Dale P Bentz - Academia.edu (original) (raw)
Papers by Dale P Bentz
Advances in Civil Engineering Materials, 2018
Characterization of fresh concrete is critical for assuring the quality of the United States' con... more Characterization of fresh concrete is critical for assuring the quality of the United States' constructed infrastructure. While fresh concrete arriving at a job site in a ready-mixed concrete truck is typically characterized by measuring temperature, slump, unit weight, and air content, here the measurement of the electrical resistance of a freshly cast cylinder of concrete is investigated as a means of assessing mixture proportions, specifically cement and water contents. Both cement and water contents influence the measured electrical resistance of a sample of fresh concrete: the cement by producing ions (chiefly potassium, sodium, and hydroxide) that are the main source of electrical conduction and the water by providing the main conductive pathways through which the current travels. Relating the measured electrical resistance to attributes of the mixture proportions, such as water-cement ratio by mass (w/c), is explored for a set of eleven different concrete mixtures prepared in the laboratory. In these mixtures, w/c, paste content, air content, fly ash content, high range water reducer dosage, and cement alkali content are all varied. Additionally, concrete electrical resistance data are supplemented by measuring the resistivity of its component pore solution obtained from five laboratory-prepared cement pastes with the same proportions as their corresponding concrete mixtures. Only measuring the concrete electrical resistance can provide a prediction of the mixture's paste content or the product w*c; conversely, when pore solution resistivity is also available, w/c and water content of the concrete mixture can be reasonably assessed.
Construction and Building Materials, 2018
Additive construction by extrusion (ACE), also known as construction-scale 3-D printing, is becom... more Additive construction by extrusion (ACE), also known as construction-scale 3-D printing, is becoming a new paradigm for concrete construction. While significant cost and manpower savings are projected, many issues remain to be solved before ACE can be viewed as a mainstream technology. Among these is the formulation of a robust and sustainable binder that meets the demanding rheological and mechanical performance requirements of an ACE application. This paper investigates the development of such a binder by exploring pastes prepared from binary blends of cement with limestone powder at various proportions , supplemented with the incorporation of a retarder/accelerator admixture combination. Performance is chiefly characterized by applying a Bingham fluid model to characterize the rheological parameters of yield stress and plastic viscosity and by assessing hydration via isothermal calorimetry measurements. Matching the surface area of the limestone powder to that of the cement that it is replacing affords the opportunity to regulate rheology via water content adjustments, while engineering the cement:limestone powder ratio to provide the desired mechanical properties. The utilization of the retar-der/accelerator combination produces a paste with a long pot life (stable rheology) that can be quickly stiffened via the injection of the accelerator (and retarded paste) into a static mixer just prior to the prin-ter's exit nozzle. Thus, these pastes demonstrate the potential to provide a robust and sustainable binder for ACE applications. Published by Elsevier Ltd.
Cement and Concrete Composites, 2018
Obtaining the mesostructure of concrete from X-ray computed tomography (CT) requires segmentation... more Obtaining the mesostructure of concrete from X-ray computed tomography (CT) requires segmentation of the data into distinct phases, a process complicated by the limited contrast between aggregates and mortar matrix. This paper explores the possibility to add baryte or hematite into the concrete mixture to enhance the contrast between cement paste and aggregates in CT, thus allowing for a semi-automatic segmentation. Raw and segmented CT images of plain and modified concrete mixtures are obtained and compared to assess the validity of the proposed approach. Characterization tests are also performed in order to ensure that the concrete characteristics are not appreciably affected by the presence of the enhancers.
Construction and Building Materials, 2018
Self-compacting concrete (SCC) has become a preferred option for many projects that should satis... more Self-compacting concrete (SCC) has become a preferred option for many projects that should satisfy strict fresh stage properties that are of major concern in quality assurance. To ensure stable and robust fresh stage properties, typically a significant amount of fine materials is incorporated, but this often increases shrinkage. For this purpose, fly ash (FA) has been used, but because it can induce delays in times of setting , it is not extensively used. Under this scenario, micro limestone powders (L) have been effectively used to counteract the delays in the times of setting of concrete with high volumes of fly ash. For a fixed water/powder ratio equivalent to a water-to-cement ratio of 0.40 in a 100% cement mixture, a total of thirteen mortars were produced to evaluate the synergetic effects of twelve portland cement substitutions by FA + L on times of setting, compressive strength, shrinkage and electrical resistivity. Results indicate appropriate FA + L combinations to counteract delays in times of setting and to significantly improve electrical resistivity and volume stability.
During steam curing of concrete, temperature and moisture gradients are developed, which are diff... more During steam curing of concrete, temperature and moisture gradients are developed, which are difficult to measure experimentally and can adversely affect the durability of concrete. In this research, a model of cement hydration coupled to moisture and heat transport was used to simulate the process of steam curing of mortars with water-to-cement (w/c) ratios by mass of 0.30 and 0.45, considering natural convection boundary conditions in mortar and concrete specimens of AASHTO Type VI beams. The primary variables of the model were moisture content, temperature, and degree of hydration. Moisture content profiles of mortar specimens (40 mm in diameter and 50 mm in height) were measured by magnetic resonance imaging. The degree of hydration was obtained by mass-based measurements of loss on ignition to 1000 °C. The results indicate that the model correctly simulates the moisture distribution and degree of hydration in mortar specimens. Application of the model to the steam curing of an AASHTO Type VI beam indicates temperature differences (between the surface and the center) higher than 20 °C during the cooling stage, and internal temperatures higher than 70 °C that may compromise the durability of the concrete.
While the influence of paste properties on concrete performance has been extensively studied and ... more While the influence of paste properties on concrete performance has been extensively studied and in many cases reduced to quantitative relationships (e.g., Abram’s law), that between aggregate characteristics and concrete performance has not been investigated in detail. Based on previous research that demonstrated significant strength differences for two similar concrete mixtures, one prepared with limestone aggregates and the other with siliceous gravel, a joint study between the National Institute of Standards and Technology (NIST) and the Federal Highway Administration (FHWA) was initiated to explore in detail the influence of aggregate source, mineralogy, and material properties on concrete performance. Eleven aggregates of differing mineralogy were identified and obtained both for bulk characterization and for incorporation into two concrete mixtures. The first concrete mixture was based on a 100 % ordinary Type I/II portland cement (OPC), while the second consisted of a ternary 60:30:10 volumetric blend of this cement with 30 % of a Class C fly ash and 10 % of a fine limestone powder. This latter sustainable mixture had exhibited exemplary performance in a previous study. Aggregates were characterized with respect to mechanical and thermomechanical properties, geometrical characteristics, and surface energies. For the prepared concretes, mechanical, thermomechanical, and electrical properties were measured at different ages out to 91 d and microstructural examinations were conducted to examine the interfaces between aggregates and cement paste. Concrete performance varied widely amongst the different aggregates, with the (range/average) ratio for 28-d compressive strength being 0.32 for the OPC concretes and 0.37 for those based on the ternary blend binder. With the exceptions of relating concrete modulus to aggregate modulus and concrete coefficient of thermal expansion (CTE) to aggregate CTE, weak correlations were generally obtained between a single aggregate characteristic and concrete performance properties. Models to predict 28-d compressive strength based on the aggregates’ CTE (and aggregate absorption in the case of the ternary blend mixtures) provided predictions with a relative standard error (standard error/mean) of about 7 %. It is suggested that aggregate and binder characteristics control the bond between aggregates and paste. Then, for most properties, concrete performance is primarily controlled by the level of this bonding, a characteristic that was only assessed in an indirect manner in the present study. Research using non-linear ultrasonic measurements to better assess this bonding in specimens remaining from the present study is currently underway.
Journal of Building Physics, 2015
Internal Curing of Concrete-State-of-the-Art Report of RILEM Technical Committee 196-ICC, 2007
Journal of endodontics, Jan 15, 2015
The setting times of commercial mineral trioxide aggregate (MTA) and Portland cements vary. It wa... more The setting times of commercial mineral trioxide aggregate (MTA) and Portland cements vary. It was hypothesized that much of this variation was caused by differences in particle size distribution. Two gram samples from 11 MTA-type cements were analyzed by laser diffraction to determine their particle size distributions characterized by their percentile equivalent diameters (the 10th percentile, the median, and the 90th percentile [d90], respectively). Setting time data were received from manufacturers who performed indentation setting time tests as specified by the standards relevant to dentistry, ISO 6786 (9 respondents) or ISO 9917.1 (1 respondent), or not divulged to the authors (1 respondent). In a parallel experiment, 6 samples of different size graded Portland cements were produced using the same cement clinker. The measurement of setting time for Portland cement pastes was performed using American Society for Testing and Materials C 191. Cumulative heat release was measured u...
In recent years, there has been great interest in reducing the cement content of concrete, due t... more In recent years, there has been great interest in reducing the cement content of concrete, due to the high energy and carbon dioxide footprints of cement production. There are numerous (waste) materials that can be substituted for cement in the concrete mixture proportions, including fly ash, slag, silica fume, metakaolin, waste glass, etc. However, a more abundant material substitute would be limestone powder, created from the same limestone that is currently heavily employed in cement production as the primary source of calcium oxide. This technical note presents an approach to replacing not only cement powder, but effectively cement paste consisting of the cement and water, with appropriately sized limestone powder(s). Such an approach effectively extends the conventional utilization of centimeter-sized coarse aggregates (rocks) and millimeter-sized fine aggregates (sand) that occupy between 65 % and 75 % of the volume of a concrete structure to include micro-aggregates ranging between about 1 µm and 100 µm in size. Here, to demonstrate the feasibility of this approach, demonstration mixtures of pastes, mortars, and concretes are each formulated with limestone powder replacement for a significant portion of their cement paste component, achieving cement reductions of up to 28 % in concrete, for example. For these mixture modifications, the water-to-cement mass ratio (w/c) is maintained at or above 0.4 to provide sufficient water to react with all of the cement, so that none of this most costly component of cement-based materials goes to waste. Meanwhile, the water-to-solids ratio (w/s) is reduced to a value in the range of 0.22 to 0.40 in order to maximize the limestone powder replacement level, while still providing sufficient flow and rheology, by using reasonable dosages of high range water reducing admixtures. The fresh, early age, and long term performance properties of these high volume limestone powder (HVLP) mixtures are contrasted with a w/c=0.4 ordinary portland cement (OPC) paste or mortar, or a w/c=0.5 OPC concrete reference, respectively. In general, the properties and performance of these more sustainable mixtures are similar or even superior to those of the corresponding reference mixture, suggesting that these new paradigm HVLP concretes could be readily substituted for existing conventional OPC mixtures. The reduced shrinkage (autogenous and drying) of the mortars with limestone powder replacement, due to their reduced paste content, is highlighted because of its likelihood to reduce concrete cracking. However, beyond measurements of electrical resistivity, this study has not specifically focused on durability issues and additional research on this topic is recommended as these new mixtures are reduced to (field) practice.
Journal of the American Ceramic Society, 1996
A series of computer simulations were developed to investigate the removal of multicomponent, the... more A series of computer simulations were developed to investigate the removal of multicomponent, thermoplastic binders from two- and three-dimensional model particulate bodies. Monosized particles with varying diameters were randomly placed in such systems, and all unoccupied pixels were assigned to the binder phase at ratios of 1:9, 1:2, or 1:1 plasticizer (volatile) to polymeric (nonvolatile) species. Simulations were carried out under isothermal conditions to study the influence of liquid-phase transport processes, i.e., plasticizer diffusion in the binder-filled pore network and capillary-driven redistribution of the binder phase, on plasticizer removal rates. Plasticizer diffusion was modeled by a random-walk algorithm, and nonplanar pore development arising from capillary-driven binder redistribution was modeled by an invasion percolation algorithm. For comparison, simulations were also carried out on systems in which binder redistribution was not permitted. In such cases, pore development was modeled as an advancing or nonadvancing planar front. Visualization of transport phenomena on a microscopic scale has provided the first quantitative assessment of plasticizer concentration profiles, C(t) and C(z), and binder-vapor interfacial development during removal. Removal rates were significantly enhanced when capillarydriven binder redistribution was assumed, and they depended strongly on initial plasticizer content under those conditions.
Journal of the American Ceramic Society, 2001
The influence of cement particle-size distribution on autogenous strains and stresses in cement p... more The influence of cement particle-size distribution on autogenous strains and stresses in cement pastes of identical waterto-cement ratios is examined for cement powders of four different finenesses. Experimental measurements include chemical shrinkage, to quantify degree of hydration; internal relative humidity development; autogenous deformation; and eigenstress development, using a novel embedded spherical stress sensor. Because the latter three measurements are conducted under sealed conditions, whereas chemicalshrinkage measurements are made under "saturated" conditions, the National Institute of Standards and Technology cement hydration and microstructure development model is used to separate the effects of differences in hydration rates (kinetics) from those caused by the different initial spatial arrangement of the cement particles. The initial arrangement of the cement particles controls the initial pore-size distribution of the cement paste, which, in turn, regulates the magnitude of the induced autogenous shrinkage stresses produced by the water/air menisci in the air-filled pores formed throughout the hydration process. The experimental results indicate that a small autogenous expansion (probably the result of ettringite formation), as opposed to a shrinkage, may be produced and early age cracking possibly avoided through the use of coarser cements.
Cement and Concrete Research, 1998
Cement and Concrete Research, 2002
Repair mortars are being used with increasing frequency to maintain the aging US infrastructure. ... more Repair mortars are being used with increasing frequency to maintain the aging US infrastructure. Durability is a key concern for such repair materials and both their volume stability and bond to the existing concrete are key attributes, the former to avoid excessive cracking of the repair and the latter to ensure that it remains in place as an integral part of the (repaired) concrete structure. This paper examines the volume stability of two commercially available repair mortars by measuring their autogenous deformation to an age of 28 d. Internal curing is examined as a strategy to mitigate the significant autogenous shrinkage encountered for both materials. The performance of pre-wetted lightweight aggregates (LWA), a superabsorbent polymer (SAP), and a superabsorbent polymer-coated sand (PCS) with respect to compressive strength and autogenous deformation are evaluated. Because these three internal curing agents have water absorptions spanning two orders of magnitude, they have differing influences on the proportioning of the mortars and on subsequent performance. In general, when proportioned with the same amount of internal curing water, the mortars based on LWA exhibit the highest compressive strengths and the greatest relative reductions in autogenous shrinkage in comparison to the controls formulated without internal curing.
The process of accelerated curing of pre-cast concrete consists in increasing the temperature wit... more The process of accelerated curing of pre-cast concrete consists in increasing the temperature within a saturated water vapor atmosphere during several hours. In this process, the cement hydration has a significant importance in the thermal behavior of concrete, since it has an effect on both the mechanical and durability properties. A Multiphysics model that describes hydration and heat and mass transport in cement-based materials was developed. The hydration reactions are described by a maturity function that uses the equivalent time concept, thereby describing the change in the degree of hydration based on the time-temperature history in the form of Domain Ordinary Differential Equations. These equations are coupled to a heat balance, using the COMSOL Heat Transfer module. Using the PDE general form, we solved the moisture content conservation equation. The numerical results were compared with the experimental temperatures obtained directly by semi-adiabatic temperature measurements. The numerical results show a good agreement
Advances in Civil Engineering Materials, 2018
Characterization of fresh concrete is critical for assuring the quality of the United States' con... more Characterization of fresh concrete is critical for assuring the quality of the United States' constructed infrastructure. While fresh concrete arriving at a job site in a ready-mixed concrete truck is typically characterized by measuring temperature, slump, unit weight, and air content, here the measurement of the electrical resistance of a freshly cast cylinder of concrete is investigated as a means of assessing mixture proportions, specifically cement and water contents. Both cement and water contents influence the measured electrical resistance of a sample of fresh concrete: the cement by producing ions (chiefly potassium, sodium, and hydroxide) that are the main source of electrical conduction and the water by providing the main conductive pathways through which the current travels. Relating the measured electrical resistance to attributes of the mixture proportions, such as water-cement ratio by mass (w/c), is explored for a set of eleven different concrete mixtures prepared in the laboratory. In these mixtures, w/c, paste content, air content, fly ash content, high range water reducer dosage, and cement alkali content are all varied. Additionally, concrete electrical resistance data are supplemented by measuring the resistivity of its component pore solution obtained from five laboratory-prepared cement pastes with the same proportions as their corresponding concrete mixtures. Only measuring the concrete electrical resistance can provide a prediction of the mixture's paste content or the product w*c; conversely, when pore solution resistivity is also available, w/c and water content of the concrete mixture can be reasonably assessed.
Construction and Building Materials, 2018
Additive construction by extrusion (ACE), also known as construction-scale 3-D printing, is becom... more Additive construction by extrusion (ACE), also known as construction-scale 3-D printing, is becoming a new paradigm for concrete construction. While significant cost and manpower savings are projected, many issues remain to be solved before ACE can be viewed as a mainstream technology. Among these is the formulation of a robust and sustainable binder that meets the demanding rheological and mechanical performance requirements of an ACE application. This paper investigates the development of such a binder by exploring pastes prepared from binary blends of cement with limestone powder at various proportions , supplemented with the incorporation of a retarder/accelerator admixture combination. Performance is chiefly characterized by applying a Bingham fluid model to characterize the rheological parameters of yield stress and plastic viscosity and by assessing hydration via isothermal calorimetry measurements. Matching the surface area of the limestone powder to that of the cement that it is replacing affords the opportunity to regulate rheology via water content adjustments, while engineering the cement:limestone powder ratio to provide the desired mechanical properties. The utilization of the retar-der/accelerator combination produces a paste with a long pot life (stable rheology) that can be quickly stiffened via the injection of the accelerator (and retarded paste) into a static mixer just prior to the prin-ter's exit nozzle. Thus, these pastes demonstrate the potential to provide a robust and sustainable binder for ACE applications. Published by Elsevier Ltd.
Cement and Concrete Composites, 2018
Obtaining the mesostructure of concrete from X-ray computed tomography (CT) requires segmentation... more Obtaining the mesostructure of concrete from X-ray computed tomography (CT) requires segmentation of the data into distinct phases, a process complicated by the limited contrast between aggregates and mortar matrix. This paper explores the possibility to add baryte or hematite into the concrete mixture to enhance the contrast between cement paste and aggregates in CT, thus allowing for a semi-automatic segmentation. Raw and segmented CT images of plain and modified concrete mixtures are obtained and compared to assess the validity of the proposed approach. Characterization tests are also performed in order to ensure that the concrete characteristics are not appreciably affected by the presence of the enhancers.
Construction and Building Materials, 2018
Self-compacting concrete (SCC) has become a preferred option for many projects that should satis... more Self-compacting concrete (SCC) has become a preferred option for many projects that should satisfy strict fresh stage properties that are of major concern in quality assurance. To ensure stable and robust fresh stage properties, typically a significant amount of fine materials is incorporated, but this often increases shrinkage. For this purpose, fly ash (FA) has been used, but because it can induce delays in times of setting , it is not extensively used. Under this scenario, micro limestone powders (L) have been effectively used to counteract the delays in the times of setting of concrete with high volumes of fly ash. For a fixed water/powder ratio equivalent to a water-to-cement ratio of 0.40 in a 100% cement mixture, a total of thirteen mortars were produced to evaluate the synergetic effects of twelve portland cement substitutions by FA + L on times of setting, compressive strength, shrinkage and electrical resistivity. Results indicate appropriate FA + L combinations to counteract delays in times of setting and to significantly improve electrical resistivity and volume stability.
During steam curing of concrete, temperature and moisture gradients are developed, which are diff... more During steam curing of concrete, temperature and moisture gradients are developed, which are difficult to measure experimentally and can adversely affect the durability of concrete. In this research, a model of cement hydration coupled to moisture and heat transport was used to simulate the process of steam curing of mortars with water-to-cement (w/c) ratios by mass of 0.30 and 0.45, considering natural convection boundary conditions in mortar and concrete specimens of AASHTO Type VI beams. The primary variables of the model were moisture content, temperature, and degree of hydration. Moisture content profiles of mortar specimens (40 mm in diameter and 50 mm in height) were measured by magnetic resonance imaging. The degree of hydration was obtained by mass-based measurements of loss on ignition to 1000 °C. The results indicate that the model correctly simulates the moisture distribution and degree of hydration in mortar specimens. Application of the model to the steam curing of an AASHTO Type VI beam indicates temperature differences (between the surface and the center) higher than 20 °C during the cooling stage, and internal temperatures higher than 70 °C that may compromise the durability of the concrete.
While the influence of paste properties on concrete performance has been extensively studied and ... more While the influence of paste properties on concrete performance has been extensively studied and in many cases reduced to quantitative relationships (e.g., Abram’s law), that between aggregate characteristics and concrete performance has not been investigated in detail. Based on previous research that demonstrated significant strength differences for two similar concrete mixtures, one prepared with limestone aggregates and the other with siliceous gravel, a joint study between the National Institute of Standards and Technology (NIST) and the Federal Highway Administration (FHWA) was initiated to explore in detail the influence of aggregate source, mineralogy, and material properties on concrete performance. Eleven aggregates of differing mineralogy were identified and obtained both for bulk characterization and for incorporation into two concrete mixtures. The first concrete mixture was based on a 100 % ordinary Type I/II portland cement (OPC), while the second consisted of a ternary 60:30:10 volumetric blend of this cement with 30 % of a Class C fly ash and 10 % of a fine limestone powder. This latter sustainable mixture had exhibited exemplary performance in a previous study. Aggregates were characterized with respect to mechanical and thermomechanical properties, geometrical characteristics, and surface energies. For the prepared concretes, mechanical, thermomechanical, and electrical properties were measured at different ages out to 91 d and microstructural examinations were conducted to examine the interfaces between aggregates and cement paste. Concrete performance varied widely amongst the different aggregates, with the (range/average) ratio for 28-d compressive strength being 0.32 for the OPC concretes and 0.37 for those based on the ternary blend binder. With the exceptions of relating concrete modulus to aggregate modulus and concrete coefficient of thermal expansion (CTE) to aggregate CTE, weak correlations were generally obtained between a single aggregate characteristic and concrete performance properties. Models to predict 28-d compressive strength based on the aggregates’ CTE (and aggregate absorption in the case of the ternary blend mixtures) provided predictions with a relative standard error (standard error/mean) of about 7 %. It is suggested that aggregate and binder characteristics control the bond between aggregates and paste. Then, for most properties, concrete performance is primarily controlled by the level of this bonding, a characteristic that was only assessed in an indirect manner in the present study. Research using non-linear ultrasonic measurements to better assess this bonding in specimens remaining from the present study is currently underway.
Journal of Building Physics, 2015
Internal Curing of Concrete-State-of-the-Art Report of RILEM Technical Committee 196-ICC, 2007
Journal of endodontics, Jan 15, 2015
The setting times of commercial mineral trioxide aggregate (MTA) and Portland cements vary. It wa... more The setting times of commercial mineral trioxide aggregate (MTA) and Portland cements vary. It was hypothesized that much of this variation was caused by differences in particle size distribution. Two gram samples from 11 MTA-type cements were analyzed by laser diffraction to determine their particle size distributions characterized by their percentile equivalent diameters (the 10th percentile, the median, and the 90th percentile [d90], respectively). Setting time data were received from manufacturers who performed indentation setting time tests as specified by the standards relevant to dentistry, ISO 6786 (9 respondents) or ISO 9917.1 (1 respondent), or not divulged to the authors (1 respondent). In a parallel experiment, 6 samples of different size graded Portland cements were produced using the same cement clinker. The measurement of setting time for Portland cement pastes was performed using American Society for Testing and Materials C 191. Cumulative heat release was measured u...
In recent years, there has been great interest in reducing the cement content of concrete, due t... more In recent years, there has been great interest in reducing the cement content of concrete, due to the high energy and carbon dioxide footprints of cement production. There are numerous (waste) materials that can be substituted for cement in the concrete mixture proportions, including fly ash, slag, silica fume, metakaolin, waste glass, etc. However, a more abundant material substitute would be limestone powder, created from the same limestone that is currently heavily employed in cement production as the primary source of calcium oxide. This technical note presents an approach to replacing not only cement powder, but effectively cement paste consisting of the cement and water, with appropriately sized limestone powder(s). Such an approach effectively extends the conventional utilization of centimeter-sized coarse aggregates (rocks) and millimeter-sized fine aggregates (sand) that occupy between 65 % and 75 % of the volume of a concrete structure to include micro-aggregates ranging between about 1 µm and 100 µm in size. Here, to demonstrate the feasibility of this approach, demonstration mixtures of pastes, mortars, and concretes are each formulated with limestone powder replacement for a significant portion of their cement paste component, achieving cement reductions of up to 28 % in concrete, for example. For these mixture modifications, the water-to-cement mass ratio (w/c) is maintained at or above 0.4 to provide sufficient water to react with all of the cement, so that none of this most costly component of cement-based materials goes to waste. Meanwhile, the water-to-solids ratio (w/s) is reduced to a value in the range of 0.22 to 0.40 in order to maximize the limestone powder replacement level, while still providing sufficient flow and rheology, by using reasonable dosages of high range water reducing admixtures. The fresh, early age, and long term performance properties of these high volume limestone powder (HVLP) mixtures are contrasted with a w/c=0.4 ordinary portland cement (OPC) paste or mortar, or a w/c=0.5 OPC concrete reference, respectively. In general, the properties and performance of these more sustainable mixtures are similar or even superior to those of the corresponding reference mixture, suggesting that these new paradigm HVLP concretes could be readily substituted for existing conventional OPC mixtures. The reduced shrinkage (autogenous and drying) of the mortars with limestone powder replacement, due to their reduced paste content, is highlighted because of its likelihood to reduce concrete cracking. However, beyond measurements of electrical resistivity, this study has not specifically focused on durability issues and additional research on this topic is recommended as these new mixtures are reduced to (field) practice.
Journal of the American Ceramic Society, 1996
A series of computer simulations were developed to investigate the removal of multicomponent, the... more A series of computer simulations were developed to investigate the removal of multicomponent, thermoplastic binders from two- and three-dimensional model particulate bodies. Monosized particles with varying diameters were randomly placed in such systems, and all unoccupied pixels were assigned to the binder phase at ratios of 1:9, 1:2, or 1:1 plasticizer (volatile) to polymeric (nonvolatile) species. Simulations were carried out under isothermal conditions to study the influence of liquid-phase transport processes, i.e., plasticizer diffusion in the binder-filled pore network and capillary-driven redistribution of the binder phase, on plasticizer removal rates. Plasticizer diffusion was modeled by a random-walk algorithm, and nonplanar pore development arising from capillary-driven binder redistribution was modeled by an invasion percolation algorithm. For comparison, simulations were also carried out on systems in which binder redistribution was not permitted. In such cases, pore development was modeled as an advancing or nonadvancing planar front. Visualization of transport phenomena on a microscopic scale has provided the first quantitative assessment of plasticizer concentration profiles, C(t) and C(z), and binder-vapor interfacial development during removal. Removal rates were significantly enhanced when capillarydriven binder redistribution was assumed, and they depended strongly on initial plasticizer content under those conditions.
Journal of the American Ceramic Society, 2001
The influence of cement particle-size distribution on autogenous strains and stresses in cement p... more The influence of cement particle-size distribution on autogenous strains and stresses in cement pastes of identical waterto-cement ratios is examined for cement powders of four different finenesses. Experimental measurements include chemical shrinkage, to quantify degree of hydration; internal relative humidity development; autogenous deformation; and eigenstress development, using a novel embedded spherical stress sensor. Because the latter three measurements are conducted under sealed conditions, whereas chemicalshrinkage measurements are made under "saturated" conditions, the National Institute of Standards and Technology cement hydration and microstructure development model is used to separate the effects of differences in hydration rates (kinetics) from those caused by the different initial spatial arrangement of the cement particles. The initial arrangement of the cement particles controls the initial pore-size distribution of the cement paste, which, in turn, regulates the magnitude of the induced autogenous shrinkage stresses produced by the water/air menisci in the air-filled pores formed throughout the hydration process. The experimental results indicate that a small autogenous expansion (probably the result of ettringite formation), as opposed to a shrinkage, may be produced and early age cracking possibly avoided through the use of coarser cements.
Cement and Concrete Research, 1998
Cement and Concrete Research, 2002
Repair mortars are being used with increasing frequency to maintain the aging US infrastructure. ... more Repair mortars are being used with increasing frequency to maintain the aging US infrastructure. Durability is a key concern for such repair materials and both their volume stability and bond to the existing concrete are key attributes, the former to avoid excessive cracking of the repair and the latter to ensure that it remains in place as an integral part of the (repaired) concrete structure. This paper examines the volume stability of two commercially available repair mortars by measuring their autogenous deformation to an age of 28 d. Internal curing is examined as a strategy to mitigate the significant autogenous shrinkage encountered for both materials. The performance of pre-wetted lightweight aggregates (LWA), a superabsorbent polymer (SAP), and a superabsorbent polymer-coated sand (PCS) with respect to compressive strength and autogenous deformation are evaluated. Because these three internal curing agents have water absorptions spanning two orders of magnitude, they have differing influences on the proportioning of the mortars and on subsequent performance. In general, when proportioned with the same amount of internal curing water, the mortars based on LWA exhibit the highest compressive strengths and the greatest relative reductions in autogenous shrinkage in comparison to the controls formulated without internal curing.
The process of accelerated curing of pre-cast concrete consists in increasing the temperature wit... more The process of accelerated curing of pre-cast concrete consists in increasing the temperature within a saturated water vapor atmosphere during several hours. In this process, the cement hydration has a significant importance in the thermal behavior of concrete, since it has an effect on both the mechanical and durability properties. A Multiphysics model that describes hydration and heat and mass transport in cement-based materials was developed. The hydration reactions are described by a maturity function that uses the equivalent time concept, thereby describing the change in the degree of hydration based on the time-temperature history in the form of Domain Ordinary Differential Equations. These equations are coupled to a heat balance, using the COMSOL Heat Transfer module. Using the PDE general form, we solved the moisture content conservation equation. The numerical results were compared with the experimental temperatures obtained directly by semi-adiabatic temperature measurements. The numerical results show a good agreement