7. Mix Placement

7.7.1 Hand-Spreading Operations

Small repairs and patching, small drainage swales, variable-width areas such as narrow tapers, radii, fillets, etc., are examples of areas that require hand-spreading and finishing.

Attention to the process is required when placing and spreading by hand. The mixture should be delivered uniformly in small piles to avoid mix segregation, placing the supply piles in a manner where shovelers and rakers must move the bulk of material the least distance. Sufficient space should be available so that workers are not required to stand in the fresh mix. Broadcasting or throwing mixture from shovels should never be allowed because segregation will result. The small piles should be spread using shovels and rakes/lutes, discarding any clumps of mixture that have formed into lumps and do not break down easily when struck with a shovel edge. After uniformly placing the mix, and before rolling starts, the surface should be checked with a straightedge and all irregularities corrected. On cool days, careful attention should be paid to the mix temperature. Hand placement of mix accelerates the cooling of the material, and compaction needs to begin as soon as possible to ensure adequate time for compaction.

It is important to understand that hand-spread mix has not been pre-consolidated by a vibrating screed or tamping bar, and the rolldown factor of the mixture may be significantly greater than mix placed by a paver. Thirty to 50 percent rolldown is not uncommon with hand-placed material.

7.7.2 Supplemental Operations and Appurtenances

Items considered supplemental paving operations include roadway widening, shoulders, and superelevated curves. In some cases, such as widening and shoulder construction, specially designed pieces of paving equipment and attachments are available to perform the work easily and efficiently. Certain other roadway appurtenances, such as asphalt curbs, dikes, ditches, spillways, and slopes, are increasingly becoming part of paving contracts. For these jobs, special equipment or special asphalt mixtures (or both) may be required.

7.7.3 Roadway Widening and Shoulders

Typical two-lane pavements are 24–28 ft (7.3–7.9 m) wide. Many thousands of miles of primary routes are inadequate in width and thickness to meet today’s standards. For details on geometric standards, refer to “A Policy on Geometric Design of Highways and Streets, 7th Edition, 2018,” commonly referred to as the “Green Book,” from AASHTO for more information.

For safety reasons, evaluating and planning can greatly improve the original alignment of substandard roadways that need widening before strengthening them with overlays. Widening may range from a few feet (greater than 1 m) on one side of a pavement to the addition of full-width traffic lanes on both sides. For safety reasons, proper traffic control must be maintained throughout construction. Work should never take place on both sides of the pavement at the same time and should avoid excessive lengths of open trench. Widening requires trenching to adequate width and depth, typically by templates or attachments to a motor grader so that the walls and base are neat and true to line and grade. For wider excavations, a continuous excavator or a milling machine that loads trucks while cutting the trench to appropriate line and grade can expedite the process. Contractors should roll the subgrade with adequate compaction equipment until it is firm and free of loose material and compaction exceeds the minimum requirements. Typical compaction references are a Modified Proctor Test or a California Bearing Ratio Test to confirm compliance with the required density. Workers should trim and clean the existing pavement edges nearly vertically and remove all dirt and foreign material, then spray the edge with an ample amount of tack coat to adhere the new mixture to the existing pavement. They should then place the asphalt pavement base in layers and compact it to the required density. Appropriate compaction equipment will need to be selected—vibrating plates for narrow trenches, special trench rollers available in different widths, or full-size rollers for wider sections.

When adding shoulders to an existing roadway, contractors should observe the same procedures and precautions used for roadway widening. For safety reasons, it is important that enough mix is placed to assure that after rolldown, the new shoulder height matches the existing pavement edge to avoid drop-offs or ponding water.

When paved shoulders are part of the initial construction, contractors should use the procedures for mainline paving. Variable-width screeds should be considered to pave shoulders at the same time as the mainline and eliminate the longitudinal edge joint.

7.7.4 Superelevation

On new construction, paving curve sections with superelevated cross slopes is generally not a problem. The prepared base and/or foundation material is usually superelevated or sloped the same as the new pavement. This allows placing the same uniform thickness on the curved or superelevated sections as on the tangent or straight sections. As the paver proceeds through the transition to the superelevated section and out again, the paver and the screed tilt accordingly on the prepared foundation, and little, if any, adjustment in the screed controls is required.

When building a superelevated pavement on a flat or crowned base or subgrade, use variable thickness layers to build the pavement. Pavers equipped with automatic slope controls allow the transverse cross slope to be “dialed in” as the paver moves through the superelevated curve. Sensors riding on an erected or traveling stringline maintain grade control for the screed nearest the sensor. The depth at the other side depends on the transverse cross-slope setting.

When the paver reaches the point where the slope begins to change and transitions into a superelevated section, stakes or other markers on adjacent lanes indicate the required cross slopes for the screed operator to dial in/out continuously as the paver proceeds through the transition. The change in cross slope is gradual, and small errors in dialing will not affect the riding quality of the pavement. Once the paver enters full superelevated cross slope, the slope setting will remain the same throughout. Then, as the paver exits the fully superelevated section, the cross slope is gradually dialed out using the same procedure, but in reverse.