Tack Coat

A tack coat is an application of asphalt to an existing paved surface to provide bond between the existing surface and the asphalt material to be placed on it. Two essential requirements of a tack coat areas follows: (1) it must be thin and (2) it must uniformly cover the entire surface to be treated. A thin tack coat does no harm to the pavement, and it will properly bond the course.

Some of the bituminous materials, used for tack coats, are rapid-curing cutbacks, road tar cutbacks, rapid-setting emulsions (may be used in warm weather), and medium-asphalt cements. Because rapid-curing cutbacks are highly flammable, safety precautions must be carefully followed.

A tack coat should be applied only when the surface to be tacked is dry; and the atmospheric temperature has not been below 35F for 12 hours immediately before application.

Before applying the tack coat to a surface that is sufficiently bonded, ensure that all loose material, dirt, clay, or other objectionable materials are removed from the surface to be treated. This operation may be accomplished with a power broom or blower, supplemented with hand brooms if necessary.

Immediately following the preparation of the surface, the bituminous material should be uniformly applied by means of a bituminous (asphalt) distributor at the spraying temperature specified. The amount of bitumen application, known as rate of application (ROA), for a tack coat should be applied in quantities not less than 0.05 or more than 0.25 gallon per square yard. The exact quantity varies with the condition of the existing pavement being tack-coated. Normally, the construction project specification denotes the ROA for the tack coat application; however, when the ROA is not included in the project specifications, the NCF uses an ROA of .15 for planning and estimating purposes.

Following the application of bituminous material, the surface should be allowed to dry until it is in a proper condition of tackiness to receive the surface course; otherwise, the volatile substances may act as a lubricant and prevent bonding with the wearing surface. Clean, dry sand should be spread on all areas that show an excess of bitumen to blot up and cure the excess effectively. After excess bitumen is set, any loose sand should be lightly broomed from the primed surface before the wearing surface is laid.

An existing surface that is to be covered by a bituminous wearing surface should be barricaded to prevent traffic from carrying dust or mud onto the surface, either before or after the tack coat is applied. Should it become necessary for traffic to use the surface, one lane may be tack-coated and paved, using the other lane as a traffic bypass. The bypass lane should be primed and sanded before it is opened to traffic and it should be swept and reprimed after the adjacent lane is completed. Doing this preserves the base and acts as a dust palliative (shelter.)

The formula for a tack coat estimate is as follows: tack coat application are as follows:

Always round your answer to the next higher number. In this case, 1478.4 is rounded to 1479 gallons.

TYPES OF ASPHALT PAVEMENT CONSTRUCTION

Two major types of asphalt pavement construction are in use today: plant mix construction (so-called because the mixture is prepared in a central mixing plant) and mixed-in-place construction (so-called because the mixture is mixed on the area to be paved).

PLANT MIX CONSTRUCTION

Asphalt-paving mixtures, prepared in a asphalt mixing plant, are known as plant mixes. Plant mix asphalt concrete is considered the highest quality plant mix. It consists of well-graded, high-quality aggregate _{and asphalt cement.} The asphalt and aggregate are heated separately from 250F to 325F, carefully measured and proportioned, then mixed until the aggregate particles are coated with asphalt. The hot mixture, kept hot during transit, is hauled to the construction site where it is spread on the roadway with an asphalt-paving machine. The smooth layer from the paver is compacted by rollers to proper density before the asphalt cools.

Asphalt concrete is but one of a variety of hot-asphalt plant mixes. Other mixes, such as sand asphalt and coarse-graded mixes, are prepared and placed in a similar manner; however, each has one common ingredient, which is asphalt cement.

Asphalt mixes, containing emulsified or cutback asphalt, may also be prepared in asphalt mixing plants. The aggregate may be partially dried and heated or mixed as it is withdrawn from the stockpile. These mixes are usually refered to as cold mixes, even though heated aggregate may have been used in the mixing process.

Both asphalt mixtures, made with emulsified asphalt and some cutback asphalts, can be spread and compacted on the roadway while quite cool. Such mixtures are called cold-laid asphalt plant mixes They are hauled and placed in normal warm weather temperatures. These mixtures, after being placed on the roadway, are sometimes processed or worked back and forth laterally with a grader before being spread and compacted. This action speeds up setting or curing.