Night Paving: Joint Density, Segregation, and a Falling Thermometer
You are inspecting a night-time mill-and-overlay of a 4-lane urban arterial. The 2-inch surface course is a 1/2-inch NMAS mixture with PG 64-22 binder. The special provision requires the mat to be rolled before it cools to 185 degrees F, a lot density of at least 92.0 percent of maximum theoretical (Rice) density, and longitudinal joint density of at least 90.0 percent. At 10:30 p.m. the air temperature is 58 degrees F and falling, wind is 12 mph, and the milled surface is 56 degrees F. The mat behind the screed reads 268 degrees F. Cores centered on the paver's centerline joint read 88.1 percent. A 25-foot segregated streak appears along the outside edge after the paver stops waiting for trucks. The contractor wants all lanes open at 5 a.m.
1. The joint cores read 88.1 percent against a 90.0 percent requirement. What do you verify, and what is your decision?
Read the worked answer
Verify before deciding:
- Core location is centered on the joint and the core diameter is adequate (typically 4 inches minimum; a small core that catches more of one lane biases the result).
- The Rice maximum theoretical density is from the correct mix design, lot, and date.
- The lab calculation and the test assignment (which lot and station the cores represent).
- Whether the joint is a hot joint or a cold joint - a long paver stop creates a cold joint, which is the usual root cause.
Decision path: Read the specification. If joint density is a rejectable quality characteristic with no pay factor, the joint is deficient and must be corrected: typically mill out a strip on each side (often 12 inches minimum, per the spec) full depth of the surface course and replace it, forming a new joint. If the specification provides a pay adjustment for joint density, you document and apply the adjustment per the schedule - but the inspector does not negotiate the pay factor in the field.
Either way: notify the engineer in writing the same night, and document stations, core numbers, densities, and photographs before any replacement or opening.
2. What do you do about the 25-foot segregated streak, and can the contractor open all lanes at 5 a.m.?
Read the worked answer
The streak is coarse-aggregate segregation - typically from paver stop/start, running the auger only partly covered, or dumping the truck bed into the hopper. A segregated mat will not make density, will ravel under traffic, and will admit water.
Actions: measure and locate it (station, offset, width, area); record it with photos; do not accept it as-is. Either remove and replace full depth, or test density in the streak and let the engineer decide - but a 25-foot ravel-prone strip in a wheel path normally comes out.
Root cause fix: keep the auger at least about 80 percent covered with mix, avoid stopping the paver, do not let trucks bump the paver, and use a material transfer vehicle if one is available.
Opening at 5 a.m.: permitted only if density acceptance is satisfied (or the specification allows opening before results), the mat has cooled enough not to shove under traffic, and the failed joint has been addressed per the engineer's direction. Opening a lane over a deficient joint or a segregated strip just converts a construction defect into a maintenance claim. If results are not in, keep that lane closed or get written direction.
3. Air is 58 degrees F and falling with a 12 mph wind and a 56-degree milled surface. How does that affect the rolling window, and what do you require?
Read the worked answer
The paver temperature of 268 degrees F is fine; the problem is how fast it drops. A thin 2-inch mat on a cold 56-degree base with wind loses heat quickly, and the time between the screed and the end of intermediate rolling shrinks to minutes.
Check the specification's weather limits first. Many agencies prohibit placing the surface course when the surface temperature is below 50 degrees F and falling (some allow 40 degrees F). If the limit is being approached, surface-course paving should stop rather than produce a mat that cannot be compacted.
Require:
- Enough rollers and a disciplined breakdown/intermediate/finish sequence immediately behind the screed, not staged at the end of the pull.
- Shorter hauls and no paver stops; trucks staged so the paver never waits.
- Temperature checks at the plant, at delivery, behind the screed, and at the end of breakdown rolling, with times recorded, so any area that fell below the 185-degree cessation temperature before rolling finished is identifiable.
Document the ambient, base, and mat temperatures and the roller pass times; if the mat cooled below the cessation temperature before rolling was complete, expect density failures and test those areas specifically.