Imperial College London

Professor Hong S. Wong

Faculty of EngineeringDepartment of Civil and Environmental Engineering

Professor of Concrete Materials



+44 (0)20 7594 5956hong.wong Website




Ms Ruth Bello +44 (0)20 7594 6040




228DSkempton BuildingSouth Kensington Campus





Publication Type

113 results found

Korec E, Jirásek M, Wong HS, Martínez-Pañeda Eet al., 2024, Phase-field chemo-mechanical modelling of corrosion-induced cracking in reinforced concrete subjected to non-uniform chloride-induced corrosion, Theoretical and Applied Fracture Mechanics, Vol: 129, ISSN: 0167-8442

A model for corrosion-induced cracking of reinforced concrete subjected to non-uniform chloride-induced corrosion is presented. The gradual corrosion initiation of the steel surface is investigated by simulating chloride transport considering binding. The transport of iron from the steel surface, its subsequent precipitation into rust, and the associated precipitation-induced pressure are explicitly modelled. Model results, obtained through finite element simulations, agree very well with experimental data, showing significantly improved accuracy over uniform corrosion modelling. The results obtained from case studies reveal that crack-facilitated transport of chlorides cannot be neglected, that the size of the anodic region must be considered, and that precipitate accumulation in pores can take years.

Journal article

Wu J, Wong HS, Zhang H, Yin Q, Jing H, Ma Det al., 2024, Improvement of cemented rockfill by premixing low-alkalinity activator and fly ash for recycling gangue and partially replacing cement, Cement and Concrete Composites, Vol: 145, ISSN: 0958-9465

Recycling mining wastes such as gangue for cemented rockfill is a key approach for sustainable development while ensuring the safety of underground structures in the mining industry. However, achieving sufficient strength for this low-cost material remains a fundamental challenge. This study explores an approach to strengthen cemented rockfill by using well-graded gangue, and by partially replacing Portland cement with fly ash and a premixed low-alkalinity activator. The effects of activator dosage and curing time on mechanical properties, composition and microstructure of cemented rockfill were investigated through uniaxial compression, XRD, FTIR, TG/DTG, MIP and SEM-EDS analyses. Results show overall improvements in the performance of cemented rockfill despite its low cement content, and the use of a low-cost low-alkaline activator. The 28-d compressive strength improved by over 30 % to reach 8 MPa with only 7.7 % wt. Portland cement and incorporating more than 60 % wt. of waste gangue in its mix composition. The main source of strength is calcium silicate hydrate and calcium aluminate silicate hydrate, the latter playing an increasing contribution with longer curing due to fly ash activation. Addition of premixed activator to fly ash enhanced the formation of Ca6Si6O17(OH)2, and CaO∙Al2O3∙xSiO2∙yH2O. Activator causes densification of the microstructure and this occurs predominantly on pores smaller than 1 μm. The optimum mix having the highest performance-cost ratio contains between 10 % and 15 % activator. This work reveals an effective and low-cost method of premixing fly ash with a low-alkalinity activator for recycling mining wastes and reducing cement.

Journal article

Zhang K, Yio M, Wong H, Buenfeld Net al., 2024, Development of more accurate methods for determining carbonation depth in cement-based materials, Cement and Concrete Research, Vol: 175, ISSN: 0008-8846

Measuring carbonation is increasingly important, especially for developing novel low-CO2 cements and carbon-capture technologies. This study shows for the first time, the feasibility and advantages of confocal Raman microscopy (CRM) for measuring carbonation depth in cement-based materials, providing high spatial resolution (down to <100 μm), by mapping CaCO3 and Ca(OH)2. Pastes and mortars of different binders (CEM I, 30 % PFA, 50 % GGBS) and w/b ratios (0.45, 0.60) exposed to natural (440 ppm CO2) and accelerated carbonation (4 % CO2) at 65 % RH, 21 °C for up to 3 months were tested. CRM shows a sharp carbonation front, without the transition zone commonly supposed. Carbonation depths measured with image analysis of CRM-CaCO3 maps and phenolphthalein-treated surfaces are in excellent agreement, however the latter is less reliable for depths <5 mm. Profile-based methods (TGA, XRD, FTIR, RS and BSE) systematically over-estimate carbonation depth; a simple method to correct this error is proposed.

Journal article

Al-Noaimat YA, Chougan M, Al-kheetan MJ, Yio MHN, Wong HS, Ghaffar SHet al., 2023, Upcycling end-of-life bricks in high-performance one-part alkali-activated materials, Developments in the Built Environment, Vol: 16, Pages: 1-15, ISSN: 2666-1659

One-part alkali-activated materials (AAMs) can preserve natural resources and lower embodied carbon of the built environment by accommodating various wastes, industrial by-products, and end-of-life materials in their composition. This study investigates the feasibility of using end-of-life bricks in two physical states, powder and aggregate, to partially replace fly ash precursor and natural aggregate in AAMs, respectively. The mechanical characteristics, microstructure, water absorption, freeze-thaw and fire resistance of the modified AAMs were evaluated. The effect of adding different ratios of nano graphite platelets was also investigated. Results showed that brick-based one-part AAMs can achieve mechanical properties, pore structure, water absorption and freeze-thaw resistance comparable to fly ash-based AAM while having 65% better fire resistivity. Incorporating bricks as aggregate resulted in a maximum improvement of 17% and 27% in the AAMs' compressive and flexural strength levels, respectively, and a general enhancement in the freeze-thaw resistance with showing no reduction in compressive strength after exposure to elevated temperature. Incorporating 0.1 wt% nano-graphite further enhanced flexural strength by 30%, decreased water absorption by 18% and improved freeze-thaw resistance compared to the mix without nano-graphite. Moreover, adding up to 0.5% nano-graphite enhanced the fire resistivity of the composite, allowing it to exhibit 19% better strength performance than before exposure.

Journal article

Suwan T, Wong HS, Fan M, Jitsangiam P, Thongchua H, Chindaprasirt Pet al., 2023, Laboratory-grade vs. industrial-grade NaOH as alkaline activator: the properties of coal fly ash based-alkaline activated material for construction, Case Studies in Construction Materials, Vol: 19, Pages: 1-14, ISSN: 2214-5095

Alkaline-activated materials (AAM) are potentially low-carbon alternative binders for the cement industry. Most research studies related to AAM have been performed with high-purity laboratory-grade NaOH alkaline activators (Lab-grade). Gaining confidence in AAM beyond the laboratory scale in real applications remains a major challenge. To make this prospect more realistic, the viability of low-purity industrial-grade (Ind.-grade) and local no-grade alkaline activators was investigated. The strength results of Ind.-grade AAM were 15–20% lower than those of Lab-grade AAM. The purity of the Ind.-grade activator should be at least 98% to achieve the performance of Lab-grade AAM. The production cost of Ind.-grade AAM was 75–85% lower than that of Lab-grade AAM at the same activator concentration. It should be noted that the use of no-grade NaOH is not recommended for engineering purposes due to its uncertain quality and low purity. The combined benefits of low cost and high availability of Ind.-grade activators are expected to enhance the commercial viability of AAM as a sustainable alternative construction material for field applications.

Journal article

Korec E, Jirásek M, Wong HS, Martínez-Pañeda Eet al., 2023, A phase-field chemo-mechanical model for corrosion-induced cracking in reinforced concrete, Construction and Building Materials, Vol: 393, Pages: 1-23, ISSN: 0950-0618

We present a new mechanistic framework for corrosion-induced cracking in reinforced concrete that resolvesthe underlying chemo-mechanical processes. The framework combines, for the first time, (i) a model forreactive transport and precipitation of dissolved Fe2+ and Fe3+ ions in the concrete pore space, (ii) aprecipitation eigenstrain model for the pressure caused by the accumulation of precipitates (rusts) under poreconfinement conditions, (iii) a phase-field model calibrated for the quasi-brittle fracture behaviour of concrete,and (iv) a damage-dependent diffusivity tensor. Finite element model predictions show good agreement withexperimental data from impressed current tests under natural-like corrosion current densities.

Journal article

Wu J, Wong HS, Yin Q, Ma Det al., 2023, Effects of aggregate strength and mass fraction on mesoscopic fracture characteristics of cemented rockfill from gangue as recycled aggregate, Composite Structures, Vol: 311, Pages: 1-116851, ISSN: 0263-8223

The discrete element model considering media properties and interface differences was constructed for revealing the mesoscopic fracture characteristic of cemented rockfill in this study. The effects of aggregate strength and mass fraction on strength, deformation, energy and cracking of cemented rockfill were investigated. The results showed that the generally concerned two-element intergranular fracture between aggregates and cement matrix ITZ only plays a leading role in one fracture characteristic, but the failure induced by damaged aggregates is more severe than this fracture characteristic. The fracture characteristic transformation induced deterioration of cemented rockfill is catastrophic. The compressive strength of the specimen produced from 12.88 MPa gangue is nearly 30% worse than that of 32.77 MPa. And this fracture characteristic transformation is related to the aggregate mass fraction. The aggregate strength corresponding to the turning aggregate to cemented matrix strength ratio decreases from 52 MPa to 32 MPa with increasing the aggregate mass fraction from 42.7% to 57.7%. Therefore, the development of cemented rockfill requires more attention to the compatibility as characterized by the aggregates to cement matrix strength ratio and aggregate mass fraction. As the strength ratio increases, the recycled aggregates transform from defect clusters to load bearing phases in cemented rockfill.

Journal article

Wang J, Yio MHN, Zhou T, Wong HS, Davie CT, Masoero Eet al., 2023, Water sorption isotherms and hysteresis of cement paste at moderately high temperature, up to 80 °C, Cement and Concrete Research, Vol: 165, Pages: 1-12, ISSN: 0008-8846

The constitutive models of concrete often consider water desorption isotherms to be near-equilibrium andsignificantly affected by moderately high temperature, 40–80◦C, typically through microstructural changes.However literature data suggest that adsorption, not desorption, is near-equilibrium and moderate temperaturesdo not cause microstructural changes. This work supports the latter theory, through dynamic vapor sorptionexperiments on cement paste at 20–80◦C. Samples were pre-conditioned at 60% relative humidity and 20◦C,and isotherms were measured for several humidity ranges and testing rates. The results, corroborated byclassical DFT simulations, indicate that adsorption is near-equilibrium and mostly unaffected by temperature,whereas desorption is out-of-equilibrium due to the ink-bottle effect at high humidity, and interlayer waterat low humidity. Starting from the second cycle, desorption at higher temperatures features a shift of thecavitation pressure and overall a smaller hysteresis. A conceptual model of pore-specific temperature-dependenthysteresis is proposed to qualitatively explain the results.

Journal article

Kia A, Wong HS, Cheeseman CR, 2022, Freeze–thaw durability of conventional and novel permeable pavement replacement, Journal of Transportation Engineering Part B-Pavements, Vol: 148, ISSN: 2573-5438

Permeable concrete pavements are becoming more common as a stormwater management system to mitigate urban flooding. However, they have several well-defined drawbacks including low permeability, high clogging potential, and low strength and durability, notably in cold climates exposed to freezing and thawing. A new generation of high-strength clogging-resistant permeable pavement replacement (CRP) has been developed, through extensive laboratory work, to address these shortcomings and advance the field of permeable pavements. This paper reports on new advances in permeable pavement systems and the performance of a range of conventional permeable concrete and the developed novel CRP (both prepared using Portland cement) of varying porosity exposed to freeze–thaw cycles. This will allow performance evaluations of both systems in a cold climate. The tests involved exposing samples to temperatures varying from −20°C to +20°C and measuring changes in mass, area, compressive strength, and ultrasonic pulse velocity after each cycle. These new results show that CRP is highly resistant to degradation caused by freeze–thaw cycles compared to conventional permeable concrete, reducing maintenance requirements and improving service life. This study presents the first high-strength clogging-resistant permeable pavement replacement that is durable under frost action, these findings will support and enable wider use of permeable pavements in cold regions.

Journal article

Ayati B, Newport D, Wong H, Cheeseman Cet al., 2022, Acid activated smectite clay as pozzolanic supplementary cementitious material, Cement and Concrete Research, Vol: 162, Pages: 1-8, ISSN: 0008-8846

This research has investigated the structural changes and pozzolanic activity produced in acid activated smectite clay. The activation treatment used HCl at different concentrations, using different times and at a range of temperatures. X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy were used to determine the acid dissolution mechanism and characterise the activated clay mineral structure. Acid activation causes dehydroxylation of smectite clay, followed by leaching of octahedral cations. This results in the formation of a silica-rich amorphous phase that exhibits substantial pozzolanic activity compared to the same clay sample that had undergone calcining treatment at 850. The optimum sample was activated for 8 h using 5 M HCl at 90 °C. This was 93 % amorphous. Mortar prisms prepared with 25 % replacement of Portland cement by acid activated smectite produced 93 % compressive strength of plain Portland cement mortar.

Journal article

Ding T, Wong H, Qiao X, Cheeseman Cet al., 2022, Developing circular concrete: Acid treatment of waste concrete fines, Journal of Cleaner Production, Vol: 365, Pages: 1-8, ISSN: 0959-6526

The development of circular concrete, to enable key components to be extracted and reused, is a key requirement to achieve sustainability in the built environment. Current industry practice for end-of-life concrete is best described as down-cycling because recycled concrete aggregate has limited use, with disposal of the associated crushed concrete fines. Acid treatment of waste concrete is being investigated to allow key concrete components to become circular and, in this work, the effect of acetic acid concentration, liquid/solid (L/S) ratio, reaction time and temperature on the leaching of waste concrete fines is reported. An acid concentration of 0.6 mol/L, an L/S ratio of 7 ml/g, and a reaction time of 6 h at ambient temperature allows clean sand to be extracted from concrete fines. This performs identically to new sand in mortar samples. We show for the first time that the dried and ground silica-rich residue produced by acid leaching has pozzolanic properties comparable to commercially available supplementary cementitious materials (SCM) such as blast furnace slag and coal fly ash. The potential for CO2 sequestration using the Ca2+-rich leached solution to form CaCO3 is calculated. The research shows that acid leaching of concrete fines can produce clean reusable sand, generates a viable SCM and sequester significant amounts of CO2 by forming precipitated calcium carbonate.

Journal article

Yio MHN, Ho YW, Abdul Wahid F, Wong H, Buenfeld NRet al., 2022, Analysis of cement paste and aggregate content of concrete using micro X-ray fluorescence, Magazine of Concrete Research, Vol: 74, Pages: 889-904, ISSN: 0024-9831

A new method for determining the cement paste, fine aggregate and coarse aggregate content of hardened concrete using micro X-ray fluorescence (μXRF) is presented. The method involves mathematical and morphological operations to extract aggregate particles from large-area (100 x 50 mm2) composite element maps acquired with μXRF. The method was tested on five concretes containing different types of aggregates including gravel, limestone, siliceous sand, and sintered lightweight aggregates. The results were compared against point count analysis and data from the actual mix design. The average errors in relation to the mix design for the measured fine aggregate, coarse aggregate, total aggregate, cement paste contents and fine/coarse aggregate ratio were 12%, 12%, 3.4%, 10%, and 27% respectively. All measured values fell within ±4% of those point-counted. An image size of > 2000 mm2 (at least five times maximum aggregate size) was found to be required to obtain representative results. The advantages and limitations of the proposed method are discussed.

Journal article

Lu D, Shi X, Wong HS, Jiang Z, Zhong Jet al., 2022, Graphene coated sand for smart cement composites, Construction and Building Materials, Vol: 346, Pages: 1-12, ISSN: 0950-0618

Unlike conventional approaches of direct addition of carbon-based conductive fillers into cement matrix in developing smart sensing composites, this study proposes a targeted and therefore more efficient approach through nano-surface engineering of the sand. Specifically, a simple method that enables uniform adsorption of graphene oxide onto the surface of the sand particles, followed by simple annealing and microwave treatment to prepare graphene-coated sand (conductive aggregates). Scanning electron microscopy indicates that about 62.2% of the sand surface area is successfully coated by graphene, with an average thickness of approximately 8.8 nm. The mortar incorporating conductive aggregates demonstrates outstanding electrical conductivity (resistivity of 960 Ω·cm) and a high fractional change in resistivity of ∼ 18% under cyclic compressive loading, which outperforms previously reported results obtained by direct addition of graphene or carbon nanotubes at equivalent concentrations. The use of conductive aggregates in mortars also results in other minor benefits such as a slight enhancement in flowability and reduction in water sorptivity. All of these were achieved without substantial reduction in 28-d compressive strength. These findings demonstrate a great potential of aggregate surface nano-engineering for developing smart cement-based composites for practical sensing applications.

Journal article

Ayati B, Newport D, Wong H, Cheeseman Cet al., 2022, Low-carbon cements: potential for low-grade calcined clays to form supplementary cementitious materials, Cleaner Materials, Vol: 5, Pages: 100099-100099, ISSN: 2772-3976

The use of low-carbon supplementary cementitious materials (SCM), such as calcined clays, to replace cement clinker has been recognized by the Cement Industry to achieve reductions in greenhouse gas emissions. This paper investigates eight low-grade clays, with <20% kaolinite, obtained from different geological formations, that have been calcined to produce potential SCMs. The clays were characterised before and after calcining at 750, 800, 850 and 900 °C, and the mineralogical changes and amorphous phase contents determined. The pozzolanic activity and the strength activity index of the different calcined clays were evaluated. The results show that calcined clays from the Oxford and Ampthill geological formations in the UK can produce SCMs with pozzolanic activity higher than conventional SCMs such as PFA. These clays were rich in illite and smectite and produced ∼60% amorphous phase when calcined at 850 °C. Mortars produced using calcined clays had higher compressive strengths than mortars containing pulverised fuel ash and achieved ∼90% of the compressive strength of 100% Portland cement mortar samples at 28 days. The research demonstrates that low-grade clay resources can be calcined to produce SCMs and that these can be used to form cementitious materials with reduced total associated CO2 emissions.

Journal article

Zhang K, Yio M, Wong H, Buenfeld Net al., 2022, Optimising confocal Raman microscopy for spectral mapping of cement-based materials, Materials and Structures, Vol: 55, Pages: 1-18, ISSN: 1359-5997

Raman spectroscopy combined with confocal imaging, i.e. confocal Raman microscopy (CRM) is a relatively new technique with huge potential for high-resolution chemical mapping of phase composition and spatial distribution in cement-based materials. However, the effects of sample preparation and various operating parameters on mapping quality has not been systematically studied. This paper optimises CRM for spectral mapping of carbonated and non-carbonated cement-based materials. The effects of sample preparation and scanning parameters on the detection of four main phases (calcite, portlandite, ettringite and unreacted cement) were investigated. Results show that although freshly cut cementitious samples can be analysed as-is, the Raman signals improve with short gentle drying and surface grinding/polishing prior to analysis. Increasing laser power, exposure time and scan accumulation, and short laser wavelength yields higher signal-to-noise (SNR) ratio in the obtained spectrum. The use of a 4.15 mW laser power, 2 s exposure time and scan accumulation of 2 with 532 nm laser represents a good operating condition for Raman analysis of cement-based materials. This produces SNR > 10 for all investigated phases at short testing time and low risk of laser-induced damage. Microcracking caused by localised heating during closely-spaced mapping can be limited by impregnating the sample with epoxy to protect the microstructure. We show for the first time that CRM can be used to quantify the volume fraction of calcium carbonate and portlandite at high resolution when combined with SEM. The advantages and limitations of CRM for mapping cement-based materials are discussed.

Journal article

Wong HS, Angst UM, Geiker MR, Isgor OB, Elsener B, Michel A, Alonso MC, Correia MJ, Pacheco J, Gulikers J, Zhao Y, Criado M, Raupach M, Sørensen H, François R, Mundra S, Rasol M, Polder Ret al., 2022, Methods for characterising the steel–concrete interface to enhance understanding of reinforcement corrosion: a critical review by RILEM TC 262-SCI, Materials and Structures, Vol: 55, ISSN: 1359-5997

The steel–concrete interface (SCI) is a complex, multi-phase and multi-scale system. It is widely known to influence the performance and long-term durability of concrete structures. However, a fundamental understanding of its properties and effects on corrosion initiation of embedded reinforcing steel remains elusive. This is attributed to its complicated heterogeneity and time-dependent nature, exacerbated by the lack of suitable techniques for systematic and detailed characterisation. This paper, prepared by members of the RILEM Technical Committee 262-SCI, critically reviews available information regarding current methods (laboratory or field-based) for characterising local properties of the SCI that have been identified as governing factors affecting corrosion initiation. These properties include characteristics of the steel such as mill scale and rust layers, and characteristics of the concrete such as interfacial voids, microstructure and moisture content. We evaluated over twenty methods and summarised their advantages, applications and limitations. The findings show a severe lack of well established, non-destructive techniques that are suitable for direct monitoring of the SCI at a representative scale with sufficiently high resolution (spatial, temporal), particularly for moisture related aspects. Several promising novel techniques with significant potential for further development and application were identified and discussed. Finally, we provide several recommendations for future research needs that are required to advance this critically important topic.

Journal article

Zhang K, Yio M, Wong H, Buenfeld Net al., 2022, Real-time monitoring of carbonation of hardened cement pastes using Raman microscopy, Journal of Microscopy, Vol: 286, Pages: 126-133, ISSN: 0022-2720

This study investigated the feasibility of Raman microscopy for monitoring early surface carbonation of hardened cement pastes in real time for up to 7 days. Samples were exposed to natural carbonation (440 ppm CO2) and accelerated carbonation (4% CO2), and the evolution of calcium carbonate (CaCO3) polymorphs, portlandite, ettringite, C-S-H gel and unreacted cement particles was followed. Results showed that calcite is the main polymorph formed under both natural and accelerated carbonation. Under accelerated carbonation, the formation of calcite on the sample surface completed within 1 day whereas under natural carbonation, the formation of calcite is expected to continue beyond 7 days. The contents of portlandite and ettringite decreased rapidly under accelerated carbonation but much more gradually under natural carbonation. However, calcium silicate minerals in unreacted cement particles remained unchanged throughout the carbonation processes. Overall, this study demonstrated that Raman microscopy is a valuable tool for non-destructive real-time imaging of surface carbonation in cement-based materials.

Journal article

Li K, Wang Y, Jiang Z, Wong HS, Zhou T, Wu J, Zhang J, Zhang Aet al., 2022, Functional building devices using laser-induced selective metallization on magnesium oxychloride cement composites, Cement and Concrete Composites, Vol: 128, Pages: 1-11, ISSN: 0958-9465

Magnesium oxychloride (MOC) cement as viable substrate candidate has unique applications in a range of functional and novel sensing applications due to their low thermal conductivity, high toughness, durability and excellent fire resistance. Here, we developed a facile strategy to fabricate metallized patterns on MOC cement composites through laser-induced selective metallization. The laser sensitiser of copper hydroxyl phosphate [Cu2(OH)PO4] was incorporated into MOC cement to prepare MOC cement composites. Then, the metallized copper patterns were obtained on MOC cement composites after 1064 nm pulsed laser activation and electroless copper plating (ECP). The obtained copper layer on MOC cement composites exhibited high electrical conductivity and excellent mechanical adhesion to the substrate. Furthermore, the rough copper patterns exhibited self-cleaning ability after superhydrophobic modification, which could prevent the surface from being contaminated. The metallized copper pattern could rapidly heat up when energised due to the Joule heating effect, and it can be applied for electric heating and automatic deicing in winter. In addition, a UV photodetector was fabricated by designing an interdigital metallized pattern combined with the UV light response characteristics of nano-ZnO, which has potential application on intelligent functional devices.

Journal article

Kia A, Cheeseman C, Wong H, 2022, High Strength Porous Cement-Based Materials, US20220010500A1


Yio MHN, Mac MJ, Yeow YX, Wong H, Buenfeld NRet al., 2021, Effect of autogenous shrinkage on microcracking and mass transport properties of concrete containing supplementary cementitious materials, Cement and Concrete Research, Vol: 150, ISSN: 0008-8846

It is well-known that supplementary cementitious materials (SCMs) and low water-to-binder (w/b) ratio increase autogenous shrinkage, but the impact on microcracking and long-term transport properties is less understood. This paper examines the effect of microcracking induced by autogenous shrinkage on transport properties of concretes cured up to ~3.6 years. Variables include SCM type (9% SF, 70% GGBS), w/b ratio (0.20–0.45), maximum-aggregate-size (MSA: 5–20 mm) and shrinkage reducing admixture (SRA). Oxygen diffusivity, permeability and water sorptivity were correlated with microcracks characterised using laser scanning confocal microscopy and 3D X-ray microtomography. Results show greater microcracking in mixes containing SCMs, low w/b ratio and large MSA. At the same w/b ratio and binder type, strong positive correlations are observed between transport and microcracking with increasing MSA, confirming the negative impact of autogenous shrinkage. SRA was effective in reducing these effects. The significance is compared with drying shrinkage and implications for durability are discussed.

Journal article

Kia A, Delens J, Wong H, Cheeseman Cet al., 2021, Structural and hydrological design of permeable concrete pavements, Case Studies in Construction Materials, Vol: 15, ISSN: 2214-5095

Permeable pavements are used to mitigate urban flooding. However, conventional concrete permeable pavements have low compressive strength and are prone to clogging, which degrades performance and reduces service life. A new type of permeable pavement, high-strength clogging resistant permeable pavement (CRP), has recently been developed that overcomes many limitations of conventional permeable pavements. This paper presents a new design methodology for CRP that takes into account both structural and hydrological considerations. This is used in 12 case studies which compare CRP with conventional permeable pavements. The results highlight several advantages of CRP and demonstrate that CRP with low porosity (∼5%) can cope with severe rainfall run-off volumes. The suitability of using CRP in both light and heavy load bearing applications is demonstrated. The research also shows that the use of CRP allows considerable reductions in pavement depth compared to conventional permeable pavements with reduced material costs.

Journal article

Muslim F, Wong H, Choo TH, Buenfeld Net 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

Journal article

Ferdous W, Manalo A, Siddique R, Mendis P, Zhuge Y, Wong H, Lokuge W, Aravinthan T, Schubel Pet al., 2021, Recycling of landfill wastes (tyres, plastics and glass) in construction – A review on global waste generation, performance, application and future opportunities, Resources, Conservation and Recycling, Vol: 173, ISSN: 0921-3449

The world is moving towards a circular economy that focuses on reducing wastes and keeping materials in use for the longest time possible. This paper critically reviewed three of the largest volume of landfill waste materials (tyres, plastics and glass) that are becoming a major concern for many countries. At present, crumb rubbers (from tyres) and glass sands (from crushed waste glass) are being used in concrete and road constructions while plastics are often used in manufacturing civil structures. However, only 10% tyres, 19.5% plastics and 21% glass are currently recycled globally. The massive volume of remaining unused wastes goes to landfill creating environmental problems. Therefore, finding new strategies of utilising these landfill wastes is vital. The global and country specific production, recycling and landfilling rates of these waste are summarised to understand the present situation of global waste crisis. Future strategies for improved waste management, potential investment and research directions are highlighted. New options for recycling wastes tyres, plastics and glass in construction are also presented to provide practical and economical solutions to extract maximum value and ensure their continued use in a closed loop system.

Journal article

Muslim F, Wong HS, Chiu TKQ, Buenfeld NRet al., 2021, Improving bond strength and mass transport properties of spacer-concrete interface with textured spacers, Materials and Structures, Vol: 54, Pages: 1-16, 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.

Journal article

Mac M, Yio MHN, Wong H, Buenfeld NRet 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.

Journal article

Mac M, Yio M, Desbois G, Casanova I, Wong H, Buenfeld Net 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.

Journal article

Muslim F, Wong H, Cheng G, Alexandrou C, Liu B, Buenfeld Net 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.

Journal article

Wong H, Poole AB, Wells B, Eden M, Barnes R, Ferrari J, Fox R, Yio MHN, Copuroglu O, Guðmundsson G, Hardie R, Jakobsen UH, Makoubi K, Mitchinson A, Raybould P, Strongman J, Buenfeld Net 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.

Journal article

Ferdous W, Manalo A, Wong H, Abousnina R, AlAjarmeh O, Schubel Pet 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.

Journal article

Maraghechi H, Avet F, Wong H, Kamyab H, Scrivener Ket 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

Journal article

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