Showing papers in "Cement & Concrete Composites in 2019"
TL;DR: In this paper, a yield stress based mixture design approach for 3D printable concretes was presented, where the mixtures were evaluated based on buildability, extrudability, robustness and tests for structural build-up.
Abstract: The current study deals with a yield stress based mixture design approach for 3D printable concretes. The mixtures were evaluated based on buildability, extrudability, robustness and tests for structural build-up. For the print parameters (such as pump type, nozzle size and extrusion velocity) used in the study, it was found that both extrudability and buildability could be achieved only when the material yield stress was within a range of 1.5–2.5 kPa. Below this range, the material lacked enough strength to achieve shape stability, while above this range, the extrudabilty of the material was difficult. The robustness of the mixtures was quantified in terms of a variability factor defined in terms of the variation in yield stress with small changes in the superplasticizer dosage. Inclusion of 10% of silica fume, 0.1% of viscosity modifying agent (VMA) and 0.1–0.3% addition of nanoclay resulted in decreasing the variability factor, hence improving the robustness. The structural changes due to thixotropy and cement hydration increased the yield stress with time. This structural build-up was assessed by measuring the yield stress with increasing rest duration. The mixture with silica fume showed the maximum structural build-up while the mixture with VMA showed the least. Heat curves from semi-adiabatic calorimetry and penetration curves were also used to assess the structural build-up. They showed a similar trend to that of the yield stress vs time plots.
222 citations
TL;DR: A literature review and experimental results on the effect of high incorporation levels of fly ash (FA) and recycled concrete aggregates (RCA), individually and jointly, on the pore system of concrete that remarkably influences its durability are presented in this article.
Abstract: This paper presents a literature review and experimental results on the effect of high incorporation levels of fly ash (FA) and recycled concrete aggregates (RCA), individually and jointly, on the pore system of concrete that remarkably influences its durability. For that purpose, apart from an extensive literature review, three tests were performed, including electrical resistivity (ER) test, which indirectly measures the interconnected porosity of concrete, and water absorption (WA) by capillarity and immersion tests that both depend on the pores number and size but in a different way. A comparison between the experimental results and the literature is also presented to show the main findings and the research needs. The results show that WA increases and ER decreases with increasing incorporation level of RCA, and the opposite occurs with the addition of FA for both tests. The reduction percentage of WA was higher in mixes with both RCA and FA when compared to the sum of reductions in mixes with only RCA or FA. Thus, it is advisable to produce concrete with both mentioned non-traditional materials in terms of WA and ER of concrete. In addition, the benefit of incorporating of FA and RCA in concrete increased even more when superplasticizers was used.
179 citations
TL;DR: In this article, the state of the art of the impact resistance of ordinary fiber-reinforced concretes (FRCs) containing various fibers is reviewed, and various types of impact test methods are addressed as well as some concerns about them based on extensive literature reviews and our perspective.
Abstract: This paper reviews the state of the art of the impact resistance of ordinary fiber-reinforced concretes (FRCs) containing various fibers. First, various types of impact test methods that are current available are addressed as well as some concerns about them based on extensive literature reviews and our perspective. Then, common properties of FRCs under impact loading regardless of fiber type, such as the reasons for their enhanced strength under impact, the effect of size on impact resistance, and several factors (i.e., matrix strength, loading conditions, and fiber existence) that influence strain-rate sensitivity, are discussed. Furthermore, the comprehensive impact resistances of FRCs with various fibers (i.e., steel, polymeric, carbon, basalt, natural, and hybrid fibers) are investigated under different loading conditions. After summarizing the impact properties of FRCs with various fibers, the comparative impact resistance of FRCs according to the fiber type is evaluated to determine which type gives the best improvement of impact resistance. Lastly, the effect of supplementary cementitious materials (SCMs), i.e., fly ash, silica fume, and slag, on the impact resistance of FRCs is examined, and some combinations of SCM and fiber types that lead to enhanced impact resistance are suggested.
142 citations
TL;DR: In this article, the authors explored the efficacy of wood waste biochar as a green admixture and assessed the influence of different physico-chemical properties of dredged sediments on the mechanical performance of cement-based sediment products.
Abstract: Routine navigational dredging generates huge quantities of marine sediment, posing significant environmental and economic burden. This research intended to explore the efficacy of wood waste biochar as a green admixture and assess the influence of different physico-chemical properties of dredged sediments on the mechanical performance of cement-based sediment products. X-ray diffraction and porosimetry analysis reflected that particle size distribution of sediments determined the pore structure formation and strength development. Thermal and calorimetry analyses showed that biochar incorporation slightly enhanced the cement hydration reaction, while its relatively large and brittle particles induced microcracks and weakened the strength of sediment products. Nevertheless, biochar addition enhanced immobilization of potentially toxic elements and organic contaminants, rendering the sediment products more environmentally acceptable. Hence, the innovative approach of this study can recycle dredged sediment and waste wood biochar to produce eco-friendly construction materials such as fill material and paving blocks.
139 citations
TL;DR: In this article, the effect of silica fume, fly ash, and metakaolin on ITZ thickness and strength was assessed using a model based on the Mori-Tanaka scheme coupled with an estimation of deviatoric stress within ITZ.
Abstract: The interfacial transition zone (ITZ) has a major detrimental impact on the structural performance of concrete. This negative impact can be modulated by introducing mineral admixtures to a concrete mix, which fill the excessive voids within ITZ and react with portlandite to form more compact products. The approach described here, consisting of characterization of phases and micromechanical modeling, enabled assessment of the effect of silica fume, fly ash, and metakaolin on ITZ thickness and strength. The proposed model was based on the Mori-Tanaka scheme coupled with an estimation of deviatoric stress within ITZ. This study suggests that silica fume is efficient in reducing ITZ thickness, while the addition of fly ash more significantly contributes to ITZ strength. Moderate replacements of Portland cement for silica fume or fly ash, up to 20%, can positively influence concrete performance; in case of metakaolin, replacement up to 10% is recommended.
132 citations
TL;DR: In this paper, the super-fine ground granulated blast furnace slag was obtained by wet grinding and an attempt to activate the super fine ground granule slag by sodium sulfate (SS) and sodium carbonate (SC) was made.
Abstract: In this study, the super-fine ground granulated blast furnace slag was obtained by wet grinding (i.e. WGBBS), and an attempt to activate the super-fine ground granulated blast furnace slag by sodium sulfate (SS) and sodium carbonate (SC) was made. SS/SC-activated WGGBS samples were prepared and cured at the room temperature. The compressive strength at the age of 3 d, 7 d, 28 d, and 56 d was tested. Hydration heat was assessed, and micro structure and hydrates were also characterized with XRD, TG-DTG, SEM-EDS, and NMR; the pore structure was assessed with MIP. The results showed that SS and SC efficiently activated the hydration of WGGBS with D50 = 3.87 μm at the room temperature, and such high activating efficiency of SS and SC under room temperature was seldom reported in the literature. The mechanism behind was mainly because of the super-fine particles of slag with greater amounts of hydration points, which were produced in the process of wet grinding. Difference in activating efficiency between SS and SC was mainly because the anionic groups acted as different roles in hydration process: SS could induce the formation of ettringite while SC could induce the formation of calcite. Such results were expected to provide guidance on designing weak base-activated slag system.
118 citations
TL;DR: In this article, a 3D-printing test device was used for this purpose, which enabled the elimination of most artefacts in the characterization of materials, and the results obtained using the proposed approach were compared with results of a simple ram-extruder, slump-flow and viscometer tests.
Abstract: Digital construction (DC) is a new process, and hence, no standard experimental methods for process-specific material characterization are as yet available. This article proposes a methodology for characterizing the extrudability of cement-based materials for DC, both quantitatively and inline. A 3D-printing test device was used for this purpose, which enabled the elimination of most artefacts in the characterization of materials. Unit extrusion energy UEE, defined as the energy consumed per extruded unit volume, was used as the measure of extrudability, lower UEE implying higher extrudability. The results obtained using the proposed approach were compared with results of a simple ram-extruder, slump-flow and viscometer tests. Two fine-grained concrete mixtures under investigation, one with ordinary sand and one with very fine sand, having respective yield stresses of 306.2 Pa and 642.7 Pa, were characterized. They showed a significant difference in their extrudability: the UEE needed in the case of material with finer sand was 1.62 times higher than that of the mixture with coarser sand. Interestingly, average ram extrusion force for the finer mixture was much lower than that of the coarser mix, underlining the challenges in material characterization and the need to consider the possible artefacts of various testing methods. Comparative analyses substantiated the significance of the proposed inline extrudability quantification method for DC.
118 citations
TL;DR: In this article, the synergetic effects of hybrid polypropylene (PP) and steel fibers on explosive spalling prevention of ultra-high performance concrete (UHPC) at elevated temperature were investigated.
Abstract: This study investigated synergetic effects of hybrid polypropylene (PP) and steel fibers on explosive spalling prevention of ultra-high performance concrete (UHPC) at elevated temperature. Permeability of UHPC was measured and correlated to the extent of spalling quantitatively. Microstructures of UHPC before and after elevated temperature exposure were examined to reveal potential mechanisms responsible for changes in permeability. Results showed that the use of hybrid PP and steel fibers completely prevented explosive spalling even at low fiber dosage of both fibers due to significant increase of permeability. Microstructural analysis revealed that such synergistic effect on increased permeability of hybrid PP and steel fiber-reinforced UHPC was attributed to enhanced connectivity of empty PP fiber tunnels by multiple microcracks generated from the thermal expansion of both fibers.
117 citations
TL;DR: In this paper, a Portland cement free, alkali-activated material was optimized in order to enhance strength and durability, achieving a w/b ratio of less than 0.25.
Abstract: Based on the principles of ultra-high performance concrete (UHPC) a Portland cement free, alkali-activated material was optimized in order to enhance strength and durability. The formulation is based on ground granulated blast furnace slag and as an activator a combination of potassium water-glass and potassium hydroxide was used. Furthermore, silica fume and metakaolin as inorganic fines were used to increase the packing density of the mixture. Quartz sand (0–2 mm) and quartz powder were added as aggregates. The results comprise an enhancement of the rheological properties by the stepwise increase of silica fume. A w/b ratio of less than 0.25 was realised using a certain mixing procedure with a high intensity mixer. The compressive strength reaches values comparable with the strength range of an UHPC. The already low capillary porosity could further be decreased by the substitution of slag with a small amount of metakaolin, which is leading to a higher polymerisation degree due to an incorporation of aluminium in the reaction products. The enhanced polymerisation degree was estimated by FTIR and XRD. Although no effective superplasticizer can be used in this high alkaline material, the low w/b ratio and a good workability can be achieved by using certain amounts of silica fume.
113 citations
TL;DR: In this paper, five different treatment techniques of aggregate concrete, such as carbonation, acid immersion, acetic acid immersion with mechanical rubbing and lime immersion with carbonation are adopted during the study.
Abstract: Recycling of construction and demolition waste in the concrete is considered a sustainable way However, recycled aggregates (RA) with inferior properties are produced after recycling as compared to natural aggregates. This study aims to improve the performance of RA by utilizing different treatment methods and to evaluate the properties of the resulting recycled aggregate concrete (RAC). For this purpose, five different treatment techniques of RA, such as carbonation, acetic acid immersion, acetic acid immersion with mechanical rubbing, acetic acid immersion with carbonation and lime immersion with carbonation are adopted during the study. Different mechanical tests are performed to investigate the effect of different RA treatment techniques on the mechanical properties of RAC with treated and untreated RA. Increase in split tensile strength and flexural strength along with improved stress-strain behavior of RAC is observed for treated RA as compared to untreated RA. The stress-strain behavior of RAC having RA treated through acetic acid immersion with mechanical rubbing and lime immersion with carbonation is observed very close to the stress-strain curves of natural aggregate concrete reflecting the positive impact of these RA treatment techniques on the performance of RAC. Moreover, empirical relations to predict different mechanical properties and stress-strain model of RAC with both treated and untreated RA are also developed in this work. A comparative study of the existing and proposed models with the test results indicates that the proposed relations and model can effectively predict the mechanical behavior of RAC with both treated and untreated RA.
113 citations
TL;DR: In this paper, a fly ash/slag composite mix and the Na2CO3/Na2SiO3 activator were monitored through Isothermal conduction calorimetry.
Abstract: Reactions between the fly ash/slag composite mix and the Na2CO3/Na2SiO3 activator were monitored through Isothermal conduction calorimetry. The resulting products were analyzed with the help of XRF, XRD and FT-IR techniques. In calorimetric response, the composite pastes had more total heat release than the fly ash paste requiring ∼50% less activation energy to yield reaction products. These products were largely amorphous as observed in the XRD patterns. Rheological studies indicated that composite pastes were very stiff above 25 wt% slag addition as its yield stress was almost doubled to fly ash paste. The compressive strength of hardened pastes increased with increasing slag content and activator dosage and decreased with increasing water-binder ratio. The deposition of reaction products onto the fly ash/slag particle surfaces and also the dense microstructures as observed in FESEM supported higher strength of geopolymer pastes at higher activator and slag contents. The developed paste with standard sand at 1:2 ratio produced mortar with a compressive strength of ∼72 MPa.
TL;DR: In this paper, a triaxial compression test setup was developed, to characterize all essential parameters to assess failure by elastic buckling, and material yielding according to the Mohr-Coulomb criterion.
Abstract: In 3D concrete printing processes, two competing modes of failure are distinguished: material failure by plastic yielding, and elastic buckling failure through local or global instability. Structural analysis may be performed to assess if, and how, an object may fail during printing. This requires input in the form of transient material properties obtained from experimental testing on early age concrete. In this study, a custom triaxial compression test setup was developed, to characterize all essential parameters to assess failure by elastic buckling, and material yielding according to the Mohr-Coulomb criterion. The results of the triaxial tests were compared to simultaneously run unconfined uniaxial compression tests and ultrasonic wave transmission tests. The correlation between these experimental methods was reviewed. It was concluded that the triaxial compression test is an appropriate method to determine all relevant transient properties from one series of experiments. Subsequently, the experimental results were used for structural analyses of straight printed walls of different lengths with a Finite Element Modelling approach. These walls have been printed up to failure during print trials and the results were compared to the numerical predictions. The failure mode is predicted accurately by the numerical model, as is the critical height at which failure occurs for relatively small objects. For larger objects and/or longer printing processes, the quantitative agreement of the critical height with the print experiments could be improved. Two possible causes for this deviation are discussed.
TL;DR: In this paper, the authors explored crack closure and recovery of mechanical and permeability properties after multiple (i.e., three) damage cycles by comparing autogenous and bio-based self-healing in concrete using a combination of steel and PVA fibers, superabsorbent polymer (SAP), and bacteria immobilized in biochar.
Abstract: Self-healing under multiple damage cycles is critical to the serviceability of concrete structures. This article explores crack closure and recovery of mechanical and permeability properties after multiple (i.e., three) damage cycles by comparing autogenous and bio-based self-healing in concrete using a combination of steel and PVA fibers, superabsorbent polymer (SAP), and bacteria immobilized in biochar. Swelling of SAP upon exposure to water and enhancement of hydration by curing action of SAP led to improved blocking and filling of cracks compared to a control (plain concrete); however, the effectiveness of autogenous crack closure (by only SAP or SAP plus fibers) was limited to narrow surface cracks ( 600 μm) and internal micro-cracks compared to that attained by the autogenous mechanism alone in control and concrete containing only SAP and fiber. Effectiveness of crack filling by immobilized spores in biochar was found to be consistently higher than concrete with directly added spores and SAP through all three cycles of damage and healing. Precipitation of calcium carbonate crystals in internal cracks and interfacial zones around PVA fiber and aggregate in concrete with biochar immobilized spores resulted in high recovery of strength and permeability compared to the autogenous healing mechanism in control and concrete with SAP and fiber. However, it was found that macro-voids formed by SAP with a larger average particle size and higher swelling capacities affected total permeability and permeability recovery after repeated healing. Overall, we conclude that cementitious systems with biochar-immobilized bacteria, SAP, and fibers can enhance self-healing and impart improved durability to concrete structures.
TL;DR: In this article, the effect of nano-silica particles on the hydration, the rheology and the strength development of cement pastes is evaluated by means of isothermal calorimetry and thermogravimetric analysis.
Abstract: The aim of the present work is to evaluate the effect of nano-silica (NS) on the hydration, the rheology and the strength development of cement pastes. The advance of chemical reactions is monitored by mean of isothermal calorimetry and thermogravimetric analysis: adding nano-silica particles speeds up the hydration of the cement paste but alter its workability. Indeed, the effect of the nano-silica particles on the hydration kinetics can be modelled by accounting for its high specific surface and a flocculation model based on the DLVO theory is proposed so as to investigate the stability of nano-silica suspensions in the fresh cement paste. As a consequence, the dosage of nano-silica can be optimized to promote the early age strength. Lastly, a ternary blend incorporating fly ash can be designed so as to provide an early age strength similar to that of the cement while lowering the induced CO 2 emissions.
TL;DR: In this article, the authors evaluated the effect of carbonated RCA on the transport properties of old cement mortars attached on recycled concrete aggregates (RCA) and found that even with the external layer (0-10mm) carbonated, a considerable improvement of transport properties could still be achieved.
Abstract: Old cement mortars attached on recycled concrete aggregates (RCA) seriously affects the durability of the recycled aggregate concrete (RAC). Revealing the variation of the transport properties of cement mortar after subjecting to accelerated carbonation treatment is crucial with respect to understanding the effects of carbonated RCA on the durability of RAC. In this paper, cement mortars were used to study the transport properties after they were treated by accelerated carbonation. The corrosion behavior of steel bars in the concrete incorporating the carbonated RCA was also evaluated. The experimental results indicated that the water absorption and sorptivity, resistance to chloride ion penetration, as well as the bulk electrical conductivity of the cement mortar was decreased after subjecting the accelerated carbonation treatment. Extending the treatment from 1 day to 7 days only resulted in a marginal improvement of the transport properties. A significant decrease in the local porosity at the edge of cement mortar determined by Scanning electron microscopy-backscattered image was observed, agreeing well with the concentrated calcium carbonate (CC) contents detected in the surface layer by TGA. The determination of the spatial gradients of the porosity and carbonates/portlandite contents illustrated that even only with the external layer (0–10 mm) carbonated, a considerable improvement of transport properties of the cement mortar could still be achieved. The corrosion test in the new concrete prepared with the carbonated RCA also confirmed that the corrosion resistance of steel bars in the RAC was improved.
TL;DR: In this article, peptone, yeast extract and Bacillus Subtilis were added as microbial adjuvant in concrete mix design, which led to a decrease in porosity resulting in an increase of strength, dynamic modulus as well as a reduction of water uptake, gas permeability and chloride permeation.
Abstract: Bacterial self-healing is an innovative technology allowing repairing open micro-cracks in concrete by CaCO3 precipitation. This bio-technology improves the durability of the structure. In this paper, peptone, yeast extract and Bacillus Subtilis were added as microbial adjuvant in concrete mix design. This led to a decrease in porosity resulting in an increase of strength, dynamic modulus as well as a reduction of water uptake, gas permeability and chloride permeation. Scanning electron microscopy, energy dispersive spectroscopy and raman spectroscopy showed that the microbial precipitations in the crack were CaCO3. Moreover, the morphology of calcite crystals was a needle-like, bouquet-like and rhombohedral-shaped. At 44 days, 400 μm crack surface width was completely filled. Hence, peptone, yeast extract and Bacillus Subtilis could be considered as a promising concrete admixture in enhancing durability and mechanical properties of concrete. Furthermore it is established that Eurocode 2 could be applied with confidence for predicting properties of bacterial concrete.
TL;DR: In this paper, the effect of using different admixtures on the one-part ambient-cured alkali activated pastes was investigated and the ATR-FTIR test was used to check the admixtures stability in high alkali media.
Abstract: This paper investigates the effect of using different admixtures on the one-part ambient-cured alkali activated pastes. Na2SiO3-Anhydrous powder was used as solid activator along with three superplasticizers including naphthalene, melamine and polycarboxylate with different water/precursor ratios. The flowability, setting time and compressive strength development were evaluated for all AAMs mixes. ATR-FTIR test was used to check the admixtures stability in high alkali media. All types of admixtures significantly improved the flowability of the Na2SiO3-anhydrous one-part AAMs and marginally affected the compressive strength. The results showed that polycarboxylate was effective for high water/precursor (≥0.36) while naphthalene performed better in case of low water/precursor (≤0.36) for Na2SiO3-anhydrous one-part AAMs. However, lowering the water/precursor ratio in the Na2SiO3-anhydrous one-part AAMs using superplasticizers showed no improvement or modest improvement in the compressive strength. The ATR-FTIR results revealed the relatively high stability of admixtures in Na2SiO3 alkali medium of one-part AAMs (for w/b = 0.4).
TL;DR: In this article, the axial stress-strain behavior of macro-synthetic fiber reinforced recycled aggregate concrete was investigated under axial compression, with the variation of three different replacement ratios of recycled aggregates and three different dosages of macro polypropylene fibers.
Abstract: This study aims to investigate the axial stress-strain behavior of macro-synthetic fiber reinforced recycled aggregate concrete. Concrete cylinders reinforced with macro-synthetic fibers were tested under axial compression, with the variation of three different replacement ratios of recycled aggregates (i.e., 0, 50 and 100%) and three different dosages of macro polypropylene fibers (i.e., 0, 0.5 and 1% of volume of recycled aggregate concrete). A comparative study of the existing stress-strain models for steel fiber reinforced normal and recycled aggregate concrete with the test results indicates that the stress-strain behavior of steel fiber reinforced normal and recycled aggregate concrete can be well predicted by these existing models. No stress-strain model for macro-synthetic fiber reinforced normal and recycled aggregate concrete has been developed. Based on the test results, a stress-strain model is developed in this work by modifying the parameters of best performing stress-strain model for steel fiber reinforced normal aggregate concrete. The proposed model can effectively predict the stress-strain behavior of both steel and macro-synthetic fiber reinforced normal and recycled aggregate concrete. Test results show that the peak stress, peak strain and ultimate strain of concrete specimens increase with the increase in fiber dosage and the addition of fibers has a better effect on recycled aggregate concrete and as compared to normal aggregate concrete.
TL;DR: In this article, the authors used biogeography-based programming (BBP) as a symbolic regression method to predict the compressive strength of silica fume concrete, while the constrained biogeographic-based optimization (CBBO) was used to estimate its optimal mix design.
Abstract: The use of silica fume in concrete mixtures has been dramatically increased in concrete industry, especially for achieving high strength concrete. An accurate model of estimating the compressive strength and optimal mix design of silica fume concrete can save in time and cost. In this study, the biogeography-based programming (BBP) was used as a symbolic regression method to predict the compressive strength of silica fume concrete, while the constrained biogeography-based optimization (CBBO) was used to estimate its optimal mix design. For this purpose, a comprehensive database was gathered from various published documents. From the collected data, about 75% of all data was employed to train the model, while the rest was used to verify the developed model. The amounts of cement, water, silica fume, coarse aggregate, fine aggregate, superplasticizer, as well as the maximum size of aggregate and concrete age were selected as the effective input variables of the model. The compressive strength of silica fume concrete was considered as the only output variable. The results show that the BBP model can be successfully used for the prediction of the compressive strength of silica fume concrete with acceptable accuracy. In addition, a graphical user interface was designed which allows the users to estimate the optimal mix design of silica fume concrete.
TL;DR: In this paper, the influence of crystalline admixtures on the strength, self-healing, and durability characteristics of concretes with two different cement types (Ordinary Portland Cement [OPC] and Portland Limestone Cement[PLC]).
Abstract: The repair of concrete structures damaged by water or water borne chemicals is estimated to cost billions of dollars annually worldwide. However, the solutions that can make concrete structures more sustainable and durable, are limited. The use of crystalline admixtures (CA) has a potential of improving the durability and reducing permeability of concrete structures especially those exposed to corrosive environments. This paper presents various investigations on the influence of crystalline admixtures on the strength, self-healing, and durability characteristics of concretes with two different cement types (Ordinary Portland Cement [OPC] and Portland Limestone Cement [PLC]). Test methods include the rapid chloride permeability (RCP), surface/bulk electrical resistivity and water permeability tests, self-healing test, compressive strength test and salt ponding test. The results indicate that the water permeability coefficient decreased by 3 times whereas the self-healing ratio increased by a higher rate by adding crystalline admixtures into the concrete mix. This paper presents empirical equations to correlate resistivity, total charge passed, or water permeability with each other. Further, the correlation between the surface and bulk resistivity is strong and the evidence from self-healing test suggests faster sealing of crack widths up to 250 μm for CA treated specimens.
TL;DR: In this paper, the authors used isothermal calorimetry and thermogravimetric analysis to determine the reactivity of supplementary cementitious materials (SCMs) by using isothermal and thermodynamic analysis.
Abstract: Tests to determine the reactivity of supplementary cementitious materials (SCMs) by using isothermal calorimetry and thermogravimetric analysis have been proposed. In one such test, the heat release and calcium hydroxide consumption of SCMs mixed with calcium hydroxide (3:1 ratio of calcium hydroxide and SCM) at 50 °C in a 0.5 M potassium hydroxide environment are measured. In this study, we show the results of such testing for a large variety of SCMs and fillers, ranging from conventional materials such as fly ash, slag, silica fume, quartz, and limestone, to alternative materials such as calcined clays, municipal solid waste incineration fly ash, basic oxygen furnace slag, ground lightweight aggregates, ground pumice, ground glass pozzolan, and basalt fines. A total of 54 SCMs are tested using this approach. Results show that even among SCMs of the same type, there is considerable difference in the heat release and calcium hydroxide consumption, likely due to differences in amorphous content, chemical composition, and fineness, leading to different reactivities. Based on the response in the test, SCMs are classified into inert, pozzolanic, and latent hydraulic; the pozzolanic and latent hydraulic materials are further classified into less reactive and more reactive. The relationship between heat release and calcium hydroxide consumption depends on the chemical composition of the SCMs, and SCMs with high calcium, high alumina, and high silica contents show different relationships (determined by the slope of the heat release vs. calcium hydroxide plot).
TL;DR: In this article, the physical and chemical characteristics of coral aggregates are studied, and the particle shape features are analyzed, and an indicator, namely texture index (TI), is proposed to characterize the surface microstructure of coral aggregate.
Abstract: With the increasing development of oceanic resources, coral aggregate concrete has wide potentials in the construction of islands and reefs, as well as the flood embankment, road and airport in coastal areas. However, the complex particle composition of coral aggregates, shape, surface structure and pores lead to unusual microstructure, workability, mechanical property and durability of resulting concrete. In this paper, the physical and chemical characteristics of coral aggregates are studied, and the particle shape features are analyzed. Quantitative parameters such as sphericity (ψ), angular number (AN), and index of aggregate particle shape and texture (IAPST) are used to characterize the features of aggregates. An indicator, namely texture index (TI), is proposed to characterize the surface microstructure of coral aggregate. The results show that the coral aggregates with rough surface and porous interior have unique tree-shaped and rod-shaped particles and the former accounts for 41.3% of the total weight. Coral aggregates have typical ‘concave hole’ characteristic, porosity of 48.2–55.6% and >12% water absorption. The average sphericity and AN of coral aggregate are 0.5–0.6 and 27.5–30.3, respectively. At the same particle size, the ψ of natural limestone aggregates is significantly larger than that of coral aggregates. The AN of coral aggregates is 2.4–3.0 times larger than that of limestone aggregates at a single grain size. At the same single-grain level, the IAPST of coral aggregates and natural limestone aggregates are between 31.6-34.3 and 18.6–19.6, respectively. The TI of the coral aggregates of 4.75–16 mm and 16–31.5 mm are 16.2 and 15.9, while the limestone aggregates are only 1.22 and 1.17, respectively. Compared with IAPST, the use of TI is more suitable to characterize the ‘concave hole’ feature of coral aggregate.
TL;DR: In this article, a new polymer consisting of black carbon and sulfur was used to glue the two layers together, which showed a considerable increase in the interlayer bonding strength and chemical cohesion depicted by scanning electron microscopy.
Abstract: This study focuses on one of the bottlenecks facing the concrete 3D printing technology, the lack of proper bonding between the two adjacent layers of 3D printed concrete. Herein, a new polymer consisting of black carbon and sulfur was used to glue the two layers together. The experimental results, verified via molecular dynamics and density functional theory calculations, showed a considerable increase in the interlayer bonding strength. Two-fold rise in interlayer tensile strength as well as chemical cohesion depicted by scanning electron microscopy proves this approach to be successful in providing enhanced bonding between two adjacent printed mortar layers without hindering the printing process. The improvement arises from different types of forces in the interlayer region of modified samples, compared to that of the interlayer region of original sample. The uniform surface provided by the hardened polymer is a good substrate for the top layer in addition to extending the time gap between printing layers. This novel method can accelerate the automation of the construction industry, while reducing the costs in terms of both human labor and capital.
TL;DR: In this paper, the tensile properties of all-grade strain hardening cementitious composite (SHCC) from normal strength to high strength were investigated, and a performance-based design concept was proposed for designing SHCC based on the required mechanical and crack pattern properties, corresponding to the ultimate limit state and serviceability limit state.
Abstract: A systematic study of the tensile properties of all-grade strain hardening cementitious composite (SHCC) from normal strength to high strength was conducted in the present research. The compressive strengths of the cylinder specimens ranged from 43 MPa to 115 MPa. Different combinations of fiber volume fractions and fiber aspect ratios (fiber length/diameter) were employed to investigate the effects of the fiber reinforcement index (the product of fiber volume fraction and fiber aspect ratio) on the tensile properties of SHCC, including mechanical properties and crack patterns. The relationships between the fiber reinforcement index and tensile properties were established at all compressive strengths and compared with the previous experimental values. Moreover, a performance-based design concept was proposed for designing SHCC based on the required mechanical and crack pattern properties, corresponding to the ultimate limit state and serviceability limit state, respectively.
TL;DR: In this article, the authors examined the implications of the fiber type and the distance between fibers on the pullout behavior of steel fibers embedded in ultra-highperformance concrete (UHPC).
Abstract: This study examines the implications of the fiber type and the distance between fibers on the pullout behavior of steel fibers embedded in ultra-high-performance concrete (UHPC). For this, three different types of steel fibers, i.e., straight, hooked, and twisted, and four different distances between fibers, corresponding to fiber volume fractions of 1%, 2%, and 7% and a fiber bundle, were considered. To evaluate the effect of the distance between fibers, four individual fibers were included in a single dog-bone specimen, and a single fiber specimen was also fabricated and tested as a control specimen. Test results indicate that the twisted steel fiber exhibited the greatest pullout resistance, followed by the hooked and straight steel fibers. Approximately 30% lower bond strengths were obtained for the specimens with multiple fibers as compared to those with a single fiber, regardless of the fiber type and distance between fibers. The average bond strengths of the hooked and twisted steel fibers were improved by decreasing the distance between fibers up to 1 mm, corresponding to a volume fraction of 7%, while the bundled fiber specimens provided the poorest pullout resistance in terms of bond strength and energy absorption capacity for all types of fibers. The reduction rate of pullout resistance was the most significant for the straight fiber, relative to the hooked and twisted fibers. Minor matrix damage was obtained for the straight fiber specimen, and its pullout performance was not influenced by the surrounding fibers. In contrast, severe matrix damage was observed for the hooked and twisted fibers, and they were overlapped, causing a larger spalling area with a closer fiber distance.
TL;DR: In this article, the individual and combined effects of polypropylene (PP) fibers, steel fibers, and aggregate size on spalling behavior and pore pressure build-up of ultra-highperformance concrete (UHPC) exposed to elevated temperature were investigated.
Abstract: This paper investigates the individual and combined effects of polypropylene (PP) fibers, steel fibers, and aggregate size on spalling behavior and pore pressure build-up of ultra-high-performance concrete (UHPC) exposed to elevated temperature. Simultaneous measurements of pore pressure and temperature were conducted at different depths in UHPC specimens under one-sided heating with a heating rate of 2 °C/min. Compressive, tensile, and permeability tests were performed to analyze spalling behavior. Addition of PP fibers fully prevented spalling and they are much more effective in increasing permeability than steel fibers and larger aggregates. The combined use of PP and steel fibers, and PP fibers and larger aggregates showed strong synergistic effect on increasing permeability. The higher the permeability, the lower was the maximum pore pressure measured in the samples. Two plateaus were observed from the temperature history due to vaporization of liquid water (between 115 and 125 °C inside the specimens) and release of water vapor (starting from 180 °C), respectively. The second plateau was identified as the functional temperature of PP fibers. Maximum pore pressures in spalled specimens were much lower than their tensile strengths, which could imply the contribution of hydraulic pressure in the region of moisture clog on spalling.
TL;DR: In this paper, the authors examined the chloride binding of hydrated cement pastes containing fly ash exposed to CaCl2 solutions of varying concentrations at 5 and 23 degrees C. They found that the amount of bound chloride by Friedel's salt is relatively independent of the exposure temperature, and as the chloride concentration [Cl−] increases, it increases until a plateau is reached at [Cl −] greater than 2
Abstract: In cementitious materials exposed to solutions containing chloride, chloride binding typically results from the chemical reactions between chloride ions and aluminate phases to form Friedel's salt, and the interaction between chloride ions and calcium silicate hydrates (C S H). Calcium oxychloride can also form when Ca(OH)2 in cementitious materials reacts with CaCl2 solutions. This paper examines the chloride binding of hydrated cement pastes containing fly ash exposed to CaCl2 solutions of varying concentrations at 5 and 23 °C. Thermogravimetric analysis was used to quantify the chloride binding associated with Friedel's salt and calcium oxychloride. The amount of bound chloride by Friedel's salt is relatively independent of the exposure temperature, and as the chloride concentration [Cl−] increases, it increases until a plateau is reached at [Cl−] greater than 2 M. The addition of fly ash results in an increase in the chloride binding due to Friedel's salt. A lower exposure temperature leads to a greater amount of bound chloride associated with calcium oxychloride. In this study, no chloride binding associated with calcium oxychloride was observed in the cement pastes with 40% and 60% fly ash. The temperature-dependent chloride binding associated with C S H is a significant portion of the total chloride binding (19.8 %–70.8%) when cement pastes are exposed to CaCl2 solutions. As the replacement level of fly ash increases, the chloride binding by C S H increases first and then decreases. The amount of bound chloride by C S H increases linearly as the pH of the exposure solution decreases.
TL;DR: In this paper, different content of Ca(OH)2 were adopted to prepare calcium containing geopolymers, and compressive strength and hydrochloric acid attack tests were conducted to evaluate the effect of calcium content on the macro-performances.
Abstract: Fly ash-based geopolymer, regarded as a eco-friendly cementitious material instead of ordinary Portland cement, has been rapidly developed and applied in practical engineering practice. Previous researches demonstrated that calcium component produces distinct effect on the formation of polymer gel products, and therefore influencing the macroscopic mechanical behaviors and microstructure of geopolymers. However, the influence mechanism of calcium component on the formation of gels product is still not clear. In this study, different content of Ca(OH)2 were adopted to prepare calcium containing geopolymers. The compressive strength and hydrochloric-acid attack tests were conducted to evaluate the effect of calcium content on the macro-performances. Then, scanning electron microscopy-energy spectrum test (SEM-EDS) was carried out to accesses the morphology and elemental components of the prepared composites. Thereafter, the microstructure of gels product was probed through Fourier transform infrared (FTIR) spectroscopy and 29Si nuclear magnetic resonance (NMR) spectroscopy. The critical value of elemental ratio (Na + K + Ca)/Al to characterize the gel product was specified. Two kinds of fly ash and two alkali-activated solutions were adopted to verify the results. The geopolymerization products will be calcium-containing geopolymer gels (C,N-A-S-H) when (Na + K + Ca)/Al≤0.95, while be coexist form of C-S-H and N-A-S-H gels when (Na + K + Ca)/Al>0.95. The results provide experimental basis and references for the application of calcium-containing solid wastes in geopolymer materials.
TL;DR: In this paper, a coupled CO2-water curing regime was employed on Ordinary Portland cement (OPC) paste samples immediately after casting, which allowed carbonation and hydration of OPC to proceed simultaneously.
Abstract: A coupled CO2-water curing regime was employed on Ordinary Portland cement (OPC) paste samples immediately after casting, which allowed carbonation and hydration of OPC to proceed simultaneously. The strength development and microstructural evolution was evaluated by using multiple-techniques. The results indicated that, compared to the normal hydrated counterpart, a lower porosity, higher amorphous phase content and overall reaction degree can be achieved in the coupled CO2-water cured OPC sample. By combining with the morphological observations, a new mechanism was proposed for the rapid hardening of OPC. It is shown that the carbonation reactions led to the formation of calcite particles, which provided more nucleating sites for C S H gel growth; and thus, an increase in the overall reaction degree of the cement paste can be achieved within the first 24 h compared to the conventional water curing process.
TL;DR: In this paper, a surface modification method for steel fibers is proposed to improve the interfacial properties of similar fibers in a cement-based composite, which reduces the possibility of a weak zone and creates denser microstructure at the fiber-matrix interface.
Abstract: In this study, a new surface modification method for steel fibers is proposed to improve the interfacial properties of similar fibers in a cement-based composite. Steel fibers were coated with a nano-SiO2 multilayer film, which was expected to react with Ca(OH)2 formed from cement hydration, reducing the possibility of a weak zone and creating a denser microstructure at the fiber-matrix interface. Several techniques, including scanning electron microscopy (SEM), backscattered scanning electron microscopy (BSEM), energy dispersive spectroscopy (EDS), and infrared absorption spectroscopy (IR) were used to characterize the interfacial microstructure improvement resulting from the surface modification at the micro-scale; in addition, a fiber pull-out test was carried out to evaluate the enhancement of the mechanical properties of the interface at the macro-scale. The experimental results indicate that the coated nano-SiO2 can react with Ca(OH)2, resulting in an increased quantity of C–S–H gel and decreased porosity at the modified steel fiber-cement interface. The dense adhesion between the modified fibers and cement matrix facilitate the establishment of a stronger interfacial bond. Both the bond strength and pull-out energy increase significantly at 3, 7, and 28 days. The results support the conclusion that the proposed surface modification method is suitable for steel fibers and will improve their reinforcement efficiency.