245 results found
Muslim F, Wong H, Choo TH, et al., 2021, Influence of supplementary cementitious materials on microstructure and transport properties of spacer-concrete interface, Cement and Concrete Research, Vol: 149, Pages: 1-14, ISSN: 0008-8846
Reinforcement spacers are a critical component of concrete structures. Their presence affects microstructure and transport properties of concrete cover though this is not widely appreciated. This paper presents the first study to determine whether the negative effects of spacers can be mitigated through the use of supplementary cementitious materials such as silica fume, fly ash and blast-furnace slag. Concrete samples (>200) with different spacers, binders, curing and drying regimes were prepared and tested for diffusion, permeation, absorption, electrical conductivity, carbonation and microstructure. It was found that spacers increase all transport properties, the extent depending on type of spacer, drying regime and transport mechanism. The spacer-concrete interface is weak, porous and micro-cracked, and this lowers the resistance of concrete to ingress of aggressive agents. The beneficial effects of SCMs (strength enhancement and densification) and prolonged curing (120-day) are insufficient to overcome the negative effects of spacers. Implications for durability are discussed
Yio MHN, Mac MJ, Yeow YX, et al., 2021, Effect of autogenous shrinkage on microcracking and mass transport properties of concrete containing supplementary cementitious materials, Cement and Concrete Research, ISSN: 0008-8846
Muslim F, Wong HS, Chiu TKQ, et al., 2021, Improving bond strength and mass transport properties of spacer-concrete interface with textured spacers, Materials and Structures, ISSN: 1359-5997
Spacers areimportant devices used in all concrete structures to support reinforcing steel and achieve the required cover. However, spacers induce a weak, porous andmicrocracked interface that facilitates ingress of aggressive agents. This paper aims to address the problem by developing a method to produce cementitious spacers with a range of small-scale surface textures including grooves, squares, rectangles, hemispheres and pyramids.The textured spacers were cast in Portland cement mortar or concrete, cured up to 28 days, and tested for tensile bond strength, accessible porosity and mass transport properties. The properties were correlated to surface characteristics to establish the effects of texture on spacer-concrete interface. Results show that textured spacers promote mechanical interlocking with concrete, thereby increasing bond strength, resistance against microcracking and pressure-induced flow. The use of certain textures can compensatefor the negative effects of spacers, achieving similar bond strength and permeability to monolithic concrete without the interface.
Mac M, Yio MHN, Wong H, et al., 2021, Analysis of autogenous shrinkage-induced microcracks in concrete from 3D images, Cement and Concrete Research, Vol: 144, Pages: 1-15, ISSN: 0008-8846
A new image analysis procedure for quantifying microcracks from three-dimensional (3D) X-ray microCT images of concrete is presented. The method separates microcracks from air voids and aggregates by combining filtering and morphological operations. It was applied to study the effects of supplementary cementitious materials (SCMs) and curing age on autogenous shrinkage-induced microcracks in low w/b ratio concretes, and to determine the representative elementary volume (REV) for various properties of microcracks and air voids. Results showed that slag and silica fume significantly increased autogenous shrinkage and related microcracking. These SCMs increased volume fraction, width, length, dendritic density, anisotropy, and connectivity of microcracks, but decreased specific surface and tortuosity. Similar trends were observed with age. Comparison between 3D and 2D measurements was made. REV analysis showed that a sampling volume of ~20 × 20 × 25 mm3 is sufficient for characterising most parameters of autogenous shrinkage microcracks and air voids in concrete.
Mac M, Yio M, Desbois G, et al., 2021, 3D imaging techniques for characterising microcracks in cement-based materials, Cement and Concrete Research, Vol: 140, ISSN: 0008-8846
Concrete inherently contains pores and microcracks that can adversely impact its mechanical properties and long-term durability. However, characterising microcracks is difficult due to their complex, multiscale and three-dimensional (3D) nature. This paper presents an evaluation of 3D imaging techniques for characterising microcracks induced by different mechanisms. Seven cement pastes, mortars and concretes subjected to drying shrinkage, autogenous shrinkage and freeze-thaw cycles were investigated using focused ion beam nanotomography (FIB-nt), broad ion beam serial section tomography (BIB-SST), laser scanning confocal microscopy (LSCM) combined with serial sectioning and X-ray microtomography (μCT). The study shows that the characteristics of microcracks vary significantly depending on exposure conditions. Yet there is no single technique that can capture the entire size range of microcracks from sub to tens of μm within a sufficiently representative sampling volume. The achievable image volume and resolution, and the advantages and disadvantages of each technique are compared and discussed.
Muslim F, Wong H, Cheng G, et al., 2020, Combined effects of vertical spacers and segregation on mass transport properties of reinforced concrete, Materials and Structures, Vol: 53, ISSN: 1359-5997
All concrete structures contain reinforcement spacers, and deep sections can be affected by bleeding and segregation without displaying visible indications during casting. However, their effects on mass transport and long-term durability are not well studied. In this paper, reinforced concrete columns were prepared with plastic and cementitious spacers to achieve 50 mm cover, and compacted at different vibration frequencies and durations. 28d cured samples were extracted along the height, conditioned to equilibrium (21 °C, 75% RH or 50 °C, 7% RH), and then subjected to water absorption, electrical conduction, epoxy impregnation and fluorescence imaging. Samples from the top of the column consistently gave higher accessible porosity and mass transport compared to samples from the bottom. Presence of spacers caused additional increases in mass transport because of preferential flow through the spacer-concrete interface which is more porous and microcracked compared to bulk concrete farther away. Image analysis on cross-sections showed that the columns experienced some aggregate segregation despite care taken to avoid over-compaction. The resistance of concrete to ingress of aggressive agents decreases with increasing height due to the combined negative effects of reinforcement spacers and segregation.
Wong H, Poole AB, Wells B, et al., 2020, Microscopy techniques for determining water-cement (w/c) ratio in hardened concrete: A round-robin assessment, Materials and Structures, Vol: 53, ISSN: 1359-5997
Water to cement (w/c) ratio is usually the most important parameter specified in concrete design and is sometimes the subject of dispute when a shortfall in concrete strength or durability is an issue. However, determination of w/c ratio in hardened concrete by testing is very difficult once the concrete has set. This paper presents the results from an inter-laboratory round-robin study organised by the Applied Petrography Group to evaluate and compare microscopy methods for measuring w/c ratio in hardened concrete. Five concrete prisms with w/c ratios ranging from 0.35 to 0.55, but otherwise identical in mix design were prepared independently and distributed to 11 participating petrographic laboratories across Europe. Participants used a range of methods routine to their laboratory and these are broadly divided into visual assessment, measurement of fluorescent intensity and quantitative backscattered electron microscopy. Some participants determined w/c ratio using more than one method or operator. Consequently, 100 individual w/c ratio determinations were collected, representing the largest study of its type ever undertaken. The majority (81%) of the results are accurate to within ± 0.1 of the target mix w/c ratios, 58% come to within ± 0.05 and 37% are within ± 0.025. The study shows that microscopy-based methods are more accurate and reliable compared to the BS 1881-124 physicochemical method for determining w/c ratio. The practical significance, potential sources of errors and limitations are discussed with the view to inform future applications.
Yio MHN, Wong H, Buenfeld N, 2019, 3D pore structure and mass transport properties of blended cementitious materials, Cement and Concrete Research, Vol: 117, Pages: 23-37, ISSN: 0008-8846
The effect of supplementary cementitious materials on three-dimensional pore structure and how this influences mass transport properties are not well understood. This paper examines the effect of silica fume, fly ash and ground granulated blastfurnace slag on 3D structure of capillary pores (>0.24 μm) within 1003 μm3 cement paste for the first time using laser scanning confocal microscopy, combined with backscattered electron imaging and mercury intrusion porosimetry. Pastes containing different binder types, w/b ratios and curing ages were tested. Results show that SF enhances 3D pore structure from early ages whereas PFA and GGBS show improvements at later ages. SCMs not only reduce the volume and size of accessible pores, but also decrease connectivity and increase tortuosity, pore coordination number and formation factor. Measured 3D pore parameters were used as modelling inputs to estimate diffusivity and permeability. Predictions to within a factor of five from measured values were obtained.
Wu Z, Wong H, Chen C, et al., 2019, Anomalous water absorption in cement-based materials caused by drying shrinkage induced microcracks, Cement and Concrete Research, Vol: 115, Pages: 90-104, ISSN: 0008-8846
This paper concerns understanding the influence of drying induced microcracking on water absorption by capillary suction. Paste, mortar and concrete samples with different binder type, w/b ratio, thickness, aggregate size, and curing age were tested. Samples were subjected to gentle stepwise drying at 21 °C/93% → 55% RH, or drying at 21 °C/55% RH, 21 °C/0% RH, 50 °C or 105 °C to induce microcracks <100 μm wide. Results show that the presence of microcracks causes cumulative water absorption to scale non-linearly with . The observed relationship is approximately sigmoidal/S-shaped, with the position of inflection point related to microcracking and the degree of non-linearity increasing with drying severity. A simple fluorescence imaging method was developed to enable continuous monitoring of the advancing wetting front and to study the effect of microcracks. Quantitative image analysis of water penetration produced results consistent with gravimetric measurements.
Lee D, Wong HS, Buenfeld NR, 2018, Effect of alkalinity and calcium concentration of pore solution on the swelling and ionic exchange of superabsorbent polymers in cement paste, Cement and Concrete Composites, Vol: 88, Pages: 150-164, ISSN: 0958-9465
Swelling kinetics of superabsorbent polymers (SAP) in fresh concrete is complex, but its understanding is crucial for optimisation in practical applications. In this study, the effect of concentration of ions common in pore solution (Na+, K+, Ca2+, Cl−, OH−, SO42−) and cyclic wetting/drying on the swelling and ionic exchange of poly(AA) and poly(AA-co-AM) were investigated. Results show that swelling is not a simple function of concentration or ionic strength. In cement paste, SAP absorbs Ca2+ and releases its counterion (Na+, K+) into pore solution. Ca2+ binds with SAP and decreases initial swelling, but the bound Ca2+ can be displaced and swelling gradually recovers. Swelling increases with increase in alkalinity, but decreases with increase in calcium concentration. The higher the degree of ion exchange, the lower the swelling of SAP. Poly(AA) is more susceptible to Ca2+ complexation and therefore achieves a lower swelling ratio and slower swelling recovery compared to poly(AA-co-AM).
Muslim F, Wong HS, Buenfeld NR, 2018, Improving the spacer-concrete interface for bond strength and durability, Pages: 669-675
Spacers are important devices in reinforced concrete that are used to support reinforcing steel during concreting in order to achieve the required concrete cover. They are placed at every meter length or less of steel reinforcement and left permanently in the structure. However, it has been shown that the interface between spacer and concrete is highly porous and microcracked. This lowers the resistance of the concrete cover to the ingress of aggressive agents causing degradation. This study aims to address this problem by improving spacer design to enhance bond strength and durability of the spacer-concrete interface. Cementitious spacers with a range of surface textures were produced prior to casting into concrete. Samples were prepared with CEM I Portland cement at a water/cement (w/c) ratio of 0.4 and cured for 1, 7, and 28 days in a fog room and then conditioned at 50°C to equilibrium moisture content. The spacer-concrete interface was then tested for tensile bond strength and mass transport properties including oxygen diffusivity, oxygen permeability, and water absorption. The measured surface properties were correlated to the measured bond strength and transport properties to establish the effects of surface texture on the spacer-concrete interface.
Yio MHN, Wong HS, Buenfeld NR, 2017, Representative elementary volume (REV) of cementitious materials from three-dimensional pore structure analysis, Cement and Concrete Research, Vol: 102, Pages: 187-202, ISSN: 0008-8846
The representative elementary volume (REV) is a fundamental property of a material, but no direct measurements exist for cementitious materials. In this paper, the REV of cement pastes with supplementary cementitious materials (GGBS, PFA, SF) was determined by analysing the three-dimensional pore structure (> 0.2 μm) using laser scanning confocal microscopy (LSCM). The effect of axial distortion inherent to LSCM on 3D pore structure was also investigated. A range of 3D pore parameters was measured using skeletonisation, maximal ball and random walker algorithms. Results show that axial distortion has insignificant effects on most parameters except Euler connectivity, average pore and throat volumes and directional diffusion tortuosities. Most pore parameters become independent of sampling volume at ≈ 603 μm3 except diffusion tortuosities and formation factor. The REV for porosity calculated based on a statistical approach at eight realisations and 5% relative error was found to be ≈ 1003 μm3.
Wu Z, Wong HS, Buenfeld NR, 2017, Transport properties of concrete after drying-wetting regimes to elucidate the effects of moisture content, hysteresis and microcracking, Cement and Concrete Research, Vol: 98, Pages: 136-154, ISSN: 0008-8846
Drying and wetting induce a number of microstructural changes that could impact transport properties and durability of concrete structures, but their significance is not well-established. This research examines pastes, mortars and concretes with different w/b ratios, binders, aggregate sizes, curing and conditioning regimes. 50 mm thick samples were dried to equilibrium at either 105 °C, 50 °C/7% RH, 21 °C/33% RH or gentle stepwise at 21 °C/93% RH → 3% RH, and then rewetted stepwise by humidification at 21 °C/33% RH → 86% RH and full saturation to produce varying degrees of damage and moisture content. Oxygen diffusivity and permeability, electrical conductivity, microcracking, accessible and total porosity were measured at different conditioning stages over 3-year period to better understand the effects of shrinkage, hysteresis and drying-induced damage on transport properties. The effect of supplementary cementitious materials (GGBS, SF) and implications of drying-wetting on concrete durability are discussed.
Yio M, 2017, Characterising the microstructure of cement-based materials using laser scanning confocal microscopy
Although the steel-concrete interface (SCI) is widely recognized to influence the durability of reinforced concrete, a systematic overview and detailed documentation of the various aspects of the SCI are lacking.In this paper, we compiled a comprehensive list of possible local characteristics at the SCI and reviewed available information regarding their properties as well as their occurrence in engineering structuresand in the laboratory. Given the complexity of the SCI, we suggested a systematic approach to describe it in terms of local characteristicsand their physical and chemical properties. It was found that the SCI exhibits significant spatial inhomogeneity along and around as well as perpendicular to the reinforcing steel. The SCI can differ strongly between different engineering structures and also between different members within a structure; particular differences are expected between structures built before and after the 1970/80s. A single SCI representing all on-site conditions does not exist. Additionally, SCIs in commonlaboratory-made specimens exhibit significant differences compared to engineering structures. Thus, results from laboratory studies and from practical experience should be applied to engineering structures with caution. Finally, recommendations for further research are made.
Wong HS, Muslim F, Buenfeld N, 2016, Microstructure of interface between reinforcement spacer and concrete incorporating supplementary cementitious materials, 16th Euroseminar on Microscopy Applied to Building Materials
Yio MHN, Wong HS, Buenfeld NR, 2016, 3D Monte Carlo simulation of backscattered electron signal variation across pore-solid boundaries in cement-based materials, Cement and Concrete Research, Vol: 89, Pages: 320-331, ISSN: 0008-8846
Three-dimensional (3D) Monte Carlo simulation was used to study the variation of backscattered electron (BSE) signal across pore-solid boundaries in cement-based materials in order to enhance quantitative analysis of pore structure. The effects of pore size, depth and boundary inclination angle were investigated. It is found that pores down to 1 nm can generate sufficient contrast to be detected. Visibility improves with larger pore size, smaller beam probe size and lower acceleration voltage. However, pixels in shallow pores or near pore boundaries display higher grey values (brightness) than expected due to sampling sub-surface or neighbouring solid material. Thus, cement-based materials may appear less porous or the pores appear smaller than they actually are in BSE images. Simulated BSE images were used to test the accuracy of the Overflow pore segmentation method. Results show the method is generally valid and gives low errors for pores that are 1 μm and greater.
Muslim F, Gu Z, Wong HS, et al., 2016, Effect of reinforcement spacers on mass transport properties of concrete containing supplementary cementitious materials, 36th Cement and Concrete Science Conference
Alzyoud S, Wong HS, Buenfeld NR, 2016, Influence of reinforcement spacers on mass transport properties and durability of concrete structures, Cement and Concrete Research, Vol: 87, Pages: 31-44, ISSN: 1873-3948
Spacers are ubiquitous in reinforced concrete, but their influence on durability is unclear. This paper presents the first study on the effects of spacers on mass transport and microstructure of concrete. Samples with different spacers, cover depths, aggregate sizes, curing ages and conditioning were subjected to diffusion, permeation, absorption and chloride penetration, and to μXRF, BSE microscopy and image analysis. Results show that spacers increase transport in all cases, the magnitude depending on spacer type and transport mechanism. Plastic spacers produced the largest increase, followed by cementitious spacers and then steel chairs. The negative effect is due to a porous spacer-concrete interface that spans the cover where preferential transport occurs. Spacers may seem low value, small and inconsequential, but because they are placed every ≤ 1 m along rebars, their overall effect on ingress of external media is significant. This is not currently recognised by standards or by most practitioners.
Abyaneh SD, Wong HS, Buenfeld NR, 2016, Simulating the effect of microcracks on the diffusivity and permeability of concrete using a three-dimensional model, Computational Materials Science, Vol: 119, Pages: 130-143, ISSN: 0927-0256
Concrete inevitably contains microcracks, but their significance on transport properties and long-termdurability is not well established. This is because of difficulties in isolating and evaluating the effect ofmicrocracks whether by laboratory experiments or computer simulations, owing to their complexheterogeneous nature. In this paper, a three-dimensional numerical approach to simulate mass transportproperties of concrete containing microcracks is presented. The approach is based on finite-element methodand adopts aligned meshing to improve computational efficiency. The mesostructure of concrete isrepresented by aggregate particles that are surface meshed by triangulation and porous cement paste matrixthat are discretised with tetrahedral elements. Microcracks are incorporated as interface elements at theaggregate-paste interface or at the cement paste matrix spanning neighbouring aggregate particles. The mainadvantage of this approach is that the smallest microcracks can be simulated independent of the discretisationsize. The model was first validated by comparing the simulations to available analytical solutions. Then, thediffusivity and permeability of a range of concretes containing different amounts of microcracking withincreasing complexities were simulated. The results are analysed and discussed in terms of the effect ofmicrocrack type (bond, matrix), volume fraction, width, specific surface area and degree of percolation ontransport properties.
Yio MHN, Wong HS, Buenfeld NR, 2016, 3D Pore Structure Characterisation of Blended Cement Pastes using Laser Scanning Confocal Microscopy, Young Researchers’ Forum III, Innovation in Construction Materials
Lee HXD, Wong HS, Buenfeld NR, 2016, Self-sealing of cracks in concrete using superabsorbent polymers, Cement and Concrete Research, Vol: 79, Pages: 194-208, ISSN: 1873-3948
Cracks in concrete can self-heal when exposed to prolonged wetting, but this is limited to narrow cracks. In practice, cracks. > 0.2. mm cause leakage and impair performance of structures. The potential of superabsorbent polymers (SAPs) to self-seal such cracks was investigated via transport experiments, microscopy and modelling. Forty samples containing SAP and through-thickness cracks were subjected to 0.12. wt.% NaCl at 4. m/m pressure gradient to simulate groundwater seepage. Results show that SAP can re-swell and seal cracks, for example in the case of 0.3. mm cracks reducing peak flow rate and total flow by 85% and 98% respectively. Increasing SAP dosage accelerates sealing, but imparts a strength penalty and this limits practical applications. Modelling suggests that the effectiveness of SAP can be enhanced by increasing its re-swelling ratio and particle size, and depressing its initial swelling. These variables increase the SAP exposed in a crack and the gel volume available to seal it.
Yio MHN, Mac MJ, Wong HS, et al., 2015, 3D imaging of cement-based materials at submicron resolution by combining laser scanning confocal microscopy with serial sectioning, Journal of Microscopy, Vol: 258, Pages: 151-169, ISSN: 0022-2720
In this paper, we present a new method to reconstruct large volumes of nontransparent porous materials at submicron resolution. The proposed method combines fluorescence laser scanning confocal microscopy with serial sectioning to produce a series of overlapping confocal z-stacks, which are then aligned and stitched based on phase correlation. The method can be extended in the XY plane to further increase the overall image volume. Resolution of the reconstructed image volume does not degrade with increase in sample size. We have used the method to image cementitious materials, hardened cement paste and concrete and the results obtained show that the method is reliable. Possible applications of the method such as three-dimensional characterization of the pores and microcracks in hardened concrete, three-dimensional particle shape characterization of cementitious materials and three-dimensional characterization of other porous materials such as rocks and bioceramics are discussed.
Wu Z, Wong HS, Buenfeld NR, 2015, Influence of drying-induced microcracking and related size effects on mass transport properties of concrete, Cement and Concrete Research, Vol: 68, Pages: 35-48, ISSN: 0008-8846
Microcracking has been suspected of influencing the transport properties and durability of concrete structures, but the nature and extent of this influence is unclear. This paper focuses on the influence of drying-induced microcracking. Samples were prepared with sample thickness/maximum aggregate size (t/MSA) ratios ranging from 2 to 20 and dried to equilibrium at 105 °C or 50 °C/7% RH or 21 °C (stepwise: 93% RH → 55% RH) prior to characterisation of microcracks and transport tests. Results show for the first time that there is a significant size effect on microcracks and transport properties. Samples with smaller t/MSA had more severe microcracking and higher gas permeability. Gas permeability decreased with increasing t/MSA (for a decreasing MSA), and remained constant beyond t/MSA of 10. However, this size effect was not seen on gas diffusivity and sorptivity. The implications of these findings particularly regarding the influence of drying-induced microcracks on the durability of concrete structures are discussed.
Abyaneh SD, Wong HS, Buenfeld NR, 2015, Modelling the Effect of Microcracks on the Transport Properties of Concrete in Three Dimensions, 10th International Conference on Mechanics and Physics of Creep, Shrinkage, and Durability of Concrete and Concrete Structures (CONCREEP), Publisher: AMER SOC CIVIL ENGINEERS, Pages: 377-386
Wu Z, Wong HS, Buenfeld NR, 2014, Effect of confining pressure and microcracks on mass transport properties of concrete, ADVANCES IN APPLIED CERAMICS, Vol: 113, Pages: 485-495, ISSN: 1743-6753
Yio MHN, Wong HS, Buenfeld NR, 2014, Fluorescence laser scanning confocal microscopy for real-time imaging of early cement hydration, 15th Euroseminar on Microscopy Applied to Building Materials
Abyaneh SD, Wong HS, Buenfeld NR, 2014, Computational investigation of capillary absorption in concrete using a three-dimensional mesoscale approach, COMPUTATIONAL MATERIALS SCIENCE, Vol: 87, Pages: 54-64, ISSN: 0927-0256
Yio MHN, Phelan JC, Wong HS, et al., 2014, Determining the slag fraction, water/binder ratio and degree of hydration in hardened cement pastes, Cement and Concrete Research, Vol: 56, Pages: 171-181, ISSN: 0008-8846
A method for determining the original mix composition of hardened slag-blended cement-based materials based on analysis of backscattered electron images combined with loss on ignition measurements is presented. The method does not require comparison to reference standards or prior knowledge of the composition of the binders used. Therefore, it is well-suited for application to real structures. The method is also able to calculate the degrees of reaction of slag and cement. Results obtained from an experimental study involving sixty samples with a wide range of water/binder (w/b) ratios (0.30 to 0.50), slag/binder ratios (0 to 0.6) and curing ages (3 days to 1 year) show that the method is very promising. The mean absolute errors for the estimated slag, water and cement contents (kg/m3), w/b and s/b ratios were 9.1%, 1.5%, 2.5%, 4.7% and 8.7%, respectively. 91% of the estimated w/b ratios were within 0.036 of the actual values.
Dehghanpoor Abyaneh S, Wong HS, Buenfeld NR, 2013, Modelling the diffusivity of mortar and concrete using a three-dimensional mesostructure with several aggregate shapes, Computational Materials Science, Vol: 78, Pages: 63-73, ISSN: 0927-0256
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