《拌砼和易性》PPT课件.ppt

第四章 新拌砼的性质,Fresh concrete is a mixture of water, cement, aggregate and admixture (if any). After mixing, operations such as transporting, placing, compacting and finishing of fresh concrete can all considerably affect the properties of hardened concrete.,第一节和易性,一、砼的和易性定义 二、和易性的测定 三、影响和易性的因素 四、调整和易性的措施,The characteristics of fresh concrete(新拌混凝土) which affect full compaction are its consistency（稠度）, mobility（流动性） and compactability（易密性）. In concrete practice these are often collectively known as workability（工作性，和易性）. The ability of concrete to maintain its uniformity is governed by its stability（稳定性）, which depends on its consistency and its cohesiveness （粘聚性）.,. For practical purposes workability generally implies the ease with which a concrete mix can be handled from the mixer to its finally compacted shape. The three main characteristics of the property are consistency, mobility and compactability. Consistency is a measure of wetness or fluidity. Mobility defines the ease with which a mix can flow into and completely fill the formwork or mould. Compactability is the ease with which a given mix can be fully compacted, all the trapped air being removed.,一 砼和易性的定义：指砼拌和物易于施工操作（拌和、运输、浇灌、捣实）并能保证质量均匀、成型密实的性能。它包括流动性(mobility )、粘聚性(cohesiveness)和保水性(water retentivity)三方面的含义。,和易性包括三个方面： 1、流动性 指砼拌和物在自重或施工机械振捣的作用下，能够流动并均匀密实地填满模板的性能。 2、粘聚性 保持整体均匀，不发生分层离析现象。 3、保水性 保持水分不使之泌出。,Three tests widely used for measuring workability are the slump, compacting factor and V-B consistometer tests. Slump test was developed by Chapman in the United States in 1913. A 300mm high concrete cone, prepared under standard conditions (BS 1881 : part 2) is allowed to subside and the slump of reduction in height of the cone is taken to be a measure of workability.,混凝土分层离析,外分层,内分层,新拌混凝土泌水示意图,二、和易性的测定方法,砼拌和物坍落度试验 维勃稠度试验 扩展度试验,二、砼拌和物坍落度的测定,The three types of slump usually observed are true slump, shear slump and collapse slump, as illustrated in figure 13.2. A true slump is observed with cohesive and rich mixes for which the slump is generally sensitive to variations in workability. A collapse slump is usually associated with very wet mixes and is generally indicative of poor quality concrete and most frequently results from segregation of its constituent materials. Shear slump occurs more often in leaner mixes than in rich ones and indicates a lack of cohesion which is generally associated with harsh mixes (low mortar content).,由坍落度的不同可将砼拌和物分为： 大流动性砼 T 160 mm 流 动 性 砼 100 150 mm 塑 性 砼 50 90 mm 低 塑 性 砼 10 40 mm,拌合物的坍落度的选择：要根据结构构件截面尺寸大小、配筋疏密和施工捣实方法等来确定。,三、影响和易性的因素Factors Affecting Workability ：,1、水泥浆的数量（水灰比不变） 2、水泥浆的稠度（水泥量不变） 3、砂率Sp（砂重占砂石总重百分率） 4、水泥的品种和骨料的性质 5、外加剂 6、时间和温度,恒定用水量法则,在配制混凝土时，当所用粗、细骨料的种类及比例一定时，为获得要求的流动性，所需拌合用水量基本是一定的，即使水泥用量有所变动(1m3混凝土水泥用量增减50100kg)时，也无甚影响。这一关系称为“恒定用水量法则”，,Sp图 水泥浆用量一定,SpC图 T相同,合理砂率：是指在用水量及水泥用量一定的情况下，能使砼拌和物获得最大流动性，且能保持粘聚性、保水性良好时的砂率值。,四、改善和易性的措施,1、掺加外加剂 2、改善骨料的级配 3、在级配良好的情况下，尽可能采用较大粒径的骨料。 4、选用合理砂率。 5、在拌和砼时，当T过小，应保持W/C不变，增加水泥浆量。当T过大时，应保持Sp不变，增加砂石用量。,第二节 含气量,拌合物中的气相分两类： 1、浇筑振捣过程产生：粗大、连通。 2、引气剂引入：均匀、稳定、封闭、独立、细小（200m）。 普通混凝土气相含量约为1，加入引气剂的混凝土气相含量约为35。,引起剂加入后含气量的作用,1、和易性：含气量每增加1时，坍落度约提高1cm，若保持原流动性不变，则可减水约610。同时由于微小气泡的存在，阻滞了固体颗粒的沉降和水分的上升，加之气泡薄膜形成时消耗了部分水分，减少了能够自由移动的水量，从而使混凝土拌合物的保水性得到改善，泌水率显著降低，粘聚性也良好。 2、强度：含气量每增加1时，抗压强度将降低46，抗折强度降低23，而且随龄期的延长，引气剂对强度的影响越显著。 3、耐久性： 抗渗性能比不掺引气剂的混凝土提高50以上， 抗冻性可提高3倍左右。,第三节 湿表观密度,第四节 凝 结 时 间 将5mm及其以上的粗集料筛去 ，将剩余 砂浆置于规定容器内，振动密实，用规定的贯入阻力仪，测量贯入阻力和时间的变化关系。贯入阻力为3.5MPa和28MPa时分别对应初凝时间和终凝时间。,假凝,混凝土固相颗粒和水拌合后迅速产生硬化，但温度不上升。如再次搅拌拌合物又恢复流动性，并呈现正常凝结的现象，强度不明显降低。 原因： 1、石膏与水泥矿物粉磨