High Cement Content Research Articles

Effect of water to cement ratio on mechanical properties of FRC subjected to elevated temperatures: Experimental and soft computing approaches

The deterioration of concrete is greatly dependent on the cracks and microcracks development owing to loading or environmental influences. An effective step to avoid the spread of cracks and microcracks is employing of different fibers in concrete and the manufacture of fiber reinforced concrete. On the other hand, fire is one of the cases that always threatens engineering structures. Elevated temperatures cause obvious chemical and physical changes that lead to concrete deterioration. In this study, the effect of adding various fiber with different volume contents in concrete mixture with various cement content was evaluated experimentally. Results indicate that spalling was more dominant in mixture containing steel fiber and with higher amount of cement, while there is not any spalling in mixture containing polypropylene (PP) fibers. Moreover, the reduction in tensile strength of fiber-free concrete specimens is less pronounced in mixtures with higher cement content. In addition, the positive performance of PP fibers compared to steel fibers was proved at higher temperatures and cement contents. Furthermore, by conducting a comprehensive and accurate survey, this research pinpoints gaps in the current literature. Moreover, the gathered dataset serves as input for training a machine learning approach known as Group Method of Data Handling (GMDH). The GMDH network demonstrates a satisfactory accuracy in predicting experimental results, with a MSE of 0.0044.

Study on Failure Characteristics and Acoustic Emission Laws of Rock-like Specimens under Uniaxial Compression

Complex underground conditions make it challenging to conduct extensive coring, and it is difficult for laboratories to carry out a large number of rock mechanics experiments due to the limited number of cores. Rock-like specimens are commonly used in the laboratory to replace coal-rock specimens. In this paper, rock-like specimens with different proportions are produced to study the mechanical properties, failure characteristics, and acoustic emission laws of rock-like specimens under uniaxial compression. The results show that when the rock-like specimen does not contain gypsum, the stress of the specimen decreases rapidly after reaching the peak stress, which is similar to the mechanical properties of hard brittle rock. When the rock-like specimen contains gypsum, the stress of the specimen decreases slowly after reaching the compressive limit, and the failure of the specimen is gentle, which is similar to the mechanical properties of soft rock. When the specimen lacks gypsum, the strength of the rock-like specimen is larger, and the strength of the specimen is positively correlated with the proportion of cement. When the specimen contains gypsum, the strength of the rock-like specimen decreases sharply, and the strength of the rock-like specimen is negatively correlated with the proportion of gypsum. The maximum acoustic emission ringing count increases with higher cement content but decreases with increased gypsum content. The cumulative count change of acoustic emissions approximately underwent four stages, aligning well with the four stages of uniaxial compression failure observed in typical rock. The research results have important reference value for the selection of rock-like materials to replace the original rock materials for laboratory research.

Sustainable and mechanical properties of Engineered Cementitious Composites with biochar: Integrating Micro- and Macro-Mechanical Insight

Engineered Cementitious Composites (ECC) are ductile, strain-hardening cementitious composites with a tightly controlled crack opening profile. A significant concern in the development and application of ECC is its high carbon emissions associated with high cement content consumption. As an emerging green additive, biochar can significantly cut down on the carbon emission of concrete products. However, research that integrates micro- and macro-mechanical insights with biochar incorporation is limited. In this study, micro-mechanical tools indicated that a certain amount of substitution biochar could enhance the tensile properties of ECC. The study assessed various properties including compressive strength, porosity, density, tensile performance, crack pattern, sustainability and cost of ECC with different biochar proportions. The findings showed that incorporating 10% to 20% biochar effectively increased the tensile strain capacity and decreased the crack opening width in ECC materials. This improvement can be attributed to the introduction of finer biochar particles (≤75μm) at the fiber/matrix interfacial transition zone, which alters the fiber-bridging force by reducing the chemical and frictional bond strength between the fiber and matrix, as revealed by micro-mechanical tests and microstructural inspection. Moreover, the utilization of biochar contributes to reducing the carbon emission of ECC, enhancing sustainability while maintaining a ∼10% minimal reduction in compressive strength. Overall, this study introduces a novel approach for reusing low-carbon biomass waste in the production of building materials, offering potential advantages in micro- and macro-mechanical performance, cost-effectiveness, and environmental sustainability.

Evaluation of cement-treated recycled concrete aggregates for sustainable pavement base/subbase construction

Rapid urbanization and population growth have caused a substantial rise in the generation of all kinds of waste globally. The use of recycled waste not only can replace the enormous volume of virgin materials used in construction but also alleviate environmental concerns by lowering landfilling loads. This study aims to evaluate the feasibility of using non-treated and cement-treated recycled concrete aggregates (RCA) in pavement base and subbase layers. A detailed study focused on the strength, durability and microstructural aspects of treated RCA. To fulfill this, experimental test program and analysis covered particle-size distribution, Los Angeles abrasion, aggregate impact value, modified Proctor compaction, unconfined compressive strength (UCS), weight loss under wet-dry cycles, and scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis. The recycled aggregates are found to show excellent physical and mechanical characteristics in terms of abrasion and aggregate impact values. Cement treatment has shown to significantly enhance the strength and durability characteristics of RCA, rendering it suitable for pavement base/subbase layer application. RCA samples treated with a cement content of 3 % and above (by weight) are found to satisfy the minimum specified unconfined compressive strength (UCS) values of 2 MPa at 7 days of curing, according to the Texas Department of Transportation (TxDOT) and Austroads guidelines for pavement base and subbase layer applications. However, a relatively higher cement contents (5 % and 7 %) is required to meet the minimum 7-days UCS criterion of 3 and 4.5 MPa for base layer applications of low- and high- volume roads, respectively, according to Indian road standards (IRC SP 72 (2015) and IRC 37(2018)). Cement-treated samples with 2 % cement or above showed minimal weight loss after 12 alternate wet-dry cycles, which was well within the maximum permissible limit (14 %) specified in various standards for stabilized materials. Additionally, microstructural investigations have clearly showed formation of hydration products corroborating the mechanism behind strength gain with time. Findings from this study provide references to promote the use of RCA in construction of pavement base and subbase layer, contributing towards the goal of net zero set by the United Nations.

Rheological, mechanical, and environmental performance of printable graphene-enhanced cementitious composites with limestone and calcined clay

As extrusion-based 3D concrete printing gains wider acceptance in construction, there is a growing imperative to incorporate supplementary cementitious materials (SCMs) into printable mixtures to address their high cement content and promote sustainability. Conventional SCMs like fly ash and slag are becoming increasingly scarce, underscoring the need for alternative solutions such as limestone and calcined clay. Additionally, the utilization of nanomaterials in printable mixtures holds potential for enhancing the mechanical properties of 3D printed structures. These properties are often compromised by interlayer interfaces and void formations during printing, resulting in lower mechanical performance compared to conventionally cast concrete. Graphene nanoplatelets (GNPs), with their high aspect ratios and exceptional mechanical characteristics, offer promising avenues for reinforcing printable cementitious composites. This study explores the rheological, mechanical, and environmental aspects of graphene-enhanced cementitious composites incorporating limestone and calcined clay. Graphene nanoplatelets (GNPs) were dispersed utilizing a surfactant-assisted sonication technique, and their dispersion characteristics were evaluated through absorbance, particle size, and zeta potential measurements. Then, the influence of varying GNP concentrations on the rheological properties of limestone-calcined clay (LC2) cementitious composites was assessed. Compressive and flexural strength tests were conducted on 3D-printed LC2 samples with different GNP ratios, alongside cast specimens for comparison. Microstructural examination of failed specimens was performed using scanning electron microscopy. Furthermore, a life cycle assessment was conducted to compare the environmental impacts of printable LC2 mixtures to conventional printable mixtures. Incorporating GNPs at a ratio of 0.05 % by weight of cement in printable LC2 mixtures enhances compressive strength by 23 %, leading to a reduction of approximately 31 % in environmental impacts compared to conventional printed mixtures.

The Role of Different Clay Types in Achieving Low-Carbon 3D Printed Concretes

Concrete 3D printing, an innovative construction technology, offers reduced material waste, increased construction speed, and the ability to create complex and customized shapes that are challenging to achieve with traditional methods. This study delves into the unique fresh-state performance required for 3D printing concrete, discussing buildability, extrudability, and shape retention in terms of concrete rheology, which can be modified using admixtures. Currently most 3D printing concretes feature high cement contents, with little use of secondary cementitious materials. This leads to high embodied carbon, and addressing this is a fundamental objective of this work. The research identifies attapulgite, bentonite, and sepiolite clay as potential concrete admixtures to tailor concrete rheology. Eight low-carbon concrete mixes are designed to incorporate GGBS at a 50% replacement level and are used to measure the influence of each clay on the concrete rheology at varying dosages. A comprehensive rheological test protocol is designed and carried out on all mixes, together with other tests including slump-flow and compression strength. The objective is to determine the applicability of each clay in improving the printability of low-carbon concrete. The findings reveal that at a dosage of 0.5%, sepiolite was seen to improve static yield stress, dynamic yield stress, and rate of re-flocculation, resulting in improved printability. The addition of attapulgite and sepiolite at a dosage of 0.5% by mass of binder increased compressive strength significantly; bentonite had very little influence. These gains are not repeated at 1% clay content, indicating that there may be an optimum clay content. The results are considered encouraging and show the potential of these clays to enhance the performance of low-carbon concrete in 3D printing applications.

Mechanical performance and permeability of low-carbon printable concrete

Common issues in 3D printed concrete arise due to its high cement content, processing requirements, and the lack of extensive research into its long-term performance. To this end, this study explores using supplementary cementitious materials from local regions to partially replace cement, aiming at reducing the carbon footprint. Various printable mixtures with different substitution rates are formulated, and corresponding mechanical and permeability tests are conducted. It is revealed that the compressive and flexural strengths of 3D printed concrete with low cement content were lower than the strength of cast concrete due to the construction process of printed concrete; the mechanical strength of 3D printed concrete showed prominent anisotropic characteristics. It was found that, compared with the other two sets of ratios, the higher contents of FA and GGBFS could effectively fill the pores of the printed concrete. Both the capillary water absorption and the content of chloride ions of the low-carbon mixture were significantly lower than that of the pure cement-printed specimens.

A case study on early-age cracking of high-strength concrete construction by coupled thermal-mechanical analysis and field monitoring

The use of high-strength concrete (HSC) in constructing high-rise reinforced concrete buildings and their deep foundations is becoming more popular because of the significant reduction in member size and, thus, self-weight in structural members. However, the higher cement content in HSC may result in increased heat of hydration and autogenous shrinkage, growing concerns about forming early-age cracks in HSC structural members. This case study assessed the early-age cracking of HSC in bored pile construction of high-rise buildings. A nonlinear transient coupled thermal-mechanical response analysis was performed to evaluate the potential of early-age cracking caused by the heat of hydration generated in the HSC using ABAQUS and two subprograms, USDFLD and UEXPAN. A 3-D finite element model was developed considering thermal effects, time-dependent evolution of early-age strength and stiffness of HSC, crack strain development, autogenous shrinkage, and stress-relaxation in the analysis. Field measurements were carried out on a 3-m diameter bored pile constructed using Grade C50/60 HSC (fck,cube = 60 MPa) for a high-rise building project in Hong Kong to evaluate the validity of the numerical model. Optical fiber sensors were used to monitor the temperature changes of the pile for approximately 380 hours after casting. The temporal and spatial temperature distributions measured in the field measurements agree well with the simulations. Large thermal gradients and high tensile stress zones were observed across the horizontal cross-sections of the pile, resulting in the formation of vertical annular cracks. Based on the results of the analysis, the maximum crack widths were predicted and compared with the durability crack width limits.

Comprehensive analysis of cement-stabilised dredged marine soil: Evaluating physical, microstructural, and mechanical characteristics

This study evaluates the index, microstructural, and mechanical behaviour of cement-stabilised soft soil. The marine soil dredged from Hangzhou is taken for the study. Different percentages of ordinary Portland cement (OPC)by dry mass were added to the soils during reconstitution, i.e., 5, 10, 15, and 20%. The reconstituted samples were analysed in the laboratory to determine basic properties such as moisture content, void ratio, density, specific gravity, consistency limits, and particle size distribution. The microstructures of the reconstituted samples were also evaluated through scanning electron microscopy. Results inferred a significant impact on the physical properties of the soil. An increase in cement content increases the consistency limits. Additionally, the specific gravity, plasticity index, and density of the treated soil initially decreased but increased upon further addition of cement content. The microstructure of the soil transformed from a dispersed to a flocculated structure, with larger and denser particles. The change in microstructure was also evident in the particle size distribution, with an increase in cement content leading to larger size particles. Finally, the unconfined compression test conducted on the reconstituted soil indicates a substantial enhancement in strength with higher cement content and prolonged curing periods.

A Comprehensive Overview of Recycled Glass as Mineral Admixture for Circular UHPC Solutions

This review article analyzes the influence of recycled glass (as sand and powder) beyond the durability, rheology and compressive strength of plain UHPC, even exploring flexural and direct tensile performance in fiber-reinforced UHPC. Interactions with other mineral admixtures like limestone powder, rice husk ash, fly ash, FC3R, metakaolin and slags, among others, are analyzed. Synergy with limestone powder improves rheology, reducing superplasticizer usage. Research highlights waste glass–UHPC mixtures with reduced silica fume and cement content by over 50% and nearly 30%, respectively, with compressive strengths exceeding 150 MPa, cutting costs and carbon footprints. Furthermore, with the proper fiber dosage, waste glass–UHPC reported values for strain and energy absorption capacity, albeit lower than those of traditional UHPC formulations with high cement, silica fume and quartz powder content, surpassing requirements for demanding applications such as seismic reinforcement of structures. Moreover, durability remains comparable to that of traditional UHPC. In addition, the reported life cycle analysis found that the utilization of glass powder in UHPC allows a greater reduction of embedded CO2 than other mineral additions in UHPC without jeopardizing its properties. In general, the review study presented herein underscores recycled glass’s potential in UHPC, offering economic and performance advantages in sustainable construction.

The Effect of Using Jute on the Compressive Strength of Concrete

Background Concrete comprises cement, fine aggregates, coarse aggregates, and water, making it the most widespread construction material globally. However, while producing cement as an important component of concrete, it generates significant CO2 emissions, worsening global warming and environmental challenges worldwide. Objective This study aims to investigate the impact of incorporating jute fiber into concrete mixes of varying proportions to enhance compressive strength. Methods Concrete mixes of three grades were prepared, with varying percentages of jute (0.5%, 1.0%, 1.5%, and 2%) by weight of cement. Compressive strength tests were conducted at 7 and 28 days of curing. Results It shows notable enhancements in strength, improvement of 15.07 % (for Nominal mix 1:1:2) and 20.29% (for Nominal mix 1:1.5:3 ) is at the addtion of 1% jute but 6.72% (for Nominal mix 1:2:4) is at addtion of 2% jute mix. Conclusion Findings suggest that adding jute fiber, particularly with higher cement content, significantly enhances concrete strength, emphasizing its potential as a reinforcing material for sustainable construction.

Enhancing riverbank stability: A case study on soil improvement through Jet grouting along Can Tho riverbank

Soil cement mixing (SCM) using a high-pressure grouting method (Jet grouting), considered an improvement solution for soft soil has been popular in countries, especially in Japan and other countries in Northern Europe. However, the implementation of Jet grouting is still quite modest in the Mekong Delta. In this study, the depth effect on the compressive strength of soil-cement mixing is conducted at the Can Tho River embankment project. The results show that the uniformity of SCM depends on the cement content. The higher cement content gives a better distribution of compressive strength from the bottom to the top of SCM. In this study, the cement content of 400 kg/m3, considered the suitable content for the Jet-grouting method in Can Tho, is much higher than in (TCVN 9906:2014).

Slope stability analysis of cement and silica fume stabilized expansive soil slope

The objective of this study was to assess the impact of chemical stabilization on slope stability using water retention test data. The analysis focused on hypothetical expansive soil with a 10 m height and a 34° angle. Two critical unsaturated hydraulic properties were considered: (1) soil water retention curve and (2) hydraulic conductivity function. Chemical stabilizers were introduced at varying percentages relative to cement content, with silica fume (SF) partially replacing portions of the cement. The influence of stabilizers on slope stability was evaluated by examining changes in the factor of safety (FOS) with different stabilizer concentrations. Generally, the FOS increased with higher cement content, with further improvement observed when cement was partially replaced by silica fume. Additionally, the addition of silica fume enhanced water retention capacity, as evidenced by altered pore pressure distribution and water table levels post-rainfall infiltration.

Digital fabrication material processing strategy for bespoke low clinker mass concrete components

Digital manufacturing techniques are seen to be very promising in concrete construction because of their potential for increasing productivity, as well as their potential to increase sustainability due to reduced material usage via nonstandard geometry. Concrete 3D printing via extrusion has received the most attention as a method, however the problem of implementing reinforcement remains unresolved; therefore, printed concrete typically serves as either a formwork or as unreinforced masonry. The recently developed technique of Digital Casting allows for digitally produced concrete components that implement standard reinforcement bars. The technique was developed especially for digitally produced, nonstandard formworks, and relies on controlled set acceleration of a self-compacting concrete to control formwork pressure during production.Digital fabrication techniques such as Digital Casting (and 3D Printing), however, have notoriously high cement contents due to processing requirements, and this may negate any sustainability benefit that can be realized through material savings in a bespoke concrete component. Research efforts to bring these cement contents closer to standard concretes have focused on: 1) reducing paste content through increased aggregate content and 2) reducing clinker content through substitutions, with most recent efforts focused on the latter. A recent project demonstrated, additionally, that elevated clinker contents negatively impact the casting of mass concrete elements due to high heat generation, which limits the applicability of digital fabrication for mass concrete applications.In this study, the use of a CEMIII (slag substituted cement) based mix design for this type of application was examined. The mix was successfully accelerated to control formwork pressure in a digital casting process of a thick (>1 m) bespoke element with overhang, and internal temperatures monitored. The reduced heat release generated temperatures of only half of those generated by a previous mix design with higher clinker content.

Reservoir Characterization of Alluvial Glutenite in the Guantao Formation, Bohai Bay Basin, East China

Alluvial glutenite reservoirs have obviously stronger heterogeneous and more complex control factors than sandstone reservoirs. Taking the Binxian Uplift area in the Boahi Bay Basin as an example, the aim of this study is to clarify the characteristics and control factors of the alluvial glutenite reservoir quality and the influence of reservoir properties on hydrocarbon accumulation. Pore types in the study area mainly include residual intergranular pores, intergranular dissolved pores, intragranular dissolved pores, and mold pores. The residual intergranular pores and intergranular dissolved pores are the main pore types. Most samples have porosity greater than 15% and permeability is mainly concentrated between 50 mD and 500 mD. It is shown that lithology type, microfacies, and diagenesis have significant impact on the reservoir quality. The reservoir qualities of very fine sandstone and fine sandstone are better than those of conglomerate and gravel-bearing sandstone. Instead of grain size, sorting affects the alluvial glutenite reservoir quality significantly. Oil-bearing samples commonly have sorting coefficient less than 2 while non-oil-bearing samples have sorting coefficient larger than 2. There are significant differences in reservoir physical properties of different sedimentary microfacies. The stream flow in mid-alluvial fan (SFMA) and braided channels outside alluvial fans (BCOA) have relatively weaker compaction and better reservoir quality than the overflow sand body (OFSB) and debris-flow in proximal alluvial fan (DFPA). Calcite cementation, the main cement in the study area, commonly developed at the base of SFMA and BCOA and near the sandstone-mudrock contacts. The source of calcium carbonate for calcite cement mainly came from around mudstone. High calcite cement content commonly results in low porosity and permeability. Individual glutenite thickness is also an important influencing factor on reservoir quality. Reservoirs with large thickness (>4 m) have high porosity and permeability. Dissolution occurred in the reservoir, forming secondary dissolution pores and improving reservoir quality. The dissolution fluid for formation of secondary pores is mainly meteoric waters instead of organic acid. The reservoir property has an important influence on hydrocarbon accumulation. The lower limit of physical properties of an effective reservoir is a porosity of 27% and permeability of 225 mD. The findings of this study can be utilized to predict the reservoir quality of alluvial glutenite reservoirs effectively in the Bohai Bay Basin and other similar basins.

PERFORMANCE EVALUATION OF LATERITE AND PULVERIZED RECLAIMED ASPHALT PAVEMENT STABILIZED WITH BASE SEAL AND CEMENT

This research delves into the effectiveness of incorporating pulverized Reclaimed Asphalt Pavement (RAP), Base Seal, and cement to enhance the stability of lateritic base materials in Nigeria. Addressing a crucial gap in pavement engineering literature, particularly the underexplored use of base seals as liquid stabilizers, the study adopts a comprehensive methodology involving literature review, field evaluation, and laboratory tests. Analyzing various material mixes, from pure Laterite to combinations with RAP, Base Seal, and varying cement contents, the research reveals substantial improvements in crucial engineering properties. Enhanced particle size distribution, reduced clay content, and optimal compaction characteristics at 0% cement content for laterite materials with pulverized RAP signify the positive impact of these additives. California Bearing Ratio analysis establishes a consistent correlation between higher cement content and improved load-bearing capacity. Notably, the synergistic effects of RAP and base seal, particularly at 5% cement content, showcase the highest CBR value (91), indicating optimal conditions for strength and stability. The study concludes with practical recommendations for road maintenance authorities, advocating for innovative approaches like full-depth reclamation techniques and stabilized full-depth reclamation, exploring different RAP to base material ratios, examining curing periods' impact, assessing long-term performance, considering environmental implications, and conducting field trials for validation. Overall, these findings contribute valuable insights to pavement design practices, fostering the development of resilient infrastructure in construction projects.

Engineered geopolymer composite (EGC) with ultra-low fiber content of 0.2%

Engineered cementitious composite (ECC) features with excellent tensile performance. However, two major drawbacks, including the high environmental impacts associated with high content of cement and the high cost caused by high fiber content, impede the wide application of ECC. This study designs and successfully develops a high ductile cement-free engineered geopolymer composite (EGC) with ultra-low content of fibers (0.2% by volume, EGC-0.2%). EGC-0.2% exhibits a tensile strain capacity above 4% and a controllable crack width around 200 µm. The compressive strength reaches 39 MPa and the density is below 1200 kg/m3, showing an excellent specific strength (the ratio of the compressive strength-to-density) of 37 kPa/(kg/m3). The matrix fracture toughness of the developed EGC is low to facilitate the high ductility of the EGC-0.2% by such low fiber content. In addition, EGC-0.2% presents extremely low environmental impacts and cost. The embodied energy and embodied carbon decrease by 23% and 67% than conventional concrete, respectively, and the cost is only 21% of the classic M45-ECC with similar mechanical performance. The current findings provide valuable references for the future advancement of ECC towards high sustainability and cost effectiveness.

Hydration mechanism and mechanical properties of a developed low-carbon and lightweight strain-hardening cementitious composites

Strain-hardening cementitious composites (SHCC) have been widely studied due to their high toughness and durability. The high cement content increases a huge burden on the cost and CO2 emissions of SHCC. To reduce the environmental impacts of SHCC, this study first develops low-carbon and lightweight SHCC by using high-volume recycled concrete powder (RCP) as supplementary cementitious materials and cenosphere waste as lightweight aggregate. The influence of RCP content (0%, 15%, 30%, and 45% by mass) on the hydration mechanism, mechanical properties, and sustainability of low-carbon and lightweight SHCC was investigated. The results showed that the incorporation of RCP in the SHCC matrix resulted in a decrease in hydration heat with increasing RCP content. The filling and pozzolanic effects of RCP were significantly lower than those of cement. The increased porosity and the presence of the interface transition zone due to RCP incorporation led to reduced compactness of the SHCC, which consequently led to decreased compressive strength and fracture toughness of the cement matrix. While the fracture toughness of the cement matrix was reduced, the SHCC still exhibited remarkable bending toughness and tensile ductility. The developed low-carbon and lightweight SHCC containing 45% RCP showed a density of 1482.5 kg/m3, a tensile strength of 3.94 MPa, and a tensile strain capacity of 6.80%, which successfully pushed the performance of low-carbon and lightweight SHCC. The replacement of cement with high-volume RCP in the low-carbon and lightweight SHCC resulted in a significant reduction in embodied carbon compared to conventional SHCC. Therefore, SHCC combines the advantages of lightweight, low-carbon, and highly ductile, making it a promising material for widespread utilization in concrete structures.

Engineering behaviour of in situ cored deep cement mixed marine deposits subjected to undrained and drained shearing

The deep cement mixing (DCM) is used to improve the capacity and reduce the settlement of the soft ground by forming cemented clay columns. The investigation on the mechanical behaviour of the DCM samples is limited to either laboratory-prepared samples or in-situ samples under unconfined compression. In this study, a series of drained and undrained triaxial shearing tests was performed on the in-situ cored DCM samples with high cement content to assess their mechanical behaviours. It is found that the drainage condition affects significantly the stiffness, peak and residual strengths of the DCM samples, which is mainly due to the state of excess pore water pressure at different strain levels, i.e. being positive before the peak deviatoric stress and negative after the peak deviatoric stress, in the undrained tests. The slope of the failure envelope changes obviously with the confining pressures, being steeper at lower stress levels and flatter at higher stress levels. The strength parameters, effective cohesion and friction angle obtained from lower stress levels (c′0 and φ′0) are 400 kPa and 58°, respectively, which are deemed to be true for design in most DCM applications where the in-situ stress levels are normally at lower values of 50–200 kPa. Additionally, the computed tomography (CT) scanning system was adopted to visualize the internal structures of DCM samples. It is found that the clay pockets existing inside the DCM samples due to uneven mixing affect markedly their stress-strain behaviour, which is one of the main reasons for the high variability of the DCM samples.

Comparative studies of LC3- and fly ash-based blended binders in fibre-reinforced printed concrete (FRPC): Rheological and quasi-static mechanical characteristics

Supplementary cementitious materials (SCMs) became in recent decades popular and promising materials as partial replacements for cement in construction engineering. These neutralise the effects of high cement content required for the extrusion process of 3D concrete printing and enhance its sustainable benefits. The crucial advantages are the reduction of CO2 emission and energy consumption of cement-based materials, high performance, and cost-effectiveness. This study compares the rheology and hardened mechanical properties of fibre-reinforced printed concrete (FRPC) containing limestone calcined clay cement (LC3-FRPC) and fly ash (FA-FRPC). Microstructural evolution, including the porosity, CT image, and pore volume distribution in mould-cast and extrusion-based 3D printed fibre-reinforced concrete was analysed by X-ray computed tomography to examine its influence on mechanical performance. Scanning electron microscopy, augmented by Energy dispersive X-ray spectroscopy was also used to analyse the macropores structure of the mixtures. SCMs mixtures considered exhibit more than 160 min printing open time, thereby displaying satisfactory material rheological performance and suitable for 3D printability. However, LC3-FRPC enhanced the workability, open time, and buildability by 1.7 %, 15.4 %, and 19 %, respectively, in comparison to FA-FRPC. The yield shear stresses and shear modulus of the mixtures, particularly LC3-FRPC increased linearly with resting time intervals and are within the minimum recommended limits for sound shape retention and buildability of materials. The results confirm higher strengths for mould-cast and intralayer concrete specimens, particularly D3 in compression and D1 in tension and flexure for FA-FRPC compared to its 3D-printed interlayer counterpart. The strengths obtained in LC3-FRPC follow the trends observed in the literature, with mould-cast > D1 > D3 by 18.4 % and 41.7 %, and mould-cast > D1 > D3 by 6.5 % and 57.4 % for tension and flexure, respectively. These could be ascribed to the low total porosity and critical pore diameter at the interface of the specimens. Although there is a slight reduction in the compressive, tensile, and flexural strengths of LC3-FRPC (due to the high-water demanded) compared to FA-FRPC, the significant levels of C-A-S-H gel polymerisation and ettringite formation in LC3-FRPC is beneficial for improving the flexural strength of the fibre-matrices. The polypropylene fibres in the macropores, as seen by SEM imaging enhance the mechanical characteristics of the mould-cast and printed specimens of both mixtures. Then, the interfacial bond strength is enhanced in LC3-FRPC compared to FA-FRPC by 8.1 % and 9.8 % for tension and flexure, respectively, thereby implying lower level of anisotropy in tension (7.2 %) and flexure (9.2 %). Conclusively, the observed mechanisms of FA- and LC3-based blend binders incorporated in FRPC strengths development, and their influence based on the macropores structure at the interfacial regions of mould-cast and printed concrete are presented.