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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? 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 ?
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!
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!
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!
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
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
Overview • Volume stability • Internal RH and drying shrinkage • Restrained stress • Case: Airport slab curling • Case: SCC segregation
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
Chemical shrinkage Ref: PCA, Design & Control of Concrete Mixtures
Self-dessication Autogenous shrinkage solid Jensen & Hansen, 2001 water air (water vapor)
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”
Measuring autogenous shrinkage • Sometimes the easiest solution is also the best…
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
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
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
Old way: New embedded sensors: Measuring internal RH
Modeling RH & Stress Add a fitting parameter NOTE: The fitting parameter is associated with creep in the nanostructure
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
Overall stress gradient in restrained cement materials Free shrinkage drying stresses Applied restraint stress T=0 ft + + + + + - External restraint stress superposed
Time to fracture (under full restraint) related to gradient severity Failed at 7.9 days Failed at 3.3 days
Shrinkage problems • Uniform shrinkage • cracking under restraint • Shrinkage Gradients • Tensile stresses on top surface • Curling behavior of slabs, and cracking under wheel loading
Evidence of surface drying damage Hwang & Young ’84 Bisshop ‘02
3 in (76 mm) 3 in (76 mm) LVDT Extensometer Load cell Actuator Feedback Control Applying restraint
Creep Cumulative Shrinkage + Creep Typical Restrained Test Data
A versatile test method • Assess early cracking tendencies
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
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
NAPTF slab cracking SLAB CURLING P HIGH STRESS Material (I) Material (II)
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
Loadings Temperature Internal RH Number are sensor locations (Depth from top surfaces of the slab)
Z Y X Deformation Deformation Ground Contacts Ground Contacted Displacement in z-axis (Bottom View)
Z Y X Stress Distribution Maximum Principle Stress What will happen when wheel loads are applied ? 1.61 MPa (234 psi) Age = 68 days
Lift-off Displacement Clip Gauge Setup Lift-off Displacement
Analysis of stresses σmax = 77 psi σmax = 472 psi σmax = 558 psi No Curling Curling Only Curling + Wheel loading
Several issues • Do SCC mixtures tend toward higher shrinkage? • How will segregation influence stresses?
We can expect problems • Typical SCC has lower aggregate content, higher FA/CA ratio, and lower w/cm ratio FA/CA Ratio
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%
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%
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
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