摘要
研究了玄武岩纤维棒(BFB)的纤维增强指数IR对玄武岩纤维棒聚合物混凝土(BFB‑PC)韧性指标T2(n-1)(n)的影响,通过分形理论探究了T2(n-1)(n)与裂缝形貌的关系,并建立了弯曲韧性计算模型.结果表明:当IR=90.0时,BFB对BFB‑PC的增韧效果最优,此时BFB‑PC的弯曲韧性比未掺BFB时提升了8.39倍;BFB的增韧阈值为IR=37.5和IR=90.0;当IR相同时,与未掺聚合物的混凝土相比,聚灰比为0.06的BFB‑PC T2(n-1)(n)更高;IR越大裂缝形貌越复杂,T2(n-1)(n)与裂缝分形维数呈二次函数关系;本文提出的弯曲韧性计算模型准确描述了BFB对PC梁的增韧作用,其试验值与理论值的相对误差小于15.00%.
作为水泥混凝土的替代品,聚合物混凝土(PC)广泛应用于排水系统、桥面铺装层等领
掺入纤维能提高复合材料的力学性
鉴于此,本文研究了不同聚灰比mP/mC(质量比,文中涉及的含量、比值等除特殊说明外均为质量分数或质量比)和IR条件下玄武岩纤维棒聚合物混凝土(BFB‑PC)的弯曲韧性,分析了IR对其韧性指标T2(n-1)(n)(n为未掺纤维试件峰值荷载对应挠度的倍数)的影响,探究了T2(n-1)(n)与裂缝形貌的关系,并建立了BFB‑PC的弯曲韧性计算模型,以期为高韧性聚合物混凝土的设计提供理论依据.
水泥(C)为P·O 42.5普通硅酸盐水泥;骨料为细度模数2.7的中砂(S),级配区为Ⅱ区;聚合物(P)为丁苯乳液,固含量为50%,黏度为2 300 mPa·s;消泡剂(DA)为聚醚改性硅消泡剂,用于减少因丁苯乳液掺入而引入的气泡;减水剂为减水率25%以上的聚羧酸减水剂;拌和水(W)为自来水.硅灰(SF)的SiO2含量为96.04%;微珠(CP)的SiO2、Al2O3和Fe2O3的总含量为82.45%.BFB由400根玄武岩纤维单丝和树脂包裹组成,长度lf为12、15 mm,直径df为0.4 mm,密度为2 000 kg/
Specimen | /% | lf/mm | Mix proportion/(kg· | |||||||
---|---|---|---|---|---|---|---|---|---|---|
C | S | SF | CP | P | W | DA | BFB | |||
P0B0 | 0 | 432.00 | 1 440.00 | 24.00 | 24.00 | 0 | 168.00 | 0.48 | 0 | |
P0B1‑12/15 | 1 | 12,15 | 432.00 | 1 440.00 | 24.00 | 24.00 | 0 | 168.00 | 0.48 | 20.00 |
P0B2‑12/15 | 2 | 12,15 | 432.00 | 1 440.00 | 24.00 | 24.00 | 0 | 168.00 | 0.48 | 40.00 |
P0B3‑12/15 | 3 | 12,15 | 432.00 | 1 440.00 | 24.00 | 24.00 | 0 | 168.00 | 0.48 | 60.00 |
P6B0 | 0 | 432.00 | 1 440.00 | 24.00 | 24.00 | 57.60 | 139.20 | 0.48 | 0 | |
P6B1‑12/15 | 1 | 12,15 | 432.00 | 1 440.00 | 24.00 | 24.00 | 57.60 | 139.20 | 0.48 | 20.00 |
P6B2‑12/15 | 2 | 12,15 | 432.00 | 1 440.00 | 24.00 | 24.00 | 57.60 | 139.20 | 0.48 | 40.00 |
P6B3‑12/15 | 3 | 12,15 | 432.00 | 1 440.00 | 24.00 | 24.00 | 57.60 | 139.20 | 0.48 | 60.00 |
将BFB、胶凝材料及砂放入搅拌锅内搅拌,待材料混合均匀且纤维分散后,缓慢加入含减水剂、丁苯乳液和消泡剂的水.成型尺寸为40 mm×40 mm×160 mm的胶砂试件,装模后覆盖1层不透水薄膜,防止水分散失.24 h后拆模放入标准养护室养护至7 d取出,再转入室内自然养护至规定龄期.
试件的荷载-挠度(P‑δ)曲线见

图1 试件的荷载-挠度曲线
Fig.1 P‑δ curves of specimens

图2 BFB‑PC的T2(n-1)(n)
Fig.2 T2(n-1)(n) of BFB‑PC
采用鞠杨
(1) |
式中:n取值3、5、7、10;Eunrein为不掺纤维试件荷载-挠度曲线下的面积;为给定挠度下荷载-挠度曲线下的面积.
试件尺寸与加载方式不变时,δmax和Eunrein为材料常数,与纤维掺量无
由
T2(n-1)(n)随IR增大而增大,同时BFB‑PC的裂缝扩展路径与裂缝形貌变得更加曲折、复杂.通过分形理论可以建立分形维数与材料特性之间的关系,有效描述混凝土裂缝形态的复杂程度,因此大量应用于断口、裂纹等领
(2) |
通过Matlab计算BFB‑PC的裂缝分形维数,提取裂缝图像,将原始图转换成灰度图,并对灰度图进行二值化处理.当mP/mC=0.06时,BFB‑PC的裂缝灰度图见

图3 BFB‑PC的裂缝灰度图
Fig.3 Grey images of cracks in BFB‑PC(mP/mC=0.06)
BFB‑PC裂缝分形维数与T2(n-1)(n)的关系见

图4 BFB‑PC裂缝分形维数与T2(n-1)(n)的关系
Fig.4 Relationships of Dbox and T2(n-1)(n) of BFB‑PC
根据文献[
无序分布的纤维取向系数可通过计算得到.由于边界效应的影响,复合材料中单根纤维的与其到试件边缘的距离有关.假定纤维几何中心到试件边缘的距离为αlf(01),α与取向系数的关系见

图5 边界效应对纤维取向系数的影响
Fig.5 Influence of boundary effect on fiber orientation coefficien
当试件宽度t 2lf时,BFB‑PC中BFB的纤维取向系数平均值与t、lf的关系
(3) |
由计算可得,当lf为12、15 mm时,BFB的分别为0.474 0、0.498 8.
当BFB‑PC失效时,BFB的拔出或拔断与其长度密切相关,文献[
当满足临界条件=时,得到BFB的临界长度为:
(4) |
式中:为BFB的抗拉强度;为BFB与基体间的平均界面黏结强度.
当BFB‑PC产生裂缝后,荷载开始向BFB上转移,直至纤维从基体中拔出或拔断.由BFB‑PC的抗拉强
(5) |
纤维增强混凝土的抗弯强度与抗拉强度呈线性关系.为简便计算,文献[
(6) |
BFB的长度有效系数
(7) |
(8) |
将单根BFB的平均
(9) |
由于试件破坏后没有直接断开,这表明并非所有BFB都参与抑制裂缝扩展,因此引入高缝比(试件高度与裂缝长度的比值)来计算BFB的数量效率.通过Image J测量试件裂缝的长度,结果取平均值.综上,得到修正后的BFB‑PC弯曲韧性增量为:
(10) |
文中挠度指LVDT测得梁的跨中变形,试验中定义荷载-挠度曲线总积分面积得到的弯曲韧性为极限弯曲韧性.根据荷载-挠度曲线的积分面积计算试件破坏时吸收的能量.
BFB‑PC梁的弯曲韧性理论参数见
Parameter | mP/mC=0 | mP/mC=0.06 | |||||||
---|---|---|---|---|---|---|---|---|---|
P0B1‑12 | P0B1‑15 | P0B2‑12 | P0B2‑15 | P6B1‑12 | P6B1‑15 | P6B2‑12 | P6B2‑15 | ||
N×1 | 6.6 | 5.3 | 13.2 | 10.6 | 6.6 | 5.3 | 13.2 | 10.6 | |
/mm | 15.17 | 20.88 | 20.77 | 28.54 | 16.45 | 20.78 | 20.65 | 27.51 | |
τ/MPa | 30.28 | 22.04 | 22.14 | 16.11 | 27.94 | 22.14 | 22.23 | 16.70 | |
ηl | 0.396 | 0.359 | 0.289 | 0.263 | 0.365 | 0.361 | 0.291 | 0.273 | |
β | 0.83 | 0.79 | 0.86 | 0.85 | 0.86 | 0.85 | 0.88 | 0.80 |
BFB‑PC梁的弯曲韧性增量见

图6 BFB‑PC梁的弯曲韧性增量
Fig.6 Increment value of flexural toughness of BFB‑PC beams
(1)BFB的增韧阈值为IR=37.5和IR=90.0.当37.5≤IR≤90.0时,纤维增强指数IR和聚灰比对BFB‑PC的韧性指标T2(n-1)(n)产生正面影响;当IR37.5时,少量BFB对BFB‑PC的韧性提升有限;当IR90.0时,BFB‑PC的T2(n-1)(n)随着IR的增大而减小.与不掺BFB相比,当IR=90.0时,BFB对BFB‑PC的增韧效果最优,其弯曲韧性提升了8.39倍.随着IR的增大,BFB‑PC从脆性断裂转变为延性破坏,部分工况表现出明显的挠曲软化行为,具有更高的剩余承载力.
(2)T2(n-1)(n)与IR之间存在二次函数关系,且T2(n-1)(n)与IR在大挠度阶段具有更好的相关性.随着n的增大,T2(n-1)(n)的增长率增大.当IR相同,BFB‑PC聚灰比为0.06且T2(n-1)(n)高于其未掺聚合物时,掺入丁苯乳液可改善BFB与基体的界面黏结性能.
(3)随着IR的增大,BFB‑PC的裂缝形貌发生变化,裂缝扩展路径从直线变得非常曲折,且裂缝形貌越复杂,BFB‑PC开裂时消耗的能量越多.T2(n-1)(n)与裂缝分形维数呈二次函数关系,当IR90.0时,T2(n-1)(n)随着分形维数的增大而增大,相关性随n的增大而增大.
(4)建立了考虑纤维取向系数、长度有效系数的BFB‑PC弯曲韧性计算模型,理论值与试验值的相对误差均小于15.00%.故本文计算模型可靠,能够很好地描述BFB对PC梁的增韧作用.
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