摘要
利用自制加压装置对不同强度等级和尺寸规格的10个钢管混凝土试件进行压力养护,研究了养护压力和持续时间对试件轴压强度、应变和破坏特征的影响,分析了水化产物发育、孔结构和微裂纹结构的发展情况.结果表明:压力养护促使体系更加致密,能够抑制微裂纹形成和发展;水化产物结晶发育良好,可填充孔隙结构,提高混凝土自身强度;当压力传导至钢管时使其产生环向拉伸应变,核心混凝土终凝后,钢管收缩对其产生预压应力,两者协同增强,从而提高了试件的轴向承载力.
水泥水化早期阶段的养护是水化产物微观结构形成的关键,决定混凝土力学性能的发
压力养护工艺可追溯至20世纪70年代,前苏联利用加压和热养护使混凝土快速凝结硬化,提高了模具使用周转
为实现流态化混凝土拌和物能够在更高压力下养护的目的,本文采用两段式钢管,利用钢管约束混凝土拌和物,通过自制加压装置直接对钢管内的混凝土施加压力,并维持此压力来养护混凝土至终凝硬化.研究压力养护条件下混凝土内部的物料分布、微观结构、水化进程和强度发展,测试钢管混凝土(CFST)试件的轴压力学性能,探索压力养护工艺对CFST材料和性能的影响机理.
采用C70SF钢纤维混凝土和C70混凝土制备钢管混凝土试件.
Type of concrete | Mix proportion/(kg· | Apparent density/(kg· | |||||||
---|---|---|---|---|---|---|---|---|---|
W | C | K | Si | SF | S | G | PCE | ||
C70SF | 140.0 | 500.0 | 100.0 | 50.0 | 10.0 | 635.0 | 950.0 | 12.2 | 2 396.5 |
C70 | 140.0 | 480.0 | 100.0 | 25.0 | 0 | 660.0 | 980.0 | 9.4 | 2 394.4 |
两段式CFST试件示意图如

图1 两段式CFST试件
Fig.1 CFST specimen welded together in two sections

图2 自制加压装置
Fig.2 Self‑designed hydraulic device
CFST试件制作步骤如下:(1)通过行程段钢管开口向钢管内浇筑混凝土,待浇筑至距开口50 mm处停止,放置可活动的密封垫片进行封口;(2)将钢管混凝土试件整体卧式放入自制加压装置中,顶推活塞杆一端伸入行程段钢管内接触密封垫片,一端接触液压千斤顶,进行压力养护;(3)压力养护结束后,取出灌注混凝土的两段式钢管混凝土试件,用盘锯将试验段和行程段切割分离,并将试验段断面打磨平整,即制备得到轴压试验用CFST试件.
试验方案如
Specimen No. | Type of concrete | D1/mm | T1/mm | L1/mm | D/mm | T/mm | L/mm | P/MPa | F/t | t/h |
---|---|---|---|---|---|---|---|---|---|---|
D2A10 | C70SF | 159.0 | 4.5 | 350.0 | 273.0 | 8.0 | 1 000.0 | 0 | 0 | 0 |
D2B10 | 12.0 | 0 | 0 | 0 | ||||||
D2C10 | 16.0 | 0 | 0 | 0 | ||||||
S2A10 | 8.0 | 16 | 29 | 24 | ||||||
S2B10 | 12.0 | 26 | 45 | 24 | ||||||
S2C10 | 16.0 | 36 | 63 | 24 | ||||||
D3B8 | C70 | 159.0 | 4.5 | 350.0 | 325.0 | 12.0 | 1 000.0 | 0 | 0 | 0 |
S3B8T04 | 26 | 45 | 4 | |||||||
S3B8T08 | 26 | 45 | 8 | |||||||
S3B8T24 | 26 | 45 | 24 |
Note: T1—Wall thickness of piston stroke steel pipe.
浇筑两段式钢管的同时,取部分混凝土制作边长为150 mm的立方体试块,同条件下养护至规定龄期.对行程段钢管中的核心混凝土和立方体试块进行钻芯取样,均制备成尺寸为70.7×70.7 mm的试件,依据JGJ/T 384—2016《钻芯法检测混凝土强度技术规程》进行抗压强度检测.
对试验段钢管进行切割,将混凝土断面打磨平整,在同条件下养护至56 d,采用30 000 kN电液伺服压力试验机进行轴压试验,加载速率为0.5 mm/min,至试件破坏后停止加载.
在试验段钢管外表面上部(1‑1)、中部(2‑2)和下部(3‑3)布置应变片和位移传感器,如

图3 试验段钢管的应变片和位移传感器布置
Fig.3 Strain gauge and displacement sensor arrangement of test steel pipe
将行程段钢管混凝土断面处的20 mm×20 mm砂浆碎片,与作为对比样品的同条件养护试块的砂浆碎片,放入无水乙醇中密闭保存24 h以终止水化;随后在105 ℃烘干24 h后制得微观试样,采用日本电子JSM‑IT300型扫描电子显微镜(SEM)进行微观结构表征.
Type of concrete | Slump/mm | Slump‑flow/mm | Emptying time/s | Air content(by volume)/% | Initial setting time/min | Final setting time/min | Cubic compressive strength/MPa | Compressive strength of cylinder core/MPa |
---|---|---|---|---|---|---|---|---|
C70SF | 240 | 635 | 18.9 | 4.8 | 660 | 1 020 | 68.8 | 72.6 |
C70 | 250 | 660 | 14.6 | 4.2 | 360 | 600 | 63.7 | 68.2 |

图4 核心混凝土的抗压强度和表观密度变化
Fig.4 Changes in compressive strength and apparent density of core concrete
结合
Specimen No. | Peak load/kN | Peak compressive strength/MPa | Range of load capacity improvement/% | Stiffness/(kN·m | Stiffness increase rate/% |
---|---|---|---|---|---|
D2A10 | 6 497 | 111.0 | — | 1 743.1 | — |
D2B10 | 6 863 | 117.2 | — | 1 940.6 | — |
D2C10 | 7 548 | 128.9 | — | 2 330.2 | — |
S2A10 | 7 806 | 133.4 | 20.2 | 1 974.9 | 13.3 |
S2B10 | 8 397 | 143.5 | 22.4 | 2 382.8 | 22.8 |
S2C10 | 9 146 | 156.2 | 21.2 | 2 787.3 | 19.6 |
D3B8 | 9 326 | 118.0 | — | — | — |
S3B8T04 | 10 928 | 131.7 | 17.2 | — | — |
S3B8T08 | 11 508 | 138.7 | 23.4 | — | — |
S3B8T24 | 11 442 | 137.9 | 22.7 | — | — |

图5 CFST试件的荷载-位移曲线
Fig.5 Load‑displacement curves of CFST specimens

图6 试件S2A10和D2A10在3‑3截面处的荷载-应变曲线
Fig.6 Load‑strain curves at sections 3‑3 of specimen S2A10 and D2A10

图7 CFST试件的破坏特征
Fig.7 Damage characteristics of CFST specimens

图8 混凝土拌和物的固-液-气三相体系
Fig.8 Solid‑liquid‑gas dispersion system of concrete mixture

图9 振动成型的混凝土立方体试块断面的孔隙分布
Fig.9 Pore distribution in the cross‑section of concrete cube specimens formed by vibration

图10 试件S2A10的切割断面及其二值化图像
Fig.10 Cutting section of specimen S2A10 and its binarization image
有研究认为,养护条件可以促使C‑S‑H凝胶的类型发生变化,从而影响混凝土的微观力学性能和形貌结

图11 同条件养护和压力养护试件的断面微观形貌
Fig.11 Section microstructures of specimens under the same curing condition and pressure curing condition
压力养护在增强核心混凝土自身密实度的同时,势必会对外围约束的钢管产生影响.以试件S2A10为例,研究了压力养护时钢管的环向应变变化情况,结果见

图12 压力养护时钢管的环向应变
Fig.12 Circumferential strain of steel pipe during pressure curing
对于振捣成型试件,由于核心混凝土硬化时的自然收缩,使其与钢管产生“剥离”趋势;在受压时钢管变形会进一步增加两者的“剥离”趋势,其核心混凝土总体处于单向受压状

图13 CFST试件的初始应力状态
Fig.13 Initial stress state of CFST specimen
(1)在混凝土凝结硬化阶段采用压力养护,可以改善混凝土自身的微观结构和力学性能.压力养护后水化产物结晶良好、形貌多样化,能够充分填充孔隙和界面过渡区;混凝土断面处的宏观孔隙率由4.70%降至0.35%.
(2)过高的养护压力和初凝后的持续加压,对继续提高核心混凝土强度的效果有限.当养护压力由16 MPa增加到 36 MPa时,C70SF钢筋纤维混凝土芯样强度仅提高1.9 MPa;当压力养护持续时间由4 h增加到 24 h时,C70混凝土芯样强度仅提高3.5 MPa.
(3)压力传导至外围钢管上,使钢管发生环向拉伸变形.随着压力养护结束和核心混凝土的自身收缩,钢管收缩趋势给核心混凝土施加环向预压应力,同时由于混凝土自身强度的提高,使得钢管混凝土试件的极限承载力提高20%以上,压力养护对试件的破坏特征无显著影响.
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