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Infrastructure Materials Science

by: Rowena Stamm

Infrastructure Materials Science CE 305

Marketplace > University of New Mexico > Civil Engineering > CE 305 > Infrastructure Materials Science
Rowena Stamm
GPA 3.73


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Class Notes
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This 2 page Class Notes was uploaded by Rowena Stamm on Wednesday September 23, 2015. The Class Notes belongs to CE 305 at University of New Mexico taught by Staff in Fall. Since its upload, it has received 15 views. For similar materials see /class/212182/ce-305-university-of-new-mexico in Civil Engineering at University of New Mexico.


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Date Created: 09/23/15
UI Concrete De nitions and Relevance 1 Ettringite De nition Ettringite is a calcium sulphoaluminate that is formed during the early hydration between C3A gypsum and water It has a brous crystalline morphology When gypsum is depleted during hydration process ettringite will partially or completely convert into monosulphoaluminate Relevance Ettringite contributes to the initial setting mechanism of concrete When hardened concrete is subjected to sulphate attack the formation of ettringite which can lead to internal volume expansion thus cracking of the concrete 2 Calcium Hydroxide CH Cgstals De nition Calcium Hydroxide CH is a crystalline compound that forms as a hydration byproduct during the reaction of C3S and CZS with water Its formula is Ca OH2 Relevance Calcium Hydroxide is primarily responsible for maintaining the high alkaline pH environment in concrete this environment protects the reinforcing steel from corrosion CH contributes little to strength and when it is present in the excessive amounts it may be the source of durability problems It can dissolve in owing pore solution then leach out of the concrete leaving behind voids CH can participate in sulphate attack and other chemical attack mechanism that lead to internal volume expansion and cracking of the concrete Heat of Hydration De nition The heat released during the exothermic reaction of the four major compounds of the Portland cement and water Relevance The industry is most concerned with heat released during the early stages of cement hydration In large concrete elements excessive early heat release can lead to large thermal gradients across the structural cross section this may lead to thermal microcracking lowering the strength and durability of concrete Engineers can take advantages of heat of hydration to maintain acceptable temperature of concrete during cold weather concreting Acceleration Period of Cement Hydration De nitionThe stage of hydration after the dormant period that starts 4 hours or so after casting Relevance The acceleration period starts when the thin layers of hydrate mostly sulphoaluminate based surrounding the cement grains rupture due to osmotic pressures Cement reacts with water normally after this point Therefore the acceleration period is the stage during which signi cant hydration strength gain of concrete occurs Pozzolanic Reaction De nition The pozzolanic reaction is one between a reactive silicates contained in waste materials such as y ash slag volcanic ash metakolin or silica fume b calcium hydroxide CH usually provided through the hydration of Portland cement and c water The reaction produces form of Calcium Silicate Hydrate CSH Relevance The combination of Portland cement with a pozzolan allows one to eliminate much of detrimental CH from the concrete system and replace it with strong CSH This results in longterm improvement of strength and durability due to lowering permeability and diffusivity of concrete Entrained Air De nition A system of ne air bubbles that is produced in concrete by air entraining admixtures The objective of air entrainment is to produce an average bubble spacing factor of less than 250 pm with a total air content of 46 depending on the projected severity of exposure to freezethaw cycles Relevance Air entrainment is primarily used to prevent the cracking of critically saturated concrete during frequent exposure to freezing and thawing Air entrainment produces a lighter weight but weaker concrete in general 1 entrainment results 3 reduction in strength The detrimental aspect of strength reduction can be partially offset however because air entrainment improves workability and cohesiveness and as wc ration can be reduced Capillary Pores De nition Capillary pores are pores of 550 mm diameter depending on concrete age that form ducts often continuous within the binding matrix and between region of hardened cement paste Relevance Capillary pores are responsible to uid transport in concrete structure Thus aggressive ions that can attack the chemical compounds within concrete enter structures through capillary pores and microcracks Capillary CE 305 iFall 2008 1 Dr M Reda Taha porosity is often a signi cant component of concrete and therefore also reduces the strength Strength increases with time as increased hydration produces reaction products that moves into and deposit within capillary pore space Transition Zoneg TZ De nition The interfacial transition zone TZ is a thin sheath of low quality materials that surrounds coarse aggregate particles and larger particles of ne aggregate The TZ varies in thickness but is thought on an average to be about 50 pm thick The transition zone has higher porosity than the bulk matrix contains higher proportion of ettringite and calcium hydroxide CH and lower proportion of hardened cement paste Relevance The transition zone is a weak phase weakest link of the concrete that is located in the regions where shear and tensile stresses are largest This zone must transfer stress from bulk matrix to the aggregate particles It is a zone where volumetric expansion may occur due to difference in shrinkage strain between the aggregate and the paste matrix This weak zone is therefore prone to microcracking which reduces strength and durability of concrete Bleeding De nition The tendency of water to migrate towards the surface of a fresh concrete mass as the solid particle within the concrete mass settle Relevance Bleeding is the natural process that is bene cial to concrete since it removes water from the concrete mass the resultant concrete mass have a lower wc ratio than it had when it was cast into the forms Bleed water also acts as a buffer between the concrete and the atmosphere and allow the surface of the concrete to cure properly Excessive bleeding that may occur in high wc ratio concrete with insuf cient nes will result in vertical bleeding channels piping that can carry unhydrated cement to the surface and produce permeable and weak surface Segregation De nition The separation of coarse aggregate from the binding matrix in fresh concrete due to the tendency of higher density so settle to the bottom of the forms Relevance Segregation normally occurs in high wc concrete or in those concrete with inadequate cohesion stickiness due to insuf cient ne materials Segregation produces a heterogeneous concrete mass with variable properties through the cross section Excessive segregation leads to honeycombing where the aggregate particles have very little cement paste bridging between them Slump Loss De nition The reduction of workability of concrete with time after water is added as measured by the loss of slump Slump loss is associated with formation of early hydration products and in particular ettringite from reaction between C3A gypsum and water Admixtures with high sulphate content eg superplasticizers can result in slump loss Relevance In normal strength concrete slump loss is usually slow enough that there is adequate period after the casting to place and consolidate the concrete When superplasticizers are used however slump loss can be rapid This means that much better site control is necessary or other special techniques need to be employed to ensure that there is adequate time for the placement and consolidation of the concrete Plastic Shrinkage De nition Shrinkage strain created by water menisci in the capillary pore in the surface at the surface of the fresh concrete Such menisci can only form when the bleed water at surface has completely evaporated or when the rate of evaporation is greater then the rate of bleed water supply to the surface Relevance Plastic shrinkage strains produce tensile strain at the concrete surface If the tensile strength of concrete is insufficient then a network of plastic shrinkage cracks will occur These cracks are unsightly and if cracking is extensive can lead to less durable concrete Superplasticizers De nition A high range very effective water reducing chemical admixture used primarily to improve the workability of concrete with relatively low watercementitious wcm materials ratios Relevance Concrete with low watercementitious wcm in the range of 02 to 03 would not be possible without superplasticizers Superplasticizers can be used to a improve the workability of non workable concrete b improve the strength of concrete by reducing the wc ratio while maintaining the required workability or c reduce the amount of mix water while maintaining the same target strength and workability Superplasticizers are expensive and their use often involves a substantial increase in the cost of the concrete CE 305 iFall 2008 2 Dr M Reda Taha


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