摘要
研究了冻融循环作用下青砖的表观形貌、质量损失率、相对动弹性模量、抗压强度及孔结构的变化规律,并结合分形理论建立了分形维数与抗压强度、孔隙率及抗冻性能的关系.结果表明:随着经历冻融循环次数的增加,青砖表面的小孔劣化为大孔,然后逐渐延伸形成裂缝,导致质量损失率不断增加,相对动弹性模量和抗压强度均呈下降趋势;经历冻融循环后青砖内部孔具有明显的分形特征,其分形维数在2.964 2~2.982 7之间;经历冻融循环后青砖的分形维数与抗压强度呈正相关,与孔隙率呈负相关,与抗冻性能具有高度的相关性;分形维数可用于评价青砖微观孔结构的变化,也可以反映经历冻融循环后孔结构对青砖宏观性能的影响;研究结果可以为寒冷地区古建筑青砖的保护及耐久性损伤研究提供理论依据.
古建筑是文化遗产的重要组成部分,是中华传统文化的重要代表.青砖是古建筑中使用数量较多、范围较广的材料,在漫长的岁月里经历了自然环境的侵蚀和人为破坏.冻融是导致寒冷地区古建筑青砖耐久性下降的重要原因之一.在寒冷环境的长期作用
国内外学者研究了古青砖的物理力学性
近年来,许多学者通过分形理论对材料孔结构的变化进行分
本文分析了冻融循环作用下青砖的宏观性能损伤及其孔隙率和孔径分布的变化特征;通过引入分形理论,建立了分形维数与抗压强度、孔隙率及抗冻性能的关系,探究了青砖宏观破坏和微观结构演化的关系,研究结果可以为寒冷地区古建筑青砖的保护及耐久性损伤研究提供理论依据.
由于古青砖十分珍贵且较难获取,故选取内蒙古呼和浩特市出售的仿古青砖作为试验材料,其主要原料为黏土,制作工艺和制作流程与古青砖相近.通过对比仿古青砖与内蒙古中部隆盛庄古镇古青砖的物理力学性能及矿物组成(质量分数)(见
Type | Density/(g·c | Water absorption(by mass)/% | Porosity(by volume)/% | Compressive strength/MPa |
---|---|---|---|---|
Ancient blue brick | 1.62 | 18.94 | 9.51 | 7.73 |
Antique blue brick | 1.74 | 17.42 | 6.24 | 11.23 |

图1 青砖的XRD图谱
Fig.1 XRD patterns of blue bricks
冻融循环试验依据WW/T 0049—2014《文物建筑维修基本材料 青砖》进行,试验设备采用MNSY‑2400L型模拟自然环境下工程材料耐久性损伤试验系统.首先,将青砖试块置于10~20 ℃水中浸泡24 h,完成饱水后取出,用湿布擦去表面水分后放入实验箱中,试块间隔大于20 mm,在-15~-20 ℃下冰冻3 h;然后,取出青砖,放入10~20 ℃水中融化2 h,即完成1次冻融循环.冻融循环次数(N)设定: 0、15、30、45、60、75、90 次,共分为7组,每组5个试块,试块尺寸为50 mm×50 mm×50 mm.每经历15次冻融循环后测量1次试块的质量(m)、相对动弹性模量(Er)和抗压强度(fc).
采用上海纽迈牌 MesoMR‑60S型核磁共振仪测试青砖的孔结构,磁场强度为(0.50±0.08) T,仪器主频率为21.3 MHz,仪器恒温32 ℃,磁体扫描范围0~60 mm.采用钻头直径为56 mm的取芯机对青砖试块钻芯取样,再使用切割机将钻取的青砖圆柱体切割成ϕ50×50 mm的青砖核磁试块.先将试块置于负压真空饱水仪中进行24 h真空负压饱水处理,然后用抹布擦拭试块表面,最后用塑料薄膜将试块包裹,以避免水分蒸发.

图2 青砖表观形貌的变化
Fig.2 Changes of appearance of blue brick
通过引用张道明
N/times | Mass loss/g | Mass loss rate/% | Mass loss per unit area/(g·c | Mass loss rate per unit area/% | ||||
---|---|---|---|---|---|---|---|---|
Antique brick | Control group | Antique brick | Control group | Antique brick | Control group | Antique brick | Control group | |
0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
15 | 0.564 | 0.013 | 0.270 0 | 0.290 0 | 0.022 56 | 0.000 18 | 0.500 | 0.135 |
30 | 0.328 | 0.020 | 0.427 1 | 0.600 0 | 0.013 12 | 0.000 27 | 0.291 | 0.207 |
45 | 0.667 | 0.027 | 0.746 0 | 0.780 0 | 0.026 68 | 0.000 36 | 0.592 | 0.280 |
60 | 1.035 | 0.027 | 1.242 0 | 0.890 0 | 0.041 40 | 0.000 36 | 0.919 | 0.280 |
75 | 0.781 | 0.043 | 1.622 0 | 1.270 0 | 0.031 24 | 0.000 58 | 0.693 | 0.445 |
90 | 1.132 | 2.158 0 | 0.045 28 | 1.000 |
相对动弹性模量的大小反映了冻融循环作用下青砖内部的损伤程度,内部损伤程度越大,相对动弹性模量的值越小.

图3 经历不同冻融循环次数青砖相对动弹性模量的变化
Fig.3 Changes of relative dynamic elastic modulus of blue brick with different freeze‑thaw cycles

图4 青砖试块受压试验的破坏形态
Fig.4 Failure morphology of blue brick block under pressure test

图5 青砖的抗压强度
Fig.5 Compressive strength of blue brick
谱分布能够反映孔隙的分布情况,值的大小能够代表孔隙的大小.

图6 青砖的谱
Fig.6 T2 spectra of blue bricks
谱峰面积的大小与其内部所含的流体数量成正比,冻融后青砖谱面积的变化反映了青砖内部孔隙体积的变化.

图7 青砖谱的面积
Fig.7 Area of spectra of blue brick

图8 青砖的孔隙率
Fig.8 Porosity of blue brick
参考吴中伟

图9 青砖的孔径分布
Fig.9 Pore distributions of blue bricks

图10 青砖孔隙半径分布的占比
Fig.10 Pore radius distribution ratios of blue bricks

图11 青砖孔隙率与抗压强度的关系
Fig.11 Relation between porosity and compressive strength of blue bricks

图12 青砖孔结构与抗冻性能的关系
Fig.12 Relationship between pore structure and frost resistance of blue brick
由
由
综合来看,抗冻性能中质量损失率较相对动弹性模量与孔隙率的关联性更强.因此,青砖微观孔隙率的变化更能反映宏观抗冻性中质量损失率的变化.
分形维数能够描述孔的结构特征,在一定程度上反映材料孔结构的疏密变化.孔体积分形维数(D,以下简称分形维数),可以表征青砖孔体积空间的复杂程度.根据盒维数计算方法,青砖分形维数的计算如
(1) |
式中:u为孔隙的体积分数;为青砖内部最大孔隙的半径, μm.
设,,通过建立线性回归方程,计算出青砖的分形维数,结果见
N/times | D | |
---|---|---|
0 | 2.982 7 | 0.962 2 |
15 | 2.980 1 | 0.973 5 |
30 | 2.979 5 | 0.973 4 |
45 | 2.974 6 | 0.992 1 |
60 | 2.969 7 | 0.932 6 |
75 | 2.966 6 | 0.989 4 |
90 | 2.964 2 | 0.964 8 |

图13 青砖分形维数与抗压强度的关系
Fig.13 Relation between fractal dimension and compressive strength of blue brick

图14 青砖分形维数与孔隙率的关系
Fig.14 Relation between fractal dimension and porosity of blue brick

图15 青砖分形维数与抗冻性能的关系
Fig.15 Relationship between fractal dimension and frost resistance of blue bricks
由
由
(1)冻融循环初期,青砖表面的孔洞逐渐增多且出现大量孔隙.随着经历冻融循环次数的增加,孔洞逐渐连接发展成为微小裂纹,然后扩展为裂缝,造成青砖表面剥落损伤加剧,外观劣化损伤现象明显.质量损失率逐渐上升,相对动弹性模量和相对抗压强度均呈下降趋势.经历90次冻融循环后, 青砖的抗压强度下降到6.77 MPa.
(2)冻融后青砖的谱分布及峰面积变化幅度有所不同,但总体上呈不断增大的趋势,说明随着经历冻融循环次数的增加,青砖内部产生了新的孔隙,孔隙所占比例逐渐增大.经历90次冻融循环后,孔隙率变为20.71%,冻融后青砖的孔隙率与抗压强度呈负相关.青砖的孔隙率与其抗冻性能相关性较高,可通过冻融后青砖的微观孔隙率推断抗压强度和抗冻性能的变化.冻融后青砖的孔径大小分布在3×1
(3)青砖具有明显的多重分形特征,分形维数在2.964 2~2.982 7之间,表明不同冻融阶段下青砖内部孔结构的复杂程度.随着经历冻融循环次数的增加,青砖的分形维数逐渐减小,青砖内部的无害孔、少害孔和有害孔的占比减少,多害孔的占比逐渐增多. 青砖的分形维数与抗压强度的关系呈正相关、与孔隙率呈负相关,相关系数分别为0.906 9和0.958 8.分形维数还与青砖的抗冻性能具有较高的关联性,说明孔隙结构的复杂程度是影响青砖宏观性能的重要因素.
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