Zachary C. Grasley

Carbon Nanotubes and Carbon Nanofibers for Enhancing the Mechanical Properties of Nanocomposite Cementitious Materials

Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) are quickly becoming two of the most promising nanomaterials because of their unique mechanical properties. The size and aspect ratio of CNFs and CNTs mean that they can be distributed on a much finer scale than commonly used microreinforcing fibers. As a result, microcracks are interrupted much more quickly during propagation in a nano- reinforced matrix, producing much smaller crack widths at the point of first contact between the moving crack front and the reinforcement. In this study, untreated CNTs and CNFs are added to cement matrix composites in concentrations of 0.1 and 0.2% by weight of cement. The nanofilaments are dispersed by using an ultrasonic mixer and then cast into molds. Each specimen is tested in a custom-made three-point flexural test fixture to record its mechanical properties; namely, the Youngs modulus, flexural strength, ultimate strain capacity, and fracture toughness, at 7, 14, and 28 days. A scanning electron microscope (SEM) is used to discern the difference between crack bridging and fiber pullout. Test results show that the strength, ductility, and fracture toughness can be improved with the addition of low concentrations of either CNTs or CNFs. DOI: 10.1061/(ASCE)MT.1943-5533.0000266.

Mechanical Properties of Nanocomposite Cement Incorporating Surface-Treated and Untreated Carbon Nanotubes and Carbon Nanofibers

To study the effects of functionalized carbon nanotubes (CNTs) and carbon nanofibers (CNFs) on the mechanical properties of cement composites, both untreated and treated CNFs and CNTs were added to cement paste in concentrations of 0.1% and 0.2% by weight of cement. The surface-treated nanofilaments were functionalized in a solution of sulfuric acid (H2SO4) and nitric acid (HNO3). The nano- filaments were dispersed by using an ultrasonic mixer and were then cast into molds. Each specimen was tested in a custom-made three-point flexural test fixture to record the mechanical properties (i.e., the Youngs modulus, flexural strength, ductility, and modulus of toughness) at the age of 7, 14, and 28 days. The microstructure was analyzed by using a scanning electron microscope. Untreated CNTs and CNFs were found to enhance the mechanical properties of cementitious materials, whereas the acid-treated CNTs and CNFs degraded the mechanical properties. DOI: 10.1061/(ASCE)NM.2153-5477.0000041.

Distribution of Carbon Nanofibers and Nanotubes in Cementitious Composites

Carbon nanofibers (CNFs) and nanotubes (CNTs) are known to be extremely strong and stiff, and their potential as reinforcement has been of interest to many investigators in the past decade. One of the most important keys for fully harnessing the properties of any type of fiber is to control the distribution in the material matrix. As far as CNFs–CNTs are concerned, the strong attraction among nanoscale fibers due to van der Waals forces makes this task difficult. This study focuses on some of the problems that prevent a uniform distribution of CNFs–CNTs in cement paste and the methods used in the past to enhance dispersion. The first phase of the experimental program investigates the effect of using superplasticizers accompanied by sonication on the dispersion of CNFs in water and paste. The second phase focuses on the problem of cement grain size and limitations that the use of fine grain cement causes. Finally, on the basis of results and past studies, suggestions are made for achieving enhanced dispersion of CNFs–CNTs in cement paste.

Challenges and benefits of utilizing carbon nanofilaments in cementitious materials

Carbon nanofibers/tubes (CNF/Ts) are very strong and stiff and as a result, are expected to be capable of enhancing themechanical properties of cementitious materials significantly. Yet there are practical issues concerning the utilization of CNF/Ts in cementitious materials. This study summarizes some of the past efforts made by different investigators for utilizing carbon nanofilaments in cementitious materials and also reports recent experimental research performed by the authors on the mechanical properties of CNF-reinforced hardened cement paste. The major difficulties concerning the utilization of CNF/Ts in cementitious materials are introduced and discussed. Most of these difficulties are related to the poor dispersibility of CNF/Ts. However, the findings from the research presented in this work indicate that, despite these difficulties, carbon nanofilaments can significantly improve the mechanical properties of cementitious materials. The results show that CNFs, even when poorly dispersed within the cementitious matrix, can remarkably increase the flexural strength and cracking resistance of concrete subjected to drying conditions.

Modeling drying shrinkage stress gradients in concrete

Cracking is a problem for the serviceability, durability, and aesthetic quality of concrete structures and pavements. The potential for drying shrinkage cracking in concrete is related, in part, to the development of a moisture gradient across the cross-section of the concrete element. This article reports on a model that has been developed that incorporates experimental measurements of internal relative humidity to investigate drying shrinkage stress gradients in concrete specimens. The authors outline the experimental and analytical procedure for using the model. The authors conclude that initial research involving application of the model has indicated that there is a relationship between drying stress gradient severity and the time to cracking under full restraint. Further development of the model could involve combination with fracture models to investigate microcrack formation and propagation.

Viscoelastic Poisson's Ratio of Asphalt Mixtures

AbstractThe Poisson’s ratio (PR) of asphalt mixtures is a fundamental material property that is an important input parameter to viscoelastic pavement models. It is typically assumed that PR of asphalt is time independent or, in cases where the time dependency of viscoelastic PR is considered, calculated as the negative ratio of the time-dependent transverse and axial strains measured in a uniaxial creep test. This paper presents analytical and experimental results demonstrating the errors involved in these assumptions. The first part of this study derives an expression for viscoelastic PR. The second part of this study reports the results of various experimental tests to measure axial and transverse strains of asphalt concrete specimens under different loading conditions. The viscoelastic PR increased with time in the tensile relaxation test and ramped tensile test, whereas it slightly increased at the beginning of the compressive relaxation test and was subsequently virtually constant. The error introduc...

Mapping Voids, Debonding, Delaminations, Moisture, and Other Defects Behind or Within Tunnel Linings

Periodic monitoring of tunnel conditions and deterioration rates is the answer to determining the appropriate schedule of maintenance or rehabilitation activities to remedy structural problems that could lead to rapid deterioration and unexpected tunnel failures. The aggressive environmental conditions in which tunnels exist, as well as the need to keep tunnels open to traffic, make their inspection a challenge. Nondestructive testingmethods that are automated, quantitative, and rapid, and that provide complete coverage compared with conventional visual inspections, could solve this dilemma. This report presents the findings of the Strategic Highway Research Program 2 (SHRP 2) Renewal Project R06G—High-Speed Nondestructive Testing Methods for Mapping Voids, Debonding, Delaminations, Moisture, and Other Defects Behind or Within Tunnel Linings. The study was divided into two phases to (1) establish testing criteria and prioritize the techniques to be developed and evaluated under the project on the basis of tunnel operators’ requirements and (2) conduct the necessary technology development for those techniques recommended. In addition to conducting technology development, the project performed proof-of-concept and field testing. Beyond this report, the deliverables for this project include two products that will be published separately: 1) a user’s manual, which provides information on three NDT technologies for inspection of tunnels; and 2) a brief manual to the analysis software Tunnelcheck, which was developed under this project.

Utilization of Silica Fume to Stabilize the Dispersion of Carbon Nanofilaments in Cement Paste

AbstractThe incorporation of carbon nanofilaments in cementitious materials has been researched extensively in the past decade. However, the poor dispersibility of these filaments remains a hurdle to harnessing their exceptional properties. This work presents a method for enhancing the dispersion of carbon nanofibers (CNFs) in cementitious matrices. The method incorporates silica fume to stabilize the distribution of CNFs in fresh cement paste. The investigations show that this method is significantly more effective than similar approaches. To examine the effectiveness of the method, specimens of CNF incorporated cement paste with different concentrations of silica fume were produced and imaged by optical microscopy. Using a novel dispersion quantification method, the dispersion values of CNFs in the specimens were measured and compared. The results show that silica fume, if used in sufficient proportions, can largely prevent the reagglomeration of CNFs in fresh cement paste, and thereby stabilize their d...

A Comparison of Test Methods for Early-Age Behavior of Cementitious Materials

A wide range of tests has been reported that can be used for probing different aspects of the chemical and structural properties of cement paste and concrete at early ages. In principle, these tests should be complementary and their results could be integrated to develop a broader understanding of early-age behaviour. Under the auspices of the Center for Advanced Cement Based Materials (ACBM), an inter-laboratory study was undertaken to compare the results of several of these different test methods on a single cement. A large quantity of cement was homogenized by the Cement and Concrete Reference Laboratory (CCRL), as part of their proficiency sample program, and distributed to several ACBM researchers for independent testing. Aggregates for making mortar were supplied from a single source (graded Ottawa sand). Tests on pastes and mortars included calorimetry, compressive strength, Vicat needle penetration, chemical shrinkage, autogenous shrinkage, AC-impedance spectroscopy, time domain reflectometry, and ultrasonic shear wave reflectometry. Numerical simulations of the same materials were undertaken for comparison. Results from these tests will be compared with respect to the way in which they capture solidification and strength development.

Mechanical Properties of Concrete Pavement Mixtures with Larger Size Coarse Aggregate

The use of larger maximum size aggregates (MSA) in concrete mixtures can increase the load transfer efficiency across aggregate interlock joints if small crack widths are achieved. Aggregate interlock joints can minimize the number of dowels required on airport and low volume concrete pavements, and thus decrease their total construction cost. This paper describes a laboratory study evaluating the strength, fracture, and shrinkage properties of airport concrete mixtures with 1 and 1.5 inch MSA and several total cementitious contents. Although the split tensile and flexural strength of the larger MSA mixtures were reduced by 20 percent, the 28-day fracture energy was similar between the mixtures. The 1-day GF was significantly greater for the larger MSA mixture indicating a greater joint shear transfer capability. The brittleness of the concrete was also reduced as the MSA increased. Finally, the concrete free shrinkage was similar for the same mix proportions but different MSA.