Search

Cement production is environmentally unsustainable due to the high anthropogenic carbon emissions produced. Supplementary cementitious materials (SCMs), derived from the by-products of different industries, have been deemed an effective way to reduce carbon emissions. The reduction in carbon emissions is achieved by lowering the clinker factor of cement, through a partial replacement with an SCM. Sugarcane Bagasse Ash (SCBA) is produced as an agricultural waste from the sugarcane industry and has gained a lot of attention for being a feasible and readily available pozzolanic material, underutilised as an SCM. This study evaluates alkali-activated sugarcane bagasse ash’s mechanical and durability performance, at varied contents, in binary blended cement concrete and ternary blended cement concrete containing silica fume (SF). Potassium Hydroxide (KOH), used as the alkali activator, is intended to enhance the reactivity of the ash, with the possibility of a high-volume SCBA content. The mechanical performance was investigated by compressive and split tensile strength tests, and durability performance was investigated using the Oxygen Permeability Index (OPI) test. In addition, a micro-CT porosity test was conducted to assess how the microstructure and porosity of the concrete affect the mechanical and durability performance. The results indicated that using SCBA in a ternary blend with SF can significantly improve the overall performance and create less porous concrete. At 30% SCBA and 10% SF replacement, the performance tests revealed the highest mechanical strength and the lowest permeability, outperforming the control concrete and the binary blended cement concrete containing only SCBA.

Alkali-silica reaction (ASR) and its associated deformation are major durability problems in concrete structures and was reported as far back as the 1940s by Stanton (2008) [1]. This deleterious reaction causes excessive expansion and cracks that can lead to severe degradation of the concrete structures. Despite the age-long discovery and numerous ASR studies, understanding the ASR mechanism remains challenging due to complex processes and reactions. This paper presents a review of ASR in concrete structures and details the factors associated with ASR, the reaction mechanism and chemistry, and its adverse effect on concrete structures. The alkalis in the pore solution, the reactive amorphous silica present in aggregates, and the presence of moisture (with other external climatic inputs) are the key factors responsible for ASR. The study also provides a critical assessment of the various test methods for ASR evaluation in the United States. A case study correlating the results (from the literature) of three prominent test methods was also carried out. From this review, the new miniature concrete prism test (MCPT) method was concluded to be rapid, reliable, and capable of determining the influence of aggregate reactivity, alkali availability, and exposure conditions as compared to other methods.

Huge embodied energy, carbon footprint and cost of fired brick gave way for compressed stabilised earth bricks (CSEB). However, in order to improve its surface erosion durability, another problem arose due to CSEB plastering, which is poor long-term bonding. This problem gave rise to this research aimed at investigating and improving the adhesive bond between mortar and compressed stabilised earth bricks. Four variants of CSEB were made from inclusions of sand and used motor oil while six variants of mortar containing cement, sand, used motor oil and earth were used. Sandcrete block served as the control. Dry and wet shear bond strengths between the CSEBs and mortars were determined by an innovative loading system. The results from the investigation show that the inclusion of sand and oil in CSEB improved the dry unconfined strength by 88.2% but reduced the wet unconfined strength by 19.5%. The inclusion of sand and oil has relatively no impact on the water absorption properties of CSEB. All four variants of CSEB have the best bond with cement mortar in dry condition but worst bond under wet condition. Cement mortar containing earth/and oil have the best bond with the four variants of CSEB in wet condition. This study concludes that CSEB plaster should contain earth in the right proportion to avoid spalling associated with plastered CSEB in water prone surfaces. Also, sand and used motor oil can be included in CSEB for heavy load masonry works.

The study sets out to evaluate the thixotropy of conventional concrete containing rheology modifiers using various methods of rotational rheometry. Five mixes were examined: the control, two mixes with viscosity modifying agent (VMA) and superplasticiser (SP) respectively, a mix with combined viscosity modifying agent and superplasticiser (VMA + SP) and the last mix with an increase in water content. These mixes were formulated to achieve concrete with varying rheological properties. Tests were carried out to examine the evolution of rheology parameters (static yield stress, dynamic yield stress and plastic viscosity), thixotropy index, shear thinning and thickening, flocculation properties, torque decay pattern (with a subsequent drop in apparent viscosity), and hysteresis loop. Results show that conventional concrete known to be a shear thinning and thixotropic material, can possess anti-thixotropic (rheopexy) and shear thickening properties depending on the condition pre-history, resting time, shearing rate and inclusion of rheology modifiers. These factors also influence the magnitude of the thixotropy but not necessarily the evolution of thixotropy. Inclusion of VMA in conventional concrete had only slight influence (both increasing and decreasing depending on the method of measurement) on its thixotropic behaviour while the inclusion of SP, VMA + SP and increased water considerably influenced concrete’s thixotropic behaviour in a similar pattern. However, each rheology modifiers had its unique influence. VMA significantly improves the evolution of concrete’s viscosity; inclusion of SP tends to result in a rheopectic concrete; addition of VMA + SP and pre-shearing of concrete may result in an unstable thixotropy, while SP can initially dominate the thixotropic behaviour of concrete containing both VMA and SP, VMA’s influence may later surpass that of SP; more water in concrete tends to result in a lesser rate of thixotropic structure breakdown.

Interlayer bond strength is one of the key aspects of 3D concrete printing. It is a well-established fact that, similar to other 3D printing process material designs, process parameters and printing environment can significantly affect the bond strength between layers of 3D printed concrete. The first section of this review paper highlights the importance of bond strength, which can affect the mechanical and durability properties of 3D printed structures. The next section summarizes all the testing and bond strength measurement methods adopted in the literature, including mechanical and microstructure characterization. Finally, the last two sections focus on the influence of critical parameters on bond strength and different strategies employed in the literature for improving the strength via strengthening mechanical interlocking in the layers and tailoring surface as well as interface reactions. This concise review work will provide a holistic perspective on the current state of the art of interlayer bond strength in 3D concrete printing process.

Web-crushing failure in beams is a brittle failure mode as it typically occurs suddenly without warning. In order to ensure ductile shear behaviour, shear strength is limited by the maximum strut capacity equation (web-crushing equation) to prevent the crushing of concrete before the yielding of shear reinforcement. The accurate prediction of the web-crushing strength of beams is an involved process. Consequently, many codes of practices have resorted to implementing simplified methods for predicting web-crushing failure in beams. Eurocode 2 uses the Variable Strut Inclination Method. This paper aims to assess the model uncertainty and bias (model error) related to the web-crushing strength of prestressed concrete beams. The characteristics of the model error were obtained by comparing experimental web-crushing strengths to predicted web-crushing strengths. Effects of shear parameters on the model error realisations were analysed. The model error was treated as a random variable and used as input in the reliability and safety performance assessment of prestressed concrete beams designed according to Eurocode (EC2) design provisions and failing due to web-crushing. The reliability analysis was conducted using the First Order Reliability Method (FORM). The assessment revealed that the reliability levels implied by the operational EC2 code for web-crushing failure in beams are not consistent over the parametric variation of shear reinforcement (ρwfywm) investigated. The results indicated an increase in estimated reliability index (β) values as ρwfywm increases and a decrease in reliability as concrete strength fcm increases. Low strength beams with ρwfywm ≤ 12.5 MPa and medium strength beams with ρwfywm ≤ 17 MPa generally failed to meet the resistance safety requirement as recommended by EN 1990. The research has contributed to existing studies on the effective application of reliability methods in deriving structural design guidelines.

The major processes that influence and constitute the plastic cracking behaviour of concrete include bleeding, evaporation, settlement, shrinkage, and capillary pressure. These underlying processes mainly occur during the plastic phase of concrete, during which time concrete has considerable rheological properties. Potentially, there should be interactions between these phenomena. The goal of this experimental study is in two phases – showcase the plastic cracking behaviour of concrete and reveal the nature of its interactions with the rheological properties. A background to the envisaged role of rheology is provided, the plastic cracking and rheological behaviours of five concrete mixes were independently investigated, and the relationship between them established. Emphases are placed on the cracking underlying process (settlement, shrinkage, capillary pressure) and the initial/evolving rheo-physical parameters. The plastic cracking behaviour is shown to have three unique stages observable on the plastic cracking behaviour plot. The settlement process is responsible for the interconnectivity with rheology and is interwoven with the plastic shrinkage and capillary pressure. Hence, the rheological interdependence is mainly valid during the settlement period. An initial settlement period is directly linked to the permeability properties while the later period is inversely influenced by the yield stress and its evolution (structuration). The plastic shrinkage and capillary pressure build-up are directly linked to the yield stress and thixotropy. The study suggests that the stiffness evolution of concrete and its suspending mortar in terms of storage modulus does not influence the plastic cracking underlying processes.

A key benefit of 3D concrete printing is the removal of the need for a mould enabling one-off and low volume production with greater variability in part geometry. The staircase effect generated by the layering is used as a feature of many applications, however for smooth surfaces and at interfaces between components, this may not always be desirable. In addition, global deformation of the part during printing may influence the accuracy of the final element. In order to compete with cast equivalents in this regard, secondary treatments such as subtractive milling and trowelling can be applied post printing. These are relatively new developments in the field of digital fabrication with concrete and there is little published work on methods and in particular, none that examine the effect of milling on the surface condition of 3D printing mortar. This paper explores these issues based on a manufacturing tooling perspective and finds that high quality surfaces can be formed, the roughness of which is affected by the size of the aggregates in the mix and the set-state of the material, which needs to be close to the solid state. In practice, this results in a ‘milling window’ which has implications for process timing and the robotic systems typically deployed for 3D concrete printing applications.

The advancement of concrete technology such as 3D concrete printing has rekindled research interest in the early-age mechanical properties of concrete. However, the non-adaptability of early-age (plastic) concrete to classical mechanical measurements causes the quest for alternative approaches. This study is a new and unique approach, using rheological methods, aimed at investigating the early development of shear and viscoelastic properties of concrete from the time of mixing up to the initial setting time. Dynamic shear rheometry (coupled with rotational tests), taking advantage of the normal stress measurement, was used to evaluate the shear and viscoelastic properties. Viscosity modifying agent (VMA), superplasticiser (SP) and increased water content were incorporated to achieve a robust test program. The Mohr-Coulomb shear parameters, shear yield parameters, shear modulus, creep and stress relaxation properties were evaluated from rheometry tests. Theoretical and experimental analyses of a vane-in-cup rheometry reveal that the resultant normal stress, during an elastic response, originates from the self-weight of the sample and is dependent on the sample's weight distribution. The results, therefore, show that it is not sufficient to estimate the self-weight of the concrete for normal stress evaluation and that the shear and creep properties of concrete mainly develop within half-time of the initial set while stress relaxation is inherent and remains fairly unchanged. Challenges and prospects of this approach were also discussed.

Sugarcane bagasse ash (SCBA) has been pitched as a feasible supplement that can ease the sustainability issues associated with cementitious materials. In this respect, SCBA has been well researched with over 100 journal publications. However, there is a scarcity of comprehensive reviews detailing the research developments so far. Therefore, this study is a holistic review with a large scope, content and coverage that emanated from a comprehensive dissection of the literature. The study systematically reviewed the SCBA's origin of pozzolanicity, processing approaches, sustainability potentials, and performance in cementitious materials. The performance entailed characterisation, pozzolanic activity, microstructure refinement, fresh and rheological properties, mechanical and durability properties. SCBA processing is key to its reactivity and performance; hence, recommendations were made on the best processing approach. Performance ranging from 80 to 160% of the control (without SCBA) exist in the literature. Such performance emanates from a conservative use of about 20–25% or further processing that yields optimum contents up to 50%. This study serves as an up-to-date appraisal of data and researches related to the use of SCBA in cementitious materials.

The global quest for sustainable cementitious binders and composites has been on the increase. Bamboo leaves and clay brick wastes were evaluated for their potential to serve as a sustainable pathway for cementitious composites. This was based on the premise that the combined chemical compositions of bamboo leaf ash (BLA) and ground clay brick (GCB) can potentially yield cementitious composites with adequate strength and durability. The BLA and GCB were blended with Portland cement (PC) in ten different proportions, and the obtained binders extensively characterised to certify their use in cementitious composites. The binders were incorporated in concrete, and their performance evaluated. Furthermore, their potentials to improve the sustainability of concrete were evaluated. The results show that though GCB outperforms the BLA, they can both be combined at 10% each to achieve acceptable strength and resistance to chemical attack. It was concluded that BLA and GCB could potentially improve the sustainability of concrete production by reducing the CO2 emission by 19%, embodied energy by 15% and cost by 10% without increasing the energy demand to achieve concrete strength similar to that of the Portland cement.

Purpose The purpose of this paper is to investigate the extent to which green building features are evident in office properties in Lagos, Nigeria; and consequently determine the degree of compliance with green standards in the country. Design/methodology/approach The study purposively sampled two (2) office properties from the management portfolio of 88 registered Estate firms in Lagos. Data were collected using physical observation on the properties and interview with two users purposively selected from each of the properties. The data were analysed with the use of frequency distribution, percentages and measures of green features availability index. Findings The result revealed a low extent of green features incorporation in existing office properties with the value of availability indices on most features falling below 2.5 on a five-point scale. Feature relating to material use and conservation is the most incorporated green feature (mean score of 2.62) while those relating to owner and occupant education were least in incorporation (mean score of 1.895). Practical implications From the findings, it is apparent that green retrofitting may be necessary in Nigeria due to the low extent of green practices in existing office properties. Also, reinforcement of existing government policies and increased sensitisation of stakeholders on impact of current building practices are pertinent to green building success in Nigeria. Originality/value Most existing studies of similar focus are based in the developed economies where stronger implementation framework exists for green building. Besides, they are mostly based on evaluation of green certified buildings using few criteria. This study differs in that it presents the existing building sustainability practices in a less pronounced green property market, with varying architectural styles using more robust criteria. Information provided is applicable in Nigeria and other emerging economies.

Production of concrete's constituent materials has led to the depletion of the environment and natural resources. This experimental study is, therefore, geared towards the sustainable production of concrete that involves the use of a large volume of industrial waste in concrete. Nigerian electric arc furnace steel slag (EAFS) as industrial waste was used as coarse aggregate in concrete for the replacement of natural aggregate. Three sizes of 5/10, 5/14 and 5/19 mm of coarse aggregate at EAFS content of 0, 25, 50, 75 and 100% were considered. The EAFS aggregate morphology was characterised with X-ray computed tomography. Tests carried out on the EAFS concrete include compressive and split tensile tests, water absorption, initial surface absorption and sorptivity. Other properties evaluated include water penetration, porosity, permeability, and resistance to sulphate attack. Results show that it is possible to completely replace the natural coarse aggregates with bigger sized EAFS without significant detrimental effects on the concrete's mechanical and durability properties. Statistical analysis showed that increased curing of the concrete tends to nullify the negative effects of EAFS aggregates' size and content on the mechanical properties. The green performance analysis shows that EAFS coarse aggregate can be used to potentially produce a greener concrete containing about 30% volume of industrial waste with a cumulative 20% savings in energy, emission and cost.

The self-settlement phase of plastic cracking is majorly shear dominated and can be characterised by shear rheometry. This phase is susceptible to microstructural damage/cracking that can be rapidly propagated by the later occurring plastic shrinkage. This study experimentally determined the material properties and behaviour of rheologically modified concrete mixes using shear rheometry and measured their plastic cracking. Von-Mises and Hencky, and Bresler-Pister theories were then used as failure criteria with hypothetically fully restrained plastic strain/stress analyses to model the occurrence of plastic cracking. The incorporated rheology modifiers influenced the shear properties, viscoelastic abilities and plastic cracking of the concrete. The predictions of the model revealed that the plastic settlement is indeed mainly shear-related, the viscoelastic behaviour influences plastic cracking, and microcracking can occur before the initial setting time. It was also concluded that plastic concrete damage tends to be strain-oriented with pressure-insensitive form of ductile failure.

The viscoelasticity of fresh fine cement-based material can be investigated by rheological means using dynamic shear rheometry, hence, the term rheo-viscoelasticity. However, readily available classical rheometers are limited to the study of paste due to the aggregate size associated with mortar and concrete. This study, therefore, sets out to experimentally investigate the rheo-viscoelastic behaviour of cement-based materials containing rheology modifiers, by progressively evaluating the viscoelasticity of cement paste-mortar-concrete. The tests carried out include strain sweep, shear rate sweep, creep and creep recovery, stress relaxation and three interval thixotropy test (3ITT). The results show a trend of improved linear viscoelastic properties of the control fresh cement-based materials due to progressive inclusion of aggregate (increasing volume fraction and size). However, the rheology modifiers widely varied this trend, tending to make it difficult to approximate the rheo-viscoelastic behaviour of mortar and concrete from that of the cement paste as generally suggested in literature. The increase in the coarse volume fraction improved the stability of the cement-based materials microstructure to varying shearing rates. The study suggests that the liquid phase has a role in improving the creep recovery and stress relaxation of the mixes. Furthermore, pseudo-strain hardening was observed for the cement-based materials and flocculation-driven structuration was found to be reduced for the cement paste due to the progressive inclusion of aggregates leaving hydration as the main driving source of concrete’s structuration.