This paper reviews studies on the effect of supplementary cementing materials (SCM) on alkali-silica reaction (ASR). SCMs control expansion due to ASR by binding alkalis and limiting their ...availability for reaction with alkali-silica reactive aggregate. The efficacy of the SCM is dependent on the composition of the SCM. Increased amounts of SCM are required to control ASR as its calcium and alkali content increase, as its silica content decreases, as the alkali contributed by the Portland cement increases and as the reactivity of the aggregate increases. There is evidence that the alumina content of the SCM also affects its alkali-binding capacity, however, the precise role and contribution of the alumina is not clear.
This paper presents the durability performance of concrete incorporating high-volumes of fly ash exposed to a harsh marine environment for 19 to 24years. Concrete specimens (305×305×915mm 1×1×3ft.) ...were cast using W/CM in the range of 0.31 to 0.46, various types of fly ash with replacement levels of 56 or 58%, and various types of normal density and lightweight aggregates.
Surface erosion/scaling was seen on all specimens; the extent of erosion was slightly worse for fly ash concrete with normal density aggregate compared to the controls (without fly ash), but fly ash concrete with two of the three lightweight aggregates exhibited severe surface erosion.
Laboratory testing included taking cores from each block and determining the existing chloride profile, compressive strength and “chloride permeability” (ASTM C1202).
The depth of chloride penetration was found to be in excess of 100-mm (4in.) for the concrete specimens without fly ash, whereas the presence of fly ash significantly decreased the depth of penetration to approximately 30 and 40mm in specimens containing either normal density or lightweight aggregate, respectively. The increased penetration in the concrete with lightweight aggregates is attributed to the surface erosion observed with these concretes. The results from the chloride permeability testing also indicate significant increases in the resistance to chloride-ion penetration for fly ash concrete.
Utilisation of potentially alkali–silica reactive aggregates requires reliable performance tests to evaluate the alkali–silica reactivity of various aggregate combinations, including their alkali ...threshold dependence on binder type. Several such performance tests have been used worldwide for more than 15years, but none of the methods have proven to be reliable for use with all aggregate types and all binders. One of the objectives of RILEM TC 219-ACS (2007–2012) is to develop and validate one or more of such performance tests.
Several parameters may influence the results obtained in an accelerated performance test compared to the field behaviour. Based on a state of the art literature review, this paper discusses which parameters must be considered to be able to develop reliable ASR performance testing methods and provides some tentative recommendations. The internal humidity in the test specimens, the extent of alkali leaching and the storage temperature are of particular importance.
Alkali–silica reaction (ASR) is a major concrete durability problem, resulting in significant maintenance and reconstruction costs to concrete infrastructures all over the world. Despite decades of ...study, the underlying chemical and physical reaction mechanisms remain poorly understood, especially at molecular to micro-scale levels, and this has resulted in the inability to efficiently assess the risk, predict the service life, and mitigate deterioration in ASR-susceptible structures. This paper intends to summarize the current state of understanding and the existing knowledge gaps with respect to reaction mechanisms and the roles of aggregate properties (e.g., composition, mineralogy, size, and surface characteristics), pore solution composition (e.g., pH, alkalis, calcium, aluminum), and exposure conditions (e.g., temperature, humidity) on the rate and magnitude of ASR. In addition, the current state of computer modeling as an alternative or supplement to physical testing for prediction of ASR performance is discussed.
Item response theory (IRT) is moving to the forefront of methodologies used to develop, evaluate, and score clinical measures. Funding agencies and test developers are routinely supporting IRT work, ...and the theory has become closely tied to technological advances within the field. As a result, familiarity with IRT has grown increasingly relevant to mental health research and practice. But to what end? This article reviews advances in applications of IRT to clinical measurement in an effort to identify tangible improvements that can be attributed to the methodology. Although IRT shares similarities with classical test theory and factor analysis, the approach has certain practical benefits, but also limitations, when applied to measurement challenges. Major opportunities include the use of computerized adaptive tests to prevent conditional measurement error, multidimensional models to prevent misinterpretation of scores, and analyses of differential item functioning to prevent bias. Whereas these methods and technologies were once only discussed as future possibilities, they are now accessible because of recent support of IRT-focused clinical research. Despite this, much work still remains in widely disseminating methods and technologies from IRT into mental health research and practice. Clinicians have been reluctant to fully embrace the approach, especially in terms or prospective test development and adaptive item administration. Widespread use of IRT technologies will require continued cooperation among psychometricians, clinicians, and other stakeholders. There are also many opportunities to expand the methodology, especially with respect to integrating modern measurement theory with models from personality and cognitive psychology as well as neuroscience.
Public Significance Statement
Item response theory (IRT) is a methodology used to develop, evaluate, and score psychological tests. This article reviews the literature for recent advancements in the use of IRT as applied to clinical research and practice. IRT has contributed to major advancements in computerized adaptive testing, multidimensional modeling, and the identification of bias, but there are still many opportunities to further develop applications of the theory.
Elevated levels of cholesteryl ester (CE)-enriched apoB containing plasma lipoproteins lead to increased foam cell formation, the first step in the development of atherosclerosis. Unregulated uptake ...of low-density lipoprotein cholesterol by circulating monocytes and other peripheral blood cells takes place through scavenger receptors and over time causes disruption in cellular cholesterol homeostasis. As lipoproteins are taken up, their CE core is hydrolyzed by liposomal lipases to generate free cholesterol (FC). FC can be either re-esterified and stored as CE droplets or shuttled to the plasma membrane for ATP-binding cassette transporter A1-mediated efflux. Because cholesterol is an essential component of all cellular membranes, some FC may be incorporated into microdomains or lipid rafts. These platforms are essential for receptor signaling and transduction, requiring rapid assembly and disassembly. ATP-binding cassette transporter A1 plays a major role in regulating microdomain cholesterol and is most efficient when lipid-poor apolipoprotein AI (apoAI) packages raft cholesterol into soluble particles that are eventually catabolized by the liver. If FC is not effluxed from the cell, it becomes esterified, CE droplets accumulate and microdomain cholesterol content becomes poorly regulated. This dysregulation leads to prolonged activation of immune cell signaling pathways, resulting in receptor oversensitization. The availability of apoAI or other amphipathic α-helix-rich apoproteins relieves the burden of excess microdomain cholesterol in immune cells allowing a reduction in immune cell proliferation and infiltration, thereby stimulating regression of foam cells in the artery. Therefore, cellular balance between FC and CE is essential for proper immune cell function and prevents chronic immune cell overstimulation and proliferation.
In the COVID-19 crisis, the science of learning has two different responsibilities: first, to offer guidance about how best to deal with the impact of the current situation, including lockdown and ...home-schooling; and second, to consider bigger questions about what this large-scale educational experiment might mean for the future. The first part of this Viewpoint summarises advice for parents on mental health, and on becoming stand-in-teachers. The second part, taking the longer view, considers the potential negative impact of the COVID-19 crisis in increasing inequality in education; but also the potential positive impact of driving innovations in technology use for educating children.
It is well understood that the partial replacement of portland cement with pozzolans in sufficient proportions results in improved long-term performance of concrete. However, dwindling availability ...of the most widely used pozzolan in the construction industry, fly ash, is forecast due to the termination of coal-fired electricity power stations around the world, necessitating the exploration of alternative pozzolans. Experimental investigations are conducted to study the performance of traditional pozzolans such as fly ash, silica fume and natural pozzolans (metakaolin, pumice, perlite and lassenite) together with novel pozzolans, including ground glasses (high-alkali and low-alkali), and ground bottom ash. Twenty-one concrete mixtures are examined for compressive strength, electrical conductivity, electrical resistivity, chloride permeability, and chloride migration coefficient. The reactivity of pozzolans studied is also determined in terms of compressive strength in lime-pozzolan mortar using a modified lime-reactivity test method. Many of the materials tested demonstrate the potential to be used in place of fly ash. However, a wide variation in the performance of these materials is evident which highlights the need for a reliable test to determine the level of reactivity of a pozzolan. The results indicate that the 7-day strength in the modified lime-reactivity test provides a good indication of the pozzolanic reactivity of the material and of how the material can be expected to contribute to the strength and permeability of concrete. The use of electrical resistivity as an indicator of the performance of pozzolans in terms of strength development and chloride penetration resistance when used to partially replace portland cement in concrete is also discussed.
Cellular membranes are not homogenous mixtures of proteins; rather, they are segregated into microdomains on the basis of preferential association between specific lipids and proteins. These ...microdomains, called lipid rafts, are well known for their role in receptor signaling on the plasma membrane (PM) and are essential to such cellular functions as signal transduction and spatial organization of the PM. A number of disease states, including atherosclerosis and other cardiovascular disorders, may be caused by dysfunctional maintenance of lipid rafts. Lipid rafts do not occur only in the PM but also have been found in intracellular membranes and extracellular vesicles (EVs). Here, we focus on discussing newly discovered functions of lipid rafts and microdomains in intracellular membranes, including lipid and protein trafficking from the ER, Golgi bodies, and endosomes to the PM, and we examine lipid raft involvement in the production and composition of EVs. Because lipid rafts are small and transient, visualization remains challenging. Future work with advanced techniques will continue to expand our knowledge about the roles of lipid rafts in cellular functioning.
The over-pruning hypothesis of autism Thomas, Michael S.C.; Davis, Rachael; Karmiloff-Smith, Annette ...
Developmental science,
March 2016, Volume:
19, Issue:
2
Journal Article
Open access
This article outlines the over‐pruning hypothesis of autism. The hypothesis originates in a neurocomputational model of the regressive sub‐type (Thomas, Knowland & Karmiloff‐Smith, 2011a, 2011b). ...Here we develop a more general version of the over‐pruning hypothesis to address heterogeneity in the timing of manifestation of ASD, including new computer simulations which reconcile the different observed developmental trajectories (early onset, late onset, regression) via a single underlying atypical mechanism; and which show how unaffected siblings of individuals with ASD may differ from controls either by inheriting a milder version of the pathological mechanism or by co‐inheriting the risk factors without the pathological mechanism. The proposed atypical mechanism involves overly aggressive synaptic pruning in infancy and early childhood, an exaggeration of a normal phase of brain development. We show how the hypothesis generates novel predictions that differ from existing theories of ASD including that (1) the first few months of development in ASD will be indistinguishable from typical, and (2) the earliest atypicalities in ASD will be sensory and motor rather than social. Both predictions gain cautious support from emerging longitudinal studies of infants at‐risk of ASD. We review evidence consistent with the over‐pruning hypothesis, its relation to other current theories (including C. Frith's under‐pruning proposal; C. Frith, 2003, 2004), as well as inconsistent data and current limitations. The hypothesis situates causal accounts of ASD within a framework of protective and risk factors (Newschaffer et al., 2012); clarifies different versions of the broader autism phenotype (i.e. the implication of observed similarities between individuals with autism and their family members); and integrates data from multiple disciplines, including behavioural studies, neuroscience studies, genetics, and intervention studies.
This articles proposes a new hypothesis of the cause of autism, based on a neurocomputational model. The over‐pruning hypothesis proposes that ASD results from over‐pruning of brain connectivity early in development, particularly impacting long‐range connections. The hypothesis generates a number of novel hypotheses that can be tested against new data emerging from studies of infants at risk of developing ASD. It proposes that a single underlying pathological mechanism interacts with population‐wide variation in neurocomputational parameters to produce different trajectories of ASD including early onset, late onset, and regression.
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