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  • 1.  Concrete Pavement - Steel Reinforcement

    Posted 10-06-2020 08:56 AM

    We're currently working on a development site in my local area where concrete pavement seems to be the best course of action to provide a cost effective and sustainable long term driving surface for large vehicles (City Buses).  

    Our geotechnical engineer has provided us a report including design recommendations given the existing subsurface characteristics of the site.  The design recommendations call for 6" thick 4000 PSI concrete underlain by a sub base layer consisting of 8" of VDOT 21A aggregate compacted to 95% std. proctor maximum dry density; the concrete will be properly air entrained to account for freeze thaw and deicing chemicals.  The maximum contraction joint spacing is indicated to be 15 feet with contraction joints minimum 1.5" in depth.  The contraction joint pattern should be relatively square with the length of any given panel not exceeding 25% of its width.  Further, the report states:  "Typically, if the maximum joint spacing does not exceed 30x the slab thickness up to 15', then the concrete pavement may remain unreinforced." The report also indicates that steel reinforcement is not required for their design, but that steel may be incorporated if concerns remain regarding shrinkage cracking. 

    I'm writing in hopes of gaining further knowledge about the potential mitigation of shrinking cracking between contraction joints.  In the past we've included rebar mats within the section and in the top third of the concrete thickness and more recently we've designed and oversaw a concrete pavement with steel fibers.  The steel fibers were very interesting to us.  Our firm did not seal the design mix with the fibers; the supplier's professional engineer signed and sealed the mix to be equivalent to our design with rebar mat.  

    Any knowledge or resources you can point me to regarding the use of steel in concrete pavement would be greatly appreciated.  Thanks in advance for anything you can offer.

    Trae Livick P.E., M.ASCE
    Civil Engineer
    Roanoke VA

  • 2.  RE: Concrete Pavement - Steel Reinforcement

    Posted 10-07-2020 08:38 AM
    Three part answer:

    1.- Mitigation of shrinking cracking.

    This is a valid application of the rebar. However, 6" is a thin slab, (almost minimum thickness) so welded wire fabric may be more effective since it better distributes the reinforcement. The reinforcement alternative carries some concerns, in that the deicing chemicals tend to affect it, so the reinforcement usually needs to be epoxy coated, but preferably galvanized, and this tends to increase the cost.
    The alternatives to be evaluated are steel fibers, polypropylene fibers and water reducing admixtures. The performance of each varies from mix to mix because the type and sieve distribution of aggregates affect the result, but in general, water reducing admixtures tend to be favored because, since shrinkage cracking is the result of the loss of water when the mix sets, the shrinkage is avoided if there is no excess water to begin with. Additionally, the cost tends to be lower, since the admixture is provided by the mix supplier, so there is no rebar subcontractor. For this alternative, you would need a contractor experienced in the placing of near zero slump concrete.

    2.- Use of reinforcement to meet design loads

    Pavement design is particular in the sense that many assumptions are used in the calculations and not necessarily reflected in the design drawings and specs. These assumptions include a certain evenly distributed subgrade bearing capacity, a certain effectiveness of the drainage, a certain compliance with maximum loads and tire pressures, etc., etc. Because real cases are rarely perfectly described by the set of assumptions, many agencies tend to favor certain design elements which have worked well for each local condition, e.g.: the Ontario Ministry of Transportation prescribes the use of subdrains to ensure adequate subgrade support, regional agencies in Spain favor soil improvement in the subgrade and the use of stabilized bases, Manitoba Infrastructure favors open graded bases, etc.

    One design feature which is found in most agencies is the use of reinforcement at the joints. This is because concrete is a long lasting material, so concrete pavement is more cost effective in projects with long design periods (25, 30 yrs). If the joints are left unreinforced, the aggregate interlock in the joint is the only thing distributing the load between slabs, so with time, the joint deteriorates, water seeps in, the slab is free to curl at the edges, and premature failure tends to occur. In my 25 yr experience, I have rarely seen concrete pavements without at least joint reinforcement, and in the cases of truck routes, continuous reinforcement as well. At the same time, I am aware that VDOT is unlike other agencies, since it prescribes unreinforced concrete (plain joints) for secondary roads and for use in subdivisions. This is probably due to the short design life considered (10 yrs). 

    In light of the above, my recommendation would be, just to be sure, to double check the design using a life cycle cost analysis, using VDOT's Pavement design guidelines (Materials division, MOI VI) and the AASHTO design procedure, and compare the current design to a joint reinforced design. Depending on the percentage of heavy vehicles and the subgrade conditions, joint reinforcing might be more appropriate.

    c.- Resources 
    American concrete pavement association (ACPA) https://www.acpa.org/
    Concrete reinforcing steel institute (CRSI) https://www.crsi.org/index.cfm/basics/pavement
    Continuously Reinforced Concrete Pavement (CRCP) http://crcpavement.org/
    National Concrete pavement Technology Center https://cptechcenter.org/


    Sergio Fernandez, P.Eng.
    Calgary, Canada

    Sergio Fernandez M.ASCE
    Senior Transportation Engineer
    Calgary AB

  • 3.  RE: Concrete Pavement - Steel Reinforcement

    Posted 10-08-2020 10:11 AM
    Hello Mr. Livick,

    The deformed bars or welded wire mesh reinforcement in concrete pavements is used to minimize crack widths in the concrete panels in a jointed-reinforced concrete pavement.  Smooth dowel bars must be used at the transverse joints to provide load transfer from one panel to the other.  In a continuously-reinforced concrete pavement, the deformed bars help control the crack width and crack spacing.  JRCP is not used much on highways anymore, but cities like Houston use it for street pavements and you can probably find standard sheets with details for the reinforcement, dowels, etc. on their websites. You can also find standards for pavement sections used at bus stops online-I have seen them for Los Angeles, San Antonio, San Francisco, Columbus, and other cities.  Slab thicknesses used are typically around 10 inches.

    You should consider having an alternate typical section using a heavy-duty asphalt pavement design if the area to be paved is more than around 7,000 sy.  Virginia DOT has specifications for materials that would work very effectively for pavements subjected to slow moving bus traffic.  VDOT SM12.5E surface (2 inches) and IM19.0E base mixtures (thickness as needed to satisfy pavement design requirements), using PG76E-28(HP) binder would provide materials that are resistant to cracking and rutting.  Depending on the soil conditions and project constraints, the subgrade can be stabilized or a crushed stone subbase can be used as per the geotech's recommendation.  

    Trenton Clark at the Virginia Asphalt Association (https://vaasphalt.org/) is an excellent source of information about asphalt applications in Virginia.  He has a great deal of experience with pavement design and materials in Virginia, and chaired the TRB Committee on Flexible Pavement Design (formerly AFD60).  The Virginia Transportation Research Council has also worked with "highly modified" asphalt binders, such as the PG76E-28(HP).

    Best wishes!

    Gary L. Fitts, P.E.  M.ASCE
    Kraton Polymers
    San Antonio, TX

    Gary Fitts P.E., M.ASCE
    Manager, Market Development
    Garden Ridge TX