In Search of Crack-Free Concrete: Current Research on Volume Stability and Microstructure David A. Lange University of Illinois at Urbana-Champaign Department of Civil & Environmental Engineering

Motivation: 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

HIGH STRESS SLAB CURLING P Motivation: Slab curling Material (I)Material (II)

Material properties are key Properties are time-dependent Stiffness develops sooner than strength Ref: After Olken and Rostasy, 1994

A “materials” approach Understand…  Cement  Microstructure  Source of stress  Nature of restraint  Structural response

Chemical shrinkage Overview Early Age Volume Change ThermalShrinkageCreepSwelling External Influences Autogenous shrinkage External drying shrinkage Basic creepDrying creep Redistribution of bleed water or water from aggregate Early hydration Heat release from hydration Cement hydration

Now put them all together… …and you have a very complex problem  All of the possible types of volume change are interrelated. For example: Temperature change affects shrinkage, hydration reaction (i.e. crystallization, chemical shrinkage, pore structure)  Even worse, the mechanisms for each type often share the same stimuli. For example: Drying effects shrinkage and creep

The goal: optimization A challenging problem Methods that improve performance in regard to one issue may exacerbate another. For example:  Lowering w/c is known to reduce drying shrinkage and increase strength, but…  Creep is reduced, autogenous shrinkage is increased, and material is more brittle. All BAD.

Applying knowledge to potential materials Methods for quantifying material properties that affect volume change and thus cracking potential

Methods of measurement Volume change:  Embedded strain gages  LVDT  Dial gage Environmental stimuli  Temperature Thermocouple or thermistor  Internal or external RH Embeddable RH sensor Field ready!

Measurements (cont’d) Creep  Tensile – uniaxial loading frames  Compressive – creep frames

Examples of field instrumentation

Bridge Deck Temperatures – 1 st week

Strain in bridge deck

Summary The primary causes of volume change have been discussed  Along with ideas for minimization and optimization The goal of our research is to provide info that aids in the development of specs that minimize problems due to concrete volume change Ultimate goal: crack free concrete Immediate goal: maximizing joint spacing and minimizing random cracking

In search of crack free concrete: Basic principles Limit paste content  Aggregates usually are volume stable Use moderate w/c  Limits overall shrinkage (autogenous + drying)  Avoids overly brittle material Use larger, high quality aggregates  Improves fracture toughness

Shrinkage reducing admixtures  Reduces drying or autogenous shrinkage Saturated light-weight aggregate  Reduces autogenous shrinkage Fibers  Reduces drying or autogenous shrinkage In search of crack free concrete: Emerging approaches

END Upcoming events sponsored by CEAT: Brown Bag Lunches -- April 7 -- Marshall Thompson May 5 -- Jeff Roesler June 9 -- Erol Tutumluer July 7 -- John Popovics Workshop on Pavement Instrumentation & Analysis May 17 at UIUC with FAA participants

Thermal dilation Some sources of thermal change:  Ambient temperature change  Solar radiation  Hydration (exothermic reaction)

Heat of hydration Setting Hardening Dormant

Mechanisms of thermal dilation 3 components:  Solid dilation – same as dilation of any solid  Hygrothermal dilation – change in pore fluid pressure with temperature  Delayed dilation (relaxation of stress) Linked to moisture content, but dominated by aggregate CTD CTD of concrete ~10 x /C

Timing of set & early heat

Thermal problems Hydration heat  early age cracking on cool-down Thermal gradients  High restraint  stresses at top of pavement  cracking  Low restraint  curling  cracking under wheel loading Buckling

Thermal gradient issues Highly restrained slab  Cracking Low restraint in slab  Curling + Wheel Load  Cracking

Can construction practices counteract thermal stress? Construct during low ambient heat  Morning hours, moderate seasons Use wet curing Use low fresh concrete temperatures Use blankets or formwork that reduce RATE of cooling Reduce joint spacing in pavements and reduce restraint of structure Avoid early thermal shock upon form removal

Shrinkage Usually divided into components:  Chemical shrinkage  Internal drying shrinkage Known as Autogenous Shrinkage  External drying shrinkage

Chemical shrinkage Ref: Neville, 1995 Typical values for PC: 7-10%

Autogenous shrinkage: Particularly a problem of HPC Internal drying (self-desiccation) associated with hydration  Only occurs with w/c below ~ 0.42 Same mechanism as drying shrinkage Reason to place LOWER limit on w/c Traditional curing NOT very effective

Autogenous Shrinkage

Autogenous shrinkage: why only low w/c?

The “traditional” shrinkage: external drying shrinkage Occurs when pore water diffuses to surface Risk increases as diffusivity (porosity) goes up  Reason to place UPPER limit on w/c (or have minimum strength requirement)

Mechanism of shrinkage Both autogenous and drying 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

Surface tensionTemperaturePore Radius Radius of meniscus curvature Underpressure in pore fluid Internal Relative Humidity Change Internal Drying External Drying Hydration Physicochemical Equilibrium Mechanical equilibrium Kelvin-Laplace Equation Shrinkage Red. Adm. (SRA) Salt Concentration r pp  2 '"  ' )ln(2 v RTRH r    RH-stress relationship Kelvin-Laplace equation allows us to relate RH directly to capillary stress development  Drying shrinkage  Autogenous shrinkage ' )ln( '" v RTRH pp   p” = vapor pressure p’ = pore fluid pressure RH = internal relative humidity R = Universal gas constant v’ = molar volume of water T = temperature in kelvins

Visualize scale of mechanism Capillary stresses present in pores with radius between 2-50 nm Note the dimensions C-S-H makes up ~70% of hydration product Majority of capillary stresses likely present within C-S-H network *Micrograph take from Taylor “Cement Chemistry” (originally taken by S. Diamond 1976)

Shrinkage problems Like thermal dilation… Shrinkage gradients  High restraint  tensile stresses on top of pavement  micro and macrocracking  Low restraint  curling  cracking under wheel loading Bulk (uniform) shrinkage  cracking under restraint

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

External restraint stress superposed ftft ++ - Free shrinkage drying stresses + + Overall stress gradient in restrained concrete + 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

Fracture related to gradient severity Grasley, Z.C., Lange, D.A., D’Ambrosia, M.D., Internal Relative Humidity and Drying Stress Gradients in Concrete, Engineering Conferences International, Advances in Cement and Concrete IX(2003). Load removed from B-44 prior to failure

Creep: our friend? In restrained concrete, creep alleviates tensile stresses  Reduces tendency to crack Many possible mechanisms including moisture movement, microscale particle “sliding”, microcracking Difficult to measure, quantify, and account for in pavement and mixture design

Creep comes in two flavors Basic creep  Time-dependent deformation that occurs in all loaded concrete Drying creep  Additional creep that occurs when load is present during drying  Occurs for both tensile and compressive loads

Swelling Bleed water readsorption  As water is consumed during hydration, bleed water may be sucked back in Crystallization pressure  Certain hydration products force expansion during formation