摘要
为探究早龄期混凝土的断裂性能,针对5种养护龄期(3、7、14、21、28 d)的混凝土试件,开展20根含预制裂缝的三点弯曲梁断裂试验,得到其荷载-裂缝口张开位移(P‑CMOD)曲线.基于该曲线,计算得到早龄期混凝土的断裂韧度(起裂韧度和失稳韧度)和断裂能.采用声发射(AE)技术,通过振铃计数和累计振铃计数与时间的关系曲线,探讨养护龄期对早龄期混凝土试件损伤断裂过程的影响;通过计算上升角(RA)和平均频率(AF),分析了早龄期混凝土的破坏模式.结果表明:早龄期混凝土的断裂韧度和断裂能均随着养护龄期的增加而增大,抗开裂能力提高,延性变差.当养护龄期由3 d增至14 d时,混凝土的断裂韧度和断裂能增速较快;当养护龄期由14 d增至28 d时,混凝土的断裂韧度和断裂能增速变缓.振铃计数和累计振铃计数均能较好地反映早龄期混凝土的损伤破坏过程,其中累计振铃计数的增速随着养护龄期的增加而减小.混凝土的剪切裂缝占总裂缝的比例随着养护龄期的增加而增加.
早龄期混凝土是指养护龄期小于标准养护龄期(28 d)的混凝
Matallah
声发射(AE)技
当前,基于AE技术开展混凝土断裂特性的成果较多,但是针对早龄期混凝土断裂特性的研究相对较少.为此,本文采用AE技术,对早龄期混凝土的断裂特性和破坏模式进行研究分析.
水泥为P·O 42.5普通硅酸盐水泥;细骨料为天然河砂;粗骨料为粒径5~20 mm碎石;拌和水为自来水.试件配合比m(水泥)∶m(砂)∶m(石)∶m(水)=1.00∶2.02∶3.03∶0.51.
设计了5组养护龄期(3、7、14、21、28 d),每组4根,共20根含预制裂缝(缝高比为0.3)的早龄期混凝土试件,强度等级为C40,尺寸为100 mm×100 mm×400 mm.所有试件均一次性浇筑完成,并在浇筑时放置厚度为3 mm的预埋钢片,以形成初始缝高比为0.3的裂缝.试件浇筑完成4 h后取出预埋钢片,24 h后脱模,标准养护至规定龄期.
早龄期混凝土断裂试验在南京水利科学研究院液压伺服试验机上完成.试件标准养护至规定龄期后,在其底部预制裂缝两侧粘贴钢片,钢片刀口处放置夹式引伸计,以测量混凝土加载过程中的裂缝口张开位移(CMOD)值.试验单调加载采用位移控制,加载速率为0.05 mm/min.为更加真实地反映试验结果,对试件进行4组重复试验.早龄期混凝土试件尺寸示意图见

图1 早龄期混凝土试件尺寸示意图
Fig.1 Dimensional diagram of early‑age concrete specimen(size:mm)
断裂韧度包括起裂韧度和失稳韧度.和能够体现混凝土的抗阻裂能力,是研究混凝土断裂性能的重要参数.参考DL/T 5332—2005《水工混凝土断裂试验规程》,早龄期混凝土的由起裂荷载和初始裂缝长度计算得到;由失稳荷载和临界有效裂缝长度计算得到.的计算式为:
(1) |
其中
(2) |
式中:为夹式引伸计钢片厚度,mm;为试件裂缝口张开位移临界值,mm;为试件弹性模量,GPa;为试件初始柔度(即CMOD/P),mm/kN,取P‑CMOD曲线上升段线性直线上任意3点的CMOD值,与P值进行计算,再取其平均值;为试件厚度,mm;为试件高度,mm.
断裂能(N/m)表示混凝土裂缝扩展单位面积所需要的能量,用于反映混凝土抵抗裂缝扩展的能力.的计算
(3) |
式中:为断裂过程中试件所做的功,J,可用P‑CMOD曲线与坐标轴所围成的面积计算得到;为试件断裂面的实际面积,
试验过程中,由于P‑CMOD曲线下降至荷载P为零时是难以实现的,需要对P‑CMOD曲线尾部进行处理修

图2 早龄期混凝土的尾部断裂能计算示意图
Fig.2 Schematic diagram of tail fracture energy calculation of early‑age concrete

图3 早龄期混凝土的P‑CMOD曲线
Fig.3 P‑CMOD curves of early‑age concretes
根据P‑CMOD曲线和相关公式,计算出早龄期混凝土试件的起裂荷载、失稳荷载,以及对应的断裂参数.

图4 早龄期混凝土的起裂荷载与失稳荷载随养护龄期的变化
Fig.4 Variation of cracking load and instability load of early‑age concretes with curing age

图5 早龄期混凝土起裂荷载与失稳荷载的比值变化
Fig.5 Change in the ratio of cracking load to instability load of early‑age concretes

图6 早龄期混凝土的起裂韧度和失稳韧度
Fig.6 Crack initiation toughness and unstable toughness of early‑age concretes

图7 早龄期混凝土的断裂能
Fig.7 Fracture energy of early‑age concretes
振铃计数是反映声发射现象是否活跃的参量,从振铃计数的变化趋势可以得到材料内部损伤发展的剧烈程度和实时变化.累计振铃计数能够反映材料在加载过程中AE活动的总量和频率,其增速越快,材料内部损伤就越快.

图8 早龄期混凝土振铃计数和累计振铃计数与时间的关系曲线
Fig.8 Ringing count and cumulative ringing count with time of early‑age concrete specimens at different curing ages
混凝土断裂过程中的裂纹可分为剪切裂缝和拉伸裂缝.这2种裂缝产生的AE信号波形有所不同,剪切裂缝产生横波,拉伸裂缝产生纵波.由于横波和纵波的传播速度不同,因此可以用信号波的上升角(RA)和平均频率(AF)来识别混凝土断裂过程中的剪切裂缝和拉伸裂

图9 早龄期混凝土RA‑AF的关系曲线
Fig.9 Relation curves of RA‑AF of early‑age concretes at different curing ages
(1)早龄期混凝土的起裂荷载和失稳荷载随着养护龄期的增加而增大,起裂荷载与失稳荷载的比值随着养护龄期的增加而增大,混凝土延性变差.
(2)早龄期混凝土的起裂韧度、失稳韧度和断裂能随着养护龄期的增加而增大.与3 d养护龄期相比,7、14、21、28 d养护龄期下混凝土的起裂韧度增加63.6%、115.0%、136.8%和149.8%,失稳韧度增加49.6%、79.5%、95.9%、104.3%,断裂能增加16.4%、27.7%、33.8%和44.1%.当养护龄期由3 d增至14 d时,早龄期混凝土的断裂韧度和断裂能增速较快;当养护龄期由14 d增至28 d,早龄期混凝土的断裂韧度和断裂能增速变缓.
(3)声发射振铃计数和累计振铃计数均能较好地反映早龄期混凝土的损伤破坏过程.与3 d养护龄期相比,7、14、21、28 d养护龄期下累计振铃数分别增长153.4%、238.3%、267.9%和300.8%,且累计振铃计数的增速随着养护龄期的增加而减小.
(4)早龄期混凝土断裂破坏过程中,剪切裂缝占总裂缝的比例随着养护龄期的增加而增加.养护龄期小于14 d时,混凝土开裂以拉伸裂缝为主;养护龄期大于等于14 d时,混凝土开裂以剪切裂缝为主.
参考文献
NEHDI M, SOLIMAN A M. Early‑age properties of concrete: Overview of fundamental concepts and state‑of‑the‑art research[J]. Construction Materials, 2011, 164(2):57‑77. [百度学术]
侯东伟, 张君. 早龄期混凝土全变形曲线的试验测量与分析[J]. 建筑材料学报, 2010, 13(5):613‑619. [百度学术]
HOU Dongwei, ZHANG Jun. Experimental measurement and analysis of overall deformation of concrete at early‑age[J]. Journal of Building Materials, 2010, 13(5):613‑619. (in Chinese) [百度学术]
LAWRENCE A M, TIA M, FERRARO C C, et al. Effect of early age strength on cracking in mass concrete containing different supplementary cementitious materials:Experimental and finite‑element investigation[J]. Journal of Materials in Civil Engineering, 2012, 24(4):362‑372. [百度学术]
MAO J W, LIANG N H, LIU X R, et al. Investigation on early‑age cracking resistance of basalt‑polypropylene fiber reinforced concrete in restrained ring tests[J]. Journal of Building Engineering, 2023,70:106155. [百度学术]
陈波, 蔡跃波, 丁建彤, 等. 基于温度应力试验的早龄期混凝土弹性模量量测[J]. 建筑材料学报, 2016, 19(4):785‑790. [百度学术]
CHEN Bo, CAI Yuebo, DING Jiantong, et al. Measurement of early age concrete elastic modulus based on thermal stress test[J]. Journal of Building Materials ,2016, 19(4):785‑790. (in Chinese) [百度学术]
王世鸣, 李夕兵, 宫凤强, 等. 静载和动载下不同龄期混凝土力学特性的试验研究[J]. 工程力学, 2013, 30(2):143‑149. [百度学术]
WANG Shiming, LI Xibing, GONG Fengqiang, et al. Experimental study on mechanical properties of concrete at different ages under static and dynamic load[J]. Engineering Mechanics, 2013, 30(2):143‑149. (in Chinese) [百度学术]
MATALLAH M, FARAH M, GRONDIN F, et al. Size‑independent fracture energy of concrete at very early ages by inverse analysis[J]. Engineering Fracture Mechanics, 2013, 109:1‑16. [百度学术]
NIKBIN I M, RAHIMI S, ALLAHYARI H. A new empirical formula for prediction of fracture energy of concrete based on the artificial neural network[J]. Engineering Fracture Mechanics, 2017, 186:466‑482. [百度学术]
ABOLHASANI A, NAZARPOUR H, DEHESTANI M. The fracture behavior and microstructure of calcium aluminate cement concrete with various water‑cement ratios [J]. Theoretical and Applied Fracture Mechanics, 2020, 109:102690. [百度学术]
张廷毅, 高丹盈, 郑光和, 等. 三点弯曲下混凝土断裂韧度及影响因素[J]. 水利学报, 2013, 44(5):601‑607. [百度学术]
ZHANG Tingyi, GAO Danying, ZHENG Guanghe, et al. Fracture toughness of concrete and influencing factors under three‑point bending[J]. Journal of Hydraulic Engineering, 2013, 44(5):601‑607. (in Chinese) [百度学术]
范向前, 胡少伟, 朱海堂,等. 非标准钢筋混凝土三点弯曲梁双K断裂特性[J]. 建筑材料学报, 2015, 18(5):733‑736. [百度学术]
FAN Xiangqian, HU Shaowei, ZHU Haitang, et al. Double‑K fracture characteristics of three point bending beams of non‑standard reinforced concrete[J]. Journal of Building Materials, 2015, 18(5):733‑736. (in Chinese) [百度学术]
王治, 金贤玉, 田野, 等. 基于双G准则的掺粉煤灰砼断裂性能研究[J]. 湖南大学学报(自然科学版), 2012, 39(3):17‑22. [百度学术]
WANG Zhi, JIN Xianyu, TIAN Ye, et al. Research on fracture characteristic of fly ash concrete based on dual‑G criterion[J]. Journal of Hunan University (Natural Sciences) , 2012, 39(3):17‑22. (in Chinese) [百度学术]
LACIDOGNA G, PIANA G, CARPINTERI A. Damage monitoring of three‑point bending concrete specimens by acoustic emission and natural frequency analysis[J]. Engineering Fracture Mechanics, 2018, 210:203‑211. [百度学术]
HU S W, LU J, XIAO F P. Evaluation of concrete fracture procedure based on acoustic emission parameters[J]. Construction and Building Materials, 2013, 47:1249‑1256. [百度学术]
KRAVCHUK R, LANDIS E N. Acoustic emission‑based classification of energy dissipation mechanisms during fracture of fiber‑reinforced ultra‑high‑performance concrete[J].Construction and Building Materials,2018, 176, 531‑538. [百度学术]
HAN Q H, YANG G, XU J, et al. Acoustic emission data analyses based on crumb rubber concrete beam bending tests[J]. Engineering Fracture Mechanics, 2018, 210:189‑202. [百度学术]
CHEN C, FAN X Q, CHEN X D. Experimental investigation of concrete fracture behavior with different loading rates based on acoustic emission[J]. Construction and Building Materials, 2020,237:117472. [百度学术]
张秀芳, 徐世烺. 采用荷载-裂缝张开口位移曲线确定混凝土三点弯曲梁的断裂能[J]. 水利学报, 2008, 39(6):714‑719. [百度学术]
ZHANG Xiufang, XU Shilang. Determination of fracture energy of three‑point bending concrete beam usino relationship between load and crack‑mouth opening displacement[J]. Journal of Hydraulic Engineering, 2008, 39(6):714‑719. (in Chinese) [百度学术]
徐慧颖, 卜静武, 吴新宇. 不同缝高比大坝混凝土断裂性能及声发射特性[J]. 水电能源科学, 2022, 40(3):100‑104. [百度学术]
XU Huiying, BU Jingwu, WU Xinyu. Fracture performance and acoustic emission characteristics of dam concrete with different crack‑depth ratios[J]. Water Resources and Power, 2022, 40(3):100‑104. (in Chinese) [百度学术]
ULFKJAER J P, KRENK S, BRINCKER R. Analytical model for fictitious crack propagation in concrete beams[J]. Journal of Engineering Mechanics, 1995, 121(1):7‑15. [百度学术]
张志刚, 尹志伟, 秦凤江. 低模量早强型高延性混凝土的力学行为[J]. 建筑材料学报, 2019, 22(5):707‑713. [百度学术]
ZHANG Zhigang, YIN Zhiwei, QIN Fengjiang. Mechanical performance of engineered cementitious composites(ECC) with low modulus and early strength[J]. Journal of Building Materials,2019, 22(5):707‑713. (in Chinese) [百度学术]
YANG L Y, XIE H Z, ZHANG D B , et al. Acoustic emission characteristics and crack resistance of basalt fiber reinforced concrete under tensile load[J]. Construction and Building Materials, 2021, 312:125442. [百度学术]