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Do Concrete Materials Specifications Address Real Performance?

Do Concrete Materials Specifications Address Real Performance?. David A. Lange University of Illinois at Urbana-Champaign. How do you spec concrete?. 1930 “6 bag mix” 1970 “f’c = 3500 psi, 5 in slump” And add some air entrainer 2010 ?.

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Do Concrete Materials Specifications Address Real Performance?

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  1. Do Concrete Materials Specifications Address Real Performance? David A. Lange University of Illinois at Urbana-Champaign

  2. How do you spec concrete? • 1930 • “6 bag mix” • 1970 • “f’c = 3500 psi, 5 in slump” • And add some air entrainer • 2010 ?

  3. 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!

  4. 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!

  5. 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!

  6. For example: Early slab cracks • Early age pavement cracking is a persistent problem • Runway at Willard Airport (7/21/98) • Early cracking within 18 hrs and additional cracking at 3-8 days

  7. 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

  8. Overview • Volume stability • Internal RH and drying shrinkage • Restrained stress • Case: Airport slab curling • Case: SCC segregation

  9. Volume stability Volume Change Thermal Shrinkage Creep External Influences Heat release from hydration External drying shrinkage Basic creep Drying creep Autogenous shrinkage Chemical shrinkage Cement hydration

  10. Chemical shrinkage Ref: PCA, Design & Control of Concrete Mixtures

  11. Self-dessication Autogenous shrinkage solid Jensen & Hansen, 2001 water air (water vapor)

  12. Chemical shrinkage drives autogenous shrinkage Note: The knee pt took place at only a = 4% Ref: Barcelo, 2000 The diversion of chemical and autogenous shrinkage defines “set”

  13. Measuring autogenous shrinkage • Sometimes the easiest solution is also the best…

  14. Autogenous shrinkage

  15. Concern is primarily low w/c 0.50 w/c Initial set locks in paste structure “Extra” water remains in small pores even at a=1 Cement grains initially separated by water 0.30 w/c Autogenous shrinkage Pore fluid pressure reduced as smaller pores are emptied Pores to 50 nm emptied Increasing degree of hydration

  16. Internal RH & Internal Drying

  17. Hydration product Hydration product 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

  18. Water surface sy p”  S S  1mm Physical source of stress We can quantify the stress using measured internal RH using Kelvin Laplace equation p” = vapor pressure = pore fluid pressure R = universal gas constant T= temperature in kelvins v’ = molar volume of water

  19. Old way: New embedded sensors: Measuring internal RH

  20. Reduced RH drives shrinkage

  21. Modeling RH & Stress Add a fitting parameter NOTE: The fitting parameter is associated with creep in the nanostructure

  22. Long term autogenous shrinkage

  23. External drying stresses

  24. RH as function of time & depth Specimen demolded at 1 d Different depths from drying surface in 3”x3” concrete prism exposed to 50% RH and 23o C

  25. Overall stress gradient in restrained cement materials Free shrinkage drying stresses Applied restraint stress T=0 ft + + + + + - External restraint stress superposed

  26. Time to fracture (under full restraint) related to gradient severity Failed at 7.9 days Failed at 3.3 days

  27. Shrinkage problems • Uniform shrinkage • cracking under restraint • Shrinkage Gradients • Tensile stresses on top surface • Curling behavior of slabs, and cracking under wheel loading

  28. Evidence of surface drying damage Hwang & Young ’84 Bisshop ‘02

  29. Restrained stresses

  30. 3 in (76 mm) 3 in (76 mm) LVDT Extensometer Load cell Actuator Feedback Control Applying restraint

  31. Creep Cumulative Shrinkage + Creep Typical Restrained Test Data

  32. A versatile test method • Assess early cracking tendencies

  33. Volume stability Volume Change Thermal Shrinkage Creep External Influences Heat release from hydration External drying shrinkage Basic creep Drying creep Autogenous shrinkage Chemical shrinkage Cement hydration

  34. 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

  35. Case: Airfield slabs

  36. Curling of Slab on Ground

  37. NAPTF slab cracking SLAB CURLING P HIGH STRESS Material (I) Material (II)

  38. 2250 mm 275 mm. 2250 mm Finite Element Model NAPTF single slab ¼ modeling using symmetric boundary conditions 1. 20-node solid elements for slab 2. Non-linear springs for base contact

  39. Loadings Temperature Internal RH Number are sensor locations (Depth from top surfaces of the slab)

  40. Z Y X Deformation Deformation Ground Contacts Ground Contacted Displacement in z-axis (Bottom View)

  41. Z Y X Stress Distribution Maximum Principle Stress What will happen when wheel loads are applied ? 1.61 MPa (234 psi) Age = 68 days

  42. Lift-off Displacement Clip Gauge Setup Lift-off Displacement

  43. Analysis of stresses σmax = 77 psi σmax = 472 psi σmax = 558 psi No Curling Curling Only Curling + Wheel loading

  44. Case: Self Consolidating Concrete

  45. Several issues • Do SCC mixtures tend toward higher shrinkage? • How will segregation influence stresses?

  46. We can expect problems • Typical SCC has lower aggregate content, higher FA/CA ratio, and lower w/cm ratio FA/CA Ratio

  47. Problems can arise Typical Concrete – “Safe Zone” ? w/b, paste% 0.41, 33% 0.40, 32% 0.39, 37% 0.34, 34% 0.33, 40%

  48. Role of paste content and w/c ratio Typical Concrete – “Safe Zone” ? w/c, Paste% 0.40, 32% 0.41, 33% 0.34, 34% 0.39, 37% 0.33, 40%

  49. Acceptance Criteria: w/c ratio • Tazawa et al found that 0.30 was an acceptable threshold • In our study, 0.34 keeps total shrinkage at reasonable levels • 0.42 eliminates autogenous shrinkage • Application specific limits • High Restraint: 0.42 • Med Restraint: 0.34 • Low Restraint: w/c based on strength or cost

  50. 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

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