SWUTC Research Project Description

Title of Project:  Assessment the Potential of Using Carbon Nanotubes Reinforcements for Improving the Tensile/Flexural Strength and Fracture Toughness of Portland Cement Paste for Damage Resistant Concrete Transportation Infrastructures

Project Number:  476660-00011

Principal Investigator:
Rashid Abu Al-Rub
(979) 862-6603
P.I. Affiliation:  Texas A&M University
rabualrub@civil.tamu.edu

Project Monitor:
Cemal Basaran
University at Buffalo, The State University of New York
Department of Civil, Structural, and Environmental Engineering
243 Ketter Hall
Baffalo, NY 14260
(716) 645-2114 ext. 2429
cjb@eng.buffalo.edu

Project Status:  Active

Date Started:  9/1/08

Estimation Completion Date:  8/31/09

Estimated Cost - Current Fiscal:  $72,750

Estimated Cost - Total Planned:  $72,750

Project Summary:
Project Abstract:
The focus of the proposed research will be on exploring the use of nanotechnology-based carbon nanotubes (CNTs) reinforcements in improving the mechanical properties of Portland cement paste as a construction material. Due to their ultra-high strength and very high aspect ratios, CNTs have been used as excellent reinforcements in enhancing the physical and mechanical properties of polymer, metallic, and ceramic composites to be used in several electronic and biomedical devices and sensors. Very little attention has been devoted on exploring the use of CNTs in the transportation industry. Therefore, this study aims to bridge the gap between the nanotechnology-based CNTs and the transportation materials. This will be achieved by testing the integration of CNTs in ordinary Portland cement paste through adapting the state-of-the-art techniques in utilizing CNTs as excellent reinforcements in other materials like polymers. Different mixes with fixed classical strengthening factors (e.g. water-to-cement ratio, air content, admixtures) and variable CNTs concentrations will be prepared. Different techniques for other materials (like polymers) will be used in achieving uniform dispersion of CNTs in the cement paste matrix and strong CNT-cement bonding. Small-scale specimens will be prepared for mechanical testing in order to measure the modified mechanical properties as a function of CNTs concentration, type, and distribution. Micromechanical characterization of the undeformed and deformed microstructures will be conducted to understand the role of CNTs dispersion, bond strength, and reinforcing mechanisms in arresting micro-crack propagation through the cement matrix.  

Project Objectives:
The main objective of the proposed research activities is in investigating the unexplored potential use of CNTs as nano reinforcements in improving the tensile/flexural strength and fracture toughness of Portland cement paste, which is one of the largest commodities consumed worldwide. CNTs are carbon fibers with diameters on the nanometer scale. They have a combination of desirable multifunctional properties such as high mechanical strength and stiffness, and thermal conductivity that make them ideal candidates for use in concrete materials. CNTs will be added to the cement past as reinforcements to act as bridges across micro-cracks to form reinforcing mechanisms and arrest cracking in the cement matrix. Due to their very high surface area to volume ratio, they can act as excellent reinforcements and in carrying the internal tensile stresses. They may improve significantly the flexural strength, tensile strength, fracture toughness, and even the compressive strength of cement matrix composites.

Task Descriptions:
Task 1. Literature Review and State-of-the-Art
A comprehensive review will be conducted in this task to identify the different methods that are currently used in dispersing CNTs in the matrix materials and in enhancing the bonding between the CNT and the matrix.

It is known that there are two main obstacles in using CNTs as excellent reinforcement fibers in matrix materials such as polymers, metals, and ceramics. The first obstacle lies in the difficulty to achieve uniform dispersion of CNTs in the matrix material. CNTs tend to adhere together after purification due to Van der Waal’s forces, making a uniform distribution of individual tubes particularly difficult to achieve. The second obstacle lies in achieving suitable CNT-matrix bonding such that typical CNT matrix composites experience fiber pullout under low loads and, therefore, do not achieve high strengths. Therefore, the aforementioned obstacles are the main problems in hindering the wide use and application of CNTs in many industries. However, there have been different methods to overcome these obstacles that will be reviewed and understood during this task.

Task 2. Cement and Carbon Nanotubes Integration
Hydrated cement is a brittle material such that it is much stronger in compression than in tension. Various forms of reinforcements, typically in the form of rods or fibers are added to the cement past to compensate for its weakness in tension. The method that will be employed in this task is the integration of CNTs in Portland cement paste. The strength of cement paste depends upon a number of factors, which include, amongst others, the water-to-cement ratio and degree of air voids. Cement in itself has a complex nanoscale structure. Therefore, some of the properties that affect the strength of cement are expected to act at the nanoscale. However, this same nanostructure and the chemical processes that produce it open possibilities for using CNTs to modify the cement tensile strength.

Different mixes of Portland cement paste will be prepared by systematically varying the weight content of CNTs and their dispersion through the cement matrix. However, all other strength improving factors such as water-to-cement ratio, air content, and use of admixtures will be kept the same for all the cement paste mixes. A plain cement paste mix will be prepared with zero CNTs concentration and will be taken as a reference to study the impact of the addition of the CNTs in the different mixes. The nanotubes will be obtained commercially in order to move quickly to mixing, specimen preparation, testing, and characterization. Single-walled and multi-walled CNTs will be purchased functionalized from a supplier such as Cheap Tubes, Inc. Functionalized CNTs will be used to achieve uniform dispersion of the CNTs within the cement matrix. Moreover, CNTs will be functionalized to chemically react with cement components, providing routes for other forms of interaction and cement system property control. The most promising techniques in achieving both uniform dispersion of CNTs and strong CNT-cement bonding, which will be identified in Task 1, will be employed in this study.

Task 3. Mechanical Testing
In this task, small-scale cylindrical, prismatic, and beam-like specimens will be prepared from the different cement paste mixes of different concentrations of functionalized CNTs. The prepared specimens will be tested under tensile, flexural (three point bending), compressive, and shear loading conditions and the associated mechanical response and properties (e.g. Young’s modulus, tensile/compressive strengths, and fracture toughness) will be compared with various CNTs concentrations and ordinary plain cement paste sample. Moreover, mechanical property testing by dynamic mechanical analysis (DMA) will be used to observe the change in the storage modulus (elastic response), loss modulus (viscous response), and loss factor (loss modulus/storage modulus). If time allows, nanoindentation testing will be carried out to measure the Young’s modulus and hardness of the cement paste composite samples. For the mechanical testing, appropriate sample preparation protocols and conditioning will be developed to impart the same preparation conditions to all of the samples. Some of the tests will be conducted in the concrete laboratory in the Civil Engineering Department and others will be tested in the Materials Characterization Facility (MCF) at Texas A&M University.

Task 4. Micromechanical Characterization
The role of CNTs in improving the control of cement paste porosity and their ability to suppress the evolution and propagation of cracks in the cement paste will be investigated in this task. Different microscopy characterization techniques that include scanning electron microscope (SEM), transmission electron microscope (TEM), atomic force microscope (AFM), and others will be used to investigate the dispersion and reinforcing mechanisms of CNTs. TEM and AFM will be used when the length scale requires it. Conclusions and recommendations will be drawn concerning the role of CNTs reinforcements in their ability to suppress the evolution and propagation of micro-cracks in the cement paste. These micromechanical characterizations will be conducted at the MCF and Microscopy and Imaging Center at Texas A&M University.

Task 5. Final Report
A final report will be prepared at the end of the project.  This final report will document the findings from this study including literature review, a detailed description of the mixing of CNTs and mechanical testing and micromechanical characterization, and recommendations for tailoring cement paste materials mechanical properties through the use of CNTs. Additionally, the testing protocols for the successful techniques will be documented. The final report will also include recommendations for future research.

Index Terms:
Portland cement concrete, Infrastructure, Flexural strength, Tensile strength, Carbon, Fracture mechanics, State of the art, Water cement ratio, Air content, Admixtures, Research projects