The　effects　of　loading　rate　and　heterogeneity　of　meso-structure　on　the　failure　pattern　and　the　macroscopic　dynamic　mechanical　properties　of　concrete　are　investigated.　Considering　the　effect　of　concrete　heterogeneity,　concrete　is　simulated　as　a　two-phase　composite　composed　of　aggregate　and　mortar　matrix　at　meso-scale　in　this　work.　The　strain-rate　effect　is　also　accounted　for　and　the　damaged　plasticity　theory　is　employed　to　describe　the　mechanical　behavior　of　mortar　matrix.　It　is　assumed　that　the　aggregate　phase　cannot　be　damaged　due　to　high　strength,　and　thus　the　aggregate　particles　are　set　to　be　elastic.　The　dynamic　tensile　failure　modes　of　a　single-edge　notched　concrete　specimen　and　an　L-shaped　specimen　are　numerically　simulated.　The　simulation　results　indicate　that　the　dynamic　failure　pattern　and　the　direction　of　crack　propagation　of　concrete　have　pronounced　loading　rate　dependency.　With　the　increase　of　loading　rate,　the　failure　mode　of　concrete　changes　from　mode-I　to　mixed　mode.　A　more　complex　meso-structure　leads　to　a　stronger　interaction　between　the　components　and　more　complicated　crack　paths,　and　a　more　obvious　crack　branching　behavior.　Furthermore,　as　loading　rate　increases　much　more　branching　cracks　occur　within　concrete　and　the　width　of　the　damaged　region　increases,　implying　that　the　fracture　process　at　high　strain　rates　requires　more　energy　demand　to　reach　failure.　This　should　be　the　main　reason　for　an　increased　dynamic　strength　of　concrete.　©,　2015,　Tsinghua　University.　All　right　reserved.