Four-Dimensional X-ray Microtomography Study.ppt
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1、Dale Bentz, Phillip Halleck, Abraham Grader, and John Roberts RILEM Conference- Volume Changes of Hardening Concrete: Testing and Mitigation August 2006,Four-Dimensional X-ray Microtomography Study of Water Movement during Internal Curing,Outline,Need for internal curing Blended cements “Undercuring
2、” with internal curing Microtomography observations of water movement during internal curing Quantitative analysis of 3-D images Mixture proportioning for internal curing,What is internal curing (IC)? Answer: As being considered by ACI-308, “internal curing refers to the process by which the hydrati
3、on of cement occurs because of the availability of additional internal water that is not part of the mixing water.” For many years, we have been curing concrete from the outside in, internal curing is for curing from the inside out. Internal water is generally supplied via internal reservoirs, such
4、as saturated lightweight fine aggregates, superabsorbent polymers, or saturated wood fibers.,http:/ do we need IC? Answer: Particularly in HPC, it is not easily possible to provide curing water from the top surface (for example) at the rate that is required to satisfy the ongoing chemical shrinkage,
5、 due to the extremely low permeabilities that are often achieved in the concrete as the capillary pores depercolate. Capillary pore percolation/depercolation first noted by Powers, Copeland and Mann (PCA-1959).,How does IC work? Answer: IC distributes the extra curing water (uniformly) throughout th
6、e entire 3-D concrete microstructure so that it is more readily available to maintain saturation of the cement paste during hydration, avoiding self-desiccation (in the paste) and reducing autogenous shrinkage. Because the autogenous stresses are inversely proportional to the diameter of the pores b
7、eing emptied, for IC to do its job, the individual pores in the internal reservoirs should be much larger than the typical sizes of the capillary pores (micrometers) in hydrating cement paste and should also be well connected (percolated).,Cement paste,Water reservoir,Blended Cements,Internal curing
8、 can be particularly important in high-performance (low w/cm) blended cement systems Increased chemical shrinkage of pozzolanic and slag reactions Cement: 0.06 to 0.07 mL/g cement Silica fume: 0.22 mL/g cement Slag: 0.18 mL/g cement Fly ash (Type F): 0.12 to 0.16 mL/g cement Possible earlier deperco
9、lation of capillary pores and reduced permeability limiting water transport distances within the hydrating blended cement paste microstructure,Autogenous Deformation Results,IC added via fine LWA to increase total “w/c” from 0.30 to 0.38 or 0.40 Note chemical shrinkage of pozzolanic reaction of sili
10、ca fume with CH is 0.22 g water/g silica fume or about 3.2 times that of cement,Autogenous Deformation Results,IC added via fine LWA to increase total “w/c” from 0.30 to 0.38 Note chemical shrinkage of slag hydraulic reactions is 0.18 g water/g slag or about 2.6 times that of cement,“Undercuring” wi
11、th Internal Curing,Hydrating cement paste is a complex and dynamic porous media and as such, internal curing mixture proportions that supply only part of the total needed water (demand) can potentially exhibit some interesting results as illustrated in the schematic on the following slide,Empty and
12、Full Pores,Saturated curing,Sealed curing,RH = 98 %,RH = 93 %,Sufficient Internal curing,IC Reservoir,Cement paste,RH = 97 %,Insufficient Internal curing,Cement paste,RH = 90 %,IC Reservoir,Better hydration Only pores in reservoirs empty,Some increase in hydration Pores in both reservoirs and paste
13、empty,Cement paste,Cement paste,Less hydration Largest pores in paste empty,Four-Dimensional X-ray Microtomography,X-ray microtomography allows direct observation of the 3-D microstructure of cement-based materials Example: Visible Cement Data Set http:/visiblecement.nist.gov In October 2005, experi
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