Investigation Into The Dynamic Behaviors Of Rolled Compacted Concrete (RCC) And Blast-Resistance Of Concrete Gravity Dams

In recent decades,terrorist attacks,accident explosions,and military conflicts are becoming large threats to infrastructures.Among all kinds of infrastructures,high dams have great political and economic benefits to the society in areas such as electricity,irrigation,flood control,and drinking water.There exists a possibility for an attack on a high-hazard dam that could result in serious injury,property damage and economic devastation.Thus,nowadays,the owners and operators of the dams find they need to change the ways they identify threats to and vulnerabilities of the dams,manage risk,and implement measures to protect the dams from security-related failures.However,this is an area that needs urgent researches,and there is not a substantive progress with regard to how explosives actually affect the dams or how well these hydraulic structures can withstand such attacks.In order to reveal the dynamic responses and damage mechanism for concrete gravity dams subjected to underwater explosions,this thesis firstly focuses on the dynamic behaviors of roller compated concrete(RCC),especially for the vertical stratification from construction technique,the interlayer effect on the shock wave propagation,and the initial damage from improper rolling compaction.Then,the dynamic size effect of RCC is investigated based on the comprehensive understanding of its strain-rate sensitivity and sizedependence.At last,some simplifications based on above researches are made to obtain the numerical inputs in the simulations,and the vulnerability of concrete gravity dams exposed to underwater explosions is studied using a finite element package,LS-DYNA,in which the safeties of dam head and dam base are emphasized to achieve a more reasonable assessment to the blast-resistance of concrete gravity dams.The main highlights of this doctoral research are summarized as below:(1)Considering the inhomogeneity of RCC due to the special construction technique,the validations of one-dimensional wave propagation theory and the homogeneity assumption for stress and strain in split Hopkinson pressure bar(SHPB)tests are discussed at first.Based on the experimental data,the vertical stratification of RCC is verified to significantly influence the dynamic mechanical properties at high strain rates.Moreover,the weak bonding interlayer in the path of shock wave propagation shows a great impact on the reflection and transmission of shock wave,and the influence of interlayer on transmitted wave can be further revealed mathematically based on the equivalent viscoelastic theory.(2)RCC material usually remains in the initial damage status inevitably due to the improper vibration compaction control,which is usually ignored in original design.Three damage measurements,i.e.,density,ultrasonic pulse velocity(UPV),and elasticity modulus,are conducted to quantify the damage degree of RCC before SHPB tests.Then,the initial damage effect on dynamic compressive behaviors of RCC under impact loadings is investigated,which can be evaluated by a suggested model.(3)Focusing on the dynamic size effect of RCC material,SHPB tests on different dimensional specimens(in diameter)are conducted under high-strain-rate loadings in this research.Then,the size-dependence of RCC material is investigated within a wide range of strain rates and is reconfirmed based on analysis of variance(ANOVA)and Weibull analysis.A modified Weibull size effect law suitable for the full strain-rate range is proposed to illustrate the observed dynamic size effect for RCC material under impact loadings.However,in the observed dynamic size effect,contributions from lateral inertial confinement and end friction confinement,named as the structural effect,are emphasized to be removed to achieve the true strain-rate effect of RCC.(4)For a deeper understanding of the dynamic size effect,the impact fragmentations of RCC specimens with different diameters are investigated by SHPB tests under various loading rates.The dynamic fragmentation process detected by high speed camera has confirmed the existence of lateral inertia confinement under impact loadings.The sieving analysis is carried out to investigate the strain-rate effect and dynamic size effect on fragment size distribution,as well as its correlation with dynamic behaviors.Furthermore,the mechanism for dynamic behaviors of RCC is discussed based on the fractal characteristics of concrete fragmentation under impact loadings.(5)After a comprehensive consideration of the major differences between dam concrete and normal concrete,in terms of strength grade,aggregate size,and construction technique,the CSC model accompanied by suitable parameters is used to simulate the dynamic behaviors of dam concrete under dynamic loads.Then,the structural dynamic responses of concrete gravity dams under different explosion scenarios are investigated with a fully coupled dam-resevoir-foundation model,as well as the damage characteristics of the dam.Since the dam head is the weakest link of the whole dam,an optimized damage evaluation criterion based on blast-induced vibration is proposed to identify the damage state of the dam head after an underwater explosion.(6)Besides the safety of the dam head,the stability against sliding of dam base is specially emphasized for a more thorough evaluation on the blast-resistance of concrete gravity dams,which underlines the importance of considering the foundation lithology.The effect of foundation lithology on shock wave propagation and damage characteristic of the dam is discussed under a typical explosion scenario,as well as the damage mechanism of the dam base.Then,a comprehensive evaluation on the stability against sliding of the concrete gravity dam under various explosion scenarios is performed,taking the foundation lithology into consideration.