Autogenous shrinkage
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Do Concrete Materials Specifications Address Real Performance? David A. Lange University of Illinois at Urbana-Champaign ILLINOIS University of Illinois at Urbana-Champaign How do you spec concrete? 1930 1970 “6 bag mix” “f’c = 3500 psi, 5 in slump” And add some air entrainer 2010 ? ILLINOIS University of Illinois at Urbana-Champaign Is concrete that simple? How simple are your expectations? Are we worried only about strength? What about … Long-term durability Crack-free surfaces Perfect consolidation in conjested forms These cause more concrete to be replaced than structural failure! ILLINOIS University of Illinois at Urbana-Champaign Seeking the Holy Grail Admixtures developed in 1970’s open the door to lower w/c and high strength Feasible high strength concrete moved from 6000 psi to 16,000 psi Feasible w/c moved from 0.50 to 0.30 Everybody loves high strength! ILLINOIS University of Illinois at Urbana-Champaign But there are trade-offs… Low w/c high autogenous shrinkage High paste content greater vol change High E high stress for given strain High strength more brittle …greater problems with cracking! ILLINOIS University of Illinois at Urbana-Champaign For example: Early slab cracks Early age pavement cracking is a persistent problem ILLINOIS University of Illinois at Urbana-Champaign Runway at Willard Airport (7/21/98) Early cracking within 18 hrs and additional cracking at 3-8 days Concrete IS complex Properties change with time Microstructure changes with time Volume changes with time Self imposed stresses occur Plus, you are placing it in the field under variable weather conditions There are a million ways to make concrete for your desired workability, early strength, long-term performance ILLINOIS University of Illinois at Urbana-Champaign Overview Volume stability Internal RH and drying shrinkage Restrained stress Case: Airport slab curling Case: SCC segregation ILLINOIS University of Illinois at Urbana-Champaign Volume stability Volume Change Thermal External Influences Shrinkage Heat release from hydration Autogenous shrinkage Chemical shrinkage Cement hydration ILLINOIS University of Illinois at Urbana-Champaign External drying shrinkage Creep Basic creep Drying creep Chemical shrinkage ILLINOIS University of Illinois at Urbana-Champaign Ref: PCA, Design & Control of Concrete Mixtures Self-dessication solid water air (water vapor) ILLINOIS University of Illinois at Urbana-Champaign Autogenous shrinkage Jensen & Hansen, 2001 Chemical shrinkage drives autogenous shrinkage Note: The knee pt took place at only a = 4% Ref: Barcelo, 2000 ILLINOIS The diversion of chemical and autogenous shrinkage defines “set” University of Illinois at Urbana-Champaign Measuring autogenous shrinkage Sometimes the easiest solution is also the best… ILLINOIS University of Illinois at Urbana-Champaign Autogenous shrinkage Autogenous Shrinkage (10-6 m/m) 50 OPC1, w/c = 0.40 SCC1, w/c = 0.39 SCC2, w/c = 0.33 SCC3, w/c = 0.41 SCC4, w/c = 0.32 HPC1, w/c = 0.25 SCC2-2 SCC2-slag 0 -50 -100 -150 -200 -250 0 20 ILLINOIS University of Illinois at Urbana-Champaign 40 60 Age (d) 80 100 Concern is primarily low w/c 0.50 w/c Cement grains initially separated by water “Extra” water remains in small pores even at a=1 Initial set locks in paste structure 0.30 w/c Autogenous shrinkage Pores to 50 nm emptied ILLINOIS Increasing degree of hydration University of Illinois at Urbana-Champaign Pore fluid pressure reduced as smaller pores are emptied Internal RH & Internal Drying ILLINOIS University of Illinois at Urbana-Champaign Mechanism of shrinkage Shrinkage dominated by capillary surface tension mechanism As water leaves pore system, curved menisci develop, creating reduction in RH and “vacuum” (underpressure) within the pore fluid ILLINOIS University of Illinois at Urbana-Champaign Hydratio n product Hydration product Physical source of stress sy S Water surface Vapor p” Diffusion S 1mm We can quantify the stress using measured internal RH using Kelvin Laplace equation ILLINOIS University of Illinois at Urbana-Champaign Pc Fy p " p ' 2s y ln( RH ) RT p " v' p” = vapor pressure = pore fluid pressure R = universal gas constant T = temperature in kelvins v’ = molar volume of water Measuring internal RH Old way: New embedded sensors: ILLINOIS University of Illinois at Urbana-Champaign Reduced RH drives shrinkage SCC4, w/c = 0.34 0 100.0 -6 Autogenous Shrinkage -20 99.5 Relative Humidity -30 -40 99.0 -50 -60 98.5 -70 -80 98.0 -90 -100 97.5 ILLINOIS 0 10 20 30 University of Illinois at Urbana-Champaign 40 Age (d) 50 60 70 80 Relative Humidity (%) Autogenous Shrinkage (10 m/m) -10 Modeling RH & Stress Drying Shrinkage (in./in./) 4.0E-04 HT 3.0E-04 ln( RH ) RT v' 3 1 RH 1 0.75(1 1 0.98 3k 3k0 2.0E-04 Add a fitting parameter 1.0E-04 Measured HT aHT Theoretical Fitted 0.0E+00 0 7 14 21 28 35 42 49 ln( RH ) RT v' 3 1 RH 1 0.75(1 1 0.98 3k 3k0 56 Time (day) NOTE: The fitting parameter is associated with creep in the nanostructure ILLINOIS University of Illinois at Urbana-Champaign Long term autogenous shrinkage ILLINOIS University of Illinois at Urbana-Champaign External drying stresses ILLINOIS University of Illinois at Urbana-Champaign RH as function of time & depth 3" x 3" Concrete Prism, 0.50 w/c 100 1/2" 1/4" 3/4" Depth from drying surface 95 Internal RH (%) 90 Specimen demolded at 1 d 85 80 75 70 65 60 0 2 4 6 8 10 12 14 Time Days Different depths from drying surface in 3”x3” concrete prism ILLINOIS exposed to 50% RH and 23 C o University of Illinois at Urbana-Champaign External restraint stress superposed Free shrinkage drying stresses Applied restraint stress Overall stress gradient in restrained cement materials ft + - + ILLINOIS University of Illinois at Urbana-Champaign + T=0 + + Time to fracture (under full restraint) related to gradient severity 6 A-44 A-44 Average B-44 B-44 Average C-44 C-44 Average D-44 D-44 Average 41 41 Average 38 38 Average 32 32 Average 5 Stress (MPa) 4 Failed at 7.9 days 3 2 1 Failed at 3.3 days 0 0 10 20 ILLINOIS University of Illinois at Urbana-Champaign 30 40 Specimen Width (mm) 50 60 70 Shrinkage problems Uniform shrinkage cracking under restraint Shrinkage Gradients Tensile stresses on top surface Curling behavior of slabs, and cracking under wheel loading ILLINOIS University of Illinois at Urbana-Champaign Evidence of surface drying damage Hwang & Young ’84 Bisshop ‘02 ILLINOIS University of Illinois at Urbana-Champaign Restrained stresses ILLINOIS University of Illinois at Urbana-Champaign Applying restraint 3 in (76 mm) 3 in (76 mm) LVDT Extensometer Load cell Actuator ILLINOIS Feedback Control University of Illinois at Urbana-Champaign Typical Restrained Test Data 200 Restrained Specimen 150 Creep 7 0 6 -50 5 Cumulative Shrinkage + Creep -100 4 -150 3 -200 2 -250 1 -300 0 2 3 4 Time (days) ILLINOIS University of Illinois at Urbana-Champaign 5 6 7 Applied Load (kN) 50 1 i 1 8 Load (kN) 0 tot el i 9 Free Specimen 100 Strain (m) n 10 c tot - sh c J c (t , t ' ) Ec (t ) el A versatile test method Assess early cracking tendencies 1.0 0.9 Stress-Strength Ratio 0.8 0.7 0.6 0.5 0.4 OPC1, w/c = 0.40 0.3 SCC1, w/c = 0.39 SCC2, w/c = 0.33 0.2 SCC3, w/c = 0.41 0.1 SCC4, w/c = 0.34 0.0 0 2 ILLINOIS University of Illinois at Urbana-Champaign 4 6 Age (d) 8 10 Volume stability Volume Change Thermal External Influences Shrinkage Heat release from hydration Autogenous shrinkage Chemical shrinkage Cement hydration ILLINOIS University of Illinois at Urbana-Champaign External drying shrinkage Creep Basic creep Drying creep Now we are ready for structural modeling! All this work defines “material models” that capture… Autogenous shrinkage Drying shrinkage Creep Thermal deformation Interdependence of creep & shrinkage ILLINOIS University of Illinois at Urbana-Champaign Case: Airfield slabs ILLINOIS University of Illinois at Urbana-Champaign Curling of Slab on Ground ILLINOIS University of Illinois at Urbana-Champaign NAPTF slab cracking SLAB CURLING P HIGH STRESS ILLINOIS Material (I) Material (II) University of Illinois at Urbana-Champaign Finite Element Model NAPTF single slab ¼ modeling using symmetric boundary conditions 2250 mm 275 mm. 2250 mm 1. 20-node solid elements for slab 2. Non-linear springs for base contact ILLINOIS University of Illinois at Urbana-Champaign Loadings Temperature Internal RH 32 25mm 100 30 262.5mm o 95 137.5mm 90 262.5mm 25mm Temperature( C) Relative humidity(%) 105 85 80 137.5mm 28 26 24 22 20 75 18 14 28 42 56 Age(day) 70 14 28 42 Age(day) 56 Number are sensor locations (Depth from top surfaces of the slab) ILLINOIS University of Illinois at Urbana-Champaign 70 Deformation Deformation Z Y Ground Contacts X ILLINOIS University of Illinois at Urbana-Champaign Displacement in z-axis (Bottom View) Ground Contacted Stress Distribution Maximum Principle Stress What will happen when wheel loads are applied ? 1.61 MPa (234 psi) Z Y X Age = 68 days ILLINOIS University of Illinois at Urbana-Champaign Lift-off Displacement Clip Gauge Setup Lift-off Displacement Lift-off displacement(mm) 4 Measured Model prediction 3 2 1 0 14 ILLINOIS University of Illinois at Urbana-Champaign 28 42 Age(day) 56 70 Analysis of stresses σmax = 77 psi No Curling ILLINOIS University of Illinois at Urbana-Champaign σmax = 472 psi Curling Only σmax = 558 psi Curling + Wheel loading Case: Self Consolidating Concrete ILLINOIS University of Illinois at Urbana-Champaign Several issues Do SCC mixtures tend toward higher shrinkage? How will segregation influence stresses? ILLINOIS University of Illinois at Urbana-Champaign We can expect problems Typical SCC has lower aggregate content, higher FA/CA ratio, and lower w/cm ratio 2.5 SCC Database 2.0 FA/CA Ratio FA/CA RATIO Mixtures studied 1.5 SCC1 SCC2 SCC3 1.0 Typical non-SCC materials, according to ACI mixture proportioning method SCC4 0.5 OPC1 ILLINOIS 0.0 50 55 60 65 University of Illinois at Urbana-Champaign 70 75 80 AGGREGATE CONTENT (%) 85 90 95 100 Problems can arise Typical Concrete – “Safe Zone” ? 20 Autogenous Shrinkage (10-6 m/m) 0 -20 w/b, paste% 0.41, 33% -40 0.40, 32% -60 0.39, 37% -80 0.34, 34% -100 -120 OPC1, w/c = 0.40 SCC1, w/c = 0.39 -140 SCC2, w/c = 0.33 0.33, 40% SCC3, w/c = 0.41 -160 SCC4, w/c = 0.32 -180 ILLINOIS 0 5 10 University of Illinois at Urbana-Champaign 15 Age (d) 20 25 30 Role of paste content and w/c ratio 0 Typical Concrete – “Safe Zone” ? -100 Free Shrinkage (x10-6) -200 -300 w/c, Paste% -400 0.40, 32% -500 0.41, 33% 0.34, 34% -600 -700 OPC1, w/c = 0.40 0.39, 37% SCC1, w/c = 0.39 SCC2, w/c = 0.33 -800 SCC3, w/c = 0.41 0.33, 40% SCC5, w/c = 0.34 -900 ILLINOIS -1000 0 5 University of Illinois at Urbana-Champaign 10 15 Age (days) 20 25 30 Acceptance Criteria: w/c ratio Tazawa et al found that 0.30 was an acceptable threshold 900 In our study, 0.34 keeps total shrinkage at reasonable levels800 0.42 eliminates autogenous 700 shrinkage 600 Application specific limits 500 Autogenous Shrinkage Strain (x10-6) High Restraint: 0.42 400 Med Restraint: 0.34 300 Low Restraint: w/c based on 200 strength or cost Autogenous Shrinkage (28d) Total Shrinkage (28d) 100 0 0.30 0.32 0.34 0.36 w/cm ILLINOIS University of Illinois at Urbana-Champaign 0.38 0.40 0.42 Acceptance Criteria: Paste Content IDOT max cement factor is 7.05 cwt/yd3 At 705 lb/yd3, 0.40 w/c = 32% paste Below 32%, SCC has questionable fresh properties Is 34% a reasonable compromise? Application specific limits High Restraint: 25-30% Med Restraint: 30-35% Low Restraint: Based on cost ILLINOIS University of Illinois at Urbana-Champaign 900 800 Autogenous Shrinkage Strain (x10-6) 700 600 500 400 Autogenous Shrinkage (28d) 300 Total Shrinkage (28d) 200 100 0 30% 32% 34% 36% 38% Paste Content by Volume 40% 42% Segregation SCC may segregate during placement Static or Dynamic How does this impact hardened performance? ILLINOIS University of Illinois at Urbana-Champaign Consider static segregation Specimen 8” x 8” x 20” prism 8 equal layers Each layer assigned: CA%, E and sh ILLINOIS University of Illinois at Urbana-Champaign Experiment ILLINOIS University of Illinois at Urbana-Champaign Cast vertically to produce a segregated cross section Laid flat to measure deflection caused by autogenous shrinkage of segregated layer Results 0.008 Deflection (in) Deflection (in) 0.007 Measured Deflection 0.006 FEM Calculated Deflection 0.005 0.004 0.003 0.002 0.001 0.000 0 2 4 6 8 10 Concrete Age (d) Concrete Age (d) ILLINOIS University of Illinois at Urbana-Champaign 12 14 16 Model validation Now run model under restrained conditions to assess STRESS Model confirms we have reasonable rules for segregation limits HVSI = 0 or 1 is OK HVSI = 2 or 3 is BAD 450 400 Max Stress Developed (psi) 350 300 250 SCC1 SCC2 SCC3 SCC4 200 150 100 50 0 0 1 2 HVSI HVSIRating Rating ILLINOIS University of Illinois at Urbana-Champaign 3 ILLINOIS University of Illinois at Urbana-Champaign Back to Specifications… What is the “real performance” we need to ensure? More that strength Spec writers need to assert more control Example: IDOT -- SCC will have limits on segregation, min. aggregate content, min. w/c ILLINOIS University of Illinois at Urbana-Champaign Specing “real performance” How do you impose long-term requirements using short-term properties? How do you impose limitation on long term cracking when factors are so extensive, including environment and loadings “beyond control of material supplier”? ILLINOIS University of Illinois at Urbana-Champaign Performance vs. Prescription Can Performance Based Specs do the whole job? Prescriptions… Min. and max w/c Min. aggregate content Aggregate gradation limits Performance requirements… Max. drying shrinkage, maybe autogenous shrinkage Permeability (RCPT ?) ILLINOIS University of Illinois at Urbana-Champaign Last thoughts “Times they are a’changing…” We have higher expectations We have new tools, new knowledge We are ever pushing the boundaries of past experience ILLINOIS University of Illinois at Urbana-Champaign
