2010 NRMCA - National Ready Mixed Concrete Association

featureInch by Inch(Reprise)Introduction by Amy Miller, National Resource Director, NRMCAArticle by Randell Riley, P.E., Executive Director/Engineer for Illinois Chapter of ACPA and consultant toIllinois Ready Mixed Concrete AssociationIntroductionIn the “old days”(prior to 2008) we wereforced to sell concrete parkinglots based primarily on longtermcosts savings, or so we thought.We would perform life cycle costs analysesshowing those savings to engineers and ownersand anyone else that would listen. We wouldtout the longevity of the product citing examplesof concrete structures from the days of JesusChrist. Short of off ering our fi rst-born, wewould resort to any means of selling concretepavement advantages only to have the fi rst-costissue thrown back in our face. We thought wewere just fi ghting price but perhaps we werereally fi ghting our own lack of education.Various economic factors have drivenasphalt prices higher so now we believe wecan fi nally face fi rst-cost competition head-on.According to Randell Riley, P.E., executivedirector/engineer for the Illinois Chapter ofACPA and consultant to Illinois Ready MixedConcrete Association, we’ve always been ableto do this, yes, even in the “old days.” Belowis a reprint of Randy’s article entitled “Inchby Inch” explaining why we always had theoption to be cost competitive on concrete parkinglots. Randy cites examples from the IllinoisDepartment of Transportation, but franklythe example is apropos for most any state asRandy uses AASHTO 1993 Guide for Designof Pavement Structures as the basis of hisdiscussion.At NRMCA we endorse use of ACI 330The Guide for Design and Construction ofConcrete Parking Lots as the fi rst option forconcrete parking lots. The Guide and associatedspecifi cation off er an all-inclusive approach todesign and construction. Often engineers citeuse of AASHTO 93 as their design choicebecause they feel there is some comfort in usingthe same source as their DOT. We feel there aremany smarter reasons to use ACI 330 instead,but I will leave that for another discussion ( orWebinar, see below). When dealing with engineersand designers that refuse to move awayfrom AASHTO 1993 in designing their concreteparking lots, Randy’s article might do justthe trick in getting a reasonable specifi cation.Inch by InchI’d like to take a couple of minutes ofyour time to show you how concrete hasalways been fi rst-cost competitive if youwere designing the sections to actually carryroughly the same traffic and getting thesame life.Let’s take a typical Illinois parking lotpavement section. How many of you haveroutinely seen a section of 3-inches of bituminoussurface on 6-inches of granularmaterial? How many of you have seen evenless? How many inches of concrete would ittake compared to what the engineers andarchitects frequently pull out of their mysteriousdesign manual? And why? Let’s startwith the why.Most engineers and architects start oneof two places in Illinois: either Chapter 54 ofthe Illinois Department of Transportation’s(IDOT) Bureau of Design and EnvironmentManual (BDE Manual) for highways orChapter 37 of IDOT’s Bureau of LocalRoads Manual. (BLR Manual) Are thesereally appropriate for parking lots? Probablynot! IDOT designs for controlling vehiclesthat are principally trucks – and usually a lotmore than we are inclined to see on a typicalparking lot.For example, if you look at either of thesereferences they start at the bottom traffic levelswith about 12 percent truck traffic. Thereare provisions if you know what you aredoing to go below this level, but even thenthe minimum thickness for concrete fromthe design charts in the BDE Manual is currentlyabout 7.5 inches for soil conditionstypical of parking lot construction and 6.5inches for the BLR Manual. Both proceduresare mechanistically based and incorporateextremely high levels of reliability. Both also16 ı JANUARY/FEBRUARY 2011