Experimental Study On The Mechanical Properties And Damage Performance Of Early Age Concrete Under Dynamic Loading

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.