Enhanced Porosity Concrete (EPC) is a relatively new class of concrete having higher than normal porosity and pore connectivity, thus facilitating applications such as storm water runoff control, and highway noise mitigation. The fundamental objective of this CAREER proposal is to link the pore structure features and performance of EPC to its material design parameters through a multi-disciplinary approach that involves stereological methods, fracture properties, porous media flow, and mathematical optimization. Geometrical data from two dimensional EPC images will be stereologically interpreted using stochastic geometry, and a novel porosity prediction method based on particle packing theory developed to establish correlations between material design parameters and pore structure features. Pore structure features will be related to: (i) mechanical behavior using a fracture criterion that involves both toughness and strength, thus providing an understanding of the competition between crack-blunting and higher void fraction, (ii) permeability using X-ray tomography, three-dimensional reconstruction of planar images, and fractal models, and (iii) pollutant retention efficiency using multi-phase flow theories and reconstructed models. Since EPC systems need to be designed for more than one performance requirement, a multi-objective optimization procedure will be used to arrive at a feasible set of material design parameters. This will facilitate the development of relationships between material design and performance through the pore structure features. This CAREER proposal will result in a much needed performance-based design methodology for EPC systems, and enable predictions of performance. This will aid in the development of test methods to determine EPC properties, and will facilitate the transition of EPC design to a scientific process from the currently employed trial and error methods. The educational plan in this proposal is strongly tied with the research component through problem-based design and learning, development of a teaching lab course, and K-12 outreach program. A conceive-implement-design-operate (CDIO) approach will be used to put the problem-based learning and design into practice where the students will be responsible for designing, constructing, and evaluating a few small EPC test bed systems. A teaching lab course, where the undergraduate students can do research on structured topics related to cement-based materials will be developed. The existing GK-12 program at Clarkson will be used as a vehicle to emphasize hands-on activities and engineering problem solving to middle and high school students using EPC as a model porous engineering material, thus integrating research and outreach.
The research carried out through this project has forwarded a fundamental understanding of pervious concrete systems used to: (i) reduce the runoff from impervious surfaces, and thus to recharge ground water, and (ii) reduce the interaction noise between tire and pavements by absorbing energy. These materials have a very open pore structure, thereby allowing them to transport water (or sound energy) through the pore network. The research carried out here developed methods in which pervious concretes could be designed for a given porosity and other features of the pore netowrk (sizes, connectivity etc - See Figure 1). A first-principles based material design has been adopted, which provides an accurate proportioning procedure based on the source materials and degrees of compaction. Several pervious concrete manufacturers in the country have adopted this method, which was introduced to them by the American Concrete Institute's (ACI) document on pervious concrete which is largely developed from the research carried out under this NSF grant. The methodologies for pore structure characterization and modeling adopted in this work (See Figure 2) are appropriate for a suite of porous materials including soils, rock, and foams. While it is well known that porosity reduces the strength of the concrete, a system that is designed to have a certain porosity (typically 20 - 25%) will have a reduced strength. Thus one of the key objectives of this study was not to develop high-strength pervious concretes, but to understand how failure occurs in a system with pore networks. This will help materials designers and field engineers to choose the right size of aggregates, cement content, water content, and other additives (like fibers) to provide the desired mechanical properties. Since cracking is an important damage mechanism in concretes, the resistance to cracking (also called toughness) as a function of the pore size, pore volume, and fiber volume fraction helps understand the interplay between these factors and consequently aids better material design (Figure 3). In an attempt to help engineers and designers develop a better understanding of this macroporous material (since pervious concretes carry out some unique functions, their comparison with conventional concretes are not always appropriate), this research program developed several models that are currently widely used. These models account for the fundamental structure of the material, and allow the users to gain an appreciation for the fluid permeability (amount of water that could be transported through the material) or elastic properties (related to carrying load) through virtual testing, thereby allowing them to implement several scenarios before actually developing and implementing the material. This necessitated new methods of characterizing the material structure. This work has contributed to the field by developing both direct methods of interrogating the pore structure (Figure 4 - 3D X-ray tomography) or indirect means where the electrical resistance of the material is monitored and suitable models applied to extract the features of then void netowrk. This study also resulted in laboratory testing methods for pervious concrete permeability. The falling head permeameter developed as part of this study is currently the most accepted method for pervious concrete permeability, and is included in the ACI doument on pervious concretes. Several educational and outreach activities formed part of this project. Among the significant educational activities were a new graduate course on microstructure and mechanics of random composites, hands-on undergraduate activities, and a construct-build-monitor activity for a pervious concrete test bed carried out in conjunction with a technical training institute. The educational outreach included three consecutive years of bringing in 50 students from the 7th and 8th grades to the university for a three-day camp on sustainable engineering materials, energy, and the environment. As part of this project, a concrete design and engineering module was also created for the the middle school students, which was ranked among the top 100 educational tools by the K-12 Engineering Pathway program.
