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.
Mitigating the effects of impact loading on critical structural systems and reducing human vulnerability in the wake of such events is challenging because the central question remains unanswered: how does concrete fail under high strain-rate loading? Fragmented concrete has long been identified as a source of injury after a blast attacks (second only to fractured glass). More recently, physicians removed concrete fragments as large as two inches from survivors of the Boston Marathon tragedy. Due to the scarcity of blast-related injuries outside of combat, few health professionals have experience treating them, exacerbating the challenge for medical personnel and first responders. The dynamic behavior of cement-based composites has been studied using a number of advanced techniques, and recent experimental and theoretical work in brittle microcracking using rectilinear slits has shown particular promise for predicting dynamic failure. While these models show good qualitative agreement with experiments, they lack a direct relationship to the sizes, shapes, and spatial arrangements of the multiscale flaws found in real concrete. The goals of this research were (1) to identify the flaws that are likely to govern failure during high-strain rate loading and quantify their sizes, shapes, and spatial arrangements mathematically; (2) to extended the library of flaw types beyond rectilinear slits to round or pore-like flaws to better account for the variety of flaw geometries present in real materials; (3) to simulate dynamic uniaxial compressive loading of mortar using a full finite element model which accounts for flaw clustering to understand failure initiation locally rather than globally; and (4) to cast companion mortar samples for full scale dynamic testing using a Kolsky bar for model validation. The influence of two flaw types - ntrained air voids (spherical pockets of air intentionally introduced to help protect against freeze/thaw damage) and the interfacial transition zone (ITZ; a locally porous area surrounding aggregates and a rich source of flaws due to particle packing and one-sided growth) - were quantified this research. A new fracture mechanics model was developed to model flaws. Round flaws were used to model entrained air voids, and slit flaws were used to model the tandem effect of fine aggregate and ITZ. Clustering of entrained air voids was also considered for the first time. Micromechanics simulations suggested cracks initiating from entrained air voids activate in tandem with cracks initiating from the ITZ. Flaws may interact in different ways, resulting in three failure modes based on damage. Larger flaws shift the dynamic strength upwards into the third (highest strength) failure mode. Air entrainment lowers the static compressive strength and the average dynamic compressive strength. However, the dynamic increase factor (the ratio of dynamic to static compressive strength) increases with increasing air content. A post-peak stress "plateau" region observed in experimental stress-time histories of air entrained mortar suggest that the damaged material can bear some additional load after failure, possibly after void collapse. This model may allow the entrained air void system to be engineered to be beneficial to a concrete element subjected to blast or impact loading. Experimental validation suggests the model is useful for predicting dynamic strength. The model is especially useful for determining quite rapidly the potential range in dynamic strengths among materials with varying microstructures. With regard to broader impacts, beyond those derived from the potential contributions of the research on novel materials characterization techniques and multidimensional concrete durability, this grand has provided full or partial support for two doctoral students (Mr. Nathan Mayercsik and Ms. Elizabeth Nadelman). Additionally, the project provided opportunity to introduce four undergraduate students (Mr. Paul Son, Ms. Jaymie Kaiser, Ms. Xenia Strosnider, Mr. Andrew DeYoung) and one high school student (Ms. Rachel Corbin) to scientific research, although they were not funded by the grant. An additional undergraduate student from Williams College, Mr. Dylan Freas, participated in this research effort with funding from NSF’s NNIN (National Nanotechnology Infrastructure Network) undergraduate research program. The research received interest from two universities (Oklahoma State University, École Nationale des Ponts et Chaussées) resulting in inter-institutional collaboration, and also from one private company (Booz Allen Hamilton), resulting in a potential avenue for private-public partnership. Finally, the research topic has been included in various outreach activities and semi- and non-technical presentations by the PI and students involved in the research effort.
