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2015SpringShotcreteEMag

Durability and Exposure Conditions of Cementitious Materials Deterioration Mechanisms By Jonathan E. Dongell Tration of concrete or shotcrete placed in other deleterious ions to move into, out of, andport mechanisms and accompanied deleteriousreactions. Mass transport allows moisture, salt, or-here is a substantial amount of literature tobe found regarding specific types of deterio various exposure conditions. This paper presents within the matrix of the cementitious structure. an introduction and methodical view of mass Salts can precipitate out of solution or crystallize, transport and its role in various exposure conditions filling voids, cracks, and space around aggregate given the presence of sufficient moisture. A simpli- created by expansion and contraction. This filling fied lineal­progression chart showing the various of matrix space creates restriction, which increases mass transport-induced deterioration mechanisms internal stress upon further expansion and contrac- that facilitate deterioration is introduced. The tion. Associated stress-induced cracking acceler- overall objective of this article is to offer a simpli- ates the rate of deterioration of the structure. Mass fied interpretation of the difficult concepts of mass transport may also provide the means by which transport and deterioration mechanisms. deleterious ions enter the matrix, or facilitate the localized buildup of certain ions, which in turn Material-Related Contributing may initiate secondary deleterious reactions. Factors Mass Transport For a cementitious material placed in an expo- In general, the term “mass transport” as it relates sure condition that is considered to be aggressive, to concrete is used to describe the overall mecha- there is a direct correlation between the suscepti- nism by which matter is moved into, out of, within, bility of that material and the rate of deterioration. or through the cementitious matrix. While mass Therefore, while this article deals only with mass transport mechanisms can proceed wherever water transport and mechanisms of deterioration, it must or sufficient moisture is present, whether or not be stated that mixture designs that minimize perme- matter is present in solution to transport, it is nev- ability and construction practices that provide ertheless the presence of deleterious material (salts uniformity in density and consolidation and mitigate or ions) or the existence of some chemical gradient cracking and other defects are key factors in pro- between the water and cementitious material that ducing a durable cementitious material. Addition- is of concern. Therefore, mass transport is typi- ally, the use of durability enhancers (pozzolanic or cally thought of as a combination of both water polymeric) within the mixture design, or the imple- or moisture and material in solution. Mass transport mentation of appropriate post-placement protective can be divided into two main transport mechanisms measures, help to ensure that the structure remains of “convection” or “wicking” (refer to Fig. 1). sound and achieves its anticipated service life. Convection Physical-Induced Deterioration Convection is the transport of a material in versus Mass Transport-Induced is a transport mechanism resulting in the random. Diffusionadvection anddiffusionsolution by Deterioration spreading or mixing of material in solution. Advec- Typically, the deterioration of a cementitious tion is a transport mechanism caused by a directional structure is the result of a multi-mechanism phe- bulk motion of fluid (water or moisture) resulting in nomenon. Freezing-and-thawing deterioration, the directional transport of material in solution. For for example, is often thought of as strictly dete- example: if dye were poured into a river, advection rioration by expansion and contraction of the would be the force (in this instance hydraulic force) material due to temperature gradient; however, carrying the dye downstream via bulk motion, and accelerated damage can occur due to mass trans- the spreading of the dye plume outward in an ever- 8 Shotcrete • Spring 2015


2015SpringShotcreteEMag
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