Particle interaction and rheological behavior of cement-based materials at micro- and macro-scales

Rheology of cement based materials is controlled by the interactions at the particle level. The present study investigates the particle interactions and rheological properties of cement-based materials in the micro- and macro-scales. The cementitious materials studied are Portland cement (PC), fly ash (FA), ground granulated blast furnace slag (GGBFS) and densified silica fume (SF). At the micro-scale, aside from the forces on particles due to collisions, interactions of particles in a flowing system include the adhesion and friction. Adhesion is due to the attraction between materials and friction depends on the properties of the sliding surfaces. Atomic Force Microscopy (AFM) is used to measure the adhesion force and coefficient of friction. The adhesion force is measured by pull-off force measurements and is used to calculate Hamaker constants. The coefficient of friction is measured by increasing the deflection set-points on AFM probes with sliding particles, thereby increasing normal loads and friction force. AFM probes were commercial Si3N4 tips and cementitious particles attached to the tips of probe cantilevers. SF was not included in the micro-scale tests due to its limiting size when attaching it to the AFM probes. Other materials included in the tests were silica, calcite and mica, which were used for verification of the developed test method for the adhesion study. The AFM experiments were conducted in dry air and fluid environments at pH levels of 7, 8, 9, 11 and 13. The results in dry air indicate that the Hamaker constant of Class F FA can be similar to PC, but Class C FA can have a high Hamaker constant, also when in contact with other cementitious materials. The results in fluid environments showed low Hamaker constants for Class F fly ashes compared to PC and also showed high Hamaker constants for PC and Class C fly ash. The results for the friction test in dry air indicated that the coefficient of friction of PC is lower than fly ashes, which is attributed to the asperities present on the particle surface. At the macro-scale, flow of cementitious materials may be in its dry or wet state, during transport and handling or when it is used in concrete mixtures, respectively. Hence, the behavior of bulk cementitious materials in their dry state and wet form are studied. In the dry state, the compression, recompression and swell indices, and stiffness modulus of plain and blended cementitious materials are determined by confined uniaxial compression. The coefficients of friction of the bulk materials studied are determined by a direct shear test. The results indicate that shape of particles has a great influence on the compression and shear parameters. The indices for PC blends with FA do not change with FA replacement, while it increases with GGBFS replacement. Replacement with GGBFS slightly decreases coefficient of friction, while replacement with FA significantly decreases coefficient of friction. At low SF replacement, coefficient of friction decreases. In wet state, unary, binary, ternary and quaternary mixes with w/b of 0.35, 0.45 and 0.55 were tested for yield stress, viscosity and thixotropy. It is found that fly ash replacement lowers the rheological properties and replacement with GGBFS and SF increases rheological properties. The distinct element method (DEM) was employed to model particle interaction and bulk behavior. The AFM force curve measurement is simulated to validate the adhesion model in the DEM. The contact due to asperities was incorporated by considering the asperities as a percentage of the radius of the contacting particles. The results of the simulation matches the force-curve obtained from actual AFM experiments. The confined uniaxial compression test is simulated to verify the use of DEM to relate micro-scale properties to macros-scale behavior. The bulk stiffness from the physical experiments is matched in the DEM simulation. The particle stiffness and coefficient of friction are found to have a direct relation to bulk stiffness

Extended Use of Limestone Fines in Various Concretes

Limestone fines are increasingly used in cement and concrete for improved material properties and sustainability. This paper presents recent research at Iowa State University on utilization of limestone fines in concrete. It includes the beneficial uses of limestone fines in: (1) limestone blended Portland cement; (2) SFSCC (semi-flowable self-consolidating concrete); and (3) HPC (high performance concrete). The research results show that using 5%~10% of limestone fines to replace for Type IP cement (with 25% fly ash) increased mortar strength. Well-designed SFSCC with 25% limestone fines (by mass of cementitious materials) displayed desirable rheological and mechanical properties required for slip-forming construction. The newly developed limestone fines-based HPC reached the one-day compressive strength of over 28 MPa

Development of Innovative, Adaptable Video Learning Modules for the Civil Engineering Classroom

As engineering and technology continue to evolve, so should the use of such innovations in engineering pedagogy. Standard course learning modules have not often utilized technology to assist in learning of engineering principles and concepts; that is, until the COVID-19 pandemic required teachers and students to use technology more frequently in a virtual teaching/learning environment. Therefore, it is even more critical now that engineering pedagogy be adapted to incorporate technology in the classroom to enhance student learning of complex engineering concepts. In this study, a team of Civil Engineering professors has set out to incorporate technology into their classrooms to help students gain a stronger understanding of the fundamental building blocks of Civil Engineering. A series of comprehensive educational video and simulation-based learning modules were created for the Civil Engineering subdisciplines of environmental, geotechnical, transportation, and structural engineering. The development and implementation of such technology-based learning modules offer new opportunities to teach students the complex concepts of Civil Engineering through visual means. The efficacy of the learning modules were evaluated through student assessment surveys for: (1) the appropriateness of the module in aiding the introduction of course content, (2) the effectiveness of the module in enhancing student understanding of course content, and (3) the overall perception of students of the module. Implementation of the modules into the classroom has shown that students responded positively to the modules, referencing the modules as both engaging and comprehensive in aiding their understanding of course content

Nonlinear Formulations of a Four-Node Quasi-Conforming Shell Element

The quasi-conforming technique was introduced in the 1980’s to meet the challenge of inter-elements conforming problems and give a unified treatment of both conforming and nonconforming elements. While the linear formulation is well established, the nonlinear formulation based on the quasi-conforming technique that includes geometric and material nonlinearity is presented in this paper. The formulation is derived in the framework of an updated Lagrangian stress resultant, co-rotational approach. The geometric nonlinear formulation provides solutions to buckling and postbuckling behaviour while the material nonlinear formulation considers the spread of plasticity within the element while maintaining an explicit construction of element matrices. Aside from the elasto-plastic constitutive relation, formulations on laminate composites and reinforced concrete are also presented. The formulations of laminate composite and reinforced concrete material are present based on the layer concept, the material properties can vary throughout the thickness and across the surface of a shell element. The various failure criteria for laminate composite are included in the formulation which makes it possible to analyses the progressive failure of fibre and matrix. For the reinforced concrete material, the nonlinearities as a result of tensile cracking, tension stiffening between cracks, the nonlinear response of concrete in compression, and the yielding of the reinforcement are considered. The steel reinforcement is modeled as a bilinear material with strain hardening

Investigation Into Shrinkage of High-Performance Concrete Used for Iowa Bridge Decks and Overlays \u2013 Phase II Shrinkage Control and Field Investigation

This Phase II research project on the shrinkage behavior of high-performance concrete (HPC) used in Iowa bridge decks and overlays evaluated several concrete mixes, building off or modifying mixes developed in Phase I. Based on shrinkage behavior and mechanical properties, the mixes studied in Phase I were characterized as having either high, medium, or low cracking potential. In the Phase II study, three concrete mixes (Mixes 6, 8, and 2, characterized in Phase I as having high, medium, and low cracking potential, respectively) were selected for further investigation. The selected mixes were modified using three shrinkage control technologies: shrinkage-reducing admixtures (SRAs), cementitious materials (CM), and internal curing (IC) agents, respectively. The modification methods were first studied in a laboratory until the optimal shrinkage behavior of each concrete mix was achieved. Two pairs of the tested concrete mixes (Mixes 6 and 8 with and without modification) were then used in a field investigation on the US 20 over I-35 dual bridge. The mixes were placed side by side for the bridge overlays, which were monitored for about one year with strain gages, temperature and moisture sensors, and regular visual examinations. The laboratory investigation confirmed positive effects for the concrete shrinkage control technologies used. The laboratory test results also provided specific details for the concrete mix modifications, ensuring optimal concrete performance and shrinkage control. The modifications included the addition of 1.0/1.25 gal/yd3 of SRA in Mix 6, the use of 10% CM reduction for Mix 8, and the use of lightweight fine aggregate as an IC material in Mix 2. The results of the field investigation suggest that environmental conditions on the casting day and the first few days of curing play an important role in the development of concrete properties. Future studies could benefit from a comprehensive stress analysis to better understand the long-term effects of the shrinkage control technologies, as well as further field tests and an extended monitoring time

Innovative Techniques and Materials for Preventing Concrete Shrinkage Cracking

NJDOT Contract ID NumberCracks weaken concrete and permit water and harmful chemical ingress into structures. Shrinkage also decreases the load-carrying capacity of bridges. This study investigates shrinkage cracking prevention of infrastructure concretes used in New Jersey. The 15 initial evaluation mixtures were first tested to evaluate their shrinkage behavior. Based on the results of the tests, control mixtures were selected for modification with shrinkage control admixtures and additives. The admixtures and additives were a shrinkage-reducing admixture, a shrinkage-compensating admixture, fibers, an internal curing agent, and two coatings. The modified mixtures were then tested for the effects of the admixtures and additives on shrinkage, mechanical properties, and durability. The cracking potential of control mixtures ranged from moderate-low to high. Initial evaluation mixture autogenous shrinkage increases with the increasing volume of fine aggregate and decrease of coarse aggregates. On the other hand, increasing the portland cement and total binder while decreasing the amount of fine and coarse aggregates tends to increase the drying shrinkage. Different types of admixtures and additives had varied effects on shrinkage. To mitigate cracking in concrete by using admixtures and additives, the standard restrained ring test ASTM C1581 can be adopted for evaluating the concrete\u2019s performance after modification. It is recommended that specifications for concrete shrinkage cracking be met by three specimens from the same batch

Particle interaction and rheological behavior of cement-based materials at micro- and macro-scales

Rheology of cement based materials is controlled by the interactions at the particle level. The present study investigates the particle interactions and rheological properties of cement-based materials in the micro- and macro-scales. The cementitious materials studied are Portland cement (PC), fly ash (FA), ground granulated blast furnace slag (GGBFS) and densified silica fume (SF). At the micro-scale, aside from the forces on particles due to collisions, interactions of particles in a flowing system include the adhesion and friction. Adhesion is due to the attraction between materials and friction depends on the properties of the sliding surfaces. Atomic Force Microscopy (AFM) is used to measure the adhesion force and coefficient of friction. The adhesion force is measured by pull-off force measurements and is used to calculate Hamaker constants. The coefficient of friction is measured by increasing the deflection set-points on AFM probes with sliding particles, thereby increasing normal loads and friction force. AFM probes were commercial Si3N4 tips and cementitious particles attached to the tips of probe cantilevers. SF was not included in the micro-scale tests due to its limiting size when attaching it to the AFM probes. Other materials included in the tests were silica, calcite and mica, which were used for verification of the developed test method for the adhesion study. The AFM experiments were conducted in dry air and fluid environments at pH levels of 7, 8, 9, 11 and 13. The results in dry air indicate that the Hamaker constant of Class F FA can be similar to PC, but Class C FA can have a high Hamaker constant, also when in contact with other cementitious materials. The results in fluid environments showed low Hamaker constants for Class F fly ashes compared to PC and also showed high Hamaker constants for PC and Class C fly ash. The results for the friction test in dry air indicated that the coefficient of friction of PC is lower than fly ashes, which is attributed to the asperities present on the particle surface. At the macro-scale, flow of cementitious materials may be in its dry or wet state, during transport and handling or when it is used in concrete mixtures, respectively. Hence, the behavior of bulk cementitious materials in their dry state and wet form are studied. In the dry state, the compression, recompression and swell indices, and stiffness modulus of plain and blended cementitious materials are determined by confined uniaxial compression. The coefficients of friction of the bulk materials studied are determined by a direct shear test. The results indicate that shape of particles has a great influence on the compression and shear parameters. The indices for PC blends with FA do not change with FA replacement, while it increases with GGBFS replacement. Replacement with GGBFS slightly decreases coefficient of friction, while replacement with FA significantly decreases coefficient of friction. At low SF replacement, coefficient of friction decreases. In wet state, unary, binary, ternary and quaternary mixes with w/b of 0.35, 0.45 and 0.55 were tested for yield stress, viscosity and thixotropy. It is found that fly ash replacement lowers the rheological properties and replacement with GGBFS and SF increases rheological properties. The distinct element method (DEM) was employed to model particle interaction and bulk behavior. The AFM force curve measurement is simulated to validate the adhesion model in the DEM. The contact due to asperities was incorporated by considering the asperities as a percentage of the radius of the contacting particles. The results of the simulation matches the force-curve obtained from actual AFM experiments. The confined uniaxial compression test is simulated to verify the use of DEM to relate micro-scale properties to macros-scale behavior. The bulk stiffness from the physical experiments is matched in the DEM simulation. The particle stiffness and coefficient of friction are found to have a direct relation to bulk stiffness.</p

Developing Green, Highly Flowable, Rapid Set, High-Performance Concrete for Pavement Patch Repair

A high-performance mortar (HPM) containing a large amount of industrial by-products such as fly ash, silica fume, and limestone fines was developed for rapid repair of concrete pavements. The HPM development included three major steps. 1. Development of the mortar mixture proportion based on the optimal hydration of binder and particle packing of the mortar system. In this step of the study, all mortar materials were systematically proportioned, and the obtained mixtures were tested for flowability, rate of hydration, set time, and strength development. The optimal mixture proportion was then selected as the HPM, as it displayed good self-consolidating ability and achieved 1-day compressive strengths greater than 6,000 psi. 2. Investigation of the mechanical properties of the new HPM, including compressive and flexural strength, elastic modulus, and slant shear and pull-off strengths of patch-substrate bonds tested at 1, 3, 7, and 28 days. In this step, the properties of the HPM were evaluated in comparison with those of a commercial repair material: the rapid-set concrete (RSC). Two types of substrates representing old concrete were used for patching repair. One was made of a typical pavement mixture (C-3WR-C20), and the other was a high-strength pavement concrete mixture (O-4WR). 3. Investigation of the durability properties of the newly developed HPM compared to those of the RSC. Durability properties included cyclic freeze-thaw (F-T) resistance, permeability, and shrinkage behavior. The results indicated that the newly developed HPM possesses excellent self-consolidating ability: highly flowable and non-segregating. Although there was delayed setting, the compressive strength of the HPM exceeded 6,000 psi at 1 day, approximately 25 percent higher than that of the RSC. At 28 days, the HPM reached 10,000 psi, while the RSC was about 7,000 psi. The HPM also displayed extremely low chloride permeability (18 coulombs) compared to the RSC (2,550 coulombs) and excellent F-T durability without the requirement for air entrainment. The F-T durability factor of HPM was kept around 100% throughout the standard F-T test, while the F-T durability factor of RSC reduced to 80% at the end of the F-T test. However, the HPM exhibited noticeably higher autogenous shrinkage and slightly lower free drying shrinkage than the C-3WR-C20 mixture, while the RSC had a little/no shrinkage during the 56-day test period. Addition of a small amount of micro-steel fibers (70 pcy) slightly reduced the shrinkage of the HPM. Further studies on fatigue and shrinkage cracking behavior of the HPM are recommended.concrete.For this and other reports, see the Midwest Transportation Center project page at http://www.intrans.iastate.edu/mtc/index.cfm/research/project/project/231283870 and the InTrans website at http://intrans.iastate.edu/</p

Developing Green, Highly Flowable, Rapid Set, High-Performance Concrete for Pavement Patch Repair

A high-performance mortar (HPM) containing a large amount of industrial by-products such as fly ash, silica fume, and limestone fines was developed for rapid repair of concrete pavements. The HPM development included three major steps. 1. Development of the mortar mixture proportion based on the optimal hydration of binder and particle packing of the mortar system. In this step of the study, all mortar materials were systematically proportioned, and the obtained mixtures were tested for flowability, rate of hydration, set time, and strength development. The optimal mixture proportion was then selected as the HPM, as it displayed good self-consolidating ability and achieved 1-day compressive strengths greater than 6,000 psi. 2. Investigation of the mechanical properties of the new HPM, including compressive and flexural strength, elastic modulus, and slant shear and pull-off strengths of patch-substrate bonds tested at 1, 3, 7, and 28 days. In this step, the properties of the HPM were evaluated in comparison with those of a commercial repair material: the rapid-set concrete (RSC). Two types of substrates representing old concrete were used for patching repair. One was made of a typical pavement mixture (C-3WR-C20), and the other was a high-strength pavement concrete mixture (O-4WR). 3. Investigation of the durability properties of the newly developed HPM compared to those of the RSC. Durability properties included cyclic freeze-thaw (F-T) resistance, permeability, and shrinkage behavior. The results indicated that the newly developed HPM possesses excellent self-consolidating ability: highly flowable and non-segregating. Although there was delayed setting, the compressive strength of the HPM exceeded 6,000 psi at 1 day, approximately 25 percent higher than that of the RSC. At 28 days, the HPM reached 10,000 psi, while the RSC was about 7,000 psi. The HPM also displayed extremely low chloride permeability (18 coulombs) compared to the RSC (2,550 coulombs) and excellent F-T durability without the requirement for air entrainment. The F-T durability factor of HPM was kept around 100% throughout the standard F-T test, while the F-T durability factor of RSC reduced to 80% at the end of the F-T test. However, the HPM exhibited noticeably higher autogenous shrinkage and slightly lower free drying shrinkage than the C-3WR-C20 mixture, while the RSC had a little/no shrinkage during the 56-day test period. Addition of a small amount of micro-steel fibers (70 pcy) slightly reduced the shrinkage of the HPM. Further studies on fatigue and shrinkage cracking behavior of the HPM are recommended.concrete.For this and other reports, see the Midwest Transportation Center project page at http://www.intrans.iastate.edu/mtc/index.cfm/research/project/project/231283870 and the InTrans website at http://intrans.iastate.edu/</p