摘要
为探究高温对页岩陶粒轻骨料混凝土(SCLAC)蠕变特性的影响,进行了室温至800 ℃后SCLAC单轴压缩试验、分级压缩蠕变试验和扫描电镜(SEM)试验,分析了SCLAC质量损失、抗压强度损失、蠕变特性及微观结构特征.结果表明:随温度升高,SCLAC的内部微观结构变得疏松,质量、抗压强度逐渐降低,800 ℃后质量损失率为9.54%,抗压强度损失率为63.88%;随温度升高和应力水平增加,蠕变应变和蠕变速率增大,蠕变历时和蠕变破坏临界应力水平减小;温度高于600 ℃时蠕变应变明显增大,在相同应力水平下,与室温相比600 ℃后的蠕变应变增加了82.76%.基于试验结果对Burgers蠕变模型参数进行辨识,所得理论曲线与减速蠕变阶段和等速蠕变阶段的试验数据吻合较好.
轻骨料混凝土因较普通骨料混凝土具有轻质高强、耐火抗冻和保温隔热等优点,被广泛应用于大跨度、高层和超高层建筑结构
以往的研究主要集中于轻骨料混凝土高温后强度特性方面,较少涉及变形特性.在荷载长期作用下,混凝土结构会出现与时间相关的变形,即蠕
本文以页岩陶粒轻骨料混凝土(SCLAC)为研究对象,进行了室温至800 ℃后SCLAC单轴压缩试验、分级压缩蠕变试验和扫描电镜(SEM)试验,分析不同温度作用后试样的质量损失、抗压强度损失、蠕变特性及微观损伤机理,为工程实际提供试验依据.
淮南八公山牌P·C 42.5级普通硅酸盐水泥,其3、28 d抗压强度分别为29.99、49.75 MPa;淮南平圩电厂生产的Ⅰ级粉煤灰,其化学组成(质量分数)见
依据JGJ51—2002《轻骨料混凝土技术规程》进行SCLAC配合比设计,见
采用50×100 mm圆柱体试件,成型24 h后拆模,置于标准养护环境下(温度(20±2)℃,相对湿度RH≥95%)养护56 d.高温处理前,将所有试件放入(105±5)℃烘箱内烘干24 h,以排除含水率对试验的影响,同时避免含湿量过大导致升温过程中发生爆裂.采用箱式电阻炉加热试件,设计目标温度200、400、600、800 ℃,加热速率5 ℃/min,升至目标温度后恒温3 h以保证炉温和试件内部温度一致,之后关闭电源,待炉内冷却至100 ℃左右,打开炉盖继续冷却至室温.将处理后的试件密封保存在聚乙烯袋中,直到试验当天.
采用CLY15016型电子蠕变松弛试验机对不同高温处理后的SCLAC试件进行单轴压缩和分级压缩蠕变试验,测试温度为25 ℃.单轴压缩试验以0.5 kN/s的速率加载获得抗压强度fc.分级压缩蠕变试验采用等量分级递增的加载方式,加载速率0.5 MPa/s,设计第1级应力σ1为抗压强度的40%,记为应力水平λ=0.4,此后逐级递增,应力水平分别为0.5、0.6、0.7、0.8、0.9,每级载荷保持12 h.试验过程中计算机自动采集应力及应变数据,加载期间采样间隔为5 s,稳压期间采样间隔为10 min.加载系统根据数据反馈自动进行应力调整,从而保证蠕变过程中施加的荷载始终恒定.试验结束后从单轴压缩试验压碎的试块中进行取样,使用日立产S‑3400N型扫描电子显微镜进行SEM分析.

图1 高温后SCLAC质量及抗压强度损失情况
Fig.1 Loss of mass and compressive strength of SCLAC exposed to high temperature
对SCLAC抗压强度进行变异性分析,计算其平均值μ、标准差δ和变异系数cv:
(1) |
(2) |
(3) |
式中:fc,i为各试件抗压强度试验值;n为试验值个数,n=3.
SCLAC高温后抗压强度的平均值μ、标准差δ及变异系数cv见

图2 高温后SCLAC分级压缩蠕变曲线
Fig.2 Multistage compression creep curves of SCLAC exposed to high temperature
由
高温后SCLAC应力水平与瞬时应变的关系如

图3 高温后SCLAC应力水平与瞬时应变关系
Fig.3 Relationship between stress level and transient strain of SCLAC exposed to high temperature

图4 20、800 ℃后SCLAC在不同应力水平下的蠕变速率
Fig.4 Creep stain rate of SCLAC under different stress levels exposed to 20,800 ℃
在最后一级应力水平下,不同受热温度的试件发生蠕变破坏规律基本一致,试件由减速蠕变阶段经等速蠕变阶段发展到加速蠕变阶段.由试件蠕变曲线可以得出试件发生破坏时的应力水平及蠕变历时,具体结果见
为进一步探究高温作用对SCLAC蠕变特性的影响,定义不同温度作用后SCLAC的蠕变εc与总应变εt的比值为φ,即
φ=εc/εt×100% | (4) |

图5 高温后试件φ值随应力水平的变化趋势
Fig.5 Variation of φ value of specimens with stress level
20、200 ℃后试件的蠕变破坏临界应力水平为λ=0.7,随温度升高曲线临界点逐渐前移,800 ℃后蠕变破坏临界应力水平为λ=0.5,这反映了SCLAC高温后更容易失稳.Yu
除最后一级应力水平外,其余各级蠕变曲线均由减速蠕变阶段-等速蠕变阶段组成.因此选用Burgers蠕变模型来描述试件的蠕变特性.Burgers蠕变模型由Kelvin模型和Maxwell模型串联组成,如

图6 Burgers 蠕变模型
Fig.6 Creep model of Burgers
Burgers蠕变模型方程为:
(5) |
式中:ε(t)为t时刻的应变;σ为轴向应力;EM为弹性模量;EK为黏弹性模量;ηM为黏弹性系数;ηK为黏性系数.该模型的第1项表示瞬时应变或与时间无关的弹性应变,第2项表示与时间相关的蠕变,第3项表示蠕变速率递减的减速蠕
利用Origin软件,以不同温度后的试件在第1级应力水平下的试验数据分别对EM、ηM、EK、ηK进行流变参数反演,绘制蠕变试验数据与Burgers蠕变模型曲线,见

图7 不同温度后SCLAC在第1级应力水平下蠕变试验数据和Burgers蠕变模型曲线
Fig.7 Creep strain curves of SCLAC after exposure to different temperatures for experimental data and Burgers creep model under the first stress level (λ = 0.4)

图8 高温后水泥基体的SEM图
Fig.8 SEM micrographs of cement matrix exposed to high temperature
轻骨料混凝土以其特有的界面过渡区显著区别于普通混凝土,

图9 高温后页岩陶粒与水泥基体界面过渡区SEM图
Fig.9 SEM micrographs of interfacial transition zone between shale ceramsite and cement matrix exposed to high temperature
从
页岩陶粒是一种优良的耐高温材料,其在20、800 ℃后的微观形貌如

图10 20、800 ℃后页岩陶粒的微观形貌
Fig.10 Micro‑structures of shale ceramsite exposed to 20,800 ℃
(1)高温作用显著降低了页岩陶粒轻骨料混凝土的质量及抗压强度,800 ℃后其质量损失率和抗压强度损失率分别达到9.54%和63.88%,且抗压强度变异系数随温度升高而增大.
(2)随温度升高和应力水平增加,页岩陶粒轻骨料混凝土蠕变和蠕变速率增大,蠕变历时和蠕变破坏临界应力水平减小.温度高于600 ℃时蠕变明显增大,在相同应力水平下,600 ℃后蠕变较室温增加了82.76%.
(3)高温作用改变了页岩陶粒轻骨料混凝土的内部微观结构,随温度升高,内部水泥凝胶由整体逐渐分散、疏松,页岩陶粒与水泥基体黏结力不断减弱、丧失,诱发了裂纹和孔隙的扩展、贯通,导致宏观强度损失和变形增大.
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