Concrete PavementsJohn HarveyUniversity of California, DavisOverview• Concrete Pavement Types• How Concrete Pavements Fail• Concrete Pavement Design• Concrete Materials for Pavements• Construction, Traffic, Delay, MoneyWhat is the Objective of Pavement Engineering and Management?• Provide adequate serviceability at minimum cost• Provide best serviceability possible with funds available• Maximum mobility at minimum costRigid Pavements - Jointed Plain Concrete PavementHydraulic Concrete SlabsBase/Subbase LayersSubgradePortland Cement ConcreteFast Setting HydraulicCement ConcreteLean Concrete Base Treated Permeable BasesAggregate BasesAsphalt Concrete BaseCement Treated BasesCompactionFabricsMineral AdmixturesChemical AdmixturesSlab dimensions designedto not crackOther Rigid Pavement Types• Jointed Reinforced Concrete Pavement (JRCP)– reinforcing steel in slabs– steel holds cracks tightly together– longer slabs than for plain concrete• Continuously Reinforced Concrete Pavement (CRCP)– no sawed joints• Prefabricated/Post-Stressed Concrete Pavement• Pre-Stressed Concrete PavementPavement Performance (Life) CurveRide QualityStructuralCapacityTraffic Repetitions(=Years?)UnacceptableField MaintenanceCapital MaintenanceFull-ScaleTesting(months)Laboratory Testing(weeks)Computer Analysis(days)Time& CostReliabilityof answersLong-TermMonitoring (10-30 years)HVS on SR14 near PalmdaleSide View of HVSWhere is Caltrans Pavement Network in its Life Cycle?• When was it built, how long was it designed for?• Mostly deployed• Mostly maintenance and rehabilitation• Some new lanes, realignments• Beginning reconstructionWhat Causes Pavement Distress?•Traffic• Environment• Interaction of traffic/environment, construction quality, materials, designEnvironment = Water, Temperature• Increase in water content– decreases soil stiffness– decreases soil shear strength– decreases resistance to erosion, pumping• Temperature– asphalt concrete stiffness/strength high at low temperatures, low at high temperatures– temperature changes cause expansion/contraction stresses in all asphalted and cemented materialsTraffic VariablesIt’s the trucks• Loads• Tire pressures• Speeds• Dynamics (interaction with roughness)• Which are most important?Big Truck - 1960Big Truck - 1960Big Truck - 2001Super Single TiresTrucks areHeavier,Faster,More NumerousDifferentSuspension,Different TiresAn Approximate Load Equivalence Factor Equation• Standard axle load = 80 kN single axle• Caltrans current LEF equation for ESALs:– ESALs = (Lsingle/80kN) 4.2– ESALs = 2*(0.5*Ltandem/80kN) 4.2– ESALs = 3*(0.33*Ltridem/80kN) 4.2• Current California legal load limits:– single axle: 89 kN– tandem axle: 151 kNRigid Pavement OverviewConcrete slabs, carrynearly all load stressLoad transfer betweenslabs importantBase must provide uniform, continuous supportto slabs, often stabilized with cement or asphaltGranular sub-base to provide support to base and slabs, without pumping, expansion/contractionCompacted subgrade, must not expand or contractto provide uniform support to layers aboveSlab Dimensions• Concrete slabs have engineered length and width• Longer slabs are more prone cracking due to shrinkage, curling and warping• Shorter slabs require more joints, which cost more to build and maintain, and can result in rougher ride• Typical slab width is 3.7 m (12 ft) = one lane• Slab length is a design variable– Caltrans joint spacing has varied over the yearsEnvironment and Loading• Tensile stresses crack concrete slabs• Environment-related mechanisms causing tensile stresses– shrinkage and warping– curling • Load related mechanisms– load mass– load location on slab• Environment and load stresses are additiveShrinkage and WarpingBaseConcrete SlabSelf-weightTensionWarping of slab:Top of slab cures faster, drier, shrinks more than bottomConcrete typically shrinks when curingUniform shrinkage causes some tensile stressesNon-uniform shrinkage causes warping, higher tensile stressesCool and moist belowHot and dry aboveShrinkage Crack (Top-Down)Slab core laid on its sideTop-Down crackBaseConcrete SlabSelf-weightCurling of slab: caused by temperature difference between top and bottom of slabNight - cooler on topBaseTensionTensionDay - hotter on topConcrete SlabSelf-weightCurlingDesert4 mmHigh Desert/MountainSouth CoastBay AreaNorth Coast2500 mmCentralValleyAverage Maximum Air Temperatures, April-September24-29 C29-35 C35-41 C18-24 CAverage Minimum Air Temperatures, October-March-1.5 to -3.5 C3.5 to 8.5 C8.5 to 13.5 C-6.5 to -1.5 CSlab Size and Environmental Region Effects• Longer slabs result in greater– shrinkage stresses– warping stresses– curling stresses• Thicker slabs have larger temperature gradients; bending resistance, weight cancel• Shrinkage, warping, curling worst where large day-night temperature changes–desert– central valleyTop-Down Thermal/Shrinkage Cracking at PalmdaleLoad Transfer• Load Transfer: – load on one slab partially carried by adjacent slabs– reduces tensile stresses in slab– reduces deflections at joints• Load transfer comes from:– aggregate interlock– tie bars (rough steel bars)– dowels (smooth steel rods)Load Transfer LocationsDowelsDowelsDowelsDowelsDowelsDowelsDowelsDowelsDowelsDowelsDowelsDowelsDowelsDowelsDowelsDowelsDowelsDowelsTiesTiesTiesTiesTiesTiesTiesTiesAggregate interlock wherever joint sawed in larger slabLongitudinal jointsTransversejointsLoad Transfer DevicesSawed transverse joint Dowel Aggregate interlockSawed longitudinal joint Tie Bar Aggregate interlockJoint Saw Cut with Aggregate InterlockDowel Bar BasketAlternative:Dowel Bar InsertersTie Bars in Longitudinal JointLoad Transfer Efficiency (LTE)LTE = deflection at Bdeflection at Awhen load is at A A BA BLTE vs RepetitionsDowelled and Undowelled HVS Sections0.000.100.200.300.400.500.600.700.800.901.000.0E+001.0E+052.0E+053.0E+054.0E+055.0E+056.0E+057.0E+058.0E+059.0E+05Load RepetitionsLoad Transfer EfficiencyDowel(90kN)Nodowels(70kN)h = 200 mmmCTB = 100 mmLoad Transfer Questions• Why are dowels smooth?– permits slabs to shrink and thermally contract with small tensile stresses• What happens if too many lanes are tied together?– shrinkage, temperature contraction can cause a crack– same when slabs are too long• Is there aggregate interlock and load transfer– with asphalt shoulders? No– with cold