Fracture Surface Roughness and Fracture Toughness of Cement-based Materials
D. A. Lange,* A. Abell
NSF Center for Advanced Cement-Based Materials

Fracture surfaces of concrete and mortar specimens are characterized using confocal microscopy and a video density technique to create 3-D computer-based topographic maps. The texture of the fracture surfaces is quantified using image analysis techniques to compute a roughness parameter and fractal dimension. Previous work has demonstrated a link between fracture surface roughness and fracture toughness. The current effort extends toward computational modeling to explain the toughening increment due to tortuousity of the crack path.

Fracture Mechanics Testing of Bond in Masonry
D. A. Lange,* C. Park
Portland Cement Assn.

The standard test of masonry bond strength is the ASTM C1072 Bond Wrench Test, which measures maximum stress but does not provide insight into deformation and fracture energy. This study will apply fracture mechanics models to masonry to more fully describe the propagation of cracks during failure. The two parameter fracture models, developed initially for concrete, will be adapted for masonry. The benefit of this "energy approach" is that we will characterize resistance to deformation and crack propagation in addition to measuring maximum stress. Application of fracture mechanics will provide a rational and more complete foundation for assessing quality of masonry bond.

Cement Composites Using Vapor-Grown Carbon Fibers
D. A. Lange,* X. Xi
Applied Sciences, Inc.

A manufacturing process developed by Applied Sciences produces carbon fibers by a vapor deposition process. The fibers are small: 0.2 microns in diameter with an aspect ratio of 40 to 200. Cements densified with small particles (DSP) and macro-defect-free (MDF) systems are candidate matrices that have very low total porosity and very fine pore structure. This project will use the carbon fibers in cast, extruded, and pressed plate forms. The mechanical, electrical, and thermal properties of the composites will be measured to assess the contribution and potential of the fibers.

Design of Masonry Mortars for Controlled Curing and Performance
D. A. Lange,* A. Werner
Portland Cement Assn.

An experimental study of bond between mortar and masonry units is helping us establish principles for design of masonry mortar to achieve superior performance. Bond is controlled by the penetration of paste into the masonry pores, the nature of the hydration products at the interface, the enhancement of bond through surface roughness, and the degree of bond across the entire masonry/mortar interface. The project studies how mortars retain water, interact with the unit and curing environment, and develop mechanical performance. The study will help us understand what parameters of mix design are relevant to superior performance.

High-Performance Concrete and Thin Bonded Overlays for Airport Pavement Systems
D. A. Lange,* L. Struble, F. Young, S. Altoubat, H. Shin
Federal Aviation Administration

Bonded overlays are a relatively fast and inexpensive technique for rehabilitation of concrete pavement surfaces. One difficult aspect of constructing bonded overlays is control of cracking and debonding. The objective of this project is to provide an understanding of early age stresses that result from thermal and shrinkage mechanisms in bonded overlay systems. Experiments to characterize early age volumetric instability are underway, and a numerical structural model of a bonded overlay will compute stresses and strains that develop within days of placement. The model will provide guidance on the effect of overlay thickness and joint spacing required to prevent slab cracking.

Microstructural Engineering of Concrete
D. A. Lange*
National Science Foundation, CMS-9623467

The microstructural engineering approach involves control of concrete microstructure through processing to yield specific material properties. The objective of this study is to enhance understanding of the relationship between concrete properties and the physical reality of microstructure and crack geometry. The experimental work addresses three main areas: pore structure, interfaces and bond, and fracture behavior. In addition, performance measures related to microstructure such as durability are of interest.

Early Age Cracking of Bonded Overlays for Pavements
D. A. Lange,* B. Bicer, N. Rau
Illinois Department of Transportation

Volumetric instability of concrete at early age is a problem that may lead to cracking. Mechanisms that cause volume changes of young concrete include thermal contraction/expansion, drying shrinkage, autogenous shrinkage, and expansive hydration products. While the isolated mechanisms have been well researched, they continue to be perplexing problems when they occur under structurally restrained conditions. This experimental project explores early age cracking of bonded overlays to more fully understand difficulties encountered at the U of I Willard Airport in Champaign, Ill., during construction in the summer of 1998.

Microcracking of Mortars Undergoing Alkali Silica Reaction
D. A. Lange,* W. Aquino
National Science Foundation, CMS-9623467

Alkali silica reaction (ASR) is a mechanism that degrades concrete materials incorporating susceptible aggregates. This project focuses on the development of stress at reaction sites and the resulting microcracking. The experiment uses mortars made with opal aggregate that is very susceptible to ASR and includes mineral admixtures such as silica fume and metakaolin to mitigate the reaction. Scanning electron microscopy is used to document the extent and nature of cracking inside the materials. Series of samples were prepared at different ages to provide information on the development of cracking. The microstructural observations will help interpret bulk length change measurements.

Fatigue of Thermite Welded Rail
F. V. Lawrence, Jr.,* J. Withee
Association of American Railroads

As the railroad industry uses ever higher axle loads, the field-welded thermite welds that connect the rails have proven to be the source of an increasing number of derailments. This study is researching the cause of the poor fracture toughness and fatigue properties of the thermite weldment and searching for ways to improve or replace the thermite welding processes in welding rail in the field.

Fatigue of Welds and Adhesive Joints
H. Sehitoglu* (Mech. & Indus. Engr.), F. V. Lawrence, Jr.,* D. F. Socie* (Mech. & Indus. Engr.), J. E. Stubbins* (Nucl. Engr.), K. J. Hsia* (Theoret. & Appl. Mech.), N. Chen, H. Hsieh, S. Andrews, T. McGreevy
Fracture Control Program

Factors that control the fatigue behavior of welded components are currently being studied. Analytical methods for estimating the total fatigue life of butt and fillet welds subjected to variable-amplitude loading histories are evaluated. Surface treatments, such as shot peening and laser dressing of the weld toe, are investigated as possible methods for improving the fatigue strength. A new model for estimating the fatigue life of weldments has been proposed for butt, T-joint, and cruciform weldments using the concepts of "crack closure" for cracks emanating from a notch. Results compare favorably with experimental data in the UIUC fatigue data bank and with experimental work in the literature.

Fatigue Crack Growth and Crack Closure
H. Sehitoglu* (Mech. & Indus. Engr.), F. V. Lawrence, Jr.,* D. F. Socie* (Mech. & Indus. Engr.), J. E. Stubbins* (Nucl. Engr.), K. J. Hsia* (Theoret. & Appl. Mech.), N. Chen, H. Hsieh, S. Andrews, T. McGreevy
Fracture Control Program

The aim of this study is to develop a life prediction methodology for fatigue crack growth based on the changes in crack opening levels with maximum stress level, crack length, geometry, mean stress, and microstructure. The primary tool for the determination of opening stress is an elastic-plastic finite-element simulation of fatigue crack growth. Stress-strain behavior in the model accounts for slip at the microlevel as well as elastic anisotrophy. Fatigue crack growth data obtained under conditions of intermediate- and large-scale yielding, including low-cycle fatigue and biaxial loading, are successfully correlated only when closure-modified parameters are employed.

Life Prediction Methods for Notched Members under Nonproportional Multiaxial Fatigue
H. Sehitoglu* (Mech. & Indus. Engr.), F. V. Lawrence, Jr.,* D. F. Socie* (Mech. & Indus. Engr.), J. E. Stubbins* (Nucl. Engr.), K. J. Hsia* (Theoret. & Appl. Mech.), N. Chen, H. Hsieh, S. Andrews, T. McGreevy
Fracture Control Program

To develop fatigue life prediction methods for notched components subjected to nonproportional multiaxial fatigue, the local stresses and strains must be related to the global stresses and strains by some approximation procedure, such as Neuber's rule. Experimental tests on notched shafts subjected to proportional and nonproportional loading in tension and torsion are being performed. The results are being used to develop and verify the approximation procedure. Fatigue life estimates will then be made using an appropriate damage model that is based upon observations made during the tests. A life prediction scheme will be developed from the approximation procedure and the appropriate damage model and will be verified from the results of the tests.

Fatigue Life Prediction of Composites
H. Sehitoglu* (Mech. & Indus. Engr.), F. V. Lawrence, Jr.,* D. F. Socie* (Mech. & Indus. Engr.), J. E. Stubbins* (Nucl. Engr.), K. J. Hsia* (Theoret. & Appl. Mech.), N. Chen, H. Hsieh, S. Andrews, T. McGreevy
Fracture Control Program

Fiber-reinforced sheet molding compound is an attractive material used in ground-vehicle structural applications. It experiences cyclic loading in service, therefore, understanding the fatigue behavior as a function of processing conditions, chemistry of constituents, and loading conditions is important. The purpose of this work is to analyze some of the available fatigue data on these materials and to conduct experiments to identify the nature of damage mechanisms and to study cumulative fatigue damage. Tension-compression testing will be considered to gain insight into mean stress effects. In all these cases, the fiber orientation of the molded part affects the progression of damage.

Durability of Advanced Materials
H. Sehitoglu* (Mech. & Indus. Engr.), F. V. Lawrence, Jr.,* D. F. Socie* (Mech. & Indus. Engr.), J. E. Stubbins* (Nucl. Engr.), K. J. Hsia* (Theoret. & Appl. Mech.), N. Chen, H. Hsieh, S. Andrews, T. McGreevy
Fracture Control Program

Recent developments in processing technology have resulted in advanced materials with lower fabrication costs and improvements in microstructural uniformity. To utilize the full potential of these materials, new design tools have to be developed in collaboration with industry. Examples of such materials include metal matrix composites and short reinforcement fibers in epoxy matrices. The metal matrix composites with higher elastic modulus, higher temperature capabilities, and lower weight compared to their counterparts represent excellent opportunities for engine, brake, and rotating components in the ground vehicle industry.

Probabilistic Methods
H. Sehitoglu* (Mech. & Indus. Engr.), F. V. Lawrence, Jr.,* D. F. Socie* (Mech. & Indus. Engr.), J. E. Stubbins* (Nucl. Engr.), K. J. Hsia* (Theoret. & Appl. Mech.), N. Chen, H. Hsieh, S. Andrews, T. McGreevy
Fracture Control Program

A comprehensive fatigue damage model is being developed to address the following issues: What governs the nucleation of a microcrack within a single grain or other suitable microstructural unit cell? What governs the growth of this microcrack into adjacent microstructural unit cells? When does the microcrack develop enough plasticity to sustain its growth? These elements will be combined into a model for the entire fatigue damage process.

Processing Existing Materials to Enhance Performance and Reduce Cost
H. Sehitoglu* (Mech. & Indus. Engr.), F. V. Lawrence, Jr.,* D. F. Socie* (Mech. & Indus. Engr.), J. E. Stubbins* (Nucl. Engr.), K. J. Hsia* (Theoret. & Appl. Mech.), N. Chen, H. Hsieh, S. Andrews, T. McGreevy
Fracture Control Program

It is no longer possible to specify a material without first considering its processing. In some applications, the so-called old materials processed in new ways are often more cost effective than some of the new advanced materials. Surface treatments such as carburizing and nitriding have been used for many years. Flexible manufacturing processes, such as those using lasers, now offer the potential to modify surfaces selectively to produce superior mechanical properties of traditional lower cost materials.

Cement Paste Rheology
L. J. Struble,* H. Zhang
NSF Center for Advanced Cement-Based Materials

The flow behavior of cement paste is being studied to understand how flow is affected by cement composition. Specific hydration products may affect flow in different ways. The objective is to be able to predict flow behavior in fresh concrete.

Modeling the Rheology of Cement and Concrete
L. J. Struble*
NSF Young Investigator Award

The flow behavior of both fresh cement paste and concrete is being studied in order to understand the relationships between initial microstructure-due to particle packing, flocculation, and early hydration reactions-and flow behavior. The initial microstructure is important because it affects the final microstructure, thereby influencing mechanical properties of hydrated paste or concrete. The flow behavior is important not only because it provides information about the initial microstructure, but also because it controls processing of concrete. A major focus throughout this research is to develop a computer simulation model to explore direct mathematical links between microstructure and flow properties.

Concrete Rheology
L. J. Struble,* R. Variankaval
National Science Foundation, MSS 92-57933 PYI; Portland Cement Assn.

The flow behavior of concrete is important both to the processibility of fresh concrete and to the mechanical behavior of hardened concrete. Methods to measure concrete flow behavior are being explored. Rheological behavior of concrete is being modeled in terms of the proportions of individual constituents, the gradation of the aggregate, and the rheological behavior of the paste. Relationships are being explored between rheology and more general aspects of concrete workability.

Use of Sewage Sludge Ash in Concrete
L. J. Struble,* L. Burley
University of Illinois

A general methodology is being developed for assessing the disposal in concrete of ash from the incineration of wastewater treatment sludge. Experimental studies are underway to determine whether such ashes affect the rate of portland cement hydration or cause formation of undesirable reaction products, and whether they affect engineering properties of the concrete.

Concrete Deterioration in Illinois Highway Pavements
L. J. Struble,* J. Pitlik
Illinois Department of Transportation

Petrographic examination is often used to assess the cause of concrete deterioration. This method has been used with a few specific Illinois highway pavements where deterioration has been observed. Some deterioration was attributed to freeze-thaw damage in the aggregate and some to alkali-silica reaction.

Cement Admixture Interactions
J. F. Young,* J. A. Lewis, H. Matsuyama
Asahi Chemical Co. (In conjunction with the Department of Materials Science and Engineering)

The interactions between superplasticizing admixtures with portland cement are being studied. Zeta potential measurements will be combined with rheological behavior to determine the mechanism of dispersion.

Dimensional Changes in DSP Cement Paste
J. F. Young,* H. Ai
NSF Center for Advanced Cement-Based Materials (In conjunction with Department of Materials Science and Engineering)

Low-porosity cement pastes containing silica fume undergo considerable autogenous shrinkage because of hydration and the pozzolanic reaction. The amount of drying shrinkage of the hardened paste depends on how much shrinkage has occurred during curing. Thermal expansion and contraction accompanying temperature change is also being studied. On heating, initial expansion is followed by a time-dependent thermal contraction which is attributed to moisture redistribution within the pore system. Both shrinkage and thermal change are being correlated with microstructural parameters.

Nanostructure of C-S-H
J. F. Young,* R. J. Kirkpatrick,* G.-K. Sun
NSF Center for Advanced Cement-Based Materials (In conjunction with the Departments of Geology and Materials Science and Engineering)

The environment of substituent atoms, such as Al and Na, in the structure of quasi-crystalline calcium silicate hydrate (C-S-H) is being studied by solid-state NMR spectroscopy. At low Ca/Si molar ratios of C-S-H, Al is in tetrahedral coordination, but at high Ca/Si ratio, octohedral coordination is also observed. The effects of concomitent substitution of both Al and Na are also be examined.

Synthesis of C-S-H/Polymer Complexes
J. F. Young,* H. Matsuyama
Asahi Chemical Co.; NSF Center for Advanced Cement-Based Materials (In conjunction with the Department of Materials Science and Engineering)

Synthetic, high-molecular-weight linear polymers can be intercalated between the layers of the quasi-crystalline C-S-H structure during its synthesis by precipitation. Nonionic, anionic, and cationic polymers can all be intercalated depending on the composition of the C-S-H and the molecular structure of the polymer. Polymers used as high-range water reducers can also be intercalated. Intercalation also occurs when C-S-H is formed from the hydration of b-Ca2SiO4.

Effect of Organic Retarders on the Early Hydration of Cement
J. F. Young,* X. Zhu
NSF Center for Advanced Cement-Based Materials

Organic retarders, such as sugars or hydroxy-carboxylic acids, can delay hydration of cement indefinitely when large doses are used. Analysis of the solution in contact with over-retarded cements confirms that this is caused by the inability of calcium hydroxide to crystalline from supersaturated solutions. The degree of supersaturation attained in retarded systems is the same as that observed just prior to setting in systems that hydrate normally.