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Shallowly discues the construction joint proceing Introduction
Maive engineering structure shakes harms the example to indicate that, the portal frame construction pitch point universal destruction is heavier, often causes the structure to expire, the Chinese and foreign scholar analyzes indicated, this is the various reason, including the design fault, has not strengthened the pitch point intensity, realizes reason and so on strong pitch point strong anchor.Simultaneously constructs also has the construction quality weakne, difficulty with guarantees the design anticipated target.In construction new old concrete construction gap time, continues the surface position, continues the surface proceing method, the new old coagulation locally born ripe ingredient and inspires pounds the influence pitch point caking intensity which the control quality all makes, therefore in portal frame construction project construction proce, because in each pillar construction inevitably and the lower level pillar existence construction joint, how does leave alone the construction joint, how procees the construction joint, causes the project quality not to receive affects, guaranteed concrete the structure security, appears especially importantly.1.current portal frame construction pitch point job practice and the flaw.(1)the early time in the general portal frame construction, after pours a pillar, carries on a mold, after the template installment completes only then enters sewer the working procedure to pour the upper formation girder concrete, therefore acts according to constructs the time limit for a project and the person, the material, machine and so on the situation, the new old concrete continues the gap time length not one, when pouring the general pitch point pours together along with the girder concrete, and the pitch point concrete intensity rank carries on the construction according to girder
(2)along with the scientific research development and the construction level enhancement, in particular specialized level higher big general contractor starts in view of steel construction the pitch point construction quality weak characteristic to take a series of measures, when pours the upper oration steel construction carries on chiseling proceing with
the drill rod front, pours carries on the watering to the old concrete to be moist, and in hands over in the composition plane with the high-grade cement mortar shop, steel construction the pitch point area concrete intensity rank generally uses the pillar(common pillar intensity and so on to rise), is pouring the pitch point zone time to strengthen inspires pounds and the strict control water-to-cement ratio.After a series of proceing measure, the construction quality obtained the enhancement, but column the pitch point area concrete still was the construction weak area, is very difficult to obtain ideally continues the effect.(3)although continued the spot to carry on proceing, but its flaw was very obvious: First uses the drill rod to carry on chiseling proceing, extremely shirt and old concrete, so that the old concrete produces the initial micro crack, seriously affects the new old concrete to continue the intensity, also chiseling proceing difficulty with controls the proceing quality specification front, pours to continues the spot moistly to carry on the watering to be very easy to work just the opposite, thus increased has handed over the composition plane place concrete water-to-cement ratio, but in handed over in the composition plane with the high-grade cement mortar shop to be very easy because the high-grade cement mortar and the green concrete hydro ma congealed and the contraction different, thus caused to continue the surface to be easier to have the initial stre and the initial micro crack.But pitch point area concrete intensity rank difference and girder, thus caused the column girder pitch point area actually to produce four frontal zones(although girder and pitch point area whole poured, but concrete intensity rank general different, thus also has created weak contact surface), when construction was very difficult to control inspires pounds the density and the time, was very easy because inspired pounds the depth but to injury and the old concrete, or because strengthened inspires pounds causes originally water-to-cement ratio to have the partial bleeding phenomenon on the quite small concrete, thus the influence concrete intensity growth, reduced the contact surface caking intensity.2.Column pitch points areas destruction mechanism and the earthquake resistance design request by shakes harms the investigation obviously, column the pitch point area destruction, mostly is because the pitch point area does not have stirrup or are few stirrup, in cuts prees under the function the concrete to appear the slanting crack even to extrude destroys, creates
the longitudinal reinforcement buckling to become the lantern shape.Therefore, guaranteed the pitch point area not premature has the shearing failure main measure is guaranteed the pitch point area concrete the intensity and the density, is enough in the pitch point area disposition stirrup.The design column frequently uses the different rank the concrete, when the construction must pay attention to column the pitch point spot concrete rank to be supposed same or is slightly low with the column concrete rank(difference not to be able to surpa 5M pa).Thus realizes the strong pitch point strong anchor.In the vertical preure and the girder end pillar end moment, under the shearing force function, the pitch point area has the more complex stre condition.Pitch point experiment which from unit and so on Beijing construction design institute carries on obviously, the pitch point destruction proce approximately may divide into two stages: The first stage for paes the crack stage.When the function and the core shearing force achieved when 60 ~ 70%, the core area appears links up the slanting crack, the crack opening approximately is 0.1 ~ 0.2m m, the reinforcement stre is very small(does not surpa 20M pa), this stage shearing force mainly undertakes by the concrete.The second stage for bursts the stage.Gradually enlarges along with repeatedly the load, links up the crack to widen, the shearing force mainly undertakes by the concrete, stirrup achieves one after another submits, crowds garrulously in the concrete front achieves the biggest bearing capacity.When design takes the limiting condition by the second stage.3.standards leave alone and the proceing request to the construction.Joint for enable the concrete structure to have a better integrity;the concrete pour should continuously carry on.If because technical or organization's reason cannot continuously carry on pours, also the middle idle time has the poibility to surpa the concrete initially to congeal, then should in front of concrete placement determine remains in the appropriate position supposes the construction joint.The construction joint was refers first irrigates the concrete to congeal the hardening, then continued the placing concrete junction plane between the new old concrete, it was the structure weak spot, thus suitably kept the structure to be smaller the shearing force also to be advantageous for the construction spot.The column should keep the water flush joint, the girder wooden fence should remain vertical joints.When starts from the construction joint place continue the concrete, after must wait the concrete compreive
strength which constructs to achieve 1.2N /m2.can carry on, moreover must make some proceing to the construction joint, strengthens the new old concrete the connection, reduces the construction joint as far as poible the adverse effect which brings to the structure integrity.The treating procees are: First in hardened on the concrete surface, eliminates the cement thin film and the loose gravel as well as the weak layer of concrete, and performs fully moist, the flushing is clean, also does not have to remain has the pending;Then first spreads cement mortar in front of the concrete in the construction joint place or and in the concrete the ingredient same cement mortar;When concrete, must carefully inspire pounds densely, causes the new old concrete union to be close.4.according to the scientific research achievement union actual construction proposed the improvement construction joint proceing method and the theory basis.(1)First maively test indicated continues the surface to carry on the rough treatment to be allowed obviously to enhance continues the surface caking intensity, but roughne enhances after the certain degree, continues the surface caking intensity the enhancement no longer to be obvious.With ordinary chiseling method existence obvious flaw, but uses the high-preured water spray treatment to be allowed to obtain a better rough contact surface, and does not injury and the old concrete, but high-preured water equipment construction cost expensive, technical content high, looked in the present stage from our country actual construction technology and the construction level, the application high preure water treatment le quantities construction joint not too is actual.Therefore myself proposed in the pouring note under story post period of time from 11 p.m.to 1 a.m, after waits at the beginning of the concrete to congeal causes on the pillar with the artificial method the surface to present the zigzag, according to the concrete thick aggregate particle size(generally is 2 ~ 4cm), the dentil depth for sticks the surface old concrete maximum aggregate particle size 1/4 ~ 1/2;grooving the mean breadth for sticks the surface old concrete maximum aggregate particle size 1 ~ 1.5 times.This law biggest merit is advantageous for the control construction quality, causes to stick in the surface roughne to have the good uniformity.Like these both has avoided chiseling, avoids the wound and the old concrete structure, and realized has
continued the surface roughne, also was easy to control the quality specification, when continuing only used to strip the loose thick aggregate.(2)in order to reduce the pitch point area weak contact surface quantity, myself thought should separate in the pitch point area two sides girder pours(even if does not separate, because pitch point concrete intensity rank is different with the girder concrete, also can produce contact surface weak level), but the pitch point pours together with the upper formation pillar, like this enabled the pitch point area to have three to continue the surface, the pillar only has to continue the surface(to be short to continue surface), was advantageous to enhances the pillar concrete anti-to cut the ability, delayed the pillar slanting crack the appearance.Simultaneously also conforms to the earthquake resistance request strong column weak girder principle, delays time which the pillar submits.Thus causes the pitch point area same the level pillar constitution whole.(3)when leaves alone the girder construction joint should retain the diagonal 45° angle(girder under construction joint nearby pillar), like this procees the result on the one hand causes the pitch point area girder end shear not with the construction joint superposition, is advantageous to anti-cuts anti-pulls, delays the time which the concrete crack appears.On the other hand acts according to had the experimental study result to indicate that, on slanting makes up compared to declines makes up sticks the surface intensity high, the same principle on slanting makes up should make up the intensity the tradition job practice in side to be higher than.Finally, because the construction joint aumes 45° the angle, thus expanded the pitch point whole to pour the region, was advantageous to realizes the strong pitch point request.(4)has when the condition may(for instance reduce medicinal preparation using the patching contact surface medicinal preparation, had experiment and theoretical analysis indicated reduces medicinal preparation to be better than plumping agent), as a result of the economy and other reasons, in the actual project construction very little uses the contact surface medicinal preparation in the small area construction joint, present constructs from the majority looked, should consider the improvement and the control interface mortar application.According to had the experimental conclusion and the theoretical analysis, myself
suggested applies makes the contact surface medicinal preparation with the pillar water-to-cement ratio same element cement mortar.(5)acts according to myself many years construction experience, because the pitch point spot girder steel bar intersects through and stirrup the encryption, factor and so on anchor causes the pitch point spot steel bar densely, constructs with difficulty, therefore the suggestion use minor diameter vibrates the stick(for instance 50sticks)to inspire pounds the pitch point spot, like this not the easy wound and the old concrete structure, also are easy to control inspires pounds the quality, reduces as far as poible secretes the water volume, the improvement junction plane tiny view structure, is advantageous to enhancing the construction joint the caking intensity.5.conclusions and forecast
This article according to tested and the theory scientific research achievement union portal frame construction concrete girder pitch point spot construction weak area carries on the discuion, key actually embarks from the present project and the full consideration is economical and the realistic factor, proposed the pitch point spot construction joint continues proceing the new method to discu its theory basis, and already carried on the Practice in the partial projects, obtained good has continued the effect.But waits for to the maive projects practice proved its validity, simultaneously is worth further carrying on the theory and the experiment analyzes.The current related scientific research mainly from macroscopic embarks to tiny view, has initially promulgated the new old concrete caking mechanism through the experiment and the theoretical analysis, myself think not only must study the new old concrete from the theory to continue the intensity, must study from the current our country construction enterprise actual level face the project application job practice, thus applies as soon as poible the theory achievement to the practice, enhances our country to construct enterprise's overall quality, guaranteed the construction quality, better satisfies the design anticipated target.浅谈施工缝的处理
引言
通过大量工程结构震害实例, 框架结构节点普遍破坏较重,往往导致结构失效, 中外学者分析表明, 这是多方面的原因, 包括设计失误, 没有加强节点强度, 实现强节点强锚固等原因。同时施工中也存在施工质量弱点, 难于保证设计预期目标。施工中新老混凝土施工间隔期, 接续面方位, 接续面处理方法, 新老混凝土生熟配料及振捣控制质量都密切的影响着节点粘结强度, 因此在框架结构工程施工过程中,由于每一层柱子施工中必然与下层柱子存在施工缝,怎样留置施工缝, 怎样处理施工缝, 使工程质量不受影响,保证砼结构的安全性,更显得尤为重要。
1、当前框架结构梁柱节点施工方法及缺陷
(1)早期在一般的框架结构施工中, 浇注完一层柱子后, 进行支模, 模板安装完成后才进入下道工序浇注上层梁混凝土, 因此根据施工工期及人、材、机等情况, 新老混凝土接续间隔期长短不一, 在浇注时一般节点(梁柱交界处)随梁混凝土一起浇注, 并且节点混凝土强度等级按梁进行施工。
(2)随着科研的发展及施工水平的提高, 尤其是专业化水平较高的大建筑公司开始针对梁柱节点施工质量薄弱的特点采取一系列措施, 在浇注上层梁柱时用钢钎进行凿毛处理, 浇注前对老混凝土进行浇水湿润, 并用高标号水泥砂浆铺在交接面上, 梁柱节点区混凝土强度等级一般采用柱子(一般柱子强度等级高),在浇注节点区时加强振捣并严格控制水灰比。经过一系列处理措施后, 施工质量得到了提高, 但梁柱节点区混凝土仍旧是施工薄弱区, 很难取得理想的接续效果。
(3)虽然接续部位进行了处理, 但其缺陷是很明显的: 首先用钢钎进行凿毛处理, 极易伤及老混凝土, 以致老混凝土产生初始微裂缝, 严重影响新老混凝土的接续强度, 且凿毛处理难于控制处理质量标准, 浇注前对接续部位进行浇水湿润很容易适得其反, 从而增大了交接面处混凝土的水灰比, 而用高标号水泥砂浆铺在交接面上很容易由于
高标号水泥砂浆与新混凝土浆体凝结及收缩不同, 从而使接续面更容易产生初应力和初始微裂缝。而节点区混凝土强度等级区别与梁, 从而使柱梁节点区实际产生了四个界面区(虽然梁与节点区整体浇注, 但混凝土强度等级一般不同, 因而也造成了薄弱界面), 在施工时很难控制振捣密度与时间, 很容易由于振捣过深而伤及老混凝土, 或者由于加强振捣而使本来水灰比就比较小的混凝土产生局部泌水现象, 从而影响混凝土强度的增长, 降低界面粘结强度。
2、梁柱节点区破坏机理及抗震设计要求
由震害调查可见, 梁柱节点区的破坏, 大都是由于节点区无箍筋或少箍筋, 在剪压作用下混凝土出现斜裂缝甚至挤压破坏,造成纵向钢筋压屈成灯笼状。因此, 保证节点区不过早发生剪切破坏的主要措施是保证节点区混凝土的强度及密实性, 在节点区配置足够的箍筋。设计梁柱常常采用不同等级的混凝土, 施工时必须注意梁柱节点部位混凝土等级应该和柱混凝土的等级相同或略低(相差不能超过5M pa)。从而实现强节点强锚固。在竖向压力及梁端柱端弯矩, 剪力作用下, 节点区存在较复杂的应力状态。
从北京市建筑设计院等单位进行的节点实验可见, 节点的破坏过程大致可分为二个阶段: 第一阶段为通裂阶段。当作用与核心的剪力达到60～70% 时, 核心区出现贯通斜裂缝, 裂缝宽度约为0.1～0.2m m ,钢筋应力很小(不超过20M pa),这个阶段剪力主要由混凝土承担。第二阶段为破裂阶段。随着反复荷载逐渐加大, 贯通裂缝加宽, 剪力主要由混凝土承担, 箍筋陆续达到屈服,在混凝土挤碎前达到最大承载能力。设计时以第二阶段作为极限状态。
3、规范对施工缝留置及处理要求
为使混凝土结构具有较好的整体性, 混凝土的浇注应连续进行。若因技术或组织的原因不能连续进行浇注, 且中间的停歇时间有可能超过混凝土的初凝, 则应在混凝土浇注前确定在适当位置留设施工缝。施工缝就是指先浇混凝土已凝结硬化, 再继续浇注混凝土的新旧混凝土间的结合面, 它是结构的薄弱部位, 因而宜留在结构受剪力较小且便于施工的部位。柱应留水平缝, 梁板墙应留垂直缝。
当从施工缝处开始继续浇筑混凝土时, 须待已浇筑的混凝土抗压强度达到1.2N /m m ⒉后才能进行, 而且需对施工缝作一些处理, 以增强新旧混凝土的连接, 尽量降低施工缝对结构整体性带来的不利影响。处理过程是: 先在已硬化的混凝土表面上, 清除水泥薄膜和松动石子以及软弱混凝土层, 并加以充分湿润, 冲洗干净, 且不得留有积水;然后在浇筑混凝土前先在施工缝处铺一层水泥浆或与混凝土内成分相同的水泥砂浆;浇筑混凝土时, 需仔细振捣密实, 使新旧混凝土结合紧密。
4、根据科研成果结合实际施工提出改进施工缝处理方法及理论依据
(1)首先大量实验表明接续面进行粗糙处理可以明显提高接续面粘结强度, 但粗糙度提高到一定程度后, 接续面粘结强度的提高不再明显。用普通凿毛方法存在明显缺陷, 而采用高压水喷射处理可以得到较好的粗糙界面, 并且不伤及老混凝土, 但高压水设备造价昂贵, 技术含量高, 在现阶段从我国实际施工技术及施工水平来看, 应用高压水处理较少数量的施工缝不太实际。
因此本人提出在浇注下层柱子时, 待混凝土初凝后用人工方法使柱子上表面呈现锯齿状, 根据混凝土粗骨料粒径大小(一般为2～4cm), 锯齿深度为粘结面老混凝土最大骨料粒径的1/4～1/2;切槽的平均宽度为粘结面老混凝土最大骨料粒径的1～1.5 倍。此法的最大优点是便于控制施工质量,使粘结面上的粗糙度具有良好的均匀性。这样既避开了凿毛, 避免伤及老混凝土结构, 又实现了接续面的粗糙, 且容易控制质量标准, 在接续时只用剥离松动的粗骨料。
(2)为了减少节点区薄弱界面数量, 本人认为应在节点区二侧把梁断开浇注(即使不断开, 由于节点混凝土强度等级不同与梁混凝土, 也会产生界面薄弱层), 而节点连同上层柱子一起浇注, 这样使得节点区存在三个接续面, 柱子只有一个接续面(少了一个接续面), 有利于提高柱子混凝土的抗剪能力, 延缓柱子斜裂缝的出现。同时也符合抗震要求的强柱弱梁原则, 延缓柱子屈服的时间。因而使节点区同上层柱子构成整体。
(3)在留置梁施工缝时应留成斜向45°角(梁底施工缝在柱子边), 这样处理的结果一方面使节点区梁端剪力不与施工缝重合, 有利于抗剪抗拉, 延缓混凝土裂缝出
现的时间。另一方面根据已有实验研究结果表明, 斜上补比斜下补粘结面强度高, 同理斜上补应比传体浇注区域, 有利于实现强节点的要求。
(4)有条件时可以应用修补界面剂(比如减缩剂, 已有实验及理论分析表明减缩剂比膨胀剂好), 由于经济及其他原因, 实际工程施工中在小面积施工缝中很少使用界面剂, 从大多数现行施工来看, 应考虑改进并控制界面砂浆的应用。根据已有实验结论及理论分析, 本人建议应用同柱子水灰比一样的素水泥浆做界面剂。
(5)根据本人多年的施工经验, 由于节点部位梁柱钢筋交叉通过及箍筋加密, 锚固等因素使得节点部位钢筋稠密, 难以施工,因此建议使用小直径震动棒(比如50 棒)振捣节点部位, 这样不容易伤及老混凝土结构, 且容易控制振捣质量, 尽量减少泌水量,改善结合面的微细观结构, 有利于提高施工缝的粘结强度。统施工方法中的侧补强度高。最后, 由于施工缝呈45°角, 从而扩大了节点。
5、结论与展望
本文根据实验及理论科研成果结合框架结构混凝土梁柱节点部位施工薄弱区进行探讨, 重点从现行工程实际出发并充分考虑经济及现实因素, 提出了节点部位施工缝接续处理的新方法讨论其理论依据, 并且已经在部分工程中进行实践, 取得了良好的接续效果。但有待于大量工程实践证明其有效性, 同时值得进一步进行理论及实验分析。
当前有关科研主要从宏观到微细观出发, 通过实验及理论分析初步揭示了新老混凝土的粘结机理, 本人认为不仅要从理论上去研究新老混凝土的接续强度, 更要从当前我国的施工企业实际水平去研究面向工程应用的施工方法, 从而尽快把理论成果应用于实践, 提高我国施工企业的整体素质, 保证施工质量, 更好的满足设计预期目标。
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