1,205 research outputs found
Mixture Proportioning for Durable Concrete: Challenges and Changes
No full textNumerous changes and innovations have occurred in concrete materials and technology during the last century. These changes have provided engineers with many advantages in design and construction of concrete structures. At the same time, however, the application of the new developments and changes in concrete mixture proportions have also generated new durability problems.This article is published as Shah, Surendra P., Kejin Wang, and W. Jason Weiss. "Mixture proportioning for durable concrete: challenges and changes." Concrete International 22, no. 9 (2000): 73-78. Copyright 2000, American Concrete Institute. Posted with permission
sj-docx-1-jop-10.1177_02698811221080165 – Supplemental material for Prospective examination of the therapeutic role of psychological flexibility and cognitive reappraisal in the ceremonial use of ayahuasca
No full textSupplemental material, sj-docx-1-jop-10.1177_02698811221080165 for Prospective examination of the therapeutic role of psychological flexibility and cognitive reappraisal in the ceremonial use of ayahuasca by Gabrielle Agin-Liebes, Richard Zeifman, Jason B Luoma, Eric L Garland, W Keith Campbell and Brandon Weiss in Journal of Psychopharmacology</p
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The Pozzolanic Reactivity Test and the Properties of Portland Limestone Cement
No full textThe pozzolanic reactivity test (PRT) is used to quantify the pozzolanic reactivity of supplementary cementitious materials (SCMs). The PRT computes reactivity by measuring heat release (Q) and calcium hydroxide (CH) consumption, as interpreted using thermodynamic modeling. The robustness of PRT is examined by experimentally varying CH-to-SCM ratio, solution-to-solid ratio, sulfate content, alkali type (Na vs. K), and alkali content. Furthermore, the similarities and differences between the PRT and the R3 test (ASTM C1897) are also evaluated. It is shown that the sulfates, as used in the R3 test, did not impact the siliceous reactions; but lead to the preferential formation of monosulfo-aluminates and ettringite with aluminous phases. The inclusion of carbonates, as used in the R3 test, also only impacted the aluminous reactions by forming hemi/mono carbo-aluminates over pozzolanic reaction products. Unlike PRT, the phase assemblage of the reacted products in the R3 test are not consistent and vary depending on the alumina content in the SCM. The PRT is recommended as robust tool to measure the degree of pozzolanic reactivity (DOR*) of the SCMs. A generalized relationship for the DOR* is also developed as a function of Q and CH consumption.
This thesis also examines the performance of cementitious systems made of the clinker of typical sulphate resistant Type II/V cements (which have low aluminate content), LS, and SCMs. The work compares the heat of hydration, shrinkage, mechanical, electrical and transport performance of the paste and mortar made using OPC with the paste and mortar made using portland limestone cement (PLC) and OPC+LS cement with and without typical commercial SCMs (silica fume (SF), fly ash (FA), and slag (SL)).
The heat of hydration, drying shrinkage, and flexural strength were investigated. When compared with OPC, the PLC and OPC+LS did not substantially affect either the extent of reaction at 7 days or the drying shrinkage. However, the degree of hydration of PLC and OPC+LS paste mixtures containing slag were approximately 10% higher than OPC paste mixtures containing slag at the age of 7 days. On the other hand, at early ages (less than 14 days) in the systems that did not contain SL or SF (i.e., plain and FA-containing systems), the flexural strength of the PLC and OPC+LS mortars are 15% and 20% lower than flexural strength of OPC mortars, respectively. The flexural strength was 7% greater at early ages for PLC and OPC+LS samples when combined with SL compared to corresponding OPC samples. The flexural strengths at later ages were similar for all mixtures.
The porosity, formation factor, and pore connectivity of mortars were also examined. The porosity of both PLC and OPC+LS mortars were 4% higher than the porosity of OPC mortar due in part to the dilution of reactive clinker. The effect of dilution was offset with increased clinker reaction. The porosity of PLC+SCM and OPC+LS+SCM mortars was also 2% to 6% higher than the porosity of OPC+ SCM mortars. The mixtures containing SCMs with reactive alumina showed less of an increase in porosity since the limestone reacted to form carbo-aluminate reaction products. Despite the increase in porosity, there was no statistically significant difference between the formation factor of the PLC, OPC+LS, and OPC mortars without SCM. The PLC+SCM, OPC+LS+SCM, and OPC+SCM mortars had a higher formation factor than the plain OPC/PLC/OPC+LS mortars due to pore refinement. Pore refinement was also observed in PLC and OPC+LS mortars containing SCMs with reactive alumina.
The results of this thesis indicate that PLCs (ASTM C595) can be used as a direct replacement for OPCs (ASTM C150) without any significant impact to hydration, shrinkage, mechanical, and transport related performance. PLCs are specifically recommended over OPCs when aluminous SCMs are incorporated in the system
Fundamental investigations related to the mitigation of volume changes in cement-based materials at early ages
No full textThe increased use of high-performance, low water-to-cement (w/c) ratio concretes has led to increased occurrences of early-age shrinkage cracking in civil engineering structures. To reduce the magnitude of early-age shrinkage and the potential for cracking, mitigation strategies using shrinkage reducing admixtures (SRAs), saturated lightweight aggregates, expansive cements and extended moist curing durations in construction have been recommended. However, to appropriately utilize these strategies, it is important to have a complete understanding of the driving forces of early-age volume change and how these methods work from a materials perspective to reduce shrinkage. This dissertation uses a first-principles approach to understand the mechanism of shrinkage reducing admixtures (SRAs) to generate an expansion and mitigate shrinkage at early-ages, quantify the influence of a CaO-based expansive additive in reducing unrestrained shrinkage, residual stress development and the cracking potential at early-ages and quantify the influence of shrinkage reducing admixtures (SRAs) and cement hydration (pore structure refinement) on the reduction induced in the fluid transport properties of the material. The effects of shrinkage reducing admixtures (SRAs) are described in terms of inducing autogenous expansions in cement pastes at early ages. An evaluation comprising measurements of autogenous deformation, x-ray diffraction (Rietveld analysis), pore solution and thermogravimetric analysis and electron microscopy is performed to understand the chemical nature and physical effects of the expansion. Thermodynamic calculations performed on the measured liquid-phase compositions indicate the SRA produces elevated Portlandite super-saturations in the pore solution which results in crystallization stress driven expansions. The thermodynamic calculations are supported by deformation measurements performed on cement pastes mixed in solutions saturated with Portlandite or containing additional Sodium Hydroxide. Further, to quantify the influence of temperature on volume changes in SRA containing materials, deformation measurements are performed at different temperatures. The results indicate maturity transformations are incapable of simulating volume changes over any temperature regime due to the influence of temperature on salt solubility and pore solution composition, crystallization stresses and self-desiccation. The performance of a CaO-based expansive additive is evaluated over a range of additive concentrations and curing conditions to quantify the reduction in restrained and unrestrained volume changes effected in low w/c cement pastes. The results suggest, under unrestrained sealed conditions the additive generates an expansion and reduces the magnitude of total shrinkage experienced by the material. However, the extent of drying shrinkage developed is noted to be similar in all systems and independent of the additive dosage. Under restrained sealed conditions, the additive induces a significant compressive stress which delays tensile stress development in the system. However, a critical additive concentration (around four percent) needs to be exceeded to appreciably reduce the risk of cracking at early-ages. The influence of shrinkage reducing admixtures (SRAs) is quantified in terms of the effects of SRA addition on fluid transport in cement-based materials. The change in the cement paste\u27s pore solution properties, i.e., the surface tension and fluid-viscosity, induced by the addition of a SRA is observed to depress the fluid-sorption and wetting moisture diffusion coefficients, with the depression being a function of the SRA concentration. The experimental results are compared to analytical descriptions of water sorption and a good correlation is observed. These results allow for the change in pore-solution and fluid-transport properties to be incorporated from a fundamental perspective in models which aim to describe the service-life of structures. Several experimental techniques such as chemical shrinkage, low temperature calorimetry and electrical impedance spectroscopy are evaluated in terms of their suitability to identify capillary porosity depercolation in cement pastes. The evidence provided by the experiments is: (1) that there exists a capillary porosity depercolation threshold around 20% capillary porosity in cement pastes and (2) low temperature calorimetry is not suitable to detect porosity depercolation in cement pastes containing SRAs. Finally, the influence of porosity depercolation is demonstrated in terms of the reduction effected in the transport properties (i.e., the fluid-sorption coefficient) of the material as quantified using x-ray attenuation measurements. The study relates the connectivity of the pore structure to the fluid transport response providing insights related to the development of curing technologies and the specification of wet curing regimes during construction
Interaction between Loading, Corrosion, and Serviceability of Reinforced Concrete
No full textThe present research studies the mutual effects between mechanical loading and corrosion of reinforcing steel, as well as their combined effect on serviceability (flexural deflection and residual loading capacity) of reinforced concrete beams. Beam specimens 10 x 15 x 117 cm in size were subjected to four-point bending at various loading levels (0 ~ 75% of the ultimate load) with different loading histories (previous loading and sustained loading). NaCl solution ponding was employed to accelerate the corrosion process. Half-cell potential and galvanized current measurements were taken to monitor time for corrosion initiation. After corrosion initiated, an external current was applied to some of the specimens to expedite corrosion propagation. Beam deflections were recorded throughout all of the tests. Residual flexural loading capacity of the beams was evaluated at the end of the experiment. The results indicate that loading history and loading level have significant effects on both corrosion initiation and the rate of corrosion propagation. The failure mode of the reinforced concrete beams appeared to shift from a shear failure of concrete to bond splitting as the degree of corrosion increased. The results suggest that for a rational service-life prediction of reinforced concrete structures, the influence of the service load on the structure performance should be considered in combination with environmental conditions and material proportions.This article is published as Yoon, Sanchun, Kejin Wang, W. Jason Weiss, and Surendra P. Shah. "Interaction between loading, corrosion, and serviceability of reinforced concrete." ACI Materials Journal 97, no. 6 (2000): 637-644.
DOI: 10.14359/9977.
Copyright 2000 American Concrete Institute.
Posted with permission
Assessing risk reduction of high early strength concrete mixtures
No full textOvernight concrete pavement patching has become a common method of rapid roadway rehabilitation utilized by the Indiana Department of Transportation (INDOT) and other transportation agencies. However, tight construction schedules cause for increased risk to contractors attempting to meet specifications for traffic opening; causing increased bid prices to account for liquidated damages. This work looks to reduce the risk of high early strength (HES) concrete patching materials through material examination and behavior under field exposed and simulated conditions. Five site visits to a pavement rehabilitation project on US HWY 30 in northern Indiana were made in order to examine field mixtures and practices. Elevated concrete temperatures and inaccurate maturity predictions due to a reduction in flexural strength development when exposed to high temperatures and accelerator dosages were found to be consistent problems during these site visits. Next, a laboratory program was developed to evaluate an observed crossover strengths in field tested concrete beams using calorimetry and fracture testing. A testing matrix evaluating four different temperatures, 10, 23, 37.5, and 50 ?C, and accelerator dosages, 0, 20, 40, 60 oz/ cwt., was implemented for examination using these techniques. The results showed a decrease in the accelerating admixtures effectiveness at high temperatures and a potential imbalance of the sulfate-aluminate reaction. Additional SO3, in the form of gypsum and plaster, was added to the original mixture as a 1-2% cement replacement by mass, to examine if performance could be improved. Additional sulfate may serve as a potential solution to behavioral changes under these conditions. Internal curing has been proven to be an effective value added methodology to improve the durability of bridge decks in Indiana. Observation has shown that at four different ICBHPC county bridge decks, internal curing vastly improves the durability, and thus the service life of concrete pavements compared historical bridge deck mixtures. Thus, internal curing could serve as an application to improving the durability, and thus the service life of HES patching mixtures. Value added methods of additional sulfate and internal curing could serve to be applied to HES mixtures to improve performance. Improved performance leads to reduced risk while increasing the service life of these repairs, ultimately saving money and time
Quantifying the performance of macrosynthetic fiber reinforcement and shrinkage reducing admixtures in large scale concrete slabs
No full textAn inherent characteristic of concrete is the tendency for it to change volume. If this volume change is restrained, residual tensile stresses will be induced, ultimately leading to crack formation. Concrete placed in slabs-on-grade applications, i.e., sidewalks, roadways, and parking lots, are especially susceptible to cracking. Welded-wire fabric is commonly used to provide reinforcement and control the cracking behavior of concrete. However, in the last several decades, the use of fiber-reinforced concrete and shrinkage-reducing admixtures have gained acceptance as an alternative to crack controlling measures. Current standard laboratory testing procedures for quantifying the shrinkage and cracking tendency of concrete provide a convenient means to evaluate these effects using small scale specimens. These small scale test methods do not fully represent the behavior of concrete in the field. This thesis describes a large scale, restrained slab testing system that is designed to simulate full scale performance under standard drying conditions. The large scale slab system allows for a direct comparison between welded wire fabric, fiber reinforcement, and shrinkage-reducing admixtures in their ability to control crack formation and transfer of tensile stresses across the crack. The restrained slab data of this research was able to show the benefits of fiberreinforced concrete and shrinkage reducing admixtures over the welded wire fabric. Furthermore, the slab system is able to monitor stress transfer across a crack through the measurement of crack width over time and provide a quantitative means to evaluate the effectiveness of crack controlling treatments
Reliability-based analysis of early-age cracking in concrete
No full textWith recent concern regarding the environmental impact of the construction and urban development, there has been an increased emphasis on understanding how the concrete industry can become more sustainable. Sustainability relates to the application of energy efficient materials with low impact on environment and ensured durability. By improving the long-term durability of concrete elements, the life of the infrastructure can be extended, saving resources and environment. One problem that leads to premature deterioration in concrete structures is the development of cracks. As a result, there is an interest in developing procedures to produce crack free concrete elements. This research describes how experiments and computer simulations can be used to relate fundamental material properties and variability to the cracking performance of cement and concrete materials. When thermal, hygral or chemical volume changes in concrete are restrained, residual stresses arise. If the residual stresses exceed the tensile strength of concrete, cracking occurs. Previous research has focused on the development of test methods and computer models to predict cracking in concrete. While these models are a great step forward, they are generally deterministic and do not consider inherent variability in material properties, construction processes, and environmental conditions. As a result, these models do not accurately capture the true risk of cracking in concrete elements. In this research, Monte Carlo method and Load and Resistance Factor Design (LRFD) approach have been applied to incorporate different sources of variability in investigating the probability of cracking in restrained concrete members. Simulations are performed to determine the extent of free shrinkage reduction that is required to minimize the probability of cracking to an acceptable level. An approach is presented that allows engineers to select and incorporate the probability of cracking during the material design process. With this information, concrete can be designed using new materials, like shrinkage reducing admixtures (SRA) or by internal curing using for example lightweight aggregates (LWA), to meet the specified shrinkage performance
Shrinkage, residual stress, and cracking in heterogeneous materials
No full textConcrete experiences volumetric changes as a result of material formation (cement hydration), thermal variations, or moisture losses. If these volume changes are prevented by the structure surrounding the concrete or volumetrically stable phases inside the concrete, residual stresses can develop. In many cases, these residual stresses may be large enough to result in cracking. While the premature cracking of concrete is significant enough to be considered in the design of concrete facilities, existing test methods and design methodologies need to be updated to better quantify the cracking potential especially when the concrete experiences non-uniform deformations. Most design and testing approaches assume that concrete behaves like a homogeneous material. However, concrete is a composite that consists of cement paste and aggregates that have dissimilar material properties. When concrete changes its volume, localized internal residual stresses develop due to heterogeneity (i.e., paste and aggregates) that could lead to microcracking and premature cracking of concrete. Therefore, to properly evaluate the cracking behavior of concrete, an analytical tool considering the heterogeneous nature of concrete should be developed. This research begins with quantifying the impact of non-uniform shrinkage on the residual stress development in the restrained ring test considering that the concrete experiences non-uniform moisture loss (drying). The role of the boundary conditions and the degree of restraint on residual stress development is also discussed. Next, the internal residual stresses that develop in a multi-phase composite system are examined by varying the series of parameters (material properties of each-phase, volume fraction of aggregate, and bond conditions between the phases). Analytical modeling is used to assess the microcracking and cracking behavior of concrete composite systems using the object-oriented finite element code. It is the hypothesis of this research that the cracking behavior of concrete can be properly evaluated by assuming that concrete is a heterogeneous multi-phase composite with the assistance of a recently developed object-oriented finite element code that enables meshing a complex material meso-structure using optical mesostructural images. Although this research focuses on shrinkage in cementitious composites only, this software could have numerous applications in determining damage development in other composites as well
Assessing the performance of antimicrobial concrete admixtures in concrete subjected to microbially induced corrosion
No full textConcrete is the mostly widely used material in wastewater storage, conveyance, and treatment. The hydrated Portland cement in concrete makes it susceptible to degradation under highly acidic conditions. The process known as microbially induced corrosion (MIC) can promote concrete deterioration and incur large maintenance costs. MIC is caused by the metabolic products of two groups of microorganisms. Anaerobic bacterial activity below the water line results in the production of aqueous hydrogen sulfide (H2S). H2S then escapes into the gas phase and is oxidized to sulfuric acid by aerobic bacteria residing on concrete surfaces above the water line. This results in sulfuric acid attack of Portland cement by dissolution of calcium hydroxide and formation of corrosion byproducts, including gypsum and ettringite. The attack gradually moves from the outer surface to the core of the concrete, causing coarse aggregate to be dislodged, reducing wall thickness and destroying structural integrity, and in some case exposing structures to wastewater. Several challenges have been associated with assessment of MIC. First, no standard method has been developed to assess concrete susceptibility to MIC; reproduction of MIC in a laboratory setting is limited by the need to culture specific bacterial strains and by handling of H2S. In-situ experiments are helpful for quantification of the effects of MIC in the field; however, the best locations for sample placement are not clearly defined, and the results are influenced by a number of factors that cannot be controlled in field experiments. In natural systems, concrete deterioration can also be influenced by other factors, including bacteria remaining on the cylinder. A combination of laboratory and field experiments was conducted to quantify the effects of MIC on a range of concrete mixtures, some of which included an antimicrobial agent designed to inactivate the bacteria that promote the process. The laboratory experiments were conducted in an isolated chamber wherein climate was controlled, including gas-phase H2S concentration. Concrete specimens representing 13 mixtures were inoculated with four species Thiobacillus bacteria (Thiobacillus thiooxidans (ATCC® 8085), Thiobacillus neapolitanus (ATCC®23641), Thiobacillus thioparus (ATCC ®8185) and Thiomonas intermedia (ATCC ® 15466). Results indicated that CAC (calcium aluminate cement) mortar, Australian cement, and incorporation of an antimicrobial agent can improve resistance to MIC under moderate MIC conditions (i.e., surface pH\u3e2.0). Rapid deterioration of specimens exposed to more aggressive conditions in the chamber indicated that degradation of concrete under the most severe MIC conditions (i.e., concrete surface pH\u3c2.0) cannot be prevented by manipulating concrete mixture proportions. The second experiment involved a field test designed to evaluate the resistance of 13 concrete mixture designs to H2S exposure of three locations within a municipal wastewater collection system. It was found that exposing to lower H2S (g) concentration can increase concrete resistance to MIC compared to higher H2S (g) concentration. However, the length of exposure was not sufficient to make clear distinctions between the performances of different mixture designs. The third set of experiments involved a test to quantify the effectiveness of the antimicrobial agent for inactivation of pure cultures of planktonic bacteria. Results indicated that the antimicrobial agent performed well for inactivation of pure cultures of Thiobacillus bacteria, except for Thiobacllus thiooxidans under the most severe condition (i.e., a solution pH\u3c2)
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