This paper describes a meta-analysis of previously published studies on the shrinkage strain of recycled aggregate concrete (RAC). The study aims at providing an analytic expression for the shrinkage ...strain of RAC to be used in conjunction with the existing
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Model Code 2010 shrinkage prediction model. For this purpose, a database of experimental results on the shrinkage of RAC and companion natural aggregate concrete (NAC), produced with the same water-cement ratio, was compiled using strict selection criteria. Results from 19 studies entered into the database, consisting of 125 shrinkage curves (39 NAC and 86 RAC) with a total of 424 data points. A comparison of RAC and companion NAC revealed that, on average, RAC displays a larger shrinkage strain. This difference increases with increasing recycled concrete aggregate (RCA) content and with decreasing compressive strength. Applying the
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Model Code 2010 shrinkage prediction model revealed that, relative to its performance on NAC, the shrinkage strain of RAC is underestimated. Finally, a correction coefficient for the shrinkage strain of RAC,
ξ
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,
RAC
, to be used in conjunction with the
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Model Code 2010 model, was proposed in the form of a bivariate power function with RAC compressive strength and RCA replacement ratio as variables.
A highly sensitive short‐wave infrared (SWIR, λ > 1000 nm) organic photodiode (OPD) is described based on a well‐organized nanocrystalline bulk‐heterojunction (BHJ) active layer composed of a ...dicyanovinyl‐functionalized squaraine dye (SQ‐H) donor material in combination with PC61BM. Through thermal annealing, dipolar SQ‐H chromophores self‐assemble in a nanoscale structure with intermolecular charge transfer mediated coupling, resulting in a redshifted and narrow absorption band at 1040 nm as well as enhanced charge carrier mobility. The optimized OPD exhibits an external quantum efficiency (EQE) of 12.3% and a full‐width at half‐maximum of only 85 nm (815 cm−1) at 1050 nm under 0 V, which is the first efficient SWIR OPD based on J‐type aggregates. Photoplethysmography application for heart‐rate monitoring is successfully demonstrated on flexible substrates without applying reverse bias, indicating the potential of OPDs based on short‐range coupled dye aggregates for low‐power operating wearable applications.
Narrowband near‐infrared organic photodiodes are reported based on a dipolar squaraine dye. J‐type coupling in the solid state allows SQ‐H thin films to combine favorable NIR absorption at 1040 nm and charge carrier mobility. The bulk‐heterojunction with PC61BM yields an organic photodiode with external quantum efficiency of 12.3% at 1050 nm with a full‐width half‐maximum of 85 nm under short‐circuit condition.
Concrete with crushed concrete aggregates (CCA) shows lesser compressive strength than reference concrete with natural aggregates. The goal of this study is to improve the strength of structural ...concrete with 53% and 100% CCA replacements without increasing the cement content. Thus, improvements in CCA quality are induced by combining mechanical and pre-soaking pre-processing techniques. Mechanical pre-processing by rotating drum is separately pursued on fine and coarse CCA for 10 and 15 min respectively. Results show, adhered mortar content and CCA water absorption reduces as pre-processing duration increases. Pre-processing influences CCA particle grading, flakiness index, shape index, void-content, unit-weight and density, jointly seen as packing density, which increases with pre-processing duration. Water amount to pre-soak CCA before concrete mixing is stable despite grading modifications, due to reduced water absorption resulting from mechanical pre-processing. Compressive strength and workability for pre-processed CCA50 and CCA100 concrete are comparable to reference concrete and show similar trends of improvement with packing density. Packing density markedly shows the quality improvements induced by pre-processing on CCA, maybe considered as one of the quality assessment indexes for CCA. Packing density should be investigated for other recipes to see the stability of the trend with workability and compressive strength.
Reading the bands: Amphiphilic pyrene trimers self‐assemble into two‐dimensional, supramolecular polymers in aqueous medium. Folding and aggregation processes are accompanied by simultaneous ...development of J‐ and H‐bands and significant changes in the fluorescence properties. The formation of sheet‐like nano‐structures is confirmed by AFM.
Alkali-silica reaction (ASR) is one of the most detrimental distress mechanisms leading to early deterioration of concrete infrastructure, creating large amounts of construction and demolition waste ...which however can be transformed into recycled concrete aggregates (RCA). Yet, as ASR is an ongoing distress mechanism, its potential of reoccurrence in RCA could cause adverse effects. Investigation of the crack propagation of RCA concrete revealed that the distress features vary widely thus, indicating different distress mechanisms in all RCA concrete. Concrete made of slightly damaged RCA shows the lease number of cracks yet, wider cracks at high expansion levels. Additionally, the distress in slightly damaged RCA concrete made of reactive coarse aggregate and RCA concrete made of reactive sand, regardless of original damage extent, is governed by cracks propagating through the residual cement paste whereas cracks propagate through the new cement paste in severely damaged RCA made of reactive coarse aggregate.
Strength development and shrinkage and creep behavior of recycled aggregate concrete as well as durability aspects such as carbonation and chloride ion penetration were investigated. Except for ...Control concrete, natural fine aggregate was replaced by recycled fine aggregate at a rate of 0%, 15%, 30%, or 50% while natural coarse aggregate was completely replaced by recycled coarse aggregate (RCA). Two different recycled fine aggregates were used: Recycled fine aggregate (RFA) and CO2 treated recycled fine aggregate (CRFA). Compressive strength of recycled aggregate concrete at all ages tested was reduced from that of the Control concrete. CO2 treatment of recycled fine aggregate improved not only the aggregate physical properties but also the strength of recycled aggregate concrete. The compressive strength reduction was 19% or less at 100% RCA + 15%–50% RFA replacement level while it was 11% or less at 100% RCA + 15%–50% CRFA replacement level after 28 days. Shrinkage and creep strains were measured for up to 300 days. Shrinkage of concrete with 100% RCA was 12% larger than that of Control concrete, while shrinkage of concretes with 100% RCA + 30% RFA or 30% CRFA replacement increased by about 25%–30%. In the creep test, creep coefficient and specific creep of all recycled aggregate concretes increased by about 20–35% over that of the Control. CO2 treatment of recycled fine aggregate resulted in up to 15% improved strength of recycled aggregate concrete, but showed little improvement on shrinkage and creep. Durability of recycled aggregate concrete evaluated in terms of carbonation and chloride ion penetration was reduced from that of the Control concrete.
•Based on current test results, the maximum amount of CRFA replacing natural fine aggregate can be suggested as 30% (by vol.). The maximum 30% utilization is also limited by availability of long-term test data such as shrinkage and creep.•It should be noted that, due to reduction of concrete durability for carbonation and chloride ion penetration, cautious use of CO2 treated recycled aggregates is recommended where the application is potentially affected by carbonation and/or chloride ion penetration.
Sand has been considered to be something of an immeasurable quantity. There are many indications that this view is no longer valid and that the limiting of natural aggregates usage is doubly ...justified. Firstly, the extraction of natural aggregates is expensive and has a huge impact on the environment. The main issues in sand and gravel mining are the large areas that are affected, ground water level changes, illegal mining, unsuitability of desert and marine sand, and costs of transport. Secondly, metallurgical waste can be used as a substitute for natural aggregates. This is doubly beneficial-the waste is recycled and the use of natural aggregates is reduced. Waste is stored in landfills that take up large areas and there is also the possibility of ground and groundwater pollution by hazardous compounds. The research presented in this article focuses on the technological conditions of using metallurgical waste in its original form and as a component of recycled concrete aggregate (RCA). The use of metallurgical sludge waste or crushed or round RCA to produce concrete deteriorates the consistency and does not significantly affect the air content and density of the concrete mix. RCA lowers the density of hardened concrete. Metallurgical sludge waste or RCA usage adversely affect the absorbability and permeability of concrete. Concrete containing metallurgical sludge waste is of higher compressive strength after 7 and 28 days, with up to 60% of waste as a sand replacement. RCA concrete achieved higher compressive strength also.
In this paper, a new recycled aggregate concrete (RAC) was produced with composite coarse aggregate and fine recycled aggregate. The composite coarse aggregate was mixed into continuous gradation by ...large particle natural aggregate with small particle recycled aggregate. To explore the time-dependent developments of the compressive strength and splitting tensile strength of this new RAC, 320 groups of cubic specimens were tested at different curing ages from 3 days to 360 days to measure the compressive and splitting tensile strengths. The amount of large particle natural aggregate varied from zero to 70% in mass of the total coarse aggregate. The water/cement ratio was taken as 0.60, 0.49, 0.41 and 0.36 to represent four strength grades of the RAC at about C20, C30, C40 and C50. Based on the tested results, the curves of the compressive and tensile strengths of the RAC that changed with curing age are plotted, which clearly exhibit that the amount of large particle natural aggregate had a rational range in different strength grades of the RAC which had better aging strength. When the RAC was no larger than C30 with a water/cement ratio of 0.60 and 0.49, the amount of large particle natural aggregate should be no more than 30%; when the RAC was no less than C40 with a water/cement ratio of 0.41 and 0.36, the amount of large particle natural aggregate should be no less than 50%. Along with the general prediction of the strength development of all the tested RAC, the optimal predictive formulas are proposed for the strength development of RAC with a rational amount of natural aggregate. Meanwhile, the strength developments of RAC with a rational amount of natural aggregate are assessed by the time-dependent models proposed by the ACI Committee 209 and CEB-FIP MC 2010.
The use of marble wastes in concrete mixtures, causing air and water pollution, has been promoted in the academic and practical spheres of the construction industry. Although the effects of various ...forms (powder, fine, coarse and mixed) of this waste on the concrete compressive strength has been subject to a decent number of studies in the literature, the difficulties in reaching specific conclusions on the effect of each test parameter constitute a major restraint for the proliferation of the use of marble wastes in the concrete industry. Most of these studies are far from underscoring all of the parameters affecting the concrete compressive strength. Due to the urgent need in the literature for comprehensive studies on concrete mixtures with marble wastes, the results of the axial compression tests on a total of 429 concrete mixtures with marble aggregates were compiled by paying special attention to reporting all test variables (form and content of marble wastes, water–cement ratio, cement content, proportion of coarse and fine aggregates in all aggregates) affecting the concrete strength. In this context, multivariate regression analyses were carried out on the existing test results. These regression analyses yielded to relationships between the change in concrete compressive strength and the test parameters for each and every form of marble waste (powder, fine and coarse aggregate). The study indicated that independent from the form of marble wastes (as powder, fine aggregate or coarse aggregate), aggregate replacements of up to 50% can yield to significant changes in the concrete compressive strength. In addition, the analytical estimates from the developed equations exhibited a high correlation (a least r value of 0.91) with the experimental results from the previous studies, yielding to rather low error values (RMSE value is 5.06 MPa at max). For this reason, the developed equations can consistently predict the changes in concrete compressive strength with varying amounts and forms of the marble aggregates as well as the other test variables.
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