Nonconventional Concrete Technologies Renewal of the Highway Infrastructure (1997) / Chapter Skim
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1 Introduction and Background
1 Introduction and Background
Pages 11-38
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From page 11... ...
systems perspective- specifically its structure and composition, synthesis and processing, properties, and performance. The advantages and disadvantages of conventional PortiancI-cement concrete are summarized, and the characteristics of an ideal concrete are cliscussed.
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Source: NRC, 1989. properties, and performance exists for concrete structures as for any other engineered material.
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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 (rebar3 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.
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Cement Paste CaTcinecl Portland cement consists of several anhyclrous oxicles, primarily tricalcium silicate (C3S) an:l clicalcium silicate (C2S)
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The specified composition ranges for Type I Normal Portland cement are given in Table 1-1. Conventional cement is produced by mixing and grinding proportionate amounts of the raw materials (i.e., limestone or chalk Epre dominantly calcium carbonate]
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, giving rise to the initially rapid and progressively slower hydration process. The topochemical reaction involves the formation of a solid product ctirectly 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 cTinker/hydrate interface.
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0 5 30 1 2 6 \_ ~/ Age: Minutes Hours C4(A F) H43 1 2 7 28 99 ~/ I Dormant period ~ | _ Setting ~ I _ Hardening _~ Hi_ _ ^, ~-~ it_ _4 -- I FIGURE 1-5 Phases of cement paste as a function of time after mixing the dry cement clinker with water.
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image of (O-day old conventions Portland cement paste. Source: Scrivenep 1984.
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Moreover, CH is a major reason for the poor acid-resistance of concrete because it has a higher acici-solubility than does C-S-H. On the other hancI, CH buffers the pore solution pH to approximately 12.5, a level at which reinforcing steel is readily passivated.
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In normal concretes, the plastic sit 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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microstructures of conventional Portland cement concrete are extremely complex. They are investigated at a variety of scales, from Me macr>leve!
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the hydration processes. At the fine end of the scale, the fractal dimensions and gel porosity of the paste are stucliecI by more sophisticated methods, such as small-angle x-ray or neutron scattering, nuclear magnetic resonance, transmission electron microscopy, ant!
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The size and distribution of the voicis must be such that the distance between any water molecule and the nearest air void is minimal. These parameters determine concrete's degree of resistance to freeze/thaw cracking.
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amorphous silica) , together with a highrange water reducer, significantly reduces the coarse porosity of the cement and the propensity for cracking clue to expansive chemical reaction between the cement and certain types of aggregate, known as alkali aggregate reactions.
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that the majority of failures of concrete structures have nothing to do with the concrete's compressive strength. Although concrete can be regarded as a clispersion-strengtheneci, ceramic-matrix composite, it is unlike other engineered composites in that its mechanical (anal others properties do not follow the law of mixtures,4 as illustrated in Figure 1 - 9.
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"Labcrete" is generally far more homogeneous than commercially macle concrete, which is produced in far larger quantities, is produced in several batches in real structures, and is very heterogeneous because as every concrete producer knows it is impossible to reproduce a concrete exactly from one batch to the next, whether in the laboratory, processing plant, or construction site. PERFORMANCE OF CONVENTIONAL CONCRETE Although, as stated above, concrete has been almost universally specified on the basis of its compressive strength at 28 days after casting, concrete structures are almost always designed with a sufficiently high safety margin that catastrophic failures due to lack of intrinsic strength (i.e., overload failures are rare.
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/ ~ ~ ~ FA (w/c = 0.56) I ~ I I I 1 11 1 1 ~1 1 1 1 25 years years years FIGURE 1-10 Compressive strength of conventional Portland cement concrete (CPC)
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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, 19951.
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The energy contents of unreinforced concrete and steel reinforcec! concrete are estimated at 450 to 750 ant!
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. 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.
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First, the inherent capillary porosity of Portland cement allows aggressive species to move into the concrete,
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Porosity can be reclucect 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 cluring hydration leads to plastic shrinkage and microcracking, which can acic!
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The brittle failure of Portland cement concrete structures is prevented by the use of steel reinforcing rods ("rebar") to absorb tensile loacls.
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The construction industry has defined some immediate neecis for conventional concrete: faster placement with smaller crews, easier forming methods, ant! faster strength gain to allow earlier stripping of forms.
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1 36 IVOlVCOlVVE1~77ONAL CONCRETE TECHNOLOGIES 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 as well as sulfate and acid water resistance resistance Shrinkage <1% of total concrete system but Zero as baseline causes matrix micro-cracking Frost resistance Requires air entrainment Resistant without air entrainment Fire resistancea Provides good insulation but can be Good insulation and nonexplosive explosive if internal moisture cannot escape Wear resistance Reasonable High Consistency of Significant inconsistencies between Various levels of strength and rates product batches of strength gain with small coefficients of variation; consistent properties between batches Field monitoring Slump, density, air-content testing Continuously controlled mixing, quality assurance transporting, and placing to achieve targeted performance using advanced sensor technology Off-line quality Compressive, flexural tests, air void Continuously tested to achieve assurance distribution, water content, cement targeted performance using content advanced sensor technology Source materials Constituent base materials from Same but with increased use of widespread regional resources, thus waste materials avoiding dependence on geographically limited sources Manufacturing Choice of locations, including Same flexibility central plant, construction site, remote locations using portable/temporary facilities Placement methods Static and moving formwork, Same extruding machines, mass placement by moving machines, pumping Placement Variety of environmental conditions, Same environments including extremes of temperature and humidity as well as under water Reinforcement Metal and rough-surface materials; Good bond to advanced com chemically inert to steel posite materials as well as steel Chemical additives Too many Preferably fewer with higher predictability Labor Existing labor/skill resources for Same full-scale processing aConcrete with low permeability explodes in fire because the moisture expands violently on evaporation and cannot escape.
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and heat of hydration (which is indicative of hardening) data for ordinary type I cement pastes.
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38 NONCONVEN77ONAL CONCRETE TECHNOLOGIES therefore susceptible to mechanical damage for a considerable perioc! after placement.
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