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

Chemical shrinkage Volume stability Volume Change ThermalShrinkageCreep External Influences Autogenous shrinkage External drying shrinkage Basic creepDrying creep Heat release from hydration Cement hydration

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

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

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

Measuring autogenous shrinkage Sometimes the easiest solution is also the best…

Autogenous shrinkage

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

Internal RH & Internal Drying

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 Hydratio n product

Physical source of stress  p” sysy S S Water surface 1m1m  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

Measuring internal RH Old way:New embedded sensors:

Reduced RH drives shrinkage

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

Long term autogenous shrinkage

External drying stresses

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

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

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

Restrained stresses

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

Typical Restrained Test Data Creep Cumulative Shrinkage + Creep

A versatile test method Assess early cracking tendencies

Chemical shrinkage Volume stability Volume Change ThermalShrinkageCreep External Influences Autogenous shrinkage External drying shrinkage Basic creepDrying creep Heat release from hydration 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

Case: Airfield slabs

Curling of Slab on Ground

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

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

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

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

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

Clip Gauge Setup Lift-off Displacement

Analysis of stresses σ max = 77 psi Curling OnlyCurling + Wheel loading σ max = 472 psiσ max = 558 psi No Curling

Case: Self Consolidating Concrete

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” ? 0.39, 37% 0.34, 34% 0.41, 33% 0.40, 32% 0.33, 40% w/b, paste%

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

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/yd 3 At 705 lb/yd 3, 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

Segregation SCC may segregate during placement  Static or Dynamic How does this impact hardened performance?

Consider static segregation Specimen 8” x 8” x 20” prism 8 equal layers Each layer assigned: CA%, E and  sh

Experiment  Cast vertically to produce a segregated cross section  Laid flat to measure deflection caused by autogenous shrinkage of segregated layer

Results Deflection (in) Concrete Age (d)

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 HVSI Rating

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

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”?

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 ?)

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