早龄期混凝土.ppt

第十章第十章 早龄期混凝土早龄期混凝土10.1 定义和意义10.2 配料、拌合与运输10.3 浇灌、捣实和抹面10.4 养护和脱模10.5 工作性10.6 坍落度损失10.7 离析和泌水.第十章第十章 早龄期混凝土早龄期混凝土10.8 早期体积变化10.9 凝结时间10.10 混凝土的温度10.11 质量控制.第十章第十章 早龄期混凝土早龄期混凝土10.1 定义和意义 大量的工程实践告诉我们：从配料、搅拌、运输、浇灌、捣实、表面修整及养护等工序所采取的措施正确与否直接关系到混凝土的质量。尽管这一时期和混凝土的使用寿命相比微不足道。所谓早龄期？开始于凝结硬化前，结束于1-2天的硬化混凝土。.第十章第十章 早龄期混凝土早龄期混凝土10.2 配料、拌合与运输 配料：计量过程 质量法 体积法 采用体积法计量时，应该注意砂子的容胀问题！坍落度损失：如何改善？.vFigure 10-1 Centrally mixed concrete in a ready-mixed concrete plant.第十章第十章 早龄期混凝土早龄期混凝土10.3 浇灌、捣实与抹面 .第十章第十章 早龄期混凝土早龄期混凝土10.3 浇灌、捣实与抹面 .第十章第十章 早龄期混凝土早龄期混凝土10.3 浇灌、捣实与抹面 .第十章第十章 早龄期混凝土早龄期混凝土10.3 浇灌、捣实与抹面 捣实：使混凝土充填模板，排出气体并使之密实的过程。常用振捣器。插入式 附着式 抹面=找平+抹平.第十章第十章 早龄期混凝土早龄期混凝土10.4 养护和脱模 养护的目的：1.防止水分蒸发；2.控制混凝土的温度。（书上译得有误，P232）The two objectives of curing are to prevent the loss of moisture and to control the temperature of concrete for a period sufficient to achieve a desired strength level.第十章第十章 早龄期混凝土早龄期混凝土10.4 养护和脱模 脱模早晚关系到模板的周转速度，影响到所需模板的数量，进而关系到工程的成本。何时脱模？能够承受自重及施工时候所施加的荷载。.第十章第十章 早龄期混凝土早龄期混凝土10.5 工作性10.5.1 定义和意义 ASTM C-125 定义：工作性是混凝土所具有的一种性质，它决定了在保证最低限度均匀性的前提下，操作一定数量新拌混凝土所需要的功。是混凝土的一项综合性能，至少应该包含以下二个方面的性质：稠度和稳定性。.第十章第十章 早龄期混凝土早龄期混凝土10.5 工作性 稠度：是简单表征新拌混凝土流动能力或流动性的指标。常用坍落度试验或维勃稠度试验测得。稳定性：反映了混凝土防止其组分相互分离能力大小的一个指标。即泌水和离析越小，稳定性越好。通常用粘聚性或内聚性来定性地衡量泌水和离析。.第十章第十章 早龄期混凝土早龄期混凝土10.5 工作性 谈论工作性的好坏与否，必须与施工条件联系起来。.第十章第十章 早龄期混凝土早龄期混凝土10.5 工作性 10.5.2 测量方法 有多种，但目前尚无理想的表征工作性的方法。常用的有：坍落度法 维勃稠度 密实系数 .第十章第十章 早龄期混凝土早龄期混凝土10.5 工作性 10.5.2 测量方法 坍落度法：所用设备非常简单，只包括一根捣棒和一个高300mm,上口直径100mm，下口直径200mm的截口圆锥筒。先将混凝土填满圆锥筒，然后慢慢将圆锥筒提起。圆锥状的混凝土因重力作用而坍落。因坍落而减小的高度就是坍落度。.第十章第十章 早龄期混凝土早龄期混凝土10.5 工作性 10.5.2 测量方法 坍落度法：1.Stand on the two foot pieces of cone to hold in firmly in the place during Steps 1 though 4.Fill cone mold 1/3 full by volume 2-5/8“(67 mm)high with the concrete sample and rod it with 25 strokes using a round,straight steel rod of 5/8(16 mm)diameter 24“(600 mm)long with a hemispherical tip end.Uniformly distribute strokes over the cross section of each layer.For the bottom layer,this will necessitate inclining the rod slightly and making approximately half the strokes near the perimeter(out edge),then progressing with vertical strokes spirally toward the center.第十章第十章 早龄期混凝土早龄期混凝土10.5 工作性 10.5.2 测量方法 坍落度法：2.Fill cone 2/3 full by volume(half the height)and again rod 25 times with rod just penetrating into,but not through,the first layer.Distribute strokes evenly as described in Step 1.第十章第十章 早龄期混凝土早龄期混凝土10.5 工作性 10.5.2 测量方法 坍落度法：3.Fill cone to overflowing and again rod 25 times with rod just penetrating into,but not through,the second layer.Again distribute strokes evenly.第十章第十章 早龄期混凝土早龄期混凝土10.5 工作性 10.5.2 测量方法 坍落度法：4.Strikes off excess concrete form top of cone with the steel rod so that the cone is exactly level full.Clean the overflow away from the base of the cone mold.第十章第十章 早龄期混凝土早龄期混凝土10.5 工作性 10.5.2 测量方法 坍落度法：5.Immediately after completion of Step 4,the operation of raising the mold shall be performed in 52 sec.by a steady upward lift with no lateral or torsional motion being imparted to the concrete.The entire operation from the start of the filling through removal of the mold shall be carried out without interruption and shall be completed within an elasped time of 2-1/2 min.第十章第十章 早龄期混凝土早龄期混凝土10.5 工作性 10.5.2 测量方法 坍落度法：6.Place the steel rod horizontally across the inverted mold so that the rod extends over the slumped concrete.Immediately measure the distance from bottom of the steel rod to the displaced original center of the specimen.This distance,to the nearest 1/4 in(6 mm),is the slump of the concrete.If a decided falling away or shearing off concrete from one side or portion of the mass occurs,disregard the test and make a new test on another portion of the sample.第十章第十章 早龄期混凝土早龄期混凝土10.5 工作性 10.5.2 测量方法 坍落度法：.第十章第十章 早龄期混凝土早龄期混凝土10.5 工作性 10.5.2 测量方法 坍落度法：此法不适用流动度过大或过小的混凝土。对稠度或者是流动性测量较实用。但不能表征混凝土的内聚性。.第十章第十章 早龄期混凝土早龄期混凝土10.5 工作性 10.5.2 测量方法 维勃稠度法：.第十章第十章 早龄期混凝土早龄期混凝土10.5 工作性 10.5.2 测量方法 密实系数法：.第十章第十章 早龄期混凝土早龄期混凝土10.5 工作性 10.5.3 影响工作性的因素及控制1.用水量2.水泥用量3.骨料特征4.外加剂.第十章第十章 早龄期混凝土早龄期混凝土10.6 坍落度损失10.6.1 定义：在实践中，如果说某一种混凝土有坍落度损失，则通常指一种很快发生且特别大的稠度损失。现象：稠度随时间流失而逐渐减小 原因：水分失去？减水剂？.第十章第十章 早龄期混凝土早龄期混凝土10.6 坍落度损失10.6.2 重要性：你能说明混凝土坍落度损失对混凝土施工的重要性吗？.第十章第十章 早龄期混凝土早龄期混凝土10.6 坍落度损失10.6.3 原因和控制：你能说出混凝土坍落度损失的原因吗？水泥？作业因素吗？环境因素？.Chapter 10 concrete at early ages10.5.3 Factors Affecting Workability and Their Control vWater content.for a given maximum size of coarse aggregate,the slump or the consistency of concrete is a direct function of the water content;i.e.,within limits it is independent of other factors such as aggregate grading and cement content.vConcrete mixtures with very high consistency tend to segregate and bleed,thereby adversely affecting the finishability;mixtures with too low a consistency may be difficult to place and compact,and the coarse aggregate may segregate on placement.Chapter 10 concrete at early agesvCement content.With conventional portland-cement concrete at a given water content,a drastic reduction of the cement content would produce a harsh mixture with poor finishability.Concrete mixtures containing a very high cement content or high proportion of fine particles show excellent cohesiveness but tend to be sticky.Chapter 10 concrete at early agesvAggregate characteristics.The particle size of coarse aggregate influences the water requirement for a given consistency(Table 9-2).Also,very fine sands or angular sands require more water for a given consistency.Alternatively,they will produce harsh and unworkable mixtures at the water content that might have been adequate with a coarse or a well-rounded sand.Chapter 10 concrete at early agesvAdmixtures.when the water content of a concrete mixture is held constant,the addition of a water-reducing admixture will increase the consistency.Entrained air increases the paste volume and improves the consistency of concrete for a given water content(Table 9-2).It also increases cohesiveness by reducing bleeding and segregation.The improvement in consistency and cohesiveness by air entrainment is more pronounced in harsh and unworkable mixtures such as those used in mass concrete,which has a low cement content.Pozzolanic admixtures tend to reduce bleeding and improve the cohesiveness of concrete.Fly ash,when used as a partial replace ment for fine aggregate,generally increases the consistency at a given water content.Chapter 10 concrete at early agesv10.6.Slump Loss v10.6.1 Definitions vSlump loss is defined as the loss of consistency in fresh concrete with elapsed time.vUnder normal conditions,the volume of hydration products during the first 30 minutes after the addition of water to cement is small and the slump loss is negligible.Thereafter,concrete starts losing slump at a rate determined mainly by elapsed time after hydration,temperature,cement composition,and the admixtures present.vUnder some conditions,a concrete mixture exhibiting an unusually large loss of slump during the first 30 minutes or 1 hour may have the effect of making the mixing,convening,placing,compacting,and finishing operations difficult or,at times,even impossible.Chapter 10 concrete at early agesv10.6.2 Significance vThe premature stiffening of fresh concrete,depending on when the problem appears,may mean an increase in the mixer drum torque,requirement of extra water in the mixer or at job site,hang-up of concrete within the drum of a truck mixer,difficulty in pumping and placing the concrete,extra labor for handling and finishing operations,and finally loss of production and quality of workmanship,loss of strength,durability,and other properties when the retempering water is excessive or is not mixed thoroughly.Chapter 10 concrete at early agesv10.6.3 Causes and Control vThe primary causes of slump-loss problems with concrete are as follows:(1)the use of an abnormal-setting cement;(2)unusually long time for mixing,transporting,placement,compaction,or finishing operation;(3)high temperature of concrete due to excessive heat of hydration and/or the use of concrete-making materials that are stored at a high ambient temperature.(see fig.10-2).Chapter 10 concrete at early agesvTABLE 10-2 Slump(in.)Concrete mix CementInitial30 min60 min 90 min120 min Concrete temperature 701A77533/421/42B71/843/431/4217/83A543/831/821/414B51/431/4213/411/45A35/831/425/817/813/86B325/8217/8Concrete temperature 85789101112ABABAB71/8755333/453/85441/8321/443/8335/82215/825/8225/817/817/813/815/813/815/811/811/83/4.Chapter 10 concrete at early agesvFrom fig.10-2,The following conclusions were drawn from the investigation:v1 In general,the amount of slump loss was proportional to the initial slump;the higher the initial slump,the higher the slump loss.For example,in the case of Cement A,at the close of the 2-hour test at 23 (70),Concrete mix 1(initial slump 180 mm or 71/2 in)lost 125 mm(5 1/4 in)slump,whereas with Concrete Mix 3(initial slump 125 mm or 5 in)lost 88 mm(3 1/2 in)slump,and concrete Mix 5(initial slump 85 mm or 3-5/8 in.)lost 57 mm(2 1/4 in.)slump.Regardless of the initial slump,the final slump values after 2 hours of hydration were of the order of 37 to 50 mm(11/2 to 2 in).In such a case the method of compensating for the expected slump loss by designing for a higher initial slump is not recommended because the retempering water required at the job site may have the effect of pushing up the water-cement ratio of the concrete mixture to an undesirable level.Chapter 10 concrete at early agesv2 In general,early slump loss tends to be directly proportional to the temperature of concrete.For example,a comparison of the 180 mm(7-in)slump concretes made with Cement A at two different temperatures i.e.,23 (70)(concrete mix 1)and 30 (85)(concrete mix 7)showed that at 30,60,and 90 minutes elapsed times,the former lost 13 mm,28 mm,and 95 mm(1/2 in,1 7/8 in and 3 3/4 in)slump,while the latter lost 44 mm,70 mm,and 114 mm(13/4 in.,23/4 in.,and 41/2 in.),respectively.v3 Regard to the effect of cement composition,greater slump loss rates were observed for all test conditions in the case of concretes made with the cement containing higher C3A and high-alkali content(Cement B).For instance,at 23(70)and 30,60,and 90 min elapsed times,concrete Mix 1 lost 13 mm,28 mm and 95 mm(1/2 in.,17/8 in and 33/4 in)slump,while the latter lost 44 mm,70 mm,and 114 mm(13/4 in.,23/4 in.,and 41/2 in.)compared to 68mm,98 mm,and 143 mm(22/3 in,37/8 in,and 45/8 in),respectively,for concrete Mix 2.Chapter 10 concrete at early agesvSlump-loss problems occur most often in hot weather.The higher the temperature at which a concrete is mixed and placed the more likely it is that slump loss turns out to be the cause of any operating problem.vIn conclusion,elimination of every possible delay in concrete handling operations,keeping the temperature of concrete as closed to the 10 to 21 range as possible,and a laboratory check on the stiffening and setting characteristics of the cement(with or without the admixtures selected for use)are the necessary preventive measures to control slump loss problems.Chapter 10 concrete at early ages10.7.Segregation and Bleeding 10.7.1.Definitions and Significance vSegregation is defined as the separation of components of a fresh concrete mixture so that they are no longer uniformly distributed.There are two kinds of segregation.The first,which is characteristic of dry concrete mixtures,consists of separation of mortar from the body of concrete.Bleeding,as explained next,is the second from of segregation which is characteristic of wet concrete mixtures.Chapter 10 concrete at early agesvBleeding is defined as a phenomenon whose external manifestation is the appearance of water on the surface after a concrete mixture has been placed and compacted but before it has set(i.e.,when sedimentation can no longer take place).Bleeding is a form of segregation because solids in suspension tend to move downward under the force of gravity.Bleeding results from the inability of the constituent materials to hold all the mixing water in a dispersed state as the relatively heavy solids settle.Chapter 10 concrete at early agesvLaitance,associated with the external manifestation of bleeding,is caused by the tendency of water rising in the internal channels within concrete,carrying with it very fine particles of cement,sand,and clay(present as a contaminant in aggregate)and depositing them in the form of a scum at the concrete surface.Chapter 10 concrete at early agesv10.7.2 Measurement vThere are no tests for measuring segregation;visual observation and inspection of cores of hardened concrete are generally adequate to determine whether segregation has occurred.There is,however,an ASTM standard test for the measurement of rate of bleeding and the total bleeding capacity of a concrete mixture.According to ASTM C-232,a sample of concrete is placed and consolidated in a cylindrical container,250-mm diameter and 280-mm high.The bleed-water accumulated on the surface is withdrawn at 10-minute intervals during the first 40 minutes,and thereafter at 30-minutes interval.Bleeding is expressed in terms of the amount of accumulated water as percentage of the net mixing water in the concrete sample.Chapter 10 concrete at early agesv10.7.3 Causes and Control vA combination of improper consistency,excessive amount of large particles of coarse aggregate with either a too high or a too low density,presence of less fines(due to a low cement content,a low sand content,or a poorly graded sand),and inappropriate placing and compacting methods are among the general causes for segregation and bleeding problems in concrete.Chapter 10 concrete at early agesvIt is interesting to point out that high-C3A and high alkali cements,which show greater slump loss,tend to reduce bleeding as a result of rapid formation of sulfoaluminate hydrates such as ettringite.Chapter 10 concrete at early ages10.8.Early Volume Changes 10.8.1.Definitions and Significance vAfter fresh concrete has been placed in deep forms,such as the forms for a tall column or a wall,after a few hours the top surface will have subsided.The tendency toward subsidence is also confirmed by the presence of short horizontal cracks.This reduction in volume of fresh concrete is known by terms such as prehardening,presetting shrinkage,or plastic shrinkage,since the shrinkage occurs while the concrete is still in the plastic state.As a result of prehardening shrinkage,cracks develop over obstructions to uniform settlement,i.e.,for instance,reinforcing bars and large aggregate particles.Chapter 10 concrete at early agesvFigure 10-7 Plastic shrinkage cracking in freshly placed concrete.Chapter 10 concrete at early agesv10.8.2.Causes and Control vA variety of causes contribute to plastic shrinkage in concrete:bleeding or sedimentation,absorption of water by sub-grade or forms or aggregate,rapid water loss by evaporation,reduction in the volume of the cement-water system,and bulging or settlement of the formwork.vSettlement cracks in columns and plastic shrinkage cra