A Partnership in Research and Outreach David A. Lange, CEAT Director Department of Civil and Environmental Engineering Chang Joon Lee, Robert Rodden, Yi-Shi Liu, Zachary C. Grasley

Moisture curling project Basic driving force  Internal capillary stresses imposed under partial saturation Measurement of material properties  Creep  Elastic properties Material modeling  Hygrothermal forces, shrinkage, creep  Time dependence, inter-dependence Structural modeling Validation using lab tests & NAPTF slab tests

Two cases Highly restrained slab  Cracking Low restraint in slab  Curling + Wheel Load  Cracking

Shrinkage measurement Specimen sliced into the dimensions of 5”x1”x1/8”  uniform moisture condition throughout the cross section of the specimen under drying Linearly varying displacement transducer (LVDT)

Experimental results The shrinkage curves share similar characteristics under various relative humidity conditions The linear responses end after one day of drying, and tend to level off

Results (cont’d) Both matrices yielded lower drying shrinkage than plain cement matrix system Shrinkage of rice husk ash matrix is lower than that of fly ash matrix Larger amount of amorphous silica and surface area in rice husk ash

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 Also plays a role in thermal dilation Hydratio n product

RH related to capillary pressure Kelvin-Laplace Equation relates RH directly to capillary pressure We can effectively calculate the internal stress by measuring RH As vapor pressure contributes little p” = vapor pressure  = pore fluid pressure R = universal gas constant T = temperature in kelvins v’ = molar volume of water

Measuring internal RH

UIUC field system UIUC field-ready system can sample RH and temperature at user prescribed interval and log to a nonvolatile external memory chip  Current system utilizes 55 channels (5 microprocessors)  Data can be collected with a laptop via a serial connection

Field-Ready RH/Temp Multiplexer In October 2005:  Cast 3 slabs (Plain, Cured, and SRA PCC) with a depth of 15” at ATREL Collected RH/Temp data at various locations above, at the surface, and through the depth of all slabs for three months continuously ALL above surface data showed little deviance from ambient

Novel Surface Sampling Technique A novel surface sampling technique was employed to measure the RH/temp at the surface An sensor was placed in a sleeve of Gore-Tex which was then troweled into the surface New technique yielded excellent repeatability

Preliminary Through Depth Data Internal RH measured from the surface of a 15” thick moist cured PCC slab

Preliminary Through Depth Data Drying occurs at the surface and base of the slab Daily cycling of RH and temp is evident RH and temp daily fluctuation is less as depth is greater

Determining creep properties

Computer Modeling

ABAQUSDIANAICON Gradient excitations YES Aging concrete properties NOYES Hygrothermal model for shrinkage NO YES Aging effect on creep NOSIMPLESOLIDIFYING NOTE: Assessments are based on the built-in functions of the codes Why is our modeling concept useful?

“Instantaneous” response - Static “Delayed” response - Creep Hygrothermal Model Concrete is an Aging Material Linear Elastic Continuum Solidification Theory [Bazant 1977] “Hygrothermal” response - Shrinkage & Thermal Expansion Material models

Uniaxial compressive test with axial & lateral strains Young’s modulus Lab Test: Stress-strain & Young’s modulus

Sealed testExposed to ambient Drying Sealed Basic creep Total deformation Lab test - creep

Lift-off displacement A VD-1 VD-4 CL-3 VD-5 CL-4 CL-2 Deformation comparison CL = Clip gauge VD = Vertical Displacement Transducer

Summary CEAT working to provide  a more complete understanding of mechanisms  new sensor technology  accurate, complete material models  useful software tool for analysis Results relate to material selection (e.g. high FA concrete) and pavement design