Cast-in-place thin layers of Ultra-High Performance Fiber Reinforced Cementitious Composites (UHPFRC) on the specific zones of existing reinforced concrete (RC) bridge decks has been demonstrated to be a technically efficient and economic rehabilitation and strengthening method. In these applications, the thin UHPFRC layer, serving as tensile reinforcement, increases the bending and shear resistance of UHPFRC-RC composite members through its in-plane tensile resistance and deformability. Bridge deck slabs are the most fatigue loaded structural elements in bridges, and the actual stress state caused by wheel loading is nearly equi-biaxial and far from uniaxial. Bridge decks are expected to be subjected to a high number of fatigue stress cycles, which may exceed several hundred million. Accordingly, this thesis is devoted to study experimentally and analytically the static and fatigue biaxial flexural response of strain-hardening UHPFRC thin slab elements. For a given UHPFRC mix, there are no intrinsic tensile properties. The representative behavior, especially the strain-hardening response, is dependent on the fiber distribution. The present thesis introduces firstly the uniformity factor for considering the local fiber distribution within an UHPFRC element. Accompanied with the fiber orientation factor and efficiency factor, the influence of uniformity on the strain-hardening response of UHPFRC under uniaxial tension is investigated quantitatively by means of experimental campaign and mechanical analysis. The direct tensile test (DTT) was carried out on dumbbell specimens, extracted from a UHPFRC slab element, to characterize the tensile behavior of UHPFRC. Before tensile testing, actual fiber distribution of each specimen was measured by the non-destructive test (NDT) method using a magnetic probe. Finally, the most likely Ό_2 values are determined for UHPFRC layers with different thickness. In a next step, the biaxial flexural response of UHPFRC thin slab elements using ring-on-ring test method is investigated experimentally and analytically. Based on the testing results, a quasi-elastic limit is introduced to characterize the flexural response of UHPFRC under biaxial stress condition. In addition, an in-depth comparison between ring-on-ring test and 4-Point-Bending-Test (4PBT) results, with special emphasis on flexural strength and matrix discontinuity development, is conducted. An original analytical inverse analysis method for determining the biaxial tensile properties of UHPFRC from ring-on-ring test is developed based on yield line and elastic slab bending theories. The inverse analysis results are validated against the experimental evidence, particularly based on DIC analysis. It is concluded that the elastic limit is 18% lower, and almost equivalent tensile strength of UHPFRC subjected to biaxial stresses under biaxial stress condition, compared with those under uniaxial condition. And a relatively small biaxial hardening strain is highlighted. Finally, four series of flexural fatigue tests under constant amplitude fatigue cycles up to Very High Cycle Fatigue (20 million cycles) are conducted using circular slab elements. The fatigue stress level S is ranging from 0.50 to 0.68, targeting the fatigue endurance limit of UHPFRC material and fatigue strength under biaxial stress condition. The fatigue damage evolution is analyzed in terms of central deflection as well as development of fictitious crack propagation and opening.