| 1. |
- Badanoiu, Alina, et al.
(författare)
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Cementitious composites reinforced with continuous carbon fibres for strengthening of concrete structures
- 2003
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Ingår i: Cement & Concrete Composites. - 0958-9465 .- 1873-393X. ; 25:3, s. 387-394
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Tidskriftsartikel (refereegranskat)abstract
- Continuous fibre wrapping technique is becoming a common repair and strengthening technique for concrete structures. In this method continuous fibre sheets are bonded to the surface of a concrete structure using organic resins. The main disadvantages of this method are: (i) the use of resins (organic compounds) which are hazardous for the manual worker, and (ii) the low permeability of this type of organic materials. These problems could be solved if a cement-based matrix is used instead of epoxy matrix. In the current development stage one of the limiting parameter in a cementitious carbon fibre composite is the bond between the fibres and the cementitious matrix. The main objective of the study presented in this paper was to improve the bond between the carbon fibres and the cementitious matrix. Two possible methods were studied: (i) the design of a complex binder based on Portland cement with polymer and silica fume additions and (ii) surface treatment of the carbon fibres prior to their casting in the cementitious composite.
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| 2. |
- Bado, Mattia Francesco, et al.
(författare)
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Characterization of concrete shrinkage induced strains in internally-restrained RC structures by distributed optical fiber sensing
- 2021
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Ingår i: Cement and Concrete Composites. - : Elsevier BV. - 0958-9465. ; 120
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Tidskriftsartikel (refereegranskat)abstract
- The present paper reports the result of an inter-university experimental investigation on concrete shrinkage induced strains on embedded rebars instrumented with Distributed Optical Fiber Sensors (DOFS). The monitoring was performed for a standard 28 days drying time and for a shorter 6 days time span (reflecting realistic constructions schedules accelerations to meet set deadlines). The tested specimens were Reinforced Concrete (RC) tensile members differing in their geometry, DOFS employed and fiber/rebar bonding techniques. Regarding the latter, a combination of cyanoacrylate (for gluing) and silicone (protection) was found to be the optimal one for deployments inside RC structures. The DOFS-reported combined effect of concrete shrinkage and creep on the embedded rebars is compared with the Model Code 2010's predictions and employed to extract conclusions on the residual performance of the RC members at the end of their drying phase.
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| 3. |
- Berrocal, Carlos Gil, 1986, et al.
(författare)
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Electrical resistivity measurements in steel fibre reinforced cementitious materials
- 2018
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Ingår i: Cement and Concrete Composites. - : Elsevier BV. - 0958-9465. ; 89, s. 216-229
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Tidskriftsartikel (refereegranskat)abstract
- This paper reports results from experiments aimed at better understanding the influence of fibre dosage and fibre geometry on the AC frequency needed to determine the DC resistivity of cementitious materials containing steel fibres. Impedance spectroscopy and DC galvanodynamic measurements were performed on mortar prisms with varying fibre reinforcement to determine the matrix resistivity (related to ionic current within the pore solution) and composite resistivity (accounting for both ionic current and electronic current through the fibres). The results showed that adding steel fibres did not significantly affect the DC nor the AC matrix resistivity of the mortar prisms. However, the steel fibres yielded a drastic reduction of the frequency associated to the AC matrix resistivity from ∼1 kHz in plain mortar to ∼1 Hz in steel fibre reinforced mortar. These findings revealed the need to adequately adjust the frequency in AC resistivity measurements of steel fibre reinforced cementitious materials.
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| 4. |
- Buasiri, Thanyarat, et al.
(författare)
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Sensing mechanisms of nanomodified Portland cement composites
- 2024
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Ingår i: Cement & Concrete Composites. - : Elsevier. - 0958-9465 .- 1873-393X. ; 151
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Tidskriftsartikel (refereegranskat)abstract
- Mortar sensors were fabricated as beams incorporating different amounts of carbon nanofibers (CNFs) synthesized in-situ on cement particles. Changes in electrical resistivity were measured and compared to recorded changes in compressive stress, temperature, and humidity. Sensing mechanisms and corresponding models were developed. The findings of the study indicate that the piezoresistive effect is influenced by the critical concentration of CNFs inside the composite matrix and the tunneling effect. In addition, water absorption and desorption, as well as the amount of chemically bound water played an important role in humidity sensing. Thermal fluctuation-induced tunneling conduction was dominant for the temperature sensitivity.
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| 5. |
- Cui, Hongzhi, et al.
(författare)
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Possible pitfall in sample preparation for SEM analysis - A discussion of the paper “Fabrication of polycarboxylate/graphene oxide nanosheet composites by copolymerization for reinforcing and toughening cement composites” by Lv et al.
- 2017
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Ingår i: Cement and Concrete Composites. - : Elsevier BV. - 0958-9465. ; 77, s. 81-85
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a discussion of the paper published by Lv et al. in Cement and Concrete Composites, 2016, 66: 1–9. The discussion is mainly focused on whether or not graphene oxide (GO) nanosheets can regulate formation of flower-like cement hydration crystals. Lv et al. in their paper proposed a regulation mechanism stating that GO nanosheets can control ettringite (AFt), monosulfate (AFm) and calcium hydroxide (CH) to form the flower-like and polyhedron crystals, whilst our experimental results show that there might be a possible pitfall in sample preparation for SEM analysis. It is here suggested that the main chemical components of flower-like and polyhedron crystals are calcium carbonates, which are not the products from cement hydration but from carbonation of cementitious hydrates. It is therefore suggested that further study is needed to verify the regulation mechanism of GO on cement hydration crystals proposed by Lv et al. in the discussed paper.
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| 6. |
- Falchi, Laura, et al.
(författare)
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The influence of water-repellent admixtures on the behaviour and the effectiveness of Portland limestone cement mortars
- 2015
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Ingår i: Cement & Concrete Composites. - : Elsevier. - 0958-9465 .- 1873-393X. ; 59, s. 107-118
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Tidskriftsartikel (refereegranskat)abstract
- Abstract Water-repellent mortars were prepared using different hydrophobic compounds as admixtures. Calcium and zinc stearates, silane/siloxane products (as liquid solution and powder) were mixed into limestone cement mortars for obtaining in-bulk water-repellent mortars suitable for building protection and resistant to the degrading action of water. The influences of the admixtures on the hydration and structure of the designed mortars were investigated by SEM, TG–DSC, FT-IR, XRD, and isothermal calorimetry. The effectiveness of these agents against water action was evaluated by using techniques and methods such as mercury intrusion porosimetry, water absorption tests and contact angle measurements. Siloxane products conveyed good water-repellent effectiveness, without strongly influencing the setting and hydration of the binder, while the zinc stearates slowed down the hydration reactions.
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| 7. |
- Hallgren, Mikael, et al.
(författare)
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Non-linear finite element analyses of punching shear failure of column footings
- 2002
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Ingår i: Cement & Concrete Composites. - 0958-9465 .- 1873-393X. ; 24:6, s. 491-496
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Tidskriftsartikel (refereegranskat)abstract
- Current design methods and code formulas for the assessment of the punching shear strength are normally based on tests on slabs with relatively high slenderness, i.e., with high shear-span to depth ratios. Column footings normally have low shear-span to depth ratios. Previous punching tests on column footings indicate that the failure mechanism for punching of slabs with low shear-span to depth ratios differs from that of slabs with high shear-span to depth ratios. In this investigation, punching tests on two circular column footings of reinforced concrete were simulated numerically. The results show how the failure mechanism differs from that of more slender slabs. A parametric study also confirms that the punching shear strength of the analysed slabs strongly depends on the compressive strength of concrete.
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| 8. |
- Hong, Shuxian, et al.
(författare)
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Determination of impressed current efficiency during accelerated corrosion of reinforcement
- 2020
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Ingår i: Cement and Concrete Composites. - : Elsevier BV. - 0958-9465. ; 108
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Tidskriftsartikel (refereegranskat)abstract
- Impressed current technique is a widely adopted method for accelerating corrosion of steel reinforcement. The mass loss of steel can be theoretically estimated by Faraday's law with the assumption that all of the impressed current participated in the corrosion reaction. In the present study, accelerated corrosion process of reinforcing steel with impressed current was non-destructively traced by X-ray micro-computed tomography (μCT) with high accuracy. The mass loss of steel at different accelerated corrosion periods was analyzed and the efficiency of impressed current was calculated. It was found that the mass loss of steel did not always follow Faraday's law, and impressed current efficiencies varied with different corrosion periods. In the early corrosion periods, the water electrolysis occurred and lowered the current efficiency due to the high anode potential. While in the late corrosion periods, the natural corrosion caused the current efficiency to exceed 100%.
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| 9. |
- Huang, Liming, 1993, et al.
(författare)
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Real-time monitoring the electrical properties of pastes to map the hydration induced microstructure change in cement-based materials
- 2022
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Ingår i: Cement and Concrete Composites. - : Elsevier BV. - 0958-9465. ; 132
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Tidskriftsartikel (refereegranskat)abstract
- The effect of the supplementary materials (SCMs) on the moisture content and ion diffusivity at different hydration time is important for the service life modelling of modern concrete. This study designed a simple but valid method to monitor the microstructure change in pastes during hydration. A procedure easy to implement was proposed to detect the water content in pastes. The electrical conductivity of pore solution was evaluated by the evaporable water content in pastes and chemical composition in the binders. Results show that the electrical properties of pastes (conductivity, formation factor and its growth rate) can effectively indicate the hydration reactivity of binder, pore connectivity and volume of pore solution in the hardened pastes. The effect of waterbinder ratio and SCMs on the structure of pastes are effectively indexed by the formation factor which is the conductivity of pore solution divided by that of paste. The inflection point of average growth rate of formation factor is a good index for the final setting of pastes. The relation between volume of evaporable water and formation factor is well demonstrated by the extended percolation theory. The real-time monitored electrical conductivity and formation factor of pastes can be used to calculate the chloride migration coefficient in hardened cement pastes.
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| 10. |
- Johannesson, Björn
(författare)
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Dimensional and ice content changes of hardened concrete at different freezing and thawing temperatures
- 2010
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Ingår i: Cement & Concrete Composites. - : Elsevier BV. - 0958-9465 .- 1873-393X. ; 32:1, s. 73-83
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Tidskriftsartikel (refereegranskat)abstract
- Samples of concrete at different water-to-cement ratios and air contents subjected to freeze/thaw cycles with the lowest temperature at about -80 degrees C are investigated. By adopting a novel technique, a scanning calorimeter is used to obtain data from which the ice contents at different freeze temperatures can be calculated. The length change caused by temperature and ice content changes during test is measured by a separate experiment using the same types of freeze-thaw cycles as in the calorimetric tests. In this way it was possible to compare the amount of formed ice at different temperatures and the corresponding measured length changes. The development of cracks in the material structure was indicated by an ultra-sonic technique by measuring on the samples before and after the freeze-thaw tests. Further the air void structure was investigated using a microscopic technique in which air'bubble' size distributions and the so-called spacing factor, indicating the mean distance between air bubbles, were measured. By analyzing the experimental result, it is concluded that damages occur in the temperature range of about -10 degrees C to -55 degrees C, when the air content is lower than about 4% of the total volume. For a totally water-saturated concrete, damages always occur independently of the use of entrained air or low water-to-cement ratios. It is, further, concluded that the length changes of these samples correspond to the Calculated ice contents at different temperatures in a linear fashion. @ 2009 Elsevier Ltd. All rights reserved.
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