MOISTURE CURLING OF CONCRETE SLABS FOR AIRFIELD APPLICATIONS

1. NAPTF SINGLE SLAB NUMERICAL SIMULATION EXPERIMENTS A A B B ▪ (Compressive only) spring elements were attached at the bottom of the slab for the ground contact simulation Displacement 1 P k k Force A ▪ A single slab with high fly ash replaced concrete was tested at NAPTF to study the moisture curling behavior (Summer 2003) Curling & ground contact change Humidity ▪ Simple linear model for the volumetric change due to the humidity Z Y σ11 X Temperature η 1 E1 ▪ Nonlinear hygrothermal model for the volumetric change due to the humidity and temperature variation “INSTANTANEOUS” MATERIAL PROPERTIES BACKGROUND APPROACH MODELS FOR AGING CONCRETE E2 η 2 ▪ Humidity, temperature, strains & deformations were measured for more than three months after casting. THE VOLUME & MATERIAL PROPERTIES VOLUME OF HYDRATED PRODUCTS ▪ FAA tested a new pavement system and experienced unexpected cracks (Spring 2000) Lift-up displacement Lift-up displacement due to curling of slab ▪ The evolution of Elastic modulus & compressive strength were defined using the VOLUME of hydrated product ▪ The volume of hydrated products can be measured by adiabatic heat evolution test of the cement hydration ▪ The degree of hydration, another representation of the volume of hydrated products in concrete, is a crucial parameter which characterize the evolution of material properties FAA pavement test & cracks map Elastic modulus Compressive strength En η n High stress region ▪ Moisture curling plays important role in these crack patterns “DELAYED” MATERIAL PROPERTY - CREEP z Degree of Hydration σ11 EFFECTIVE TIME ON HYDRATION High stress due to moisture curling & external load [ Cutsch and Rostassy, 1995 ] y x ▪ Bazant’s Solidifying material model for creep was adapted for this study. This model includes a creep magnification factor, non-aging creep and effective load bearing volume. PREDICTION OF SLAB CURLING ▪ The effective time is for the temperature & humidity effect on the hydration of cement paste OBJECTIVE Temperature ▪ History of internal humidity and temperature defines the VOLUME of hydrated product in concrete so the VOLUME defines material properties of concrete Humidity Creep magnification GKM for non-aging creep Effective load bearing volume Time ▪ To further our understanding of slab curling and behavior of high fly ash concretes for airport pavements. ▪ To predict slab curling with the full range of material properties, environmental conditions, slab configuration, and other factors that contribute to the curling problem Effective time TEMPERATURE DEFINE APPLY HYGROTHERMAL STRAIN CURLING REDUCED TIME ON CREEP Volume Structural model UNDERSTANDING ▪ History of internal humidity and temperature in slab system defines the structural behavior as well as the material properties DEFINE PREDICT ▪ The reduced time is for the temperature & humidity effect on the creep rate ▪ Hygrothermal strains due to the temperature & humidity change. PREDICTION Moduli & Strength Deformation DEFORMATION Creep Stress Hygrothermal strains HUMIDITY STRESS Reduced time Hygrothermal strain EXPERIMENTAL PROGRAM CONCLUDING REMARKS MECHANICAL TEST ▪ Internal humidity is the most important factor that characterize the curling behavior of slab. A simple linear relation for the hygral strain is not a proper model for our application. The non-linear hygrothermal model used for the numerical simulation shows a good result ▪ “Instantaneous” material properties such as elastic modui and strengths can be measured by mechanical test at different age of concrete. “Delayed” material property is measure by uniaxial creep test. Figure from PCA ▪ Material models for aging concrete and a 3D FEA code was developed for this project. The models and the code provide us an effective tool for learning about the role of material in slab curling problem. Mechanical test Creep test DRYING SHRINKAGE TEST ▪ Volumetric strain change due to internal humidity & temperature is one of the most important relations required for the prediction of material and structural behavior FUTHER ISSUES Experimental ▪ Complete tests for bulk property of high fly ash concrete Material modeling ▪Refine the model for RH-shrinkage relationship ▪ Address drying creep & Creep Poisson’s ratio Application ▪ Use model to learn about role of material in slab curling problem Internal humidity measurement Dying shrinkage test with paste & mortar specimens RESTRAINED SHRINKAGE TEST ▪ Restrained shrinkage test involves non-uniform temperature and humidity profile through the cross-section. It also produce a creep due to the restrained stresses Fully restrained shrinkage test Partially restrained shrinkage test MOISTURE CURLING OF CONCRETE SLABS FOR AIRFIELD APPLICATIONS Chang Joon Lee, Yi-Shi Liu, Ben Birch, David A. Lange, Jeffery R. Roesler