1

Introduction and Background

This chapter provides an overview of conventional concrete technology from a materials science and engineering (MSE) systems perspective —specifically its structure and composition, synthesis and processing, properties, and performance. The advantages and disadvantages of conventional Portland-cement concrete are summarized, and the characteristics of an ideal concrete are discussed.

The focus of the MSE systems approach is the manipulation of the ways that materials are synthesized and processed to control their microstructures at various length scales to achieve certain bulk properties and system performance. The strong interrelationship among synthesis/processing, structure/composition, properties, and performance is depicted in Figure 1–1 and was the main conclusion of the 1989 NRC report Materials Science and Engineering for the 1990s.

The sensitivity of the properties of materials to microstructural changes can be demonstrated by numerous examples with widely varying chemistries (NRC, 1989, 1991). Even in single crystal form, structural nonuniformities like lattice defects (e.g., dopant elements, dislocation, twins, stacking faults, and second-phase precipitates) affect the physical properties of materials (e.g., conductivity, hardness, magnetic susceptibility, and strength). Most materials are used in polycrystalline forms, however, since there are myriad possibilities of modifying their extrinsic physical and chemical properties through the modification of their structures. These modifications may span the entire material structure, from the nanometer to the micrometer to the macro scale.

The MSE systems approach also applies to concrete. The same strong correlation among synthesis/processing, structure/composition,



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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure 1 Introduction and Background This chapter provides an overview of conventional concrete technology from a materials science and engineering (MSE) systems perspective —specifically its structure and composition, synthesis and processing, properties, and performance. The advantages and disadvantages of conventional Portland-cement concrete are summarized, and the characteristics of an ideal concrete are discussed. The focus of the MSE systems approach is the manipulation of the ways that materials are synthesized and processed to control their microstructures at various length scales to achieve certain bulk properties and system performance. The strong interrelationship among synthesis/processing, structure/composition, properties, and performance is depicted in Figure 1–1 and was the main conclusion of the 1989 NRC report Materials Science and Engineering for the 1990s. The sensitivity of the properties of materials to microstructural changes can be demonstrated by numerous examples with widely varying chemistries (NRC, 1989, 1991). Even in single crystal form, structural nonuniformities like lattice defects (e.g., dopant elements, dislocation, twins, stacking faults, and second-phase precipitates) affect the physical properties of materials (e.g., conductivity, hardness, magnetic susceptibility, and strength). Most materials are used in polycrystalline forms, however, since there are myriad possibilities of modifying their extrinsic physical and chemical properties through the modification of their structures. These modifications may span the entire material structure, from the nanometer to the micrometer to the macro scale. The MSE systems approach also applies to concrete. The same strong correlation among synthesis/processing, structure/composition,

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure FIGURE 1–1 The interrelationship of the four elements of materials science and engineering: synthesis/processing, structure/composition, properties, and performance. Source: NRC, 1989. properties, and performance exists for concrete structures as for any other engineered material. STRUCTURE AND COMPOSITION OF CONVENTIONAL PORTLAND CONCRETE Conventional 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 the material (i.e., casting rather than hewing and carving). As such, it was initially expected to resist only compressive loads. As highway systems developed and expanded after World War II, however, concrete started to find new uses in roads and bridges, where it was subjected to tensile-bending stresses as well. A comparison of the mechanical properties of concrete with those of other materials is presented in Figure 1–2.

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure FIGURE 1–2 Fracture toughness versus strength for concrete and other structural materials. Source: Ashby, 1992. In MSE terms, conventional concrete is a particulate-strengthened, ceramic-matrix-composite material (Figure 1–3). 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 reinforcing steel bar (rebar) acting as the “fiber.” One fundamental difference, however, between conventional concrete and other engineering composites is that the composition; and hence the properties, of the cement paste do not remain constant after processing but vary with time, temperature, and relative humidity. A second difference is concrete's porosity. The pores of concrete are filled with a highly alkaline solution with a pH of between approximately 12.5

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure FIGURE 1–3 Macrophotograph of plain polished section of concrete showing sand and stone particles in a cement paste matrix. Source: Hansson, 1995. and 13.8 at normal relative humidity. This solution can be regarded as a separate phase of the microstructure and plays a major role in determining the strength and durability of concrete. The dimensions of the different structural features in concrete span 10 orders of magnitude (Figure 1–4), from nanometer-sized pores and gel “particles” to rebar that can be tens of meters in length, and to paste, sand, and stone particles of all sizes in between. Although the performance of concrete is affected by the properties (e.g., density and porosity) of its sand and stone components, these properties are determined by nature. Suitable aggregate must be selected from available sources. Therefore, it is the cement paste in conventional concrete that is the most important MSE systems component because it can potentially be tailored to fit the job. Cement Paste Calcined Portland cement consists of several anhydrous oxides, primarily tricalcium silicate (C3S) and dicalcium silicate (C2S), with

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure smaller amounts of tricalcium aluminate (C3A) and calcium aluminoferrite (C4AF). Cement also contains small amounts of impurities, such as magnesium, sodium, potassium, and sulfur compounds. The specified composition ranges for Type I Normal Portland cement are given in Table 1–1. FIGURE 1–4 Dimensional hierarchy of structures in concrete.

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure Conventional cement is produced by mixing and grinding proportionate amounts of the raw materials (i.e., limestone or chalk [predominantly calcium carbonate] and clay or shale [predominantly aluminum silicates]) to give a Ca:Si ratio of approximately 3:1. The mixing and grinding were traditionally carried out in a water slurry, but modern cement plants usually use a more energy-efficient dry process that eliminates the need for water evaporation. The constituents are generally represented as a ternary system of CaO+SiO2+Al2O3, with some substitution of iron for aluminum. The ternary phase diagram exhibits a peritectic reaction between Ca8SiO5, Ca2SiO4, Ca3Al2O6, and liquid at 1455°C (Osborn and Muan, 1964). The addition of Fe2O3 to the system results in the formation of a lower melting peritectic eutectic, with the compound 4CaO·Al2O3·Fe2O3 in equilibrium with the above four phases at 1338°C (Lea and Parker, 1964). The constituents undergo a partial reactive melting and liquid-phase sintering during subsequent heating to between 1500°C and 1600°C in a rotary kiln. This process results in the formation of “clinker,” which consists of hard, shiny globules of C3S and C2S that are held together by the peritectic mixture of mostly C3A and C4AF. There are two environmental problems associated with this rocess. First, a considerable amount of energy is required to produce the clinker—approximately 1400 and 800 cal/g for the wet and dry processes, respectively. Second, the decomposition of CaCO3during the process results in TABLE 1-1 Major Constituents and Composition Ranges of Type I Normal Portland Cement (ACI, Section 225R, 1995) Compound Composition Abbreviation Wt. percent Tricalcium silicate 3CaO·SiO2 C3S 42 to 65 Dicalcium silicate 2CaO·SiO2 C2S 10 to 30 Tricalcium aluminate 3CaO·Al2O3 C3A 0 to 17 Calcium aluminoferrite 4CaO·l2O3·Fe2O3 C4AF 6 to 18 Other Mg, Na, K, and S oxides   Balance

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure significant CO2emissions.1 The cooled clinker is then ground to the required fineness to produce commercial cement. Other materials are added during this final grinding stage, particularly gypsum (CaSO4·2H2O), which controls set time. When mixed with water, the cement powder hydrates to form cement paste, which is an interconnected or interlocking network of solid and semi-solid phases that gives concrete its strength and stability. The rate at which the constituents of clinker react with water varies, as shown schematically in Figure 1–5. The logarithmic time-scale on this diagram should be noted. The reactions start within seconds of the cement being mixed with water but may not reach completion for many years. Figure 1–5 is a representative diagram and does not take temperature into account. The reactions are exothermic, and the temperature of the concrete is therefore not constant throughout the hydration processes but initially rises and then gradually falls. In massive structures, the temperature can rise in excess of 60°C. If the temperature were constant, this figure could be regarded as an isothermal section of a time-temperature-transformation (TTT) diagram. Unlike the processing of steel, however, there is generally little control of the processing of concrete. Although some newer methods for controlling the curing process are being introduced, particularly for high-performance concrete (e.g., the use of nitrogen or embedded cooling pipes in the structure to cool the aggregates or concrete), temperature control is primarily limited to the use of hot water for mixing in cold weather and the addition of ice to the mixing water in hot weather. It is often postulated that the initial reaction is “through-solution” in that the reactant solids dissolve in the water, react, and then precipitate as the hydrated product. The reaction is subsequently topochemical (Mehta, 1986), giving rise to the initially rapid and progressively slower hydration process. The topochemical reaction involves the formation of a solid product directly on the surface of the reactant, which then demands either the diffusion of the solid reactant outward to the surface of the forming product or the diffusion of water inwards to the clinker/hydrate interface. Observations of hollow shells of reactant products containing unhydrated clinker, known as “Hadley grains” (Figure 1–6), are cited as evidence for the topochemical reaction. 1   The CaCO3 is more thermodynamically stable than the hydrated cement products, which reacts with the CO2 in the atmosphere to convert again into carbonates.

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure FIGURE 1–5 Phases of cement paste as a function of time after mixing the dry cement clinker with water. Source: Soroka drawing from Locher and Richartz personal communication as printed in Soroka, 1979.

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure FIGURE 1–6 Partially hydrated clinker particle surrounded by hydrated calcium-silicate-hydrate gel (C-S-H): (a) image of 7-day old conventional Portland cement paste; (b) image of 60-day old conventional Portland cement paste. Source: Scrivener, 1984.

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure The aluminate phases C3A and C4AF are the first components of the clinker to hydrate, and they react with the gypsum to initially form (ettringite). This is a metastable product, however, which then further reacts to form a monosulfate, . The calcium sulfoaluminates constitute about 15 to 20 percent of the volume of the paste but do not play a major role in the structure –property relations (Mehta, 1986). The principal constituent of the paste is calcium-silicate-hydrate (C-S-H), which is 50 to 60 percent by volume of the solids and is formed by the hydration of C3S and C2S. C-S-H is largely responsible for the strength and cementing properties of the paste. Calcium hydroxide (CH), a product of the calcium silicate hydration reactions, comprises 20 to 25 percent of the solid volume but does not significantly contribute to the strength of the paste. Moreover, CH is a major reason for the poor acid-resistance of concrete because it has a higher acid-solubility than does C-S-H. On the other hand, CH buffers the pore solution pH to approximately 12.5, a level at which reinforcing steel is readily passivated. Cement has the fluidity of a slurry when first mixed with water and can easily flow into a formwork. However, as the sulfoaluminates form interlocking crystals and the C-S-H begins to form, the resulting network of solids causes the cement to set (i.e., to transform from a slurry into a stiff mass).2 The material has no strength at this stage of hydration and consequently is very friable. As stated above, the length of time required for initial setting is determined by the amount of gypsum present in the clinker. The onset of strength gain (“hardening”) occurs only on the formation of C-S-H, which starts some hours after mixing (Figure 1–5). The water/cement (w/c) ratio is the most important parameter in determining the properties of hardened concrete. A w/c ratio of approximately 0.23 is theoretically needed for complete hydration of the cement components but is not actually sufficient. C-S-H is usually classified in concrete R&D terminology as a “gel” and defined as having “particles” that are extremely small (of micrometer dimensions) and poorly crystalline. Within the gel itself, there are water-filled spaces referred to as “interlayer spaces” or “gel pores.” The whole mass is thought to be held together by either van der Waals forces or, more probably, hydrogen bonds. The total volume of water in the gel pores 2   In the terminology of the physical chemist, this setting is defined as a “gel.”

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure corresponds to a mass fraction of approximately 0.19, giving a theoretical total w/c ratio requirement for complete hydration of 0.42. It is important to note that this meaning of the word “gel” is different than that used by physical chemists and discussed above. In practice, however, a w/c of 0.42 is only sufficient if a “water-reducing” chemical admixture is used, because additional water is required to create a workable mix. Thus, w/c ratios of at least 0.50, and often considerably higher, were common until recently, when the negative effects of high w/c ratios on the durability of concrete were recognized. The excess water exists in the large spaces between the cement particles and between the cement and the aggregate that do not become completely filled with gel. These spaces form a network of “capillary pores” with cross-sectional dimensions on the order of micrometers. Because of the small dimensions of the pores (approximately 0.2 to 3.0 (m) and the polar nature of water molecules, the water is chemisorbed on the gel. Thus, the interlayer water behaves quite differently from both the water of hydration (which is bound by primary chemical bonds) and the water in the capillaries (which can flow freely). The pore water is a concentrated ionic solution that is saturated in Ca(OH)2, contains NaOH, KOH, and soluble sulfates, and has a pH in the range of 12.5 to 13.8. The solution is always in intimate contact with the solid phases of the paste and therefore also contains equilibrium concentrations of the species comprising the solids. Hydrated cement occupies a smaller volume (by approximately 11 percent) than the sum of the volumes of the dry cement and the added water that becomes chemically bound. Taking into account the presence of the gel pores, the total contraction of theoretically fully hydrated cement with a w/c of 0.42 is approximately 8 percent.3 The contraction due to this volume change is known as chemical or autogenous shrinkage and can result in the development of microcracks. This is aggravated at the surface by the loss of water due to evaporation, resulting in so-called plastic shrinkage because it occurs when the mix is still plastic. As a consequence, cracking at the surface may be particularly severe. In normal concretes, the plastic 3   It should be noted that this figure pertains solely to the cement matrix. The shrinkage of a concrete structure is less than one percent, since the cement matrix is only a small volume fraction of the entire concrete mass.

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure FIGURE 1–10 Compressive strength of conventional Portland cement concrete (CPC) and Portland fly ash cement concrete (FA). Unpublished data from H. Caratin, Ontario Hydro Technologies, 800 Kipling Avenue, KR 252, Toronto, Ontario M87 5S4, Canada. calcium (and sodium and potassium) hydroxide component reacts with CO2 to form calcium carbonate, even though the carbonation reaction actually strengthens and hardens the cement “paste.” A more severe problem is chloride-induced corrosion. Chlorides can diffuse through the capillary pore and/or micro-crack network to the rebar, where they cause instability in the passive film on the steel, leading to local film breakdown and corrosion pit formation. The resulting expansive corrosion layer causes cracking in the matrix. Environmental changes are also responsible for other problems: freezing results in expansion of the pore solution and causes internal stressing, while repeated drying and wetting can cause both absorption of undesired species from the environment and leaching of critical components from the concrete. Thus, normal environmental conditions interact with concrete's microstructure to cause the material to deteriorate over time (Figure 1–11, Figure 1–12, Figure 1–13, Figure 1–14, through 1–15). Concrete structures are failing at an alarmingly increasing rate and at earlier stages of their specified service lives. This can be partly attributed to exposure to increasingly hostile environments (e.g., marine environment, chemical industrial use, and exposure to deicing-salt). Nevertheless, it is becoming increasingly clear that

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure FIGURE 1–11 Corrosion of reinforcing steel in the support structure of an elevated highway caused by deicing salts seeping from the deck. The expansion of the corrosion products has caused spalling of the concrete cover. Source: Hansson, 1995. the predictions of laboratory studies are not being borne out in practice. The extent of the problem is such that finding ways to improve concrete durability has been described as a “multibillion-dollar opportunity” (NRC, 1987). The U.S. Department of Transportation (DOT) estimates that the annual cost to maintain overall 1993 highway conditions is $49.7 billion and that additional improvements would require an average annual investment of approximately $65.1 billion. For bridges, the cost to maintain overall 1994 conditions is estimated at $5.1 billion, and the average annual cost to improve them at $8.9 billion (DOT, 1995). ASSESSMENT OF CONVENTIONAL CONCRETE TECHNOLOGY This section summarizes the advantages and disadvantages of conventional concrete technologies.

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure FIGURE 1–12 Spalling of concrete surface caused by repeated freezing and thawing. Courtesy of R.D. Hooton, University of Toronto. Advantages The reasons for the widespread use of concrete are manifold: Concrete is the lowest-cost structural material by an order of magnitude (roughly 10 cents a kilogram) when compared with other engineered construction materials. The raw materials for concrete are available in almost every corner of the world, enabling it to be produced for local markets with minimal transportation cost. Concrete is a relatively low consumer of power in its production. The energy contents of unreinforced concrete and steel reinforced concrete are estimated at 450 to 750 and 800 to 3,200 kWh/ton, respectively, whereas the energy content of structural steel is approximately 8,000 kWh/ton. Concrete is extremely versatile. It is used in high-tech engineering constructions (e.g., offshore oil platforms, high-rise buildings), low-tech construction (e.g., sidewalk paving), and artworks (e.g., sculpture). Concrete is more chemically inert than other structural materials and exhibits excellent resistance to water, making it an

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure FIGURE 1–13 Salt scaling on concrete steps within six months of being cast caused by the use of deicing salts. ideal material for such water-control structures as pipelines and dams. Contrary to popular belief, water is not deleterious to plain or reinforced concrete. It is the chemical species dissolved in water (e.g., chlorides, sulfates, and carbon dioxide) that cause deterioration. Concrete readily lends itself to reinforcement because it is strong in compression and high in stiffness. Although relatively weak in tension, the tensile strength of concrete can be increased by the use of steel reinforcing bars (rebars). Concrete provides good protection for rebar by acting as a physical barrier to the ingress of corrosive species and by supplying chemical protection in the form of a highly alkaline environment. The high level of passivation allows low-quality recycled steel (e.g., black or mild steel) with high levels of impurities to be used. Supplementary materials (e.g., fly ash, blast furnace slag, condensed silica fume, rice-husk ash) have been added to concrete in recent years to: (1) reduce raw-material costs, (2) turn waste materials into useful products, and (3) improve the properties of concrete, if added and cured appropriately.

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure FIGURE 1–14 Cracking in concrete paving caused by an expansive reaction between the aggregate and the alkalis in the cement paste. Source: Hansson, 1995. Disadvantages From the microstructural viewpoint, conventional concrete has two main disadvantages. First, the inherent capillary porosity of Portland cement allows aggressive species to move into the concrete,

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure FIGURE 1–15 Backscattered electron SEM image of the surface layers of concrete exposed to 1.5 percent sulfate solution. Source: Crumbie et al., 1989. constituents to be leached out, and damage to occur from repeated freezing and thawing of the pore solution. Porosity can be reduced significantly by limiting the w/c ratio, but this results in incomplete hydration, the long-term effects of which are unknown. Second, the reduction in volume during hydration leads to plastic shrinkage and microcracking, which can add to the problem of porosity as well as reduce overall strength. From the processing viewpoint, conventional concrete also has two main disadvantages. The first is complexity. The large number of components makes it difficult to replicate mixes exactly and to control homogeneity. The second disadvantage is the temporal separation of the setting and hardening processes. The incubation period before setting is not only desirable but essential to allow for placement and compaction. However, since the setting or stiffening of concrete is largely controlled by the hydration of the aluminate phases and the hardening is produced by the hydration of the more slowly reacting calcium silicate phases, there is a period in which concrete is stiff but

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure unable to bear any load. In this state, it is easily damaged by vibration or unintentional loading. This problem is exacerbated by the use of high-range water reducing agents (superplasticizers), which lengthen the period between setting and the onset of hardening. Many of the problems associated with concrete in practice stem from inadequate curing. It is essential that concrete be prevented from drying out during the early stages of hardening because this leads to inadequate hydration and shrinkage cracking in the surface layers (i.e., in precisely those parts of the structure most susceptible to degradation by salt, ices, sulfates, and so forth). The chief mechanical weaknesses of conventional concrete are lack of intrinsic toughness and tensile strength. New “high-performance” concretes have compressive strengths far in excess (from two to five times) of conventional concretes, but tensile strengths are not improved in proportion, and the lack of strain capacity can lead to significant amounts of autogenous shrinkage cracks. Moreover, the lack of bleeding in these mixes can lead to severe plastic shrinkage in the absence of appropriate wet curing. Laboratory development of “macro-defect free cements” has shown that lack of toughness and tensile strength can be attributed to the presence of large defects, such as cracks and voids. Elimination of these flaws from normal engineering structures is not practical because of the large scale, however. The brittle failure of Portland cement concrete structures is prevented by the use of steel reinforcing rods (“rebar”) to absorb tensile loads. As mentioned above, however, the steel may corrode if the concrete cover is neutralized by carbonation or if chlorides penetrate it and break down the passive film on the steel. Oxides and hydroxides, the products of the corrosion, occupy a much larger volume than the original metal. Expansion of corrosion products puts the concrete into tension locally, resulting in cracking and spalling. It is often the effects of the production of these corrosion products that limits the life of a structure rather than a reduction in the load-bearing capacity of the rebar itself. The ability of steel and concrete to work together is also dependent on the bond between the two materials and the joinery details that enable forces to flow through the structures to the ground. The properties of reinforced concrete, whether pre-cast or cast-in-place, are critical to the design of effective joints to divert lateral forces, brittle failure, normal tolerance control, expansion and contraction, and corrosion. These characteristics require a

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure combination of structural design as well as material innovations for optimum performance. CHARACTERISTICS OF AN IDEAL CONCRETE Attempts to overcome the disadvantages of conventional concretes have been approached with what might be called a “Band-Aid” philosophy. Solutions to specific problems have been sought without considering the entire structure from an MSE systems perspective. In each case, the solution to a specific problem has been to add another component to the mix, thus increasing its chemical and microstructural complexity. The interaction of all these additives has sometimes created further problems. Consequently, the first priority in the design of the construction material of the future should be to apply an MSE systems approach that takes into account all of the problems to which structures are exposed. This may mean that different material designs will be required for different applications, that the surface layers of a structure may have to be different from the rest of the structure, or that a multilayered structure is needed. The construction industry has defined some immediate needs for conventional concrete: faster placement with smaller crews, easier forming methods, and faster strength gain to allow earlier stripping of forms. Large projects continue to be planned and executed around the world, and large-scale production would be essential before any nonconventional concrete could find widespread use. Processing performance improvements must also be attainable by the less-sophisticated smaller construction companies, whose huge number of smaller projects demand a wide variety of types of concrete deliveries. New techniques must also be sufficiently flexible to accommodate a wide range of applications in order to limit the number of techniques available and not confound the different users. Processing and netshape forming of a nonconventional concrete material must at least emulate the capabilities of current concrete materials as well as exceed them in as many areas as possible (Table 1–2). The following goals should be given high priority in the design of a “new improved concrete”: Developing a system in which setting and hardening coincide, as illustrated in Figure 1–16 and Figure 1–17. This would eliminate the current problem of the mix initially being stiff but friable and therefore susceptible to mechanical damage for a considerable period after placement.

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure TABLE 1–2 Notional Comparison of Conventional Concrete with Ideal Concrete Property Conventional Concrete Ideal Concrete Porosity Porous Impermeable as baseline Weight variations Heavy Light as baseline Workability Variable with insufficient control Controlled variability Chemical resistance Poor acid resistance but excellent water resistance Water as well as sulfate and acid resistance Shrinkage <1% of total concrete system but causes matrix micro-cracking Zero as baseline Frost resistance Requires air entrainment Resistant without air entrainment Fire resistancea Provides good insulation but can be explosive if internal moisture cannot escape Good insulation and nonexplosive Wear resistance Reasonable High Consistency of product Significant inconsistencies between batches Various levels of strength and rates of strength gain with small coefficients of ariation; consistent properties between batches Field monitoring quality assurance Slump, density, air-content testing Continuously controlled mixing, transporting, and placing to achieve targeted erformance using advanced sensor technology Off-line quality assurance Compressive, flexural tests, air void distribution, water content, cement content Continuously tested to achieve targeted performance using advanced sensor technology Source materials Constituent base materials from widespread regional resources, thus avoiding dependence on geographically limited sources Same but with increased use of waste materials Manufacturing flexibility Choice of locations, including central plant, construction site, remote locations using portable/temporary facilities Same Placement methods Static and moving formwork, extruding machines, mass placement by moving machines, pumping Same Placement environments Variety of environmental conditions, including extremes of temperature and humidity as well as under water Same Reinforcement Metal and rough-surface materials; chemically inert to steel Good bond to advanced composite materials as well as steel Chemical additives Too many Preferably fewer with higher predictability Labor Existing labor/skill resources for full-scale processing Same a   Concrete with low permeability explodes in fire because the moisture expands violently on evaporation and cannot escape.

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure FIGURE 1–16 Decrease in fluidity (setting) and the onset of strength (hardening) as a function of time after mixing. Solid lines indicate the properties of conventional concrete. Dotted lines indicate the desired properties of a nonconventional concrete. FIGURE 1–17 Electrical conductivity (which is indicative of setting) and heat of hydration (which is indicative of hardening) data for ordinary type I cement pastes. Source: Gorur et al, 1982. Reprinted with kind permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kidlington OX5 1GB, UK.

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Nonconventional Concrete Technologies: Renewal of the Highway Infrastructure Developing a system in which the microstructure—and hence properties—can be manipulated and controlled during processing (as in conventional metallurgy) in order to tailor the properties of the final product to the application.