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Fly ash is a by-product of coal combustion, made up of particles that are collected through various methods. This by-product has been used successfully as a partial Portland cement replacement in concrete, but the performance predictions of fly ash in concrete have been difficult to predict, especially at high fly ash replacement rates. This study focuses on comparing the performance of concrete with a variety of fly ash mixtures as well as the particle distribution and chemical makeup of fly ash. The slump, unit weight, compressive strength, and isothermal calorimetry tests were used to measure the performance of concrete at 0%, 20%, and 40% fly ash replacement levels. The particle distribution of fly ash was measured with an automated scanning electron microscope. Additionally, the major and minor oxides from the chemical makeup of fly ash were measured for each mixture and inputted into a table. The particle distribution and chemical makeup of fly ash were compared to the performance of slump, unit weight, compressive strength, isothermal calorimetry, and surface electrical resistivity.

Concrete mixtures with different aggregate gradations and workability were prepared and X-ray computed tomography (XCT) is used to study the aggregate packing in hardened concrete. The workability of the mixtures was measured and then the aggregate spacing and distribution in the hardened concrete were investigated with an XCT scan of 40 μm resolution. The workability measurements showed segregation or poor workability in the mixtures with more than 20% of the coarse aggregates retained on a single sieve. In addition, in these mixtures, it was observed that ≈75% of the aggregates have a spacing

This work compares the sequential air method (SAM) and the results from the hardened air-void analysis (ASTM C 457) for 488 different concrete mixtures from the lab and field. The results show that there is a wide variation of air contents that correlate with a Spacing Factor of 200 μm. These results show the inadequacy of using air content to decide the quality air void system of the concrete. In fact, 25% of the field data was shown to have a Spacing Factor higher than the recommended values. The results from the SAM, Spacing Factor, and volume of fine air voids (chords less than 300 μm) exhibited good agreement for both the laboratory and field data. Since the SAM can be used to test concrete before it sets and it can give important insight into the bubble size and spacing, this makes it a valuable tool to design and evaluate the air void system of fresh concrete and provide insight into the air void system in the hardened concrete.

While the stress-induced dissolution of various minerals has gained attention as an important time-dependent deformation mechanism, this has only sparingly been investigated in portland cement systems. In this paper, X-ray Computed Tomography (XCT) is used to make direct observations of the microstructural evolution in cement paste samples under different levels of stress during their first 60 h of hydration. Stiffness and creep measurements are also made while imaging the changes in the microstructure. The results show that stress applied at ages between 24 h and 60 h caused an increase in stiffness, increase in early age creep, and a statistically higher amount of dissolution of individual particles near the point of load application. Keywords: X-ray Computed Tomography, Microstructure evolution, Cement Hydration, Creep, Dissolution

A new classification approach is presented that uses individual fly ash particle measurements to provide improved and more in-depth information about the properties of the concrete. The technique uses machine guided X-ray microanalysis to measure the compositions of 2000 randomly selected particles from 20 different fly ashes. This paper details the methods used to acquire the particle composition data and the derivation of the representative groups of the fly ash particles or classification groups. This method is named the Particle Model. To investigate the utility of these groups, 12 fly ashes were used at a 20% mass replacement of the cement in a series of concrete mixtures, which were tested for compressive strength at various times over 180 d of curing. Seven of the nine particle compositions identified were found to influence the compressive strength of the concrete with a linear model R-squared value of 0.99. The Particle Model showed a statistically significant improvement over the Class C or F classification from ASTM C618 and EN450 . This work aims to establish the Particle Model and show that the classification shows promise to be used as a method to predict the physical properties of concretes that contain fly ash.

This paper gives a quantitative comparison of how different curing methods impact the rate of drying and subsequent penetration of lime water and chloride penetration of concrete. Laboratory work is used to investigate a bridge deck concrete mixture cured by two different curing compounds, wet curing of different lengths, and then no curing. The results confirm that wet curing methods reduce the ingress of external chemicals more effectively. The wet curing for even one day provided significant improvement over both curing compounds and no curing. To confirm the findings in the field eight bridge decks were investigated that were cured with a curing compound and wet curing. The field investigation confirms the findings of the laboratory testing and emphasizes the importance of wet curing for long term durability of concrete. This paper provides important quantitative data that can be used to compare these methods and help with making decisions about different curing practices and the impact on the service life of concrete.

Air entraining admixtures (AEAs) are surfactant-based solutions and powders that are used to stabilize air voids in cement paste that protect concrete against frost damage. After reviewing the chemical and structural features of AEA that govern their performance, we discuss: (i) how they establish a stabilizing shell around air voids, (ii) methods for quantifying the distribution of voids after mixing and after hardening, (iii) mechanisms by which voids protect against frost damage, (iv) effects of AEA on other properties of wet and hardened concrete, and (v) interactions of AEA with other chemical and supplementary cementitious materials.

This paper follows the hydration of both portland cement and tricalcium silicate pastes between 30 min and 16 h of hydration. In-situ fast X-ray computed tomography (fCT) was used to make direct observations of the air-filled void formation in w/s of 0.40 to 0.70 with a micron resolution.The results show that over the first hour of the acceleration period the volume of air-filled voids reaches a maximum value and then decreases during the acceleration period and stays constant. The void distribution changes from a few coarse voids to a large number of smaller and more uniformly distributed voids. This behavior is suggested to be controlled by changes in the ionic strength that cause exsolution of dissolved air from the pore solution. Keywords: X-ray computed tomography, Cement hydration, Void evolution, Calorimetry, Induction period, Exsolution

Electrical resistivity shows promise in predicting the mass transport of ions in concrete. This could be of great value because of the low cost, speed, and convenience of performing electrical resistivity measurements. This study uses the Nernst-Einstein equation to correlate the apparent iodide diffusion coefficient (Dic) of fly ash–cement paste and the surface electrical resistivity (ρsr) of fly ash concrete of the same age. This relationship is investigated at three different ages for 19 different fly ash sources at 20% and 40% fly ash replacement to the cement. A factor K which shows the correlation between ρsr and Dic is calculated using the Nernst-Einstein equation, and the relationship is evaluated with a regression analysis. While different K factors were found to relate ρsr and Dic, it was not possible to only use one K factor to relate Dic and ρsr for 20% or 40% replacement of fly ash for the materials and mixtures investigated. Despite there not being a single relationship, this work suggests a practical approach to use the ρsr in a specification to obtain a Dic of the desired value or lower.

The Sequential Air Method (SAM), AASHTO TP 118, is the only test that can quantify the air void volume and spacing in fresh concrete but using the results to design a concrete mixture can be challenging. This work introduces a new tool that uses the outputs from the SAM to determine how different ingredients or changes to the environment impact the volume and spacing of the air void system. This tool is known as the Efficiency Chart. The Efficiency Chart can help producers create more reliable and consistent air void systems that also meet the suggested freeze thaw requirements. The Efficiency Chart is made by using 227 diverse concrete mixtures to establish an upper and lower bound of performance. To show the usefulness of this method several examples are presented to show how different admixtures and cements impact the air void systems. Agreement is shown between the predictions of the Efficiency Chart and the results from a hardened air void analysis (ASTM C457). Because the SAM can evaluate fresh concrete, the concrete industry can use the Efficiency Chart to help design and troubleshoot the air void systems in their concrete mixtures.

Alternative cementitious materials (ACMs) are receiving increasing attention worldwide but there is a lack of knowledge around the resistance of these materials against harmful ion intrusion. In addition, current accelerated test methods that measure ionic diffusion under an electric field are not reliable when comparing binders with vastly different pore solution chemistry. This paper overcomes these issues by using laboratory transmission X-ray microscopy (TXM) and micro X-ray fluorescence (μXRF) imaging to make real-time measurements of ion diffusion in paste and mortar samples for five commercially available ACMs and an ordinary portland cement. The results compare the apparent ion diffusion rate, quantify the change in the ion diffusion rate over time, and give insights into ion binding. The results show that after 42 d of ion exposure that the samples made with calcium aluminate cement had the lowest rate of ion penetration while the samples with alkali activated and calcium sulfoaluminate cement had the greatest rate of ion penetration. The portland cement had an ion penetration level that was between these two. Also, both the alkali-activated and calcium sulfoaluminate samples showed a decrease in the rate of penetration over time. These measurements are important to quantify the sustainability of ACMs, better justify their uses where durability are a concern, and guide future durability testing for these promising materials.

A new approach is presented to determine the air void size and spacing in fresh air entrained concrete by measuring the change in response of the concrete to a series of sequential pressures. This information is used to calculate a parameter called the Sequential Air Method (SAM) Number. Comparisons are made to hardened air void analysis (ASTM C457 ) from laboratory and field tests for 303 mixtures as well as rapid freeze-thaw testing (ASTM C666 ) for 68 mixtures to the SAM Number with over 85% agreement. Furthermore, the SAM Number showed a higher correlation to rapid freeze-thaw testing than current recommendations for the Spacing Factor. The variability of the SAM Number is compared to other direct and indirect measurements of air void size and spacing. Finally, several applications of the method are discussed along with guidance on how to specify the method.

The external penetration of ions into concrete plays an important role in long-term durability of concrete structures. In addition, the degree of saturation (DoS) of the paste has a large impact on rate and mechanisms of fluid penetration into concrete. There is little knowledge of the dynamic process of fluid penetration into these materials because few experimental techniques are able to give quantitative spatial measurements of the fluid movement without damaging the sample. This paper uses laboratory transmission X-ray microscopy (TXM) to investigate the in-situ penetration of an iodide based tracer into cement paste with water-to-cement ratio of 0.40 conditioned at four different relative humidities (RH). This technique can non-destructively image the distribution of the dissolved ions at 8.8 μm resolution in minutes. In addition, micro X-ray fluorescence (µXRF) imaging and conventional gravimetric measurements were used to compare I and Cl penetration rate and validate the results from TXM. Mathematical models have been proposed to account for ion penetration in partially-saturated samples. The results show that samples that were ponded for two days after being conditioned in 35% and 70% RHs (DoS of 40.8% (±0.1%) and 68.8% (±0.5%) respectively) show a 40× and 10× increase respectively in the penetration coefficient over the saturated sample. Furthermore, important insights about combined ion transport mechanisms have been observed from concentration profiles by TXM.

The external penetration of ions into cementitious materials plays an important role in long-term durability of concrete structures. Current test methods to study bulk diffusion of ions in cement-based materials are typically destructive, time consuming and labor intensive. This research work uses laboratory transmission X-ray microscopy (TXM) and X-ray computed microtomography (μCT) to image iodide diffusion in cement paste. These techniques can non-destructively image the distribution of the dissolved ions at 8.8 μm resolution in minutes with a 2D analysis and a few hours for 3D. This method does not aim to measure the size of the pores, instead it aims to image the movement of the fluid through the pores by imaging an electron dense tracer with similar behavior as chloride ions. The tests were completed on cement paste mixtures with water-to-cement ratios (w/c) of 0.35, 0.40, and 0.45. The findings were validated with micro X-ray fluorescence (μXRF) imaging. Iodide and chloride diffusivity is also compared for paste samples with w/c of 0.40 and shown to provide comparable results. Results are also presented of 3D ion diffusion and void filling at high degrees of saturation.

Corrosion of steel reinforcement in concrete is not easy to detect until cracking has initiated. When there is a question about Cl intrusion into concrete, cores are taken, the sample is ground in controlled thicknesses, and then the powder is analyzed by titration. This paper uses micro X-ray fluorescence (μXRF), a non-destructive chemical imaging technique, to obtain equivalent or better information than profile grinding methods with less human involvement. Fifteen different comparisons are made between profile grinding and μXRF from the same concrete and the results are comparable. Examples are also presented where the spatial measurements made by μXRF provide insights that are not possible with the conventional profile grinding method. This means the Cl profile can be obtained with μXRF with less human effort while providing additional insights not possible with the typical profile grinding analysis.

The Freeze-thaw (FT) performance of cementitious materials is dependent on the degree of saturation (DOS). It has been proposed that FT damage can occur during freezing when the DOS is greater than a critical DOS (DOSCR). In a previous study, a systematic methodology has been developed to determine the DOSCR of cement paste samples using high precision length change measurements during freezing and thawing in a thermomechanical analyzer (TMA). This paper explores the extension of the established methodology to mortar samples to determine their DOSCR. The influence of the drying temperature and the characteristics of FT cycle (temperature range, heating and cooling rate) were investigated to determine their impact on the FT performance of the mortar samples. The mortar samples tested in this study were obtained by making concrete and then wet sieving the concrete to obtain mortar. Mixtures with different air void content and air void quality were prepared in this study in order to determine the impact of air void content and air void quality on the FT performance of the samples using TMA testing. The results confirm that length change measurements can be used to determine DOSCR of mortar. A correlation considering the probability of failure was found to exist between the air void quality and the FT performance of mortar samples. The approach presented in this paper provides an alternative method that can complement other existing tests to determine the DOSCR for use in the sorption-based model.

Mass transport in concrete is an important factor to control the durability of concrete structures. Fly ash is commonly used as an additive to help reduce the mass transport of concrete. Since fly ash is a waste product, it is challenging to predict the performance of fly ash in concrete. This work uses a technique called the Particle Model to predict the reactivity and modification of the diffusion coefficient of the paste with fly ash at a 20% and 40% replacement level. The results show that the diffusion coefficient can be correctly predicted for 86% of the mixtures for 20% fly ash replacement and 95% of the mixtures for 40% fly ash replacement within the variation of the test method. This work provides a deeper understanding of the impacts of fly ash on the properties of concrete and provides a new method to predict the performance of the diffusion coefficient at different hydration times.

The reasons for the start and end of the induction period of cement hydration remain topic of controversy. One long-standing hypothesis is that a thin metastable hydrate forming on the surface of cement grains significantly reduces the particle dissolution rate; the eventual disappearance of this layer re-establishes higher dissolution rates at the beginning of the acceleration period. However, the importance, or even the existence, of this metastable layer has been questioned because it cannot be directly detected in most experiments. In this work, a combined analysis using nano-tomography and nano-X-ray fluorescence makes the direct imaging of early hydration products possible. These novel X-ray imaging techniques provide quantitative measurements of 3D structure, chemical composition, and mass density of the hydration products during the induction period. This work does not observe a low density product on the surface of the particle, but does provide insights into the formation of etch pits and the subsequent hydration products that fill them.

Silane is a commonly used surface treatment to reduce water entry into concrete. Only limited work has been done to show the effective lifespan of silane treatments. This work uses 360 cores taken from 60 bridge decks treated with silane that have been in-service between 6 and 20 years. Optical staining techniques showed that the average silane thickness started decreasing after 12 years of service. The deterioration primarily occurs from the bulk of the concrete towards the surface. This suggests that deterioration from the alkali pore solution of the concrete may be important.

Currently, bulk chemical analysis techniques are typically used to determine the composition of fly ash. While bulk analysis provides useful information, the bulk averaging process of a method removes the specificity of the data. Since the individual particles in the fly ash are participating in different reactions, the information about individual particles may be more useful for predicting the performance of fly ash within concrete. This paper presents a method to characterize fly ash on a particle-by-particle basis using automated scanning electron microscopy (ASEM). This technique can simultaneously measure the particle size and chemical composition. The data compares well with bulk powder measurements while also providing single particle information on thousands of samples. This paper also presented advanced characterization of fly ashes using this ASEM method, which cannot be observed from the bulk measurement of fly ash.

A novel approach called the Particle Model is used to predict the compressive strength of fly ash concrete. Thousands of fly ash particles are classified into the nine groups based on the chemical information of each particle. Predictive models for compression strength for 20% and 40% fly ash replacement levels are developed for seven different curing times between 3d and 180d. The R-squared value for the compressive strength prediction of the Particle Model is ≈ 0.99, while the Class C and F classification model is

The electrical resistivity performance of fly ash in concrete is not easy to predict due to the diverse sources of fly ash and their varying reactivity. Electrical resistivity gives direct evidence of the rate of corrosion within the concrete and provides indirect clues into the fluid transport into the material. This paper aims to develop predictive models for the electrical resistivity of fly ash concrete by applying the Particle Model. The Particle Model rapidly examines individual fly ash particles without human intervention and is used to derive predictive models for 20% and 40% fly ash replacement levels in concrete at seven different periods of hydration. The R-squared values of predictive models in the Particle Model show significant improvement over using the classification method based on Class C and F for both fly ash replacement at all investigated time periods. The derived predictive models are able to accurately estimate the electrical resistivity within +/- 10% for 80% of all measurements at 20% fly ash replacement and within +/− 10% for 75% of all measurements at 40% fly ash replacement. These investigations provide important insights into how the Particle Model can help predict the electrical resistivity of fly ash concrete even at different mixtures and hydration times.

The water-cement ratio (w/cm) is one of the most influential parameters to determine the quality of concrete. A new test method has been developed that uses external heat to evaporate the water from the concrete before it has hardened. Data are presented for 258 mixtures with 23 aggregates, 9 cements, 5 supplementary cementitious materials, and 15 different admixtures. For the laboratory testing, the average measured w/cm is within 0.01 from the batched w/cm with a coefficient of variation (COV) of 3.2%. A subset of these mixes was evaluated with the AASHTO T 318 microwave test and the measured w/cm is 0.05 higher than the expected value and the COV is almost three times higher (8.9%). Field data is also presented from 27 mixtures and the measured w/cm shows good agreement with the batched values. The method, calculation, and practical applications of this new test method are presented.

This work uses X-ray computed micro tomography (XCT) to investigate the role of the critical degree of saturation (DOS) and air void system on the crack propagation of portland cement mortar subjected to freeze-thaw cycles. Three-dimensional imaging before and after freezing allow quantitative measurements on the location and volume of the formation of new cracks and the infilling of air voids. Cracks present before freeze-thaw cycles are observed to extend and new cracks initiate from the paste-aggregate interface. In addition, calcium-rich products were observed to fill >20% of the volume of the air voids with a diameter

This paper uses a new approach where fly ash is characterized on a particle-by-particle basis using automated scanning electron microscopy. The particle data is then analyzed with principal component analysis (PCA) to find interrelationships among the particle chemical composition for 20 different fly ashes. Consistent trends were observed in 20 fly ashes. These trends from PCA were verified by making comparisons of these trends for individual particles. One application is introduced using this particle data. Each particle is categorized into four broad groups with a limited chemical composition. These four groups were found in different proportions in the different fly ashes investigated. Compressive strength and surface electrical resistivity were measured from concrete mixtures made with these fly ashes and the four groups correlated with the performance in concrete. This finding is an important step to develop a more general classification of fly ash based on the individual particle make-up, which helps to optimize the mixture design and also benefits the sustainable concrete by increasing the effectiveness of industrial by-waste usage.

Many State Highway Agencies have been working to develop performance-based specifications for concrete pavements and concrete bridge decks in freeze-thaw environments. A time to reach critical saturation (TTRCS) model has been proposed to estimate the freeze-thaw performance of concrete. This study evaluates the TTRCS model for thirty different concrete mixtures with varying w/c, air volumes, and quality of air void (size and spacing). Simple quality control test procedures are used to determine the input parameters for the TTRCS model. The estimated time to reach critical saturation is compared with the measured durability factor using ASTM C 666–15. Results indicate that 86% of the mixtures with air volume above 4.5% and a Sequential Air Method (SAM) Number below 0.30 have a normalized time to reach critical saturation of greater than 20, and a durability factor that is greater than 75%. The mixtures with a high range water reducer require a higher volume fraction of entrained air to satisfy the recommended limit for the durability factor. This appears to be due to an interaction between the high range water reducer and air entraining admixture which results in greater air void spacing (i.e., larger air voids). However, the addition of high range water reducer was also found to increase the time to reach critical saturation in the mixtures with a low w/c due to a refined pore structure with a reduction in the connectivity of the matrix pores. Reducing the w/c improves the freeze-thaw performance due to a reduction in the pore volume, connectivity, and absorption rate of the concrete. A relationship is developed to estimate the time to reach critical saturation based on SAM Number and apparent formation factor. In addition, a relationship is proposed to estimate the critical degree of saturation based on air void content and quality.

Disagreements about the mechanisms of cement hydration remain despite the fact that portland cement has been studied extensively for over 100 years. One reason for this is that direct observation of the change in microstructure and chemistry are challenging for many experimental techniques. This paper presents results from synchrotron nano X-ray tomography and fluorescence imaging. The data show unprecedented direct observations of small collections of C3S particles before and after different periods of hydration in 15 mmol/L lime solution. X-ray absorption contrast is used to make three dimensional maps of the changes of these materials with time. The chemical compositions of hydration products are then identified with X-ray fluorescence mapping and scanning electron microscopy. These experiments are used to provide insight into the rate and morphology of the microstructure formation.

Choosing an aggregate gradation is an important step in the design of concrete mixtures. This work presents a systematic laboratory study to find practical limits for the workability and fi...

This paper examines the impact of wet curing with different durations on differential drying shrinkage of concrete beams. The work shows that in severe drying conditions the magnitude of curling increased with the duration of wet curing of concrete specimens. Testing results are presented along with an explanation for the mechanisms observed. These findings provide guidance for wet curing durations for concrete slabs that must be resistant to volume change from drying.

Concrete samples obtained from deteriorated bridge structures in Texas were cultured in growth medium containing thiosulfate and investigated for microbial production of acid and subsequent biodeterioration of concrete. Mixed cultures obtained from a corroded concrete oxidized thiosulfate to sulfuric acid causing the pH to drop from 6.3 to 3.4. Interestingly, the decrease in pH was minimal when the organisms were cultured in the presence of concrete and thiosulfate in spite of an 84% oxidation of thiosulfate to sulfuric acid, presumably due to the buffering capacity of fresh concrete. Biodeterioration of concrete was also evident from the 2-fold higher amount of calcium leached from concrete compared to un-inoculated controls. Further, a pure culture was isolated and identified as a Streptomyces sp. The isolate oxidized more than 96% of the added thiosulfate to sulfate (sulfuric acid) and decreased the culture pH from 6.4 to 3.5. Under acidic conditions, an appreciable amount of calcium leached from concrete (nearly 4-fold higher than the control) suggesting deterioration of concrete. Scanning electron micrographs of the concrete exposed to both mixed cultures as well as Streptomyces sp. revealed distinct cracks, presumably due to the formation of expandable products, gypsum and ettringite.

The purpose of the experiments described in this chapter was to apply acoustic emission technology in a passive mode to identify crack development in concrete specimens exposed to thermal cycling. Acousto-ultrasonics techniques, or using the acoustic emission sensors in an active mode to function as both pulse generators and receivers, were applied to periodically measure the speed of an ultrasonic wave sent across the specimen throughout thermal cycling. The observed reduction in wave velocity can be correlated to the degrading elastic properties of the concrete matrix due to damage development. These experiments were performed on concretes with a wide range of entrained air volumes which were conditioned to various degrees of saturation to identify the critical degree of saturation (S crit) required for deterioration. As predicted, concrete specimens conditioned to high levels of saturation showed more damage than samples at or below the critical degree of saturation. This was made clear by studying the activity parameters of the recorded hit data from passive acoustic emission methods as well as by comparing the damage parameter determined from the acousto-ultrasonic method. Large amounts of acoustic emission activity were observed during periods of freezing resulting from cracking due to volume expansion as well as during thawing due to friction during crack closure. Small fluctuations in pulse velocity which clearly correlated to the temperature cycles were observed during testing of a control aluminum sample, showing a minor dependence of pulse velocity on temperature effects on the coupling agent alone. The combined approach using both active and passive acoustic emission proved to be highly informative, making it possible to quantify bulk material damage development while observing trends in acoustic emissions throughout multiple freezing and thawing cycles.

Evaluating the workability of concrete pavements prior to paving can be challenging. In this paper a novel test was developed using a simplistic and economic approach to measure the response of the concrete to vibration and the ability of the concrete to hold an edge. The variability of the test was evaluated and two comparisons were made to slip formed pavers. Also, a procedure was developed using the test for comparing the workability performance of different mixtures. The procedure was used to briefly investigate the impacts of aggregate gradation on the workability of slip formed paving mixtures.

While there is great interest in characterizing and modifying materials at the nanoscale, progress has been slow because few techniques allow for critical observations at this length scale. This work presents a data fusion technique that combines synchrotron-based X-ray nano computed tomography and nano X-ray fluorescence to non-destructively investigate complex nanoscale materials and provide combined three-dimensional (3-D) renderings of microstructure and chemistry. The technique has been named nano tomography-assisted chemical correlation (nTACCo) and is demonstrated on fly ash particles with nanoscale chemical inhomogeneities. Our findings show that nTACCo is capable of providing the concentration and location of seven different nano-inclusions within a particle. This work also provides direct observations of reactivity and chemical distribution of fly ash. This ability to combine 3-D structure and chemistry at the nanoscale will provide unprecedented tools for nanoscience in material science, biology, chemistry and medical science.