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1、Ch4 Concrete and Mortar,4.1 Properties of fresh concrete（新拌砼）4.2 Mixing 拌合,Placing 浇筑 and Curing养护4.3 Properties of Hardened Concrete4.4 Mix proportioning and design,A Definition of Portland Cement Concrete（PCC）,A mixture of:Portland CementFine AggregateCoarse AggregateWaterAirCement and water combi
2、ne,changing from a moist,plastic consistency to a strong,durable rock-like construction material by means of a chemical reaction called“hydration”.,Volumetric proportions of constituents of PCC,Based on apparent density,Based on apparent density,concrete can be divided into:Normal weight concrete(23
3、50-2500kg/m3)Lightweight concrete(up to 1900kg/m3)Heavyweight concrete(up to 3200kg/m3),4.1 Properties of fresh concrete,4.1.1 Properties of fresh concrete 4.1.2 Factors affecting Consistency(稠度)and Workability(和易性)4.1.3 Segregation(离析)and Bleeding(泌水),4.1.1 Properties of fresh concrete,Workability（
4、和易性）Consistency（稠度）Slump（坍落度）,Workability（和易性）,Workability can be defined as the ease with which a fresh-concrete mix can be handled from the mixer to the final structure.Expressed by three characteristicsConsistency 稠度 Mobility 流动性Compatibility 相容性;保水性和易性是指新拌混凝土易于各工序施工操作（搅拌、运输、浇灌、捣实等）并能获得质量均匀、成型密实的
5、性能。影响和易性的主要因素：1、水泥浆的数量。与流动性以及粘聚性相关 2、水泥浆的稠度。3、砂率 此外水泥品种、骨料种类、粒形和级配、外加剂、时间、温度等，都对混凝土拌和物的和易性有一定影响,Consistency and Slump（稠度）（坍落度）,Consistency is a measure of concretes wetness or fluidity,and depends on the mix proportions and properties of ingredients.Measured bySlump test 坍落度试验,Slump test,Slump test,
6、Mold:slump cone坍落度筒Bottom diameter of 200mmtop diameter of 100mmheight of 300mm,Procedure of Slump test,a.Fill the cone using standard procedure,b.Strike off excess,c.Raise the mold and measure the displacement of the original center of the specimen,Slump,In the slump test,the number of mm the origi
7、nal center of the cone sinks or settles uniformly is measured to the nearest 5 mm.The measurement is called slump.E.g.slump can be 75,80,85mm,etc.,Types of slump,Near-zero slumpVery dry mixNormal slumpProperly proportioned concreteShear slumpConcrete lacks plasticity and cohesionCollapse slumpLean,h
8、arsh or extremely wet concrete,Recommended slumps for various type of construction,4.1.2 Factors affecting Consistency and Workability,Internal Factors Cement propertiesProportion of cement,Types of cement,FinenessAggregate propertiesSurface area of aggregate,shape,gradation,maximum size of aggregat
9、eWater contentWater-to-Cement ratio 水灰比AdmixturesWater-reducing admixture,air-entraining admixture减水剂、引气剂,4.1.2 Factors affecting Consistency and Workability,External FactorsEnvironmentTemperature,humidity,wind speed TimeDelay in placing,The relationship of slump and temperature,The relationship of
10、slump and time,4.1.3 Segregation and Bleeding,Segregation 离析 and Bleeding 泌水 results inNonhomogeneity 不均匀性Excessive bleeding-Weak top layer,Segregation,Segregation is the tendency for separation between large and fine particles of fresh concrete.Segregation results inRock pocket/honeycomb 蜂窝Sand str
11、eaks 砂条痕Crazing 龟裂Surface scaling 表面剥落,Segregation,Reasons of segregationVery wet mixtures,lack of fine particles,high slump concrete,OvervibrationControlling segregation Air-entainmentFinely divided materialsDropped vertically,Bleeding,Bleeding-A concrete mix that does not possess proper consistenc
12、y is unable to hold the mix water,which slowly gets displaced and then rises to the top of the form.Reasons of Excessive bleedingOvervibration,overtroweling,lean mixes,lack of fine sand,premature finishingControlling bleedingAir-entainmentIncrease the fineness of cementDecrease water-to-cement ratioRich mixes,Bleeding,
