Executive Summary

Conventional Portland cement concrete is a conglomerate of hydraulic (Portland) cement, sand, stone, and water. It was developed approximately 150 years ago to imitate natural stone while providing less labor-intensive methods of shaping building materials (i.e., casting rather than hewing and carving). In materials science and engineering (MSE) terms, conventional concrete is a particulate-strengthened, ceramic-matrix composite material. The sand and stone are the dispersed particles in a multiphase matrix of cement paste. Reinforced concrete can then be considered a “fiber-reinforced” composite, with the rebar acting as the “fiber.” One fundamental difference, however, between conventional concrete and other engineering composites is that the composition and properties of the cement paste do not remain constant after processing but vary with time, temperature, and relative humidity. Another issue is the porosity. At normal relative humidities, the pores in concrete are filled with a highly alkaline solution (with pH between approximately 12.5 and 13.8) that can be regarded as a separate phase of the microstructure and plays a major role in determining the strength of the concrete and the durability of the structure.

The dimensions of the different structural features in concrete span 10 orders of magnitude, from nanometer-sized pores and gel “particles ”; to rebar reinforcement that can be tens of meters in length; to paste, sand, and stone particles of all sizes in between. Although the performance of concrete is influenced by the properties (e.g., density and porosity) of its sand and stone components, these properties are fixed by nature. Therefore, it is the cement paste in conventional concrete that is the most important MSE-systems component because



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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure Executive Summary Conventional Portland cement concrete is a conglomerate of hydraulic (Portland) cement, sand, stone, and water. It was developed approximately 150 years ago to imitate natural stone while providing less labor-intensive methods of shaping building materials (i.e., casting rather than hewing and carving). In materials science and engineering (MSE) terms, conventional concrete is a particulate-strengthened, ceramic-matrix composite material. The sand and stone are the dispersed particles in a multiphase matrix of cement paste. Reinforced concrete can then be considered a “fiber-reinforced” composite, with the rebar acting as the “fiber.” One fundamental difference, however, between conventional concrete and other engineering composites is that the composition and properties of the cement paste do not remain constant after processing but vary with time, temperature, and relative humidity. Another issue is the porosity. At normal relative humidities, the pores in concrete are filled with a highly alkaline solution (with pH between approximately 12.5 and 13.8) that can be regarded as a separate phase of the microstructure and plays a major role in determining the strength of the concrete and the durability of the structure. The dimensions of the different structural features in concrete span 10 orders of magnitude, from nanometer-sized pores and gel “particles ”; to rebar reinforcement that can be tens of meters in length; to paste, sand, and stone particles of all sizes in between. Although the performance of concrete is influenced by the properties (e.g., density and porosity) of its sand and stone components, these properties are fixed by nature. Therefore, it is the cement paste in conventional concrete that is the most important MSE-systems component because

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure its structure and composition can be controlled to modify the properties of the overall material. Conventional concrete has many advantages over other structural materials: (1) it is the lowest-cost structural material by an order of magnitude; (2) it can be produced from materials found in abundance all over the world, meaning that transportation costs are low; (3) it is a relatively low consumer of power during its production; (4) it is extremely versatile, with essentially the same material being used in both high and low technology constructions; (5) it is more chemically inert than other structural materials; (6) it is resistant to water; (7) it readily lends itself to reinforcement; and (8) it provides good protection for steel (used as reinforcement) because of its high alkalinity. Unfortunately, conventional concrete also has limitations: (1) its network of capillary pores and microcracks can allow aggressive species (e.g., chlorides) to enter and cause corrosion of steel reinforcement, which in turn causes the concrete to crack and spall; (2) it is degraded by repeated freezing and thawing because of its free-water content; (3) its hydration reaction produces a decrease in volume that results in shrinkage cracking; (4) its setting and strength development processes are difficult to control, making placement, compaction, and curing critical stages in ensuring a durable final product; and (5) it is brittle and must be reinforced to improve mechanical strength and toughness. As each of these problems has been identified, conventional concrete research and development (R&D) has advanced a new additive to solve it. Although the various chemical and mineral additives solve a specific problem, they may also react with each other to enhance or diminish the value of each, cause new problems, or create a mixture so complex that it is difficult to control and reproduce. It is recognized that significant near-term research is being pursued within the cement and concrete research community. The objectives of the NRC study that produced this report were to look beyond near-term developments in concrete technology to identify R&D opportunities in innovative, nonconventional materials and processes that have the potential to accelerate the construction process, improve the durability of highway pavement and bridges, and enhance the serviceability and longevity of new construction under adverse conditions. Meeting even one of these three objectives could save billions of dollars in construction and maintenance costs. The U.S. Department of Transportation estimates that the cost of maintaining 1993 highway conditions is $49.7 billion per year and the

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure cost of improving them is $65.1 billion per year. For bridges, the cost to maintain 1994 conditions is estimated at $5.1 billion per year and the cost of improving them at $8.9 billion per year. The main thrust of this report is that concrete, as a material as well as the structure created from it, must be viewed not as the simple conglomerate of constituent materials assembled through a sequence of unit processes but as a single integrated system.1 The committee recognizes that the systems approach inherently requires an interdisciplinary framework. In turn, such a framework requires effective program management to enable and assure resonance of multiple viewpoints (i.e., from the specialized research levels at one end to the applications level at the other). Enhanced performance and reduced life-cycle costs can only be realized from this perspective. The synergies between constituent materials and unit processes must be understood quantitatively and manipulated to improve performance at reduced life-cycle cost. To realize a systems approach to concrete design, the materials and processes must be understood at a level of detail far in excess of the current state of the art. The following sections of this Executive Summary present conclusions and recommendations developed from a systems-approach perspective that can aid in the development of nonconventional concrete technologies with superior properties. It should be noted that the committee neither attempted to prioritize the conclusions and recommendations presented in this report nor considered the potential increases in initial cost for implementing nonconventional techniques. The committee believed that such considerations would have detracted from the main objective of the report and that the research community would best be served by the presentation of the widest range of innovative ideas without the overlaying of potential cost constraints. POTENTIAL MATRIX MATERIALS AND THEIR SYNTHESIS The major conclusion and recommendation of the committee regarding potential matrix materials and their synthesis is that a sustained R&D effort is required to obtain a thorough understanding of the development and behavior of the cement matrix and its 1   Thus, the innovative aspect introduced by the committee is the examination of the effects of these materials on concrete within a systems approach.

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure microstructure on the atomic and molecular level. The committee believes that such an understanding is the primary path toward the development of more reliable methods to control the micro-, meso-, and macro-morphology of concrete and to produce a nonconventional concrete superior to conventional concrete. This knowledge would require establishing detailed, nonempirical relationships between the microstructures produced by processing methods and the resulting properties and performance of the concrete. The committee also formulated the following conclusions and recommendations on nonconventional concrete matrix materials and their synthesis: The processing of conventional Portland cement can be considered as a reaction of a basic oxide (e.g., CaO) with an acidic monomer (e.g., SiO2) to form a polymer. Although there are numerous other methods for synthesizing a cementitious material by this approach, very few are sufficiently low-cost or widely enough available for use in roads and bridges. The committee concludes that novel cement materials must be based on one or a combination of the following three strategies: (1) use the next least expensive basic oxides other than CaO to produce the initial acidic monomer (e.g., Na2O); (2) use the basic calcium oxide and silica in conventional Portland cement in conjunction with recycled or by-product materials that already contain a substantial amount of acidic monomer (e.g., glass, blast-furnace slag, fly ash, silica fume) to reclaim the energy previously invested in producing these materials or to produce different reactions and products; or (3) use basic calcium oxide and silica in conventional Portland cement but refine and control the reaction sequence. Theory and experiment have shown that all inorganic gels fall into the same universality class, the fundamental physics of which is independent of the length scale of the internal structure of the gel. The study of large-scale percolation effects involving the interaction of aggregate and the theory of gelation and critical phenomena suggest possible mechanisms of rheology control. The committee recommends that research be pursued on the use of surfactants, surface charge controllers, pH controllers, nucleating agents, and superplasticizers for the manipulation or elimination of gelation and the introduction of new, more stable phases. Certain synthetic polymers, cellulose derivatives, and clays absorb large amounts of water. The use of such materials as organic

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure polymers, wood, paper waste, and swelling clays may have a beneficial effect on the rheology and workability of the concrete and possibly increase its strength at the critical point of the sol-gel transition but with the possible (and potentially detrimental) consequence of requiring a larger proportion of water. The committee recommends that research be conducted on the application of these materials to maintaining more constant hydration reaction conditions. Few techniques are currently available to control the temperature of the gel network formation reaction. The temperature of the pour at different stages of the setting process could potentially be maintained at desired levels by the use of sequential endothermic and exothermic side-reactions. The committee recommends that research be pursued on the development of sequential endothermic and exothermic side-reactions to improve the thermal control of the gel network formation reaction. The reactive inorganic additives (supplemental cementitious materials [SCMs]; e.g., fly ash, silica fume, and blast-furnace slag) that are being increasingly added to concrete, need to be viewed as integral components of the cement mix rather than as mineral additives. An understanding of the behavior of additives that are aluminum-rich as opposed to those that are aluminum-poor is critical. The committee believes that characterization of the chemistry of SCMs and exploration of their interactions with other matrix components with the goal of exploiting these interactions and consistently improving concrete properties are particularly important areas for further research. The committee also recommends that research be conducted on the possible surface activation by pretreatment of these additives to improve desirable chemical reactivity and physical properties. The committee believes that another particularly promising area of research is the possibility of extending the life of steel reinforcement by changing the matrix of the concrete either to maintain reducing conditions at the interface (e.g., the use of oxygen scavengers, such as sulfides, organics, and other agents) or to prevent significant amounts of water from coming in contact with the rebar. Smart matrix passivation exploiting the selective migration of ionic species caused by functional chemical gradients should be explored as an alternative approach to rebar protection. The addition of nuclei to conventional Portland cement paste at key points in its synthesis might permit the incubation time of

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure setting to be controlled, the spatial and temporal homogeneity of the material to be improved, or even the gel phase to be crystallized into more chemically durable phases. The committee recommends that research be undertaken to increase understanding of the linkages among synthesis and processing, structure and composition, properties, and performance in concrete in order to develop such new technologies as crystalline or macromolecular nucleating agents and growth modifiers. Lessons learned from successful synthetic (e.g., metals, ceramics, polymers, and their composites), biological, and geological materials should also be applied to concrete R&D to permit the possible tailoring of microstructures to control properties for specific long-term performance. REINFORCEMENT AND LAYERED STRUCTURES The committee recommends that concrete reinforcement R&D focus on an integrated concrete system of component materials, reinforcements, and multiple reinforcing phases. A potential design for a nonconventional concrete system would be a layered structure. Layered concrete structures can be designed for particular functions by stacking and combining material layers with different chemical, structural, or mechanical properties. The internal layers could include: visco-elastic materials, to dampen structural vibrations shape memory alloys, which can exhibit high damping capacity and might improve earthquake resistance in large constructions (Van Humbeeck et al., 1996) foams, porous materials, or three-dimensional woven structures to absorb the energy and stress waves produced by continuous pressure (the materials could also be infiltrated by a liquid medium to provide further dissipation and absorption of energy) fiber reinforcements, to bridge, deflect, and arrest crack growth The committee concludes that such a design approach would have to be based on models of the properties of the constituent materials and the interfaces between these materials, such as: thermodynamic and kinetic behavior, including phase transformations and rates of chemical reactions

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure interfacial interactions within the microstructure effects of local damage on structural integrity and durability energy and momentum transfer and dissipation energy absorption capacities and resiliency during service loadings The committee also believes that a particularly promising area of research that should be pursued concerns the extension of reinforcement lifetime in low-pH, nonconventional concrete technologies by: (1) the application of coatings to protect the rebar, such as sacrificial reducing agents to prevent oxygen from reaching the steel rebar; or (2) the replacement of the carbon-steel rebar with a nondegrading continuous reinforcement, similar to stainless steel or composite materials. PROCESSING The committee concluded that processing and net-shape forming capabilities of a nonconventional concrete must at least emulate or exceed the capabilities of conventional concrete. The following issues must be taken into consideration when examining a potential nonconventional concrete process: (1) processing requirements that correspond to new material developments; (2) processing and constructability constraints that influence the success of a construction material; (3) scale-up possibilities to attain acceptable performance and required amounts from novel process plants for expected construction scenarios; (4) robustness and simplicity of the process controls that are needed in a production facility to produce the performance variables and strengths desired; and (5) parameters that influence the placing, finishing, and curing requirements of new concrete material (e.g., flow and workability, resistance to mix separation under a variety of conditions, tolerance of environmental conditions, and the need for and interaction with a range of typical reinforcing materials). The committee formulated the following conclusions and recommendations concerning the development of nonconventional processing technologies: The production processes for nonconventional concrete will involve the selection, preparation, mixing, and delivery of source materials, including the use of cement binders, the mixing or

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure blending of raw materials in controlled amounts, the delivery of blends either as slurries or dry mixes, and the placement of the mixes, usually as slurries, into forms or machines for final net-shaping. The committee concludes that nonconventional concrete technologies will require higher levels of process control than are currently available and strongly recommends that R&D be undertaken in every aspect of the production process to permit better placement and to optimize the performance and properties of potential nonconventional concretes. The major areas of improved process control include cement feedstock production, cement hydration, concrete mixing, and fiber mixing. The committee concludes that quality assurance procedures will be critical during the delivery of a nonconventional concrete from the processing facility and its placement in a precasting yard or construction site. Realistic and routine tests must be available to provide specific information about the material, especially when current experience is superseded by the use of new materials. The committee recommends that R&D should focus on the development of test methodologies that: (1) are seamless throughout the entire process; (2) improve the availability and reliability of the data collected during the production and construction process for use by all parties; (3) ensure the delivery and use of the proper raw concrete or concrete elements; and (4) allow the implementation of model-based design for the concrete system within an MSE-systems approach. Suitable tests are largely unavailable at the present time. Concrete is usually placed far from its initial batch mixing location. The committee believes that particularly important is the development of test methods and sensors that will better describe the dynamics of the setting process in continuous fashion from initial batch mixing to transportation to the construction site, pouring and compaction in workforms, and final finishing and curing operations. Although current tests may assure a certain level of quality at placement, the number of variables and parameters tested are so numerous and difficult to quantify that it takes experience and insight to determine why a test failed and how to rectify the deficiency. The committee recommends research into nonconventional continuous processes with the potential of yielding a more consistent product than possible with batch processes. Continuous mixing,

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure especially on-site, would appear to offer a more consistent product. Real-time monitoring of rheological and chemical properties with the goal of process monitoring and chemical modification is needed to achieve continuous control over the properties of the delivered concrete. SYSTEMS APPROACH The committee recommends that future concrete-systems R&D be considered within the context of “model-based design.” Model-based design is predicated on the fundamental understanding, at all length scales, of materials and their behavior from the processing of the raw materials to the preparation of the concrete to the setting process to the understanding of its behavior during in-service life. A materials understanding at all length scales means that the behavior of the material is understood quantitatively from the atomic level to the microscopic level to the macroscopic or continuum level. Such an understanding would allow the materials and their processing to be continuously controlled. Model-based design, often called “Smart Processing,” relies extensively on a sensor-rich environment to allow computer-controlled adjustments to materials and processing parameters. The committee concludes that the most fundamental need for realizing a systems approach to concrete and the design of concrete-based structures is more extensive basic knowledge of materials and processes. The physics and chemistry of the gelation process needs to be understood both qualitatively and quantitatively, as do the interactions between cement and aggregate and between cement and reinforcement. The global and local constitutive properties (e.g., mechanical behavior) of the cement, aggregate, reinforcements, and their interfaces also need to be determined. Further work on solution thermodynamics of the relevant hydrate systems and kinetic modeling is also warranted. Without this basic understanding, any significant advance in the science of concrete and its applications will not be forthcoming. Model-based design within the construct of a systems approach will not happen until the fundamental knowledge base is significantly expanded. The committee recommends that the final product should also be monitored through discrete embedded and/or external sensors. This would allow the formulation of an explicit quantitative understanding of degradation processes that could then be fed back into the

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure model-based design methodology. It would also permit cost-effective, localized repairs before damage accumulation takes the material or structure out of service. The goal is to minimize maintenance costs and maximize in-service time by eliminating routine preventative maintenance. The committee concludes that the final overarching requirement to implement a model-based design methodology is to view these materials and the derived structures within a total life-cycle cost context. Although the use of sensors and on-line computational tools to adjust materials and processes and to monitor in-service performance is expensive, the use of this approach will greatly increase the life of the material and structure, and thereby potentially reduce total life-cycle costs. For this approach to succeed, the costs of the structure must be viewed from the standpoint of total life-cycle costs and not simply from the perspective of the lowest cost for original construction.