Multiscale structure-property relationships of ultra-high performance concrete
Ultra-High Performance Concrete (UHPC) is defined as having a compressive strength over 150 MPa and a tensile strength over 8 MPa [Habel et al. 2006]. This strength is accomplished by the exclusion of coarse aggregate, inclusion of silica fume, and reduction of the water to binder ratio [Zadeh et al. 2008]. Figure 1.1 shows the macroscopic differences between ordinary concrete and UHPC. The adjustments made to UHPC also improve the density of the mixture, making it less permeable than ordinary concrete. The impermeability makes the material useful in highly corrosive environments and environments subjected to high strain rates [Charron et al. 2006]. 
The structure-property relationships of Ultra-High Performance Concrete (UHPC) were quantified using imaging techniques to characterize the multiscale hierarchical heterogeneities and the mechanical properties. UHPC’s composite-like structure called for the inclusion of different sized inclusion types such as steel fibers, sand grains, unhydrated cement grains, and voids. Through image analysis the average size, percent area, nearest neighbor distance, and relative number density of each inclusion type was determined and then used to create Representative Volume Element (RVE) cubes for use in Finite Element (FE) analysis. Three different size scale RVEs at the mesoscale were found to best represent the material: the largest length scale (35 mm side length) included steel fibers, the middle length scale (0.54 mm side length) included large voids and silica sand grains, and the smallest length scale (0.04 mm side length) included small voids and unhydrated cement grains. By using three length scales of mesoscale FE modeling, the bridge of information to the macroscale cementitious material model is more physically based.
Authors: Megan Burcham, Dr. Mark Horstemeyer, Youssef Hammi
Mechanical testing was accomplished using the equipment at the Construction Materials Research Center at Mississippi State. Cylinders of Cement Paste (CP), Mortar (M), Fiber-Reinforced Paste (FRP), and UHPC were subjected to testing to obtain elastic moduli, stress-strain behavior, compressive strengths, and tensile strengths. The materials involved in each of the constituent mixtures are outlined in Table 2.2. Compressive tests were conducted according to ASTM C39, tensile tests were conducted according to ASTM C496, and elastic modulus tests were conducted according to ASTM C469. Figure 2.2 shows the setup that was used in the compression and tension tests. The testing of these combinations of constituents enabled more accurate FE modeling, as the characteristics of each constituent were known.
Mechanical testing enabled appropriate modeling by revealing information about constituents in UHPC. The results from compression and tension tests are shown in Figure 4.4. As cracks developed in the specimens, the stress-strain behavior became unreliable; therefore, only the earlier (mostly linear) portions of the stress-strain curves were plotted. A direct comparison of each constituent’s compressive and tensile properties is shown in Figure 4.5 to highlight the different stress state dependences.
During testing, the elastic modulus was also found using a compressometer. These values were compared to the elastic moduli determined from compressive stress-strain testing, and the results are shown in Table 4.4.
- ↑ Burcham, M. (2016). Multiscale structure-property relationships of ultra-high performance concrete. Unpublished master’s thesis, Mississippi State University, Starkville, Mississippi.