Low Longitudinal Joint Density on a Butt Joint
A 2 in (50 mm) hot-mix asphalt overlay is being placed in 12 ft lanes against a lane paved two days earlier. The joint is a butt joint with a 1.5 in overlap. Tack coat was applied at 0.05 gal/sy. The contractor uses a 10-ton steel drum roller and four passes. The roller operator keeps the drum 8 in away from the unconfined edge to avoid tipping. The joint mix temperature at the start of rolling is 170°F (77°C). The mat density from six random cores averages 94.2% of Gmm and meets the 92.0% minimum. Five joint cores centered on the joint average 89.4% of Gmm; the specification requires 91.0% minimum at the joint. The joint cores are 2.0 in tall and show a visible vertical seam. The contractor wants to accept the lot because the mat density passes and argues the joint cores are not representative. The mix is a 19.0 mm base course with 4.8% AC.
1. Evaluate the likely cause of the low joint density and state whether the joint cores should be considered representative.
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The joint cores are representative if they were centered on the joint, cut full depth, trimmed, and tested consistently with the mat cores. A centered core includes equal portions of the old and new lanes, so it can be biased by the old lane density, but the old lane was paved two days earlier and should be well compacted. A joint average of 89.4% is therefore a real signal that the new lane side of the joint is undercompacted. The two main causes are visible in the construction details. First, the roller stayed 8 in from the unconfined edge, leaving the joint zone without effective confinement and compaction. Second, the joint temperature at the start of rolling was only 170°F, which is below the effective compaction range for most 19.0 mm mixes. The 1.5 in overlap and tack coat help bond, but they do not replace compaction. The mat density passing does not rescue the joint because the specification has a separate joint criterion.
2. What repair method is appropriate for the failed joint, and what would you require before accepting the repair?
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Saw-cut a vertical strip centered on the joint wide enough to remove the low-density material, typically at least 6 in wide or the width needed to expose sound, dense mix on both sides. Remove the strip to full depth, clean the vertical faces, apply tack coat, place fresh hot mix, and compact immediately with a joint compactor or edge roller while the mix is in the proper temperature range. Do not simply overlay or seal the low-density joint. Before accepting the repair, take cores centered on the repaired joint, test them by AASHTO T 166, and verify that the joint density meets 91.0% of Gmm. Also inspect the repaired joint for a tight vertical seam, proper surface texture, and no signs of segregation or flushing. Document the repair location and core results.
3. What changes should be made to the rolling and joint construction sequence on the next lane?
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Roll the joint first, while the mix is hottest, using a joint compactor or a small drum roller that can work close to the unconfined edge. Roll from the hot side with about 6 in of drum overlapping the cold lane so the hot mix is pushed into the joint, not away from it. Keep the roller within about 3 in of the edge, or use an edge roller, so the unconfined edge receives compaction. Start rolling at the specified initial compaction temperature and finish before the mix cools below the effective range; 170°F is too cool to rely on for joint density. Maintain the tack coat and overlap, but do not let the overlap cool before rolling. Check joint density early with a gauge or cores so the rolling pattern can be adjusted before the lane is completed.