摘要
采用80 ℃下恒温12 h的蒸养制度对砂浆进行了蒸养,蒸养结束后采用自然养护、水养和标养3种方式进行后续养护,同时设置了全程标养砂浆作为对照组.研究了不同养护制度下砂浆的早期强度发展规律,并基于X射线计算机断层扫描(X‑CT)技术对砂浆的孔结构特征进行了分析.结果表明:80 ℃下恒温12 h蒸养使砂浆孔隙率增大,最终导致蒸养砂浆28 d抗压强度低于全程标养砂浆;蒸养砂浆抗压强度随孔隙率增大而快速降低,两者之间呈现良好的线性关系;蒸养后水养和标养可以有效降低蒸养砂浆的孔隙率,使其28 d抗压强度高于蒸养后自然养护的砂浆;砂浆孔隙的孔体积与球形度存在明显的负相关关系,且蒸养会使孔体积-球形度分布规律中的异常值增多.
关键词
蒸汽养护(蒸养)是混凝土预制构件工业化生产中一种常用的养护方
矿物掺合料的火山灰效应可以改善混凝土孔结构,从而降低蒸养过程中造成的热损
然而,目前关于高温蒸养及后续养护方式对大掺量矿物掺合料砂浆孔结构影响的相关研究较少.X射线计算机断层扫描(X‑CT)是一种具有较高分辨率的无损检测技术,能清晰直观地检测砂浆内部细观孔隙分布和孔隙形
C50是预制构件混凝土最常见的强度等级之一,掺加矿物掺合料可以降低混凝土成本且对其耐久性有利,因此大掺量矿物掺合料混凝土用于生产混凝土预制构件具有极大优势.高温蒸养会使水泥砂浆孔结构劣化,这是其不利影响的主要表现形式之一.而X‑CT的高精度扫描需要样品体积足够小,为了避免粗骨料的存在对小体积样品均匀性的影响,本试验设计了C50强度等级的大掺量矿物掺合料砂浆,对其进行蒸养和孔结构研究.
水泥采用海螺牌P·O 42.5普通硅酸盐水泥,矿物掺合料为S95粒化高炉矿渣粉,粉煤灰为Ⅱ级粉煤灰,砂子采用细度模数为2.6的河砂,水为实验室自来水,减水剂采用聚羧酸高效减水剂.砂浆配合比为m(水泥)∶m(矿渣粉)∶m(粉煤灰)∶m(砂子)∶m(水)∶m(减水剂)=250.0∶150.0∶100.0∶1 350.0∶160.0∶3.5.胶凝材料的化学组成见

图1 砂子的粒径级配
Fig.1 Grading curve of sand particle size
参照GB/T 17671—2021《水泥胶砂强度检验方法(ISO法)》拌制砂浆,将搅拌均匀后的砂浆装入尺寸为40 mm×40 mm×160 mm的钢模中振捣成型.对于成型后的砂浆,其中1组放入标养箱内进行标养(编号为SD),另外3组放入蒸养箱内进行蒸养.蒸养制度的制定主要考虑2个因素:一方面考虑实际生产过程中为了加快模具的循环利用,应尽量在24 h之内结束整个蒸养过程;另一方面考虑预制构件后续预应力的张拉要求,在蒸养结束后强度应尽量达到设计强度的70%左右,这样可以实现在拆模后2~3 d内进行预应力张拉工序.最后,通过预试验确定了基本满足上述需求的蒸养制度,即:20 ℃下静置3 h、升温2 h、80 ℃下恒温12 h和降温2 h共4个阶段,如

图2 蒸养制度
Fig.2 Steam curing system
X‑CT扫描采用德国Vtomexs微焦点X‑CT系统,设置电压为120 kV,电流为150 μA,功率为18 W.砂浆养护28 d后取样进行X‑CT扫描,样品采用边长20 mm的立方体,获得的2D图像分辨率为38.8 μm/体素,像素数量为1 024×1 024.
由于射束硬化,CT图像会出现中间黑、边缘亮的伪影,此时若直接通过一个固定的灰度阈值分割来提取孔隙,将使CT图像上较亮区域的部分孔隙被漏掉或者较暗区域的孔隙被过度放大.为了更好地对孔隙进行提取分割,采用活动轮廓模型(active contour model)对图像进行预处理.该方法是通过在目标周围区域构造1条可变形的初始活动轮廓曲线,并建立相应的能量泛函方程,曲线在自身变形能和轮廓外部势能的共同作用下,通过最小化能量泛函,使轮廓曲线收敛到目标图像的边缘,找出最佳可能的边界轮廓曲
(1) |
式中:为内部能量,控制着活动轮廓曲线的弹性变形;为外部势能,外部势能吸引活动轮廓曲线到图像上的目标特征区域;为归一化弧长;,为活动轮廓曲线控制点的二维坐标.
实际计算过程中,常采用能量函数的离散方程来计算Eto
(2) |
式中:为活动轮廓曲线上第i个离散的控制点;为活动轮廓曲线控制点的1阶导数;为活动轮廓曲线控制点的2阶导数;n为控制点数;α为弹力系数;β为强度系数.
利用欧拉方程求解能量函数的最小值:
(3) |
式中:、分别为控制点在图像空间中的2阶微分和4阶微分.
此时,图像分割问题转化为变分问题,在灰度数字图像离散条件下,通过构造线性方程组,采用Matlab软件进行迭代计算求解.通过活动轮廓模型分割出孔隙后,对孔隙结构进行了三维重构,并对孔结构特征进行了统计分析.上述CT图像处理流程如

图3 CT图像处理流程
Fig.3 CT image processing flow
为了分析不同养护制度对大掺量矿物掺合料砂浆孔隙率的影响,沿着竖直方向(z轴)由表及里对4种养护制度下砂浆的二维孔隙率进行了分析,结果如

图4 沿竖直方向的二维孔隙率
Fig.4 2D porosity along vertical direction
4种养护制度下,砂浆不同龄期抗压强度测试结果如

图5 砂浆不同龄期抗压强度
Fig.5 Compressive strength of mortar at different ages
为了验证砂浆孔隙率与抗压强度的相关性,在每个扫描的砂浆样品中提取4个部位的代表体积元素(REV),计算其孔隙率,并与对应砂浆28 d抗压强度进行相关性分析,如

图6 砂浆28 d抗压强度与孔隙率的关系
Fig.6 Relationship between 28 d compressive strength and porosity of mortar
采用X‑CT技术分析了不同养护制度下砂浆的孔径分布特征,结果如

图7 基于X‑CT的孔径分布图
Fig.7 Pore diameter distribution diagram based on X‑CT
为了从三维视角分析不同养护制度下砂浆的孔隙特征,对砂浆孔体积分布特征进行了分析,结果如

图8 砂浆孔体积分布
Fig.8 Pore volume distribution of mortars
水泥基材料中的孔隙形态非常复杂,并不都是圆形或椭圆形孔隙.为了评价养护制度对孔隙三维形态的影响,采用球形度(S)对孔隙进行统计分析.孔隙的球形度是孔隙的表面积与孔体积对应的球体表面积之
(4) |
式中:V为孔隙三维体积;A为孔隙三维表面积.

图9 砂浆孔隙球形度与孔体积的关系
Fig.9 Relationship between sphericity and pore volume of mortar

图10 砂浆孔隙球形度分布
Fig.10 Sphericity distribution of mortar pore
为了研究3种蒸养砂浆与全程标养砂浆孔隙球形度的区别,采用箱线图分析了孔隙球形度与孔体积的关系,如


图11 球形度-孔体积关系箱线图
Fig.11 Box diagram of sphericity‑pore volume relationship
(1)相对于全程标养砂浆,80 ℃下恒温12 h蒸养会使砂浆孔隙率增大,对砂浆的抗压强度产生不利影响,导致蒸养砂浆的28 d抗压强度低于全程标养砂浆.蒸养后水养和标养对后续水化反应有促进作用,可以有效降低蒸养砂浆的孔隙率,有利于抗压强度的增长.
(2)蒸养有利于消除500 μm以上的大孔,蒸养后再水养或标养可以有效减少蒸养砂浆中的微米级孔隙.因此合理的后续养护制度对蒸养砂浆的孔结构改善具有重要意义.
(3)砂浆孔隙的孔体积与球形度存在明显的负相关关系.孔隙的孔体积越小,球形度越好,而蒸养会使孔体积-球形度分布规律中的异常值增多,这可能是蒸养导致砂浆性能劣化的原因之一.
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