85 results found
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.
Ferdous W, Manalo A, Wong H, et al., 2020, Optimal design for epoxy polymer concrete based on mechanical properties and durability aspects, Construction and Building Materials, Vol: 232, ISSN: 0950-0618
Polymer concrete has shown a number of promising applications in building and construction, but its mix design process remains arbitrary due to lack of understanding of how constituent materials influence performance. This paper investigated the effect of resin-to-filler ratio and matrix-to-aggregate ratio on mechanical and durability properties of epoxy-based polymer concrete in order to optimise its mix design. A novel combination of fire-retardant, hollow microsphere and fly ash fillers were used and specimens were prepared using resin-to-filler ratios by volume from 100:0 to 40:60 at 10% increment. Another group of specimens were prepared using matrix-to-aggregate ratios from 1:0 decreasing to 1:0.45, 1:0.90 and 1:1.35 by weight at constant resin-to-filler ratio. The specimens were inspected and tested under compressive, tensile and flexural loading conditions. The epoxy polymer matrix shows excellent durability in air, water, saline solution, and hygrothermal environments. Results show that the resin-to-filler ratio has significant influence on the spatial distribution of aggregates. Severe segregation occurred when the matrix contained less than 40% filler while a uniform aggregate distribution was obtained when the matrix had at least 40% filler. Moreover, the tensile strength, flexural strength and ductility decreased with decrease in matrix-to-aggregate ratio. Empirical models for polymer concrete were proposed based on the experimental results. The optimal resin-to-filler ratio was 70:30 and 60:40 for non-uniform and uniform distribution of aggregates, respectively, while a matrix-to-aggregate ratio of 1:1.35 was optimal in terms of achieving a good balance between performance and cost.
Maraghechi H, Avet F, Wong H, et al., 2019, Correction to: performance of limestone calcined clay cement (LC3) with various kaolinite contents with respect to chloride transport (Materials and Structures, (2018), 51, 5, (125), 10.1617/s11527-018-1255-3), Materials and Structures/Materiaux et Constructions, Vol: 52, Pages: 124-124, ISSN: 1359-5997
Khotbehsara MM, Manalo A, Aravinthan T, et al., 2019, Effect of elevated in-service temperature on the mechanical properties and microstructure of particulate-filled epoxy polymers, Polymer Degradation and Stability, Vol: 170, ISSN: 0141-3910
In civil engineering applications, epoxy-based polymers are subject to different environmental conditions including in-service temperature, which might accelerate their degradation and limit their application ranges. Recently, different particulate fillers were introduced to enhance the mechanical properties and reduce the cost of epoxy-based polymers. This paper addresses the effect of in-service elevated temperature (from roomtemperature to 80o C) in particulate-filled epoxy based resin containing up to 60% by volume of fire retardant and fly ash fillers through a deep understanding of the microstructure and analysis of their mechanistic response. An improvement in the retention of mechanical properties at in-service elevated temperature was achieved by increasing the percentages offillers. The retention of compressive and split tensile strength at 80o C for the mix containing 60% fillers was 72% and 52%, respectively, which was significantly higher than the neat epoxy. Thermo-dynamic analysis showed an increase in glass transition temperature with the inclusion of fillers, while these mixes also experienced less weight loss compared to neat epoxy, indicating better thermal stability. Scanning electron microscopy images showed the formation of dense microstructures for particulate-filled epoxy based resin at elevated temperatures. This indicates that the particulate filled epoxy resin exhibits better engineering properties at in-service elevated temperatures, increasing their durability and therefore their suitability for civil engineering applications. A simplified prediction equation based on power function was proposed and showed a strong correlation to the experimental compressive and splitting tensile strength at different levels of in-service elevated temperature.
Angst UM, Geiker MR, Alonso MC, et al., 2019, The effect of the steel–concrete interface on chloride-induced corrosion initiation in concrete: a critical review by RILEM TC 262-SCI, Materials and Structures, Vol: 52, ISSN: 1359-5997
The steel–concrete interface (SCI) is known to influence corrosion of steel in concrete. However, due to the numerous factors affecting the SCI—including steel properties, concrete properties, execution, and exposure conditions—it remains unclear which factors have the most dominant impact on the susceptibility of reinforced concrete to corrosion. In this literature review, prepared by members of RILEM technical committee 262-SCI, an attempt is made to elucidate the effect of numerous SCI characteristics on chloride-induced corrosion initiation of steel in concrete. We use a method to quantify and normalize the effect of individual SCI characteristics based on different literature results, which allows comparing them in a comprehensive context. It is found that the different SCI characteristics have received highly unbalanced research attention. Parameters such as w/b ratio and cement type have been studied most extensively. Interestingly, however, literature consistently indicates that those parameters have merely a moderate effect on the corrosion susceptibility of steel in concrete. Considerably more pronounced effects were identified for (1) steel properties, including metallurgy, presence of mill scale or rust layers, and surface roughness, and (2) the moisture state. Unfortunately, however, these aspects have received comparatively little research attention. Due to their apparently strong influence, future corrosion studies as well as developments towards predicting corrosion initiation in concrete would benefit from considering those aspects. Particularly the working mechanisms related to the moisture conditions in microscopic and macroscopic voids at the SCI is complex and presents major opportunities for further research in corrosion of steel in concrete.
Kia A, Wong H, Cheeseman C, 2019, High-strength clogging resistant permeable pavement, International Journal of Pavement Engineering, ISSN: 1029-8436
Permeable pavement is utilised in order to alleviate flooding in towns, cities and other urban areas, but it is prone to clogging, has relatively low strength and requires regular maintenance. We have developed a novel permeable pavement with low tortuosity pore structure that can be cast on-site that is not only resistant to clogging, but also has high permeability and strength. This high strength clogging resistant permeable pavement (CRP) was prepared by introducing straight pore channels of varying size and number into self-compacting mortar. Samples with porosity ranging from 2 to 32% were tested. In all cases, permeability and compressive strength were substantially higher than conventional permeable concrete. More significantly, CRP can be engineered with low porosity (5%), high strength (> 50 MPa) and high permeability (> 2 cm/s), but does not clog despite extensive cyclic exposure to flow containing sand and clay. A simple method to model the permeability of CRP from the pore structure is described. We report for the first time a high strength clogging resistant permeable pavement capable of retaining sufficient porosity and permeability for storm-water infiltration without requiring frequent maintenance. This innovative system will help alleviate urban flooding and contribute towards a more sustainable urbanisation.
Kia A, 2019, Control of clogging in permeable concrete pavements
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.
Kia A, Wong H, Cheeseman C, 2018, High strength porous cement-based materials, P120039GB
Maraghechi H, Avet F, Wong HS, et al., 2018, Performance of limestone calcined clay cement (LC3) with various kaolinite contents with respect to chloride transport, Materials and Structures, Vol: 51, ISSN: 1359-5997
The durability of mortar and paste mixtures with respect to chloride ion ingress was investigated for binary blends of Portland Cement Calcined Clay, and ternary systems of Limestone Calcined Clay Cement (LC3). Five clays from various sources with different kaolinite content (17–95%) were studied. The main factor controlling the diffusivity of LC3 systems was found to be the kaolinite content of the clay. Resistance to chloride ingress increased to intermediate levels of kaolinite content and then stabilized. An intermediate kaolinite content of around 50% resulted in two orders of magnitude reduction in diffusivity compared to PC, indicating that the use of high grade (expensive) clays is not necessary to obtain good durability. The chloride binding capacity and distribution of bound chloride between Friedel’s salt and C–A–S–H were quantified for the different systems at fixed water to binder ratio of 0.5. The chloride binding capacity appeared to be a minor factor compared to the porosity refinement in the improved durability of LC3 systems.
Olsson N, Abdul Wahid F, Nilsson LO, et al., 2018, Wick action in mature mortars with binary cements containing slag or silica fume – The relation between chloride and moisture transport properties under non-saturated conditions, Cement and Concrete Research, Vol: 111, Pages: 94-103, ISSN: 0008-8846
Moisture and ionic transport under non-saturated condition is an important, but poorly understood transport phenomena particularly for mature systems containing supplementary cementitious materials. This paper investigates the moisture and chloride profiles of 3-year old mortars containing Portland cement (OPC), slag and silica fume (SF) after long-term (30–48 months) wick action exposure in 1.09 M NaCl solution. Moisture profiles were measured with 1H NMR relaxometry and chloride profiles with microXRF. The measured profiles were discussed in relation to moisture dependent material properties such as chloride diffusion coefficients, moisture diffusion coefficients, and desorption isotherms. Results show that the combination of different cementitious materials, e.g. the cementitious binder, is the key factor affecting chloride penetration depth. The cementitious binder also strongly affects chloride diffusion coefficient, moisture diffusion coefficient and chloride binding properties, which are all important parameters for the prediction of chloride ingress.
Kia A, Cheeseman CR, Wong H, 2018, Control of clogging in conventional permeable concrete and development of a new high strength clogging resistant permeable concrete pavement, 38th Cement and Concrete Science Conference
Kia A, Wong HS, Cheeseman C, 2018, Defining clogging potential for permeable concrete, Journal of Environmental Management, Vol: 220, Pages: 44-53, ISSN: 0301-4797
Permeable concrete is used to reduce urban flooding as it allows water to flow through normally impermeable infrastructure. It is prone to clogging by particulate matter and predicting the long-term performance of permeable concrete is challenging as there is currently no reliable means of characterising clogging potential. This paper reports on the performance of a range of laboratory-prepared and commercial permeable concretes, close packed glass spheres and aggregate particles of varying size, exposed to different clogging methods to understand this phenomena. New methods were developed to study clogging and define clogging potential. The tests involved applying flowing water containing sand and/or clay in cycles, and measuring the change in permeability. Substantial permeability reductions were observed in all samples, particularly when exposed to sand and clay simultaneously. Three methods were used to define clogging potential based on measuring the initial permeability decay, half-life cycle and number of cycles to full clogging. We show for the first time strong linear correlations between these parameters for a wide range of samples, indicating their use for service-life prediction.
Kia A, Wong HS, Cheeseman CR, 2018, Development of clogging resistant permeable concrete, 13th International Symposium on Concrete Roads
Kia A, Wong HS, Cheeseman CR, 2018, Development of clogging resistant permeable concrete, Proceedings of the 9th International Workshop on Research and Innovations for Design of Sustainable and Durable Concrete Pavements
Kia A, Wong HS, Cheeseman CR, 2018, Examining the clogging potential of permeable concrete and development of a high strength clogging resistant system, 4th Young Researcher’s Forum
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
© ICDCS 2018. All rights reserved. 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.
Wong HS, 2017, Concrete with superabsorbent polymer, Eco-efficient Repair and Rehabilitation of Concrete Infrastructures, Pages: 467-499, ISBN: 9780081021811
© 2018 Elsevier Ltd. All rights reserved. Superabsorbent polymers (SAPs) are cross-linked polymers with several unique characteristics that can be exploited for a number of applications in concrete technology. Indeed, recent years have seen a growing interest in the use of SAP to improve the performance and long-term durability of concrete structures. A key property of SAP is their ability to absorb and retain large amounts of fluid, swell to form an insoluble gel and subsequently release the absorbed water back into hardened concrete when internal humidity drops. The absorption and desorption kinetics of SAP can be controlled by a number of factors. This chapter presents an overview of these properties and their effects on fresh and hardened concrete. It then discusses potential applications in concrete, including the use of SAP as an admixture for internal curing to mitigate autogenous shrinkage, improve freeze-thaw resistance and induce self-sealing/healing of cracks. The focus is to review recent studies of SAP in concrete, identify gaps in knowledge and highlight future research needs.
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.
Zhou D, Wang R, Tyrer M, et al., 2017, Sustainable infrastructure development through use of calcined excavatedwaste clay as a supplementary cementitious material, Journal of Cleaner Production, Vol: 168, Pages: 1180-1192, ISSN: 0959-6526
Major infrastructure development projects in London produce large quantities of London clay and use significant volumes of concrete. Portland cement (CEM I) in concrete is normally partially replaced by supplementary cementitious materials such as ground granulated blastfurnace slag or pulverised fuel ash. The supply of supplementary cementitious materials is critical to the production of sustainable concrete. This study has investigated use of waste London clay as a supplementary cementitious material. The optimum calcined clay was produced at 900 °C and concrete made with 30 wt% of CEM I replaced by calcined clay had 28-day strengths greater than control samples. Compressive strengths of concrete containing calcined London clay were similar to concrete containing ground granulated blastfurnace slag and pulverised fuel ash. The production of calcined London clay emits ∼70 kg CO2/tonne and this is 91% lower than CEM I. 30 wt% replacement of CEM I by calcined London clay therefore produces concrete with ∼27% lower embodied carbon. London clay can be calcined to form a technically viable supplementary cementitious material and use of this in concrete would enable major civil infrastructure projects to contribute to a circular economy.
Kia A, Wong H, Cheeseman CR, 2017, CLOGGING POTENTIAL OF PERMEABLE CONCRETE, 37th Cement and Concrete Science Conference
Permeable concrete is used to reduce local flooding in urban areas. However, it is prone to cloggingby particulate matter and requires regular maintenance. This paper reports on the performance ofpermeable concrete exposed to different clogging test methods to further understand this complexphenomena. New methods were developed to study the clogging effect and to define a cloggingpotential. The tests involve applying flowing water containing sand and/or clay in cycles throughthe sample and measuring the change in flow rate. Clogging depends on the applied solution andexposure method used. Significant permeability reductions were observed in all samples,particularly when simultaneously exposed to sand and clay. This is because flocculated clayadhered to surface of sand particles and this caused increased clogging.
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: 1873-3948
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.
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.
Kia A, Wong HS, Cheeseman CR, 2017, Clogging in permeable concrete: a review, Journal of Environmental Management, Vol: 193, Pages: 221-233, ISSN: 0301-4797
Permeable concrete (or “pervious concrete” in North America) is used to reduce local flooding in urban areas and is an important sustainable urban drainage system. However, permeable concrete exhibits reduction in permeability due to clogging by particulates, which severely limits service life. This paper reviews the clogging mechanism and current mitigating strategies in order to inform future research needs. The pore structure of permeable concrete and characteristics of flowing particulates influence clogging, which occurs when particles build-up and block connected porosity. Permeable concrete requires regular maintenance by vacuum sweeping and pressure washing, but the effectiveness and viability of these methods is questionable. The potential for clogging is related to the tortuosity of the connected porosity, with greater tortuosity resulting in increased potential for clogging. Research is required to develop permeable concrete that can be poured on-site, which produces a pore structure with significantly reduced tortuosity.
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.
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