摘要
采用试验和三维随机骨料细观模型,研究了全珊瑚海水混凝土(CASC)在高温前后的静动态力学性能、抗侵彻和防爆性能. 结果表明:添加剑麻纤维能有效改善CASC的脆性;随着温度的提高,CASC的残余抗压强度先增后减;经历高温后CASC的静动态力学性能明显降低,温度越高,高温弱化效应越明显. 提出了描述CASC高温后受压应力-应变关系的两段式方程.建立了一种适用于CASC的三维随机骨料细观模型,数值模拟结果与试验结果较为吻合.
中国南海诸岛大部分为珊瑚岛礁,具有丰富的珊瑚资源.自中国提出“一带一路”和“海洋强国”等战略后,对南海诸岛基础设施的建设已初具规模. 为降低成本,缩短建设周期和解决原材料来源等问题,在不破坏岛礁现有生态坏境的前提下,就地取材,充分利用珊瑚碎屑和珊瑚砂来代替普通砂石骨料,用海水加以拌和制备全珊瑚海水混凝土(CASC). 在对工程结构,尤其是一些军事防护工程及重要设施进行设计和分析时,既要考虑基本受力情况,也要考虑冲击、爆炸等因素. 因此,深入研究CASC高温前后的静动态力学性能、抗侵彻和防爆性能,对保证岛礁工程的结构安全具有重要理论意义和较高实用价值.
鉴于此,本文介绍了本课题组自2018年以来开展的CASC系列研究成果,其中包括CASC的静态力学性
水泥选用南京江南-小野田水泥有限公司产P·Ⅱ52.5硅酸盐水泥和沈阳市嘉宝环球实业有限公司产52.5型碱式硫酸镁水泥,其中后者的主要成分是轻烧氧化镁、硫酸镁、磨细矿渣与核心外加剂,具有较高的韧性.粉煤灰(FA)为南京电热厂生产的Ⅰ级粉煤灰.矿渣(SG)为江苏江南粉磨公司S95级磨细矿渣.珊瑚和珊瑚砂均来自南海某岛礁,其中珊瑚为5~15 mm连续级配,珊瑚砂为中砂,细度模数2.44.减水剂为江苏苏博特新材料有限公司聚羧酸高效减水剂.人工海水为3.5%的NaCl溶液.剑麻纤维为广西剑麻集团产剑麻纤维,直径为0.03 mm.

图1 4种混凝土弹性模量与轴心抗压强度的关系
Fig.1 Relationship between elastic modulus and axial compressive strength of four kinds of concrete

图2 CASC与SFCASC的静态压缩韧性指数
Fig.2 Static compression toughness indices of CASC and SFCAS

图3 不同种类混凝土的应力-应变全曲线
Fig.3 Stress‑strain curves for different types of concrete

图4 CASC的相对残余轴心抗压强度与温度的关系
Fig.4 Relationship between relative residual axial compressive strength and temperature of CAS
弹性模量是衡量混凝土材料变形能力的重要指标.

图5 CASC的初始弹性模量随温度的变化曲线
Fig.5 Variation curves of the initial elastic modulus with temperature of CAS

图6 高温后3种强度等级CASC的应力-应变全曲线
Fig.6 Stress‑strain curves of CASCs with three strength grades after high temperature
根据试验数据曲线具体情况,确定高温后CASC的准静态单轴压应力-应变全曲线,表达式如下:
(1) |
式中:x = ε/ε0,ε0为峰值应变;y = σ/σ0,σ0为峰值应力;a、b分别为上升段和下降段曲线的控制参数.

图7 各混凝土的静态轴心抗拉应力-应变曲线
Fig.7 Static axial tensile stress‑strain curves of various concrete

图8 C30强度等级CASC和SFCASC的应力-应变曲线
Fig.8 Stress‑strain curves of C30 strength grade CASC and SFCAS

图11 C50强度等级CASC、SFCASC与OPC的冲击劈拉强度对比
Fig.11 Comparison of the impact split strength of C50 strength grade CASC, SFCASC and OP
CASC侵彻试验中,子弹的总质量为(180.0±1.5) g,子弹速率基本保持在300 m/s. 通过高速摄影机确定弹丸在着靶前的弹姿为水平状态,弹丸仅对靶体侵彻面形成宏观破坏,破坏形态呈漏斗状,靶体侵彻背面和侧面无任何变化. 不同强度等级的CASC靶体被侵彻后,弹坑直径及深度均随着混凝土强度等级的提高有所减小.

图13 CASC侵彻试验及模拟
Fig.13 Penetration experiment and numerical simulation of CAS

图14 CASC爆炸试验
Fig.14 Explosion experiment of CAS
混凝土强度等级对CASC靶板的爆坑直径有显著影响,随着强度等级的升高,弹坑直径减小;而强度等级对弹坑的深度影响较小. 从背爆面的破坏破坏情况可以看出,高强CASC背爆面不仅会产生以板中心向外扩散的径向裂纹,还存在多条横裂纹,部分区域甚至出现层裂现象. 由此说明珊瑚混凝土强度等级越高,脆性就越大,背爆面的破坏效果也越显著.
CASC的细观建模方法可参考文献[

图15 C30强度等级CASC 准静态压缩应力-应变曲线
Fig.15 Quasi‑static compression stress‑strain curves of C30 strength grade CAS

图16 CASC 准静态压缩破坏过程
Fig.16 Quasi‑static compression damage process of CAS
针对CASC圆柱体试件进行冲击压缩试验,采用细观模型模拟不同应变率下CASC的破坏形态和破坏过程,研究其动态应力-应变关系及应变率效应.

图17 CASC的动态压缩应力-应变曲线
Fig.17 Dynamic compression stress‑strain curves of CAS

图18 不同应变率作用下CASC的压缩破坏形态
Fig.18 Compression damage patterns of CASC under different strain rate
(1)得到不同强度等级全珊瑚海水混凝土(CASC)的应力-应变全曲线,其上升段斜直线线段较长,在应力达到峰值应力90%以上时出现拐点,呈现出明显脆性;添加剑麻纤维可有效增强CASC的韧性.
(2)随着温度的升高,CASC的残余抗压强度和弹性模量均呈现先增后减趋势,温度越高,高温弱化越明显;CASC力学性能的临界温度是300 ℃,对于岛礁工程中防火防灾具有重要意义.建立了高温后的CASC应力-应变全曲线方程,可反映CASC在高温后受压后的全部特征.
(3)CASC在动态冲击压缩、动态冲击劈拉及动态直接拉伸试验中均表现出明显的应变率硬化效应,应变率越高,其应力越大.剑麻纤维的添加可以有效改善CASC的脆性,增加CASC的韧性,使其破坏程度明显降低.
(4)随着混凝土强度等级的提升,CASC靶体侵彻深度和开坑面积不断减小,且伴随大量破碎剥落;爆炸弹坑直径减小,但弹坑深度变化较小.
(5)建立了一种适合于CASC的具有随机形状和尺寸的三维随机骨料模型,并模拟了CASC在动静态作用下的破坏形态与破坏机理.
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