Concrete Design Research Articles

Experimental investigation of the mechanical performance of novel rigid connections in prestressed circular composite precast concrete columns under cyclic lateral loading

The present study introduces an advanced design of Prestressed Circular Composite Precast Concrete Columns (PCCPCCs), which have been applied in real-world structures, equipped with two novel rigid connection types (i.e., the reinforcing-cage connection and the welded-plate connection) to address existing challenges in the design, construction, and quality control of traditional precast concrete (PC) columns. Five full-scale specimens were experimented individually to assess the mechanical performance of PCCPCCs subjected to cyclic lateral loading and constant axial loading. The results indicate that the specimens exhibited no signs of collapse or loss in axial load capacity during the tests. Furthermore, all specimens met or exceeded the performance criteria set by relevant standards, such as GB50011-2010 and ACI 374.1-05, regarding drift ratio, moment capacity, energy dissipation, and ductility. It demonstrates that the novel design of PCCPCCs proposed in this study can be integrated into practical construction projects designed as lateral-force-resisting systems, such as moment-resisting frames and bridges, to accommodate diverse construction project requirements.

Biochar-enhanced concrete mixes: Pioneering multi-objective optimization

The incorporation of biochar (BC) as a partial substitute for cement in concrete formulations provides a promising pathway for mitigating the environmental impacts associated with carbon dioxide emissions during cement production. To achieve optimal composition of BC-enhanced concrete, multiple objectives including mechanical strength, economic factors, and embodied CO2 must be balanced while considering a multitude of variables constrained by complex non-linear relationships. Here, we introduced an intelligent hybrid optimization algorithm that combines Particle Swarm Optimization (PSO), Least Squares Support Vector Machine (LSSVM), and Non-dominated Sorting Genetic Algorithm II (NSGAII) to predict the performance of BC-enhanced concrete and optimize its mix proportions for multiple objectives. Our results demonstrate that the PSO optimization algorithm for searching LSSVM hyper parameters outperforms other optimization algorithms, exhibiting higher generalization performance and improved overall accuracy (R2 = 0.95). Furthermore, the proposed model framework effectively presents a complete Pareto front for the BC-enhanced concrete mix ratio. This triangular model system for concrete mix ratio can determine the optimal solution, tailored to the preferences of the owner's unit. Ultimately, PSO-LSSVM-NSGAII intelligent hybrid optimization algorithm enhances the efficiency of mix proportion design for BC-enhanced concrete.

Predicting the mechanical properties of plastic concrete: An optimization method by using genetic programming and ensemble learners

This study presents a comparative analysis of individual and ensemble learning algorithms (ELAs) to predict the compressive strength (CS) and flexural strength (FS) of plastic concrete. Multilayer perceptron neuron network (MLPNN), Support vector machine (SVM), random forest (RF), and decision tree (DT) were used as base learners, which were then combined with bagging and Adaboost methods to improve the predictive performance. In addition, gene expression programming (GEP) was used to develop computational equations that can be used to predict the CS and FS of plastic concrete. An extensive database containing 357 and 125 data points was obtained from the literature, and the eight most impactful ingredients were used in the model’s development. The accuracy of all models was assessed using several statistical measures, including an error matrix, Akaike information criterion (AIC), K-fold cross-validation, and other external validation equations. Furthermore, sensitivity and SHAP analysis were performed to evaluate input variables' relative significance and impact on the anticipated CS and FS. Based on statistical measures and other validation criteria, GEP outpaces all other individual models, whereas, in ELAs, the SVR ensemble with Adaboost and RF modified with the Bagging technique demonstrated superior performance. SHapley Additive exPlanations (SHAP) and sensitivity analysis reveal that plastic, cement, water, and the age of the specimens have the highest influence, while superplasticizer has the lowest impact, which is consistent with experimental studies. Moreover, GUI and GEP-based simple mathematical correlation can enhance the practical scope of this study and be an effective tool for the pre-mix design of plastic concrete.

Designing formative feedback in collaborative online international learning

BACKGROUND: The adequate delivery of formative feedback in higher education is vigorously discussed to support the development of students’ learning activities. Most of the literature refers to individual feedback to students in the context of web-based training and teaching in higher education. However, concrete design recommendations are scarce, especially regarding collaborative online international learning modules. OBJECTIVE: This paper aims to identify and systematize the need for formative feedback from students in Virtual Exchange modules and concludes with implications for the design of formative feedback activities. METHODS: The research employs a two-step sequential explorative mixed methods and longitudinal approach. A quantitative pre-test is followed by a qualitative self-reflective journal survey with two data collection dates using a thematic co-occurrence analysis. RESULTS: Based on 11 abductively coded themes, findings include the effects of agents on students and barriers and prerequisites for implementing formative Feedback in COIL modules. CONCLUSIONS: The effects of feedback can vary depending on the agent. While e-tutors need to build trust and give orientation, teachers support reflection, and peers affect motivation. Applicable and quick responses are fundamental for a positive perception of formative feedback. Peer feedback can potentially improve learning and reduce the teacher’s workload.

Analysis of equivalent substitution of rectangular stress block for nonlinear stress diagram

This article deals with the analysis of equivalency of the substitution of rectangular stress block for nonlinear stress diagram with descending branch for the calculation of flexural, eccentrically compressed and eccentrically tensioned reinforced concrete members of rectangular cross-section. The method for equivalent substitution of these diagrams is proposed. Analytical relationships of area and its centre for the nonlinear diagram with descending branch are presented. Coefficients for equivalent substitution of the said diagrams for the standard concrete strength classes are determined and given. It gives the opportunity to design reinforced concrete members using rectangular stress block model. Substitution of the diagrams applied in EC2 and in STR 2.05.05:2005 is compared with the equivalent substitution of rectangular stress diagram for nonlinear stress diagram with descending branch. It is shown that in STR 2.05.05:2005 description of the ratio between the depth of the rectangular diagram and that of the equivalent nonlinear one with descending branch is very poor. An explicit analytical relationship for this ratio and its linear approximation in respect to the concrete characteristic and design strengths are presented in this article as well.

Off-site 3D printed concrete beam design and fabrication

This study identifies the need for a prefabricated reinforced 3D printed concrete (3DPC) beam that can integrate into a building system. A novel 3DPC beam for a specified target structure is designed, fabricated, and experimentally tested. The beam design is experimentally investigated by fabricating the beam with design-determined print parameters and performing a set of two large-scale 4-point bending tests which determine the shear and flexural resistance of the beam. Modified Eurocode design equations successfully predict the flexural resistance within 6%, however the shear design equations overpredict the shear resistance by 51%. The study provides a replicable design and fabrication workflow for 3DPC elements, and a feasible 3DPC beam concept that requires shear strengthening before application in the target frame. 3D concrete printing designers are provided with a 3DPC beam solution with the potential to integrate into a building system and are further equipped to design and fabricate future 3DPC building elements.

Experimental study and numerical simulation of a structural concrete column for building heating integration and energy storage

A steel pipe assemblage is integrated into a structurally reinforced precast concrete column to demonstrate thermal energy storage (TES) and space heating capabilities. This thermal energy column is heated via hot water at varying flow conditions. The system is used as a thermal storage medium, as a means of reducing the demands on the building heat pump, and for regulating the thermal comfort of the building interior environment. The thermal performance of the energy column is examined experimentally and numerically in both charging and discharging phases. Two examples are used to demonstrate the improvements in the operation of building thermal equipment and in the building thermal environment offered by either passive or active discharging of the energy column. Following the conclusion of thermal evaluation, the structural performance of the column under axial compression was evaluated by loading the system to failure. The experiment was paused for visual observation at both the service level and nominal capacity milestones. The longitudinally embedded steel pipes developed comparable strains to the column longitudinal steel reinforcing bars. The structural capacity of the energy column can be accurately predicted using standard concrete design approaches that account for the mechanical influence of the embedded steel pipe assemblage. It is demonstrated that the developed concrete energy column provides a viable option for improving the building thermal energy efficiency and maintaining structural performance.

Mix design optimisation of self-sealing concrete containing microcapsules with polyurethane shell and water repellent cargo

Despite the robust ability of microcapsules (MIC) to break and release agent for healing/sealing efficacy, the reduction of concrete strength has been a major issue when using MIC. In this study, the optimisation of concrete mix design was initiated with employing three main parameters: MIC size (56, 93 μm), MIC dosage (0, 3, 6% by weight of cement) and water-cement (w/c) ratio (0.40, 0.50, 0.60). The used microcapsules consisted of water repellent agent core acting as a sealing agent and polyurethane as the shell material. Results confirmed the MIC dosage as the most significant factor affecting the mechanical properties, while no significant effect on the MIC size. Further, the MIC effect can be reduced by changing the concrete mix design from low to high w/c. The sealing efficiency of concrete was remarkably improved with the inclusion of MIC and a good bond between the capsules and concrete matrix was attained.

Engineering punching shear strength of flat slabs predicted by nature-inspired metaheuristic optimized regression system

Reinforced concrete (RC) flat slabs, a popular choice in construction due to their flexibility, are susceptible to sudden and brittle punching shear failure. Existing design methods often exhibit significant bias and variability. Accurate estimation of punching shear strength in RC flat slabs is crucial for effective concrete structure design and management. This study introduces a novel computation method, the jellyfish-least square support vector machine (JS-LSSVR) hybrid model, to predict punching shear strength. By combining machine learning (LSSVR) with jellyfish swarm (JS) intelligence, this hybrid model ensures precise and reliable predictions. The model’s development utilizes a real-world experimental data set. Comparison with seven established optimizers, including artificial bee colony (ABC), differential evolution (DE), genetic algorithm (GA), and others, as well as existing machine learning (ML)-based models and design codes, validates the superiority of the JS-LSSVR hybrid model. This innovative approach significantly enhances prediction accuracy, providing valuable support for civil engineers in estimating RC flat slab punching shear strength.

Exploring the physical and mechanical characteristics of multi-generation recycled aggregate concrete at equivalent compressive strengths

The concrete industry has been responsible for significant depletion of natural resources and at end-of-life significantly contributes to global waste production. To date experimental studies have typically explored the use of single generation recycled aggregates (RA) in concrete production, however, to move towards a circular economy the concrete industry needs to embrace the use of multi-generation recycled aggregate concrete (MGRAC). In this paper the physical and mechanical performance of 45 concrete mix designs over 4 generations of concrete production is explored. At each generation a series of mixes are conducted at various water to cement ratios (w/c) and with a range of blends of virgin and recycled aggregates from differing generations to capture the impact on material properties. The physical and mechanical properties are quantified for both the aggregates and the concretes at each generation. The results show that the physical properties and some mechanical properties of the concrete and RA decrease with respect to virgin aggregate concrete and virgin aggregate but, implementing a 50% replacement of VA can slow down intergenerational degradation. Furthermore, the blending RA from a range of generations can also slow intergenerational degradation but to a lesser extent. Effective aggregate properties are determined, and direct relationships confirmed indicating potential for informed mix design based on an understanding of a few simple aggregate properties.

Comparison of the Variation of Coarse Aggregate Gradation on the Strength of Concrete

Concrete as the main construction material has an important role in determining the strength and reliability of a construction project. The quality of concrete is influenced by the properties of its constituent materials, such as gravel, sand, cement and water. Testing the compressive strength of concrete is an important step to assess its quality and ensure it meets construction standards. Although previous research has compared the compressive strength of concrete and changes in aggregate volume, this research still covers limited dimensional variations and further exploration of aggregate dimensional variations is needed. This laboratory study used a conventional concrete design according to SNI 03-2834-2000, with compressive strength tests carried out at the Material Testing Laboratory, part of the Civil Engineering Department at Politeknik Negeri Banyuwangi Banyuwangi. The experimental results recorded in the fourth chapter show that at 7, 14, and 28 days, type AG 10 has the highest compressive resistance value, statistically significant mean value, and also shows that type AG 40 has the lowest compressive strength value. Therefore, it can be concluded that the smaller the size variation of coarse aggregate, the higher the compressive strength of concrete. These findings provide a comprehensive picture of the compressive strength characteristics of concrete with varying coarse aggregate sizes.

High-performance lightweight aggregate concrete research and mechanical property analysis: A review

In the current era where prefabricated construction is dominant, under the trends of industrialization, mechanization, and green construction, a series of experiments and explorations have been conducted in a multidisciplinary approach of materials engineering, structural engineering, and civil engineering. This paper presents an overview of existing domestic and international research on high-performance lightweight aggregate concrete used for prefabricated interior partition walls. This includes: research and design of high-performance lightweight aggregate concrete and initial mechanical property analysis; determination of its significant optimization effect for preparing prefabricated interior partition panels; seeking ways to further enhance the performance of lightweight aggregate concrete-made interior partition panels and increase their applicability in building assembly. Taking into account engineering practical applications, the future application prospects of high-performance lightweight aggregate concrete in prefabricated interior partition panels are discussed, alongside further research directions and suggestions. The research outcomes of this paper are of great significance in promoting the development of industrialized construction and green buildings.

Symmetric prism characterisation test to infer heat generation and heat conduction properties of concrete

Performance-based concrete design can accelerate the acceptance of emerging lower carbon concretes to reduce the environmental impact. A semi-adiabatic symmetric concrete prism experimental and analytical approach is proposed as a pathway for concrete thermal characterisation. The prism design promotes 1-D heat flow conditions using a single insulated specimen. The prism dimensions (700 mm ×100 mm ×100 mm) are rationalised to suit usage as a characterisation specimen and mix compositions that replicate those of in-situ concretes can be assessed. The experimental spatial and temporal thermal profiles of a CEM I mix and a high GGBS replacement mix were measured during hydration and through a step change in temperature in the hardened state. Thermocouples and a distributed fibre optic sensor along the central axis of the symmetric prism were used for this purpose. Multi-physics modelling provided a platform to address thermal measurement data uncertainty, extract fundamental concrete thermal parameters and gain insight into the heat generation and heat flow physics in the concrete prism specimens. The complete heat generation and maturity time histories were accurately modelled and ‘concrete thermal footprints’ derived.

Analytical evaluation of stress–strain behavior of rubberized concrete incorporating waste tire crumb rubber

The advancement of rubberized concrete technology and its application in engineering can significantly promote the complete recycling and safe disposal of waste tire rubber. Integrating crumb rubber into concrete alters its compressive stress–strain behavior. This study investigated the stress–strain characteristics of rubberized concrete containing waste tire crumb rubber, addressing the absence of standard guidelines for the design of rubberized concrete. By examining the complete stress–strain curves of rubberized concrete, key parameters such as peak stress, initial elastic modulus, peak strain and secant modulus were meticulously evaluated. The study delved into the mechanisms through which crumb rubber influences the stress–strain behavior of concrete, considering factors including variations in rubber content and pretreatment methods. The experimental data were analyzed using conventional concrete constitutive theories to quantify the impact of crumb rubber on rubberized concrete's stress–strain relationship. Significant variations were observed, including a notable decline in strength with increasing rubber content. For instance, increasing the rubber content from 0.0% to 5.0% led to reductions of 44.2% in peak stress and 47.0% in secant modulus compared to the reference concrete. In addition, pretreating the crumb rubber markedly improved rubberized concrete strength, and the peak stress of concrete with 1.0%–5.0% pretreated crumb rubber increased by an average of 7.0% compared to that with untreated crumb rubber. The study identified that crumb rubber's inclusion significantly altered the coordination of stress and strain in concrete, thereby affecting its stress–strain behavior. A set of evaluation equations for the stress–strain relationship of rubberized concrete was developed, offering a reliable method for predicting its behavior. This research fills a critical gap in the analytical assessment of rubberized concrete and marks a significant stride towards its broader use in sustainable construction practices.

Exploring the interrelationships between composition, rheology, and compressive strength of self-compacting concrete: An exploration of explainable boosting algorithms

This study introduces a novel methodology for enhancing the compressive strength of self-compacting concrete (SCC) via the use of the Explainable Boosting Machine (EBM), a sophisticated and interpretable machine learning algorithm. It presents a data-driven model that aims to accurately predict the strength of SCC by considering the intricate interactions among its various elements. Additionally, the model provides insights into the variables that influence SCC's compressive strength. By using EBM in conjunction with XGBoost and CatBoost algorithms, this study conducts a comparative examination of predictive abilities using datasets related to composition and rheology. The findings reveal that CatBoost has greater predictive performance using rheology dataset, as shown by an R2 value of 0.977. Conversely, XGBoost exhibits a higher predictive capability using the composition dataset, as indicated by an R2 value of 0.947. The EBM can provide comprehensive explanations at both global and local levels. It effectively identifies the key factors that have a significant influence on compressive strength. These factors include the coarse aggregate content, cement content, water content, viscosity, and V-funnel flow time. The study findings provide more evidence to support the notion that including rheological data into the model leads to a notable improvement in its accuracy. This outcome further confirms the existence of a direct correlation between rheological properties and compressive strength. The explanatory insights provided by EBM give practical instructions for customising SCC mixes to attain desired strengths. This facilitates quality control and enables personalised concrete design in the field of construction. This study highlights the potential of interpretable machine learning algorithms in improving the predictive modelling of SCC features. This advancement may lead to the development of more durable, efficient, and customised building materials.

Environmental impact evaluation of low-carbon concrete incorporating fly ash and limestone

This work examines the environmental impact of low-carbon concrete that incorporates supplementary cementitious materials (SCMs). After reviewing near-zero carbon SCMs and low-carbon concrete, a life cycle assessment (LCA) was undertaken for concrete mix designs with normal-to-high compressive strengths, incorporating limestone and fly ash as cement replacements. The analysis includes relevant region-specific life cycle inventory parameters for raw materials, energy production, and transportation. A comparative assessment between embodied carbon emissions and the material mechanical performance is then made. The results of this paper indicate that incorporating limestone and fly ash in concrete can reduce carbon emissions, yet at a proportional decrease in mechanical properties compared to conventional cement concrete. The combination of cement and fly ash produced, on average, a higher strength concrete by 20.5% and lower CO2-eq values by 21.1% when compared to limestone cement blends. The CO2-eq emissions associated with transportation of the main constituents for concrete production were on average below 4% of the total CO2-eq per mix. In addition to eco-mechanical quantitative assessments, the study offers insights and recommendations for the development of concrete materials considering global resource availability of near-zero carbon concrete constituents.

Probabilistic effect of non-potable mixing water on characteristic strength of concrete

Across the world, the acceptance criterion for concrete made with non-potable water (SA) is stipulated for mean strength with reference to similar concrete made with potable water (SP). The basis for the strength design of concrete is the characteristic strength. The effect of the acceptance criteria on the characteristic strength of SA and SP was examined using statistical simulations, for the first time. It was found that, compared with a two-sample set, using a three-sample set would improve the characteristic strength of SA. A higher allowable ratio between the means of SA and SP and a lower deviation of individual samples from the mean would improve the characteristic strength of SA. The effect of the coefficient of variation (CoV) of the original population on the characteristic strength of SA varied. On the one hand, a higher CoV reduced the characteristic strength of SP, thereby increasing the probability of SA strength being higher than SP strength. On the other, a higher CoV of SP meant a wider spread of the samples and that could increase the CoV of SA, thereby reducing the characteristic strength of SA. Probabilistic charts were developed for the characteristic strengths of SA and SP for different combinations of the aforementioned variables, with guidance for practical application.

Recycled glass powder as an alternative to fly ash in non-proprietary UHPC: A comparative study of resource-efficient design, mechanical and durability properties

Fly ash (FA) is one of the preferable supplementary cementitious materials used in the design of resource-efficient ultra-high performance concrete (UHPC). Recent changes in energy production have led to a reduced availability of FA which increases the need to find an alternative replacement. One suitable, innovative, and sustainable pozzolan with higher consistency in material properties is recycled glass powder (RGP), produced from 100% post-consumer glass. To fill the knowledge gap whether RGP is a suitable alternative to FA in non-proprietary UHPC, this research emphasizes the comparative investigation of material properties of UHPC using FA versus RGP. The comprehensive material characterization and analysis included workability, mechanical behavior such as compressive strength, single fiber pull out and direct tensile behavior, static elastic modulus, and long-term strength. Durability properties included electrical surface resistivity and freeze-thaw resistance. In addition, a comparative analysis for their resource-efficiency and service life was also performed. The results were compared to two commercial UHPCs, available in the United States. This research showed that UHPC with RGP can be designed to achieve comparable mechanical and durability properties as UHPC with FA. Compressive strength up to 176 MPa (25.5ksi) were obtained without the use of special treatment, at current material cost of about US$1145/m3 with 2% of fiber volume fraction of steel fibers. Over time, the compressive strength of UHPC with RGP increased by 15%, resulting in more than 200 MPa (29.0ksi) after three years. Both UHPCs designed with either FA or RGP showed strain-hardening under direct tension with maximum tensile strength of about 14.8 MPa (2.1ksi). Both the UHPC with RGP and FA achieved electrical surface resistivity of more than 870kΩ × cm and 822kΩ × cm in 28 days and relative dynamic modulus of 102% after 600 freeze-thaw cycles that ensure long term durability with design service life of 364 and 344 years respectively.

Impact of Recycled Concrete and Brick Aggregates on the Flexural and Bond Performance of Reinforced Concrete

The construction industry strongly relies on concrete and clay bricks for various applications. The escalating demand for these materials, driven by rapid population growth, has led to resource depletion and increased construction and demolition waste (CDW). Recycling CDW into construction materials, particularly in the form of recycled concrete aggregates (RCAs) and recycled brick aggregates (RBAs), has emerged as a promising solution. This study deals with the structural performance of concrete incorporating RCAs and RBAs. The experimental program encompasses material characterization, concrete mix design, and several tests to assess density, compressive strength, bond behavior, and flexural properties. The results indicate that the replacement of fine natural aggregate (NA) with fine RCAs or RBAs has a negligible impact on density, while the partial replacement of coarse NAs with RAs yields modest reductions in compressive strength. Notably, the bond strength between steel rebar and concrete is influenced by the type and content of RA, with specimens containing RCAs exhibiting a higher bond strength than those with RBAs. Empirical models used to predict bond strength generally align with experimental results, with conservative predictions by some models, such as ACI 318, and overestimation by others, such as models proposed by AS-3600 and CEB-FIB. The flexural tests of beams highlight the variation in stiffness and load-bearing capacity with the proportion of NAs replaced by RAs. While beams with 50% NA replacement demonstrate comparable performance to control beams, those with 100% RA replacement exhibit lower cracking and yielding stiffness. Cracking patterns in beams with RAs differ from control beams, with RA-containing beams showing more cracks and an altered crack distribution. The findings underscore the feasibility of using recycled aggregates in construction, with partial NA replacement offering a balance between sustainable material usage and desired structural properties.

Research on Practical Teaching Reform of Applied University Courses: A Case Study of "Concrete Structure Design"

By analyzing the current situation and deficiencies of the teaching of concrete structure design courses, summarizing teaching experience, and combining with the characteristics of our university's "applied" talent training, this study explores the teaching reform of concrete structure design courses for the 2021 level civil engineering majors. The exploration primarily focuses on optimizing teaching content, improving teaching methods, strengthening practical components, and diversifying assessment methods. Practice has shown that this approach has a demonstrative effect on undergraduate teaching in civil engineering at our university.