Checking can be defined as short transverse cracks, usually 1 to 4 inches in length and 1 to 3 inches apart, which occur in the asphalt concrete mat. These surface cracks, or checks, are not visible when the paver places the material. The cracks may occur after the second or third pass of the compaction equipment over the mix or after several passes of the roller when the mix is approaching the design density level, particularly when a static steel wheel roller is employed for the breakdown rolling. Rarely does the checking appear after only one pass of the breakdown roller. Checking typically does not occur when a pneumatic tire roller is used for breakdown rolling. If a pneumatic tire roller is followed by any type of static steel wheel roller when the mat temperature is too high, however, that steel wheel roller may introduce the check marks into the mix. Checking is not normally caused by “over-rolling” of the mix by the compaction equipment. The checks typically do not extend completely through the course but normally are only ½ to 3⁄8 inches in depth.
Checking can be caused by two primary factors:
- excessive deflection of the pavement structure under the compaction equipment
- a deficiency in the asphalt concrete mix design. In the former case, the pavement on which the new asphalt concrete layer is being placed is weak. The weight of the rollers causes the pavement layers to bend excessively, placing the new mix in tension at the surface. The check marks are then formed when the surface of the new mixture is pulled apart as the pavement deflects during the rolling operation.
A more prevalent cause of checking is a deficiency in the asphalt concrete mixture:
- an excess of fluids in the mix: too much asphalt cement or too much moisture in the mix, or both
- a non-uniform sand gradation: too much middle-size sand [No. 16 and No. 30 (1.18 mm and 600 m) sieve size material] and too little fine size sand [No. 50 and No. 100 (300 m and 150 m) sieve size material]
- a lack of room in the aggregate gradation for the asphalt cement (low voids in mineral aggregate-VMA). The excess of fluids makes the mix tender and allows it to be displaced easily by the compaction equipment. The hump in the fine aggregate gradation curve also causes the mix to be tender. Mixes that are low in voids in mineral aggregate content will generally be tender and move easily under the force of a vibratory or static steel wheel roller. Further, the various characteristics of the aggregate such as surface texture, crushed content, and amount of dust coating can play a role in the amount of checking that occurs.
- The mix tends to be shoved by the roller instead of being tucked under the drums or tires of the compaction equipment. The potential for checking is characterized by a bow wave that occurs in front of the drums on a steel wheel roller. The mix deficiency is compounded, and the amount of checking that occurs is increased, when the mix temperature is too high. As the mix temperature increases, the viscosity of the asphalt cement decreases, causing the mixture to be more tender.
Depending on the cause of the mix tenderness, some mixes can be adequately compacted at lower than normal mix temperatures. In some cases, because of the properties of the aggregate and the asphalt cement in the mix, the effective compaction temperature range may shift downward by as much as 50°F and adequate density can be obtained even at relatively low compaction temperatures. This means that the rolling zone (the distance between the paver and the breakdown roller) is increased and the mix is permitted to cool more before the initial compactive effort is applied. The feasibility of this solution to the checking problem is determined by two factors: whether checking is eliminated or significantly reduced during the delayed rolling procedure and whether an adequate level of density is obtained. A roller combination test strip should be constructed to determine the proper rolling zone to be used. The density level obtained should be checked with both a nuclear density gauge and with cores cut from the roadway. If the density level achieved is proper and if the amount of checking obtained is significantly reduced, the delayed rolling procedure can be used to overcome the problem.
Another solution is to change the roller type and pattern being used to compact the mix. If a static steel wheel roller is being used in the breakdown position, it should be removed and either a pneumatic tire roller or a vibratory roller used as the initial compaction machine. On some very tender mixes, even when a pneumatic tire roller or a vibratory roller is used in the breakdown position, checking may still occur if a steel wheel roller is used as the intermediate roller. Some benefit, in terms of “healing” the cracks, may also be obtained by using the pneumatic tire roller in the intermediate or in the finish rolling position. In general, however, it is often more effective and efficient to use the pneumatic tire roller in the breakdown position because checking rarely occurs under this piece of compaction equipment. The level of density obtained with the change in roller type and pattern must be checked through the use of a roller compaction test strip together with nuclear density gauge readings and core density determinations.
If the mix delivered to the paver is too hot, the mix should be allowed to cool to normal laydown temperature before the compaction process is started. Improper rolling techniques should be corrected. The surface of the underlying pavement surface should be clean and properly tack coated before placement of the new mix commences.