Field RHTemp Investigation

1
A Partnership in Research and Outreach David A.
Lange, CEAT DirectorDepartment of Civil and
Environmental Engineering Chang Joon Lee, Robert
Rodden, Yi-Shi Liu, Zachary C. Grasley
2
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

3
Two cases

• Highly restrained slab
• Cracking
• Low restraint in slab
• Curling Wheel Load ? Cracking

4
Shrinkage measurement

• Specimen sliced into the dimensions of 5x1x1/8
  
• uniform moisture condition throughout the cross
  section of the specimen under drying
• Linearly varying displacement transducer (LVDT)

5
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

6
Results (contd)

• 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

7
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

8
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
9
Measuring internal RH
10
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

11
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

12
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

13
Preliminary Through Depth Data

• Internal RH measured from the surface of a 15
  thick moist cured PCC slab

14
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

15
Determining creep properties
16
Computer Modeling
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Why is our modeling concept useful?
NOTE Assessments are based on the built-in
functions of the codes
18
Material models
Concrete is an Aging Material
Instantaneous response - Static
Linear Elastic Continuum
Delayed response - Creep
Solidification Theory Bazant 1977
Hygrothermal response - Shrinkage Thermal
Expansion
Hygrothermal Model
19
Lab Test Stress-strain Youngs modulus
Youngs modulus
Uniaxial compressive test with axial lateral
strains
20
Lab test - creep
Sealed test
Exposed to ambient
Drying
Total deformation
Basic creep
21
Deformation comparison
Lift-off displacement
VD Vertical Displacement Transducer
CL Clip gauge
22
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