This proposal outlines a joint experimental and computational program aimed at developing the framework for a multi-scale model of cement-based materials, which will provide for advances in quantitative descriptions of their heterogenous structure and an ability to predict the effect of loading rate on failure in materials ranging from normal strength concrete to fiber-reinforced ultrahigh performance concrete. Specifically, the proposed activities are divided into four tasks: 1) quantitative characterization of multi-scale structure and meso-scale fluctuations in structural descriptors; 2) development of micromechanical models for predicting strain-rate dependent behavior of concrete/cementitious materials; 3) stochastic simulation-based modeling of variations in meso-scale constitutive properties of concrete/cementitious materials; and, 4) full-scale finite element model of samples under uniaxial/biaxial compression at varying strain rates, along with experimental validation using Kolsky bar equipment.
The research conducted through the proposed activities will address the important problem of impact loading on concrete materials, by providing new modeling paradigms and experimental techniques to address the strain-rate-dependent failure of cement-based materials. These protocols can form the basis for improved blast-resistance of materials and more effective structural design. In addition, the techniques are also expected to be applicable to the broader family of brittle materials under high strain rates, such as modeling failure of rocks under mining blasts or ceramic armors under ballistic loading. The proposed activities represent a multi-institutional collaboration, which will provide students mentored by the PIs exposure not only to the expertise of both PIs, but also to the educational research environments at each of the involved institutions. Each PI will mentor one Ph.D. student and undergraduate researchers who will all be encouraged to travel to the other institution on a regular basis to share findings.
Unfortunately, the effect of blast loading on structures is becoming increasingly of concern to our societal safety and well-being. As the most common structural material, concrete requires a much better understanding of its behavior under dynamic loads in order for us to be able to propertly mitigate injuries and damage due to blast. This work provides a fundamental understanding of the dynamic failure of concrete and the framework for computational models that will be able to predict failure of concrete structural elements. In this way, we can better design against blast and/or understand which elements of a structure are most vulnerable. This research addresses the major challenge of understanding dynamic failure of cement-based materials, by identifying and characterizing the microstructural characteristics that drive compressive failure, by developing models that can represent failure more generally, and by validating these models through dynamic testing. Specifically as proposed here, the planned activities consisted of 4 tasks: 1. Characterization of multi-scale structure and meso-scale fluctuations in structural descriptors: The team at Georgia Tech identified flaw and inclusion locations, sizes, shapes, and orientations, using serial sectioning and/or tomography to quantify the pore and aggregate populations in air-entrained mortar, which was selected as a model material for this project. The JHU team developed techniques to translate the resulting two dimensional measures of microstructure into three dimensions, and vice versa. The characterization results show that the pore size distributions fit well into a power-law distribution. 2. Development of micromechanical models for predicting strain rate dependent behavior of concrete and cementitious materials: The JHU team developed both a 2D and a 3D micromechanical model for brittle materials that contain both pores and slit-like flaws. The model takes as input the pre-existing flaw population (as characterized experimentally in Task 1 above) and identifies an associated compressive strength for a given strain rate. Based on the pore population and the sand grain sizes used in the air-entrained mortar characterized at Georgia Tech, this model was used to predict dynamic strength of the air-entrained mortar. The micromechanical model is able to predict strengths that are consistently less than 15% higher than the actual strength, which is excellent agreement given the level of uncertainty in some model parameters and the error associated with the dynamic testing. Beyond matching a particular value of strength, the model is able to predict the qualitative effects of changes to the microstructure. In other words, the model tells us which pore populations will lead to higher (or lower) over all strength values, even if we don't exactly match the actual value of strength. 3. Stochastic simulation-based modeling of variations in meso-scale constitutive properties of concrete/cementitious materials: GA Tech and JHU worked together to use stochastic simulations as the basis for a study of local variations in failure strength. The idea is that failure strength may be more of a local than a global measure, and therefore one should consider the local minimum of strength rather than a global measure. Using local strengths as a measure of failure, the predicted strength is reduced, enabling improved prediction of dynamic strength. 4. Experimental validation using Kolsky bar equipment. The GA Tech team mixed a number of air-entrained mortar samples with varying levels of air entrainment. These samples were tested using the Kolsky bar apparatus at JHU, and the resulting measures of compressive strength were compared to the predicted strengths from the micromechanics model, with very good success. Without resorting to any parameter tuning, the uniaxial compressive strength predicted by the model and that measured from the tests agree to within 10-15%. Beyond this direct quantitative validation, the micromechanics model predicted relative changes in strength from one sample to the next. The results of this research have been disseminated through multiple journal publications (one currently in print, two currently under review, and two more currently in preparation). The results were also presented at a number of conferences, including but not limited to the 2011 - 2013 ASCE Engineering Mechanics Institute Conference, the 2013 and 2014 Mach Conference, and the 2014 US National Congress on Theoretical and Applied Mechanics. 3 Ph.D. students have received partial funding under this project, one of which has completed her degree. Participants in this grant have served in a number of outreach activities, including the Maryland Wood Bridge Challenge for high school students for 3 years, the JHU VEX Round-Up World for middle and high school students for 2 years, and the Bryn Mawr STEM advancement program for high school girls for 1 year.
