摘要
为了研究高温和应变率对超高性能混凝土(UHPC) 劈裂抗拉性能的影响,对不同温度(20、105、200、300、400 ℃)作用后UHPC的质量损失率、抗压强度、弹性模量、静态和动态(应变率为1.8~6.8
超高性能混凝土(UHPC)在火灾或高温后易发生爆裂破坏,主要原因是UHPC基体致密难以释放孔隙蒸汽压力,同时各组分存在热不相容性,会导致高温后基体内产生裂纹损
本文对钢纤维体积分数φSF=2.0%的UHPC试件进行高温剥落试验,设置升温速率为2、4、10 ℃/min,对20、105、200、300 ℃作用后的试件进行质量损失率、抗压强度、弹性模量、静态和动态劈裂抗拉强度测试,结合微观分析和现有理论研究温度和应变率对UHPC相关性能的作用机理.
水泥为PⅡ 52.5硅酸盐水泥,化学组成(质量分数,文中涉及的组成、水胶比等除特别说明外均为质量分数或质量比)见
CaO | SiO2 | Al2O3 | Fe2O3 | MgO | SO3 | K2O | TiO2 | IL |
---|---|---|---|---|---|---|---|---|
67.88 | 19.56 | 4.48 | 2.94 | 1.36 | 2.52 | 0.75 | 0.18 | 0.33 |

图 1 原材料的粒径分布
Fig.1 Particle distribution of raw materials
Length/mm | Diameter/mm | Density/(kg | Strength/MPa | Elastic modulus/GPa |
---|---|---|---|---|
13 | 0.2 | 7 850 | 2 000 | 220 |
Cement | Silica fume | Sand | Water | Superplasticizer | Steel fiber |
---|---|---|---|---|---|
788 | 200 | 1 100 | 182 | 21 | 156 |
首先按照配合比将水泥、硅灰和细骨料放入搅拌机中干拌2 min,然后倒入称量好的水和减水剂搅拌2 min,最后缓慢加入钢纤维,继续搅拌至钢纤维均匀分散.将搅拌完成的浆料装入模具中,振捣至表面浮浆泛起后用保鲜膜密封,静置24 h后脱模,在(20±2)℃、相对湿度95%以上的标准养护环境中养护至28 d.
在升温前先将试件放入烘箱内在(105±5)℃下烘干24 h,采用2、4、10 ℃/min的升温速率对直径100 mm、高度50 mm的圆柱体试件进行加热,高温剥落试验目标温度为105、200、300、400 ℃.达到目标温度后保温2 h,观察试件损伤状态.自然冷却至室温后进行力学性能测试.利用X射线衍射仪(XRD)进行物相分析,通过手持显微镜观测不同温度作用后UHPC基体与钢纤维界面处的形貌.依据GB/T 17671—2021《水泥胶砂强度检验方法(ISO法)》,使用100 kN微机控制万能试验机对尺寸为40 mm×40 mm×40 mm的立方体试件进行抗压试验,加载速率为2.4 kN/s.根据GB/T 50081—2019《混凝土物理力学性能试验方法标准》对直径100 mm、高度200 mm的圆柱体试件进行弹性模量测试.静态和动态劈裂抗拉试验均选用直径100 mm、高度50 mm的圆柱体试件.静态劈裂抗拉强度通过公式计

图 2 分离式霍普金森杆试验装置示意图
Fig.2 Schematic diagram of SHPB testing system(size:mm)
Temperature/℃ | 2 ℃/min | 4 ℃/min | 10 ℃/min | ||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
N | N | N | |||||||||
105 | 6 | 0 | 0 | 6 | 0 | 0 | 6 | 0 | 0 | ||
200 | 6 | 0 | 0 | 6 | 0 | 0 | 6 | 2 | 0 | ||
300 | 6 | 2 | 0 | 6 | 3 | 0 | 6 | 6 | 0 | ||
400 | 6 | 0 | 6 | 6 | 0 | 6 | 6 | 0 | 6 |

图3 不同升温速率下400 ℃时UHPC试件的高温爆裂实物图
Fig.3 Morphology of explosive spalling of UHPC specimen at 400 ℃ with different heating rates
综上,当升温速率较小时,UHPC内孔隙蒸汽压力较均匀地分布在试件中部,温度梯度较低且随温度升高变化幅度较小;当孔隙蒸汽压力积聚产生的拉应力超过UHPC极限抗拉强度后,试件发生程度较轻的爆裂性剥

图 4 UHPC试件的质量损失率
Fig.4 Mass loss rate of UHPC specimens

图 5 不同温度下UHPC试件的抗压强度和弹性模量
Fig.5 Compressive strength and elastic modulus of UHPC specimens at different temperatures

图 6 不同温度下UHPC试件的静态劈裂抗拉强度
Fig.6 Static splitting tensile strength of UHPC specimens at different temperatures

图 7 主裂纹处钢纤维的微观形貌
Fig.7 Microscopic morphology of steel fibers at the major crack
不同温度和应变率下的UHPC试件在动态劈裂抗拉荷载下的破坏形态见
Temperature/℃ | 1.8-2.2 | 3.4-3.7 | 4.9-5.2 | 6.3-6.8 |
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20 |
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105 |
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200 |
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300 |
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不同温度作用后UHPC在应变率为1.8~6.8

图 8 不同温度和应变率下UHPC试件的应力-时程曲线
Fig.8 Stress‑time curves of UHPC specimens at different temperatures and strain rates

图 9 不同温度和应变率下UHPC试件的动态劈裂抗拉强度
Fig.9 Dynamic splitting strength of UHPC specimens at different temperatures and strain rates
动态增强因子(DIF)为动态劈裂抗拉强度与静态劈裂抗拉强度的比值.不同温度和应变率下UHPC试件的DIF值见

图 10 不同温度和应变率下UHPC试件的DIF值
Fig.10 DIF values of UHPC specimens at different temperatures and strain rates

图 11 不同温度和应变率下UHPC试件的耗散能
Fig.11 Dissipated energy of UHPC specimens at different temperatures and strain rates

图 12 高温和应变率对UHPC劈裂抗拉性能的作用机理
Fig.12 Mechanism of high temperature and strain rate on the splitting tensile properties of UHPC

图 13 不同温度下UHPC的XRD图谱
Fig.13 XRD spectra of UHPC at different temperatures

图 14 不同温度作用后UHPC基体与钢纤维界面图像
Fig.14 Images of UHPC matrix and steel fiber interfaces exposed to different temperatures
(1)当温度为20~105 ℃时,只有毛细孔间的自由水和水化产物凝胶颗粒表面的吸附水蒸发,且高温试验前试件经烘干处理,部分自由水已经蒸发,因此温度为105 ℃时UHPC试件的质量损失率仅为0.42%.C‑S‑H凝胶在熟料颗粒间可发挥桥连作用填充孔隙与微裂纹,使UHPC基体堆积更紧
(2)当温度为105~300 ℃时,凝胶颗粒分解释放出化学结合水,水分蒸发速率随温度升高而加快,因此当温度为200、300 ℃时试件质量损失率分别达到了1.34%、4.23%.水泥的水化反应和硅灰的火山灰反应继续生成C‑S‑H凝
(3)当温度达到400 ℃时,Ca(OH)2开始分解为CaO和
(1)当温度不超过300 ℃时,超高性能混凝土UHPC的高温损伤随温度升高而增大;当温度达到400 ℃后,试件均在保温阶段发生爆裂性剥落.升温速率为2、4、10 ℃/min时试件发生爆裂性剥落的平均保温时间分别为25、18、8 min.高温作用后C‑S‑H凝胶表面的吸附水、孔隙内自由水、凝胶结合水先后蒸发,增大了试件的质量损失.
(2)温度从20 ℃上升到300 ℃,UHPC的抗压强度、弹性模量和静态劈裂强度均增大,分别提高了13.2%、19.1%和17.3%.300 ℃以内的高温作用可以促进水泥水化反应和硅灰火山灰反应,有利于提升UHPC基体的致密程度和增强钢纤维的桥接作用.
(3)UHPC的劈裂抗拉损伤程度随温度升高而降低,主裂纹宽度和端部三角形破坏区面积均随应变率升高而增大,动态劈裂抗拉强度和耗散能均具有明显的应变率效应.当应变率从1.8~2.2
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