《FourDimensional Xray Microtomography Study.ppt》由会员分享，可在线阅读，更多相关《FourDimensional Xray Microtomography Study.ppt（25页珍藏版）》请在三一办公上搜索。
1、Dale Bentz,Phillip Halleck,Abraham Grader,and John Roberts RILEM Conference-Volume Changes of Hardening Concrete:Testing and MitigationAugust 2006,Four-Dimensional X-ray Microtomography Study of Water Movement during Internal Curing,Outline,Need for internal curingBlended cements“Undercuring”with in
2、ternal curingMicrotomography observations of water movement during internal curingQuantitative analysis of 3-D imagesMixture 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 hydration of cement occ
3、urs 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 as saturated lightwei
4、ght fine aggregates,superabsorbent polymers,or saturated wood fibers.,http:/,Why 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,due to the extremely low
5、 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 the entire 3-D concrete microstruc
6、ture 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 being emptied,for IC to do its job,th
7、e 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 can be particularly important in high-pe
8、rformance(low w/cm)blended cement systemsIncreased chemical shrinkage of pozzolanic and slag reactionsCement:0.06 to 0.07 mL/g cementSilica fume:0.22 mL/g cementSlag:0.18 mL/g cementFly ash(Type F):0.12 to 0.16 mL/g cementPossible earlier depercolation of capillary pores and reduced permeability lim
9、iting 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 0.40Note chemical shrinkage of pozzolanic reactionof silica fume with CH is 0.22 g water/g silica fume or about 3.2
10、 times that of cement,Autogenous Deformation Results,IC added via fine LWA to increase total“w/c”from 0.30 to 0.38Note chemical shrinkage of slag hydraulic reactionsis 0.18 g water/g slag or about 2.6 times that of cement,“Undercuring”with Internal Curing,Hydrating cement paste is a complex and dyna
11、mic 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 Full Pores,Saturated curing,Sealed curing,RH=98%,RH=93%,Sufficient
12、 Internal curing,IC Reservoir,Cement paste,RH=97%,Insufficient Internal curing,Cement paste,RH=90%,IC Reservoir,Better hydrationOnly pores in reservoirs empty,Some increase in hydrationPores in both reservoirsand paste empty,Cement paste,Cement paste,Less hydrationLargest pores in paste empty,Four-D
13、imensional X-ray Microtomography,X-ray microtomography allows direct observation of the 3-D microstructure of cement-based materialsExample:Visible Cement Data Set http:/visiblecement.nist.govIn October 2005,experiments were conducted at Pennsylvania State University to monitor three-dimensional wat
14、er movement during internal curing of a high-performance mortar over the course of two days(time is the 4th dimension),Mixture Proportions,w/c=0.35Blend of four sands(Ferraris)to improve particle packingLWA added in saturated surface dry(SSD)conditionSSD specific gravity of 1.7Commercial cement no p
15、articles larger than 30 m diameterHydration conducted at 30 oCmaintained by circulating fluid from a temperature controlled bath,After mixing,1 d hydration,2 d hydration,Subtraction:1 d after mixing,Aqua indicates dryingRed indicates wetting,All images are 13 mm by 13 mm,Three-dimensional subtracted
16、 imageof 1 d hydration initial microstructureshowing water-filled pores that have emptied during internal curing(4.6 mm on a side),2-D image with water evacuatedregions(pores)overlaid on original microstructure(4.6 mm by 4.6 mm),Four-Dimensional X-ray Microtomography,Quantitative Analysis,Four-dimen
17、sional image sets analyzed to estimate volume of water moving from LWA to cement paste during first 2 d of hydrationAnalysis based on changes in greylevel histogram with timeResults compared to conventional measures of hydration including chemical shrinkage,non-evaporable water content,and heat rele
18、ase,Preprocessing of 3-D Image Data,Median filter applied to remove noise and sharpen greylevel histogram,Temporal Analysis of Greylevel Histograms,Change in“empty”pores with time quantified,Tomography Water Movement vs.Hydration Measures,Good”quantitative”agreement between estimated water movement
19、volume and other measures of hydration,Four-Dimensional X-ray Microtomography,Empty porosity within LWA from analysis of 3-D microtomography data sets scales“exactly”with measured chemical shrinkage of the cement for first 36 h of curing,Mixture Proportioning for Internal CuringQuestions to Consider
20、 When Using IC,How much water(or LWA)do I need to supply for internal curing?How far can the water travel from the surfaces of the internal reservoirs?How are the internal reservoirs distributed within the 3-D concrete microstructure?AnswersMay be found at the NIST internal curing web site:http:/cik
21、s.cbt.nist.gov/lwagg.html,How much water(or LWA)do I need to supply for internal curing?,Answer:Equation for mixture proportioning(Menu selection#1)MLWA=mass of(dry)LWA needed per unit volume of concreteCf=cement factor(content)for concrete mixtureCS=(measured via ASTM C 1608-05 or computed)chemical
22、 shrinkage of cementmax=maximum expected degree of hydration of cement,(w/c)/0.36 or 1S=degree of saturation of LWA(0 to 1 when added to mixtureLWA=(measured)absorption of lightweight aggregate(use desorption measured at 93%RH(potassium nitrate saturated salt solution)via ASTM C 149804a),How far can
23、 the water travel from the surfaces of the LWA?,Answer:Equation balancing water needed(hydration)vs.water available(flow)(Menu selection#2)“Reasonable”estimates-early hydration-20 mmmiddle hydration-5 mmlate hydration-1 mm or less“worst case”-0.25 mm(250 m)Early and middle hydration estimates in agr
24、eement with x-ray absorption-based observations on mortars during curing,How are the internal reservoirs distributed within the 3-D concrete microstructure?,Answer:Simulation using NIST Hard Core/Soft Shell(HCSS)Computer Model(Menu selections#3 and#4)Returns a table of“protected pastefraction”as a f
25、unction of distance from LWA surface,Yellow Saturated LWARed Normal weight sandBlues Pastes within variousdistances of an LWA,10 mm by 10 mm,Mortar from CT experiment97%of paste within 2 mm of LWA,Summary,Internal curing especially critical in high performance blended cement systemsToo little internal curing can actually result in a lower internal RH than in a system with no internal curingX-ray microtomography can be used to“observe”water movement during internal curing in four dimensionsInternet tools exist to assist in mixture proportioning for internal curing,
