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后浇带施工技巧

  后浇带在日前房屋越来越高,体量越来越大的情况下,已经被普遍使用。虽然后浇带带来千般好处,但如在施工过程中稍有大意,就会使后浇带起不到应有的作用,反而给施工带来负面作用,给结构造成隐患。后浇带施工时,以下几个细节应该注意,确保施工效果。

1、模板支撑系统

  作为温度后浇带,一般要求砼浇注的间隔时间不少于60天沉降后浇带要求高层部分主体结构施工完成后再浇注砼,间隔时间可能要几个月甚至十几个月。在这期间,后续工程还要往上施工,已施工过的后浇带两侧面的梁板结构就变成了悬挑构件,且要承受上部其他结构层的自重和施工荷载,单靠正常施工的模板支撑系统远远不够。因此,后浇带下及其两侧各1m范围内的模板及支撑必须在编制施工组织设计时,经过计算独立设置,确保受力和稳定,否则,可能会造成梁板上部裂缝或后浇带部位下挠,顶板面不平下沉等质量事故。这部分支撑尽量采用上下可调支撑,待结构砼强度达到设计要求后,逐根卸荷随即再旋紧撑牢,以避免原支撑层层满负荷往下传力,致使最下层无支撑结构受力和挠度过大的不合理情况。下面来说说模架体系的注意要点。

1)后浇带两侧保留受荷支撑不少于两排,排距不大于1m(包括梁板支撑均应保留)。该部分模板支撑系统要相对独立,以便于其他模板及支撑的正常拆除和周转。
2)被后浇带断开而形成悬臂的结构梁,要留有早拆支撑,以备其他支撑拆除后留作受荷支撑(其他支撑的拆除时间以该梁砼强度达到设计值的100%和上一层梁砼强度≥75%设计值为准)。早拆支撑的间距应以计算确定,计算时可考虑扣除该悬挑梁所能承担的荷重,其他荷载(包括本层结构部分荷载和上一层结构施工荷载)由早拆支撑承担。如果一般φ48壁厚3mm普通钢管受力不够(或间距过密时),应换撑120×120或150×150方木。点这免费下载施工技术资料
3)后浇带保留的支撑,水平方向应可靠拉结,以防失稳。
4)对地下室较厚底板、大梁等属大体积砼的后浇带,两侧必须设置专用模板和支撑,以防止砼漏浆而使后浇带底部断不开。对地下室有防水抗渗要求的,还应留设止水带或作企口模板,以防后浇带处渗水。

2、后浇带内钢筋应区别后浇带性质认真处理

后浇带按使用功能划分有温度后浇带和沉降后浇带两种,施工中要区别后浇带性质,分别采取不同的措施予以处理。

1)温度后浇带主要是为了解决后浇带砼浇灌前,其两侧结构砼的收缩变形。因此,为了保证在这期间砼的自由变形,后浇带内垂直于后浇带的受力钢筋必须断开;这一点在施工过程中往往被忽视,致使后浇带起不到应有的作用。断开钢筋的位置,应尽量相互交错在后浇带两侧面,以利于再连接时接头位置相互错开。为了方便施工,一般可按正常配筋穿过后浇带,施工完该层砼后3天内用气割将垂直于后浇带的主筋割断,断隙一般在5mm左右,待满足后浇带间隔时间后,采用水平钢筋窄间隙电弧焊工艺把断开的钢筋再焊接起来。

2)沉降后浇带主要是考虑高层部分结构和裙房之间沉降差而设置的。后浇带宽度一般在0.8~1.0m。在这个区段内,两侧的结构沉降差反应到钢筋上所产生的钢筋应力和应变,相对来说不太大,不致影响到结构的沉降。因此,沉降后浇带钢筋一般可不考虑断开。但在施工中应注意,基础或地下室底板及地梁的后浇带,其下垫层砼不宜浇注太厚(宜减薄3cm左右),下部钢筋必须垫加厚垫块抬起,离开垫层不小于5cm,以使两侧结构能够自由沉降,且沉降后钢筋仍有足够的砼保护层厚度。

3、加强后浇带砼浇注前的防护处理

1)对大体积砼后浇带,两侧砼浇注后,后浇带内侧砼应作为大体积砼养护防裂的重点,因为最大温差往往出现在后浇带两侧砼面,而两侧砼一旦出现裂缝,就是结构内部深层裂缝,有时会贯通整个结构面。因此,对大体积砼后浇带,应在带端侧面或顶面钢筋断开的合适部位预留进人孔,施工人员直接进入后浇带内支模顶撑,拆除清理和养护等。在养护期内上面覆盖塑料薄膜和岩棉被(或草苫),必要时用碘钨灯照射侧面砼加温,以确保大体积砼养护温差不超过25℃。
2)后浇带两侧砼浇注时,顶面砼留5~10cm宽企口。后浇带上面覆盖彩条塑料布,在企口宽度范围内用1:3水泥砂浆压缝找平,上砌120宽护堤,高120~180mm,外侧粉刷,作为钢筋保护和施工现场安全防护,也为了防止杂物进入后浇带内难于清理,防止雨水或施工养护用水进入后浇带。尤其是地下室,水渗地基土会影响地基承载力等结构性能,该道防护必须认真处理。过道处应加铺6cm厚木过道板,护堤适当降低。

  3)对地下室底板水平后浇带端头和外墙壁竖向后浇带,应砌筑240砖墙封堵,砌筑砂浆标号不小于M5.0,并用防水砂浆粉刷;地下室外壁进行防潮或防水处理。这样,一是防止雨水或施工用水进入后浇带;二是该墙可作为挡土墙,不影响基坑及时回填,有利于赶工;三是可以作为后浇带永久外模使用,减少施工环节。点这免费下载施工技术资料
  4)后浇带砼浇注前,要对后浇带进行清理,凿除浮石浮浆,调整钢筋等。

4、后浇带砼配置浇捣和养护

1)后浇带内的砼,一般要求较两侧已浇注砼强度等级提高一至二级(由设计院设计确定)。一般选用无收缩或微膨胀砼,配合比设计应经过计算试拌试验,常用措施主要有以下三种:
    1.1减小水灰比,控制砼坍落度。掺加FDN—2000等高效早强型减水剂(减水率一般在15%左右),控制水灰比不大于0.4,坍落度控制在1~3mm(泵送砼可控制在10~14)为宜。
    1.2掺加UEA膨胀剂,配置收缩自补偿砼。
    1.3配置微膨胀砼,配合比为水:微膨胀水泥:砂:石子=0.55:1:1.98:2.65,其中微膨胀水泥由普通水泥和膨胀组份组成,膨胀组份由钒上水泥和生石膏以1:1组成。施工时调整膨胀组份的掺量,以获得0.1%~0.2%的理想膨胀率。

2)砼的拌制浇捣必须认真,严格配比计量,并适当延长搅拌时间1~3分钟,浇注顺序宜从一端向另一端分层斜面赶进(因为可不考虑砼振捣侧压力的影响),采用泵送砼更佳,有利于析水排出和砼的结合,不留施工缝。

3)砼的早期养护,主要是表面适时收面。对大体积后浇带砼,收面后还要注意覆盖塑料薄膜和草苫等保温养护,以减少温差,防止裂缝。使用微膨胀砼灌注后浇带,要注意蓄水养护3~7天,以获得理想膨胀率。

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