Friday, November 20, 2015

Fifty year concrete

Disintegrating concrete.  This is a molded product and the source chose to use round aggregate.

This is a follow-up to the post on Durable Road Surfaces.

I had a comment come in via email.  Rather than post the entire comment, I will quote a portion of it.

I do like your Auto Supplier analogy and I’d be very interested to see how/if that is something we could implement for MDOT.  My first worry is contractors would cry about government overreach and too strenuous of regulation.  My other concern is MDOT currently tells the contractors what to do in pretty specific detail.
The devil is in the details.  I am not a "concrete guy" so I may give examples that will make real concrete guys snicker.  I offer these examples to illustration of how auditing might work.

Suppose that the most common failure modes for concrete road surfaces are
  • Disintegration of the slab at the expansion strips.
  • Spalling that occurs due to voids that are near the surface.
  • Heaving/shattering due to ice formation below the concrete slab.

Further, suppose engineers determined that disintegration of slab near the expansion strip is highly dependent on two factors, the precise geometry of the edge (corner-vs-chamfer-vs-radii) and how soon it is worked after the pour.
A close-up of the molded product shown in the header.  You can see that the rocks are no longer bonded to the matrix.  You can also see a "skin" of concrete delaminating from the core in the upper-right corner of the photo.

This is a section of curb near a handicapper access that was shaved by a snow plow.  You can see that the primary aggregate is crushed limestone.  You can also see that it is more angular than the aggregate shown above and that it is not segregated...i.e., it extends all the way to the surface.  All photos taken at Grand Valley State University.

Also suppose that voids are due to an interaction between the aggregate shape (round-vs-angular/crushed), time of settling and vigor of tamping.

Finally, suppose that heaving/shattering is dependent on many things, primarily the depth of the gravel bed, the uniformity of the gravel size, the slope of the surface the gravel is laid on and the degree of tamping of that surface.

Implementing inspection


Every story is a combination of who-what-how-where-when-why.

  • Who checks the tools used to profile the corners of the slab at the expansion strip?
  • What do they check...the tool....the profile of the concrete?
  • How do they do it?  Is the gage mastered against NIST traceable standards?  It is one thing to eyeball a radius but eyeballing is subject to drift.
  • Where is the inspection done?  Back at the shop or checking the actual tools/radii out in the field?
  • When is it done, at the start of a shift, once a week, at the letting of the contract?  Bear in mind that tools wear out and non-inspected tools are likely to drift onto the job site.  Also, concrete is abrasive and wears tools.

Why?

There are two costs to doing road work.  One cost is easy to audit.  It is the cost of letting the contract.  That cost is called "Internal Cost".

The other cost is called "Externalized Costs".  Those are costs that are incurred but are not directly borne by the Department of Transportation or by the contractor.

Pollution is a classic case of an externalized cost.  The exact magnitude of that cost is almost impossible to calculate because it is spread out over thousands, perhaps millions of people.

While externalized costs are impossible to calculate with precision, it is usually possible to estimate some of the larger pieces.

In the case of road construction, one portion of the cost is due to the congestion caused by closing down lanes.

This summer, it was not uncommon to sit in a traffic stoppage on I-94.  The one time I was paying attention to mile markers, it was 11 miles long and it took an hour to get through.  The congestion was two lanes wide and there was a vehicle every thirty feet.  According to Mr Excel, that means there were approximately 3800 vehicles impacted.  Assuming single occupancy (an underestimate) that means that 3200 incremental manhours were consumed in the congestion.

Placing a value on manhours is tricky.  I will go with $25/hour because that is the cost to the firm for a person being paid $10/hour due to the cost of benefits and mandatory reporting.  If Acme Manufacturing values a production employee's time as being worth $25/hour  then I will do the same.

Multiplying out the numbers, the value of the commuter's time that was gobbled up in daily congestion amounts to $80,000/hour or almost a half million dollars a day, figuring 6 hours of peak traffic per day.

Other costs include depreciation of vehicles due to overheating (idling is rough on cooling systems, belts and cam shafts).  There are more fender-benders per mile...that is another cost.  Fuel consumed and additional greenhouse gasses produced are more externalized costs.

On a six lane highway, "twenty year" concrete means that the residents get punched in the nose once every three years (twenty years/six lanes).  "Fifty year" concrete means that the residents will get punched in the nose once every eight years.

We get what we inspect


We communicate our expectations by what we inspect.  It is very easy to specify the heck out of processes.  But specification mean nothing unless somebody checks to ensure that those specifications are being met.

Repeat picture.  By all accounts this looks like "50 year concrete".  It would be interesting to look at the contract to see what specifications were included.

Attributes controlled by interactions are the most insidious.  Minimal tamping might work fine for rounded aggregate, the round gravel falls into place like greased ball bearings.  The folks making the pour become convinced that the spec for tamping is bullshit.  Then a lot of concrete comes in with crushed aggregate and the crew pours miles of porous, voided concrete and does not even realize it.

I don't propose adding more specifications.  I advocate focusing on the specifications that strongly influence the outcome.  I propose auditing conformance to those specifications.  I advocate "strong" adherence to the reaction plans that are probably already written into the contracts.

I don't think building good roads is magic.  The Romans knew how to do it.  I think building good roads is a matter of minding the Ps-and-Qs and not taking shortcuts.

3 comments:

  1. There is also the issue of concrete strength in PSI... 3000-5000 are all different mixes.

    ReplyDelete
  2. There is also the issue of concrete strength in PSI... 3000-5000 are all different mixes.

    ReplyDelete
  3. perhaps a long view private company could buy I 94 across Michigan, rebuild it as a 100 year road, and charge travelers to use their road.

    ReplyDelete