Proper use of a 16' straightedge for pavement smoothness 1

[Hoaokapohaku]

Does anybody know an authoritative source indicating how to properly use a 16' straight edge? The use is for measurement of asphalt pavement smoothness at an airport under the FAA P401 specification. I'm finding it surprising hard to demonstrate that the proper use is to measure under the straight edge when placed on the pavement supported along its length rather holding one end down and using it to extend over a grade break and then measure.

 

[Ron]

The proper way to use a straightedge is to support it on each end with precision blocks that are the thickness of the tolerance you want to check. For instance, if your tolerance is 1/4" in 16 feet, then you need two blocks that are 1/4" thick to rest the ends of the straightedge on. Then you use a check block that is twice the thickness of the tolerance, or in this case, 1/2".

When you have the straightedge resting on the 1/4" blocks, there should be no high spot that touches the underside of the straightedge. Correspondingly, there should be no low spot that would allow the insertion of the 1/2" check block. You can actually purchase shims of various thicknesses that are configured like a "feeler gage" to do this, but you should still support on each end with blocks the thickness of the tolerance.

Allowing the straightedge to teeter on the high spots or cantilever over sections is a poor way to check.

 

[Hoaokapohaku]

Thanks Ron! I haven't seen it done quite that way but that is definately better than any other way I have seen. Do you know any place this is documented?

 

[Ron]

Hoa...don't know of any. I started doing that about 30 years ago.

The Florida DOT has a similar procedure for calibrating a rolling straightedge against a manual straightedge, but it doesn't give this as a procedure for measuring.

I like it because it is logical and intuitive.

 

[chicopee]

Have you tried to read the manufacturer's instruction pamphlet?

 

[Ron]

chicopee...good idea if they would provide good instructions; however, most instructions I've read just say to set the straightedge on two high spots on the pavement and measure the dips. That's insufficient in my opinion.

Using the shim method or shim/feeler gages you can appropriately quantify both the high and low spots relatively.

 

[eea]

In my DOT days, we used to stretch a stringline between the bottom of the two rolling wheels on the 16' rolling straight edge and adjust the "set pegs / bolts" for the required spec over that 16' length (should be three set points). Any scrapes from the pegs in the warm mat indicate a bump or dip out of spec tolerance.

You'll want to try to "bump the mat" alongside the finish roller in an attempt to work out any bumps if possible. Elsewise we were just dinging them after the fact with a set deduction, say 1 ton per bump. I have seen some recent projects where the Contractor bump ground after the fact but that really doesn't look good on a fresh overlay.

The 16' rolling straight edge is also handy to check against the 30' rolling profilograph when you have to go back and grind bumps out on concrete pavement after you reduce those results. Much easier to lug around than that A-framed monster.