Recent nanoscience research improves understanding of cement and concrete properties and looks to the next generation of highway pavements.

 

 

Even as traffic on the Nation's highways has increased from 65 million cars and trucks in 1955 to almost 246 million today, the condition of U.S. highways and bridges has deteriorated. According to estimates by the U.S. Department of Transportation, the current backlog of unfunded but needed repairs and improvements totals $495 billion.

The increased traffic volume has generated an escalating need for high-performance, durable construction materials for roadway pavements. This need, in turn, is driving research to develop the next generation of materials.

In a research project supported by the Federal Highway Administration (FHWA), rather than designing a concrete mix by trial and error, researchers at the Center for Advanced Cement-Based Materials (ACBM) took a back-to-basics approach. That is, they chose to pursue a systematic study of concrete at the micro- and nanoscales to understand the properties of how materials in concrete interact with one another. This study at the smaller scale could enable them to develop more effective solutions that achieve the desired performance.

At all scales, concrete is a heterogeneous material. Concrete's strength and durability depend on structural elements and phenomena at micro- and nanoscales. (A nanometer is one billionth of a meter.) To understand concrete properly, to better control its properties, and to design new materials with specific properties, starting at the smallest scale is necessary—that is, understanding the micro- and nanostructure is the first step. Nanotechnology research today provides the necessary tools for establishing the relationships between the processing, properties, and performance of concrete.

The research project, which started in September 2004 and ended in August 2006, was funded by FHWA and involved a number of research themes. The outcomes of this project led to further research focused on additional themes that are supported by multiple organizations. In the recent research, the first theme involved understanding the micro- and nanostructures of concrete using advanced experimental tools such as atomic force microscopy, which uses a high-resolution probe to measure properties, and nanoindentation, which consists of a set of tests for investigating hardness and other mechanical properties of materials in small dimensions.

The second theme is development of a new type of self-consolidating concrete (SCC) for slipform (SF) paving processes by adding materials such as nanoclays (very small, plate-like, water-absorbent minerals) and fly ash to the composition. In Self-Consolidating Concrete (ACI 237R-07 Emerging Technology Series), the American Concrete Institute (ACI) defines SCC as "highly flowable, nonsegregating concrete that can spread into place, fill the formwork, and encapsulate the reinforcement without any mechanical consolidation [or vibration]." ACI continues, "SCC has also been described as self-compacting concrete, self-placing concrete, and self-leveling concrete." For SF applications, the placed concrete mixture also must become firm enough to hold the vertical pavement edge as the paving machine moves forward. Therefore, the new materials used to make SF-SCC (that is, slipform self-consolidating concrete) require cutting-edge technological and scientific developments for studying the fresh-state properties of SF-SCC. Fundamental research on particle packing and flocculation (particles aggregated to form a structure) mechanisms provided insight on how to eliminate internal vibration and durability issues associated with longitudinal cracking along the vibration trail.

Third, the researchers are using nanofiber (fibers with diameters less than 100 nanometers)-reinforced concrete to develop the next generation of highway pavements.

Fourth, nanotechnology shows promise in the development of smart sensors. Because concrete develops its properties (such as strength) with time and chemical reactions (called hydration), it is critical to monitor concrete at early ages. In this effort, the researchers developed a new method to monitor the properties of concrete pavement at its early ages just after it is placed. This new method relies on measuring the amount of ultrasonic sound wave returned after striking the surface of concrete pavement.

Characterization of Cement And Concrete at the Nanoscale

Hydration of cement produces a rigid, heterogeneous microstructure. As water is introduced to cement to make a paste, which hardens over time, the main microstructural phases in the hydrated cement paste are: (1) calcium silicate hydrate gel, C‑S‑H; (2) calcium hydroxide, CH; (3) ettringite (a sulfoaluminate hydrate); (4) monosulfate; (5) unhydrated cement particles; and (6) air voids. These microstructural phases govern the macroscopic properties of cementitious materials, such as strength, ductility (pliability), early-age rheology (flow), and durability. Controlling the macroscopic properties demands a detailed knowledge of the structure of these phases at the smallest size level. Among the various phases, the first one—C‑S‑H—is the most important product of hydration and accounts for 50 to 70 percent of the total paste volume. This main binding phase governs the macroscopic properties of the cement paste, but the micro- and nanoscale structure of C-S-H is not well established.

 

 

In the study, the research team used an atomic force microscope (AFM) to image the surface of 6-month-old cement paste samples. The samples were from type I portland cement with a water-to-cement ratio of 0.5. The AFM images show C-S-H as nearly spherical particles of different sizes in different areas.

The research team also combined nanomechanical testing with imaging at the nanoscale. The team did this pioneering research using a special type of nanoindenter. In any indentation technique, one material of known properties is pushed inside the material of unknown mechanical properties. This technique originated from the Mohs scale of mineral hardness developed in 1812, in which one material is considered to be harder if it can leave a permanent scratch on another material. In nanoindentation, a researcher pushes a small probe into a hardened concrete sample and then plots the load applied by the probe versus its displacement in the sample. Next, the researcher analyzes the data obtained from the plots to estimate the elastic modulus (slope of stress versus strain curve) and hardness of the sample. At the normal scale, this is similar to a strength test and modulus of elasticity run on concrete pavement cores or molded cylinders made with the mixture.

 

 

Results from the study showed that the elastic modulus of the C-S-H gel in different areas varied within a wide range (~10-35 GPa, where 1 GPa equals approximately 145.038 kips per square inch, and a kip stands for a thousand pound-force). The team found the unhydrated cement particles to be almost 10 times harder than the C-S-H, with modulus in the range of 100-130 GPa. This finding is significant because the unhydrated cement particles act at this small scale as hard aggregates within a more yielding matrix, which may adapt to stress without brittle fractures.

In addition, for the first time in any study, the researchers measured experimentally the nanoscale mechanical properties of the interfacial transition zone in mortar and concrete. In the concrete at the juncture of the cement paste and aggregate particle surfaces, there is a zone with a high porosity and a tendency to develop microcracking. This zone is called the interfacial transition zone. "The interfacial transition zone is considered to be the weakest link in normal strength concrete, and it affects concrete's strength and durability," says Suneel Vanikar, team leader for concrete in the FHWA Office of Pavement Technology. "It is widely accepted by researchers that the properties of the interfacial transition zone must be taken into account in modeling the overall mechanical and permeability properties of concrete. But the modulus values used in current models lack theoretical or experimental evidence because of the practical problems in measuring such a small, narrow region of only 10 to 20 micrometers around coarse aggregate particles. The researchers successfully used the imaging feature of a nanoindenter to view this phenomenon."

 

 

Findings from this research provide more accurate input for models and serve as the first step in designing improved materials for pavements. Further research is ongoing to understand the effect of the addition of nanosilica on the microstructure. This research is of significant interest because previous research by one of the international collaborators showed that nanosilica is useful in reducing the impact of calcium leaching, which is one of the main durability issues with concretes in general.

 

 

Self-Consolidating Concrete For Slipform Paving Processes

In current practice, concrete pavement construction uses dry stiff concrete with slump (subsidence of a mold slip) of less than 5 centimeters (2 inches). The slump test provides a measure of consistency (or flowability of the concrete), where typical slumps for normal concrete range from 7.5 to 10 centimeters (3 to 4 inches). Greater slumps indicate more flowable mixes, while smaller slumps indicate stiffer mixes. The right mixture for the concrete is needed so that the pavement has high quality and the required strength; however, the moldability and placeability of the concrete in the intended application is critical also.

A slipform paving machine processes the fresh concrete, including placement, leveling, casting, consolidation, and finishing. The paving machine moves with a constant speed over the fresh concrete deposited in front of the machine, and at the end of the process, the fresh concrete slab can hold its shape without any edge support after the slipform has moved forward.

 

Mean Particle Size and Mineral Additives in SF-SCC Compositions Compared to Typical Cement and Fly Ash

  

 

Equally spaced vibrators in the paving machine introduce extensive internal vibration to consolidate and compact the fresh concrete (pack the materials and remove larger sized trapped air voids). These internal vibrators may cause overvibration of the stiff concrete if the vibration frequency is set incorrectly or the paving machine moves too slowly. Overvibration leads to segregation of aggregates and significant reduction of smaller sized entrained air in the concrete along the path of the vibrators. (Note that in concrete, some entrained air in the mixture is required for the best performance; therefore, the vibration process must not remove all the air.) When such a pavement is subjected to heavy traffic loading and/or freeze-thaw weather cycles during its service life, so-called vibrator trails (surface defects indicating segregation of aggregates, leaving a cement-rich layer) can occur, or longitudinal cracks can form.

To eliminate the need for internal vibration in the paving process, the researchers collaborated with the Center for Portland Cement Concrete Pavement Technology (now called the National Concrete Pavement Technology Center) at Iowa State University to extend self-consolidating concrete technology to slipform pavement applications. The key to slipform paving is that the material must be workable enough to be consolidated, yet stiff enough to stand without formwork after the paver moves on at the end of the processing. The challenge to develop SF-SCC is that the material must change from very fluid to very stiff during the slipform process.

The development of SF-SCC required changing the microstructure by combining concepts from particle packing (how particles of different sizes are arranged and how that affects compressive strength), admixture technology (the combination of different mineral and chemical admixtures), and rheology (the study of how materials flow). Specifically, the addition of different materials such as nanoclays and fly ash to the composition made it possible to maintain a balance between flowability during compaction and stability after compaction. The researchers used scanning electron microscopy to evaluate the particle microstructure of the clays used for the experiments.

For this research, the Iowa State team developed a model minipaver that simulates the slipform paving process without the application of internal or external vibration. At the end of the process, concrete slabs of modified mix with fly ash or fly ash and clay showed much better shape stability and surface smoothness than the slab with a standard slipform concrete mix.

Study of Flocculation

Flocculation is the grouping or clumping together of suspended particles within a fluid or liquid. The shape stability of any material in suspension depends on the rate at which flocs (groups of particles) form and on the strength of the bonds between particles. The study of flocculation is important in a wide variety of applications, such as nondrip paints, extruded ceramics, and emulsions. In the case of cement-based materials, understanding flocculation and the ability to control it are necessary for developing stable suspensions of self-consolidating concrete with good workability. In the slipform paving process, flocculation is important because the formulation of stable flocs indicates that the concrete mixture will retain its shape better without the need for molds and has more stability because more applied shear stress is required to break apart the bonds. Likewise, if the particles clump or group together in smaller flocs, the pavement mixture will not keep its shape as well. Research efforts include understanding the interactions among particles at the nano and micro levels.

 

 

The researchers relied on experimental observation of particle flocculation and deflocculation as they occurred to understand the actual physical processes within the microstructure. To monitor the flocculation process, the research team used focused beam reflectance measurement (FBRM), an experimental technique that provides in situ measurement of the evolution and size distribution of particle flocs. FBRM instruments operate by scanning a highly focused laser beam across particles in a suspension and measuring the time duration of back-scattered light from the individual particles. The chord length is determined by the time (pulse width) rather than the intensity (pulse amplitude). Measuring the chord length based on pulse width makes the measurement less sensitive to influences due to color or reflectivity. The researchers also used a centrifuge method to determine the compressive stress on a suspension of settling particles. In this method, the researchers centrifuged cement paste samples derived from SF-SCC concrete mixes at a particular speed until an equilibrium height was achieved. In terms of shape stability, a mix with a high compressive stress requires more energy to break bonds, indicating a greater floc strength and higher shape stability. Similarly, a mix with larger flocs under a given stress indicates a stronger floc strength because more applied shear stress is required to break apart the bonds.

 

 

New Generation Of Materials

The researchers are developing new construction materials using microfibers and hybrid fiber systems in cementitious (concrete, extruded materials, and mortar) materials. Currently, the researchers are working on nanofiber-reinforced systems that could lead to the next generation of fiber-reinforced concrete. Engineers use nanofibers when applications require superior mechanical and thermal properties, such as ultralight weight, superior strength, increased toughness, and enhanced electrical and thermal conductivities.

The researchers also are investigating the use of carbon nanotubes in cementitious materials. Carbon nanotubes are cylindrical carbon molecules with very high length- to-diameter ratios and novel properties. Scanning electron microscopy has shown that carbon nanotubes have the ability to bridge cracks in cement systems. This potential might increase the flexural strength significantly and increase the ductility of concrete.

 

 

Dispersion of nanotubes in cementitious material, however, is a major issue because they do not disperse easily through the cement paste as it flows into place. To address this problem, current research by the team is exploring alternative processing methods, such as coating fibers with additives, varying the rheological properties of the matrix, or changing the mixing procedure. Research efforts also include reducing the number of nanotubes used in the cement mix to make the product cost effective. Future research should include exploring the use of agricultural waste fibers and cellulose nanofibers, both of which have potential as economical alternatives to carbon nanotubes.

Sensors

A reliable testing method is needed to perform in situ monitoring of mortar or concrete properties at early ages to prevent the failure or cracking of pavements during construction or shortly thereafter. Development of new materials also demands improved sensing systems for better quality control during construction, and nanotechnology can play a significant role in developing these smart sensors.

 

 

In the past, the researchers developed a nondestructive testing method called ultrasonic wave reflection (UWR). This method measures the shear wave reflection loss at an interface between the hydrating cement paste and a buffer material. The researchers used this measurement to predict the mechanical properties of early-age concrete and to monitor macrostructural parameters, such as setting of the cement paste and its viscosity, dynamic shear modulus, and compressive strength.

The results of the current project show that the UWR method is accurate in determining the viscosity of cement pastes at fresh state and the shear modulus of pastes at the hardened state. The researchers also developed a relationship between the reflection loss and the compressive strength of cement paste, independent of both the curing temperature and the water/cement ratio.

Next Steps

To meet the increasing need for high-performance, durable construction materials for roadways, the researchers took a back-to-basics approach to improve understanding of the properties of cement-based materials at a small scale and to develop new materials. Nanoscale characterization of cement paste samples showed that the mechanical properties of the C-S-H gel—the glue in concrete—vary in a wide range, requiring complex modeling. Furthermore, the researchers observed that the residual cement particles are almost 10 times harder than the glue produced when mixed with water. This finding signifies that engineers might be able to design the material with the minimum glue (C-S-H) necessary to bind the harder particles or phases together.

Research on transferring SCC technology to pavement construction is in progress by the research team. Combining the concepts of particle packing and flocculation, admixture technology, and rheology, the researchers have developed SF-SCC that changes from very fluid to very stiff during the paving process. Experimental observation of particle flocculation and deflocculation as they occur should be included in future research, which could significantly contribute toward increasing the durability of pavements by eliminating segregation and cracking due to overvibration of concrete during construction.

Preliminary research also has shown that nanofibers and nanotubes potentially can make cement itself super ductile, with more ability to accommodate tension without cracking (more tensile strain capacity), which could increase flexural strength significantly. However, dispersion of nanotubes in cementitious material remains a major challenge, and the researchers are exploring various processing methods to optimize the number of nanotubes and their dispersion needed to develop cost-effective concrete for the next generation of highways.

 

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Surendra P. Shah is the Walter P. Murphy Professor with the ACBM Center at Northwestern University in Evanston, IL.

Paramita Mondal is a Ph.D. candidate with the ACBM Center.

Raissa P. Ferron is a Ph.D. candidate with the ACBM Center.

Nathan Tregger is a Ph.D. candidate with the ACBM Center.

Zhihui Sun is an assistant professor with the civil and environmental engineering department at the University of Louisville, KY.

For more information, contact Surendra P. Shah at 847-491-3858 or s-shah@northwestern.edu. This research was supported by an FHWA grant (Award No. DTFH61-05-C-0001).