durability - Durability Construction Project Life Cycle...

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Durability Dr. Kimberly Kurtis School of Civil Engineering Georgia Institute of Technology Atlanta, Georgia Construction Project Life Cycle Time Concept Engineering and Design Procurement Construction Start-up Operation Decomissioning Construction Costs Other 55% Materials 45% Source: Engineering News-Record Indexes of Costs Trend, 1949-1979. Infrastructure Durability Cost of Environmental Remediation, 1987-2000: air pollution $26.7 - 44 billion radiation pollution $327 - 896 billion water pollution $34.4 - 57.5 billion Cost of Bridge Repair, 1981-2000: $102.6 billion Forms of Attack Chemical –leaching, sulfate attack, delayed ettringite formation, acid attack, carbonation, alkali-aggregate reactions Electrochemical – corrosion Physical Attack – freeze/thaw, abrasion, erosion, fire Permeability Permeability - measure of the rate of flow through a material; related to interconnectivity of pores, microcracks, and cracks Permeability of the paste Permeability and gradation of the aggregate Quality of the paste and ITZ Relative proportion of aggregate to paste
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Permeability Largely influenced by the structure (I.e., size and distribution of voids and cracks) and porosity of the HCP, which in turn is strongly influenced by the w/c or w/cm Permeability Related to capillary pore volume Capillary space in hydrated cement pastes increase markedly for w/c>0.42 Size of the pores is very important; water moves through capillary pores (10-50+nm), NOT gel pores (<10nm) In concrete, ITZ is also important for transport Permeability: Cement Paste Flow of water through cement paste obeys D’Arcy’s law for flow through porous media: dq/dt = v = K p (h/x) Where dq/dt or v is the rate of flow of water K p is the permeability coefficient h is the hydraulic pressure exerted by the water x is the specimen thickness K p is strongly dependent on the capillary porosity, which is related to the w/c or w/cm and the degree of hydration Typical values for K p Hardened cement paste, kept moist 0.1-120x10 -12 cm/s (for w/c 0.30-0.70) Coarse aggregate 1.7-3.5x10 -13 cm/s High strength concrete 1x10 -13 cm/s Mature, good quality concrete 1x10 -10 cm/s Why is the permeability in normal strength concrete so much larger? Permeability: Concrete • Influence of curing Permeability: Concrete • Influence of SCMs Silica fume reduces the number of pores < 10 µ m
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Chemical Transport In addition to flow of water through the concrete, we must also consider chemical interactions between the ingressing water or solution and the pore solution and solids in the concrete (I.e., adsorption, precipitation, dissolution) The moisture content of the concrete also affects the rate of ingress; D’Arcy’s Law is applicable to saturated concrete. Capillary draw is important in dry or semi-dry concrete Fick’s Second Law Often used to describe transport in near-saturated concrete where no pressure head exists dC/dt = K d ( δ 2 C/ δ x 2 ) Where C = concentration t = time K d = diffusion coefficient x = depth For semi-infinite slab, the solution to this equation is C(x,t) = C o {1-erf[x/(2 K d t)]}
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This note was uploaded on 08/23/2011 for the course CEE 8813b taught by Professor Kurts during the Spring '07 term at Georgia Tech.

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durability - Durability Construction Project Life Cycle...

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