Crack Free Concrete Made With Nanofiber Reinforcement

Advanced technological aspects of cement based materials have recently focused on developing high performance cement composites, which possess high compressive strength. However, such composites exhibit extremely brittle failure, low tensile capacity and are sensitive to early age microcracking as a result of volumetric changes due to high autogenous shrinkage. These characteristics of cement based materials are serious shortcomings that not only impose constraints in structural design, but also affect the long term durability of structures. To overcome the aforementioned disadvantages reinforcement of cementitious materials is typically provided at the millimeter and/or the micro scale using macrofibers and microfibers, respectively [1]. Cementitious matrices, however, exhibit flaws at the nanoscale, where traditional reinforcement is not effective. The goal of the proposed research is to take a leap forward to the nano level. It is envisioned that problems which cannot be solved with existing technology can be addressed with the use and manipulation of nano component materials. Nanofibers, such as carbon nanotubes (CNTs) and carbon nanofibers (CNFs), are expected to have several distinct advantages as a reinforcing material for cement based materials as compared to more traditional fibers. First, they exhibit significantly greater strength and stiffness [2-3] than conventional fibers, which should improve overall mechanical behavior. Second, their higher aspect ratio is expected to effectively arrest the nanocracks and demand significantly higher energy for crack propagation. Thirdly, provided that are uniformly dispersed, and due to their nanoscale diameter, nanofibers are expected to be closely spaced in the cement matrix. As illustrated by Akkaya et al. [4], the tensile strength of the composite is increased when the fiber free area is decreased. Nanofibers are expected to control the formation of nanocracks in the matrix, producing a high-performance cementitious nanocomposite. Additionally, CNTs exhibit very interesting electromechanical properties [5-6]. When subject to stress/strain, the electrical properties of CNTs change, expressing a linear and reversible piezoresistive response. Previous work on polymers has shown that CNT nanocomposites could be used as stress/strain sensors [7-8]. These superior mechanical, electrical and chemical properties of CNTs offer exciting opportunities for the research and development of new advanced high performance cementitious nanocomposites. Research conducted in the previous year has focused on solving the two major drawbacks associated with the incorporation of nanofibers in any type of material, which are poor dispersion and cost. Nanofibers tend to adhere together due to Van der Waal forces and it is particularly difficult to separate them individually [9]. To achieve good reinforcement in a composite, it is critical to have uniform dispersion of the nanofibers within the matrix [10]. The results have shown that by applying ultrasonic energy and use of a surfactant, CNTs and CNFs can be effectively dispersed in the cementitious matrix. As a result of effective dispersion the fracture properties of cement matrices were substantially increased by adding a very low amount of CNTs/CNFs [0.025wt% to 0.08wt% of cement]. The use of CNTs at this very low percentage makes the cost of the material very attractive. The scanning electron microscopy results have shown that the addition of this small quantity of CNTs enables the control of the matrix cracks at the nanoscale level. The nanoindentation results indicate that CNTs can strongly modify and reinforce the nanostructure of the cementitious matrix by increasing the amount of high stiffness C-S-H and reducing the nanoporosity. Besides the benefits of reinforcement, autogenous shrinkage results indicate that CNTs can also have beneficial effect on the transport properties of cementitious materials, which leads to improved durability of the cement matrix. 2 During the previous funding year a U.S. Formal Patent Application was filed, with the contribution of ITI. This is an important commercialization tool that dramatically increases the exposure of this work to companies and practice. Already, various companies have been interested in commercializing the product. The goal for the current proposal year is to further investigate the practical use of the material. In that aspect, the use of nanofibers in concrete will be studied. Additionally, nanofibers will be used to improve other properties such as conductivity, piezoresistivity and the transport properties, for applications such as reduction of cracking due to shrinkage and structural health monitoring. Considering the keen interests in practice, the research team has set its priority to concentrate on the following tasks: 1. Study and optimization of hybrid mixes with CNTs and carbon nanofibers to make recommendations for the highest quality product 2. Investigate the interaction between C-S-H and nanofibers using techniques such as FTIR, TEM, MIP to discover the bonding mechanisms and further improve the properties at the nanoscale 3. Study the effect of nanofibers on autogenous shrinkage cracking for concrete 4. Explore the electrical conductivity of the nanocomposites (the use of carbon fibers has shown to increase the electrical conductivity of cement paste composites by orders of magnitude) 5. Developing and optimizing hybrid mixes containing CNTs, nanofibers and microfibers 6. Developing and optimizing hybrid mixes with CNTs, nanofibers, microfibers and macrofibers for the ultimate goal of reaching a crack free material 7. Explore the piezoresistive response of the nanocomposites and the capability of CNTs to be used as sensors in concrete 8. Develop a quality control test to be used in the industry 9. Study the failure mechanism of the hybrid mixes to explore the methods by which different fibers interact in cementitious matrices ACBM will focus on delivering the tasks 1-5 and 9 for the proposal year. This new composite material will have revolutionary impact on economics, environment, and structural durability, which is extremely important to the overall public view. The potential benefits of infrastructure having such a new type of composite material includes the following: * Enhancement of the quality of cement and improve material durability by delaying the crack formation and increase the likelihood of multiple cracking * Reduced maintenance and construction costs * Development of a high performance material which will produce durable, aesthetically pleasing and cost-effective repairs * Development of a nanocomposite material with improved piezoresistive response to monitor the structural health The research results will find wide applications for highway structures, bridges, pavements, runways for airports, continuous slab-type sleepers for high speed trains and in general in all applications of conventional and high strength concrete, as well as in manufactured precast elements for residential and commercial buildings. 2 Introduction Concrete is heterogeneous at all length scales with a very complex micro- and nano-structure [11]. A significant portion of today's civil infrastructure is partially or completely constructed out of cementitious materials, such as concrete. Cementitious materials are typically characterized as brittle materials with low tensile strength and strain capacity and are sensitive to microcracking. Fibers are incorporated into cementitious matrices to overcome these weaknesses, producing materials with increased tensile strength, ductility, toughness and improved durability [1]. Fibers influence the fracture behavior of cementitious composites by interacting with the matrix they reinforce. To understand which shapes, sizes and types of fibers are most effective, one must consider the me