【作者】 王世鸣；

【导师】 李夕兵；

【作者基本信息】 中南大学 ， 岩土工程， 2014， 博士

【摘要】 摘要：混凝土作为应用最广泛的建筑材料之一,在基础工程建设中发挥着重要的作用。随着实验设备和实验手段的改进,人们对混凝土展开了全方位的研究：从宏观到微观,从静载荷到动载荷,从稳定龄期到早龄期,从添加剂到钢纤维等等,在不断完善对混凝土认识,并促进了混凝土力学特性的改善。然而混凝土从制成到其稳定发挥作用-般需要28天,随着社会的进步,高效建设已是现代建筑理念的核心,在建设过程中,混凝土在养护期间不可避免会受到外界间断性甚至是持续性的影响。。在一些大断面开挖工程中,尤其是隧道和矿山巷道工程施工过程中,钻爆掘进法仍然是最主要的施工手段。因此,当隧道或巷道存在多个工作面同时进行钻爆掘进时,对整个隧道或巷道断面(包括围岩、初期喷混凝土层和二次衬砌结构)来说,实际上要受到多次爆破的动力扰动作用。且在工程中,为了加快施工进度,经常在喷射混凝土之后不久就立即进行爆破开挖,初期支护结构在距离爆源较近的情况下,爆破施工会使结构体产生累积损伤,影响混凝土支护结构的长期强度,使之达不到预期的支护效果。纵观以往混凝土力学研究,主要集中在稳定龄期后混凝土力学研究,对于早期混凝土力学研究较少,且局限于静载条件。爆破等工序的扰动作用不可避免的对早龄期混凝土结构造成损伤,这不仅对工程设计提出了新的要求,例如不同的爆破药量对不同龄期支护结构造成不同的损伤值,那么设计过程中就应将这一因素考虑进去,确保遭遇外界扰动后混凝土支护结构依旧能满足工程要求,同时在施工中,对爆破等工序应如何采取措施来防护支护结构也提出了相应的指导,达到科学服务于工程的目的。本文正是基于工程和科研的需要,针对上述情况下混凝土的力学性能展开研究,主要研究内容如下：(1)早龄期混凝土静载和动载下单轴压缩试验表明：静态和动态混凝土的强度与龄期呈对数关系增长；混凝土是一种应变率敏感性材料,在应变率20-75s-1范围内,动态强度提高因子为1.19-2.53,且动态混凝土强度与应变率呈指数关系增长,其应变率指数随着龄期的增长而迅速降低,直至接近脆性材料应变率指数；静态和动态峰值应变随龄期的增长整体上呈减小的趋势,其中动态峰值应变随应变率的提高而增大,但其增长幅度随着龄期增长而变小；动载下混凝土能耗与入射能和应变率都呈线性关系,且变化模式相近,均为在龄期7天以前,混凝土单位体积吸收能对外界冲击敏感性不高,但随着龄期的增长快速提高；而在龄期7天以后,随着龄期的变化,单位体积吸收能受外界冲击影响不大。(2)50-75%临界入射能多次冲击条件下,早龄期混凝土经过2-4次冲击基本上都发生了不同程度的破坏：龄期7天以前,混凝土具有一定的粘弹性,峰值应变随着冲击次数的增加而减小,抗压强度和弹性模量随着冲击次数的增加而提高,且多次冲击中,冲击入射能越小,提高作用越明显；随着龄期的增长,混凝土逐渐显现准脆性材料的特性,峰值应变随着冲击次数的增加整体变化不明显,强度和弹性模量随着冲击次数和冲击入射能的增加虽然有一定的提高,但整体变化不大；多次冲击破坏时混凝土的累积吸收能随着冲击次数的增加而增加,随着龄期的增长,每次冲击的吸收能也随之增长,与单次冲击破坏所需的能量相比,早龄期混凝土破坏累积吸收能受冲击次数影响不大。(3)冲击条件下混凝土的破坏模式实质上均为拉伸破坏,破坏模式整体受龄期影响较小,其中多次冲击下混凝土的破坏形态随龄期的变化整体上是由存在大块到大块度碎裂再到存在大块的一个过程,与龄期7天以“碎”而不裂的大块不同,龄期28天混凝土大块布满裂纹,整体上已经碎裂。(4)早期冲击养护模式下,混凝土的力学特性主要受到冲击作用和养护作用共同影响。龄期7天以前,当外界入射能不超过各龄期临界入射能的75%时,冲击养护后,混凝土表现出受冲击压实的作用。其中在早期单龄期冲击条件下,在各自50%临界入射能冲击后养护达到稳定龄期后破坏所需的能量影响并不大,而当冲击入射能达到或是超过临界入射能达到75%时,早期冲击作用对试样达到稳定龄期后抵抗外界入射能力具有明显的降低作用；龄期7天以后,随着混凝土脆性特性逐渐显现,冲击压实作用和养护作用减弱；在50%临界入射能冲击条件下早期单龄期冲击对混凝土力学特性影响不大,达到或超过75%后会明显弱化试样达到稳定龄期后抵抗外界入射能力。而早期多龄期冲击条件下,龄期7天以后的冲击过程中容易产生裂纹,导致混凝土各方面力学性能有所下降,且随着冲击入射能的增大下降越明显。除了在龄期3天以前破坏时两者的单位体积吸收能相近外,随着龄期的变化,单位体积吸收能与龄期呈不同模式变化；多龄期冲击破坏累积单位体积吸收能远远大于单次冲击破坏单位体积吸收能,但在多龄期持续冲击后,混凝土在后期冲击破坏时单位体积吸收能远小于单次冲击破坏的单位体积吸收能,下降了近50%。(5)根据早期混凝土力学特性随龄期存在由有粘弹性向脆性转变这一特性,将龄期变量引入Weibull分布统计损伤模型中,结合基于Kelvin模型改进的岩石时效损伤模型,提出了能反映混凝土材料力学性能随龄期变化的本构模型,并通过模型曲线和实测曲线的对比对模型进行了验证。结果表明该模型能较好的反映出混凝土在冲击荷载作用下其力学特性随龄期变化的规律。(6)采用基于龄期的损伤本构模型对不同冲击条件下早龄期混凝土的损伤特性进行了计算分析,再次验证了该模型能较好的反映出早龄期混凝土在不同条件下的损伤特性,且符合试验中混凝土不同龄期表现出的力学特性,并指出在研究材料的力学特性时,与单一的宏观力学特性相比,结合材料宏观力学特性和内部变量更能反映出材料在外界荷载下真实的力学特性。更多还原

【Abstract】 Abstract:As one of the most commonly used building materials, concrete plays an important role in the construction of infrastructure projects.With the improvement of laboratory equipment and experimental method,the researchers launched a comprehensive study of the concrete:from macro to micro,from static load to dynamic load,from the stable early age to age, from an additive to steel fibers, etc, in constantly improve the understanding of concrete,while also largely contributed to improve the mechanical properties of concrete.While the mechanical properties of concrete would basically meet design requirement after28days curing, which is in contradiction with the modern construction methods that was dedicated to shortening the construction period.And during curing, concrete would be inevitably affected by intermittent or even lasting outside loading. In some large section excavation projects,especially in tunnels and mine workings project, drilling and blasting method is still the usual way to excavate tunnel.Therefore,when the tunnel section is divided into multiple sections were carried out using the drilling and blasting method,the entire tunnel section(including surrounding rock, the initial layer of shotcrete and the Secondary lining structure) is,in fact, subject to repeated blasting disturbances.In order to accelerate the construction progress,blasting operation is often conducted following the spraying concrete operation in a very short time interval,while the repeated blasting impact loading will probably cause damage to the concrete that is newly formed or has not reached to stable age near the excavation face,affecting the strength of early age concrete,decreasing the effectiveness of concrete support structure.Previous studies have focused a lot on the mechanical properties, strain-stress curve,mechanical properties prediction model of early age concrete subjected to static loading.However, there is little information in the literature on the mechanical properties under dynamic loading conditions.As early age concrete would be inevitably affected by disturbance of blasting processes, it is not only required new requirements for the engineering design,such as different doses causing different damage of different ages concrete supporting structure,and this factor should be taken into account in engineering design to ensure that concrete supporting structure is still effective after subjected to external disturbances, which can also to improve the utilization of the material, but also proposed appropriate guidance to take measures to protect the supporting structure from blasting in the construction of the process,and ultimately achieve the purpose of scientific services in engineering. Therefor, base on the engineering and scientific requires,this paper investigated the mechanical properties of early age under the cases mentioned above.The main contents are as follows:(1)The static and dynamic uniaxial compression tests show that concrete is sensitive to strain rate,the dynamic increase factor(DIF) increases from1.19to2.53at strain rate range of20～70s-1,and the static and dynamic strength of concrete have a positive logarithmic relationship with age.The growth trend of early age concrete dynamic strength with strain rate can be presented with an index function,while the strain rate index decreases with the growth of age,and at age of28days,it is close to the strain rate index of brittle materials.The static and dynamic critical strain(strain corresponding to maximum stress) of concrete decrease with the growth of age,and the dynamic strain grows with the increases of strain rate, while the growh magnitude decreaces with age.Under dynamic load,the unit volume absorbed energy of early age concrete has a positive linear relationship with incdent energy and strain rate.And before age of7days, the unit volume absorbed energy shows low sensitive to the impact loading, but it is improved with the growth of age; while there is no obvious change with the growh of impact loading after age of7days.(2) Early age concrete was fragmentation with large degrees after impact2-4times under the impact level of50～75%critical incident energy (EIC). Before age of7days,as the concrete shows viscoelasticity, and compaction after proper multiple impact which can be presented that the critical stain decreases with impact times, dynamic strength and elastic modulus increase with impact times,the smaller impact incident energy,the bigger growth magnitude.However, with the growh of age, the concrete begins to reveal the characteristics of quasi-brittle materials, the critical strain,dynamic strength and elastic modulus have no obvious change with the increase of impact times and impact incident energy.The cumulative unit volume absorbed energy of concrete subjected multiple impact increases with impact times,and the unit volume absorbed energy of each impact also increases with the growth of age.While compare with single impact, the influence of impact times on the cumulative damage absorbed energy is not obvious.(3)The failure modes of concrete under impact load are tensile failure,and there is little effect of age on the failure mode of concrete under impact conditions.Under multiple impact condition, the faluire shape of concrete vration with age is from existing large pieces to fragmention with large degree,and then to existing large pieces.However, compare with failure modes that existing large pieces without cracking before age of7days, the large pieces at age of28days covered with cracks, and which has been broken as a whole.(4)The mechanical properties of early age concrete are con-determined by the role of impact and curing under the impact-curing mode, and the role of impact on concrete presents compaction and produce cracks.Before age of7days, the concrete specimens are compact after impact of75%corresponding’s EIC, and the effect of early age impact on the dynamic mechanical properties of specimens at stable age is little when the specimens previously subjected to impact at age of1,3and7days respectively under the impact level of50%corresponding’s EIC,.While under the impact level of75%corresponding’EIC,there is an evidential effect on the mechanical properties of specimens at stable age. However, as concrete begins to show the characteristics of quasi-brittle materials, there is little change of mechanical properties when the specimens previously subjected to impact after age of7dyas under the impact level by no more than75%corresponding’s EIC.And specimens are prone to crack under multiple age impact condition after age of7days, which woud lead to the mechanics performance of concrete degradation, and the bigger incident energy, the more degradation. In terms of absorbed energy, except age of3days, the unit volume absorded energy under single age impact condition is similar to multiple age impact condition. The cumulative damage unit volume absorbed energy under multiple age impact condition is far greater than the cumulative damage unit volume absorbed energy under single age impact condition, and magnitude increase with the growth of age.However, the absorbed energy ability of stable age concrete previously subjected multiple age impact is also far smaller than that under single age impact condition, dropped by nearly50%.(5)Based on statistical damage theory and Weibull distribution, combining the analysis of the change laws of stress-strain curves and viscosity coefficient of concrete with age, a damage constitutive model that can reflect the variation in dynamic mechanical properties with age was proposed. And the new-built constitutive model was verified by comparing stress-strain curves obtained from the new model with experimental stress-strain curves, indicating that there was a good agreement with two groups of stress-strain curves.(6)The analyses of early concrete damage mechanical under different impact conditions are conducted based on the new-built constitutive damage model.And the model is once again proven that can reflect the damage characteristics of early age concrete under different conditions, which agree with mechanical properties of different age concrete performence in the tests. What is more, in the study of the mechanical properties of materials, compare with single macroscopic mechanical properties, the combining macroscopic mechanical properties and internal variables can better reflect real mechanical properties of material under external load.更多还原