Study On Rate Independence Of Concrete Strength

The dynamic strength of concrete is an aged but hot problem, which has beenattracting sustaining and extensive attention worldwide. Awareness of the problem willdirectly affect the dynamic safety design of concrete engineering structures. In thisthesis, the existing experiments for dynamic strength of concrete are investigatedutilizing molecular dynamics method, continuum dynamics theory and finite elementmethod. And the cognition of rate independence for concrete strength is graduallyrefined and deepened. The main research achievements of this thesis are listed asfollows:1. The two main misunderstandings of the traditional dynamic strengthexperiments of concrete are analyzed and figured out. Firstly, the one dimensionalstress assumption of traditional dynamic strength experiments is not strictly correct,and thus, the measured nominal strength is not able to represent the true properties ofmaterial strength of concrete unless necessary information filtering and screening oftest data are employed. Secondly, the molecular dynamics numerical simulation resultsof ideal homogeneous material indicate that the main source of the rate dependence fornominal strength is not the material heterogeneity, but the material inertia confinementeffect and the multi-axis stress state caused by it.2. Based on the dynamic continuum theory, two theoretical models appropriate foranalyzing inertia effect of the dynamic strength experiments for concrete areestablished. Under dynamic loading, the axial and radial inertia effects of concretespecimen will cause additional axial stress in the two ends, making the measurednominal strength larger than material strength. Moreover, the radial inertia confinementeffect under dynamic compression will interact with the pressure dependence ofcompressive strength for concrete material, and this coupling effect will drasticallyenhance the measured nominal strength with the increase of strain rate.3. The experiments for tensile and compressive dynamic strength of concrete aresimulated respectively based on their finite element models, and the influence laws ofdifferent experimental conditions are investigated in detail and summarized. With theselaws, previous experimental results are filtered and extracted to get the true material strength of concrete, by doing this, the rate independence of concrete strength issynthetically proved. The simulation results indicate that the measured nominalstrength is influenced by many factors, such as specimen size, end restraint effect, pureinertia effect, radial inertia confining pressure coupling effect, end friction anddamping ratio. Although the concrete strength remains unchanged, with the increase ofstrain rate, the combined effects of many different experimental conditions headed byinertia effect will cause corresponding transition and transformation of the stress andstrain state of the specimen, thereby the simulation results perform the same ratedependent laws of nominal strength as previous tests.4. The rate independence of concrete strength brings an important impact toseismic designs of dams, and the earthquake damage to Koyna Dam in India is selectedfor a case study. Different seismic failure patterns of Koyna Dam are simulated andcompared, and the pattern obtained with static concrete strength is closer to the realsituation than that obtained with dynamic concrete strength which is stipulated inChinese seismic design norm for hydraulic structures (DL5073-2000). For this reason,static concrete strength should be used in seismic designs, and definitely effective andreliable safety for dams can be realized in exchange for a small increase of concreteconstruction volume.