treatment for everything from residential driveways to public roads, highways, airport runways, parking lots, and a multitude of other paved surfaces, slurry surfacing systems are now used extensively throughout the world. Local, state, and federal agencies — including the military — have a growing and ongoing commitment to the use of slurry surfacing systems in their pavement preservation programs, attesting to its effectiveness and economy.

Slurry surfacing systems are mixed in/by specially designed application equipment which includes compact, truck-mounted, trailer-mounted, or continuous run pavers. Each type of placement machine carries a quantity of unmixed materials which are blended in a continuous flow pugmill. The component materials are measured volumetrically and fed into the pugmill based on job mix formula (JMF) supplied by a laboratory accredited in slurry surfacing mix design.

Slurry surfacing systems are made quickly and accurately at the project site. Mixing and spreading are accomplished in one continuous operation.

The materials, including aggregate, mineral filler, emulsified asphalt (often polymer-modified ), additives, and water are mixed in the pugmill. From the pugmill, the mixture flows into the spreader box at the rear of the application equipment, where it is applied to the pavement surface. The operator can adjust the amount of water in order to maintain proper mixture consistency. If necessary, additive may be used to control the mix time and setting characteristics of the mixture, impacting return to traffic time. Compaction/rolling is not normally needed or applied. Changes in temperature, humidity, and texture of the existing surface can lead to a change in the amount of water and/or additive required.

The spreader box is capable of distributing the slurry surfacing system over the width of a traffic lane in a single pass. There are many types and variations of spreader boxes, but they all perform the same function, which is to spread the mixture in a uniform manner onto the existing pavement surface. Spreader boxes range from simple, light, non-adjustable units to large boxes equipped with augers, special runners, and hydraulic controls. Specialized boxes may be used for rut filling, special shoulder work, and variable width-spreading. The spreader box choice depends on the slurry surfacing treatment type and specified application rate of the material.  

Experienced operators continually monitor the mixture for consistency, making minor adjustments (within the mix design parameters) as needed for a uniform mixture consistency. Prior to application, other personnel notify the public of the upcoming treatment plan, clean the existing road surface before the slurry surfacing application begins, barricade the street, inspect the operation in progress for uniformity, protect/uncover utility structures, and perform any necessary handwork in any area inaccessible to the spreader box.

ISSA specifies three standard gradations, each with a designated use for the surface condition to be addressed.

ISSA Type I: This aggregate gradation is used to fill surface voids, address moderate surface distresses, and provide protection from the elements. Although not a substitute for crack sealing, the fineness of this mixture provides maximum crack penetration. It is frequently used on airfields, parking lots, and residential streets where a tighter, finer surface texture is desired.

ISSA Type II: This aggregate gradation is used to seal/fill surface voids, address more severe surface distresses, and provide a durable wearing surface. It is the most common gradation used in slurry surfacing systems. It is often used on pavements to correct distresses caused by weathering and raveling while producing an adequate wearing surface for low, medium, and heavy traffic volumes. This gradation is commonly used on airfields, parking lots, municipal streets, county roads, and highways.

ISSA Type III: This aggregate gradation, which is the most coarse, gives maximum coefficient of friction and improved durability due to the depth of the application, as the larger aggregate size increases the thickness of the mat placed. It is best-suited to higher-traffic pavements such as expressways, major highways, and arterials. When used in micro surfacing, this gradation is ideal for rut filling and reestablishing profiles with minor surface irregularities.

Pavement preservation is a set of practices designed to extend pavement life, improve safety, and reduce life cycle costs. Because pavement preservation, also referred to as pavement preventive maintenance, saves tax dollars, it is a top priority of the Federal Highway Administration. Studies by the FHWA have shown that using Slurry Surfacing is an effective strategy to accomplish all of these goals. The National Cooperative Highway Research Program Synthesis 223 is a survey of 60 state, province, and local transportation agencies. This survey confirms that the most cost-effective strategy for managing pavements, which also results in the highest pavement rating, is to perform preventive maintenance on better rated pavements first and then fund rehabilitation of the poorer rated pavements. Slurry Surfacing is the perfect tool for preventive maintenance.

Asphalt pavement condition is adversely affected by oxidation from the sun, wear and tear from traffic and damage from salt and chemicals in colder climates. Slurry Surfacing is the right choice to extend the life and improve the performance of existing pavements. Consider all the factors and see why Slurry Surfacing should be your first choice. Benefits include:

Correct Problems. Slurry Surfacing will protect the pavement from further abuse and will improve the overall condition by addressing minor surface cracking, raveling and loss of matrix. Roadways that have unacceptable surface texture due to polished aggregates or flushing lead to unsafe conditions. Slurry Surfacing decreases water permeability and improves skid resistance, which leads to a safer roadway for the traveling public.

Save Money. Slurry Surfacing is economical and highly cost-effective when compared to other pavement options. If applied at the correct time, it will extend the service life of the pavement and reduce the total life cycle cost.

Convenience. Quick and easy application means less inconvenience to customers and residences. Most slurry surfacing applications are traffic ready within one to two hours.

Enhance Aesthetic Value. With one simple application, the existing pavement receives a new wearing surface of uniform black color and texture. Enhanced appearance means higher property values, better marketing potential, and citizen approval. Whether a subdivision street, shopping center, or commercial parking lot, your pavement will be more attractive when treated with Slurry Surfacing.

Be Green – Save Green. The pavement preservation process of Slurry Surfacing as promoted by ISSA—slurry seal and micro surfacing—stretch your budget while also using fewer natural resources, consuming less energy, and reducing overall greenhouse gas (GHG) emissions. Saving money and resources allows you to balance the needs of people, nature, and the economy, while in turn addressing a national demand for sustainable living. The environmental impact of ISSA’s pavement preservation processes is less than that of traditional pavement repair methods.

According to BASF Corporation’s recent “Micro Surfacing Eco-Efficiency Analysis (EEA)”, verified by NSF International (NSF International is a not-for-profit, non-governmental organization that develops national standards and provides third-party conformity assessment services), the use of micro surfacing results in the following environmental benefits:

• 50% Fewer Resources Consumed - Over a 40-year life cycle, a road treated with pavement preservation consumes less aggregate and binder, even when considering the use of recycled asphalt pavement (RAP) in mill and fill.
• 40% Less Energy Used - Since pavement preservation uses cooler production and application rates and less asphalt and aggregate than some traditional pavement repair methods, it consumes about 40 percent less primary energy.
• 45% Fewer Greenhouse Gases (GHG) Emitted - Pavement preservation requires half the amount of material to be shipped to and from manufacturing and job sites, reducing emissions. It also uses a cooler process, which means roads are drivable within a short period after application, reducing traffic delays and their associated GHG emissions.