How many hours worth of paving is in the trucks on the road

Reference no: EM131579353

Summer Highway Materials, Construction Assignment

Compaction Problem

This is a mainline paving project on a remote new highway section not yet open to traffic. It is clear and sunny: 70°F air temperature and 60°F base temperature. The project team has been placing a 2-inch lift of binder course, 14 ft wide, since 8:00 a.m. Today is the first day placing binder. The paving plan calls for 10 hours of paving today - a total of 2,500 tons.

The HMA facility, mix delivery (20 ton capacity, 2 hour cycle time), and paver production rates have been balanced for 250 tons per hour. Both the paver and rollers normally operate 80% of the time. The 500-foot test strip (completed yesterday) achieved the target density with three coverages of the vibratory roller at 2,900 vpm, low amplitude, at an effective speed of 261 feet per minute.

This is a QC/QA project with a penalty on density. The density specification is 92 to 97 percent of theoretical. An average target density of 93.5% is needed to maintain a 100 percent pay factor with this particular mix. The theoretical density for this mix is 153.7 pcf. Mix properties and mix temperature behind the screed (275°F) have been very uniform all day.

The roller train consists of a 66 inch wide vibratory roller, a 7-tire pneumatic, and a 12-ton static steel wheel roller for breakdown, intermediate, and finish rolling, respectively. Note: The pneumatic is required by the specifications for surface texture and is operating at a low ground contact pressure. No other rollers are available today.

The vibratory roller was just checked and was currently running in low amplitude, consistently hitting the rolling pattern (with a rolling zone of about 300 ft), at an actual speed of 295 fpm. Your handy reed tachometer measured the roller drums operating at 2,550 vpm, and you estimate it is working about 50 minutes per hour (re-watering has not been a problem). The actual paver speed was checked and was running about 32 fpm, and 48 minutes per hour on average. For once, the haul trucks are delivering right at 250 tons per hour. You can add 2 more trucks if you need them.

It is 12:00 noon and the weather service has just issued a revised weather forecast calling for mostly cloudy skies, air temperature dropping to 50°F over the next 90 minutes, and northwest winds increasing from 10 to 25 knots; no rain is predicted. At last report, approximately 11 loads of mix have been shipped and are on the road to the project from the plant 40 miles away.

You have a QC technician on site with a nuclear gauge. The nuclear tests (properly correlated to cores and taken every 500 feet) show that in-place readings have been steadily decreasing all morning (93.8% to 92.2%). The paving team members have voiced some concern about the declining density, and about the pending weather change. However, there is pressure on both sides to get this project completed and the road opened for the holiday weekend in 2 weeks (i.e. keep paving as long as you can).

You, as paving team leader, must decide what action, if any to take. Consider the density situation, the weather, and the pressure to get this project completed. Blank production rate sheets are enclosed if needed.

The following questions should be considered as you work your way to a conclusion.

Some of these may not apply and they are listed in no distinct order.

1. What should you notice first in this situation?

2. What is the first step you should take?

3. Was the process in balance starting this morning? Yes or No

Check F-RATE to H-RATE?

Check P-RATE to C-RATE?

4. Is the process in balance now? Yes or No

Check F-RATE to H-RATE?

Check P-RATE to C-RATE?

5. What could be the causes of the density problem?

6. How many hours worth of paving is in the trucks on the road?

7. Should you add additional trucks? Yes or No

8. What is your time available for compaction?

9. What will it be in two hours? _________ Why?

10. How many total passes of the breakdown roller are required for density?

11. What effect will increasing the speed of the roller have?

12. How fast can the roller operate and under what conditions?

13. How long is the rolling zone? ______ How long should it be?

Attachment:- Compaction Problem.rar

Reference no: EM131579353

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