- Slurry Seals – What and Why
The slurry seal is a thin surface treatment that does not add any structural capacity to the existing pavement. It is applied as a maintenance treatment to improve the functional characteristics of the pavement surface; i.e., the ride quality (smoothness) and skid resistance (safety). The slurry seal is a mixture of asphalt emulsion, graded aggregates, mineral filler, water and other additives. The asphalt emulsion serves as the binder, holding the crushed aggregate together and bonding the slurry to the old surface to which it is being applied. The aggregate must be clean, crushed, durable, properly graded, and uniform. The asphalt emulsion is a three-part system consisting of asphalt, water, and emulsifier. Fillers such as Portland cement, hydrated lime, or aluminum sulfate liquid are often used in small quantities as stabilizers or chemical modifiers.
In the early 1930s, a coating consisting of a mixture of very fine aggregates, asphalt binder, and water was laid out on a road in Germany. It proved to be a novel approach, a new and promising technique in maintaining road surfaces, and marked the beginning of slurry seal development. Later in that decade, extensive worldwide experimentation began in earnest. But it was not until the 1960s, with the introduction of improved emulsifiers and continuous flow machines that real interest was shown in the usage of slurry seal for a wide variety of applications.
Continuing advancements in mixing methods, emulsions and machinery have made slurry seals a common choice for durable, relatively low cost paving and surface maintenance. As a treatment for everything from residential driveways to public roads, highways, airport runways, parking lots, and a multitude of other paved surfaces, slurry seal is now used extensively throughout the world. Local, state, and federal agencies — including the military — routinely use slurry seals in their maintenance programs, attesting to their effectiveness and economy.
How is Slurry Made and Applied?
Slurry is made in specially designed equipment either truck-mounted or self-propelled. This equipment, shown below, carries a quantity of unmixed materials which are blended together in a continuous flow pugmill. The use of this machinery ensures a smooth, consistently uniform mixture. Slurry can be made quickly and accurately at the project site. Mixing and spreading are accomplished in one continuous operation, with the surface being reopened to travel within a few hours.
A slurry seal is applied to an existing pavement surface by means of a spreader box linked to the surface slurry-mixing unit. Slurry is introduced into the spreader box, which then lays down the slurry coating as the mixer/spreader is driven forward. The box is capable of spreading the slurry seal over the width of a traffic lane in a single pass, and is constructed so that close contact with the existing surface is maintained.
- Purpose and Types?
The main use of slurry surfacing materials is for pavement preservation as a part of a program of periodic surfacing before distresses appear.
A slurry seal is used to do the following:
A slurry seal should not be used to do the following:
Emulsions of varying composition and setting times are mixed with any one of three grades of aggregates to create slurry seal mixes for specific purposes. Aggregate types are I (fine), II (general), and III (coarse). The primary difference is the aggregate top size. The type I slurries are the finest and are used for lightly trafficked roads or parking lots. Type II slurries are coarser and are suggested for raveling and oxidation on roadways with moderate to heavy traffic volumes. Type III slurries have the coarsest grading and are appropriate for filling minor surface irregularities, correcting raveling and oxidation, and restoring surface friction. Type III slurries are typically used on arterial streets and highways. Types and typical uses of slurry seals are summarized in table below.
Caltrans, Maintenance Technical Advisory Guide, Volume I, Flexible Pavement Preservation, Second Edition, 2008