5th Pan American Conference for NDT 2-6 October 2011, Cancun, Mexico
Scanning Electron Microscopy to Examine Concrete with Carbon Nanofibers Shane M. PALMQUIST 1, Edward KINTZEL 2, Keith ANDREW 2 1
Department of Engineering, Western Kentucky University; Bowling Green, Kentucky, United States Phone: +1 2707 452919, Fax: +1 2707 455856; e-mail:
[email protected] 2 Department of Physics & Astronomy, Western Kentucky University; Bowling Green, Kentucky, United States Phone: +1 2707 454357, Fax: +1 2707 452014; e-mail:
[email protected],
[email protected] Abstract Mechanical properties of concrete are most commonly determined using destructive tests including: compression, flexure, and fracture notch specimen tests. However, nondestructive tests exist for evaluating the properties of concrete such as ultrasonic pulse velocity and impact echo tests. One of major issues with concrete is that unlike steel it is quasi brittle material. It tends to want to crack when tensile stresses develop. These cracks generally develop at the interfacial transition zone, ITZ, between the cement paste and the aggregate. Fibers have been added to concrete for many years to help with temperature and shrinkage cracks. In more recent years, the concepts of adding fibers with enhanced properties such as carbon nanofibers, CNFs, to concrete have been explored. Some possibilities include developing concrete that may be more durable, flexible, stronger, less permeable, and potentially “crack free” than traditional concrete. Based on SEM images and quantitative data from the literature, this paper examines the ITZ of concrete made with CNFs. Results provide greater understanding on the nature of the ITZ region in concrete made with CNFs. Keywords: Concrete, cementitious materials, fibers, carbon nanofibers (CNFs), interfacial transition zone (ITZ)
1. Introduction For many years, fibers have been added to concrete mixes in low volume dosages for the purposes of reducing plastic shrinkage cracking [1]. However, they do not significantly affect the free shrinkage of concrete nor do they provide increased ductility. Of particular importance is the understanding and interaction of small fibers with the cementitious mix at the micro and nanoscales. Figure 1 shows the surface of a cementitious material at the micron level, which was taken at Western Kentucky University by a Large Scanning Electron Microscope, LSEM. This LSEM is one of only two in the world that are capable of testing full size members and specimens. In the figure, darkened regions throughout indicate locations of potential microcracks, voids and areas of higher porosity. Connecting more structured regions to other weaker regions can improve the performance of concrete. Adding fibers at the smaller scales and in greater volume dosages can have an impact at the macroscale in terms of crack lengths, crack widths, degree of spalding, and crack pattern [2]. Many factors affect the performance of fibers in concrete and cementitious materials. These include: fiber material, fiber length, fiber diameter, surface texture of the fiber, fiber orientation in the cementitious composite, fiber distribution in the composite, and bond between the fiber and the composite. While any one factor may have a significant effect on the composite performance, bond is critical.
2. Fibers in Concrete 2.1 Fiber Bond Developing a compatible bond between the fibers and the cementitious material is crucial to improving the properties of the concrete. If a perfect bond or overly strong bond develops
between the fibers and the cementitious material, the resulting concrete will not perform well. Fibers in the concrete should bridge the crack widths that form and close them as they try to develop and open. A near perfect bond results in a highly localized strain in a very small region of a fiber causing it to potentially break prematurely. Likewise, if a poor bond or overly weak bond develops between the fibers and the cementitious material, the resulting concrete will not perform well either. The fibers, which bridge the crack widths, will not get activated when cracks develop and open. Fiber pull-out will result. Figure 2 shows the bottom side view of a four point bend test where a good bond between the fibers and the composite developed. As soon as a crack appeared and began to grow, the bridging microfibers closed the crack. Afterward another crack appeared and was closed. This repeated until final failure occurred in a dominating crack. In this case, adding compatible microfibers improved the ductility of the concrete [3].
Figure 1. SEM image of the surface of a cementitious material
Figure 2. Four point bend test showing multiple cracks on underside
2.2 Fiber Dispersion Fiber distribution refers to the ability of the fibers to evenly disperse in the cementitious mix. Failure of the fibers to properly disperse evenly throughout the mix results in fiber clumping. Clumping can severely weaken the strength of the overall material and cause the concrete to be inhomogeneous. Random pockets of clumped fibers will result in voids. Fiber distribution is generally handled at the product level, where fibers are engineered to segregate evenly into the concrete upon being added to the mix. However, verification is required and should not be assumed. 2.3 Carbon Nanofibers Failure of concrete materials is caused by microcracks that propagate and grow. These cracks often occur much earlier and can only be observed at the nanoscale [4]. Carbon nanofibers, CNFs, have many advantages as a reinforced material for cements as compared to traditional fibers. CNFs have superior tensile strengths (ranging from 10 to 63 GPa) compared to other types of traditional fibers. This can improve the mechanical properties of the concrete. CNFs have very small diameters which can be distributed more widely in the concrete with reduced fiber spacing than traditional larger fibers. CNFs have very high length-to-width ratios, requiring significantly higher energies for crack propagation around the fibers. Figure 3 shows CNFs bridging a crack [4]. However, proper dispersion of CNFs in concrete is critical.
Figure 3. CNFs bridging a crack [4]
3. Interfacial transition zone 2.1 Wall effect Concrete and cementitious materials generally consist of two distinctly different regions, the cement paste and the aggregate. Fibers act like a specialized aggregate and only occupy a very small portion of the total volume. In general, there is a tendency to assume that the properties of the cement paste and the aggregate in the mix are unaffected by the presence of
the other. However, the presence of the two in the mixture will result in an interaction between them that corresponds to a negative impact on the performance of the hardened concrete material. Cement particles in fresh concrete cannot efficiently pack together when they are in close proximity of larger solid objects, such as aggregates or fibers. However, much smaller cement and fibers associated with micro and nano materials may potentially improve this. The potential packing problem due to size compatibility is generally referred to as the “wall effect.” For typical concrete with normal sized aggregates and common temperature and shrinkage fibers, this effect is magnified by the shearing stresses exerted on the cement paste by the aggregate and fibers during the mixing process, which tend to cause the water to separate from the cement particles near the surfaces of the aggregate and fibers. The result is a narrow region surrounding the aggregate and fibers with fewer cement particles, and thus more water. This region is often called the interfacial transition zone (ITZ). Due to the varying types, amounts and sizes of the components in concrete materials as well as the mixing and casting processes involved, the ITZ are not specific zones but are regions of transition. The formation of the ITZ also occurs along mold interfaces since the mold itself acts like a very large aggregate as compared to the cement paste. Figure 4 shows the ITZ regions around all aggregates, fibers and mold faces. ITZ around aggregate Aggregate
Cement paste
ITZ along face of mold
Fiber ITZ around fiber
ITZ along face of mold
Figure 4. Cementitious material showing ITZ regions 2.2 ITZ properties The ITZ is a region with a higher w/c ratio than the cement paste, and thus a higher porosity than the cement paste. The ITZ region is not uniform but varies by location as the distance away from the surface of the aggregate or fiber increases as shown in Figure 5 [5]. In this figure, the aggregate is shown on the left. Thin white lines running vertically in this figure indicate distances of 20 and 50 µm from the surface of the aggregate. As mentioned earlier and as seen in Figure 5, the ITZ is not a region but a zone of transition. There is no discrete boundary between the ITZ region and the cement paste. The size of the ITZ region is generally believed to be 15 to 20 µm in size [5,6]. The size of the ITZ region tends to be
larger around larger aggregates and fibers and smaller around smaller aggregates and fibers. Because of the larger pores, the ITZ regions are characterized by the presence of larger crystals, particularly of calcium hydroxide, CA(OH)2, than are found in the cement paste. Figure 6 presents some of the data presented by Scrivener et al. [5]. This data gives the distribution of unhydrated cement in concrete at various ages for a sample of concrete with a w/c ratio of 0.4. Due to the “wall effect” upon mixing of the concrete, there are less cement grains in the fresh state near the surface of an aggregate or fiber than further away in the cement paste region. Thus, the water-to-cement ratio in the ITZ region is higher than in the cement paste. This is shown in Figure 6. At mixing (calculated), the volume fraction of unhydrated cement increases rapidly from 0 to 31 percent by volume as the distance from the surface of the aggregate increases from 0 to 3 µm. The high water-to-cement ratio of the ITZ indicates that this region is weaker than the cement paste. As the distance from the surface of the aggregate increases, the volume fraction of unhydrated cement increases. Thus, the ITZ is a region of transition. At a distance of 20 µm from the surface of the aggregate, the change in the volume fraction of unhydrated cement for mixing, 1 day, 28 days, and 1 year are: 45, 21, 13, and 4.5 percent per µm, respectively. For the data corresponding to mixing and 1 year, these values occurred earlier at approximately 11 µm from the surface of the aggregate.
Figure 5. Aggregate on left, white lines are at distances of 20 and 50 µm from interface [5] From Figure 6, the initial change (increase) in volume fraction of unhydrated cement per µm from the surface of the aggregate can be calculated. These rates correspond to: 4.75, 1.33, 0.67, and 0.27 (change in percent per µm) for mixing, 1 day, 28 day, and 1 year, respectively. Clearly, there is a significant change in the rate from mixing and 1 day. A significant amount of hydration begins upon mixing where larger crystals develop. Upon hydration, the cement particles are porous in structure with a size distribution that ranges from the nanometres to millimetres. These pores are the weak areas providing pathways for chemical attacks which result in cracking and deterioration [7]. These weak areas affect the performance of the cement paste as well as the ITZ regions. High
performance fibers such as CNFs may be able to fill these voids and make these regions more structured. The CNFs have ideal mechanical and fracture properties which will benefit the resulting cementitious material.
Figure 6. Distribution of unhydrated cement in concrete at various ages (w/c = 0.4) [5]
3. Discussions The ITZ directly affects the properties of concrete, since the ITZ acts as the "weak link” in the chain when compared to the cement paste or the aggregate or the fibers. Thus, as the strength and stiffness of the ITZ regions decrease, the strength and stiffness of the resulting concrete also decrease. The total volume of the ITZ regions increases as the total amount of aggregate increases and as the average size of the aggregate increases. Decreasing the volume of the ITZ regions is important for enhancing the structural properties of concrete. This is clearly evident in Figure 5 where areas of voids are visible in the ITZ region. If the ITZ regions could be tailored to have the same properties as the cement paste or very close, this would be ideal. Using very fine aggregates and CNFs that are closer in size to the cement particles offers a good approach to decreasing the total volume of the ITZ regions, and this would increase the mechanical performance of the resulting concrete material. However, CNFs are expensive, but prices are coming down.
4. Conclusions This paper examined the ITZ regions in concrete with CNFs. ITZ is a region of transition from a lower w/c ratio to a value found in the cement paste. The size of the ITZ region ranges from about 15 to 20 µm in thickness. Based on data provided by Scrivener et al. (2004), the initial rate of increase in percent of unhydrated cement per µm from the surface of the aggregate for standard concrete was calculated to be 4.75, 1.33, 0.67, and 0.27 for mixing, 1 day, 28 days, and 1 day, respectively. At mixing, the water-to-cement ratio at the surface of the aggregate was very high.
The ITZ regions act as the “weak link” so making these regions stronger will enhance the overall performance of the material. One way is to use CNFs. These fibers have superior mechanical properties; however, fiber dispersion and bonding are still challenging. Another issue with the CNFs is cost. References 1.S H Kosmatka, B Kerkhoff, and W C Panarese, „Design and Control of Concrete Mixtures‟, Portland Cement Association, 14th edition, Skokie, Illinois, 2006. 2.S P Shah, W J Weiss, and W Yang, „Shrinkage Cracking – Can it be prevented?‟, Concrete International, American Concrete Institute, Farmington Hills, Michigan, pp 51-55, April 1998. 3.S M Palmquist, „Ductile Concrete using Structural Fibers‟, Proceeding, Precast Concrete Institute (PCI), 54th Annual Convention and Exhibition, 2008. 4.A Keyvani, „Huge Opportunities for industry of Nanofibrous Concrete Technology, International Journal of Nanoscience and Nanotechnology, Vol. 3, No. 1, pp 3-11, December 2007. 5.K L Scrivener, A K Crumbie, and P Laugesen, „The Interfacial Transition Zone (ITZ) Between Cement Paste and Aggregate in Concrete‟, Interface Science, pp 411-421, 2004. 6.J F Young, S Mindess, R J Gray, and A Bentur, „The Science and Technology of Civil Engineering Materials‟, Prentice-Hall, pp 186, 1998. 7.A Keyvani, S Noboru, and et. al, „Effect of Galvanized Steel Fibers on Corrosion Protection of Reinforced Concrete‟, J. of the The Japan Soc. of Civil Engineers (JSCE), pp 35-46, August 2000.
