MOISTURE CURLING OF CONCRETE SLABS FOR AIRFIELD APPLICATIONS Chang Joon Lee, Yi-Shi Liu, Ben Birch, David A. Lange, Jeffery R. Roesler z x y ▪ 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 OBJECTIVE BACKGROUND P ▪ Moisture curling plays important role in these crack patterns FAA pavement test & cracks map ▪ FAA tested a new pavement system and experienced unexpected cracks (Spring 2000) High stress due to moisture curling & external load High stress region UNDERSTANDING PREDICTION HUMIDITY TEMPERATURE CURLING STRESS DEFORMATION APPROACH Volume Temperature Humidity Time DEFINE Moduli & Strength DEFINE Creep Hygrothermal strains APPLY Structural model PREDICT Deformation Stress ▪ History of internal humidity and temperature defines the VOLUME of hydrated product in concrete so the VOLUME defines material properties of concrete ▪ History of internal humidity and temperature in slab system defines the structural behavior as well as the material properties [ Cutsch and Rostassy, 1995 ] ▪ 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 THE VOLUME & MATERIAL PROPERTIES PREDICTION OF SLAB CURLING NAPTF SINGLE SLAB EXPERIMENTS A B Humidity Temperature Lift-up displacement ▪ A single slab with high fly ash replaced concrete was tested at NAPTF to study the moisture curling behavior (Summer 2003) ▪ Humidity, temperature, strains & deformations were measured for more than three months after casting. A B NUMERICAL SIMULATION Force Displacement k k 1 ▪ (Compressive only) spring elements were attached at the bottom of the slab for the ground contact simulation ▪ Nonlinear hygrothermal model for the volumetric change due to the humidity and temperature variation X Z Y Curling & ground contact change Lift-up displacement due to curling of slab A ▪ Simple linear model for the volumetric change due to the humidity Fully restrained shrinkage test Partially 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 ▪ Volumetric strain change due to internal humidity & temperature is one of the most important relations required for the prediction of material and structural behavior Internal humidity measurement Dying shrinkage test with paste & mortar specimens EXPERIMENTAL PROGRAM Creep test ▪ “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. Mechanical test MECHANICAL TEST DRYING SHRINKAGE TEST RESTRAINED SHRINKAGE TEST Figure from PCA MODELS FOR AGING CONCRETE EFFECTIVE TIME ON HYDRATION REDUCED TIME ON CREEP ▪ The reduced time is for the temperature & humidity effect on the creep rate ▪ The effective time is for the temperature & humidity effect on the hydration of cement paste VOLUME OF HYDRATED PRODUCTS ▪ The volume of hydrated products can be measured by adiabatic heat evolution test of the cement hydration Degree of Hydration Effective time Reduced time CONCLUDING REMARKS ▪ 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 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 ▪ 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. “INSTANTANEOUS” MATERIAL PROPERTIES Elastic modulus σ 11 η 1 η 2 η n E1E1 EnEn E2E2 “DELAYED” MATERIAL PROPERTY - CREEP Creep magnification Effective load bearing volume GKM for non- aging creep ▪ 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. HYGROTHERMAL STRAIN ▪ Hygrothermal strains due to the temperature & humidity change. ▪ The evolution of Elastic modulus & compressive strength were defined using the VOLUME of hydrated product Compressive strength Hygrothermal strain